#  DS communications: Trends, Challenges, and Solutions

Sarah Karmous, Nadia Adem, Mohammed Atiquzzaman, and Sumudu Samarakoon Sarah
Karmous and Nadia Adem are with the Department of Electrical and Electronic
Engineering University of Tripoli, Libya (e-mail<EMAIL_ADDRESS>y). Nadia Adem is also with the Authority of Natural Science Research and
Technology, Tripoli, Libya. Mohammed Atiquzzaman is with the School of
Computer Science, University of Oklahoma, USA (e-mail: atiq@ou.edu). Sumudu
Samarakoon is with the Faculty of Information Technology and Electrical
Engineering, University of Oulu, Finland (e-mail: Sumudu.Samarakoon@oulu.fi).

# Demystifying DS communications: State of the Art, Challenges, and Future
Directions

Sarah Karmous, Nadia Adem, Mohammed Atiquzzaman, and Sumudu Samarakoon Sarah
Karmous and Nadia Adem are with the Department of Electrical and Electronic
Engineering University of Tripoli, Libya (e-mail<EMAIL_ADDRESS>y). Nadia Adem is also with the Authority of Natural Science Research and
Technology, Tripoli, Libya. Mohammed Atiquzzaman is with the School of
Computer Science, University of Oklahoma, USA (e-mail: atiq@ou.edu). Sumudu
Samarakoon is with the Faculty of Information Technology and Electrical
Engineering, University of Oulu, Finland (e-mail: Sumudu.Samarakoon@oulu.fi).

#  Demystifying DS communications: Advances, Challenges, and Opportunities

Sarah Karmous, Nadia Adem, Mohammed Atiquzzaman, and Sumudu Samarakoon Sarah
Karmous and Nadia Adem are with the Department of Electrical and Electronic
Engineering University of Tripoli, Libya (e-mail<EMAIL_ADDRESS>y). Nadia Adem is also with the Authority of Natural Science Research and
Technology, Tripoli, Libya. Mohammed Atiquzzaman is with the School of
Computer Science, University of Oklahoma, USA (e-mail: atiq@ou.edu). Sumudu
Samarakoon is with the Faculty of Information Technology and Electrical
Engineering, University of Oulu, Finland (e-mail: Sumudu.Samarakoon@oulu.fi).

# Connecting the Universe: Enablers, Challenges, and Mitigation Techniques

Sarah Karmous, Nadia Adem, Mohammed Atiquzzaman, and Sumudu Samarakoon Sarah
Karmous and Nadia Adem are with the Department of Electrical and Electronic
Engineering University of Tripoli, Libya (e-mail<EMAIL_ADDRESS>y). Nadia Adem is also with the Authority of Natural Science Research and
Technology, Tripoli, Libya. Mohammed Atiquzzaman is with the School of
Computer Science, University of Oklahoma, USA (e-mail: atiq@ou.edu). Sumudu
Samarakoon is with the Faculty of Information Technology and Electrical
Engineering, University of Oulu, Finland (e-mail: Sumudu.Samarakoon@oulu.fi).

# Deep Space Optical Communications: A Survey on Enabling Technologies,
Challenges, and Mitigation Techniques

Sarah Karmous, Nadia Adem, Mohammed Atiquzzaman, and Sumudu Samarakoon Sarah
Karmous and Nadia Adem are with the Department of Electrical and Electronic
Engineering University of Tripoli, Libya (e-mail<EMAIL_ADDRESS>y). Nadia Adem is also with the Authority of Natural Science Research and
Technology, Tripoli, Libya. Mohammed Atiquzzaman is with the School of
Computer Science, University of Oklahoma, USA (e-mail: atiq@ou.edu). Sumudu
Samarakoon is with the Faculty of Information Technology and Electrical
Engineering, University of Oulu, Finland (e-mail: Sumudu.Samarakoon@oulu.fi).

# How Can Optical Communications Shape the Future of Deep Space
Communications? A Survey

Sarah Karmous, Nadia Adem, Mohammed Atiquzzaman, and Sumudu Samarakoon Sarah
Karmous and Nadia Adem are with the Department of Electrical and Electronic
Engineering University of Tripoli, Libya (e-mail<EMAIL_ADDRESS>y). Nadia Adem is also with the Authority of Natural Science Research and
Technology, Tripoli, Libya. Mohammed Atiquzzaman is with the School of
Computer Science, University of Oklahoma, USA (e-mail: atiq@ou.edu). Sumudu
Samarakoon is with the Faculty of Information Technology and Electrical
Engineering, University of Oulu, Finland (e-mail: Sumudu.Samarakoon@oulu.fi).

###### Abstract

With a large number of deep space (DS) missions anticipated by the end of this
decade, reliable and high capacity DS communications are needed more than
ever. Nevertheless, existing technologies are far from meeting such a goal.
Improving current systems does not only require engineering leadership, but
also, very crucially, investigating potential technologies that overcome the
unique challenges of ultra-long DS links. To the best of our knowledge, there
has not been any comprehensive surveys of DS communications technologies over
the last decade. Free space optical (FSO) is an emerging DS technology, proven
to acquire lower communications systems size, weight and power (SWaP) and
achieve a very high capacity compared to its counterpart radio frequency (RF),
the currently used DS technology. In this survey, we discuss the pros and cons
of deep space optical communications (DSOC) and review physical and networking
characteristics. Furthermore, we provide, for the very first time, thoughtful
discussions about implementing orbital angular momentum (OAM) and quantum
communications (QC) for DS. We elaborate on how these technologies among other
field advances, including interplanetary network, and RF/FSO systems improve
reliability, capacity, and security. This paper provides a holistic survey in
DSOC technologies gathering 200+ fragmented literature and including novel
perspectives aiming to setting the stage for more developments in the field.

###### Index Terms:

Deep space optical communications (DSOC), delay tolerant networking (DTN),
detection technologies, interplanetary network (IPN), link capacity,
modulation and coding schemes, orbital angular momentum (OAM), quantum
communications (QC), radio frequency (RF)/free space optical (FSO) systems,
networking protocols.

##  I Introduction

Space exploration offers an opportunity to understand the universe, resulting
in endless benefits for human life diverse aspects such as technologies,
transportation, environment, etc [1]. There is an increased investment from
several worldwide governmental space agencies [2] as well as private companies
[3] aiming to expand human presence in the Solar system [4, 5]. A number of
missions have been recently conducted, e.g. [6, 7, 8] to understand Mars
atmosphere and search for life there. Many other missions are scheduled for
the next several years, such as Rosalind Franklin [9]. There are also plans to
send crews to Mars as early as 2026 [10].

The success of these missions relies heavily on advances in different fields
such as physics, astronomy, and telecommunications. Therefore, advances in
communications technology are natural drivers for achieving these goals.

###  I-A Motivation

Space exploration is built upon reliable broadband interplanetary
communications [11], which are only possible through developing dedicated
technologies suitable for facing the unique deep space (DS) communications
challenges. Unlike Earth and near-Earth, DS communications suffer from non-
guaranteed line-of-sight, ultra-long time-changing distances, variability of
Sun-Earth-probe angles, and the coronal or Solar wind plasma, referred to as
Solar scintillation [12]. These challenges tremendously deteriorate the
quality of the links and hence, limit capacity [13]. Current missions to Mars,
for example, have only a few megabits per second communications rates at the
minimum distance [14, 15, 16]. Fig. 1 shows a timeline for a number of Mars
orbiters, landers, and rovers along with their data rates [7, 17, 18, 19, 20,
21, 22, 23, 24, 25, 26, 8]. Capacity drops by orders of magnitudes as the
distance increases [7]. Moreover, higher attenuation is seen beyond Mars
missions. For example, the Voyager 1 spacecraft, which is located
approximately at 24.1 billion km distance as of October 2023, operates at a
maximum data rate of few kilo bits per second for less than a minute, only a
few times a week [27].

Figure 1: Mars missions timeline with their communications links to Earth data
rates.

To build high capacity DS communications, research efforts have been recently
devoted to exploring the potentials of free space optical (FSO) communications
[28, 13, 29, 30, 31, 32]. Compared to radio frequency (RF), FSO links enjoy
larger bandwidth, smaller apertures at final stations, much-improved security,
less transmit power and higher immunity against interference. Although most
recent DS Mars missions are RF-based [16], many projects have already adopted
FSO for Moon, and Mars communications with plans to expand to other planets
such as Jupiter [33, 34, 35, 36, 37]. In addition, a number of implementation-
based studies have been done to evaluate the technology transition, e.g. [30,
38]. A tremendous work is yet to be done to make FSO technology ready for
future missions [39].

Adopting FSO in DS will play an important role in implementing interplanetary
broadband networks. Achieving such a goal, though, will depend on the
reliability, effectiveness, and efficiency of so many other technologies,
including modulation, coding, detection, and networking protocols. The
criteria for selecting DS communications schemes differ tremendously from
those for terrestrial, airborne, and even near-Earth communications. In this
survey, we highlight the various DS optical communications (DSOC) design
criteria and present the latest research and innovations in the area.

###  I-B Related Surveys

There are few surveys available on DS communications [29, 31, 36, 40, 30, 41,
42, 43, 44], most of which are outdated and/or do not address the field
comprehensively. Although [29] covers various aspects of DSOC, an updated
survey that reflects the last decade field demands, challenges, and advances
is needed. [31] is another survey that covers optical communications in DS,
but only partially. Biswas et al. in [36], on the other hand, focus merely on
the progress made and status of DSOC national aeronautics and space
administration (NASA) projects. In the same direction, [40] overviews mainly
laser-based DS demonstration projects and future missions. [30], however,
provides only studies about DS systems architects and discussions about the
transition from RF to optical technology. In addition, while [41] focuses only
on DS networking aspects, [42] covers mainly some aspects of RF-based DS
communications systems. Although [43] addresses FSO and RF challenges, and
discusses DS systems implementations and demonstrations, it misses presenting
research discussions and future perspectives in the field. Finally, [44] gives
an inspiring relatively-recent overview on DS networking and implementations,
however, it lacks any further details in the other aspects, including physical
characteristics and field advances.

Despite of their valuable contributions, none of the work above covers the DS
communications holistically. In addition, most of these surveys are not
recent. Our work, however, aims to cover the key aspects of DSOC and add the
missing areas from the aforementioned related work while discussing the latest
advances in the field, as we demonstrate in Table I.

###  I-C Contributions

In this survey, we cover existing DSOC systems' limitations, recommended
technologies to cope with them, and challenges facing their adoption. The
contributions of this paper relative to the literature are summarized as
follows.

  1. 1)

We cover major challenges of DS communications. We also discuss DSOC promises,
challenges, and potential solutions and compare it with existing RF
technology.

  2. 2)

We survey the widely used modulation and coding schemes, and the opportunities
for enhancing performance by fusing them with emerging detection techniques.

  3. 3)

We review recommended networking architecture models and present the key
characteristics of promising protocols and identify related open research
directions.

  4. 4)

We present an overview of the interplanetary network (IPN) and study the
visions for its future.

  5. 5)

To the best of our knowledge, we provide the first thoughtful discussion about
implementing orbital angular momentum (OAM) for DS and identify challenges and
potential solutions.

  6. 6)

We discuss the advantages, efforts, and challenges for implementing QC and
partnering RF and FSO technologies in DS.

With these contributions, we aim to pave the way for empowering more rapid
advancements toward robust communications services in DS.

TABLE I: DS COMMUNICATIONS SURVEYS COMPARISON Main theme | Key aspects |  [29] |  [30] |  [41] |  [42] |  [31] |  [40] |  [36] |  [43] |  [44] | This work
---|---|---|---|---|---|---|---|---|---|---|---
Overview | Missions timeline | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\surd$ | $\surd$ | $\times$ | $\surd$ | $\surd$
| Demonstrations | $\surd$ | $\surd$ | $\times$ | $\times$ | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\surd$
| Existing systems | $\times$ | $\surd$ | $\times$ | $\times$ | $\times$ | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\surd$
| Limitations | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\surd$
| Implementations | $\times$ | $\times$ | $\surd$ | $\times$ | $\times$ | $\times$ | $\surd$ | $\surd$ | $\surd$ | $\surd$
| Future prospective | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\times$ | $\surd$ | $\surd$ | $\times$ | $\surd$ | $\surd$
Basics | Link engineering | $\surd$ | $\times$ | $\times$ | $\surd$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\surd$
| Overall challenges | $\surd$ | $\surd$ | $\times$ | $\surd$ | $\times$ | $\times$ | $\times$ | $\surd$ | $\times$ | $\surd$
Enabling technologies | Protocols | $\times$ | $\times$ | $\surd$ | $\surd$ | $\surd$ | $\times$ | $\times$ | $\times$ | $\surd$ | $\surd$
| Modulation | $\surd$ | $\surd$ | $\times$ | $\surd$ | $\surd$ | $\times$ | $\times$ | $\surd$ | $\times$ | $\surd$
| Coding | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\surd$ | $\times$ | $\times$ | $\times$ | $\times$ | $\surd$
| Receiver technologies | $\surd$ | $\surd$ | $\times$ | $\surd$ | $\surd$ | $\times$ | $\surd$ | $\surd$ | $\surd$ | $\surd$
Performance analysis | Capacity | $\surd$ | $\surd$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\surd$
Advances | OAM | $\times$ | $\times$ | $\times$ | $\times$ | $\surd$ | $\times$ | $\times$ | $\times$ | $\times$ | $\surd$
| IPN | $\times$ | $\times$ | $\times$ | $\surd$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\surd$
| Quantum | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\surd$
| RF/FSO | $\times$ | $\surd$ | $\times$ | $\times$ | $\surd$ | $\surd$ | $\surd$ | $\times$ | $\times$ | $\surd$
DS context | Fully | Fully | Fully | Fully | Partially | Fully | Fully | Partially | Fully | Fully
System technologies | FS0 | RF&FSO | RF&FSO | RF | FSO | FSO | FSO | RF&FSO | RF&FSO | RF&FSO
Time relevance | 2011 | 2011 | 2011 | 2011 | 2017 | 2018 | 2018 | 2020 | 2021 | 2023

###  I-D Organization

The remainder of the paper is structured as follows. Section II outlines DSOC
advantages over RF, and implementation efforts and challenges. Section III
analyzes DS links physical characteristics and discusses related research
directions. While Section IV characterizes DSOC capacity limits, Section V
surveys DS networking protocols and related research efforts. Section VI
investigates on state-of-the-art and challenges of DSOC advances including,
IPN, OAM, QC, and RF and FSO integration. Finally, Section VII concludes the
survey.

##  II DSOC Advantages and Challenges

Optical communications promise tens to a hundredfold increase in data rates,
thanks to the high available bandwidth in the license-free spectrum in which
the technology operates [31, 45]. Such a capacity improvement has already been
proven in demonstrations [46]. To illustrate further the gain of using DSOC
compared to RF, we, based on [39], define the figure of merit (FoM) metric as

$FoM\mathrel{\ensurestackMath{\stackon[1pt]{=}{\scriptstyle\Delta}}}{10}\times\log\left({\frac{Dis^{2}\times{R}}{{Dia}\times{P}}}\right),$
(1)

where $Dis$ is the end-to-end DS link distance in the astronomical unit (AU).
$R$, $P$, and $Dia$ are the data rate in bits per second, transmit power in
Watts, and receiver diameter size in meters, respectively. As it accounts for
space terminal performance, measured through data rate, and cost, reflected
through receiver aperture size and communications power, at any distance, the
FoM metric provides a direct comparison between the RF and FSO technologies at
the system level.

