# Unleashing the Potential of Networked Tethered Flying Platforms for B5G/6G:
Prospects, Challenges, and Applications

Baha Eddine Youcef Belmekki  IRIT Laboratory, School of ENSEEIHT, Institut
National Polytechnique de Toulouse, France,  Mohamed-Slim Alouini  Computer,
Electrical and Mathematical Sciences and Engineering (CEMSE) Division, King
Abdullah University of Science and Technology (KAUST), Kingdom of Saudi
Arabia,

###### Abstract

Researchers are currently speculating about what the role of beyond fifth-
generation (B5G) or sixth generation (6G) wireless systems will be. Several
use cases and applications are proposed, ranging from enhanced mobile
broadband communications and massive ultra-reliable low-latency Communication
to holographic telepresence and tactile communications. One crucial and
pivotal prospect of 6G is worldwide connectivity. Today, nearly 4 billion
people do not have access to an internet connection. 6G not only intends to
hyper-connect the already connected, but to bridge the digital divide by
connecting the unconnected. For that purpose, 6G architectures will integrate
airborne communications as they are considered key enablers for the next
generation of wireless systems. Airborne communications can be done via free-
flying platforms and tethered flying platforms. Free-flying platforms, such as
unmanned aerial vehicles (UAVs) and high-altitude platforms (HAPs) have
received great interest during the last decade. However, they lack endurance
and backhaul capacity. Networked Tethered Flying Platforms (NTFPs), on the
other hand, are enjoying the benefits of free-flying platforms while
overcoming their limitations by providing a continuous supply of power and
data via the tether. They are also cost-efficient and have good green
credentials. Tether platforms have a wide range of applications beyond
communications, such as energy harvesting, entertainment, science, research,
public safety, disaster relief, government, and defense. In this survey paper,
we intend to provide an extensive and comprehensive overview of NTFPs. First,
we provide a general overview of this solution for all readers interested
irrespective of their background by reviewing all the existing types of NTFPs,
including their components, their characteristics, their applications,
advantages, challenges, and regulations. We also show several case studies in
various fields and applications. Then, we investigate NTFPs from a wireless
communications perspective. We briefly provide a basic geometry analysis of
NTFPs with respect to Earth. Then, we present the works dealing with the
performance of NTFPs. We also show how NTFPs will be key enablers in 6G for
various types of communications, such as flying car communications, maritime
communications, and vehicular communications. We present an economic analysis
of NTFPs with a capital expenditure (CAPEX) and operating expenditure (OPEX)
analysis to emphasize their cost-efficiency. Finally, we present channel
models for NTFPs considering different altitudes, that is, low-altitude
platforms (LAPs) and HAPs, and for different links, that is, radio frequency
(RF) and free-space optics (FSO). Although these channel models are presented
in this survey for NTFPs, they are applicable for free-flying platforms.

###### Index Terms:

Tethered Networked Flying Platforms, NTFPs, tethered balloons, tethered
blimps, Helikites, Unmanned Aerial Vehicles (UAVs), tethered UAVs (tUAVs), Low
Altitude Platforms (LAPs), High Altitude Platforms (HAPS), tethered HAPs,
lighter-than-air platforms, heavier-than-air platforms, aerostats platforms,
aerostatic, aerodynamic platforms, Buoyant Airborne Turbines (BATs), hybrid
airship, flying cars, wireless communications, sixth generation (6G), free-
space optics (FSO).

##  I Introduction

Since the dawn of time, humans always had the need to communication and
connect with one another. In the 1980s, the first generation (1G) of wireless
communications allowing voice communication was launched. Forty years later,
three generation of wireless communication networks have been launched, namely
the second, third, and fourth generation (2G, 3G, and 4G) of wireless
networks. At the time of writing this paper, the fifth generation (5G) of
wireless communications is being commercialized in some country, such as
Switzerland, South Korea, United States, and United Kingdom. Evolving
standards throughout this saga of communications have resulted in increased
data rates and decreased latency from each generation to next.

Now, researchers have begun speculating about what beyond fifth-generation
(B5G) or the sixth generation (6G) will be, or more exactly, what 6G should be
[1, 2, 3, 4, 5, 6, 7]. The reasons why researchers have already started
thinking about 6G are the expected massive growth of mobile traffic in the
next decade and the new type of disruptive services that are envisioned. Some
of the applications and services that 6G can offer, such as, enhancing the
conventional mobile communications, increasing the accuracy of indoor
positioning, providing holographic telepresence, tactile communications,
extended reality, worldwide connectivity, and integrated networking have been
presented elsewhere [1].

Although these applications and use cases are propelling us into the future,
more than half of the people on Earth will not have access to this
applications. That is what makes worldwide connectivity a major concern,
especially since 3.9 billion people remained unconnected at the end of 2018
[8]. Indeed, a large proportion of people around the word do not have access
to an internet connection, especially in rural, sparse, and poor areas.
Services that are lacking in these areas, but can be further facilitated by
internet connection, include eHealth, eEducation, farming, eCommernce, and
eGovernment. 6G has the potential not only to hyper-connect the already
connected, but to bridge the digital divide by connecting the unconnected [9,
10].

In order to facilitate worldwide connectivity, 6G will rely on the following
trifecta: terrestrial communications, satellite communications, and airborne
communications. Terrestrial communications and tower masts are expensive for
telecommunication companies in poor or rural areas. Plus, it takes time to
construct terrestrial communications. Also, terrestrial communications are
only suitable for two-dimensional scenarios in which the height of the users
is relatively negligible. For instance, terrestrial communications would be
ill-suited to connect flying cars [11].

On the other hand, satellite communications, especially Low Earth orbit (LEO)
satellites, have ubiquitous coverage and have lower channel loss compared to
Geosynchronous Equatorial Orbit (GEO) satellites. However, the cost of
launching a large-scale constellations of LEO satellites is extremely high.
Additionally, satellites take time to be deployed, and their communications
are subject to latency due to the distance between the satellite and the
users, which cannot be overlooked for critical communications especially for
autonomous vehicles and flying cars. At the time of writing this paper, there
are two companies working on solutions to connect regular phones with
satellites: Lynk Global [12] and AST & Science [13]. However, development is
in the early stages, and these companies plan to offer global satellite
communications via regular phone in a decade or two.

Figure 1: Outline and organization of this survey paper.

The last type of aforementioned communications is airborne communications
which are considered as key enablers in the 6G architecture [14, 15]. Recently
this type of communications has recently drawn considerable attention from
researchers due to the intrinsic flying properties, which allow borad
coverage. These airborne communications include different types of flying
platforms. Some fly at a lower altitude, such as unmanned aerial vehicles
(UAVs) [16]. Others fly at a higher altitude, such as high-altitude platforms
(HAPs) [17]. These flying platforms overcome some of the limitations of
satellite and terrestrial communications, such as high cost, delay, and slow
deployment. However, these flying platforms also have some limitations in
persistence, endurance, and backhaul connection. To deal with these
limitations, another type of flying platform is currently used by government,
military, and telecommunications companies: Networked Tethered Flying
Platforms (NTFPs).

NTFPs, as their name suggests, are flying platforms tethered to a ground unit.
The tether continuously supplies the flying platforms with data and power.
They are cost-efficient and inexpensive to operate compared to free-flying
platforms. Also, from a wireless communications perspective, they are cheaper
than other communication infrastructure, such as tower masts and satellites.
Another aspects that makes NTFPs an attractive solution compared to free-
flying platforms are their endurance and persistence which are crucial to
telecommunication and surveillance missions. They are also quick and
relatively easy to deploy. But, the most relevant properties of NTFPs are
their backhaul capacity and constant power supply. Applications of NTFPs
beyond communications include energy harvesting, entertainment, science,
research, public safety, disaster relief, government, and defense.

Although these platforms have great potential as key enablers for 6G, a
comprehensive and unifying documentation on this subject is lacking. We
intend, through this paper, to provide a comprehensive overview about this
solution for readers interested in this solution irrespective of their
backgrounds. We also provide a comprehensive analysis on NTFPs from a wireless
communications perspectives. To the best of our knowledge, there are no papers
surveying NTFPs in the literature, although there are some papers dealing with
tethered aerostats from a design and manufacturing perspective, such as [18],
no prior works have considered a comprehensive overview of NTFPs, and we hope
that our survey fills this gap.

(a)

(b)

Figure 2: Different shapes of balloons.

The organization of this paper is depicted in Fig.1, and structured as
follows:

  * •

Section II: we provide a general and broad overview of NTFPs for readers
interested in these type of platforms irrespective of their background and
their field. We review the existing types of NTFPs, their main components,
what are their different anchor points, and their altitudes. Also, we
highlight their various and divers applications from different fields as well
as their main advantages and disadvantages. Finally, we present the current
regulations regarding NTFPs.

  * •

Section III: we show the numerous and divers use cases of NTFPs from real-life
scenarios. We aim through this section to highlight the various applications
from different field. Also, we list the major companies that manufacture that
manufacture and sell NTFPs.

  * •

Section IV: we address NTFPs from a wireless communications perspective.
First, we carry out a geometrical modeling between a given NTFP and the Earth.
Second, we present the works that analyze the performance NTFPs in wireless
communications. Then, we show how NTFPs are the key enablers to interconnect
heterogeneous networks and communications envisioned in 6G. For the sake of
completeness, we added a section that deals with the economic aspect of NTFPs
in a wireless communications context, with a capital expenditure (CAPEX) and
operating expenditure (OPEX) analysis to emphasize their their cost-
efficiency.

  * •

Section V: We provided a comprehensive channel modeling for NTFPs. Although
this section is titled channel modeling for NTFPs, the model presented are
valid for all type of platforms whether they are tethered or untethered (free-
flying). We split the models according to the altitude of the platforms, that
is, low-altitude platforms (LAPs) and HAPs, and the type of link used, that
is, radio frequency (RF) and free space optics (FSO).

  * •

Section VI: we conclude the survey paper with a brief summary.

(a) Tactical TCOM blimp (17M).

(b) Operational TCOM blimp (28M).

(c) Strategic TCOM blimp (71M).

(d) Strategic TCOM blimp (74M).

Figure 3: Different types of TCOM blimps [19].

##  II Overview of NTFPs

###  II-A Types of NTFPs

Airborne platforms are vehicles that can fly in the air by opposing the force
of gravity either using a static lift or a dynamic lift. For instance,
balloons and blimps use static lift and they belong to the lighter-than-air
(LTA) category, whereas airplanes and UAVs use dynamic lift and they belong to
the heavier-than-air (HTA) category. There is also a hybrid category of
platforms that use both static lift and dynamic lift. In this survey, we are
only interested in NTFPs; hence, free-flying platforms are out the scope of
this paper.

####  II-A1 LTA NTFPs

The LTA platforms that use static lift or aerostatic lift are called
aerostats. They are filled with a low-density LTA gas such as helium. The
difference between the density of the air outside the envelope of the aerostat
and the density of the LTA gas produces buoyancy according to Archimedes’
principle. The most popular LTA NTFPs are balloons and blimps.

##### Balloons

Spherical balloons have been the most commonly used for NTFPs, or more
precisely, tethered aerostats (Fig.2). They are easy to design and manufacture
at a lower cost than the other tethered aerostats. They are also easy to
deploy. The maximum altitude a tethered balloon can reach is around 600 m to
700 m, and the maximum payload they can carry is around 50 kg. However, they
are not designed to sustain high speed wind since their shape is not designed
to cope with it. Tethered balloons can sustain wind speeds around 20 km/h to
40 km/h.

##### Blimps

Blimps, also known as streamlined aerostats, are high performance platforms
that can sustain high speed wind, carry heavy payloads, and stay aloft at high
altitudes (Fig.3). There are several categories of blimps, and they differ in
size, altitude, and maximum payload [19]. For instance, TCOM categorizes their
blimps into three classes, tactical class, operational class, and strategic
class, as depicted in Fig.4:

  * •

Tactical Class: Tactical class blimps are compact and can be deployed rapidly.
Their envelope size ranges from 12 m and 17 m. They are suitable for
surveillance missions with tactical needs. They have been used in Iraq and
Afghanistan by the U.S. army, and for border surveillance between the U.S. and
Mexico. They can carry 27 kg of payload and reach an altitude of 300 m. Also,
they can stay aloft for seven days and sustain wind speeds up to 100 km/h (see
Fig.3(a)).

