the user end and FSO ensure high data rate that can operate in the same
environment without causing interference among each other. Hybrid technology
can play a significant role in link-reliability enhancement, energy-efficient
operations, seamless wireless connectivity in remote distances, interference
minimization, and security issues. The goal of combined wireless systems to
attain better performance through eliminating the limitations of individual
technologies. Some technical hurdles like shadow effects, inter-symbol
interference (ISI), phase induced noise, multi-access interference (MAI) and
multipath fading reduces the intensity of the received signal. The RF networks
performs as a backup path whenever blockage occurs, NLOS conditions, antenna
misalignment, or the severe degradation along FSO link. The transmission link
switch back to optical link when the signal is recovered.

Figure 10: Demonstration of hybrid RF/FSO mechanism.

In the response to escalating FSO link reliability, RF link serving as a
backup path in parallel with the FSO channel propagation path offers a
potential solution during FSO link outage. Thermal expansion, heavy wind
loads, and little earthquake causes the vibration, thereby break the tight
pointing (i.e. misalignment) of the FSO transmitter toward the photodetector
position [365, 366]. The combined use of RF systems like microwave and mmWave
with FSO can suppress these restrictions. Although RF system exhibits inferior
data rate performance in compared to FSO link, it can guaranteed the
connectivity while FSO channel fail. However, RF channels is less affected by
pointing errors, fog and atmospheric turbulence. Notably RF link is typically
configured in the millimeter (mm) wave bands ranging around 60 GHz that
facilitate high bandwidth incorporating with FSO link. Moreover, RF-based
communications enable improved performance in NLOS conditions and support
better mobility, but RF technology is electromagnetic interference sensitive.
For the long-span combined RF/FSO systems, RF link can be used for beam
pointing and acquisition for ensuring better reliability [64]. These special
features of RF technology mitigate the limitations of FSO deployment. The
energy efficiency (bps/Watt) of FSO system is comparatively higher than an RF
system. Shifting the traffic load to optical networks from RF minimize the
overall energy consumption. Provisioning the RF/FSO systems is the best choice
for spectrum utilization particularly for the indoor user’s performance.
Mobility support with extremely high speed is paramount benefits of the RF/FSO
hybrid systems. Hybrid FSO scheme offers excellent communication distance for
vehicular systems (V2X) handling dense traffic densities over the wider
geographical region [152]. An FSO networks plays as a primary link and RF
system used as a secondary backup path to establish high reliability. The
wireless connectivity through relay nodes is termed as mixed RF/FSO network.

The problem encountered in hybrid FSO system, a re-transmission approach is
adopted in literature [367] where the lost packets are retransmitted via RF
link. The proposed technique based on throughput-optimal scheduler that has
the capability of handling handover between optical network and RF
architecture efficiently. The function of hybrid infrastructure gateway is
scheduling, re-transmitting and handling all the incoming data from data
packet scheduler. However, the key challenge of vertical handover is to
maintain expected QoS. Markov decision process [368] and fuzzy-logic approach
[369] is proposed to manage vertical handover issue. Markov decision scheme
minimize the pin-pong effects i.e. the redundant switching is avoided when
blocking occurs in LOS link. The optical link interruption is primarily
depends on the user distribution based on space, time and duration. In
accordance to the user mobility predictions, the packet scheduling of hybrid
FSO networks can be determined using Markov chain process. On the other hand,
fuzzy-logic process address the uncertainty of decision metrics. Reference
[370, 371, 372] demonstrates the physical layer security of hybrid RF/FSO
systems formulating the minimum power consumption satisfying the users secrecy
rate. Fig. 10 demonstrates the fundamental hybrid RF/FSO communication system.

##### WiFo (WiFi-FSO) Communication

WiFo is a hybrid (i.e. a combination of both RF and FSO technologies) high
capacity indoor wireless communication systems based on femtocell architecture
to increase spectral efficiency and reduce interference [373, 374]. WiFo can
be used to handle multi user FSO communication problems where end users
received multiple FSO signals simultaneously in overlapped areas. With the
added benefits of high modulation bandwidth of VCSEL, the WiFo typically adopt
embedded coding PAM technique for short distance indoor communications
resulting low energy consumption and PiN photodiode [374]. A set of FSO
transmitters are comprised in WiFo directly under the roof. All FSO
transmitters generate invisible light that are connected to high speed 100
Gbps Ethernet network controlled by the access point (AP). Whereas, AP manages
the simultaneous data communications of FSO signals and the existing WiFi
links. WiFo permits denser deployment of light sources where a single receiver
can capture all FSO transmitted signals like MISO systems. These crowded
installations enable end users in overlapped areas to achieve higher data
rate.

Underwater optical wireless communications (UOWC) by means of the aggregate
technology of RF, acoustics and optics has drawn deep attention for multiple
potential applications for instance, oil pipe monitoring, underwater
environment and offshore investigation. UOWC based of RF and acoustic
technology cannot support long distance communication and high speed data rate
as well. Acoustic waves enables maximum 20 km underwater link distance with
bit rate ranging from tens Hz to hundred of kHz [375]. On the other hand, RF
systems also exhibits extremely poor performance that are limited to short
link distance. The excellent technical benefits of hybrid RF/acoustic/optical
technology can overcome limitations such as link delay, low coverage, and
strong signal attenuation.

Recently, optical/optical hybrid wireless technologies such as FSO/VLC,
FSO/OCC showing different characteristics has also received much attraction
among researchers. FSO/OCC hybrid solution provides long distance V2X
communications while maintain stable link performance eliminating their
individual limitations. OCC support comparatively short distance
communications between vehicles with low data rate. In contrast, FSO networks
provides communication between cars far apart. Therefore, the hybrid system
satisfy user demand and increase reliability as well. However, the performance
is considerable suffered by the atmospheric conditions in outdoor particularly
precise control of the transceiver [376]. To address precise indoor and
outdoor localization, pointing, navigation, interference effect, handling of
diverse data rates, bit rate requirement, coverage area, a hybrid FSO/VLC
heterogeneous interconnection is proposed in literature [377, 378]. The
implementation and deployment of FSO/VLC hybrid network coordination together
with user mobility support, localization, routing algorithm, traffic
management, and handover issues are clearly discussed in [378].

In order to measure the effectiveness of any system, performance analysis is a
vital issue. The performance metrics assessment can be carried out in various
ways such as analytical evaluation, numerical method, simulation environments,
and experimental verification. Different works used different performance
metrics such as throughput analysis, BER, spectral efficiency, outage
probability, secrecy rate analysis, symbol error rate, ergodic capacity, SINR,
and power efficiency to investigate received signal quality, system capacity
and reliability. Some applications of hybrid FSO systems with brief
description are presented in Table VII. Table VIII and Table IX summarizes the
key objectives and detailed discussion on different aspects on recent research
trends of hybrid FSO.