Based on projections available in [14, 40, 46], Fig. 2 provides a comparison
of RF and FSO Lunar and Mars systems. Unsurprisingly, a Lunar FSO system
represents more than 15 dB gain over the RF counterpart. The improvement seems
to be even way more substantial over Mars distances. However, implementing FSO
in DS communications faces a number of unique challenges.

Below we discuss different challenges facing DS in general at the link level,
the DSOC link budget, and the optical communications challenges.

Figure 2: FoM of DS communications systems.

###  II-A DS Communications Challenges

Ensuring high capacity and reliable communications in DS faces the challenges
listed below.

  * •

Ultra-long, which based on the International Telecommunications Union exceeds
2 million kilometers, and time-changing distance between Earth and probes
(satellites in other planets' orbiters, rovers, etc.).

  * •

Interference from communications with other missions, radiation from the Sun,
other celestial bodies, and cloud cover.

  * •

Solar scintillation, atmospheric conditions, and other channel impairments.

  * •

Variability of Sun-Earth-probe angles and the effect of that on blocking line-
of-sight links.

  * •

The burden of size, weight, and power (SWaP) constraints of spacecraft
systems.

  * •

The readiness and availability of transmitting and receiving technologies and
communications protocols.

The communications with targets in DS require sending signals over very long
distances, leading to strong power dissipation and hence a huge signal
attenuation. Even with relatively large gain transmit and receive antennas,
the received power can still be very weak. Also, the long-distance links
result in an inevitable significant latency that prevents real-time
communications and control. For example, a two-way communications with Mars
may undergo 22 minutes transmission delay.

Furthermore, the Solar flares are shown to make communications signals weaker
and noisier [29]. Besides, Solar scintillation and turbulence lead to
performance degradation. It is important to carefully account for the near-Sun
effect on the signal strength to compensate for any losses while preventing
any damage to receiver circuits. Also, due to the movement of Earth and
probes, links capacity get hugely affected. The Sun may completely block
communications path.

In addition, SWaP constraints are of a major concern not only for efficient
spacecraft design but also for communications performance improvement.
Designing a space mission with ultra-lightweight resources is thought of as a
way to utilize a better communications performance [29]. There are
technologies such as Gossamer that has the premise to design spacecraft
structures with thin, low-modulus, and low-mass materials. An example of
Gossamer technology application is the inflatable antennas that could have an
areal density of a fraction of a kilogram per square meter [47]. Also, in the
direction of optical communications, the development of ultra-lightweight
telescopes is realized, such as Cassegrain telescope that has composite
mirrors, with areal density in the range of 1–10 kg/m2 [48].

Lastly, existing terrestrial technologies, especially at the physical and
protocol levels, can not be adopted directly in DS. Digital modulations
schemes are preferred, in DS RF-based communications for example, as they are
simpler in implementation, less lossy, and more power and bandwidth efficient.
Nevertheless, amplitude modulations that can not be generated with nonlinear
amplifiers are not suitable for DS as working in saturation mode is needed to
maximize power efficiency [49]. Also, in the networking, commercial
terrestrial network architecture, for example transmission control protocol
(TCP)/internet protocol (IP), are not efficient to ensure data delivery in the
DS time-changing environments [50].

Advanced techniques are needed to overcome DS challenges and account for
unique link characteristics which we elaborate on next.

###  II-B Link Engineering of DSOC

Designing DS optical links need to account for all link parameters and circuit
characteristics. This includes the end-to-end distance between Earth and final
targets, mission requirements, required capacity, atmospheric conditions,
transmitter-Sun-receiver angle, etc. The received power at the target, after
the receiver telescope, and before the photo-detector, can be expressed as
[28]

${{P}}_{{r},{ap}}{=\
}{{P}}_{{t}}\;{{G}}_{{t}}\;{{G}}_{{r}}\;{{L}}_{{fs}}\;{{L}}_{{pt}{}}\;{{L}}_{{a}}\;{{L}}_{{c}}\;{{L}}_{{s}}\;{{\textrm{$\eta$}}}_{{t}}\;{{\textrm{$\eta$}}}_{{r}}$
(2)

where:

  * •

$P_{t}$ (Watt) is the transmitted power.

  * •

$G_{t}$ and $G_{r}$ are the transmitter and the receiver aperture gains,
respectively [51, 52].

  * •

$L_{fs}$ is the free space loss factor. Being inverse proportional with the
link distance squared, $L_{fs}$ represents the link dominant loss factor.

  * •

$L_{pt}$ is the pointing loss factor. A pointing error of $\lambda/D$ radians,
where $\lambda$ and $D$ are the signal wavelength and the transmitter
diameter, respectively, corresponds to approximately 85% intensity reduction
[53, 51].

  * •

$L_{a}$ is the atmospheric transmittance due to the aerosol and molecular
particles in the atmosphere that cause the absorption and scattering of the
signal. The recommendation is to operate close to the 1550 nm wavelength range
for low attenuation transmission [54, 55, 56].

  * •

$L_{c}$ is the transmittance of the semi-transparent ice clouds, cirrus,
present in the communications path[57].

  * •

$L_{s}$ is the scintillation loss factor, which refers to the random optical-
power fluctuations caused by atmospheric turbulence (AT) [58, 59].

  * •

$\eta_{t}$ and $\eta_{r}$ are the transmitter and receiver efficiencies,
respectively [60].

The various DSOC impairments reflect the importance of the link engineering in
deriving the needed transmit power to meet a certain detection sensitivity and
hence a service quality. It worth mentioning that the atmospheric and weather
limitations are not of concern for optical space-to-space links [29, 31, 61,
62, 32]; making choosing the FSO over the RF in these links to be of a great
value. This fact can be reflected in the drop of $L_{a}$, $L_{s}$, and $L_{c}$
loss terms in (2).

More investigations about potentials of the FSO technology in improving DS
communications along with challenges facing its implementation are provided
below.

###  II-C DSOC Promises and Challenges

RF technology is currently dominating space communications facilities. The RF
systems maturity and the cost of implementing FSO are delaying the
technological migration. The nonobligatory of RF line-of-sight link
availability [63] along with its tolerance to atmospheric effects and weather
conditions, for example fog and snow, which can affect communications with
ground stations [64] are also contributing factors to deferring the transition
to FSO.

Nonetheless, the RF bandwidth while being fairly limited requires licensing,
and is susceptible to eavesdropping [63]. In addition, having the demand of
high data rate achieved would come at the cost of increasing SWaP requirements
which can be unfeasible and hence restricting RF systems performance
upgrading. For instance, if we are acquiring several giga bits per second as
downlink data rate from the closest distance to Mars, according to [30], we
would need the Mars spacecraft to have a 5 m antenna with a transmitting power
around 200 W, and a multiple of 34 m antennas in an array as the ground Earth
receiver. In contrast, for optical, the Mars spacecraft would have a 1 m
aperture and a transmitting power of 50 W, and the receiver telescope aperture
size would be in the range of 10 m.

FSO, however, is emerging to address the shortcomings of conventional RF
systems. Since FSO beams are narrow divergence, it requires significantly
lower SWaP requirements than RF. Hence providing flexible systems upgrades and
more secure communications [63]. Not to mention the almost limitless bandwidth
with no license requirements the FSO offers [65].

Nonetheless, achieving a required performance over an optical link design
increases in difficulty with its distance [29]. An optical signal power
degrades proportionally to the square of the link distance. A high gain is
required to make up for the very long distances loss and along with other
losses discussed in previous subsection. DSOC hurdles can be compensated
through a number of approaches, that come with their challenges though, which
we identify below along with potential solutions.

  * •

Increasing the power efficiency by relying on laser beams with large gain
transmitter and large aperture diameters. Nonetheless, the cost of building
large optical ground transmitters is considered to be an implementation
concern [38]. Especially that the average power requirement for a DS link
typically would be in the range of 1–20 W, and peak power is around 100–1k W
[29].

  * •

Accurately tracking the space terminals using beacons to point lasers towards
them before sending data. Narrow beam requires an accurate pointing in order
for the signal to be received at the destination, which is challenging
considering DS distances [66]. Particularly, regarding flight systems, high
accuracy of the beam pointing is needed, especially for downlink signals from
DS that are typically with a beam-width in the range of 2–8 $\mu$rad. Adequate
beam acquisition at destinations can be achieved, though, at the cost of
computational and hardware complexities [67]. In addition, tracking and
pointing beams toward mobile receivers in space require efficient design of
accurate tracking algorithms.

  * •

Relying on large gain receivers. The received power to spacecrafts would be
around or less than 10-10W [28]. As a consequence, that requires a high gain
receiver telescopes, which can make it a challenge to obtain a functional, yet
optimized design of the a spacecraft resources [29].

  * •

Considering advanced signaling and detection techniques that maximize photon
efficiency, i.e. number of bits per photon, that for instance would include,
obtaining high detectors sensitivity and acquiring efficient modulation with
coding schemes [68].

  * •

Merging other technologies, including OAM for example, with FSO to get the
most out of its high capacity and further enhance the quality of DS data
returns [31]. More investments though need to be put forward to facilitate
such a merge.

There are a number of efforts being brought forth to facilitate the full
implementation of the FSO technology in DS [33, 34, 30, 35, 69, 36, 70],
addressing the DS FSO technology engineering challenges and maturity
requirements [29, 39]. In this paper, we survey the efforts done in this
direction and identify open research directions.

##  III DSOC Physical Layer Key Characteristics

This section reviews the different physical techniques that mitigate a number
of the previously discussed challenges to ensure DSOC reliability.

###  III-A Modulation Schemes for DSOC

Conventional RF-based space communications relies commonly on binary phase
shift keying (BPSK) modulation [71], where a carrier modulates data using two
phases that are 180 degrees apart. BPSK has been favored in such an
environment, thanks to its low bit error rate (BER) in the low signal-to-noise
(SNR) ratio regime. However, for DSOC, there are other direct detection based
modulation schemes of interest including on-off keying (OOK), pulse position
modulation (PPM), wavelength modulation, and pulse intensity modulation. OOK
and PPM are the most recommended among the methods mentioned above, thanks to
their simplicity and power efficiency [31, 29]. Even though they are similar
in concept, PPM is more favored than OOK for DSOC as the former is more power-
efficient. On the other hand, OOK is more suitable for near-Earth links as it
is cost-effective and bandwidth efficient [31].

While, OOK modulates one as a pulse and zero with the absence of a signal,
M-ary PPM modulation, which is often used with photon-counting (PC) detector
[29, 54, 72], works by dividing each channel symbol period into $M$ time
slots. $\log_{2}M$ bits are modulated by transmitting a single pulse in one of
the $M$ possible time slots of a symbol. The information in the PPM is encoded
in the position of the pulse, making it robust against noise. PPM requires a
high bandwidth though [73]. The good news, however, is that high bandwidth is
readily offered by FSO and hence is not of much concern.

To characterize the effectiveness of PPM for DS applications, we provide the
capacity offered by the PPM-PC receiver in the presence of noise and using a
Poisson channel model, typically used for DS system characterization [74]. The
approximate system capacity, denoted by $C_{PCR}$, is given by [75]

$C_{PCR}=\frac{1}{\mathrm{{ln}(2)}{E}}\Bigl{(}\frac{{P_{r}}^{2}}{P_{n}\frac{2}{M-1}+\frac{P_{r}}{\mathrm{ln}(M)}+{P_{r}}^{2}\
\frac{M\ T_{slot}}{{\mathrm{ln}(M)\ }{E}}}\Bigr{)},$ (3)

where the photon energy $E=(hc)/\lambda$, $h$ is Planck's constant, $c$ is the
light velocity, and $\lambda$ is the signal wavelength. $M$ is the modulation
order. $P_{r}$ and $P_{n}$ are the received and noise power, respectively.
$T_{slot}$ is the pulse duration. The three terms in the denominator of the
second term of (3) correspond to three different behaviors when dominating. As
$P_{n}\frac{2}{M-1}$, which is a noise-limited term, dominates, the capacity
scales as a quadratic function of the signal power. On the other hand,
$P_{r}\frac{1}{\ln(M)}$, which is the quantum-limited term, leads to a linear
scaling of the capacity. Finally, the bandwidth-limited term,
${P_{r}^{2}}\frac{MT_{slot}}{\ln(M)E}$ leads to capacity saturation. Note that
when the received $P_{r}$ is sufficiently high, the system capacity can be
approximated as

$C_{PCR}\approx\frac{1}{\ln(2)}\frac{\ln(M)}{M\ T_{slot}}.$ (4)

If the received power is sufficiently high, one can easily observe that it is
best to use low PPM modulation order.

In Fig. 3(a) and 3(b), we show the PPM systems achieved capacity for Mars and
beyond Mars distances. Here we pick the antenna diameter for transmitter and
receiver as 0.22 m and 4 m [28, 38, 68], respectively. The figure shows that
as the modulation order decreases, the capacity improves significantly but
only up to a certain distance. That is intuitive as increasing the modulation
order implies a reduction in the transmission rate and BER. As depicted in
Fig. 3(b), as a signal travels a longer distance, in our settings close to
2$\times$109 km, the trend changes. As distance gets larger and larger,
however, to maintain a certain BER performance, the modulation order needs to
be increased. Fig. 3 indicates how crucial the increase in DS communications
distance is in determining the PPM system parameters and hence performance,
for a given channel model, detector, and other system parameters including
receiver antenna diameter and transmission power, as discussed in [70] and a
number of references therein. The impact of combining the PPM modulation
technique with coding is also of crucial importance as we will discuss.

(a) Mars Distances Capacity

(b) Beyond Mars Capacity

Figure 3: Link capacity versus distance.

###  III-B Coding for DSOC Links

Channel coding is a very important block in the transmission chain to enhance
performance and ensure reliability. Applying ideal error control coding (ECC)
may lead to meeting a link capacity limit, maximum possible achievable
capacity. However, such ideal codes are hard to construct in practice.
Therefore, it is important to consider the code efficiency, which refers to
the difference between the capacity limit and the error code threshold at a
specific BER, when designing communications links [76].

In the early missions to DS, around sixties, a decision was needed on whether
to rely over a coding method or the other, namely block codes or convolutional
codes. During the design of Mariner '69 Mars mission, for example, block codes
were the best options, given that developed sequential decoding algorithms
were not that efficient for DS. Such a choice led to a poor data rate, though.
Thanks to the development of a more efficient sequential decoding algorithm,
convolutional decoding methods, for example Turbo codes, have been adopted to
various communications channels, including DS [77], starting from Pioneer 9
mission [78], which was designed after Mariner '69 but lunched earlier. Such a
change offered a coding gain, which is the power gain over the uncoded case,
of 3 dB [79] compared to that of 2.2 dB [78] in the case of block code used by
Mariner '69.