  * •

Operational Class: Operational class blimps have a medium-sized envelope that
ranges between 22 m and 28 m. They combine portability and flexibility for
quick deployment and retrieval. They can carry a payload up to 200 kg and can
reach an altitude of 1 km. This class of blimps is suitable for surveillance
and monitoring operations where land-based surveillance is infeasible. Also,
they are suitable for maritime surveillance and border surveillance.
Operational class blimps can stay aloft for two weeks and can sustain wind
speeds up to 130 km/h (see Fig.3(b)).

  * •

Strategic Class: Strategic class blimps are arguably the largest NTFPs on the
market. Their envelope size ranges from 71 m to 74 m and they can carry a
large payload of 2300 kg. They are ideal for long surveillance and monitoring
missions since they can stay in the air for 30 days and sustain a maximum wind
speed of 166 km/h. They can also reach an altitude of 4,6 km allowing them to
cover a large area. They have been used to detect low-flying aircraft or
cruise missiles (see Fig.3(c) and Fig.3(d)).

Figure 4: TCOM blimps and their type of missions.

##### Buoyant Airborne Turbines (BATs)

BATs are wind turbines manufactured by Altaeros [20]. They can reach an
attitude of 600 m, where wind speeds are higher than on the ground. They can
harvest twice as much energy compared to land-based tower turbines [21, 22,
23, 24]. Their envelope is filled with helium, and they have a tether that
holds them in place while they transmit the harvested power to a ground
station (see Fig.5).

Figure 5: Altaeros BATs [20].

####  II-A2 HTA NTFPs

The HTA platforms that use dynamic lift or aerodynamic lift are called
aerodynes. Their lift is produced by the relative motion between the HTA
platform and the air, which pushes the platform upwards by Bernoulli’s
principle. The most popular HTA NTFPs are tethered UAVs (tUAVs) and airborne
turbine kites.

##### tUAVs

tUAVs are UAVs with a physical link called a tether that supplies them with
power and data. They can reach an altitude of 200 m and carry a payload up to
15 kg (Fig.6). They usually have a battery pack in case the tether is damaged
or if there is a cut in power. Since tUAVs have a constant power supply, they
can, in theory, stay in the air for an indefinite period of time. However, the
main limiting factor is the motor of the UAV, which starts to overheat after
two to four days aloft.

Figure 6: tUAVs by Elistair [25].

##### Airborne Turbine Kite

Airborne turbine kites are wind turbines used to harvest wind power in the
air, since wind speed is higher at higher altitudes (see Fig.7). Thus, they
can harvest more energy than a tower, and they are also cheaper to construct.
Their electrical generator can be land-based (on the ground) or airborne (in
the air). The tether transmits the harvested energy to the ground. They can be
maintained aloft at lower or higher altitudes up to 4600 m [26, 27, 28].

(a) Kitewinder [29].

(b) Makani power air turbine [30].

Figure 7: Airborne wind turbines.

####  II-A3 Hybrid NTFPs

As mentioned before, hybrid platforms use both static lift and dynamic lift.
The static lift is produced by buoyancy (Archimedes’ principle), and the
dynamic lift is produced by the relative motion between the aircraft and the
air pushing the aircraft upwards (Bernoulli’s principle). The hybrid NTFP used
most often is the Helikite. Also, hybrid airships can be tethered to the
ground.

##### Helikites

Helikites are hybrid aerostats that benefit from both static lift and dynamic
lift. The term Helikite is a portmanteau of helium and kite. Helikites were
designed and patented by the company Sandy Allsopp in 1993 (see Fig.8). A
Helikite is composed of an oblate-spheroid balloon filled with helium to
provide static lift, and a kite structure to provide dynamic lift. Combining
these two lifts reduces the amount of helium required compared to other
similarly sized aerostats, and Helikite can fly at much higher altitudes than
other aerostats of the same size. Helikites also offer several advantages
compared to other NTFPs: 1) Their compact design allows them to be deployed by
fewer personnel. 2) They are not brought down by high speed winds since winds
force them to go upward. 3) They are smaller, thus, they have few problems
with helium leakage and use less helium because they benefit from dynamic
lift. Helikites can stay in the air for two weeks at an attitude of 1.5 km
carrying a payload of 23 kg. Allsopp claims that their Desert Star Helikites
can carry a payload of 220 kg at an altitude of 3.4 Km, but qualify that claim
as the estimated performance [31].

Figure 8: Desert Star Helikites by Allsopp [31].

##### Hybrid airships

Hybrid airships are hybrid aircraft, which means 60% of their lift comes from
buoyant lift (aerostatic lift), and the remaining 40% comes from aerodynamic
lift (see Fig.9). Hybrid airships do not need airports since they can take off
and land anywhere with a large open and flat field. They can reach an altitude
of 6000 m and carry a payload of 60,000 kg [32, 33]. Although the main usage
of hybrid airships is to transport passengers and deliver heavy cargo, they
can still be used for other purposes. They can also be tethered to the ground,
then removed and deployed elsewhere if needed. They are expected to in service
by 2024 [34, 35].

Figure 9: Airlander hybrid airship by Hybrid Air Vehicles [32]. Figure 10:
Different categories and types of NTFPs.

###  II-B Components of NTFPs

We detail in this section, the different components of NTFPs, which are also
summarized in Fig.11 and Fig.12.

Figure 11: Different NTFPs and their components (from left to right : TCOM
blimp [19], Drone Aviation Corp. balloon [36], Helikite [31], and Elistair
tethered drone [25]. Figure 12: Components of NTFPs.

####  II-B1 Envelope/Shell

The envelope of NTFPs contains gas, which allows the platforms to soar and
stay aloft. Some envelopes have spherical forms (balloons), as shown in
Fig.13(a), other have a fish-shaped or streamlined form (blimps), in
Fig.13(b). The lifts of those envelopes rely solely on buoyant gas. Other
envelopes have kites attached to them, which provide an aerodynamic lift to
improve their performance in the presence of strong winds, such as Helikites,
shown in Fig.13(c). Envelopes are usually made from a synthetic material, such
as polyester, polyurethane, or polyvinyl. Envelopes may contain materials such
as laminates to prevent degradation from ultraviolet light exposure or
materials to prevent the envelope from abrasions.

tUAVs have shells or frames instead of envelopes, as shown in Fig.13(d). The
shell defines the shape and look of the UAV and also contains the necessary
components, such as the motor, blades, the protection cover, landing gears,
etc. The motors provide force and lift to the tUAVs. Generally, UAVs have
between four and eight motors. An UAV with four motors is called a quadcopter,
a UAV with six motors is called a hexacopter, and a UAV with eight motors is
called an octacopter. The number of motors used depends on the type of
mission. Also, landing gear are used for UAVs that require larger ground
clearance.

(a) Balloon envelope [36].

(b) Blimp envelope [19].

(c) Helikite envelope [31].

(d) UAV shell [25].

Figure 13: Illustration of various NTFP envelopes.

####  II-B2 Lifting Gas

The envelope of the aerostat is filled with a lifting gas, also called a LTA
gas, which has a lower density than the air (atmospheric gas); hence it
creates buoyancy according to Bernoulli’s principle. Hydrogen and helium are
the most common and lightest gases used for aerostats. However, each gas has
pros and cons, as shown in Table I.

The main advantages of hydrogen are that it is the lightest existing gas, and
it can be easily produced. However, its main disadvantage is its high
flammability. Some countries have prohibited its use, especially after the
Hindenburg incidents [37, 38]. Helium, on the other hand, is the second
lightest gas, and unlike hydrogen, is a nonflammable gas. However, helium is
expensive, scarce, and a non-renewable resource. Although helium is the gas
most commonly for NTFPs, its aforementioned disadvantages are leading
researchers and manufacturers to reconsider hydrogen usage and cope with its
related safety issues. Also, some vendors are designing aerostat that use both
hydrogen and helium.

TABLE I: Comparisons between hydrogen and helium.  Gas |  Hydrogen |  Helium
---|---|---
Advantages |  \- Lightest existing gas
\- Easily produced in
large quantities |  \- Second lightest gas
\- Non-combustible
Disadvantages |  \- Highly flammable |  \- Expensive
\- Scarce
\- Non-renewable resource

####  II-B3 Payload

The payload is the weight that the NTFP can carry while in the air (Fig.14).
The payload differs from one NTFP to the next as shown in Table IV. To be more
specific, we define the total capacity payload as the total weight the
platform can lift, excluding its own weight and the weight of its tether, at
the desired altitude. The total capacity payload is divided into

  * •

The supporting system payload, which includes all the equipment necessary to
operate the platform, such as the power system, communication repeaters,
backup batteries, lights, etc, and

  * •

The operational payload, which includes equipment related to the mission, such
as high definition (HD) camera, telescope, electronics, panchromatic imaging
camera, electro-optical/infra-red sensors, acoustic detectors. The type of
equipment varies from one mission to another.

(a) Payload of 22M TCOM blimp [19].

(b) Payload of Elistair tethered drone [25].

(c) Payload of tethered balloon [39].

(d) Payload of Allsopp Helikite [31].

Figure 14: Payload of different NTFPs.

####  II-B4 Tether

The tether is rolled up around a winch at one extremity, and is attached to
the envelope/shell at the other. Fig.15 shows pictures of a tether cable.
Tethers are usually made of synthetic fibers and, depending on the type and
size of the NTFP, the tether’s length, diameter, resistance and weight will
differ. For a large platform, several tethers may be used. A tether has the
following purposes:

  * •

Maintain and stabilize the platform in the air to the ground;

  * •

Provide power to the platform through a power line; and

  * •

Provide data to the platform through optical fibers.

Also, the tether must be weather-proof to withstand extreme temperatures,
humidity, rain, snow, lightning, and other weather conditions.

|
---|---
(a) UAV tether cable [40]. |
|
(b) Aerostats tether cables [41]. |
Figure 15: Aerostats and UAVs tether cables.

####  II-B5 Mooring Stations and Anchor Units

##### Mooring stations

The mooring station is the system that holds the envelope of the aerostat
while it is inflated before the launching process, while it is deflated after
the flight, and during maintenance. The moorings stations differ in size,
form, and complexity, as depicted in Fig.16. For instance, large blimps
require large and heavy mooring stations, while smaller platforms, such as
Helikites, require lighter and simpler mooring stations. The mooring stations
also depend on the environment in which the NTFPs will be used in. For
instance, NTFPs can be deployed over the sea or ocean; therefore, they must
have mooring stations designed for maritime applications, as depicted in
Fig.17.

##### Anchor points

The anchor point or anchor unit is the unit that the platform is anchored
into, and it maintains the platform in place while aloft. There are different
types and sizes of anchor units, which we will present in more detail in
section II-C. Mooring stations can also serve as anchor points.

(a) Helikite mooring station (Helibase type 1) [42].

(b) Helikite mooring station (Helibase type 2) [43].

(c) tUAV mooring station [25].

(d) Blimp mooring station [44].

Figure 16: Mooring stations for different types of NTFPs.

(a)

(b)

Figure 17: Maritime mooring station and anchor unit [19].

####  II-B6 Winches

A winch is the device used to let out the tether during the launching process,
adjust its tension while the platform is aloft, or pull it back in during the
recovery process. The tether is wound around a drum called the winch drum. The
size and type of winch varies. Smaller NTFPs can be winched manually using a
crank, whereas larger NTFPs require a power or motored winch. The winch can be
attached to a mooring station or mounted on a trailer, such as a flat bed or a
truck bed. Fig.18 shows different types of winches.

|
---|---
(a) |
|
(b) |
|
(c) |
Figure 18: Winches for a NTFP. Source: (a) [45]; (b) [46]; (c) [25].

####  II-B7 Ground Control Station

Ground control stations serve as the operation base for NTFPs, as shown in
Fig.19. They can be used to

  * •

Control the altitude of the platforms;

  * •

Monitor and control the platforms and the equipment they carry; and

  * •

Store, and in some cases process, the data related to the mission, such as
videos and images.