TABLE VII: Summary of Hybrid FSO Applications Hybrid type |  Objective |  Contribution |  References
---|---|---|---
Microwave/FSO and mmWave/FSO |  Backhaul connectivity |  Seamless microwave photonic link composed of RoFSO, RoF and RF, traffic shifted from RF to FSO network. Effect of channel impairments like chromatic dispersion, atmospheric turbulence, and multi-path induced fading are considered. Complementary properties of the FSO and mmWave channels, diversity selection combining technique improves the link reliability in terms BER of under strong turbulence |  [379, 380, 381, 382, 383, 41, 47, 48, 274, 384, 385, 386, 387]
RF/FSO |  Backhaul connectivity |  Hybrid RF/FSO improve the link reliability for long distance communication. Extensive simulation is conducted to evaluate SINR, BER taking into account of limiting factors under different network scenarios |  [385, 303, 300, 235, 236, 237, 238, 239, 301, 285, 294, 242, 293, 297]
Acoustics UWC/FSO |  High bit rate, link reliability enhancement |  Based on the link distance, oceanic turbulence induced scintillation and misalignment in underwater links, and pointing, acquisition, and tracking (PAT) establishment, LOS/NLOS condition, a UWC/FSO system is established for reliable communication |  [142, 143, 275, 388, 273, 145]
TABLE VIII: Summary of Hybrid FSO Research Trends Reference |  Metrics |  Objectives |  Description |  Remarks
---|---|---|---|---
[48] |  Throughput, Outage probability |  Data rate maximization of vehicular ad-hoc network (VANET) |  Developed a joint channel allocation and rate control to enhance throughput and solve the cross layer design problem incorporating carrier sense multiple access (CSMA). Capacity and outage probability are also satisfied through alternating detection method |  System capacity
[237] |  Throughput |  Orthogonal frequency division multiple access (OFDMA) based scheme ensure the backhaul performance of FSO link |  Proposed a relay assisted OFDMA based throughput maximization of hybrid RF/FSO link considering multiuser resource allocation scheme including power and subcarrier allocation |  System capacity, signal quality
[301] |  Throughput |  Network control approach for throughput optimization based on various channel conditions |  Studied the achievable throughput enhancement using mixed linear integer problem. Also, proposed a heuristic scheduling and traffic demand routing adopting physical channel interference model |  System capacity, reliability
[286] |  Outage probability, SINR |  Power adaption technique driven by truncated channel inversion for ensuring fixed SINR at receiver |  Determined the improvement of outage performance assuming two distinctive power adoption strategies with analytical derivations |  System capacity, signal quality
[287] |  Throughput, outage probability, power efficiency |  Performance evaluation assuming precise channel state information at receiver |  Derived the closed form expressions for the signal decoding. Also, analyzed the throughput and outage probability in consideration of adaptive power allocation and state of channel conditions |  System capacity
[302] |  Throughput, power efficiency |  Throughput maximization using parallel relay assisted networks |  Focused on the optimal system design according to sub-optimal buffer aided mechanism such as the optimal relay selection strategies and optimal time allocation |  System capacity, reliability
[235] |  Ergodic capacity, outage probability, blocking probability, SINR |  System performance optimization with opportunistic scheduling |  Closed-from expressions are derived to evaluate the system performance analyzed varying different atmospheric turbulence conditions. Monte-Carlo simulations are conducted to justify the exact and asymptotic results in the presence of turbulence fading and pointing error |  System capacity, signal quality, reliability
[280] |  Ergodic capacity, power efficiency, outage probability |  Investigate the performance of multiuser SIMO hybrid relay networks with opportunistic user scheduling |  Outage probability, symbol error probability and ergodic channel capacity are assessed based on closed-from expressions using maximal ratio combining and selective combing in the presence of different fading model |  System capacity, signal quality
[281] |  Ergodic capacity, outage probability, BER |  Performance evaluation of asymmetric link of dual-hop relay transmission system with pointing error |  Develop the closed-from expressions of CDF, PDF, and moments of SINR validated with the help of different metrics |  System capacity
[282] |  Ergodic capacity, outage probability, BER |  Performance investigation of switching-based transmission scheme |  Investigate average BER, ergodic capacity and outage probability considering gamma-gamma fading and Nakagami fading |  system capacity
[256] |  Ergodic capacity, SINR, SER, outage probability |  Hardware impairments for heterodyne IM/DD with fixed gain relay scheme |  Impact of hardware impairments for different SNR is illustrated. Analytical expressions for different performance parameters are derived and compared the numerical results through the numerical integration method |  system capacity, signal quality
[303] |  Throughput, delay tolerant performance |  Cascading of small cell networks to hybrid RF/FSO backhaul link |  Developed optimal fixed and adaptive link allocation algorithms based on channel state information to investigate delay limited and delay tolerant transmission schemes |  system capacity, signal quality
[236] |  Throughput, BER, outage probability, ergodic capacity, SNR |  Cost reduction and scalability improvement of mmWave RF/FSO |  Examined the performance metrics considering the effect of pointing errors owing to the misalignment between the transmitter and receiver. Note that the cascaded channels are modeled by the Rician and Malaga distributions |  system capacity, reliability
[296] |  Throughput, BER, outage probability, SINR |  Mitigation technique of multipath fading and atmospheric turbulence for a dual hop hybrid system |  Proposed a transmit diversity at the sending end and selection combining technique at the receiver end to investigate the effect of pointing errors on the FSO link. Also, examine the the system performance and provide a light of way to mitigate limiting factors |  System capacity, reliability, signal quality
[123] |  Outage probability, power efficiency |  Determine the optimal transmit power and co-channel interference |  Exact closed-form expression is derived to find an optimal power allocation. The secrecy performance and physical layer security performance is studied under different constraints |  Reliability
TABLE IX: Summary of Hybrid FSO Research Trends (Continued) Reference |  Metrics |  Objectives |  Description |  Remarks
---|---|---|---|---
[283] |  Ergodic capacity, outage probability, BER |  Estimation higher link reliability under extreme atmospheric constraints |  To avoid needless switching, a single FSO link is considered to active whereas an extra mmWave RF/FSO link is used as a backup. The primary link is modeled by gamma-gamma distribution assuming strong turbulence |  Reliability, system capacity
[274] |  SNR, BER, Outage probability |  Switching strategies to minimize the bandwidth wastage maintain the same data rate |  Performance investigation using selection combining diversity based on closed-form expression of BER and outage probability. Ensure the same data with the same data rate over the FSO link without having CSI |  Reliability, system capacity
[289] |  BER, Outage probability |  Impact of outdated CSI based relay selection on the system performance |  Considered partial amplify-and-forward relay selection. RF link is subject to Rayleigh fading and FSO link is modeled by gamma-gamma distribution. Mathematical expressions of various performance metrics are derived |  Reliability
[290] |  Outage probability, power efficiency |  Cost minimization due to power consumption while ensuring packet success probability |  Performance investigation by joint consideration of power allocation and dynamic link selection for guaranteeing the long-term reliability requirements. The closed form of power allocation policy is derived for link selection and Lyapunov optimization algorithm is used to solve the problem |  Reliability
[291] |  BER, Outage probability |  Effective link quality scheduling using multiuser diversity |  Analyzed a hybrid RF/FSO point-to-multipoint system assuming multiple FSO users, RF users and access points. The asymptotic closed form expressions for average BER, outage probability are derived considering multiuser diversity gain |  Reliability, signal quality
[284] |  SINR, SER, ergodic capacity, Outage probability |  Two way multiuser relay networks with opportunistic scheduling and non-uniform channel fading is investigated |  An efficient optimal power transmission algorithm is developed for a heterodyne detection scheme. The system performance is evaluated for two way relaying network and single way relaying network in the presence of weather turbulence and pointing error effect |  System capacity, reliability, signal quality
[257] |  SINR, BER, Outage probability |  Dual hop hybrid system based on cognitive amplify and forward relay networks |  Fixed gain and channel assisted relaying policy is derived according to the asymptotic expressions. Heterodyne IM/DD detection receiver with double gamma fading channel is considered. In addition, the diversity order and diversity multiplexing trade-off is carried out to investigate the overall system performance |  System capacity, reliability
[292] |  BER, Outage probability |  Controlling the FSO link fragility and reduction of outage probability |  Analytical expressions of outage probability and BER are derived varying different modulation scheme with its different channel impairments |  System capacity, reliability
[275] |  SINR, BER, Outage probability |  Performance analysis of dual hop RF/UWOC hybrid systems under fixed and variable gain relaying schemes |  Evaluate SINR, BER, and outage probability under the effect of water bubble levels, temperature gradient, water conditions and detection techniques of underwater wireless optical communication |  Reliability
[273] |  Throughput, BER |  Boosting of total channel capacity with FSO backbone UWOC |  Total achievable throughput of FSO/UWOC link is 500 Gbps integrating PAM4 modulation with five (5 R/G/B LD transmitters of two stages) wavelength-polarization multiplexing scheme |  System capacity, signal quality

##  VII Radio over FSO (RoFSO) System

Figure 11: Simplified Block Diagram of RoFSO System.