Reed-Solomon (RS) codes are commonly used with convolutional codes for DS
application as they are efficient in correcting complex and bursty errors [41,
54, 29, 80], and have in the order of 2.5 to 3 dB efficiency [53]. As a
reference, an uncoded PPM-based system would have a code efficiency in the
order of 5 dB [73]. Nonetheless, the most recommended and efficient DS coding
scheme is the serially concatenated PPM (SCPPM) [81, 82, 73]. This coding
technique has a code efficiencies between 0.5 and 1 dB [53, 73]. Fig. 4 shows
the BER of SCPPM versus average signal power measured in signal photons
detected per modulation order and denoted by $P_{av}$, compared to the
capacity limit, Reed-Solomon PPM (RSPPM), and uncoded PPM. The figure confirms
the superiority of the SCPPM over RSPPM, as it is clearly demonstrating the
lower coding efficiency and higher gain SCPPM achieves over RSPPM.

Recently, there have also been some suggested codes in the field. For example,
Divsalar et al. in [83] show that coded optical wavelength division multiple
access (WDMA) with PPM can be a good candidate for DS. Using the coding gain
metric, WDMA has a 7 dB gain for a 0.8 coding rate, and a 12 dB coding gain
for a coding rate of 1/6 at a frame error rate (FER) of 10-6 [83].

Since fixed-rate codes like low-density parity-check (LDPC) and Turbo codes
are not efficient for DS communications as they fail to adapt to the changes
in communications link due to time variability, Liang et al. in [84] proposed
the use of rate-less codes such as raptor codes, and hence achieve a better
performance than LDPC and Turbo codes. The authors propose to further combine
spinal and polar coding to achieve even better performance.

Figure 4: Coding techniques BER compared to uncoded communications and
capacity limit [54, Fig. 4-35].

Further improvement can be offered through the use of efficient detection
method as we will elaborate next.

###  III-C Detection Techniques for DSOC

Advances in detection technology play an important role in providing reliable
DSOC. As indicated in Section II, having improved sensitivity and efficient
detection are regarded as means to overcome faint DS signals issues and design
limitations aspects. Detection receivers can either be coherent or non-
coherent. Coherent receivers can have a higher spectral and dimensional
information efficiency in one state system, examples of which are heterodyne
and homodyne receivers. Dimensional information efficiency, according to [85],
is defined as the efficiency of information transfer per a unit dimension or
optical communications degree of freedom. Even though coherent receivers
suffer from limited photon efficiency, they tend to be more practical when
very high data rates are required. Non-coherent receivers, on the other hand,
are energy efficient but come at the cost of an exponentially decreasing
dimensional efficiency [85]. Rendering to their energy efficiency, non-
coherent receivers which are alternatively called direct detectors are
commonly used in DS [53, 86]. There are different types of direct detectors
among which are avalanche photodiode detector (APD), and PIN (p-type,
intrinsic, and n-type) diode, and PC detector.

TABLE II: DSOC DETECTORS Detectors/ Receivers | Type |  Implementation |  Sensitivity (PPB) |  Data rate (Mbps) |  Advantages |  Disadvantages
---|---|---|---|---|---|---
InGaAs |  Direct | PIN |  Cubesats for DSOC systems |  - |  $163$ |  Relatively high temperature operation |  Relatively high received power requirement, not a good option as a ground receiver due to atmospheric losses
GmAPD | APD |  Mars laser communications demonstration (MLCD) project, imaging system for DSOC Project 2022 |  2 |  $34.9$ |  No read out noise penalty |  Limited bandwidth
SNSPD | PC |  Lunar laser communications demonstration (LLCD) demonstrations ,DSOC Project 2022 |  0.5 |  $781$ |  High detection efficiency ($90\%$) |  Requirement for cooling down to 2–4 K, high photon rate detection (multiple of Gbps) can be hard to attain
SQ |  Coherent | Homodyne |  Near-Earth and DS applications |  1.5 |  156 |  Energy efficient |  Limited bandwidth
SPC | Heterodyne |  EDRS, DS communications (suggested) |  10 |  1800 |  Robust to severe background noise, near-Sun communications |  High bandwidth requirement
PSA | Pre-amplified |  DS application demonstrations |  1 |  10500 |  Noise-free amplification |  High performance in limited spectral efficiency range

Due to their high sensitivity compared to other conventional direct detection
methods, PC detectors are gaining great attention and are recommended along
with PPM modulation to be used in the future DSOC. These detectors can count a
single photon incident on them. PC detectors can be viewed as an extension of
APD detectors with infinite gain in which a digital output signal is generated
for each detected photon [86]. They can offer up to 20 dB gain along with PPM
over APD and pre-amplified direct detectors [29], and sensitivities of a few
photons per bit [74]. The PC detection process, though, is affected by noise
sources which are $i)$ background noise that comes from the diffused energy
from the sky, planets, or stars in the field of view of the receiver, $ii)$
dark counts electrons generated by the detector, and $iii)$ receiver thermal
noise [28]. In addition to noise, the losses that degrade the PC detection
efficiency are $i)$ Blocking loss which results from the fact that a detector
gets blind after a detection event missing all incident photons until reset.
Blocking loss limits a detector to count at most one photon per reset time or
dead time [60]. $ii)$ Jitter loss which is the random delay elapses from the
incident of a photon on a detector to the time an electrical output pulse is
generated in response to that photon. A significant loss occurs if the
standard deviation of the jitter is in the order of the slot width of the
pulse [87].

The choice of the detection and modulation schemes is essential in mitigating
other losses resulting from link impairments. Detectors that handle a small
duty cycle in the range of a few nanoseconds, for example, are resilient to
long DSOC channel losses [88]. An example of a detector that can provide such
a requirement is the superconducting nanowire single photon detector (SNSPD)
[89], which is a commercialized highest-performing PC detector in the
frequency range of 30–30,000 THz [90]. This detector can support slot widths
down to 200 ps with detection efficiencies of 50–78% [89]. There are some
other examples of detectors and receivers that are significant in DS
communications, such as indium gallium arsenide (InGaAs) [91], Geiger-mode
avalanche photodiode (GmAPD) [92, 93, 94], SNSPD [95, 96, 97, 98, 90], single-
quadrature (SQ) [99], European data relay system (EDRS) with single photon
coherent (SPC) heterodyne detection [100, 101], and phase-sensitive optical
amplifiers (PSA) [74].

In Table II, we include the main characteristics of the aforementioned
detectors and receivers, including sensitivity measured in photon per
information bit (PPB), supported data rate, advantages, and shortcomings.

###  III-D Discussions and Outlook

We stated earlier the different potential modulation, coding, and detection
schemes for DS communications. It is worth reiterating that yet
experimentation with the efficiency of existing schemes needs to be conducted.
DSOC systems are expected to operate at the speed of several tens of gigabits
per second and beyond and hence will require a major improvement over existing
receiver technology in terms of both data rate and sensitivity [54]. Thanks to
their energy efficiency and simplicity, direct-detection systems are preferred
over coherent systems. Nevertheless, coherent detection has recently started
to get more interest due to its ability to increase the capacity in DS
communications [74].

While using single-quadrature homodyne receivers, without a pre-amplifier,
resulted in a sensitivity of 1.5 PPB at 156 Mbps [99], demonstrations have
shown that pre-amplified erbium-doped fiber coherent receivers can achieve
0.51 PPB increase in sensitivity and 10 Gbps rate [102, 103]. In addition, it
is also worth mentioning that there is some focus on in-phase and quadrature
modulation homodyne detectors in optical transmission systems for terrestrial
as well as DS communications as it promises to achieve higher spectral
efficiency than other schemes [104].

##  IV DSOC Physical Layer Performance Analysis

In order to decide on a modulation type and order, a coding technique, and a
detection approach, the channel capacity needs to be determined or at least
approximated for a given system setup.

###  IV-A Capacity Limit

Usually, the Shannon capacity gives a channel maximum error-free data rate in
classical information theory [105]. However, in optical communications,
channels are best described as quantum channels, shown to accurately describe
capacity limit of a corresponding system. For extremely power-limited
communications, as it is the case in DS, the capacity limit is best
characterised by Holevo theorem, which approximates the solution of the
quantum channels capacity limit [106, 107]. Based on Holevo theorem [85], the
analytical closed-form expression of the quantum capacity, which
asymptotically expresses the dimensional information efficiency, measured in
bits/dimension, is given by

${C_{d}}^{Hol}=ec_{p}2^{-c_{p}},$ (5)

where $c_{p}$ is photon information efficiency and e is the exponential
constant. In theory, the optimal coherent-state modulation achieves the
ultimate quantum limit, which is the capacity limit that can possibly be
approached by a system based on Holevo limit approximation. Still, practical
sub-optimal coherent receivers are operating in near the ultimate Holevo limit
but in the low photon efficiency region as they hit a brick wall of $1.44$
bits/photon and hence they are not efficient for DS applications [85].

In the case of PPM-PC receivers, the capacity can be approximated with respect
to the Holevo limit as [106]

$C_{Holevo}=2.561C_{PCR},$ (6)

where $C_{PCR}$ is given by (3). Dolinar et al. in [108] proposed using
coherent states with PC to get closer to the Holevo limit; thanks to its
ability to produce coherent states. This is challenging and not practical from
an implementation perspective, though. Quantum number state (NS), also known
as Fock state [109], communications could be an alternative, where the quantum
state is with a well-defined number of photons are shown to be able to reach
the Holevo limit [108]. This could be challenging in practice, again though,
since it requires high channel transmissivity, which is the probability of
transmitted NS photon being received at the detector. To further illustrate
this, considering OOK modulation, the single-photon NS capacity at high photon
information efficiency can be written in terms of the channel transmissivity,
denoted by $\eta$, asymptotically as

${c_{d}}^{1NS}={C_{d}}^{Hol}(2^{f(\eta)}),$ (7)

where if ${\eta}<<1$, then $f(\eta)=\eta/e$ and when $\eta{\sim}1$,
$f(\eta)=({\eta}-1)^{(\eta-1)}/e^{1-{\eta}}$. In the case of PPM, we have

${c_{d}}^{1NS}={C_{d}}^{Hol}(\eta/e).$ (8)

As shown in Fig. 5, PPM cannot reach the Holevo limit even when $\eta=1$. With
a transmissivity of one, NS along with OOK gets to the ultimate limit. As the
quantum NS transmissivity gets higher, the capacity gets closer to the Holevo
limit in the case of OOK. We also show in the same figure the capacity limit
for PPM-PC, and quantum NS modulation with OOK and PPM.

Figure 5: Asymptotic capacity limit of different modulation schemes.

As a way to analyze the DSOC links performance for a PPM-PC detector, in Table
III, we show the soft-decision capacity at various planets average distance.
Soft-decision capacity corresponds to implementing a soft-decision receiver
which passes on slot counts and use them to determine the probability of
sending a certain sequence of bits [54]. Here we assume that the transmitter
and the receiver's aperture diameters to be 0.22 m and 4 m, respectively, a
transmitted power of 4 W [28, 38, 68], a 0.25 ns time slot, a PPM modulation
order of 16, and a noise power of 1.1620$\times$10-16 W. The losses of Earth
to Mars communications, for example in the considered setup, are as follows: a
path-loss of 366 dB, the sum of other losses for the received power not
including the detection loss is 6.6 dB, and the detection loss is 4.35 dB. We
notice that the capacity dramatically gets smaller as we get to further planet
distances.

TABLE III: DIFFERENT PLANETS CAPACITY Planet | Average | Average | Received | Reached
---|---|---|---|---
| distance | delay | power | capacity
| (km) | (Minutes) | ($\mu$W) | (Mbps)
Mercury | 58 million | 3.22 | 5.1856e-6 | 139.45
Mars | 225 million | 12.5 | 4.5053e-6 | 123.42
Jupiter | 778 million | 43.22 | 1.7741e-6 | 52.527
Saturn | 1.2 billion | 66.66 | 9.2772e-7 | 28.173
Neptune | 4.5 billion | 250 | 7.8951e-8 | 2.4598
Pluto | 5.9 billion | 327.77 | 4.6219e-8 | 1.4409

###  IV-B Discussions and Outlooks

Although direct detectors can not reach capacity limit, the capacity can be
improved by increasing the detector sensitivity, selecting a modulation format
with a high peak to average power ratio like PPM, and reducing the amount of
background light detected. Compact array detectors, which decrease blocking
and pointing losses and help with beam acquisition and tracking process [110],
achieve higher data rates than a single detector, only for limited distance
though [28]. In addition, while being flexible and scalable, multiple
receivers are considered more cost-effective than single receivers for data
rate improvement [92]. However, the single receiver could outperform the array
in case of no AT [111].

##  V DSOC Networking Protocols

In addition to the different physical layer techniques and their impact on
DSOC performance as we discussed in the sections III and IV, networking
protocols are of paramount importance to make the communications links
reliable and overcome a number of different complex challenges facing DSOC
discussed in Section II. Seamless coordination between satellites, ground
communications systems, and DS probes is needed. Matured terrestrial network
protocols are mostly inefficient in DS environments where distances are
extreme and connections are intermittent; hence, as suggested in [112],
calling for well-tailored network protocols is essential. NASA, for example,
relies on delay/disruption tolerant networking (DTN) [50, 113, 114]
architecture as an intuitive solution for space missions. The architecture can
handle excessive delays and fast-changing environment while maintaining
reliability.

The Consultative Committee for Space Data Systems (CCSDS), which is one of the
most important organizations that discuss and set standards for space data
systems, recommends architectures for space networks, such as space
communications protocol specifications (SCPS) architecture [115], space packet
protocol (SP)-based architecture [116], and IP architecture. Due to its
ability to handle long and variable delays and link intermittent connectivity
issues, DTN, however, is the most persuasive architecture. The automatic
store-and-forward feature of DTN, which allows packets to be forwarded to the
next destination only if the availability of the corresponding link is
ensured, is making the DTN architecture a natural choice for severe
environments [117, 118].

###  V-A DTN Architecture Layers and Protocols

DTN architecture consists of the application, bundle, transport, internet,
data link, and physical layers. Below we discuss some of the fundamentals and
functionalities of these layers, along with corresponding protocols designed
for DS internetworking.

Application layer: In this layer, CCSDS file delivery protocol (CFDP) is a
widely used protocol [119]. In CFDP, the data units are delivered
independently from each other, aiming to increase the amount of communicating
data [120, 121]. Space missions, however, do not only rely on file transfer
for all their communications requirements. CCSDS proposed messaging exchange
protocol called asynchronous message service (AMS), which is suitable, for
example, in real-time control and robotic operation collaboration. AMS is
designed to reduce the communications burden of space missions and hence their
cost and risk of failure [122].

Bundle layer: Next to the application layer, there is the bundle layer. This
layer distinguishes DTN from the conventional TCP/IP architecture. Due to
frequent disruptions with long transmission delays, in a hop-by-hop transport
fashion, the bundle layer that only has the bundle protocol (BP) adds a store-
and-forward mechanism for each DTN node, i.e., every engine that runs BP in
the network, and sends data in large packets called bundles. The BP mechanism
is different than end-to-end protocols such as TCP, where reliability is
achieved through end-to-end acknowledgments and retransmission excluding the
intermediate nodes [123]. TCP or end-to-end protocols have a low transport
efficiency if used in the DS network [50]. The hop-by-hop principle, however,
enhances transport efficiency in such challenging environments [50]. To
enhance reliability and reduce congestion, BP supports custody transfer which
is an automatic retransmission request (ARQ) mechanism that takes custody of a
bundle, node by node, until it reaches its destination [124]. ARQ mechanisms
are implemented in the data link layer protocol as they are commonly used,
however, in addition to that, they are implemented in the upper layers of the
DTN network architecture [123], to ensure reliability of data delivery. An
optional end-to-end acknowledgment feature [125] could be used for situations
where original data senders are hard to reach for a request of retransmission
[126]. Furthermore, bundles are carried out through convergence layer adapters
(CLAs) which allows DTN to encompose different underlying protocols and hence
work well in a heterogeneous DS networks [127]. Conventional routing
protocols, most built for the TCP/IP architecture, are not efficient for time-
variant DS networks [50]. In efforts to tackle these challenges, contact graph
routing (CGR) protocol has been developed for space networks based on DTN
architecture [128]. CGR exploits the mission communications operation plans to
construct a contact graph that maps time-varying network connections which is
used as a consequence for bundles' best route calculation.