Depending on the mission type, a ground control station may be a building, a
tent, a vehicle, a container, or any place that serves as a shelter.

|
---|---
(a) tUAV ground control station [47, 48]. |
|
(b) TCOM blimp ground control station [19]. |
Figure 19: Ground control stations for a tUAV and blimp.

####  II-B8 Transportation

The components of NTFPs (mooring stations , winch, envelope, etc.) must be
transported to the deployment location. Trucks may be used to transport NTFPs
on the ground, whereas for maritime applications, ships are used as
transportation. Fig.20 depicts ground and sea transportation options for
NTFPs.

(a) Transportation by sea [49].

(b) Transportation on land [31].

Figure 20: Transportation of NTFPs by sea and land.

###  II-C Types of Anchor Units

####  II-C1 Ground Anchor Unit

  * •

Mooring stations: NTFPs can be anchored to the mooring station; this is
usually the case for blimps.

  * •

Building: NTFPs can be anchored to a building rooftop. UAVs, which can reach a
maximum altitude of 150 m, gain more altitude when placed on top of buildings
[50, 51, 52].

  * •

Vehicles: NTFPs can be anchored to a vehicle, such as a truck, and thus
benefit from the mobility of the vehicle [53].

  * •

Ground: tUAVs can be placed on the ground since they are very compact and,
unlike other NTFPs such as blimps or balloons, do not require additional
infrastructure [54].

####  II-C2 Sea Anchor Unit

  * •

Ship: When NTFPs are used in a maritime context, they use the ship or another
offshore floating structure as an anchor unit. The anchor point must be
designed to satisfy the requirements of maritime applications [55, 56].

  * •

Buoy: When deployed over the sea or ocean, NTFPs can be anchored to
oceanographic buoys [57].

  * •

Drag Sail: Drag sails can be used as an anchor unit for NTFPs. For instance,
the authors in [58] proposed a configuration where the platform, a balloon in
this case, was launched with its tether wound around a reel. The tether had a
drag sail at the end. After reaching the desired altitude, the tether anchored
the platform into the sea. At the end of the mission, the tether was released
and the balloon became a free-flying platform.

####  II-C3 Air Anchor Unit

Although it has never been tested before, the authors in[59] proposed an
interesting setup to cope with winds that blow in different directions across
the stratosphere. The idea is to tether the platform to an HTA glider flying
at a lower altitude than the platform itself.

###  II-D Altitudes

The different altitudes at which NTFPs can fly are shown in Fig.21. We
classify NTFPs as Ultra LAPs (U-LAPs), LAPs, Medium-Altitude Platforms (MAPs),
HAPs, and Ultra HAPs (U-HAP).

####  II-D1 U-LAPs (50 m–150 m)

Generally, tUAVs, balloons, Helikites, and BATs fly at this altitude. This
altitude is suitable for quick operations such as disaster relief by because
NTFPs can be rapidly deployed with decent range and coverage [51, 25].

####  II-D2 LAPs (200 m–600 m)

tUAVs do not reach this altitude, but balloons, Helikites and blimps can,
although balloons usually do not fly higher than 400 m. This altitude is
suitable for missions that last between three and seven days, and the NTFPs
have more coverage than at U-LAP altitudes [20, 31, 19].

####  II-D3 MAPs (0.7 km–5 km)

Only Helikites and blimps can reach this altitude. Although Helikites usually
do not fly beyond 1.5 km, estimates available on their website claim they can
reach 3 km [31]. Blimps can reach an attitude of around 4.6 km for long-
duration operations and surveillance missions. At these altitudes, NTFPs
benefit from higher range and coverage [31, 19].

####  II-D4 HAPs (15 km–22 km)

To this day, no NTFP has reached these high altitudes. However, there are
several studies that demonstrate the feasibility of high altitude NTFPs [60,
61, 59, 62, 63, 64, 65, 66, 67, 58].

####  II-D5 U-HAP (45 km–50 km)

Although these altitudes seem extreme and unreachable by NTFPs, there is one
paper that studied the feasibility of NTFPs flying at an ultra-high altitude
[68].

TABLE II: Comparisons among different NTFPs.  Properties |  UAVs |  Ballons |  Helikite |  Blimps |  BATs
---|---|---|---|---|---
Payload |  1 kg–15 kg |  5 kg–50 kg |  2 kg–25 kg |  16 kg–2600 kg |  NA
Altitude |  150 m–200 m |  150 m–700 m |  100 m–1.5 km |  100 m–5 km |  150 m–600 m
Wind Speed |  40 km/h–55 Km/h |  20 km/h–40 Km/h |  90 km/h |  75 km/h–165 km/h |  160Km/h
Flight Duration |  2–4 days |  1 day–7 days |  2–4 days |  1 day–30 days |  NA
Deployment Time |  Fast |  Moderate |  Fast |  Slow |  Fast
Cost |  Low/Moderate |  Low/Moderate |  Low/Moderate |  Moderate/High |  NA

###  II-E Applications of NTFPs

####  II-E1 Government & Defense

##### Tactical Operations

NTFPs helps military to conduct tactical operations via accurate environment
perceptions and real-time imaging [69, 70, 71, 19].

##### Observation and Surveillance

NTFPs offers continuous aerial surveillance and reconnaissance by day or night
for up to several days, enabling target tracking for enemies and reduced
exposure of allies [72, 70, 71, 19, 73, 74].

##### Telecommunications

NTFPs play a crucial role during military operations as they extend
communication in areas where cellular coverage is lacking [75, 69].

##### Border Surveillance

NTFPs are used to track illegal aliens, unauthorized personnel, arms
smugglers, and narcotics traffickers, and to detect and prevent enemy forces
from crossing borders [70, 71, 19, 73].

##### Surveillance of Sensitive Sites

NTFPs allow the aerial surveillance of sensitive sites, such as military
bases, nuclear plants, industrial sites, offshore platforms, harbors, and
airports [74, 70, 19, 73].

##### Detection of Aircraft

Depending on their altitude, NTFPs are capable of detecting low-flying
aircraft within their area of coverage [71, 19].

####  II-E2 Public Safety and Disaster Relief

##### Search and Rescue Missions

For search and rescue missions, NTFPs increase the search area. Also, they
provide a cellular coverage in areas such as deserts or mountains [70, 76, 77,
78, 79].

##### Firefighting Missions and Wildfire Monitoring

During firefighting missions, NTFPs not only bring cellular coverage to the
area, but can also take remote-sensing aerial infrared images for temperature
maps in order to detect and identify a critical hot spot. This helps managers
make decisions and give precise directions their the crew [80, 70].

##### Emergency Communications

Cellphone connectivity and coverage are paramount in the aftermath of natural
disasters. earthquakes, tsunamis, hurricanes, and floods can destroy cell
phone towers. Bringing cellphone coverage via NTFPs can help rescue crews
communicate and make decisions. This also help identify and prioritize the
areas most affected by the disaster [76, 81, 82, 77, 78, 79].

##### First Operations

During rescue operations to find and help victims, first responders need to
communicate with each other to coordinate their tasks. NTFPs help first
responders to assess the situation by providing communication coverage and
visual coverage [76, 82, 77, 78].

##### Crowd Control and Management

NTFPs are very useful when it comes to crowd management and riot control [74,
70].

##### Aerial Observations

For aerial observations and surveillance tasks, height is a huge advantage.
Hence, NTFPs can be used for surveillance purposes, such as control of illegal
fishing activities or for homeland security and anti-terrorism activities [70,
74, 73, 83].

##### Traffic Regulation, Accidents Management, and Vehicle Surveillance

NTFPs can be useful for traffic regulations. They can also help with
anticipating traffic jams, and alerting vehicles about accidents happening in
their vicinity. Finally, they can be used to locate suspect vehicles or track
vehicles during car chases.

Figure 21: Altitudes and anchor units of NTFPs.

####  II-E3 Communications

##### Cellular Coverage

NTFPs overcome the limitations of terrestrial communication towers. Due to
their higher altitude, they have a larger coverage area and better line-of-
sight (LOS). Plus, they are less costly to deploy and construct than cell
towers and satellites [46, 84, 85, 86, 51].

##### Coverage in Rural and Remote Areas

Almost half of the global population lives in remote or rural areas. It is not
economical or profitable for phone operators to erect cell towers in these
areas; consequently, most of the people living in these areas do not have
access to an internet connection. NTFPs can bridge this gap by providing
internet connection to these people while being economically profitable for
phone operators [87, 45, 46, 84, 78].

##### Temporary Communications

In certain situations, there is a need for temporary communications. NTFPs can
be used as temporary transmitters or as relays [69, 76, 81, 82, 77, 78, 79].

##### Remote Sea and Ocean Area Coverage

Seas and oceans are lacking cellular coverage, which makes NTFPs useful in
these areas to sailors, fishermen, personnel on floating structures [57, 56,
52].

Figure 22: Comparisons among Helikites, balloons, UAVs, and larger blimps.

####  II-E4 Entertainment

##### Coverage of Major Sport Events

Major sport events, such football, baseball, rugby, etc, gather massive crowds
of people, making NTFPs an excellent choice for broadcasting these events, and
bringing coverage to these areas.

##### Surveillance of Large Public Events

The need for surveillance of large public events cannot be overstated. NTFPs
permit a wide view of those events [70, 73, 74].

##### Aerial Recording and Photography

NTFPs can be used to take aerial photos from a perspective that is hard to
access from a regular height. They can also be used to record videos and
movies [70].

##### Advertisement

NTFPs are also used for advertisement purposes. The NTFP may have a written
sign on it, or it can lift an advertisement sign. The NTFPs can be illuminated
at night for a better visibility of the advertisement or deployed during major
sport events to attract people’s attention.

Figure 23: Applications of NTFPs.

####  II-E5 Science, Research and Environments

##### Remote Sensing

NTFPs can be equipped with remote-sensing. The sensor can be deployed to
collect data for various applications, such as detecting landslides and
habitat destruction [88, 89, 90, 91, 55, 92, 93].

##### Education

NTFPs can be used as educational tool to show students phenomena that can only
be see from a certain altitude.

##### Meteorological Data Recording and Aerial Seismology

NTFPs are used to gather meteorological data, such as atmospheric temperature,
wind speed, air pressure, and humidity [94, 95, 96]. They are also used in
aerial seismology to detect earthquakes [97, 98].

##### Telescopes

Since they have a better view at higher altitudes, NTFPs are used as aerial
telescopes [99, 100, 101, 102].

##### Agriculture and Farming

NTFPs help farmers identify several types of plants via aerial images, and may
prevent farming fraud [90, 103, 104].

##### Deforestation and Vegetation Mapping

NTFPs can be used to capture aerial images from a high altitude, which are
used to detect and prevent deforestation. These aerial images can also be used
to understand changes in biodiversity over time [105, 90].

##### Pollution

NTFPs are used to detect oil spills and floating debris. They are also used to
detect and monitor light pollution [106].

####  II-E6 Energy Harvesting

##### Solar Energy

The power harvested by ground photovoltaic panels depends heavily on the
weather. Thus, the efficiency of these panels decreases drastically in
countries with fewer sunny days such as the United Kingdom. However, NTFPs can
fly above the clouds, and get more solar exposure [60, 107, 108, 53, 109].

##### Wind Energy

Wind speed is much higher at an altitude of 150 m than at the ground. Hence,
NTFPs at high altitudes can harness more powerful winds and can generate twice
the energy of comparable ground-based turbines [26, 27, 24].

###  II-F Advantages of NTFPs

####  II-F1 Cost-Efficient

NTFPs are cost-efficient compared to free-flying platforms and other
communication infrastructure such as tower masts and satellites.