Radio over free space optics (RoFSO) has emerged as a prominent technologies
in future communications due high data transmission and low energy
consumption. The concept of RoFSO system is composed of radio over fiber (RoF)
and FSO system where the optical signal is modulated with the RF signal at a
transmitter end before transferring to the receiver. In other words, RoFSO is
technique that allows simultaneous data transmission of broadband RF signals
in bidirectional path over FSO link. A technique of modulating RF subcarriers
onto an optical carrier for transmission over an optical fiber link is known
as RoF. However, RoFSO system can overcome the last mile problems for
broadband services with desired reliability of communication link in area
where the installation of fiber cable is restricted [381]. To meet the
unprecedented growth of user demands in 5G and B5G communications, a telecom
operators need to install more number of base stations (BSs) in order to avoid
spectral congestion in lower RF frequency bands which significantly increases
the net present cost and system complexity. Whereas RoFSO provides the greater
level of reliability, flexibility in deployment, low signal loss, immunity to
electromagnetic interference with the advantage of high optical bandwidth in a
cost effective way [389]. The principle of RoFSO is similar the operation of
WDM technique. The generalized block diagram of radio over FSO (RoFSO) system
is clearly presented in Fig. 11. Under the RoFSO scheme, the central base
stations distribute optical signals through free space to multiple remote
stations. In general, Mach–Zehnder modulator (MZM) is used to perform the
modulation of an optical beam as a function of a RF signal by combining phase-
delayed optical beams. In MZM, a beam-splitter separated the incident laser
light waves into two paths to illustrate a linear electro-optic (EO) effect
that produces birefringence in an optical medium when the electric field is
applied. By tuning the electric field i.e., RF input signals the phase delay
of upper and lower path can be controlled and then the beams are combined at
the output either constructively of destructively. The EO mechanism changes
the refractive index of the optical medium and thus, the MZM can be used for
amplitude, phase, frequency modulations [222].

##  VIII Multi-user FSO Communication

Multiple users transmit different signals, which is mixed in the wireless
propagation medium. Blind source separation (BSS) technique is used to extract
the original information from the mixed signals. This problems exist in
numerous applications such as biomedical data analysis, speech signals
identification, machine learning based communications, etc. [390]. Since users
are randomly distributed over space, different channels will experience
independent fading. Exploiting multiple antenna diversity technique can
considerably improve the FSO wireless transmission performance. Multiuser
diversity offers some inherent benefits over antenna diversity as it explores
independent fading channels. It also support a simple receiver structure where
a single antenna is equipped per receiver. Introducing a number of access
points through long-range FSO link that amplifies the received signal via
short distance RF link and forward to next hop increase the transmission
distance and reliability of data services as well. Authors [276] reported a
multi user multi hop combined RF/FSO communication systems that offers lower
energy consumption, low delay processing and minimum complexity. Amplify and
forward relaying is used for fixed gain amplification when channel state
information (CSI) is unknown, in contrast, adaptive gain is a suitable choice
when CSI is known. Chen et. al. [291] studied the multi-user diversity RF/FSO
point-to-multipoint MISO communication systems consisting hybrid access points
aiming to minimum error probability and BER. A multiuser mixed RF/FSO two way
relaying (TWR) method considering asymmetric channel gain is presented in
[391] to examine the outage probability and energy-efficient power allocation.
However, Multiuser MIMO transmission is a better choice over spatial
multiplexing as it can considerably extend the capacity of wireless
communication systems [392]. Fig. 12 shown the basic multiuser MIMO
communications in FSO perspectives.

Figure 12: Generalized Multiuser MIMO FSO System.

It is essential to sense the optical spectrum periodically in order to avoid
interference among multiple users particularly to detect the channel
occupancy. Spectrum sensing plays a significant role in multi-user FSO
communications that allow user to monitor the available optical frequency
spectrum in their operating range [393]. A generalized blind spectrum sensing
[i.e. applicable to all optical wavelengths] policy based on SNR estimation
for an unknown optical signal over strong atmospheric turbulence conditions is
reported in [394]. An extensive investigation of energy detection method based
optical free spectrum sensing for FSO communication over exponentially
distributed channel is conducted in [395].

##  IX MIMO FSO Systems

Diversity techniques e.g. spatial diversity realized multiple beams at either
the transmitter or multiple apertures at the receiver end can considerably
improve the wireless FSO transmission over fading induced channels. The
spatial diversity technique helps to enhance the FSO transmission reliability
introducing inherent signal redundancy [396, 231]. It is worthy to mention
that turbulence-induced fading considerably deteriorates single input single
output (SISO)-FSO link [397]. As an indication of the context, additional
power margin is employed to achieve desire QoS, this power penalty can be
alleviated by increasing the transmission power. However, this approach is
impractical due to eye safety regulations and relatively lower reliability.
Typically, a conventional Gaussian light signal can experience beam wandering
effect, causes power fluctuations and wavefornt distortions due to unwanted
atmospheric turbulence. In addition, misalignment between transceivers causes
significant power loss at the receiver end as explained beforehand. There have
been several approaches to handle possible signal degradations. Aperture
averaging is a potential technique of improving system performance by means of
incorporating wider lens at the receiver side to mitigate fading effect via
averaging intensity fluctuations. This technique is efficient under moderate-
to-strong atmospheric turbulence when the receiver aperture is greater than
fading correlation length, $\sqrt{\lambda}l$, with $l$ denote the transmission
distance and $\lambda$ represents the wavelength [64]. Using multiple small
apertures instead of large aperture at the receiver is a suitable way to
reduce fading with taking the advantage of aperture averaging. Employing
several apertures is more advantageous under strong turbulence regime but
involves implementation complexity. For single input multi output (SIMO)
spatial diversity systems, equal gain combining (EGC) method is used that
offer less complexity but shows comparatively inferior performance to optimal
maximal ratio combining (MRC) [392]. On the other hand, multi input single
output (MISO) FSO scheme at the transmitter end is mostly used to transmit the
same signal on the separate beams, which is referred as a repetition coding
(RC). Authors in [321] pointed out that MISO FSO based spatial diversity
technique combat the fading effects implementing the multiple laser sources at
the receiving end. Moreover, generalized selective combining (GSC) method
recognized as a low complexity diversity scheme to select the best possible
path from subset and combining them using MRC fashion. However, the weak
channel estimation limits the merits of MRC scheme that may degrade the system
performance considerably. A simplified illustration of MIMO FSO system is
displayed in Fig. 13.

Figure 13: Simplified Illustration of MIMO FSO System.

Likewise RF systems, MIMO technique is very popular to combat turbulence
induced multipath fading increasing the bit rate and quality of signal
transmission through spatial multiplexing [392]. Under this scenario the FSO
transceiver serve multiple end users simultaneously using the same frequency
band where multiple co-located antennas provide the larger multiplexing gain.
In order to control inter-user interference, several precoding techniques are
studied for multiuser MIMO transmission. Typically, intensity
modulation/direct detection (IM/DD) method is implemented in MIMO-FSO systems,
where IM is used for signal transmission and DD is employed at the receiver.
According to [392], nonlinear precoding schemes attain better performance over
linear precoding method, which consider the total rate capacity of all users
but involves increased implementation complexity. For instance, the dirty
paper coding (DPC) cancel out inter-user interference prior to data
transmission with full knowledge of CSI [398]. On the other hand, zero-forcing
beamforming (ZFBF) [399] and random unitary beamforming (RUB) [400] are
popular linear precoding schemes have significantly lower implementation
complexity. The precoding matrix is modeled pseudo inverse matrix of selected
users in ZFBF, whereas, the precoding matrix in RUB is designed based on the
channel quality of randomly selected users on distinctive beamforming
directions. Reference [401] studied the orthogonal space time block code
(OSTBC) and repetition code (RC) under log-normal fading channels conditions.
However, the log-normal atmospheric channel model is not sufficient when the
huge number of heterogeneous devices are connected in 5G [176]. Recently,
numerous research works conducted focusing on MIMO-OSTBC FSO applications due
to higher diversity gain and wide range of FSO applications [402, 403, 404,
405]. Besides the OSTBC, the RC scheme outperforms than OSTBC in terms of
diversity gain but the RC applications with MIMO is challenging because of
timing asynchronous alignment between transceivers and thereby, resulting the
time difference of signal propagation [406]. The use of repetition coding in
MIMO FSO systems endeavoring to combat turbulence effects by introducing
redundancy is studied in [407]. However, several laser beams having identical
frequency are interfere among each other when simultaneously received at the
same aperture.