TABLE IV: DTN ARCHITECTURE AND A CORRESPONDING CCSDS PROTOCOL STACK Layers | Protocol |  Functions & Mechanism |  Advantages |  Disadvantages
---|---|---|---|---
Application | CFDP |  It is capable of file transfer, delivery, and storage management services, namely loading, dumping, and controlling [129]. It assumes that communications entities have storage mediums called filestore. The protocol operates by copying data between these mediums. It has a delivery process that gives the capability of transferring files in arbitrary networks and works in a reliable as well as an unreliable mode |  Scalable, minimizes the resources required to only the necessary elements for file transfer operation [129] |  Causes overhead in the network when used along with some other subsequent protocols [41, 119], does not preform well for one-directional transmission long-delay communications link [130]
| AMS |  AMS provides a asynchronous message exchange services [131]. It is designed to make the functioning parts of an application, called modules, operate in isolation and make communications between these modules self-configuring. Hence complexity of the modular data system is reduced |  Simple to use, highly automated, flexible, robust, scalable, efficient [122] |  -
Bundle | Bundle |  It applies overlay network principle which provides hop-by-hop transmission for data delivery and status reporting [124]. Information is sent in bundles, large groups of data, in a store and forward fashion with optional custody transfer [132] |  Data accountability, reliability in heterogeneous networks [124] |  Relatively high memory consumption [124], not 100% reliable [133]
Transport | LTP |  It provides optional reliable communications, as it divides blocks of data into red-part and green-part where the former need to be delivered reliably but unnecessarily the latter [134] |  High throughput for the application level data, i.e. high good- put [135] |  In-order delivery is not supported, low performance for short distance and small delay, with low BER link [135]
Internet | IPoC |  IPoC specifies the implementation of the IP protocol over CCSDS SDLPs. The protocol data units (PDU) of IP get encapsulated by EP and then sent for framing by SDLP [136] |  IPv4 and IPv6 are commercially available technologies directly implemented in space, with good transfer performance and cost-effectiveness for short links [123] |  Routing fails in long transmission delays [136]
Data link | USLP |  It utilizes features and functions of existing CCSDS data link protocols which are Prox-1, TC, TM, and AOS, with added advances. It was developed to meet the requirements of space missions for efficient transfer of data [137] |  Designed to reduce cost and complexity, increases data rate, and meet the required data rate expected for DSOC, which can not be reached with the previous CCSDS standards [137] |  -
Physical | CCSDS-HPE |  It specifies the characteristics of the transmitted laser signal [138] |  - |  -

Transport layer: TCP is considered an option for reliable data transmission
over proximity links in the transport layer. User datagram protocol (UDP)
could be another option for loss-tolerant connections [139]. On the other
hand, the Licklider transmission protocol (LTP) [134] comes as an alternative
transport protocol (TP) designed to overcome long transmission delays and
frequent interruptions. However, BP needs CLAs to interface between the
different corresponding TPs [140]. Examples of such adapters are TCP, UDP, and
LTP-based CLA s [141].

LTP is the most common transport layer protocol used in DS. It has most of the
UDP and TCP functions; LTP, however, does not perform handshakes or congestion
control like TCP [142]. Some efforts have been made to improve LTP in memory
consumption [143], reliability and energy efficiency [144].

Internet layer: When it comes to addressing nodes and packet delivery, i.e.,
internet layer functionalities, nodes that are communicating in the same
region, protocols like IP can be used [145]. In an attempt to make IP more
compatible with the space environment, however, IP over CCSDS space links
(IPoC) is recommended [136]. If a sender is located in a region different than
the destination, however, e.g. two different planets, bundles are addressed
differently for more effective routing. Addressing schemes like IP in general
indicate the geographic location of the host. Such methods, however, provide
no information about the host itself. In the case of interplanetary
communications, the location of the host might change during the course of
delivering the data, and hence information about the host itself is beneficial
for accurately assigning destinations [146]. A naming structure called
universal resource identifiers (URI), also referred to as endpoint identifiers
(EID), where each DTN node gets associated with an identifier, a type of
address that specifies the region, application, and host [127], plays an
important rule for achieving efficient interplanetary communications.

Data link layer: Packets from the internet layer get encapsulated either by
space packets (SP) [116] or encapsulation packets (EP) protocols [147].
Afterwards, packets go through the framing process in the data link layer with
protocols provided by the space data link protocols (SDLPs). These protocols
are telecommand (TC) [148], telemetry (TM) [149], proximity-1 (Prox-1) [150],
advanced orbiting systems (AOS) [151], and unified space data link protocol
(USLP) [152]. CCSDS SDLPs are responsible for transferring data with different
types and variable lengths in space links. TM, AOS, and USLP can be used for
optical communications [138]. Frames go through coding and synchronization
data link sub-layer to prepare them to the physical layer, which transmits
them to the medium based on the physical layer technical standard.

Physical layer: For DSOC, high photon efficiency (HPE) specifications are
recommended by CCSDS to be used in the physical layer. This standard
characterizes the transmitted laser signal, including used modulation and
related parameters, for space-ground and ground-space links [138].

In Table IV, we present some other aspects of the most significant and
advanced DS protocols discussed above corresponding to their DTN layers.

Figure 6: BP custody transfer, CLAs, and CGR dynamics visualization

To help visualize how DS DTN defining features work, Fig. (6) further explains
the concept of CLA, CGR, EIDs, and custody transfer. Let us build a scenario
for transmitting information from a mission control node located on Earth N1
to a Mars rover N5. To have that happen, a bundle, denoted by B, is created
with the destination EID that includes destination region and host in addition
to the underlying application, App. A contact plan is also created giving an
estimation of future connection to nodes based on their locations and
characteristics. The contact plan can be kept at N1 along with the routing
table for a centralized routing [153]. The CGR algorithm at N1 uses the
contact plan to estimate possible routes by evaluating parameters such as
local time, delivery time, bundle best transmission time, bundle priority, and
bundle queue status. In this scenario, it is assumed that the best route to
reach N5 is through node N2, N3, and then N4. As the bundle B travels over
networks, it gets handled with CLAs to integrate different transport and other
underlying protocols if any. At N3, B checks N4 which is assumed to be
unavailable at first contact, time t1. As a consequence, B is held in custody
until the node becomes, at time t2, accessible and hence B finds its way to
reach the destination.

###  V-B Discussions and Outlook

When it comes to DTN protocols, it has been observed from literature that some
protocols in different layers may not work efficiently with one another.
Hence, lack of integration in the network stack may lead to internetworking
issues. In this regard, although CFDP accomplishes reliable file transmission,
for example, it results in overhead when combined with BP and LTP. To address
this, Kim et al. in [119] attempt to derive an optimization model to decrease
the DTN network overhead for space missions caused by CFDP. However,
investigating the performance of various protocols integration in the context
of DS both in research and deployment is needed.

The reliability of communications in DS is essential and important to ensure,
but that comes at the cost of overhead. In addition to BP and its custody
transfer scheme, which is the core of the DTN, a number of coding mechanisms
have been adapted at the link layer. To ensure reliability even further, other
coding techniques are needed even in upper layers. For example to handle the
performance degradation of CFDP over long-delay and one-directional
communications links, erasure coding, which helps overcoming packet loss due
to DS link degradation, is suggested to be implemented [130]. Erasure coding
can be used in addition to or instead of ARQ to improve data delivery as the
latter has been shown to be inefficient, specially in long round-trip links
[154], and is known to increase processing complexity and storage utilization
unlike erasure coding which speeds up delivery process [155]. There are
encouragements to conduct further research to optimize the use of erasure
coding in the DTN architecture though and investigate other schemes to enhance
overall DS communications performance. Even though the adoption of hop-by-hop
transmission through BP is inevitably required for handling the intermittent
availability of links while keeping throughput reasonable, end-to-end
transmission mechanism may also be revisited along with other transport layer
schemes to improve reliability avoiding permanent bundle loss in case of a
node failure. One of these protocols is the deep-space transport protocol (DS-
TP) which serves fast file delivery, two times faster than other conventional
TPs under specific conditions [156]. However, when there is a difference in
error rate estimation between a sender and a receiver, DS-TP, as discussed in
[157], causes a malfunction in the re-transmission process leading to
inefficient use of bandwidth. Despite the efforts made to analyze this
protocol performance, much work is needed to fully understand its operation.
Stream control transmission protocol (SCTP) is another TP, suggested in [158],
that, in certain conditions, surpass conventional TCP and can be utilized in
orbiters that operate in the same region. Some other related protocols are
also proposed e.g. the seamless IP diversity based network mobility (SINEMO)
suggested, in [159, 160], to be adopted by mobile entities in space while
addressing the drawbacks of the network mobility (NEMO) basic support protocol
(BSP), proposed to be used by NASA. These mobility protocols pose security
risks, though, as in some cases the information of node locations cloud be
shared in space networks. If this information is eavesdropped, connections
could be exposed to major security threats. These issues need to be addressed
when utilizing the mobility protocols, there are some efforts to investigate
related aspects for example, authors in [161], suggested various schemes to
withstand mobility protocols security threats, however, more security along
with integration issues with other protocols require more investigation.

CGR, due to its accuracy and efficiency, has been gaining attention recently
and is shown to be the most effective routing protocol for space networks.
Nonetheless, CGR faces many challenges, making it an intriguing area for
researchers. Despite of the contributions and efforts done in the topic, by
e.g. [153] and the references therein including [162], CGR resiliency is still
a concern and subject to enhancement. In support of this claim, improvements
over CGR have been shown to be possible, e.g. in [163] where earliest arrival
optimal delivery ratio (EAODR) routing algorithm has been proposed. Being able
to account for the earliest available and future contacts EAODR seems to offer
a 12.9% delay reduction over CGR. In the same regard, addressing CGR
scalability and computational complexity that may incur due to networks size
increase are still research concerns. As a matter of fact, DTN scalability in
general is still an open research challenge. Despite the efforts in developing
DS internetworking protocols, more advanced protocols that account for the
high delays, intermittent losses, network topology dynamicity, and links and
entities heterogeneity while still guarantying high capacity are needed.
Furthermore, DS internetworking protocol needs to account for future services
such as multicast/broadcast and be efficient as satellite-to-Earth
transmission to meet the demand for expanding human presence in the Solar
system. Achieving redundancy by deploying more relaying systems orbiting over
the different planets and moons can contribute toward achieving such a goal.

Figure 7: Next-generation IPN.

##  VI Advances OF DSOC

Having discussed the key physical layers techniques and networking protocols
along with their performance in the field of DSOC in Sections III-V, we in
this section address a number of other field emerging technologies and
advancements that promise to address a number of the challenges, discussed in
Section II, and take DS communications to the next level. The section reviews
some research and implementation efforts done in the direction of the IPN,
OAM, QC, and RF/FSO systems and identifies gaps to be explored.

###  VI-A Interplanetary Network

NASA has three operational networks that support missions: deep space network
(DSN), near-Earth network (NEN), and space network (SN). DSN is the primary
network that is responsible for most of the space missions communications
worldwide. While DSN consists of three stations distributed around Earth
separated by 120° for full coverage, NEN consists of ground stations that
provide communications and tracking services from low-earth orbit to lunar
orbit [164]. SN has a constellation of geosynchronous relays, which consists
of 10 satellites called tracking and data relay satellite system (TDRSS) along
with two ground facilities, which will undergo a complete redesigning of the
architecture as arranged by the Space Network Ground Segment Sustainment
(SSGS) project, to modernize the system to be capable of handling a few
gigabits per second data rate capabilities [165].

The goal of the IPN is to provide connectivity all around the Solar system,
meeting the demand for high data volumes, including video-streaming and high
quality images transmitted by the ever-increasing number of missions. The IPN
will ultimately need to be accessible by the public [166]. Connecting DSN,
NEN, and SN networks is the first step in deploying the IPN.

Based on studies provided in [167, 168, 61], we demonstrate in Fig. 7 the
vision for the IPN architecture. Currently, through RF links, Mars and the
Moon elements have merely an access to the main Earth network. In the future,
however, planets will have their own networks, and the IPN will become a
network of networks. Each planet, as depicted in Fig. 7, will have a dedicated
relay that offers a continuous connection to Earth through optical and RF-
based links, referred to as a trunk links [167]. Local services between planet
elements are to be made possible through other, more available, optical and
RF-based, links referred to as proximity links. The orbiting satellites around
Mars, Mars Atmospheric and Volatile EvolutioN (MAVEN) and Mars Odyssey, shown
in the figure, are all part of the current Mars relay system. They are
displayed to demonstrate Earth-Mars uplink and downlink communications.
Terminal satellites may act as a link to convert optical signals to RF and
vise versa; providing a significant advantage in overcoming atmospheric
effects [61]. To avoid interplanetary link unavailability due to obstruction
and Sun-Earth conjunction, spare interplanetary communications links through
spacecrafts located at Lagrangian points are to be included [168].

In addition, to implementing FSO-based relays, which are, for example,
expected to offer 300 Mbps for Mars distances [167], FSO-based cubesats, which
are SWaP reduced satellites and, in some cases, landers, are emerging to
provide a 2 Gbps rate when thousands of them are implemented [169, 170]. The
first two interplanetary cubesats launched by NASA in 2018, Mars Cube One
(MarCO) probes, have successfully been deployed for navigation and monitoring
[171]. The MarCO s mission, which is still operational, suggests more
utilization of cubesats for future missions [172]. Lately, to improve the IPN
further, it has been suggested to use asteroids as interplanetary relays
[173]. It is proposed to use cubesat landers to form multiple antennas on the
surface of an asteroid. Based on the numerical analysis provided in [173], an
antenna of 400 m2 size provides up to 80 Mbps in Mars distances. Although
cubesat is a promising technology, networking issues resulting from long
communications distances can become even more severe as the number of IPN
nodes becomes large. For example, when cubesats are implemented in large
quantities, network protocols (NP s) like CGR, and similarly EAODR, become
inefficient [174]. To mitigate that, however, rate adaptation with a bandwidth
fair-sharing mechanism is suggested, for example, to enhance the process of
bundle transfer and management [174].

###  VI-B Orbital Angular Momentum

To better utilize the tremendous capacity offered by FSO, a great deal of
attention is being given to exploring the potential of merging FSO with OAM,
which emerges as a technology to solve the high data rate demands of future
terrestrial [175, 176, 177, 178, 179, 180] and DS communications [62, 181,
182, 183]. OAM does not only have the potential to significantly enhance the
spectral efficiency, but also to offer a great deal of improvement over
communications energy and security.