NTFPs have a low operation cost overall compared to free-flying platforms. For
instance, they are cheaper to purchase, maintain, and service compared to
free-flying platforms. Additionally, they require less training to operate and
fewer operators. Compared with tower masts, deploying a NTFP costs ten times
less than erecting a tower mast. Also, the cellular coverage provided by one
NTFP flying at an altitude of 250 m equals the coverage provided by 14 tower
masts. Tower masts consume large amounts of energy and fuel compared to NTFPs,
which consume less energy and require no fuel. Finally, compared to
satellites, deploying a NTFP is much cheaper than launching a satellite into
orbit, especially considering that satellites only operate for approximately
ten years. NTFPs offer a great alternative to free-flying platforms, tower
masts, and satellites.

####  II-F2 Endurance & Persistence

One key advantage that NTFPs have over free-flying platforms is endurance and
persistence. This is more pertinent to surveillance missions and
telecommunications during which the flying platform must stay aloft for a
prolonged period (e.g., days or weeks) and/or in a stationary position. The
persistence of free-flying platforms is only several hours, and it is very
hard for free-flying platforms to remain in a stationary position.

####  II-F3 Green Credentials

Another advantage that NTFPs have low consumption of fuel and power compared
to free-flying platforms and especially tower masts. In India for instance,
nearly 2 billion liters of diesel are burned each year to operate tower masts
[46]. This number will increase to reach 15 billion to cover rural and remote
areas in India which will dramatically increase the carbon footprint and
pollution in the atmosphere.

####  II-F4 Coverage & Backhaul Link

NTFPs have wide coverage compared to tower masts due to their higher
altitudes, which allow them to cover a larger area. When compared with
satellites and terrestrial networks, NTFPs have a higher and stronger LOS than
terrestrial networks and a shorter propagation delay than satellites.
Additionally, NTFPs have great backhaul link capacity compared to free-flying
platforms, whose wireless backhaul is more prone to interference and higher
latency. A wired (tethered) backhaul, i.e., have a wired data-link via the
tether, allowing reliable and high data rates communications.

(a) Advantages of NTFPs.

(b) Disadvantages of NTFPs.

Figure 24: Advantages and disadvantages of NTFPs.

####  II-F5 Quick Deployment

One of NTFPs biggest advantages is their fast and quick deployment, which
makes them suitable for safety missions and disaster relief operations. In
addition, NTFPs can be moved and relocated. In contrast, tower masts, once
erected, cannot be moved elsewhere. NTFPs can also be used in areas where it
is not feasible to erect a tower mast, such as during a disaster situation
where the communications infrastructure is severely damaged or destroyed or
over land that is not suitable for erecting a tower mast. In summary, NTFPs
can be quickly deployed, easily reconfigured, and rapidly relocated.

####  II-F6 Constant Power Supply

The role of the tether is to supply the platform with power so it can stay
aloft for days or weeks, whereas free-flying platforms have limited power
supply. Also, the constant power supply allows NTFPs to carry larger payloads
compared to free-flying platforms. In addition to power, the tether offers
high data rate and increased backhaul link capacity via a fiber connection to
the ground station compared to free-flying platforms.

###  II-G Disadvantages of NTFPs

####  II-G1 Mobility

Although NTFPs are rapid to deploy and have better mobility than ground
stations, they are still limited in their movement due to the physical
constraint imposed by the tether. The tether offers continuous power and data
supply to the platforms, but at a cost to mobility. Compared to free-flying
platforms, NTFPs cannot move beyond the radius of the tether’s length.

####  II-G2 Tether Constraints

One constraint that we mentioned before is the limited mobility imposed by the
tether. Also, the tether itself can be an issue if it sustains damages
(intentionally or unintentionally), preventing the supply of data and
communications. A damaged tether may hinder an ongoing operation, especially
critical operations such as military missions and disaster relief operations.
Hence, it is always recommended to protect and attend to the tether, and also
have several tethers attached to the platform for redundancy.

####  II-G3 Optimal Placement

Free-flying platforms can hover or fly freely in the air. Hence, they can move
to optimize their positions. However, tether platforms are constrained by the
tether preventing them from optimizing their position.

####  II-G4 Intervention Operations

NTFPs are not designed for intervention operations since they cannot move
freely or quickly in the air compared to aircraft. The capabilities of NTFPs
must match the requirements of the mission.

Fig.25 summarizes the advantages and disadvantages of using NTFPs. In
Fig.25(a), we see a comparison among NTFPs, satellites, and tower masts in
terms of the communications aspect. NTFPs have more advantages in
communications than disadvantages. For instance, if we compare NTFPs with
satellites, we notice that satellites have more coverage and endurance than
NTFPs, but with a far greater cost, greater delay, and longer time to deploy.
Also, if we compare NTFPs with tower masts, we can see that NTFPs outperform
tower masts in terms of mobility (tower masts are static), coverage due to
their altitude, cost, and LOS probability.

From Fig.25(b), we see the comparison between NTFPs and free-flying platforms.
Free-flying platforms outperform NTFPs only in mobility and LOS probability.
On the other hand, NTFPs have a lower cost, better backhaul link capacity,
better endurance and persistence, constant power and data supply, and less
pollution.

(a) NTFPs, satellites, and tower masts.

(b) NTFPs and free-flying platforms.

Figure 25: Comparisons among NTFPs and other flying platforms and
communications infrastructures.

###  II-H Regulations

####  II-H1 Socio-Technical Concerns of NTFPs

Although NTFPs offer several advantages and applications, there are several
concerns that have to be taken into account regarding theses platforms such as
privacy, data protection, and public safety. Before we explain the regulations
related to NTFPs, we describe the socio-technical concerns related to these
platforms [110]:

  * •

Privacy: Since NTFPs have a great coverage over the area of interest, they can
be unintentionally (or intentionally) a threat to the privacy of individuals
and businesses. Therefore, legislation and regulations must be established to
protect privacy.

  * •

Data Protection: Due to their coverage, NTFPs can collect massive amounts of
data from the public, such as images, videos, and personal data. These data
must be protected from abuse according to data protection laws, and operations
of NTFPs should be subject to regulation that protects personal information
[111].

  * •

Public Safety: Another critical issue of NTFPs is public safety. Although
NTFPs present fewer safety issues to the public than free-flying platforms,
regulations must still be issued to protect the public. Some possible risks
include a NTFP falling on the public, a cut in the power supply, the platform
landing procedure when the tether is cut, a collision with flying aircraft,
etc.

Like free-flying platforms, NTFPs are subject to regulations. However, tUAVs
and tethered LTA aerostats are not subject to the same regulations. Tethered
LTA aerostats and Helikietes fall under part 101 of the Federal Aviation
Administration (FAA) aeronautics and space regulations, whereas tUAVs fall
under part 107 of aeronautics and space regulations [112].

We will outline below the regulations as they apply to tUAVs and tethered LTA
aerostats, and highlight the main differences between tethered and free-flying
regulations.

####  II-H2 tUAVs

Whether tUAVs should be considered as kites or balloons is debated,; however,
the U.S. FAA states that tUAVs fall under the same category as free-flying
UAVs when it comes to regulations. However, some countries do not classify
tUAVs as free-flying UAVs, since they are tethered to an anchor point. Hence,
the classification of tUAVs depends on the aviation laws of each country
[113].

Table III shows the main requirements related to UAVs. We can see from Table
III that tUAVs are not under the same constraints safety-wise as their free-
flying counterparts, making tUAVs easier to deploy. We can also see that from
the set of requirements presented in Table III, three of them have no impact
on the tUAVs:

TABLE III: Different requirements between free-flying UAVs and tUAVs [25] |  Requirements |  Free-flying UAV |  tUAV
---|---|---|---
Captive system |  Have a physical connection with a mechanical strength. |  Not Required |  Compulsory
Max. altitude |  90 m–152 m (see Table IV). |  Compulsory |  Compulsory
Loss of data link |  Restore the data line or interrupt the flight to reduce the effect on third parties in the air or on the ground. |  Compulsory |  Not Required
Identification |  Remote direct identification system. |  Compulsory |  Not Required
Registration |  Have a unique physical serial number. |  Compulsory |  Compulsory
Geo-fencing |  Load and update data containing information on airspace limitations in relation to UAV position and altitude imposed in relation to geographical areas. |  Compulsory |  Compulsory
Security |  Data link system protected against unauthorized access to command and control function. |  Compulsory |  Not Required
Endurance |  Clear warning signal when the battery of the UAV or its control station reaches a low level. |  Compulsory |  Compulsory
Visibility |  Equipped with positioning and navigation lights. |  Compulsory |  Compulsory
TABLE IV: Regulations regarding UAV heights for commercial purposes [114]. Country |  Max altitude |  Min distance to people
---|---|---
Canada |  90 m |  150 m
Germany |  100 m |  not over crowds
Spain |  120 m |  not over groups
United States |  122 m |  N/A
Chile |  130 m |  30 m
Japan |  150 m |  30 m
Colombia |  152 m |  N/A

  * •

Loss of data links: In this situation, a free-flying UAVs can fly away with
all the risks and safety issues involved. tUAVs, on the other hand, cannot fly
away since they are tethered to the ground station. Therefore, there is no
need to restore the lost data link or abort the flight.

  * •

Identification: Free-flying UAVs have remote direct identification systems
that allow the UAV to be identified in the case of flyaway situations.
However, tUAVs are exempted from this requirement since their flying perimeter
is limited by the tether length (between 90 m and 160 m).

  * •

Security: In the case of free-flying UAVs, the data are transmitted though the
air, which makes them vulnerable to eavesdroppers and subject to interference.
In contrast, tUAVs transmit data via their tether, decreasing signal loss,
signal attenuation, and interference. However, the tether has to be protected
from physical harm and hijacking.

Another aspect worth noting about tUAVs is pilot requirements. Free-flying
UAVs require a qualified pilot with a flying certificate. But in the case of
tUAVs, the pilot does not need to possess any certification. Plus, the tether
makes it easier to stabilize the UAV’ movements.

Furthermore, when there is a ground power cut, a safety mechanism activates a
battery to keep the UAV in the air. This mechanism also triggers an alarm that
alerts the pilot so they can land the UAV.

####  II-H3 Tethered LTA platforms and Helikites

The regulations related to NTFPs can be divided into three types: regulations
related to flight and aviation, regulations related to communications, and
regulations related to the equipment that the platform is tethered with.

  * •

Flight and Aviation Regulations: The FAA requires that all tethered aerostats
have a rapid deflation device that will automatically and rapidly deflate the
aerostat if the tether is cut. If the device does not respond or does not
function properly, the nearest air traffic control must be notified about the
location and time of the escape of the aerostat. The deflation devices are
activated when the tethered aerostat exceeds a predetermined distance from a
given location monitored by a global positioning system (GPS), or when the
aerostat has exceeded a predetermined altitude monitored by a barometric
pressure sensor. The aerostat must be illuminated if it is flying from sunset
to sunrise. For further information about the regulations concerning tethered
aerostats, the reader is advised to read the electronic code on the federal
regulation website [112].

  * •

Communications Regulations: Missions or operations that require communications
via tethered aerostats are regulated by the U.S. Federal Communications
Commission (FCC). For further information about the regulations concerning
tethered aerostats, the reader is advised to read the electronic code on the
federal regulation website [115].

  * •

Transportation Regulations: Regulations may also apply to the vehicle to which
the aerostat is attached.

##  III Case Studies, Projects, and Companies Related to NTFPs

In this section, we explore the numerous applications of NTFPs. We present
projects and case studies from real-life scenarios involving NTFPs. Also, for
completeness, we present the major companies that manufacture and sell NTFPs
worldwide.

###  III-A Project and Case Studies

####  III-A1 Paris Airport Maintenance

The Precision Approach Path Indicator (PAPI) is a system with four lights (two
white lights and two red lights) placed beside landing runways that helps
pilots assess their landing slope (as shown in Fig.26(a)). If the PAPI
displays two white lights and two red lights, then the airplane has the
correct slope. If the PAPI shows three white lights or more, then the slope is
too high; if it shows three red lights or more, then the slope is too low
(Fig.26(b)).

To calibrate the precision of this system, maintenance is carried out using
elevating work platforms, which block access to the runway. Closing the runway
causes time loss, complicated maintenance logistics, and risks associated with
the ground operators on the runway and the operators on the elevating work
platforms.