To address timing misalignment of asynchronous MIMO FSO channels, OSTBC based
MIMO scheme offer better performance and robust to inter-symbol interference
(ISI) maintain the orthogonality conditions. However, the system performance
gap between RC and OSTBC schemes is more significant for higher order of MIMO
FSO systems. Saidi and Hamdi [408] presented BER performance for multihop
hybrid RF/FSO systems using M-ary pulse positon modulation (MPPM) and MIMO
technique under gamma-gamma turbulence channels. The enhancement of average
secrecy capacity and improvement of secrecy outage probability for MIMO-FSO
link with equal gain combing reception technique over gamma-gamma atmospheric
turbulence channels is demonstrated first time in literature [409]. Ref [410]
investigated satisfactory BER, power penalty reduction, and outage probability
performance for 10 Gb/s QPSK FSO links using aperture averaging receive
diversity and MIMO spatial technique. Eghbal and Abouei [411] studied received
signal intensity of MIMO FSO systems accounting destructive interference with
small variations of path length between optical transmitter and photodetector.
A disaster management method for MIMO-OFDM FSO links employing polarization
division multiplexing (PDM) with Malaga channel model is presented in [412].
With PDM, the two high speed OFDM signals are mutually orthogonal
polarizations is an excellent way to uplift spectral efficiency. Unlike fibre-
based communications, PDM is a suitable choice for FSO systems as it is immune
from cross polarization modulation, polarization mode dispersion and
associated losses. Under the proposed scheme, two different OFDM subcarriers
i.e., eve and odd subcarriers are multiplexed using PDM that enable to
transfer two independent data signals on a single wavelength. Das et. al.
[413] employed Almouti STBC transmit diversity and switch-and-examine
combining (SEC) scheme at the receiver side for MIMO-FSO communication systems
to investigate ergodic capacity, BER and outage probability over the Malaga
turbulence wireless channels.

##  X FSO Transmission in TCP Layer

Numerous research works are conducted focusing on signal re-transmission, re-
routing, cross layer design, delay tolerant networking in the perspectives of
transmission control protocol layer in conjunction with the physical layer
[414, 415, 416].

  * •

Re-transmission: An automatic repeat request (ARQ) is a re-transmission
protocol that is commonly used in reliable data communication. Under this
scheme, data transmission is carried out in the set of certain packet frame
lengths. The packet is re-transmitted when the receiver does not acknowledge
after certain period of time. This process is continuing until the positive
acknowledgement is received at the transmitter end. As a result, this go-back-
stop-wait N-ARQ re-transmission scheme leads to high bandwidth penalty, big
delay and large power consumption as well. Another technique of selective
repeat ARQ (SR-ARQ) where the packets are repeatedly send from the transmitter
without any feedback response from the receiver end either at data link level
or at transport layer. Here, the receiver unit will continue to accept data
packets and acknowledge to the transmitter. If the acknowledgement is not
received after certain time, it presumed to be data lost and transmit again.
On the hand, hybrid ARQ (H-ARQ) combining FEC coding and ARQ error control
mechanism in FSO communications is studied in [416]. It is found that superior
performance in terms of outage probability and BER is obtained under strong
turbulence but it suffers high delay latency and bandwidth penalty. In
contrary, cooperative diversity ARQ (C-ARQ) have shown remarkable performance
combating turbulence induced fading in FSO links. In addition, modified C-ARQ
(MC-ARQ) outperforms a lot better by means of lower transmission delay and
higher power efficiency [229]. Rateless Round Robin algorithm is used for
error controlling in practical FSO deployment under strong atmospheric
turbulence [417].

  * •

Re-routing: Data path re-routing in between optical link or low bit rate RF
link increase the FSO link reliability during adverse weather conditions. An
autonomous dynamic path reconfiguration including topology control mechanisms
and beams re-routing algorithms in the context of hybrid RF/FSO link is
demonstrated in [296]. The proposed method considerably improve the system
reliability but increase the processing delay and net present cost. Therefore,
the routing algorithm should be designed in such a way to minimize re-routing
delays or minimum number of hops with least cost. A common routing protocol
called proactive routing where all the possible routes are calculated in prior
and stored in the routing table. However, this scheme is not appropriate for
large-scale networks as it exploit heavy overhead that makes it less bandwidth
efficient. Another routing protocol named reactive routing produce less
overhead to the networks in comparison with former proactive routing. This
scheme compute new paths based on real-time demands during the malfunctions of
existing route but it causes prolonged latency of data transmission.
Therefore, hybrid routing scheme combing this two protocols can be employed
into clusters of networks for example proactive algorithms should apply within
each cluster and reactive routing policy apply among different clusters [296].

  * •

QoS control: QoS can be defined in terms of throughput, jitter delay, latency,
signal loss, spectral efficiency and energy efficiency. In order to achieve
reliable FSO communication between two nodes, the designed system should meet
the specified requirements satisfying QoS. The primary challenges to maintain
desired QoS in the FSO systems are the fluctuations of delay, data packet
rejection ratio, and overhead. The idea of buffer introducing a IP packet
investigation and scheduling processing according to the QoS specifications
and link availability. Reference [418] proposed 10 Gbps buffer encountered QoS
to minimize the packet rejection rate under strong atmospheric conditions in
hybrid RF/FSO networks. According to [418], QoS based buffer scheme provides 8
dB less fading margin. An efficient scheduling and buffer algorithm, tight
error controlling and proper channel access policies improves the QoS level in
medium access control (MAC) layer.

##  XI Next Generation FSO Communication Networks

In recent years, FSO systems as an alternative wireless communication
technology has gained incredible attention to cope up high traffic demand in a
cost effective way. In addition, FSO technology address the interference
issues and reliable connectivity via high level of security over the existing
RF infrastructure. The FSO inter-channel interference can be limited by the
controlling the optical beam width at the transmitter station. Exploiting the
coexistence of RoFSO, OWC/FSO or RF/FSO compensate the weakness of each
channels for the application of intelligent and secured data transmission,
battlefield environment monitoring. FSO established a heterogeneous structure
(RF, Coax, FSO, optical fibre) to provide extreme capacity of wireless
services both in ground and space [172]. The optoelectronic devices for
example 100 Gb/s optical source, mach-zehnder modulator (MZM), wavelength
division multiplexing (WDM), photodetector etc. that are being used for high
speed fiber optic infrastructure can also be used in FSO systems. The design
of integrating the conventional high data rate signals into FSO systems allows
to transfer terrestrial digital HD TV signals, WLAN, perform 5G communication
protocols, and IoT/IoE wireless services. A performance evaluation of FSO link
in terms of BER, SNR, power-in-bucket using rectangular coherent flat-topped
beam in consideration of atmospheric turbulence has been investigated in
[419]. To handle the effect of atmospheric scattering and absorption, all
optical relaying and intermediate repeater approach is demonstrated.
Reliability analysis contemplating polarization shift keying modulated FSO
system and optimization of FSO enabled cellular backhauling for the next
generation mobile networks are also investigated in [168]. A simplified
illustration of next generation FSO communication systems is depicted in Fig.
14.