The light that carry OAM constructs a twisted beam with a helical phase front
making a singularity at the center of the beam resulting in a doughnut shape
wavefront intensity profile. One of the OAM beam characteristics, commonly
exploited, is the topological charge or mode number which is a positive or a
negative integer that represents the number of phase twists per wavelength and
the sign indicates the direction of the twist. Thanks to their orthogonality,
OAM modes with infinite distinguished phase fronts provide an additional
degree of freedom that results in significant capacity returns.

There are various OAM generation methods that are being used in the context of
DS including spiral phase plate (SPP) [183], computer-generated holograms (CGH
s), and spatial light modulators (SLM s). These schemes are also considered
for detection. SLM, though, has shown to have a number of advantages over CGH
and SPP schemes including having a higher control accuracy and resolution
[182]. SPPs, in addition, has shown to have limitations when larger mode
numbers are considered, accordingly, Zhang et al. in [183] suggested to use
Dammann optical vortex grating (DOVG) method as it can modulate more OAM modes
than the SPP and generate independent collinear OAM channel [184]. Other
methods including holographic or interferometric methods are recommended to be
used for obtaining high photon efficiency in DS [62, 181].

Aside from [62, 181, 182, 183], to the best of our knowledge, no other
references analyzed DS OAM performance. Djordjevic et al. in [62] showed that
OAM can achieve 100 Gbps in DS, however, looking at the state-of-the-art of
OAM technologies, data rates can be in the range of hundreds of tera bytes per
second and even higher in some cases. [177], for example, showed that
multiplexing quadrature amplitude modulation (QAM) signals with OAM results in
100 Tbps for a distance of 1 m. In [185], on the other hand, where 54.139 Gbps
orthogonal frequency-division multiplexing (OFDM) 8 QAM signals over 368 WDM
pol-muxed 26 OAM modes resulted in a capacity of 1.036 Pbps and spectral
efficiency of 112.6 b/s/Hz. [185] has not relate such a high achievable rate
to a distance though. In the context of DS, Wang et al. in [182] proposed
encoding OAM states of a single photon with the deployment of relay nodes to
enable a reliable and secure transmission. Analyzing the performance of the
idea is missing though. In [183], weak beam intensity detection in presence of
a high background noise has been realized using PC detectors and a parallel
coding method evaluated with a BER as low as 1.27$\times$10-5, which is order
of magnitude better than the traditional PPM.

As indicated, many recent research and experimental work have,
unprecedentedly, exhibited very significant results in the possibility of
establishing high capacity. In addition to the general OAM implementation
challenges [175], incorporating OAM in DS faces unique hurdles. We below
demonstrate these hurdles and discuss attempts to address number of them.

Atmospheric turbulence: OAM modes can be subject to propagation effects, which
can lead to crosstalk between the modes and affect the orthogonality between
OAM states. Even though AT is not of concern in space-to-space links, as
stated previously, it is a significant challenge for probe-to-Earth links.
When it comes to limited distance communications, mainly up to several
kilometers, one can find a variety of work done for compensating the AT, see
e.g [186, 187, 188, 189, 190]. In [186, 187], for example, authors have shown,
through the use of deep learning techniques, 100% OAM mode recognition is
possible in medium to weak turbulence levels, while between 50% [186] and 80%
[188] recognition can be achieved in the case of strong turbulence. However,
there are only few efforts done in the context of DS; as an example,
Djordjevic et al. in [62] proposed combining the LDPC code with OAM-based
modulation schemes to overcome AT effects. Considering the DS links AT
mitigation methods presented in [62], which are also recommend for near Earth
communications, the authors of [181] shown the possibility of the system to
achieve a spectral efficiency of $N^{2}/\log_{2}(N)$ over that of LDPC-PPM
modulation, where $N$ is the number of OAM modes. For the best of our
knowledge, other than [181] and [62], there is no more work done addressing AT
in OAM-DSOC.

Spatial spread: OAM properties can be utilized in different types of beams,
example of which are Laguerre-Gaussian (LG) beams which have the self-healing
property, propagate for far distances [191], and can be generated easily and
hence are preferred over other types [182, 181]. LG beams intensity profile
exhibits a donut-like shape that gets bigger, i.e., spreads more spatially or
diverges, as they travel in space making their detection over DS links
distances challenging. In Fig. (8), we show the beam intensity profile and
spatial distribution for modes
$L=0,\mathrel{{\ooalign{\hss\raisebox{-2.15277pt}{$-$}\hss\cr\raisebox{2.15277pt}{$+$}}}}1$
,$\mathrel{{\ooalign{\hss\raisebox{-2.15277pt}{$-$}\hss\cr\raisebox{2.15277pt}{$+$}}}}2$
at the communications distance of 410$\times$106 km when the transmitter
diameter is considered to be 1 m. We can notice from the figure that OAM beams
preserve their shape even after traveling very long distances and that beams
spread more as the mode number increases. The spread and shape of different
OAM modes are of crucial importance when it comes to locating DS
communications nodes so that it is guaranteed that the singularity, in the
middle of a beam, is avoided. It has been proven experimentally that OAM modes
can be detected even if an LG beam is partially received [192, 193, 194].
Surprisingly, partially receiving a beam can lead to reduction in crosswalk
effects as indicated in [193], which shows that two reduced-area receivers
achieve 12 b/s/Hz higher at SNR 30 dB than a complete area receiver. Also,
Deng et al. in [194], used an approach that resulted in a 46.2% reduction in
crosstalk intensity, achieving a crosstalk intensity as low as 7.71% with a
four non-adjacent OAM states and no more than 22% for a two adjacent modes.
When it comes to partially receiving beams at DS, though, the minimum portion
of the beam that the mode can be recognized could be relatively large. The
partial signal detection of the OAM beams at DS distances, for our best
knowledge, has never been addressed and yet to be investigated.

Lateral displacement error: Keeping an LG beam axis and its receiving entity
fully aligned is difficult. For long-distance transmissions, this requirement
is even more challenging. An alignment error does not only render optical
power loss but also, more crucially, cause expansion of the spiral spectrum
resulting in crosstalk effect and mode detection difficulty [195]. Even though
it has been shown that the lateral offset can improve resiliency against
eavesdropping [196], such an analysis is lacking in DS communications.

Despite the efforts done, a significant contribution yet still need to be done
handling DS challenges considering the accomplishments done in limited
distances moving forward to facilitating OAM implementation in DS.

Figure 8: OAM beam dimensions and intensity at the farthest Earth-to-Mars
distance.

###  VI-C Quantum Communications

Some of the quantum properties have gained attention for being used in space
[197], DS [198], and terrestrial [197] communications.

The quantum technology is expected to break through the limit of classical
technologies in the aspects of security, computational capabilities, as well
as information transmissions. Theoretically, quantum cryptography promises to
obtain an absolute secure channel. Theoretical and experimental results have
shown that QC can increase the capacity of classical channels with methods,
for example, superdense coding [199, 200], and superadditivity [201, 200].
Rengaswamy et al. in [202] demonstrated that quantum-optimal joint detection
receiver which is based on the belief propagation with quantum messages (BPQM)
algorithm [203], through the superadditivity concept, can enhance classical
communications. Such a receiver detects complete codewords getting, for BPSK
pure-loss channel, to the possibility of attaining Holevo limit, hence becomes
suitable for DSOC. In the same direction, [204] showed that joint detection
receivers outperform one at a time codeword BPSK demodulators reaching five
times classical decoders rate. Kwiat et al. in [200] attempted to make QC
possible in DS, taking the advantage of the less lossy channel through
superdense coding which enhances the classical communications capacity by a
factor of two without the need for preparing entanglement, which is not easy
to set up. Realization of superdense teleportation protocol (SDT), proposed in
[200] as a candidate in DS and long distance communications in general, which
is implementable in a high fidelity, is in progress [205]. Going beyond those
accomplishments and establishing a quantum link by entangled photons in DS is
still a goal, for example for the Deep Space Quantum Link (DSQL) project
[198], which would require high performance QC system to be achieved [206].
Within the next decade, a full quantum teleportation from the Earth to Lunar
Gateway, which is a space station that would be orbiting the Moon, though, is
expected to be achieved [207].

###  VI-D Radio Frequency/Free Space Optical Systems

Even though the FSO potentials in DS are tremendous, full implementation of
the technology is yet deferred. RF technology is still dominating space
communications facilities, as discussed in Section II-C. Some efforts in the
literature support the idea of combining RF and FSO in DS as a starting point
to emerge the FSO and to get the best of both worlds.

The two schemes for integrating these technologies are hybrid RF/FSO and dual-
hop RF/FSO. While a hybrid RF/FSO system combines the RF and FSO technologies
in parallel, dual-hop RF/FSO implement them in series. The ability of hybrid
RF/FSO to switch between RF and FSO leads to overall DS system performance
improvement [208] as it does in terrestrial [209]. In addition, RF/FSO dual-
hop method increases coverage, enhances DS communications reliability, and
lowers the overall system cost. In [210], for example, the authors
demonstrated the benefits of using dual-hub RF/FSO in lowering the probability
of link outages in the presence of Solar scintillation. Similarly, authors of
[211] analyzed the performance of RF/FSO systems and showed that they
outperform both FSO and RF links in weak to moderate coronal turbulence. They
also showed that BPSK modulation outperforms some other commonly used schemes
in such an environment. [212] has also shown that combining RF/FSO with OAM
increases spectral efficiency. To boost capacity and reduce turbulence effects
in DS, furthermore, [213] suggested combining OAM multiplexing in a MIMO
system, which operates with modified PPM modulation with spatial mode
multiplexing in a hybrid RF/FSO system.

There are, besides, dedicated efforts to make RF/FSO operational for DS, the
Integrated Radio and Optical Communications (iROC) project [214], for example,
features a hybrid RF/FSO aperture antenna, called Teletanna, that reduces SWaP
burden and utilizes a beacon-less and high data rate system. iROC is
anticipated to be operational around Mars [215] with SWaP comparable to Mars
Reconnaissance Orbiter (MRO), and data rate of 267 Mbps and 10 Gbps if the
same technology is used for lunar distances.

###  VI-E Discussions and Outlook

The term IPN is now used to categorize networks that connect a number of
spacecrafts placed in different regions of space. For example, a network
consisting of a number of Mars and Earth satellites is referred to as a third
IPN [216]. However, the envisioned IPN is a network that connects all planets
elements analog to the Internet on Earth reliably. The IPN will need to handle
the growth of the multimedia data volume [217] and support the human presence
in DS.

OAM is getting a lot of attention outside the scope of DS, as being a strong
candidate for the upcoming big data era [218]. OAM has a remarkable
performance in fiber optics as it can offer a large capacity for long-distance
transmission [219]; the number of modes the fiber could handle, for example,
can reach 394 [220]. When it comes to wireless communications, nonetheless,
OAM performance can be significantly impacted by signal transmission distance,
AT, and the ability to detect partially received beams. Hence, although some
aspects indicate the interest in this technology in DS [181], more
investigation and maturity for the technology will be needed until it becomes
applicable to DS.

Among the other advantages QC offer, the improvements they promise in terms of
capacity, making them attractive to be implemented in DS to overcome the
ultra-long distances losses and meet the demand for the high rate. It will be
even of more interest to integrate, for channel multiplexing, OAM with QC, a
similar idea was implemented in [221] for a fiber optic link, boosting the
capacity even further. Scalable QC techniques are significantly challenging to
realize, mainly due to the fact that quantum repeaters, generally speaking,
are challenging to deploy [222, 223, 224, 225], as they require high accuracy
of quantum measurements, operations, and memory efficiency. Despite the
attempts that promise the practicality of quantum repeaters [226] from memory
enhancing prospective, interestingly, however, studies have shown deploying QC
over Earth-to-space links reaching a distance of 1200 km have been possible
without the need of repeaters [227, 228], as they suffer less than their
terrestrial counterparts from turbulence. As the turbulence effects are even
less between space-to-space links, it would be of interest to expand such
studies to the DS.

RF/FSO systems have shown to be an effective method to offer reliability
enhancement and communications improvement, for example, in terms of BER and
outage probability [229], spectral efficiency [212], and link outage
probability [210]. The gain of the technology merge seemed to be encouraging
the space industry in the start of implementing FSO beyond the scope of
research and development, e.g. in iROC and Mars Perseverance rover local
communications [8], to meet the future demand for DS connectivity and capacity
enhancement, especially when considering the RF technology maturity and RF-to-
FSO transition cost.

##  VII Concluding Remarks

###  VII-A Lessons Learned

Although existing RF systems are unable to meet future DS communications
demands as opposite to its FSO counterpart, implementing the FSO still faces
major impediments. Starting off with the physical layer challenges of DSOC,
there are two primary challenges, namely the beam required pointing accuracy
and AT. While the former can be tolerated by several techniques for instance,
pointing aid tracking beacons, to avoid AT, space-to-space links can be
promising. Moreover, modulation, coding, and detection schemes play important
rules on mitigating DS environment impairments. PPM modulation schemes is
recommended and commonly used schemes for DSOC. Nevertheless, as PPM have
performance limitations when high data rates are required, other schemes
should be investigated. The detection of a DSOC system significantly affects
the capacity limit and modulation performance outcomes. The more sensitive the
detector the better it is for DSOC, however sensitivity could inherently be
limited by the choice of modulation, coding, and quantum noise. Moving to the
networking aspect, DTN is the most recommended architecture in DS mainly
defined by the bundle layer and its related addressing mechanism. More
attention is needed, though, to enhance the DS protocols to achieve Solar
system exploration goals and ultimately universify the network, for which IPN
is certainly one of the solutions. Cubsats technology, on the other hand, will
be a major contributor for accomplishing the future IPN, we believe. OAM, in
addition is a promising technology for achieving data rates in the range of a
hundred gigabyte. Not like any other technology, however, there is a gap in
the literature in regard to applying OAM in DS. Partial detection, pointing
and accuracy, and AT challenges are yet to be overcome. Merging RF and FSO in
DS communications, which has been partially applied in DS, could be the
starting point for applying FSO. RF/FSO system is shown to preform better than
applying FSO alone in a number of cases.

###  VII-B Conclusion

The success of space exploration missions relies heavily on the reliability
and efficiency of DS communications. That is achieved through different
technologies and protocols that can overcome the unique challenges of DS. In
this paper, we provided an overview of different technologies and protocols
and discussed related challenges and suggested solutions. We presented a
detailed discussion and outlook regarding covered DSOC physical layer
characteristics, physical performance analysis, protocols, and the field
advances. We covered the future requirements that need to be fulfilled to cope
with the changes in space mission, including compromising between matured
existing technologies and recent advances. The paper should provide a basis
for researchers interested in learning about and contributing underlying
technologies.

## Acknowledgement

The authors would like to thank Abderrahmen Trichili, and Bassem Khalfi for
their valuable inputs and feedback for the paper.