To solve this problem, the French airport authority Groupe ADP used a tUAV at
the Paris airport Charles-de-Gaulle. An Elistair hexacopter was used to
perform the maintenance task and calibrate the PAPI. The tUAV allows a clear
view of the Paris airport runways. The advantages of using a tUAV to perform
this task are: 1) it can stay in a stationary position for prolonged periods
of time, 2) the tUAV can take off and land on a very small surface (1,2 m
diameter), 3) the tUAV can detect precisely the boundary of each color, which
is depicted in pink (a mix of white and red) as shown in Fig.26(d). To further
increase the accuracy of the PAPI, two tUAVs are placed at different distances
and altitudes; hence, by using the required threshold angle $\alpha_{A}$, the
threshold can be verified by checking the accurate altitude of the pink color
detected by the tUAVS at distances $d1_{A}$ and $d2_{A}$, as shown in
Fig.26(c).

The use of a tUAV prevents any risk to the operators, such as falling down
from the elevated work platforms. Since the UAV is tethered, it is directly
linked to the control tower, which permits secure and interference-free
communications. Also, the tUAV can be deployed over the runway with the
necessary safety measures, while the rest of the airport continues running
without interruption. Finally, the tUAV calibrates all the thresholds in one
hour.

(a) PAPI lights beside the runway.

(b) The PAPI red and white lights.

(c) Two tUAVs at two different distances to calibrate the PAPI.

(d) The tUAV calibrating the transition between each color.

Figure 26: The PAPI mode of operation, and its calibration by a tUAV [116].

####  III-A2 Road Traffic Monitoring in Lyon

To monitor traffic in Lyon, Elistair proposed a solution using their tUAV to
continuously monitor a roundabout. Lyon, which is the second largest urban
area in France, has a big concentration of traffic flow, especially in the
suburban areas.

The tUAV was equipped with a Full HD camera, and the operation lasted 3 hours
during rush hours. The tUAV was placed 110 m away from the roundabout. For
security reasons, the tUAV was located within a 50 m secured radius, as
depicted in Fig.27(b), to ensure that the operation would not be interrupted
or jeopardized.

Using the tUAV offered several advantages. For example, the tether cable
allowed the tUAV to maintain a steady position when controlling the camera.
Also, the tether allowed safe data transfer and data display in real-time. The
cloud-based platform DataFromSky was used to analyze the road-traffic data
collected from the tUAV with artificial intelligence and machine learning
tools [117].

The tUAV recorded the traffic flow in the roundabout, then the videos were
uploaded to DataFromSky. After processing and analyzing the data, DataFromSky
sent a video and metrics, such as speed, acceleration, and trajectory of the
vehicles, as shown in Fig.27(a). It also provided the quantity, categories,
and the types of the vehicles: cars, motorcycles, trucks or buses.

Thus, using a tUAV for traffic monitoring has an easy configuration and fast
deployment. The tUAVs also outperformed the traditional traffic monitoring
method in terms of mobility and coverage. By using a tether cable: 1) the UAV
maintained a persistent and steady position to record the traffic; 2) the
communications and data were secured; 3) the tUAV could stay aloft for several
hours (days if needed) thanks to the constant supply of power.

(a) A snapshot of the traffic video after DataFromSky processing.

(b) The tUAV placement.

Figure 27: An aerial view of the roundabout and the placement of the tUAV
[118].

####  III-A3 Border Security in Southern Texas

Rio Grande Valley, located in southern Texas along the Mexican border, has
accounted for the highest number of apprehended illegal immigrants in the U.S.
since 2016 [119]. The U.S. Department of Homeland Security (DHS) and Customs
and Border Protection (CBP) are facing a huge challenge in monitoring this
very long border. Also, the terrain in this area is challenging, limiting the
capabilities of ground-based surveillance systems, and pushing the DHS and CBP
to seek an elevated or aerial solution.

Figure 28: Borders control and illegal immigrants tracking with a tethered
TCOM blimp [120].

To deal with this challenge, the DHS and CBP used TCOM tethered blimps. The
tethered blimps provided large coverage and continuous surveillance of the Rio
Grande Valley to the border authorities. The advantage of tethered blimps is
that they can stay aloft for several weeks while providing real-time videos
and monitoring, which helps the authorities make better decisions. Also, the
blimps have a high degree of mobility, can be rapidly deployed, are battle-
proven, and low-cost.

After using the tethered blimps, the border authorities have witnessed a
decrease in illegal immigrant crossings, thanks to the tethered blimps’ wide
coverage and long persistence.

####  III-A4 Oil-Spill Detection in the Arctic Ocean, Norway

The Norwegian Clean Seas Association for Operating Companies, also called
NOFO, is an oil-spill response organization. NOFO works with 30 offshore
operators, providing and managing oil-spill preparedness plans. An aerial
solution can detect oil spills better than at sea level. Using an aerial
camera offers wide coverage for assessing the extent as well as the thickness
of the oil spill. Although aircraft and UAVs can be a solution, they lack
persistence and steadiness in the air, especially for long oil sweeping
missions.

Figure 29: NOFO using Helikite to detect and clean up oil spills [121].

To overcome these limitations, NOFO used a Helikite system called ”The Ocean
Eye” to detect oil spilling. Helikites have the advantage of being small and
compact; they can be easily handled, rapidly deployed, and can sustain harsh
sea weather. The Helikite can be anchored to the cleaning ship or nearby
boats, as shown in Fig.29. The Helikite provides real-time video of the oil
spill, which helps the cleaning boat locate accurate positioning and, thus,
extract more oil in a shorter time from the sea.

Figure 30: Altaeros SuperTower setup [20].

####  III-A5 Aerial Photographic Survey of Armarna, Egypt

A Cambridge University research group wanted to conduct an archaeological
photography of the ancient site Armarna in Egypt, where the famous Tutankhamen
was born. They wanted to take still stereo-images of the site. They used
Helikites equipped with two 35 mm single-lens reflex (SLR) cameras. The
Helikites provided ultra-sharp, still stereo-images of the whole area.

The Helikite is a low-cost solution, allowing to get closed to the site with
low vibrations, and with a steady position to take still images. Additionally,
the Helikite performed very well under the challenging hot weather conditions
in Egypt.

####  III-A6 Aerostats All Australia (AAA) Mobile Coverage

According to [122], nearly $70\%$ of Australia does not have mobile coverage.
To deal with that problem, a project named AAA has offered a plan to extend
mobile coverage throughout Australia and the surrounding sea areas. In order
to do so, AAA project proposes the use of NTFPs to bring wide coverage at a
lower cost than other alternative solutions. The AAA envisions mobile coverage
with low latency for all mobile users in remote areas. In the short term (5
years), AAA proposes doubling Australian coverage from one-third to two-thirds
of the total land areas. Over the long term, AAA aims to provide mobile
coverage to all Australia.

TABLE V: LTA NTFP companies.  Type |  Company |  Contact
---|---|---
Blimps |  • TCOM [19]
• Lindstrand Technologies [123]
• CNIM Air Space [124]
• ADASI [125]
• Altaeros [20] |  • NA
•<EMAIL_ADDRESS>
• +33 5 34 43 04 09
• NA
•<EMAIL_ADDRESS>
Balloons |  • Vigilance [126]
• Drone Aviation Corp [127]
• SkyDoc [128] |  • +31 402 340 600
•<EMAIL_ADDRESS>
•<EMAIL_ADDRESS>
BATs |  • Altaeros [20] |  •<EMAIL_ADDRESS>
TABLE VI: HTA NTFP companies.  Type |  Company |  Contact
---|---|---
tUAVs |  • Elistair [25]
• Equinox Innovative Systems [129]
• Tethered Drone Systems [130]
• Hoverfly tech [131]
• Drone Aviation Corp [127]
• Fotokite Sigma [132] |  • +33 9 83 57 06 39
• NA
•<EMAIL_ADDRESS>
•<EMAIL_ADDRESS>
•<EMAIL_ADDRESS>
•<EMAIL_ADDRESS>
Air Wind Turbines |  • Makani [30]
• Airborne Wind Europe [133] |  • NA
• +32 27396212
TABLE VII: Hybrid NTFP companies.  Type |  Company |  Contact
---|---|---
Helikite |  •Allsopp Helikite [31] |  •<EMAIL_ADDRESS>
Hybrid Airship |  • Hybrid Air Vehicles [32]
• Lockheed Martin [33] |  • +44 (0) 1234 336400
• NA
TABLE VIII: tUAV solution companies.  Company |  Contact
---|---
Spooky Action [134] |  \+ 952-649-1637
AeroMana [135] |<EMAIL_ADDRESS>

####  III-A7 Altaeros SuperTower

The Altaeros SuperTower is a solution proposed and developed by Altaeros to
provide cellular coverage in rural areas (Fig.30). Standard infrastructure
solutions have the disadvantage of being expensive and not lucrative in areas
with few subscribers. The SuperTower is a tethered blimp that flies at an
altitude of 240 m. The coverage gained by flying at this high altitude allows
one SuperTower to replace 15 cell towers, reducing costs by 60%. Hence, using
this solution can accelerate the implementation of a mobile network more
quickly and efficiently than standard cell towers, with significantly less
cost [136].

###  III-B Companies Related to NTFPs

Here we present the major companies that manufacture and sell NTFPs. Table V
shows the companies that manufacture LTA platforms, i.e., blimps, balloons,
and BATs.

Table VI shows the companies that manufacture HTA platforms, such as tUAVs and
airwind turbines.

Finally, Table VII shows the companies that manufacture hybrid platforms, such
as Helikites and hybrid airship. We recall that hybrid airships have the
possibility to be tethered, but are not systematically NTFPs.

For the sake of completeness, we present two companies that propose solutions
for UAVs, namely, Spooky Action and AeroMana. These companies propose a tether
configuration that can be plugged into existing UAVs without any
modifications. This will give an additional freedom of choice to free-flying
UAVs, by allowing them to be tethered when needed.

##  IV NTFPs from a Wireless Communications Perspective

###  IV-A Geometric Analysis

In order to evaluate the performance of NTFPs, we have to investigate the
geometrical aspect between a given NTFP (e.g., blimp) and the Earth. This can
be carried out following an approach similar to that used for LEO satellites
[137, 138]. Fig.31 shows the geometrical aspect and the coverage surface
between a blimp and a user located on the ground. In Fig.31, $R_{e}=6378$ km
denotes the Earth’s radius at sea level, $h$ is the height of the blimp above
the Earth, $d$ is the distance between the blimp and the ground user, also
known as the slant range, $\alpha_{n}$ is the nadir angle, that is, the angle
under which the blimp views the ground user, $\beta_{c}$ is central angle,
that is, the geocentric angle between the user and blimp nadir, and $\theta$
is elevation angle, that is, the angle between the slant range and the horizon
plane.

Figure 31:  Coverage of NTFPs from a geometric perspective.

Following the same approach as [137, 138], we obtain the following equations:

$\alpha_{n}+\beta_{c}+\theta=\frac{\pi}{2},$ (1)

$d\cos(\theta)=\left(h+R_{e}\right)\sin(\beta_{c}),$ (2)

and

$d\sin(\alpha_{n})=R_{e}\sin(\beta_{c}).$ (3)

When distance $d$ is required, to compute path loss, we apply the law of
cosines for the triangle in Fig.31, which yields

$(h+R_{e})^{2}=R_{e}^{2}+d^{2}+2R_{e}d\cos\left(\frac{\pi}{2}+\theta\right).$
(4)

Solving equation (4) with respect to $d$ yields the following solution:

$d=R_{e}\Bigg{[}\sqrt{\Big{(}\frac{R_{e}+h}{R_{e}}\Big{)}^{2}-\cos^{2}(\theta)}-\sin(\theta)\Bigg{]}.$
(5)

Hence, $d$ reaches its maximum value when $\theta=0$, which is given by

$d_{\max}=d\left(\theta=0\right)=\sqrt{h^{2}+2hR_{e}}.$ (6)

Also, from applying the law of sines in Fig.31, we get

$\frac{\sin(\alpha_{n})}{R_{e}}=\frac{\sin\left(\frac{\pi}{2}+\theta\right)}{R_{e}+h}$
(7)

Then, we have

$\sin(\alpha_{n})=\frac{R_{e}\cos(\theta)}{R_{e}+h}.$ (8)

Finally, the angle $\alpha_{n}$ is given by

$\alpha_{n}=\sin^{-1}\Bigg{(}\frac{R_{e}\cos(\theta)}{R_{e}+h}\Bigg{)}.$ (9)

To calculate $\beta_{c}$, we use equation (2):

$\beta_{c}=\sin^{-1}\Bigg{(}\frac{d}{R_{e}+h}\cos(\theta)\Bigg{)}.$ (10)

Also, we can use equations (1) and (9) to calculate $\beta_{c}$; hence,

$\beta_{c}=\frac{\pi}{2}-\theta-\sin^{-1}\Bigg{(}\frac{R_{e}}{R_{e}+h}\cos(\theta)\Bigg{)}.$
(11)

Finally, the surface coverage achieved by the blimp can be computed as
follows:

$S_{cov}=2\pi R_{e}^{2}\left[1-\cos(\beta_{c})\right].$ (12)

To better assess the impact of $h$ and $\theta$ on different metrics, such as
surface coverage and the rang $d$, we plotted different curves in Fig.32 and
Fig.33.