Figure 14: Next Generation FSO Networks.

The perfect FSO channel estimation is a challenging task as it involves high
cost and power consumption. It is not an appropriate choice in cellular
communications system since pilot transmission deteriorates the data rate.
Inspired from this encountered problems, authors [420] proposed deep learning
based low cost and low complexity channel estimation for FSO system. Although
blind channel estimation does not require pilot transmission, in addition, it
offers lower complexity but this approach is not realistic because of fixed
channel conditions [421]. Recently, machine learning [422] has attracted to
investigate optical performance monitoring, which is very useful when the
system model is complex. On the other hand, deep learning [423] could solve
the complex problems when the input is not enough for computation and provides
dynamic statistical signal processing tools to generate perfect probabilistic
channel model for received data impairments. Deep neural network (DNN) [424,
425] algorithms can be used in high-level simulation to analyze input data,
offering fast response. DNN algorithms is employed for best modulation scheme
identification [426], controlling of optical amplifiers [427], routing
paradigms [428], and quality evaluation of transmitted data signals [429].

From the aforementioned discussion, machine learning (ML) for future
generation FSO networks becomes a hot topic. Few researches are conducted in
ML based FSO communication such as detection purpose [430], error correction
for advanced optics [244], and perfect demodulator design for angular optical
beams [431, 432].

##  XII Summary

With the proliferation of diversified multimedia applications, RF systems is
not capable to fulfil the unprecedented demand by IoT, 5G, and beyond
communication systems. Various wireless access technologies have variety range
of appealing features with some limitations. It is widely known that RF and
optical wireless technologies (e.g. FSO) perform reciprocal characteristics.
The key benefits of the RF technology are mobility support, transmission
performed under LOS/NLOS conditions; provide wider coverage area, whereas it
significantly suffered due to bandwidth shortage, electromagnetic
interference, and smaller throughput performance. On the other hand, FSO
systems among all other OWC technologies offers the potential advantages of
longer coverage area and enormous bandwidth. Unlike RF signal, the optical
beam cannot be detected by spectrum analyzers and laser beams cannot penetrate
walls. Moreover, FSO transceiver are easy to deploy, lightweight, compact,
easily expandable. The narrow optical beams between FSO transceiver has the
potential to establish electromagnetic interference free longhaul ultra-speed
data communication guaranteeing a greater level of energy efficiency. However,
the reliability issue of FSO communications is degraded due to some limiting
factors such as fog, heavy rain, dense snowfall, thick cloud, pointing errors,
scintillation effect, and atmospheric molecular absorption. An extensive
comparison of different optical wireless technologies is carried out at the
beginning of this article to pointing out the key difference of FSO technology
compared to other OWC schemes. Thereafter, the wide range of FSO applications
in different perspectives such as space communications, cyber security,
solution of last mile problems, and core networks are clearly described. The
recent advancement of optical light source e.g. solid-state based
monochromatic LASER transmitter and the performance of various photodetectors
as optical receivers are explicitly presented pointing on merits and demerits.
The coherence property of laser diodes (LDs) enables fast data transmission
with less interference against LEDs. At the receiver end, PiN photodiode (PD)
is a preferable choice for short distance FSO links; in contrast, Avalanche PD
(APD) exhibits satisfactory system performance because of improved level of
receiver sensitivity for longer link distance. The goals of FSO systems are
different, and numerous approaches have been proposed to address reliability
issues. Pointing, Acquisition, and Tracking (PAT) technique to maintain LOS
directivity between two endpoints [228, 229, 230], adaptive modulation schemes
[258, 259, 260, 261, 262, 263, 264, 265, 266, 267], appropriate channel coding
based on different optical wireless link models [322, 323, 324, 325, 326, 327,
328, 329, 330], background noise reduction technique [354], spatial diversity
mechanisms [227, 228, 229, 230, 231], coherent FSO systems [331, 332, 333,
334, 335], aperture averaging for eddies induced fading [218], adaptive optics
[245, 246] for wavefront fluctuations, relay nodes [247, 248, 249] to attain
high channel gain, and hybrid FSO systems [367, 368, 369, 370, 371, 372, 373]
to overcome the limitations of a single technology. A number of research works
have been performed to address several technical issues, however, some issues
remain. For the successful deployment of FSO systems, several issues have been
counted as paramount research concerns, including interference mitigation
[433, 434, 435], automatic error correction [436], throughput enhancement
[274, 384], energy efficiency [437, 438, 439] for 5G, B5G and IoT
applications. In addition, the efficient FSO system design issues for quantum
and THz communications [440, 441], multi-hop, multi-user and cooperative
communications based on relay transmission [442, 443, 444, 445, 446, 447,
448], WDM FSO communications [449, 450, 451, 452, 453, 454], adaptive
switching mechanism in underwater optical wireless communications [187],
horizontal and vertical handover [455] issues in heterogeneous networks,
adaptive user association policies for optimal load balancing for hybrid FSO
systems [456, 457], physical layer security [458, 459, 460, 461, 462, 463,
464, 465, 466, 467, 468], software defined FSO networking [469, 470, 471, 472,
473, 474] issues are also discussed. Thereafter, radio over FSO (RoFSO)
technology is presented in 5G and other communications perspectives [381]. The
comprehensive study of MIMO FSO spatial diversity is studied in [402, 403,
404, 405, 406, 407, 408, 409, 410, 411] targeting to achieve robust system
performance. In light of existing research works, we focused the prospect of
the future generation FSO networks incorporating machine learning, deep neural
networks, efficient routing algorithms, QoS controlling methods.

Table X and Table XI discusses the summary of different FSO systems in terms
of distinctive performance metrics.