## Abbreviations

AMS

    Asynchronous Message Service
AOS

    Advanced Orbiting Systems
APD

    Avalanche Photo Diode
ARQ

    Automatic Retransmission ReQuest
AU

    Astronomical Unit
BER

    Bit Error Rate
BP

    Bundle Protocol
BPQM

    Belief Propagation with Quantum Messages
BPSK

    Binary Phase-Shift Keying
BSP

    Basic support protocol
CCSDS

    Consultative Committee for Space Data Systems
CFDP

    File Delivery Protocol
CGH

    Computer-Generated hologram
CGR

    Contact Graph Routing
CLA

    Convergence Layer Adapters
DOVG

    Dammann Optical Vortex Grating
DS

    Deep Space
DSN

    Deep Space Network
DSOC

    Deep Space Optical communications
DSQL

    Deep Space Quantum Link
DS-TP

    Deep Space Transport Protocol
DTN

    Delay/Disruption Tolerant Networking
EAODR

    Earliest Arrival Optimal Delivery Ratio
ECC

    Error Correction Code
EDRS

    European Data Relay System
EID

    Endpoint Identifiers
EP

    Encapsulation Packets
FER

    Frame Error Rate
FoM

    Figure of Merit
FSO

    Free Space Optical communications
GmAPD

    Geiger-Mode Avalanche Photodiode
HPE

    High Photon Efficiency
InGaAs

    Indium Gallium Arsenide
IP

    Internet Protocol
IPN

    Interplanetary Network
IPoC

    IP Over CCSDS Space Links
iROC

    Integrated Radio and Optical Communications
LDPC

    Low Density Parity Check
LG

    Laguerre Gaussian
LLCD

    Lunar Laser communications Demonstration
LTP

    Licklider Transmission Protocol
MarCO

    Mars Cube One
MAVEN

    Mars Atmospheric and Volatile EvolutioN
MIMO

    Multiple Input Multiple Output
MLCD

    Mars Laser communications Demonstration
MRO

    Mars Reconnaissance Orbiter
NASA

    National Aeronautics and Space Administration
NEMO

    Network mobility
NEN

    Near-Earth Network
NP

    Network Protocol
NS

    Number State
OOK

    On-off Keying
PC

    Photon-Counting
PDU

    Protocol Data Units
PPB

    Photon Per Bit
PPM

    Pulse Position Modulation
Prox-1

    Proximity-1
PSA

    Phase-Sensitive Optical Amplifiers
QAM

    Quadrature Amplitude Modulation
QC

    Quantum communications
RF

    Radio Frequency
RS

    Reed-Solomon
RSPPM

    Reed-Solomon Pulse Position Modulation
SCPPM

    Serially Concatenated Pulse Position Modulation
SCPS

    Space Communications Protocol Specifications
SCTP

    Stream control transmission protocol
SDLP

    Space Data Link Protocols
SDT

    Superdense Teleportation Protocol
SINEMO

    seamless IP diversity based network mobility
SLM

    Spatial Light Modulators
SN

    Space Network
SNR

    Signal to Noise Ratio
SNSPD

    Superconducting Nanowire Single Photon Detector
SP

    Space Packets
SPP

    Spiral Phase Plate
SPC

    Single Photon Coherent
SQ

    Single-Quadrature
SWaP

    Size Weight and Power
TC

    Telecommand
TCP

    Transmission Control Protocol
TDRSS

    Tracking and Data Relay Satellite System
TM

    Telemetry
TP

    Transport Protocol
UDP

    User Datagram Protocol
URI

    Universal Resource Identifiers
USLP

    Unified Space Data Link Protocol
WDMA

    Wavelength Division Multiple Access

## References

  * [1] International Space Exploration Coordination Group, “Benefits stemming from space exploration,” https://www.nasa.gov/sites/default/files/files/Benefits-Stemming-from-Space-Exploration-2013-TAGGED.pdf, 2013.
  * [2] National Aeronautics and Space Administration (NASA), “NASA perseveres through pandemic, looks ahead in 2021,” https://www.nasa.gov/feature/nasa-perseveres-through-pandemic-looks-ahead-in-2021, 2021.
  * [3] SpaceX, “The Moon: Returning humans to Lunar missions,” https://www.spacex.com/human-spaceflight/moon, 2020.
  * [4] B. L. Edwards, R. Daddato, K.-J. Schulz, C. Schmidt, J. Hamkins, D. Giggenbach, B. Robinson, L. Braatz, and R. Alliss, “An update on the CCSDS optical communications working group,” in _2017 IEEE International Conference on Space Optical Systems and Applications (ICSOS)_. IEEE, 2017, pp. 1–9.
  * [5] International Space Exploration Coordination Group, “The global exploration roadmap,” https://www.globalspaceexploration.org/wordpress/wp-content/isecg/GER_2018_small_mobile.pdf, 2018.
  * [6] China National Space Administration, “Tianwen 1 makes orbital correction as Mars arrival draws near,” http://www.cnsa.gov.cn/english/n6465652/n6465653/c6811227/content.html, 2021.
  * [7] H. J. Kramer, “Hope Al-Amal EMM (Emirates Mars mission),” https://www.eoportal.org/satellite-missions/emm-hope#spacecraft, 2021.
  * [8] National Aeronautics and Space Administration (NASA), “Mars 2020 mission communications,” https://mars.nasa.gov/mars2020/spacecraft/rover/communications/, 2020.
  * [9] European Space Agency, “From the Earth to the Moon and on to Mars ESA and NASA take decisions and plan for the future,” https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Exploration/ExoMars/ExoMars_2022_rover, 2022.
  * [10] S. Cao, “Elon musk reveals Spacex timeline for landing humans on Mars,” https://observer.com/2020/12/elon-musk-says-spacex-will-land-humans-on-mars-in-a-few-years-if-theyre-lucky, 2020.
  * [11] European Space Agency, “Exploring Mars,” https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Exploration/Mars.
  * [12] G. Xu and Z. Song, “Effects of solar scintillation on deep space communications: challenges and prediction techniques,” _IEEE Wireless Communications_ , vol. 26, no. 2, pp. 10–16, 2019.
  * [13] A. Seas, B. Robinson, T. Shih, F. Khatri, and M. Brumfield, “Optical communications systems for NASA's human space flight missions,” in _International Conference on Space Optics-ICSO 2018_ , vol. 11180. International Society for Optics and Photonics, 2019, p. 111800H.
  * [14] P.-D. Arapoglou, M. Bertinelli, P. Concari, A. Ginesi, and M. Lanucara, “Benchmarking the future of RF in space missions: From low Earth orbit to deep space,” in _2017 IEEE MTT-S International Microwave Symposium (IMS)_. IEEE, 2017, pp. 388–390.
  * [15] D. S. Network, “DSN Telecommunications Link Design Handbook, 810–005 Rev. E,” JPL, Pasadena, CA, 2009.
  * [16] C. D. Edwards, R. Gladden, C. H. Lee, and D. Wenkert, “Assessment of potential Mars relay network enhancements,” in _2018 IEEE Aerospace Conference_. IEEE, 2018, pp. 1–8.
  * [17] A. Zak, “Robotic missions to Mars,” http://www.russianspaceweb.com/spacecraft_planetary_mars.html, 2020.
  * [18] The Planetary Society, “Every mission to Mars, ever,” https://www.planetary.org/space-missions/every-mars-mission, 2021.
  * [19] Wikipedia, “List of artificial objects on Mars,” https://en.wikipedia.org/wiki/List_of_artificial_objects_on_Mars, 2021.
  * [20] Wikipedia, “List of Mars orbiters,” https://en.wikipedia.org/wiki/List_of_Mars_orbiters, 2021.
  * [21] Jet Propulsion Laboratory (JPL), “Telecommunications - NASA Mars exploration,” https://mars.nasa.gov/mro/mission/spacecraft/parts/telecommunications/.
  * [22] spaceflight101, “Spacecraft Information MAVEN,” https://spaceflight101.com/maven/spacecraft-information/.
  * [23] National Aeronautics and Space Administration (NASA), “Viking 2 lander,” https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTelemetry.action?id=1975-083C, 2021.
  * [24] J. Taylor, A. Makovsky, A. Barbieri, R. Tung, P. Estabrook, and A. G. Thomas, “Mars exploration rover telecommunications,” in _Deep Space Communications_. Wiley Online Library, 2016, pp. 251–356.
  * [25] National Aeronautics and Space Administration (NASA), “Mariner 4,” https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-077A, 2021.
  * [26] National Aeronautics and Space Administration (NASA), “Viking 1 orbiter,” https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTelemetry.action?id=1975-075A, 2021.
  * [27] Jet Propulsion Laboratory (JPL), “Voyager mission status,” https://voyager.jpl.nasa.gov/mission/status/, 2023.
  * [28] N. K. Lyras, A. D. Panagopoulos, and P.-D. Arapoglou, “Deep-space optical communication link engineering: Sensitivity analysis,” _IEEE Aerospace and Electronic Systems Magazine_ , vol. 34, no. 11, pp. 8–19, 2019.
  * [29] H. Hemmati, A. Biswas, and I. B. Djordjevic, “Deep-space optical communications: Future perspectives and applications,” _Proceedings of the IEEE_ , vol. 99, no. 11, pp. 2020–2039, 2011.
  * [30] R. Cesarone, D. Abraham, S. Shambayati, and J. Rush, “Deep-space optical communications,” in _2011 International Conference on Space Optical Systems and Applications (ICSOS)_. IEEE, 2011, pp. 410–423.
  * [31] H. Kaushal and G. Kaddoum, “Optical communication in space: Challenges and mitigation techniques,” _IEEE communications surveys & tutorials_, vol. 19, no. 1, pp. 57–96, 2016.
  * [32] A. Trichili, M. A. Cox, B. S. Ooi, and M.-S. Alouini, “Roadmap to free space optics,” _J. Opt. Soc. Am. B_ , vol. 37, no. 11, pp. A184–A201, Nov 2020. [Online]. Available: http://opg.optica.org/josab/abstract.cfm?URI=josab-37-11-A184
  * [33] K. Wilson, P. Leatherman, R. Cleis, J. Spinhirne, and R. Fugate, “Results of the compensated Earth-Moon-Earth retroreflector laser link (CEMERLL) experiment.” NASA JPL, 1997.
  * [34] A. Biswas, D. Boroson, and B. Edwards, “Mars laser communication demonstration: what it would have been,” in _Free-Space Laser Communication Technologies XVIII_ , vol. 6105. International Society for Optics and Photonics, 2006, p. 610502.
  * [35] M. E. Grein, A. J. Kerman, E. A. Dauler, M. M. Willis, B. Romkey, R. J. Molnar, B. S. Robinson, D. V. Murphy, and D. M. Boroson, “An optical receiver for the Lunar laser communication demonstration based on photon-counting superconducting nanowires,” in _Advanced Photon Counting Techniques IX_ , vol. 9492. International Society for Optics and Photonics, 2015, p. 949208.
  * [36] A. Biswas, M. Srinivasan, S. Piazzolla, and D. Hoppe, “Deep space optical communications,” in _Free-Space Laser Communication and Atmospheric Propagation XXX_ , vol. 10524. International Society for Optics and Photonics, 2018, p. 105240U.
  * [37] D. Y. Oh, S. Collins, T. Drain, W. Hart, T. Imken, K. Larson, D. Marsh, D. Muthulingam, J. S. Snyder, D. Trofimov _et al._ , “Development of the Psyche mission for NASA's discovery program,” 2019.
  * [38] L. J. Deutsch, S. M. Lichten, D. J. Hoppe, A. J. Russo, and D. M. Cornwell, “Creating a nasa deep space optical communications system,” _Space Operations: Inspiring Humankind’s Future_ , pp. 43–62, 2019.
  * [39] K. B. Fielhauer, B. G. Boone, and D. E. Raible, “Concurrent system engineering and risk reduction for dual-band (RF/optical) spacecraft communications,” in _2012 IEEE Aerospace Conference_. IEEE, 2012, pp. 1–7.
  * [40] W. Wu, M. Chen, Z. Zhang, X. Liu, and Y. Dong, “Overview of deep space laser communication,” _Science China Information Sciences_ , vol. 61, no. 4, pp. 1–12, 2018.
  * [41] T. De Cola, E. Paolini, G. Liva, and G. P. Calzolari, “Reliability options for data communications in the future deep-space missions,” _Proceedings of the IEEE_ , vol. 99, no. 11, pp. 2056–2074, 2011.
  * [42] G. Zhang, Z. Xie, D. Bian, and Q. Sun, “A survey of deep space communications,” _Journal of Electronics (China)_ , vol. 28, no. 2, p. 145, 2011.
  * [43] S. Hall, “A survey of free space optical communications in satellites,” 2020.
  * [44] A. Könsgen and A. Förster, “Current state and future challenges in deep space communication: A survey,” _it-Information Technology_ , vol. 63, no. 4, pp. 219–234, 2021.
  * [45] M. A. Khalighi and M. Uysal, “Survey on free space optical communication: A communication theory perspective,” _IEEE communications surveys and tutorials_ , vol. 16, no. 4, pp. 2231–2258, 2014.
  * [46] D. M. Cornwell, “NASA's optical communications program for 2017 and beyond,” in _2017 IEEE International Conference on Space Optical Systems and Applications (ICSOS)_. IEEE, 2017, pp. 10–14.
  * [47] A. Chmielewski and C. Jenkins, “Gossamer spacecraft,” _WIT Transactions on State-of-the-art in Science and Engineering_ , vol. 20, 2005.
  * [48] B. G. Boone, J. R. Bruzzi, B. E. Kluga, E. W. Rogala, R. Hale, and P. C. Chen, “Development and testing of an actively controlled large-aperture cassegrain telescope for spacecraft deployment,” in _Optical, Infrared, and Millimeter Space Telescopes_ , vol. 5487. SPIE, 2004, pp. 1042–1053.
  * [49] M. K. Simon, _Bandwidth-efficient digital modulation with application to deep space communications_. John Wiley & Sons, 2005.
  * [50] S. Burleigh, A. Hooke, L. Torgerson, K. Fall, V. Cerf, B. Durst, K. Scott, and H. Weiss, “Delay-tolerant networking: an approach to interplanetary internet,” _IEEE Communications Magazine_ , vol. 41, no. 6, pp. 128–136, 2003.
  * [51] B. J. Klein and J. J. Degnan, “Optical antenna gain. 1: Transmitting antennas,” _Applied optics_ , vol. 13, no. 9, pp. 2134–2141, 1974.
  * [52] S. L. Booth and R. M. Manning, “Optical communication link assessment utilizing a modulated retro-reflector on mars,” 2018.
  * [53] H. Hemmati, _Near-Earth laser communications_. CRC press, 2020, vol. 1.
  * [54] H. Hemmati, _Deep space optical communications_. John Wiley & Sons, 2006, vol. 11.
  * [55] A. G. Alkholidi and K. S. Altowij, “Free space optical communications theory and practices,” _Contemporary Issues in Wireless Communications_ , pp. 159–212, 2014.