In Fig.32(a), we plotted the surface coverage as a function of $h$ for several
values of $\theta$. We can see that, as the altitude of the blimp increases
($h$ increases), the surface coverage increases as well. This remark is
intuitive, because as altitude of the blimp increases, the blimp can cover a
greater surface. We can also notice that, the difference between the surface
coverage when $h=100$ m and when $h=40$ km is four orders of magnitude.

The range $d$ is plotted in Fig.33(b) as a function of the elevation angle
$\theta$ for several values of $h$. We can see that the range $d$ decreases as
the elevation increases. For instance, when $\theta=90$, the user is exactly
below the blimps, which corresponds to the shortest distance possible between
the blimp and the user. Inversely, when $\theta=0$, the user is the farthest
from the blimp, which corresponds to $d_{\max}$.

(a) Surface coverage as a function of $h$ for several values of $\theta$.

(b) Surface coverage as a function of $\theta$ for several values of $h$.

Figure 32: Surface coverage as a function of $\theta$ and $h$.

(a) The range $d$ as a function of $h$ for several values of $\theta$.

(b) The range $d$ as a function of $\theta$ for several values of $h$.

Figure 33: The range $d$ as a function of $\theta$ and $h$.

###  IV-B Performance Analysis

Very few works have investigated the performance of NTFPs in wireless
communications. Most focus on free-flying platforms, such as HAPs and UAVs.
For example, in the absence of terrestrial infrastructure, UAVs are used to
assist cellular networks. But two major issues limiting the connection between
UAVs and the core network are backhaul constraints and limited energy.

To address this issue, the work in [139] proposed a multihop connection using
several UAVs to alleviate the backhaul constraint. However, by increasing the
hops, the latency increased and the spectral efficiency of communications was
reduced.

Hence, the authors in [140] proposed a configuration with a tethered balloon
connected to the core network via fiber, as shown in Fig.34. The tethered
balloon acted as a “flying base station” located at higher altitude than the
UAVs, which created a strong LOS backhaul connection. The proposed
configuration showed an increase in the achievable end-to-end data rate of the
users. Plus, they proposed a framework that optimized the transmit power,
placement and association of the UAVs.

Regarding the use of tUAVs, the authors in [141] proposed a hybrid solution to
overcome these challenges. The solution consisted of three different types of
UAVs: UAVs that acted as communication drones, tUAVs that provided a backhaul
connection to the communication drones via a RF/FSO hybrid link [142, 143],
and UAVs that powered the communication UAVs by providing on-the-fly battery
charging. The authors showed that unlimited cellular communications could be
provided while guaranteeing a minimum rate for all users.

Figure 34: Backhaul connection between UAVs and tethered balloons.

Using tUAVs as airborne base stations has huge potential to extend network
capacity and coverage for 6G [144]. In [54], the authors investigated the
optimal placement of a tUAV tethered to a rooftop to minimize the path loss
between the tUAV and a ground user; constraints included the limited length of
the tether and the inclination angle of the tUAV for safety issues. The
authors in [145] compared the performance of UAVs and tUAVs under heavy
traffic conditions. Their results showed that tUAVs outperformed UAVs.

NTFPs can also be used to connect other flying platforms, such as future
flying cars, with the core network. Indeed, flying cars require a reliable
aerial wireless communications network. The communications technologies
currently used in vehicular communications [146, 147] are ill-suited for
flying cars due to their lack of aerial coverage [11].

Finally, NTFPs can be used as a relay between airborne platforms and ground
stations. For instance, in [148], the authors considered communications
between free-flying HAPs and ground-based stations. However, in this case,
channel state information (CSI) were hard to obtain due to the high altitude
and mobility of the HAPs. Thus, without CSI knowledge, the performance in
terms of sum-rate was significantly degraded. To overcome the absence of CSI
and maximize the sum-rate, the authors proposed an interference alignment
scheme that used a tethered balloon as a relay between the HAPs and the
ground-based stations. The proposed scheme achieved the maximum sum-rate
without CSI. Additionally, they showed that there is an optimal altitude for
the tethered balloon that maximized the achievable sum-rate.

###  IV-C NTFP Communications for 6G

Now we show how NTFPs will be the key enablers to interconnect heterogeneous
networks and communications for 6G. To this end, they will be the bridge
between land, air, sea, and space by being connected to ground communications
(e.g., cellular communications and vehicular communications), air
communications (e.g., UAV communications and HAP communications), maritime
communications (underwater communications, and remote-sea communications) and
satellite communications, as shown in Fig.35. One of key aspect of 6G is that
everything will be connected. In that context, more than 4 billion do not have
access to an internet connection, NTFPs will help bridge the digital divide
and bring connection to rural and remote areas. They will also provide last
mile connectivity, ensuring seamless connection to vehicles and flying cars,
interconnecting airborne platforms, providing coverage remote sea areas, and
serving as relay between satellite communications and ground/sea
communications.

Figure 35: Various type of communications used by NTFPs.

####  IV-C1 Remote communications and last mile connectivity

To connect the unconnected, NTFPs are a cost-efficient solution to bridge the
digital divide and provide high data rate connection to remote areas. They are
used as aerial base stations with a huge backhaul capacity to provide last
mile connectivity for hard-to-reach areas [144]. Additionally, this solution
is ready to be used immediately, can be quickly deployed, and costly
efficient.

####  IV-C2 Vehicular communications

With the upcoming rise of autonomous vehicles, the need for seamless
connectivity, high reliability, and low latency communications is of utmost
importance [149, 150]. Autonomous and driverless vehicles need to be
constantly connected and aware of their surrounding. NTFPs allow these
vehicles to be connected all the time with their wide mobile coverage and high
data rate connection. They also guarantee seamless connectivity at the edge
and under-connected areas.

####  IV-C3 Flying cars

Flying cars in general, and electric vertical take-off landing (eVTOL) in
particular, are expected to be part of our daily lives in the next decade and
6G envisions to serve the eVTOLs as of 2030 [151]. Flying cars need to be
constantly connected, in order to avoid collisions, to acquire other vehicles’
positions, or to change their itinerary when necessary. However, terrestrial
networks, as they are now, are not suitable for eVTOL communications since
flying car fly at an altitude of 300 m, which is way above tower masts. In
that context, NTFPs are the perfect solution since they have great coverage
and can provide aerial connectivity for eVTOLs [11].

####  IV-C4 Airborne communications

Airborne platforms will be ubiquitous in 6G forming a three dimensional
networks. Hence, they need to be connected with other platforms (such as
vehicles), but also connected among themselves whether they are tethered or
free-flying. NTFPs stay aloft for a prolonged period of time at the same
position, which allows them to be used as relay to interconnect different
airborne solutions flying at different altitudes, such as, HAPs and UAV [152].

####  IV-C5 Maritime communication

Maritime economy and maritime activities have witnessed a significant growth
with various activities and applications, such as fisheries, maritime
transportation, sea monitoring, deep sea mining, surveillance, and inspection
missions (e.g., oil and gas facilities). However, as important as these
activities are, they still lack broadband communications capable of
withstanding the needs of theses activities and applications. For instance,
underwater missions rely heavily on a massive number of autonomous underwater
vehicles (AUVs), remote operated vehicles (ROVs), and underwater sensors
(UWSs). These vehicles need to transmit large amounts of data among themselves
and between the shore. Another example is that fishers or travelers need an
internet connection in their cruise ship. NTFPs are a suitable solution to
connect the sea (above and under the surface) with the shore by providing
broadband connectivity and mobile coverage. They are also less costly compared
to satellite solutions, and with lower latency [57, 56, 52].

####  IV-C6 Satellite communications

Finally, NTFPs help connecting satellite with ground platforms. In fact,
satellite communications suffer attenuation and delay. Hence, NTFPs can act as
relay by converting a long-range transmission using one single hop into a
short-range transmission using multiple hops. This will alleviate the overall
propagation delay and improve the data rate [152].

###  IV-D Economics of NTFPs

One of the several advantages of NTFPs is that they are cost-efficient.
Therefore, it is relevant to investigate the economic aspects of using NTFPs
in a wireless communications context. To this end, we present two analyses:
the first one was conducted in the AAA project [122], and the second one was
done by Altaeros for their SuperTowers [20, 136]. The analysis carried out in
the AAA project provide a detailed deployment plan for 250 NTFPs over four
years. The analysis done by Altaeros compared the cost of one single NTFP with
several cell towers during their respective lifetime.

####  IV-D1 AAA Analysis

As mentioned in Section III, the AAA project aimed to extend mobile coverage
in Australia by using tethered aerostats, especially in remote areas [122].
One of the main advantages of using tethered aerostats is that they cost less
than alternative solutions. We will show the costs of using tethered solutions
from a CAPEX/OPEX perspective, as demonstrated in [122]. Also, we review the
different stages proposed in the AAA project and the level of support.

The AAA project proposed a three-stage deployment strategy spread over four
years!

  * •

Stage 1: Aerostat center for excellence,

  * •

Stage 2: Aerostat co-location with existing remote Australia cell towers, and

  * •

Stage 3: Public Safety Agency points-of-presence (POPs) and point-to-point
links.

##### Stage 1

The tasks and responsibilities of the aerostat center for excellence include
designing and testing the aerostats, carrying out trials across Australia,
designing radio access network (RAN), obtaining regulation approvals, and
training the operators and the support personals, etc. For more details, we
invite the reader to read the AAA report [122].

##### Stage 2

The advantage behind co-locating tethered aerostats with an existing cell
tower is that there are existing access roads, fiber optic backhaul and
electricity, which, according to [122], reduce the cost of a tethered aerostat
from $2 million to $500k. However, they recommend some security measures such
as the length of the tether has to respect a given distance in case the
tethered aerostat cannot be taken down (for instance, due to winch
malfunction), hence protecting the communications structure in its vicinity.
Also, they recommend that the tethered aerostat have a dedicated radio
spectrum in order to avoid interference with the already existing cell site.

##### Stage 3

New tethered aerostat sites will act as new Public Safety Agency POPs. They
should be erected on the major Australian islands and external territories.
The sites on islands and territories that have high strategic importance
should be erected jointly with the defense department and border surveillance.
In that context, larger tethered aerostats can carry heavier payloads.
Therefore, they can carry both communication payloads as well as surveillance
payloads, such cameras, sensors, etc., allowing the tethered aerostats to be
used for both communications and surveillance.