TABLE X: Summary of FSO channel capacity and link design Reference |  Modulation |  Link model |  Ergodic capacity |  Outage Probability |  BER/SER |  Throughput
---|---|---|---|---|---|---
[104] |  OCDMA-WDM, PPM |  Non fading |  No |  No |  Yes |  Yes
[105] |  OCDMA-WDM, PPM |  Gamma-Gamma |  No |  No |  Yes |  Yes
[295] |  OOK |  Non fading |  No |  No |  Yes |  Yes
[301] |  OFDMA OOK |  Rayleigh |  No |  No |  No |  Yes
[286] |  M-QAM |  Gamma-Gamma, Nakagami-m |  No |  Yes |  No |  Yes
[287] |  IM/DD BPSK |  Gamma-Gamma, log-normal |  No |  Yes |  No |  Yes
[302] |  OOK |  Gamma-Gamma |  No |  No |  No |  Yes
[235] |  BPSK |  Gamma-Gamma, Rayleigh |  Yes |  Yes |  No |  Yes
[280] |  BPSK |  Gamma-Gamma, Nakagami-m |  Yes |  Yes |  No |  No
[281] |  DBPSK, CBPSK, CBFSK |  Gamma-Gamma, Rayleigh |  Yes |  Yes |  Yes |  No
[288] |  OOK |  Rayleigh, Nakagami-m |  No |  Yes |  Yes |  No
[282] |  QAM |  Log-normal |  Yes |  Yes |  Yes |  No
[256] |  SIM-BPSK |  Malaga |  Yes |  Yes |  Yes |  No
[303] |  OOK |  Rician |  No |  No |  No |  Yes
[236] |  M-QAM, M-PSK |  Rician, Malaga |  Yes |  Yes |  Yes |  Yes
[296] |  BPSK |  Rayleigh |  No |  Yes |  Yes |  Yes
[123] |  IM/DD,BPSK |  Gamma-Gamma, Nakagami-m |  No |  Yes |  No |  No
[283] |  OOK |  Gamma-Gamma, Rayleigh |  Yes |  Yes |  Yes |  No
[274] |  BPSK, QPSK, 8-PSK |  Gamma-Gamma |  No |  Yes |  Yes |  No
[276] |  DPSK |  Gamma-Gamma |  No |  Yes |  Yes |  No
[289] |  DBPSK, SIM BPSK |  Rayleigh |  No |  Yes |  Yes |  No
[239] |  DBPSK, CBPSK, CBFSK |  Gamma-Gamma, Nakagami-m |  No |  Yes |  Yes |  No
[290] |  OOK |  Nakagami-m |  No |  Yes |  No |  No
[291] |  BPSK |  Gamma-Gamma |  No |  Yes |  Yes |  No
[284] |  BPSK |  Gamma-Gamma |  Yes |  Yes |  Yes |  No
[257] |  SIM (CBFSK, CBPSK, DBPSK) |  Nakagami-m |  No |  Yes |  Yes |  No
[292] |  OOK |  Gamma-Gamma |  No |  Yes |  Yes |  No
[238] |  OSTBC |  Nakagami-m |  No |  Yes |  No |  No
[285] |  IM/DD BPSK |  Nakagami-m, Malaga |  Yes |  Yes |  Yes |  No
[293] |  IM/DD BPSK |  Gamma-Gamma, Rayleigh |  No |  Yes |  No |  Yes
[294] |  BPSK, DBPSK, BFSK |  Gamma-Gamma |  No |  Yes |  Yes |  No
[297] |  IM/DD BPSK |  Nakagami-m, Malaga |  No |  No |  Yes |  Yes
[275] |  OOK, BPSK, QPSK, M-QAM |  Generalized Gamma, Nakagami-m |  No |  Yes |  Yes |  Yes
[273] |  four-level PAM |  Non fading |  No |  No |  Yes |  Yes
[298] |  PAM4 |  Non fading |  No |  No |  Yes |  Yes
[299] |  M-PSK |  Non fading |  No |  No |  Yes |  No
[300] |  M-PSK, DPSK |  Gamma-Gamma |  No |  No |  Yes |  No
[234] |  CBFSK, CBPSK, DBPSK |  Generalized Gamma, Nakagami-m |  Yes |  Yes |  Yes |  No
TABLE XI: Comparison of throughput and latency [5, 202] Technology |  Data rate (Gbps) |  Latency (ms) |  Communication link
---|---|---|---
Optical fiber |  100 |  $<$1 ms |  E2E
FSO |  40 |  $<$1 ms |  E2E
mmWave |  10 |  $<$1 ms |  E2E
Microwave |  1 |  $<$1 ms/hop |  P2P/P2mP

##  XIII Research Challenges, Directions and Open Issues

Diverse type of multimedia applications and high data rate digital wireless
services offered by 5G and B5G, the existing RF system is incapable to fulfill
the intended data transmission rate. As a result, FSO scheme offering an
ultra-band frequency range of license free spectrum is an attractive viable
solution. FSO communication is typically take place with a narrow directional
beam using line-of-sight (LOS) propagation technique over a long-haul link
distance. A 40 GB/s data rate through FSO communication has already been
implemented. Recently, FSO system can cover link distance ranging from nm
(inter-integrated circuit design) to 10000km (ground base to satellite FSO
link) [172]. Despite the potential benefits of FSO, to handle ever-increasing
growth of smartphones, cloud big-data, electronic devices, IoT/IoE devices,
artificial intelligence, etc. developing a robust infrastructure is become
challenging. We discussed different application scenarios ranging from
residence to space for the FSO links and systems, still research on fully
harvesting the inherent potential are immature. A number of important
challenging problems are need to be figured out for the efficient deployment
of FSO systems. In this section, we shed the light on research challenges,
future research directions and open problems related to FSO systems and
application.

###  XIII-A 5G/B5G and IoT/IoE solutions

It is obvious that FSO is the most emerging technology to support next
generation cellular networks and gigantic connectivity of IoT devices. The
substantial deployment of LDs for the FSO technology creates high inter-cell
interference in the 5G/B5G and IoT/IoE networks which become a challenging
issue. In addition, different modulation techniques of LDs may cause
flickering (i.e. fluctuation of modulated signals) [151], thereby avoiding
this issue also challenging. A coordinated multipoint (CoMP) enabled cellular
network remarkably reduce the inter-cell interference assuring throughput
maximization [437, 438, 439]. It is commonly accepted that with the
augmentation of deployed OWC nodes lead to increase the inter-cell
interference (ICI). ICI coordination and mitigation techniques in OWC domain
have been studied utilizing the recognized approached used in the RF domain
[433, 434, 435]. It is important to realize how to manage interference in
optical wireless link. A last mile bottleneck problem may arise if the
backhaul system for next generation mobile networks fails to handle an
enormous volume of data traffic to support high speed services at the end
users. Hence, the enhancement the FSO backhaul capacity for data rate
improvement to address last mile problem is become a challenging issue.
Furthermore, a massive amount of throughput demand by the access network is
generated owing to support huge connectivity of high data rate devices to the
network. Therefore, an extensive backhaul capacity is required to solve
aforementioned issues. Notably user traffic backhauling via distinctive
backhaul networks needs precise synchronization. Some of the future FSO
applications include both at cellular backhaul and front haul levels at the
transport and access network levels. Therefore, FSO system optimization for
mobile network backhauling by means of cost and reliability metrics is an
important issue.

Finally, machine-learning/deep-learning based networking is one of the prime
key in future generation cellular networks. Nowadays the demand of artificial
controlling and remote decision making increasing which can be tackled by
supervised learning such as smart healthcare and smart home load controlling
[49]. Integrating reinforcement learning and machine learning in 6G networks
enhance data rate, manage network traffic, enables intelligent network re-
assignment, automatic error correction and efficient decision making among
surrounding networks [436].

The seamless deployment of IoT devices trigger a demand to incorporate and
inter-operate a variety of hybrid connectivity technologies. For example,
numerous applications need the integration of technologies like RFID, wireless
sensor networks (WSN), cloud network using Bluetooth, WiFi, and ZigBee
connectivity on to a single or hybrid network. As a consequence, conventional
RF spectrum is more congested to support variety of IoT applications. One
promising key technology is hybrid FSO under OWC technology since it do not
interfere with RF spectrum and using the visible light spectrum more than
thousand times the modulation bandwidth of RF signal [475, 476]. To realize
the IoT/IoE paradigm, OWC technologies must evolve to accommodate the huge
data volume and expected transmission speed. Because of the aforementioned
benefits exploited by an unregulated OWC technology, FSO will play a key role
in the future IoT connectivity. The co-deployment of FSO and VLC ensure the
capability of physical systems of 5G/B5G network, smart cities (connectivity
of heterogeneous wireless services and IoT devices to urban infrastructure)
with a number of thins in synergy to connect and reach [477, 478]. Moreover,
connecting smart devices to IoT and beyond IoT network, FSO plays a core
backbone network in a convenient way adopting OFDM modulation scheme
particularly in an indoor environment [479, 480]. Hence, an efficient and
practical solution of FSO schemes for the smart approach of internet of
everything (IoE) is a major design factor.

###  XIII-B THz and Quantum Communications

FSO communication provide high data rates over a long coverage distance of LOS
connections especially in outdoor applications. FSO support some indoor
applications like connect devices inside offices, residences, hospitals,
shopping malls etc. The THz spectrum can provide the concrete solution for the
data transmissions in non-LOS (NLOS) applications where the transmitted signal
reelected off by various objects. However, THz wireless system can be applied
for both indoor and outdoor applications with very high data rate including
LOS and NLOS links. Therefore, the efficient design of NLOS FSO in
consideration of shadow fading and non-linear channel response is a critical
design issue. On the other hand, the aspect of quantum computing applications
achieve extensive secure are reliable communication in atmosphere and space.
So, FSO in optical wireless communications integrating quantum communication
will enter a new era in inter-space links [440, 441].