  * [56] C. 141.0-B-1, “Optical communications coding and synchronization. blue book. issue 1,” August 2019.
  * [57] J. J. Degnan, “Millimeter accuracy satellite laser ranging: a review,” _Contributions of space geodesy to geodynamics: technology_ , vol. 25, pp. 133–162, 1993.
  * [58] G. R. Osche, “Optical detection theory for laser applications,” 2002.
  * [59] D. Giggenbach and H. Henniger, “Fading-loss assessment in atmospheric free-space optical communication links with on-off keying,” _Optical Engineering_ , vol. 47, no. 4, p. 046001, 2008.
  * [60] B. Moision and J. Hamkins, “Downlink budget for Mars: Modulation and coding,” _The Interplanetary Network Progress Report_ , pp. 42–154, 2003.
  * [61] W. J. Hurd, B. E. MacNeal, G. G. Ortiz, R. Moe, J. Z. Walker, M. Dennis, E. Cheng, D. Fairbrother, B. Eegholm, and K. J. Kasunic, “Exo-atmospheric telescopes for deep space optical communications,” in _2006 IEEE Aerospace Conference_. IEEE, 2006, pp. 12–pp.
  * [62] I. B. Djordjevic and M. Arabaci, “LDPC-coded orbital angular momentum (OAM) modulation for free-space optical communication,” _Optics express_ , vol. 18, no. 24, pp. 24 722–24 728, 2010.
  * [63] M. Mukherjee, “Wireless communication-moving from RF to optical,” in _2016 3rd International Conference on Computing for Sustainable Global Development (INDIACom)_. IEEE, 2016, pp. 788–795.
  * [64] A. Kaur and M. Singh, “Performance evaluation of free space optics (FSO) and radio frequency communication system due to combined effect of fog and snow,” in _International conference on recent advances and future trends in information technology_ , 2012, pp. 32–36.
  * [65] M. M. Hassan and G. Rather, “Free space optics (FSO): a promising solution to first and last mile connectivity (FLMC) in the communication networks,” _IJ Wireless and Microwave Technologies_ , vol. 4, no. 1, 2020.
  * [66] B. V. Oaida, W. Wu, B. I. Erkmen, A. Biswas, K. S. Andrews, M. Kokorowski, and M. Wilkerson, “Optical link design and validation testing of the optical payload for lasercomm science (opals) system,” in _Free-Space Laser Communication and Atmospheric Propagation XXVI_ , vol. 8971. SPIE, 2014, pp. 235–249.
  * [67] M. S. Bashir and M.-S. Alouini, “Signal acquisition with photon-counting detector arrays in free-space optical communications,” _IEEE Transactions on Wireless Communications_ , vol. 19, no. 4, pp. 2181–2195, 2020.
  * [68] A. Biswas, M. Srinivasan, R. Rogalin, S. Piazzolla, J. Liu, B. Schratz, A. Wong, E. Alerstam, M. Wright, W. T. Roberts _et al._ , “Status of nasa’s deep space optical communication technology demonstration,” in _2017 IEEE International Conference on Space Optical Systems and Applications (ICSOS)_. IEEE, 2017, pp. 23–27.
  * [69] D. Y. Oh, S. Collins, D. Goebel, B. Hart, G. Lantoine, S. Snyder, G. Whiffen, L. Elkins-Tanton, P. Lord, Z. Pirkl _et al._ , “Development of the Psyche mission for NASA's discovery program,” in _35th International Electric Propulsion Conference_ , 2017, pp. 1–19.
  * [70] S. Karmous, “Connecting the universe: Challenges, mitigation, advances, and link engineering,” B.S. Thesis, Department of Electric and Electronic Engineering University of Tripoli, Tripoli, Libya, 2019.
  * [71] J. I. Statman and C. D. Edwards, “Coding, modulation, and relays for deep space communication mars rovers case study,” 2004.
  * [72] J. Li, A. Hylton, J. Budinger, J. Nappier, J. Downey, and D. Raible, “Dual-pulse pulse position modulation (DPPM) for deep-space optical communications: Performance and practicality analysis,” in _2012 International Conference on Wireless Communications and Signal Processing (WCSP)_. IEEE, 2012, pp. 1–7.
  * [73] B. Moision and J. Hamkins, “Coded modulation for the deep-space optical channel: Serially concatenated pulse-position modulation,” _IPN Progress Report_ , vol. 42, no. 161, pp. 1–26, 2005.
  * [74] R. Kakarla, J. Schröder, and P. A. Andrekson, “One photon-per-bit receiver using near-noiseless phase-sensitive amplification,” _Light: Science & Applications_, vol. 9, no. 1, pp. 1–7, 2020.
  * [75] R. K. Tyson, “Adaptive optics and ground-to-space laser communications,” _Applied optics_ , vol. 35, no. 19, pp. 3640–3646, 1996.
  * [76] B. Moision, J. Wu, and S. Shambayati, “An optical communications link design tool for long-term mission planning for deep-space missions,” in _2012 IEEE Aerospace Conference_ , 2012, pp. 1–12.
  * [77] D. Divsalar and F. Pollara, “Multiple turbo codes for deep-space communications,” _TDA progress report_ , vol. 42, no. 121, pp. 66–77, 1995.
  * [78] J. L. Massey, “Deep-space communications and coding: A marriage made in heaven,” in _Advanced Methods for Satellite and Deep Space Communications_. Springer, 1992, pp. 1–17.
  * [79] S. Lin and D. Costello Jr, “Error control coding: Fundamentals and applications, pretince-hall inc,” _Englewood Cliffs, New Jersey_ , 1983.
  * [80] R. J. McEliece and L. Swanson, “Reed-Solomon codes and the exploration of the solar system,” 1994.
  * [81] M. K. Cheng, B. E. Moision, J. Hamkins, and M. A. Nakashima, “Sat05-4: Implementation of a coded modulation for deep space optical communications,” in _IEEE Globecom 2006_. IEEE, 2006, pp. 1–5.
  * [82] I.-R. R. SA.1742, “Technical and operational characteristics of interplanetary and deep-space systems operating in the space-to-Earth direction around 283 THz,” _Geneva, Switzerland_ , 2006.
  * [83] D. Divsalar, S. Dolinar, W. Farr, D. J. Israel, M. Mohageg, M. Thill, E. Alerstam, M. Peng, M. Shaw, and E. Wollman, “Wavelength division multiple access for deep space optical communications,” in _2020 IEEE Aerospace Conference_. IEEE, 2020, pp. 1–13.
  * [84] H. Liang, A. Liu, X. Tong, and C. Gong, “Raptor-like rateless spinal codes using outer systematic polar codes for reliable deep space communications,” in _IEEE INFOCOM 2020-IEEE Conference on Computer Communications Workshops (INFOCOM WKSHPS)_. IEEE, 2020, pp. 1045–1050.
  * [85] S. Dolinar, K. M. Birnbaum, B. I. Erkmen, and B. Moision, “On approaching the ultimate limits of photon-efficient and bandwidth-efficient optical communication,” in _2011 International Conference on Space Optical Systems and Applications (ICSOS)_. IEEE, 2011, pp. 269–278.
  * [86] D. O. Caplan, “Laser communication transmitter and receiver design,” in _Free-Space Laser Communications_. Springer, 2007, pp. 109–246.
  * [87] B. Moision and W. Farr, “Communication limits due to photon detector jitter,” _IEEE Photonics Technology Letters_ , vol. 20, no. 9, pp. 715–717, 2008.
  * [88] B. Book, “Optical communications coding and synchronization,” 2019.
  * [89] J. A. Stern and W. H. Farr, “Fabrication and characterization of superconducting NbN nanowire single photon detectors,” _IEEE transactions on applied superconductivity_ , vol. 17, no. 2, pp. 306–309, 2007.
  * [90] Jet Propulsion Laboratory (JPL), “Superconducting nanowire single photon detectors for DSOC,” https://microdevices.jpl.nasa.gov/news/superconducting-nanowire-single-photon-detectors-for-dsoc/, 2021.
  * [91] A. Rajguru, N. Jerred, S. D. Howe, and A. Faler, “Laser space communication concept for deep-space interplanetary missions using cubesats,” in _45th AIAA Plasmadynamics and Lasers Conference_ , 2014, p. 2234.
  * [92] J. A. Mendenhall, L. M. Candell, P. I. Hopman, G. Zogbi, D. M. Boroson, D. O. Caplan, C. J. Digenis, D. R. Hearn, and R. C. Shoup, “Design of an optical photon counting array receiver system for deep-space communications,” _Proceedings of the IEEE_ , vol. 95, no. 10, pp. 2059–2069, 2007.
  * [93] N. Pestana, H. Clark, J. MacDonald, Z. Kranefeld, F. O’Donnell, H. Pretorius, K. McIntosh, and E. Duerr, “Evaluation of asynchronous Geiger-mode avalanche photodiode arrays for deep-space optical communications,” in _Advanced Photon Counting Techniques XV_ , vol. 11721. International Society for Optics and Photonics, 2021, p. 117210Q.
  * [94] B. W. Horkley, “Optical communications with a Geiger mode APD array,” Massachusetts Institute of Technology, Lexington, Tech. Rep., 2016.
  * [95] D. M. Boroson, B. S. Robinson, D. V. Murphy, D. A. Burianek, F. Khatri, J. M. Kovalik, Z. Sodnik, and D. M. Cornwell, “Overview and results of the Lunar laser communication demonstration,” in _Free-Space Laser Communication and Atmospheric Propagation XXVI_ , vol. 8971. International Society for Optics and Photonics, 2014, p. 89710S.
  * [96] B. S. Robinson, A. J. Kerman, E. A. Dauler, R. J. Barron, D. O. Caplan, M. L. Stevens, J. J. Carney, S. A. Hamilton, J. K. Yang, and K. K. Berggren, “781 Mbit/s photon-counting optical communications using a superconducting nanowire detector,” _Optics letters_ , vol. 31, no. 4, pp. 444–446, 2006.
  * [97] J. Huang, W. Zhang, L. You, C. Zhang, C. Lv, Y. Wang, X. Liu, H. Li, and Z. Wang, “High speed superconducting nanowire single-photon detector with nine interleaved nanowires,” _Superconductor Science and Technology_ , vol. 31, no. 7, p. 074001, 2018.
  * [98] M. Shaw, E. Wollman, J. Allmaras, A. Beyer, R. Briggs, F. Marsili, A. Velasco, B. Korzh, M. Srinivasan, R. Rogalin _et al._ , “Superconducting nanowire single photon detectors for deep space optical communications,” 2018.
  * [99] M. Stevens, D. Caplan, B. Robinson, D. Boroson, and A. Kachelmyer, “Optical homodyne PSK demonstration of 1.5 photons per bit at 156 Mbps with rate-$1/2$ turbo coding,” _Optics express_ , vol. 16, no. 14, pp. 10 412–10 420, 2008.
  * [100] F. Heine, G. Mühlnikel, H. Zech, S. Philipp-May, and R. Meyer, “The european data relay system, high speed laser based data links,” in _2014 7th Advanced Satellite Multimedia Systems Conference and the 13th Signal Processing for Space Communications Workshop (ASMS/SPSC)_. IEEE, 2014, pp. 284–286.
  * [101] M. Reine, “Encyclopedia of modern optics 2005,” 2005.
  * [102] D. Lavery, S. Liu, Y. Jeong, J. Nilsson, P. Petropoulos, P. Bayvel, and S. J. Savory, “Realizing high sensitivity at 40 Gbit/s and 100 Gbit/s,” in _OFC/NFOEC_. IEEE, 2012, pp. 1–3.
  * [103] D. J. Geisler, T. M. Yarnall, W. E. Keicher, M. L. Stevens, A. M. Fletcher, R. R. Parenti, D. O. Caplan, and S. A. Hamilton, “Demonstration of 2.1 photon-per-bit sensitivity for BPSK at 9.94-Gb/s with rate-$1/2$ FEC,” in _2013 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference (OFC/NFOEC)_. IEEE, 2013, pp. 1–3.
  * [104] S. N. Pottoo, R. Goyal, and A. Gupta, “Development of 32-GBaud DP-QPSK free space optical transceiver using homodyne detection and advanced digital signal processing for future optical networks,” _Optical and Quantum Electronics_ , vol. 52, no. 11, pp. 1–17, 2020.
  * [105] C. E. Shannon, “Communication in the presence of noise,” _Proceedings of the IRE_ , vol. 37, no. 1, pp. 10–21, 1949.
  * [106] K. Banaszek, L. Kunz, M. Jachura, and M. Jarzyna, “Quantum limits in optical communications,” _Journal of Lightwave Technology_ , vol. 38, no. 10, pp. 2741–2754, 2020.
  * [107] A. S. Holevo, _Quantum systems, channels, information: a mathematical introduction_. Walter de Gruyter, 2012, vol. 16.
  * [108] S. Dolinar, B. Moision, and B. Erkmen, “Fundamentals of free-space optical communications,” 2012.
  * [109] U. Leonhardt, _Essential quantum optics: from quantum measurements to black holes_. Cambridge University Press, 2010.
  * [110] M. S. Bashir, “Free-space optical communications with detector arrays: A mathematical analysis,” _IEEE Transactions on Aerospace and Electronic Systems_ , vol. 56, no. 2, pp. 1420–1429, 2019.
  * [111] V. Vilnrotter, C. Lau, M. Srinivasan, R. Mukai, and K. Andrews, “An optical array receiver for deep-space communication through atmospheric turbulence,” _JPL Publication: IPN Progress Report, Report_ , pp. 42–154, 2003.
  * [112] A. Hooke, “The interplanetary internet,” _Communications of the ACM_ , vol. 44, no. 9, pp. 38–40, 2001.
  * [113] Consultative Committee for Space Data Systems (CCSDS), “Space Communications Protocol Specification (SCPS) Transport Protocol,” _CCSDS 714.0-B-2_ , 2006, https://public.ccsds.org/pubs/714x0b2.pdf.
  * [114] G. Papastergiou, I. Alexiadis, S. Burleigh, and V. Tsaoussidis, “Delay tolerant payload conditioning protocol,” _Computer Networks_ , vol. 59, pp. 244–263, 2014.
  * [115] I. F. Akyildiz, O. B. Akan, C. Chen, J. Fang, and W. Su, “The state of the art in interplanetary internet,” _IEEE Communications Magazine_ , vol. 42, no. 7, pp. 108–118, 2004.
  * [116] Consultative Committee for Space Data Systems (CCSDS), “Space packet protocol,” _CCSDS 133.0-B-1_ , 2003, https://public.ccsds.org/Pubs/133x0b2e1.pdf.
  * [117] A. V. Vasilakos, Y. Zhang, and T. Spyropoulos, _Delay tolerant networks: Protocols and applications_. CRC press, 2016.
  * [118] E. P. Jones and P. A. Ward, “Routing strategies for delay-tolerant networks,” _Submitted to ACM Computer Communication Review (CCR)_ , vol. 1, 2006.
  * [119] J.-W. Kim, K.-R. Lee, J.-H. Jo, and I. Joe, “Performance modeling of DTN-based file transmission for space internet,” _Advanced Science Letters_ , vol. 25, no. 1, pp. 75–78, 2019.
  * [120] S. C. Burleigh, “Operating CFDP in the interplanetary internet,” in _SpaceOps Conference_ , 2002, p. 22.
  * [121] C. R. Haddow, M. Pecchioli, F. Keck, M. Schmidt, and F. Flentge, “File based operations-architectures and the euclid example,” in _SpaceOps 2014 Conference_ , 2014, p. 1750.