TABLE IX: Total OPEX from year 1 to year 4. OPEX | Annual Cost
---|---
L1 Support | $ $25$M
L2 Support | $ $15$M
L3 and L4 Support | $ $10$M
Total L1 to L4 Support | $ $50$M
Center for Aerostat Excellence | $ $10$M
Amortization | $ $10$M
OPEX | $ $75$M
TABLE X: Combined CAPEX and OPEX for Year 1–4  Year | Stage | Description | CAPEX | OPEX | Total
---|---|---|---|---|---
Year 1 | Stage 1 | Setup center of excellence | $ $10$M | $ $10$M | $ $20$M
Year 2 | Stage 2 | 80 aerostats cell-sites co-located x $500K | $ $40$M | $ $25$M | $ $65$M
Year 3 | Stage 2 + Stage 3 | 70 aerostats x $500K + 50 aerostats x$2M | $ $135$M | $ $50$M | $ $185$M
Year 4 | Stage 3 | 50 aerostats x $2M | $ $100$M | $ $75$M | $ $175$M
Year 1-4 | All stages | Combined CAPEX and OPEX | $ 160 M | $ 285 M | $ 445 M
Figure 36: Components Altaeros SuperTower System [153]. TABLE XI: CAPEX/OPEX analysis between cell towers and a single NTFP. | 16 x | 1 x
---|---|---
| Cell Towers | SuperTower
RAN/Site Setup | $ $1280$K | $ $105$K
Backhaul Setup | $ $560$K | $ $35$K $
CAPEX | $ $1840$K | $ $140$K
Tower Rent | $ $240$K | $ $400$K
Site Operations & Maintenance | $ $112$K | $ $10$K
Backhaul Service | $ $256$K | $ $60$K
Power & Other | $ $128$K | $ $25$K
OPEX | $ $736$K | $ $495$K
NPV during their Lifetime | $ $9.6$M | $ $4.7$M

They also plan to offer four levels of support, depending on the task and
requirements:

  * •

Level 1: Weekly on-site inspections and recovery of aerostats before/after
cyclones

  * •

Level 2: Support for monthly service and setting up new aerostat sites

  * •

Level 3: Major service, technology upgrades, and major repair performed
annually

  * •

Level 4: Joint collaboration with vendors requiring expertise

##### Level 1 - Support to Inspect and Protect Aerostat

Each site requires two people, called the L1 crew, for a weekly inspection
that includes winching the aerostat down, visually inspecting it, applying the
necessary adjustments, and then winching it back. These weekly inspections
extend the operation life of an aerostat to ten years. Although tethered
aerostats perform better in adverse weather than other airborne solutions,
such as HAPs, aircraft, UAVs, and free-flying platforms, the L1 crew have to
winch down the aerostat when there are cyclones and strong winds, check if
there are any damages, apply the necessary repairs if needed, and winch the
aerostat up. The L1 crew may be regular contractors or trusted locals. While
the aerostat is winched down, the L1 crew will have access to satellite
communication.

##### Level 2 - Monthly Service and Setup of New Aerostats

Ten L2 crews of two people each will provide basic repairs and refill the
aetostats with helium. They will also erect new aerostat sites, train the L1
crews, and prepare the aerostats for the L3 crews.

##### Level 3 - Support for Aerostat Annual Overhaul and Technology Upgrade

The L3 crew offers necessary technological and physical upgrades and an annual
overhaul. They also perform repairs and on-demand facilities.

##### Level 4 - Expert Support

The L4 expert support will provided by vendors and the Australian Center for
Aerostat Excellence.

In total, the cost estimates provided by [122] for 250 aerostats are $25
million per annum (PA) L1 support, $15 million PA for L2 support, and $10
million PA for L3 plus L4 support. Hence, the total cost of all support is $50
million PA. The Australian Center for Aerostat Excellence requires a budget of
$10 million PA for administration fees and for research and development. A
further amortization budget of $10 million is dedicated to refurbishing the
tethered aerostats every five to ten, which is 20% PA of the total support
budget. The total OPEX is shown in Table IX, and the total CAPEX and OPEX for
year 1 through year 4 (until a full fleet operation is completed) is shown in
Table X.

It is noted in [122] that the funding allocated to the AAA project would save
the government and National Broadband Network $1 billion in a Sky Muster
satellite which is twice the cost of the AAA project. Also, the AAA bandwidth
is 250 Gbps, whereas the satellite bandwidth is only 135 Gbps, that is double
the aggregate bandwidth [154].

Figure 37: Comparison between the coverage of NTFPs and cell towers.

####  IV-D2 Altaeros SuperTower Analysis

The SuperTower made by Altaeros is a high capacity and long-endurance NTFP,
that delivers data and provides coverage [20]. The components of the Altaeros
SuperTower are shown in Fig.36. Altaeros conducted a cost analysis with
CAPEX/OPEX comparisons between NTFPs (SuperTowers) and cell towers [136]. The
aim of this comparison was to investigate whether NTFPs are cost-effective
compared to cell towers over their lifetime. A single cell tower is 40–60 m
tall with a coverage radius of 10–15 km. This means that the tower has a
service area ranging between 300–700 km2. On the other hand, a NTFP that flies
at a low altitude, for instance 250 m, has radius of 40–60 km with a service
area ranging between 5,000–10,000 km2. Hence, a single NTFP has a coverage
equivalent of 16 cell towers, as shown in Fig.37. Table XI shows a CAPEX/OPEX
analysis between 16 cell towers and a single NTFP and their Net Present Value
(NPV) during their lifetime. We can see from Table XI that one NTFP flying at
an altitude of 250 m is equivalent to 16 cell towers in terms of coverage at
50% of the cost. Furthermore, we can see that the NTFP has 90% lower CAPEX and
30% lower OPEX compared to 16 cell towers. This analysis considers that the
NTFP flies at altitude of 250 m. If we consider that the NTFP flies at 900 m
such as the platforms described in the AAA project, the coverage of the NTFP
will be the equivalent of 160 cell towers in terms of coverage (as depicted in
Fig.37).

##  V Channel Modeling of NTFPs

In this section, we provide a comprehensive channel modeling framework for
NTFPs. Furthermore, for this section, the term LAP refers to all the platforms
that fly at ultra low-altitudes (50 m–150 m), low-altitudes (200 m–600 m), and
medium-altitudes (0.7 km–5 km). Also, we do not include channel modeling for
U-HAP since there is only one paper that considers the feasibility of NTFPs
flying at such altitudes [68]. For the sake of completeness, we consider both
RF links and FSO links. Indeed, FSO links can provide a high speed connection
via air-to-air (A2A) communications between NTFPs at different altitudes, and
with ground stations or ground users via air-to-ground (A2G) communications
[155]. Finally, at the end of this section, we summarize all channel models in
Table XII with the relevant references.

###  V-A LAPs Channel Modeling using RF Links

For LAPs channel modeling considering RF links, we present both large-scale
fading and small-scale fading.

Figure 38: Connection between a LAP and a mobile user in an urban environment.

####  V-A1 Large Scale Path Loss

The International Telecommunication Union (ITU) proposed a model for deriving
LOS probability in an urban environment between a transmitter at $h_{TX}$ and
a receiver at elevation $h_{RX}$ [156]. The LOS probability is a function of
the following parameters:

  * •

$p_{\alpha}$: the ratio of the built-up land area to the total land area,

  * •

$p_{\beta}$: the mean number of buildings per km2,

  * •

$p_{\gamma}$: which is a scale parameter that models the buildings’ heights.
The distribution of the buildings’ heights follows a Rayleigh distribution
given by
$f(H)=\frac{H}{\gamma^{2}}\exp\Big{(}\frac{H^{2}}{2\gamma^{2}}\Big{)}$, where
$H$ is the building height in meters.

Hence, according to [156], the LOS probability denoted by
$\mathrm{P}(\mathrm{LOS})$ is expressed as

$\mathrm{P}(\mathrm{LOS})=\prod_{n=0}^{m}\left[1-\exp\left(-\frac{{\left[h_{TX}-\frac{-(n+\frac{1}{2})(h_{TX}-h_{RX})}{m+1}\right]}^{2}}{2\gamma^{2}}\right)\right],$
(13)

where $m=\mathrm{floor}(r\sqrt{p_{\alpha}p_{\beta}-1})$ and $r$ is the ground
projection of the distance between the transmitter and the receiver as shown
in Fig.38. In the case of LAPs, $h_{RX}$ can be neglected since the height of
the receiver is much lower than the heights of buildings and the LAP altitude.
In [157], the authors showed that (13) can be expressed as a function of
$\theta$ and the environment parameters as follows:

$\mathrm{P}(\mathrm{LOS},\theta)=\frac{1}{1+a_{1}\exp(-b_{1}(\theta-a_{1}))},$
(14)

where the parameters $a_{1}$ and $b_{1}$ are a function of the environment
variables $p_{\alpha}$, $p_{\beta}$, and $p_{\gamma}$. We note that the non-
line-of-sight (NLOS) probability equals
$\mathrm{P}(\mathrm{NLOS},\theta)=1-\mathrm{P}(\mathrm{LOS},\theta)$.

The expression of path loss is then expressed as

$\displaystyle\mathrm{PL}(\alpha_{\mathrm{PL}},\theta,d)=$
$\displaystyle\left[\mathrm{Loss}_{\mathrm{LOS}}\cdot\mathrm{P}(\mathrm{LOS},\theta)+\mathrm{Loss}_{\mathrm{NLOS}}\cdot\mathrm{P}(\mathrm{NLOS},\theta)\right]\>d^{-\alpha_{\mathrm{PL}}(\theta)},$

where $\mathrm{Loss}_{\mathrm{LOS}}$ and $\mathrm{Loss}_{\mathrm{NLOS}}$
denote the mean additional losses for the LOS and NLOS transmissions,
respectively, $d$ is the distance between the transmitter and the receiver,
and $\alpha_{\mathrm{PL}}(\cdot)$ is the path loss exponent.

The path loss exponent is a function of the density of buildings and an
obstacle between the transmitter and the receiver. For instance, larger values
of $\alpha_{\mathrm{PL}}$ are assumed in dense urban areas, whereas lower
values of $\alpha_{\mathrm{PL}}$ are assumed for rural areas. Hence, the path
loss exponent $\alpha_{\mathrm{PL}}(\theta)$ can be modeled as a function of
$\mathrm{P}(\mathrm{LOS},\theta))$, which, in turn, is a function of $\theta$.
Consequently, the path loss exponent is defined as [158]

$\alpha_{\mathrm{PL}}(\theta)=a_{2}\mathrm{P}(\mathrm{LOS},\theta)+b_{2},$
(15)

where the expression of $a_{2}$ and $b_{2}$ are given by

$a_{2}=\frac{\alpha_{\frac{\pi}{2}}-\alpha_{0}}{\mathrm{P}(\mathrm{LOS},\frac{\pi}{2})-\mathrm{P}(\mathrm{LOS},0)}\cong,\alpha_{\frac{\pi}{2}}-\alpha_{0}$
(16)

and

$b_{2}=a_{0}-a_{2}\mathrm{P}(\mathrm{LOS},0)\cong a_{0},$ (17)

where $\alpha_{\frac{\pi}{2}}$ and $\alpha_{0}$ are the path loss exponent
values when $\theta=\pi/2$ and $\theta=0$, respectively. The values of
$\alpha_{\frac{\pi}{2}}$ are usually smaller since the transmitter is in LOS
with the receiver, whereas the values of $\alpha_{0}$ are usually larger since
there are more obstacles when $\theta=0$. Also,
$\mathrm{P}(\mathrm{LOS},0)\rightarrow 0$ and
$\mathrm{P}(\mathrm{LOS},\frac{\pi}{2})\rightarrow 1$. For more comprehensive
path loss models of LAPs, we direct the readers to reference [159].

####  V-A2 Small Scale Fading

##### Rayleigh distribution

This distribution is widely and extensively used in the literature. It is used
to model a transmission when the path between the transmitter and the receiver
is heavily obstructed, and when LOS is not available. It can be used when the
cooperative transmission between LAPs is considered [160]. Also, it has been
shown in [161] that the fading model in urban environments with large
elevation angles follows the Rayleigh distribution. We recall the Rayleigh
distribution is given by

$f(y)=\frac{y}{\sigma^{2}}\exp\left(\frac{-y^{2}}{2\sigma^{2}}\right).$ (18)

##### Rician distribution

Rician distribution is used to model fading of transmission with LOS for LAPs
[162, 163, 164, 158]. We recall the Rician distribution is given by

$f(y)=\frac{y}{\sigma^{2}}e^{\frac{-(y^{2}+a_{\textrm{LOS}}^{2})}{2\sigma^{2}}}I_{0}\left(\frac{y\>a_{\textrm{LOS}}}{\sigma^{2}}\right),$
(19)

where $y\geq 0$, $a_{\textrm{LOS}}$ and $\sigma$ denotes the magnitude of the
LOS and the diffuse multipath components, respectively, and $I_{0}(\cdot)$ is
the zeroth–order modified Bessel function.