###  XIII-C Relay-enabled FSO networks

A terrestrial FSO link performance can be severely degraded by beam
misalignment, attenuation and atmospheric turbulence [481]. In this regard,
relay assisted FSO networks has the capability to cope with atmospheric
turbulence through allowing the transmitted data to an intermediate relay node
in order to avoid direct link [303]. A high transmission power can be forced
to overcome the degradation of the end-to-end performance due to such
impairments. Nonetheless, such additional transmitted power may affect the
level of secrecy. So, optimal power allocation in RF/FSO hybrid networks is a
key concern for the relay assisted optical wireless systems. The relay
assisted buffer based heterogeneous networks for unmanned area vehicles (UAVs)
with fixed and moving relay stations are presented [482]. Multi hop
transmission (i.e., serial) and cooperative diversity (i.e., parallel)
relaying are this two types of relay configuration reported in [311, 173].
Multi hop transmission relaying technique extend the coverage range where the
signal transmitted in between relay node to destination node in a serial
fashion. Under parallel relaying approach, the same information is transmitted
to the both receiving node and relay node from the sending end and thereafter
relay node retransmit the data again to the destination node. Data forwarding
protocols using the relay nodes particularly amplify and forward [442, 443]
and decode and forward are discussed in [444, 445]. With the introduction of
all optical relaying has the potential to avoid the need of optical-
electrical-optical (OEO) conversion at relay each node [446, 447, 448]. The
cascaded use of saturation gain semiconductor amplifier (SOA) in FSO systenms
are capable of eliminating scintillation, atmospheric turbulence even without
having the knowledge of channel state information (CSI) [446, 483, 484].
However, the idea of relay assisted networking is established for RF
technology, still the scope of relaying is immature in optical wireless
communication literature.

###  XIII-D WDM FSO Links

The integration of wavelength division multiplexing (WDM) techniques with FSO
communication pushes toward a new dimension by expanding the capacity of FSO
links. With the introduction of dense WDM (DWDM), FSO systems capacity can be
greatly enhanced [449, 450, 451, 452, 453]. A realization of 320 Gbps LOS/Long
WDM FSO links were developed and experimented using OOK modulation [449] and a
FSO link of 200 Gb/s bit rate is demonstrated in [453]. The outage probability
of WDM FSO links in the presence of inter-channel crosstalk and turbulence is
analyzed [450]. On the other hand, optical CDMA technique received more
attention for future FSO networks to support ever-increasing user demand
especially fiber to the home (FTTH) service via optical access network. A few
research works [454, 295] have been devoted on OCDMA-WDM optical ring networks
in the context of FSO communication considering pointing error effect using
received diversity which provides augmented capacity and overall system
performance as well. Multi wavelength (MW) OCDMA offers an attractive solution
to enhance channel capacity by means of reducing multi access interference
(MAI) and crosstalk. A combined solution of WDM-OCDMA support massive number
of simultaneous users over conventional WDM scheme and also enable dynamic
add/drop function for the next generation access network [454]. The study of
hybrid DWDM-MIMO FSO communications [485] is conducted aiming to increase
transmission coverage with minimum BER and outage probability taking into
account severe weather conditions. Despite the recent advances, more research
is needed to contemplate low cost and high speed integrated FSO WDM links
under different scenarios such as terrestrial, point coverage, cellular
coverage, LOS, transmission range, fixed or mobile relay structure.

###  XIII-E Channel Characterization

Over the past few decades, various channel models namely log-normal,
lognormal-Rician and Gamma-Gamma distributions have been proposed to quantify
the impact fading properties and turbulence effect of FSO links [167]. The
performance of FSO links is heavily affected by atmospheric turbulence such as
scattering, scintillation, air absorption, free space loss and refraction.
Besides, storm, fog, heavy rain and dust may severely degrade the successful
communication link between transmitter and receiver in the outdoor
environment. Hence, the mitigation technique to address atmospheric turbulence
is difficult issue. Statistical modeling of combining the effect of
atmospheric turbulence, mobility of obstacles, and pointing error of intensity
fluctuation of receive optical signal has been a challenging research issue.
The power allocation and the link performance is strongly depends on
atmospheric loss [61] which may lead to scope to research for the
counterbalancing of the atmospheric loss. A high speed NLOS data communication
in UV band is a significant option [211]. A methodological assessment in
consideration of transceiver geometrical configurations, atmospheric
conditions and effective channel modeling is still the issue of future
research. However, modeling of optical scattering communication (OSC) channel
is more difficult than traditional LOS FSO links. This is due to the longer
link range pushes the complexity of combined modeling attenuation and multiple
scattering [9]. Therefore, designing the channel and system models that
capture different limiting factors of OSC is become a great interest
particularly for connecting distributed objects in IoT devices. The
application of MIMO with optical wireless technology is quite challenging due
to the characteristics of IM/DD channel [25]. The application of MIMO could
lead increase the hardware complexity and may limit its application. However,
the deployment of MIMO with accurate channel modeling is a good research
issue.

###  XIII-F Pointing, Acquisition, and Tracking (PAT)

In general, FSO transmitter have a broadband and point-to-point directional
link in a beeline between the sending node and destination. PAT scheme are
typically incurred in static and mobile FSO communication systems when
transceiver shoot out narrow beams and the divergence of the beams is smaller
than a few µrad (micro-radian) [170]. This ultra-narrow beam property results
high speed data rate, long reach span, less interference, and more energy
efficient along the FSO link. The properties of narrow spectral beam make the
FSO link more difficult to build between two endpoints. A precise LOS
direction of transmission link is required to maintain connectivity between
endpoints. However, PAT problems have not completely addresses despite its
promising significance. For the successful deployment of PAT mechanism, an
integrated and flexible hardware architecture is need to be developed for
dynamic PAT.

Under the pointing mechanism, FSO node starts to coordinate the potential
nodes existing in the free space prior connection procedure in mmWave networks
[486, 487]. Therefore, synchronizing between coordinating nodes is an inherent
network design issue related to the pointing mechanism. The acquisition
mechanism associated with the modulation-demodulation techniques. FSO receiver
aperture is designed such a way that it can accepts multiple optical beams,
then the receiver decides the desired signed for decoding. As an indication of
physical architecture, the dimension of receiver aperture required to be
adjusted based on the divergence angle emitted from the laser beam the
distance [488]. Such technique substantially reduce outage probability, high
date transfer rate and enhance the efficiency of power budget. The tracking
mechanism is related to the problem raised by narrow spectral beam property.
The link performance is heavily depends on the geometric alignment of
transmitter and receiver and beam shape as well. Tracking mechanism under
mobile transceivers, a misalignment of optical beams results to increase the
outage probability and reduce the available capacity [489]. On the other hand,
very high pointing accuracy is to be maintained between stationary
transceivers.

The recent growing interest of an intelligent transportation system such as
V2V, V2I, or Vehicle-to-everything (V2X) require to keep LOS connectivity with
vehicle [490, 455]. The optical wireless technology can be applied for traffic
management [491]. Such scenario cause a challenge for signal acquisition and
tracking in the presence of dense building in an urban areas. The high speed
moving vehicle for instance, cars, electric trains, or unmanned area vehicles
may be the challenging task for FSO system in the area of V2X communications.
An agile PAT mechanism can keep track with the high speed vehicles to handle
the optical link between V2X [492]. In addition, the development of PAT scheme
in smaller size and low complexity required by the mobile systems (e.g.,
battery powered drone) is a challenging issue. The design of a suitable PAT
model to provide multi-directional coverage and has the ability to mitigate
vibration is another interested area. Moreover, a directive PAT mechanism is
an important feature where the laser beam focused towards the moving direction
of the tracked vehicle is another scope to research. This directive mechanism
for mobile FSO communication reduce reduces the handover time with the advance
knowledge of trajectory object. On disadvantage of this mechanism the error
incurred due to mismatch of the actual and expected speed of vehicle,
therefore, it needs GPS or other localization technology support. A time
synchronization between two transceivers in PAT process is another crucial
issue. As a solution, transceivers allow to exchange control information,
channel state information such as location, mobility, and time for
synchronized alignment.