  * [122] Consultative Committee for Space Data Systems (CCSDS), “Asynchronous message service,” _CCSDS 735.1-B-1_ , 2012, https://public.ccsds.org/Pubs/735x0g1.pdf.
  * [123] K. Zhao and Q. Zhang, “Network protocol architectures for future deep-space internetworking,” _Science China Information Sciences_ , vol. 61, no. 4, pp. 1–16, 2018.
  * [124] Consultative Committee for Space Data Systems (CCSDS), “Bundle protocol specification,” _CCSDS 734.2-B-1_ , 2015, https://public.ccsds.org/Pubs/734x2b1.pdf.
  * [125] K. Fall, W. Hong, and S. Madden, “Custody transfer for reliable delivery in delay tolerant networks,” _IRB-TR-03-030, July_ , vol. 198, 2003.
  * [126] G. Araniti, N. Bezirgiannidis, E. Birrane, I. Bisio, S. Burleigh, C. Caini, M. Feldmann, M. Marchese, J. Segui, and K. Suzuki, “Contact graph routing in DTN space networks: Overview, enhancements and performance,” _IEEE Communications Magazine_ , vol. 53, no. 3, pp. 38–46, 2015.
  * [127] K. Fall and S. Farrell, “DTN: An architectural retrospective,” _IEEE Journal on Selected areas in communications_ , vol. 26, no. 5, pp. 828–836, 2008.
  * [128] S. Burleigh, “Dynamic routing for delay-tolerant networking in space flight operations,” in _SpaceOps 2008 Conference_ , 2008, p. 3406.
  * [129] Consultative Committee for Space Data Systems (CCSDS), “File delivery protocol (CFDP),” _CCSDS 727.0-B-5_ , 2020, https://public.ccsds.org/Pubs/727x0b5.pdf.
  * [130] T. De Cola, H. Ernst, and M. Marchese, “Performance analysis of CCSDS file delivery protocol and erasure coding techniques in deep space environments,” _Computer Networks_ , vol. 51, no. 14, pp. 4032–4049, 2007.
  * [131] S. Burleigh, “Asynchronous message service for deep space mission operations,” in _SpaceOps 2006 Conference_ , 2006, p. 5969.
  * [132] K. Zhao, R. Wang, S. C. Burleigh, A. Sabbagh, W. Wu, and M. De Sanctis, “Performance of bundle protocol for deep-space communications,” _IEEE Transactions on Aerospace and Electronic Systems_ , vol. 52, no. 5, pp. 2347–2361, 2016.
  * [133] G. Papastergiou, C. V. Samaras, and V. Tsaoussidis, “Where does transport layer fit into space DTN architecture?” in _2010 5th Advanced Satellite Multimedia Systems Conference and the 11th Signal Processing for Space Communications Workshop_. IEEE, 2010, pp. 81–88.
  * [134] Consultative Committee for Space Data Systems (CCSDS), “Licklider transmission protocol (LTP),” _CCSDS 734.1-B-1_ , 2015, https://public.ccsds.org/Pubs/734x1b1.pdf.
  * [135] R. Wang, P. Parikh, R. Bhavanthula, J. Hou, L. Zhou, and T. Taleb, “Licklider transmission protocol (LTP)-based DTN for long-delay cislunar communications,” in _2010 IEEE Global Telecommunications Conference GLOBECOM 2010_. IEEE, 2010, pp. 1–5.
  * [136] Consultative Committee for Space Data Systems (CCSDS), “IP Over CCSDS space links,” _CCSDS 702.1-B-1_ , 2012, https://public.ccsds.org/Pubs/702x1b1c1.pdf.
  * [137] Consultative Committee for Space Data Systems (CCSDS), “Overview of the unified transmission space data link protocols,” 2020, https://public.ccsds.org/Pubs/700x1g1.pdf.
  * [138] Consultative Committee for Space Data Systems (CCSDS), “Optical communications physical layer,” _CCSDS 141.0-B-1_ , 2019, https://public.ccsds.org/Pubs/141x0b1.pdf.
  * [139] M. Sanchez Net, “Support of latency-sensitive space exploration applications in future space communication systems,” Ph.D. dissertation, Massachusetts Institute of Technology, 2017.
  * [140] C. Caini, R. Firrincieli, and M. Livini, “DTN bundle layer over TCP: Retransmission algorithms in the presence of channel disruptions.” _JCM_ , vol. 5, no. 2, pp. 106–116, 2010.
  * [141] J. Hu, R. Wang, X. Sun, Q. Yu, Z. Yang, and Q. Zhang, “Memory dynamics for DTN protocol in deep-space communications,” _IEEE Aerospace and Electronic Systems Magazine_ , vol. 29, no. 2, pp. 22–30, 2014.
  * [142] M. Sarkar, K. Shukla, and K. Dasgupta, “A survey of transport protocols for deep space communication networks,” _International Journal of Computer Applications_ , vol. 31, no. 8, pp. 25–32, 2011.
  * [143] K. Zhao, R. Wang, S. C. Burleigh, M. Qiu, A. Sabbagh, and J. Hu, “Modeling memory-variation dynamics for the Licklider transmission protocol in deep-space communications,” _IEEE Transactions on Aerospace and Electronic Systems_ , vol. 51, no. 4, pp. 2510–2524, 2015.
  * [144] Z. Shi, S. Peng, Y. Zhou, L. Yang, R. Wang, H. Hu, and K. Zhao, “Study on checkpoint timer optimal setup of Licklider transmission protocol,” _IEEE Aerospace and Electronic Systems Magazine_ , vol. 35, no. 5, pp. 4–13, 2020.
  * [145] Consultative Committee for Space Data Systems (CCSDS), “Overview of space communications protocols,” _CCSDS 130.0-G-3_ , 2014, https://public.ccsds.org/Pubs/130x0g3.pdf.
  * [146] L. Clare, S. Burleigh, and K. Scott, “Endpoint naming for space delay/disruption tolerant networking,” in _2010 IEEE Aerospace Conference_. IEEE, 2010, pp. 1–10.
  * [147] Consultative Committee for Space Data Systems (CCSDS), “Encapsulation service,” _CCSDS 133.1-B-2_ , 2009, https://public.ccsds.org/Pubs/133x1b2sc2.pdf.
  * [148] Consultative Committee for Space Data Systems (CCSDS), “TC space data link protocol,” 2015, https://public.ccsds.org/Pubs/232x0b4.pdf.
  * [149] Consultative Committee for Space Data Systems (CCSDS), “TM synchronization and channel coding,” _CCSDS 131.0-B-3_ , 2003, https://public.ccsds.org/Pubs/131x0b3e1.pdf.
  * [150] Consultative Committee for Space Data Systems (CCSDS) , “Proximity-1 space link protocol - data link layer,” _CCSDS 211.0-B-6_ , 2020, https://public.ccsds.org/Pubs/211x0b6.pdf.
  * [151] Consultative Committee for Space Data Systems (CCSDS), “AOS space data link protocol,” _CCSDS 732.0-B-2_ , 2006, https://public.ccsds.org/Pubs/732x0b2c1s.pdf.
  * [152] Consultative Committee for Space Data Systems (CCSDS), “Unified space data link protocol,” _CCSDS 700.1-G-1_ , 2018, https://public.ccsds.org/Pubs/700x1g1.pdf.
  * [153] J. A. Fraire, O. De Jonckère, and S. C. Burleigh, “Routing in the space internet: A contact graph routing tutorial,” _Journal of Network and Computer Applications_ , vol. 174, p. 102884, 2021.
  * [154] D. Hou, K. Zhao, and W. Li, “Application layer channel coding for space dtn,” in _Machine Learning and Intelligent Communications: Second International Conference, MLICOM 2017, Weihai, China, August 5-6, 2017, Proceedings, Part I_. Springer, 2018, pp. 347–354.
  * [155] Q. Yu, X. Sun, R. Wang, Q. Zhang, J. Hu, and Z. Wei, “The effect of dtn custody transfer in deep-space communications,” _IEEE Wireless Communications_ , vol. 20, no. 5, pp. 169–176, 2013.
  * [156] I. Psaras, G. Papastergiou, V. Tsaoussidis, and N. Peccia, “DS-TP: Deep-space transport protocol,” in _2008 IEEE Aerospace Conference_. IEEE, 2008, pp. 1–13.
  * [157] G. Papastergiou, I. Psaras, and V. Tsaoussidis, “Deep-space transport protocol: A novel transport scheme for space DTNs,” _Computer Communications_ , vol. 32, no. 16, pp. 1757–1767, 2009.
  * [158] S. Fu and M. Atiquzzaman, “Sctp: State of the art in research, products, and technical challenges,” _IEEE Communications Magazine_ , vol. 42, no. 4, pp. 64–76, 2004.
  * [159] P. K. Chowdhury, M. Atiquzzaman, and W. Ivancic, “Sinemo: An ip-diversity based approach for network mobility in space,” in _2nd IEEE International Conference on Space Mission Challenges for Information Technology (SMC-IT’06)_. IEEE, 2006, pp. 7–pp.
  * [160] M. S. Hossain, M. Atiquzzaman, and W. Ivancic, “Cost analysis of mobility management entities of sinemo,” in _2011 IEEE International Conference on Communications (ICC)_. IEEE, 2011, pp. 1–5.
  * [161] M. Atiquzzaman and M. S. Hossain, “Security issues in space networks,” in _NASA Earth Science Technology Conference, Pasadena, CA_ , 2011.
  * [162] G. Wang, S. C. Burleigh, R. Wang, L. Shi, and Y. Qian, “Scoping contact graph-routing scalability: Investigating the system's usability in space-vehicle communication networks,” _IEEE Vehicular Technology Magazine_ , vol. 11, no. 4, pp. 46–52, 2016.
  * [163] S. El Alaoui and B. Ramamurthy, “Eaodr: A novel routing algorithm based on the modified temporal graph network model for DTN-based interplanetary networks,” _Computer Networks_ , vol. 129, pp. 129–141, 2017.
  * [164] C. Roberts, D. L. Carter, J. Hudiburg, R. N. Tye, P. Celeste, and P. Peskett, “Evolving the NASA near Earth network for the next generation of human spaceflight,” in _SpaceOps 2014 Conference_ , 2014, p. 1689.
  * [165] National Aeronautics and Space Administration (NASA), “Scan 101,” https://www.nasa.gov/sites/default/files/files/SCaN_101_2013.pdf, 2013.
  * [166] W. J. Weber, R. J. Cesarone, D. S. Abraham, P. E. Doms, R. J. Doyle, C. D. Edwards, A. J. Hooke, J. R. Lesh, and R. B. Miller, “Transforming the deep space network into the interplanetary network,” _Acta Astronautica_ , vol. 58, no. 8, pp. 411–421, 2006.
  * [167] R. C. Reinhart, J. S. Schier, D. J. Israel, W. Tai, P. E. Liebrecht, and S. A. Townes, “Enabling future science and human exploration with NASA's next generation near Earth and deep space communications and navigation architecture,” in _International Astronautical Congress (IAC) 2017_ , no. IAC-17-B2. 1.1. 41830, 2017.
  * [168] A. Alhilal, T. Braud, and P. Hui, “The sky is not the limit anymore: Future architecture of the interplanetary internet,” _IEEE Aerospace and Electronic Systems Magazine_ , vol. 34, no. 8, pp. 22–32, 2019.
  * [169] J. Velazco, “An inter planetary network enabled by smallsats,” in _2020 IEEE Aerospace Conference_. IEEE, 2020, pp. 1–10.
  * [170] N. Saeed, A. Elzanaty, H. Almorad, H. Dahrouj, T. Y. Al-Naffouri, and M.-S. Alouini, “Cubesat communications: Recent advances and future challenges,” _IEEE Communications Surveys & Tutorials_, vol. 22, no. 3, pp. 1839–1862, 2020.
  * [171] T. J. Martin-Mur and B. Young, “Navigating marco, the first interplanetary cubesats,” 2019.
  * [172] K. Oudrhiri, O. Yang, D. Buccino, D. Kahan, P. Withers, P. Tortora, S. Matousek, N. Lay, J. Lazio, J. Krajewski _et al._ , “Marco radio occultation: How the first interplanetary cubesat can help improve future missions,” in _2020 IEEE Aerospace Conference_. IEEE, 2020, pp. 1–10.
  * [173] H. Kalita, A. Ravindran, and J. Thangavelautham, “Exploration and utilization of asteroids as interplanetary communication relays,” in _2018 IEEE Aerospace Conference_. IEEE, 2018, pp. 1–9.
  * [174] F. De Rango and M. Tropea, “DTN architecture with resource-aware rate adaptation for multiple bundle transmission in interplanetary networks,” _IEEE Access_ , 2022.
  * [175] A. Trichili, K.-H. Park, M. Zghal, B. S. Ooi, and M.-S. Alouini, “Communicating using spatial mode multiplexing: Potentials, challenges, and perspectives,” _IEEE Communications Surveys & Tutorials_, vol. 21, no. 4, pp. 3175–3203, 2019.
  * [176] J. Wang, J.-Y. Yang, I. M. Fazal, N. Ahmed, Y. Yan, H. Huang, Y. Ren, Y. Yue, S. Dolinar, M. Tur _et al._ , “Terabit free-space data transmission employing orbital angular momentum multiplexing,” _Nature photonics_ , vol. 6, no. 7, pp. 488–496, 2012.
  * [177] H. Huang, G. Xie, Y. Yan, N. Ahmed, Y. Ren, Y. Yue, D. Rogawski, M. J. Willner, B. I. Erkmen, K. M. Birnbaum _et al._ , “100 Tbit/s free-space data link enabled by three-dimensional multiplexing of orbital angular momentum, polarization, and wavelength,” _Optics letters_ , vol. 39, no. 2, pp. 197–200, 2014.
  * [178] Y. Ren, Z. Wang, P. Liao, L. Li, G. Xie, H. Huang, Z. Zhao, Y. Yan, N. Ahmed, A. Willner _et al._ , “Experimental characterization of a 400 Gbit/s orbital angular momentum multiplexed free-space optical link over 120 m,” _Optics letters_ , vol. 41, no. 3, pp. 622–625, 2016.
  * [179] B. B. Yousif and E. E. Elsayed, “Performance enhancement of an orbital-angular-momentum-multiplexed free-space optical link under atmospheric turbulence effects using spatial-mode multiplexing and hybrid diversity based on adaptive MIMO equalization,” _IEEE Access_ , vol. 7, pp. 84 401–84 412, 2019.
  * [180] J. Wang, J. Liu, X. Lv, L. Zhu, D. Wang, S. Li, A. Wang, Y. Zhao, Y. Long, J. Du _et al._ , “Ultra-high 435-bit/s/Hz spectral efficiency using N-dimentional multiplexing and modulation link with pol-muxed 52 orbital angular momentum (OAM) modes carrying Nyquist 32-QAM signals,” in _2015 European Conference on Optical Communication (ECOC)_ , 2015, pp. 1–3.
  * [181] I. B. Djordjevic, “Deep-space and near-Earth optical communications by coded orbital angular momentum (OAM) modulation,” _Optics express_ , vol. 19, no. 15, pp. 14 277–14 289, 2011.
  * [182] X. Wang, Y. Liu, L. Guo, and H. Li, “Potential of vortex beams with orbital angular momentum modulation for deep-space optical communication,” _Optical Engineering_ , vol. 53, no. 5, p. 056107, 2014.
  * [183] Q. Zhang, C. Pang, L. Yang, Z. Li, H. Huang, and G. Wu, “Parallel coded optical vortex beam free-space communication based on single-photon detection,” _IEEE Photonics Journal_ , vol. 14, no. 3, pp. 1–6, 2022.
  * [184] T. Lei, M. Zhang, Y. Li, P. Jia, G. N. Liu, X. Xu, Z. Li, C. Min, J. Lin, C. Yu _et al._ , “Massive individual orbital angular momentum channels for multiplexing enabled by dammann gratings,” _Light: Science & Applications_, vol. 4, no. 3, pp. e257–e257, 2015.