###  XIII-G Underwater communication (UWC)

Nowadays UWC received much attention for oil pipe investigation, offshore
monitoring, and object detection. Long coverage distance and high speed
optical links is the much priority in many UWC applications. The adaptive
modulation and coding (AMC) technique in underwater environments is an
important issue [143]. Moreover, the adaptive switching mechanism between an
acoustic and optical mode for hybrid optical/acoustic networks in underwater
is needed for various applications [187].

###  XIII-H FSO Networking

5G/B5G technology is comprised with ultra-dense heterogeneous networks (a
combination of mixed networks such as macrocell-pico cell, macrocell-femto
cell, macrocell-microcell) to increase the data transportation capacity. A
small cell dense heterogeneous networks (HetNet) will generate frequent
handover between optical and RF networks which could lead many needless
handover [493]. Thus, controlling an unwanted handover and the ping-pong
effect is a crucial issue. Supporting of two different type of receivers
(heterogeneous) is important for hybrid FSO networks. The properties of RF-
based transceiver and the optical transceiver are different, the both networks
must be active simultaneously for the hybrid framework. Thus, combining the
two different architectures in the same platform and the same data
transmission over the distinctive systems simultaneously is a vital issue. On
the other hand, the distinctive nature of physical layer properties and data
link layer for the hybrid optical and RF wireless networks arise a major
challenge for the mobility support. A suitable handover algorithm may overcome
the continuity user mobility applications. It is important to enable
horizontal handover [24] and vertical handover [146] mechanisms to allow
seamless user mobility. For example, a user can seamlessly move around LiFi
cells (horizontal handover) and among LiFi and WiFi networks (vertical
handover) under LiFi—WiFi hybrid systems [46]. Meanwhile, the effect of user
mobility based on channel estimation and handover is also challenging. The
time taken for handover process should be short enough to meet the
specification of 5G/B5G. The time taken for exchanging the information between
user equipment and central station depends on the algorithms [494, 495]. Note
that a smaller coverage area incurred large handover and wireless optical
channel is vulnerable to channel barrier due to handover. Also, a precise
handover is required for switching among the backhauling networks. The
challenging of backhauling include the continuous change of pointing of
backhaul link for the moving objects.

###  XIII-I Load Balancing

The effective user association policy among the different available access
network is a research concern. An optimal user association mechanism solve the
joint resource allocation and user allocating problem [456]. User equipment
(UE) may need to be transferred to different access point for better
performance under load balancing technique [457]. Therefore, optimal load
balancing can be considered as a potential technical issue.

###  XIII-J Physical Layer Security

According to [458], optical wireless communications e.g. FSO networks are
vulnerable to cyber-attacks at the physical layer. In recent years, the
security and privacy issues of FSO communications received growing interest in
the presence of external eavesdropper. Unlike the conventional encryption in
top layer, no secret code is needed in physical layer security for long FSO
communications. Authors in [459] analyzed secrecy outage probability (SOP) of
FSO schemes taking into account of an external eavesdropper with generic
orientation at the physical layer. A new framework focusing on misalignment
error and corresponding approximate and asymptotic solutions for the non-
orthogonal light beams at the eavesdropper receiver is proposed. An external
eavesdropping cause the larger beam waist compared to the receiver size due to
the pulse spreading of optical beam through wireless link. An extensive
analysis of FSO physical layer security for multiple input single output
(MISO), MIMO systems, and cooperative systems are reported in [460, 461, 462]
respectively. The state-of-the-art of physical layer security is still
immature for FSO communications. The SOP analysis over the Malaga turbulence
channels without accounting pointing errors is pointed out in [463, 464]. The
impact of channel impairments on physical layer security for mixed RF/FSO
relay architectures is clearly presented in [465]. The average secrecy
capacity (ASC) analysis under active eavesdropping for mixed RF/FSO relay
networks based on decode-and-forward (DF) is studied in [466, 467]. The
performance of ASC is examined over Malaga turbulence for zero boresight
misalignment errors without considering eavesdropper location in [468]. It is
important to investigate nonzero boresight pointing errors distributions in
the FSO channels including eavesdropper’s location and orientation in order to
realize realistic scenarios on secrecy performance.

###  XIII-K Software Defined Networking (SDN) Control

SDN can effectively control and manage the hybrid optical networks through SDN
controller under dense deployment [469]. However, several OWC applications
based on network demand such as traffic re-routing, network flaw management
and security issue can be handled by the first application layer in SDN system
[470]. On the other hand, updating the flow control, network selection
mechanism along with other essential control can be performed in the control
layer [471]. SDN technology can be incorporated into FSO system for the
purpose of minimization of energy consumption through data traffic
controlling. The approach of real time applications and virtualizing of
network function in SDN system can be the emerging research issue.

Extending the optical spectrum beyond UV band allows the advantages from high
power and lower cost light sources [9] which is another research issue. The
RF-enabled indoor positioning systems are greatly affected by multipath fading
and penetration loss [472, 473]. Both indoor and outdoor positioning with
greater accuracy using free space optics can be one of the potential research
issue. In order to guarantee data loss reduction, selection of optimal
transmitter among multiple nodes, and delay minimization, the seamless
steering of transferred information become an eminent challenging problem. The
optical wireless system is usually designed for downlink communications
purpose because of some challenges in uplink direction such as energy
constraint of portable devices and the limitations of narrow beam controlling
for mobile devices. Hence, the uplink transmission is open research issue that
need to be addressed. A few research works have conducted of RF/optical hybrid
wireless from the perspective of physical layer whereas barely focus on data-
link level metrics [474]. The QoS analysis tool of cross layer design between
physical and data link layer is an important research zone.

##  XIV Conclusions

The design of pervasive and efficient wireless systems for a wide range of
transmission links is essential to meet ever-increasing higher data rate
demands in the next generation communication networks. Owing to spectrum
scarcity of radio-frequency (RF) counterparts, free space optical (FSO)
communication has been recognized as a promising option for next generation
optical networking that can support tremendous traffic demand initiated from
the internet of things/everything (IoT/IoE) devices and next-generation
cellular communication systems. The realization of unlicensed extremely wide
optical band can fulfill the exponential growth of traffic demand and
complementarily overcome the RF spectrum shortage in a cost effective way. The
aggregate deployment of hybrid wireless system offering different features can
suppress the limitations of a single network either standalone RF or optical
wireless and make a prominent alternative over RF based communication systems.
In this review paper, we have discussed the brief overview of promising OWC
technologies from various viewpoints. An extensive research on the deployment
issues of different FSO wireless networks addressing the related challenges
are underway. Principle, advantages, limitations, design issues, mitigation
techniques, significance, worldwide achievements, state-of-the-art of recent
developments and revolutions free space optical communications are presented.
Advancements of hybrid FSO, coherent FSO, FSO link budget, research challenges
of FSO infrastructure deployment of different application scenarios such as
acoustic underwater communication, 5G and B5G communication, FSO-IoT/IoE
interface are demonstrated. Provisioning of quality of experience (QoE),
massive connectivity, high capacity, tight security, low power consumption are
also focused in perspective of the coexistence FSO and RF wireless technology.
The opportunities triggered by FSO schemes as well as network architecture,
application scenarios, the key research directions, the solutions of major
challenges for a successful deployment of the considered framework in the
context of wireless communication and IoT paradigms are also briefly pointed
out. It can be concluded that FSO is increasingly becoming a prominent
technology for future generation communication system and extending the realm
of FSO technology in indoor, healthcare, industry, offices, shopping malls,
offices, stadiums, residences, transportation, space and terrestrial
environments. This survey paper provides valuable resource for clear
understanding the recent research contribution optical free space
communication in different implications and is anticipated to persuade further
effort for the eminent deployment of other OWC systems in the forthcoming
years.

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