Also, the Rician parameter $K$ is defined as

$K=\frac{a_{\textrm{LOS}}^{2}}{2\sigma^{2}}.$ (20)

In Rician fading, the Rician factor, commonly noted by $K$, is a parameter
that measures the severity of the fading. For instance, $K=5.29$ dB when the
LAP is in the takeoff and landing phase, whereas $K=19.14$ dB when the LAP
flies at an altitude of 20–30 m o[162]. In [163], the authors proposed a
piece-wise model of $K$ as a function of the altitude. The authors in [158]
proposed a model of $K$ as a function of the elevation angle:

$K(\theta)=a_{3}\cdot\exp(b_{3}\theta),$ (21)

where

$a_{3}=k_{0}\>,\>\>\>b_{3}=\frac{2}{\pi}\ln\left(\frac{k_{\frac{\pi}{2}}}{k_{0}}\right),$
(22)

where $k_{\frac{\pi}{2}}$ and $k_{0}$ are the values of $K$ at
$\theta=\frac{\pi}{2}$ and $\theta=0$, respectively. Hence, larger values of
$\theta$ lead to higher values of $K$, which characterizes fewer multipath
scatters. In contrast low values of $\theta$ lead to low values of $K$, which
characterizes severe multipath scatters [158]. Also, the A2A channel for LAPs
can also be modeled as a Rician fading distribution [164].

##### Nakagami-$m$ distribution

Nakagami-$m$ distribution offers great flexibility to model LAP channels,
since it embodies several other different distributions thanks to its Nakagami
shape and spread-controlling parameters defined by $m$ and $\Omega$,
respectively [160, 165]. The Nakagami-$m$ distribution is given by

$f(y)=\frac{2m^{m}}{\Gamma(m)\Omega^{m}}y^{2m-1}e^{\frac{-my^{2}}{\Omega}},$

where $\Gamma(\cdot)$ denotes the Gamma function. The authors in [166] showed
that the Nakagami–$m$ distribution fits the empirical measurement better than
the Rayleigh distribution.

##### Loo Model (Rice+Log-Normal) distribution

Loo Model is composed of the Rician and log–normal distributions. The authors
in [167] showed that the Loo model fits the empirical data of A2G narrowband
channels in urban areas. The Loo model is given by

$\displaystyle f(y)=\frac{y}{\sigma^{2}\sqrt{2\pi\Sigma^{2}_{A}}}\times$
$\displaystyle\int^{\infty}_{a_{\textrm{LOS}}=0}\frac{1}{a_{\textrm{LOS}}}e^{\frac{-(20\log
a_{\textrm{LOS}}-M_{A})^{2}}{2\Sigma^{2}_{A}}}e^{\frac{-(y^{2}+a_{\textrm{LOS}}^{2})}{2\sigma^{2}}}I_{0}\left(\frac{y\>a_{\textrm{LOS}}}{\sigma^{2}}\right)da_{\textrm{LOS}},$
(23)

where $M_{A}$ and $\Sigma$ denote the mean and standard deviation of the
Gaussian distribution for the direct LOS signal, respectively, and
$a_{\textrm{LOS}}$ is the amplitude of LOS signal.

###  V-B HAPs Channel Modeling using RF Links

For HAPs channel modeling considering RF links, we present both large-scale
fading and small-scale fading.

####  V-B1 Large-Scale Path Loss

The large-scale fading of a communication between a HAP and a given receiver
located on the ground is mainly caused by free-space path loss. The path loss
equation is given by

$\mathrm{PL}_{\mathrm{Free\>Space}}={\left(\frac{c}{4\pi df}\right)}^{2},$
(24)

where $c$ is the speed of light and $f$ is the operating frequency. We recall
that $d$ is the distance between the transmitter and the receiver.

####  V-B2 Small-Scale Fading

##### Rayleigh

Although the Rayleigh distribution is not often used to characterize small-
scale fading for HAPs, some works have considered Rayleigh fading for HAPs
[168, 169]. For instance, the authors in [169] used a two-states channel model
with the assumption that when the channel is considered ”good”, the Rician
model is used to model the fading, whereas when the channel is considered
”bad”, the Rayleigh model is used.

##### Rician

The Rician model is commonly used in the literature to model small-scale
fading for HAPs [169, 11, 168].

##### Loo Model

In [170], the authors provided a multi-states model considering a Ka-band
channel for HAPs. First, they modeled the effect of tropospheric weather on
the amplitude of the transmitted signal as a Gaussian distribution defined by

$f_{w,r}(r)=\frac{1}{\sqrt{2\pi\sigma_{w,r}^{2}}}\exp\left(-\frac{\left(r-m_{w,r}\right)^{2}}{2\sigma_{w,r}^{2}}\right),$
(25)

where $m_{w,r}$ and $\sigma^{2}_{w,r}$ respectively denote the mean and
variance of the Gaussian distribution of amplitude.

Then, to model impairment related to the ground environment, they provided a
three-states model, with $S\in\\{s_{1},s_{2},s_{3}\\}$ defining the different
states. The first state, $s_{1}$, describes a LOS state, the second one,
$s_{2}$, describes a moderate shadowing state, and the last one, $s_{3}$,
describes a deep shadowing state. The Loo model for these three states is
given by

$\displaystyle f_{S,r}(r)=$
$\displaystyle\frac{8.686}{\sigma_{S}^{2}\Sigma_{dB,S}\sqrt{2\pi}}$
$\displaystyle\int_{0}^{\infty}\frac{1}{a_{\textrm{LOS}}}\>\exp\left(-\frac{\left(20\log(a_{\textrm{LOS}})-M_{dB,S}\right)^{2}}{2\Sigma_{dB,S}^{2}}\right)$
$\displaystyle\times\>\exp\left(-\frac{r^{2}+a_{\textrm{LOS}}^{2}}{2\sigma_{S}^{2}}\right)\>I_{0}\left(\frac{ra_{\textrm{LOS}}}{\sigma_{S}^{2}}\right)da_{\textrm{LOS}},$
(26)

where $a_{\textrm{LOS}}$ is the amplitude of LOS signal, and $M_{dB,S}$ and
$\Sigma_{dB,S}$ denote the mean and standard deviation, respectively, of the
log-normal distribution for the state $S$.

###  V-C Channel Modeling using FSO Links

An FSO signal undergoes different types of fluctuation and attenuation due to
atmospheric turbulence, pointing errors, path loss attenuation, and
atmospheric attenuation [171]. Atmospheric turbulence is caused by random
fluctuations of the refractive index, causing, in turn, fluctuation in the
intensity and phase of the received signal. Pointing errors arise from the
misalignment of the transmitter and the receiver. Finally, path loss and
atmospheric attenuation depend on the atmospheric and weather conditions, such
as, fog, rain, snow, and dust.

####  V-C1 Atmospheric Turbulence

The FSO signal undergoes atmospheric turbulence causing fluctuations in the
intensity and phase of the received signal. Several models are used to
characterize atmospheric turbulence for an FSO link. For instance, in weak-to-
moderate turbulence, the log-normal distribution is used [172]. In moderate-
to-strong turbulence, the Gamma-Gamma distribution [173] or the
$K$-distribution [174] is used. A negative exponential distribution is used
when the turbulence effect is very strong [175]. Finally, the Málaga
distribution, also called the $M$-distribution, embodies all the
aforementioned distributions [176]. Fig.39 shows the probability density
function (PDF) of different atmospheric turbulence distributions.

##### Log-Normal Distribution

Considering weak turbulence conditions, a log-normal distribution is used. The
PDF of the log-normal distribution modeling atmospheric turbulence, denoted by
$I_{a}$, is given by

$f_{I_{a}}(I_{a})=\frac{1}{2I_{a}\sqrt{2\pi\sigma^{2}}}\exp\left(-\frac{(\log(I_{a})+2\sigma^{2})^{2}}{8\sigma^{2}}\right),$
(27)

where $\sigma^{2}$ is the variance, which is given by

$\sigma^{2}=\frac{\sigma_{\textrm{R}}^{2}}{4}=0.3k^{\frac{7}{6}}C_{n}^{2}(h)d^{\frac{11}{6}},$
(28)

where $\sigma_{\textrm{R}}^{2}$ is the Rytov variance for a plane wave
propagation
($\sigma_{\textrm{R}}^{2}=1.23k^{\frac{7}{6}}C_{n}^{2}(h)d^{\frac{11}{6}}$),
$C_{n}^{2}(h)$ is the index of refraction structure parameter at an altitude
$h$, and $k=2\pi/\lambda$ is the optical wavenumber. The expression of
$C_{n}^{2}(h)$ is given by

$\displaystyle C_{n}^{2}(h)$
$\displaystyle=0.00594\left(\frac{v_{\textrm{wind}}}{27}\right)^{2}(10^{-5}h)^{10}$
$\displaystyle\times\exp\left[-\frac{h}{1000}\right]+2.7\times
10^{-16}\exp\left[-\frac{h}{1500}\right]+C_{n}^{2}(0),$ (29)

where $v_{\textrm{wind}}$ is the wind speed and $C_{n}^{2}(0)$ is the
structure constant at level ground.

##### Gamma-Gamma Distribution

For moderate-to-strong turbulence, the Gamma-Gamma fading is used to model
atmospheric turbulence. Hence, the PDF of the irradiance, $I_{a}$, is given by

$f_{I_{a}}(I_{a})=\frac{2(\alpha\beta)^{(\alpha+\beta)/2}}{\Gamma(\alpha)\Gamma(\beta)}I_{a}^{(\alpha+\beta)/2-1}K_{\alpha-\beta}(2\sqrt{\alpha\beta
I_{a}}),$ (30)

where $K_{v}(\cdot)$ is the modified Bessel function of the second kind and
order $v$. $\alpha$ and $\beta$ are the turbulence fading parameters, and
under the plane wave approximation, are expressed as follows:

$\alpha=\Bigg{[}\exp\Bigg{(}\frac{0.49\sigma_{R}^{2}}{{(1+1.11\sigma_{R}^{12/5})}^{7/6}}\Bigg{)}-1\Bigg{]}^{-1},$
(31)

$\beta=\Bigg{[}\exp\Bigg{(}\frac{0.51\sigma_{R}^{2}}{{(1+0.69\sigma_{R}^{12/5})}^{5/6}}\Bigg{)}-1\Bigg{]}^{-1}.$
(32)

(a) Log-normal distribution

(b) Gamma-Gamma distribution

(c) Negative exponential distribution

(d) $K$-distribution

Figure 39: Different atmospheric-turbulence distributions.

#####  $K$-Distribution

The $K$-distribution can be achieved as a multiplication of a Gamma
distribution and an exponential distribution. The PDF is therefore given by

$f_{I_{a}}(I_{a})=\frac{2\alpha}{\Gamma(\alpha)}{(\alpha
I_{a})}^{(\alpha-1)/2}K_{\alpha-1}(2\sqrt{\alpha I_{a}}).$ (33)

##### Negative Exponential Distribution

The negative exponential distribution is used to model strong turbulence
conditions. The PDF of its distribution is given as

$f_{I_{a}}(I_{a})=\frac{1}{I_{a}(0)}\exp(-I_{a}/I_{a}(0)),\>\>I_{a}(0)>0,$
(34)

where $\mathbb{E}[I_{a}]=I_{a}(0)$ is the mean of receiver optical irradiance.

##### Málaga Distribution ($M$-Distribution)

Hence, we employ an FSO link that experiences $M$ turbulence, for which the
PDF of the irradiance $I_{a}$ is given by

$f_{I_{a}}(I_{a})=A\sum_{m=1}^{\beta}a_{m}I_{a}K_{(\alpha-m)}\Bigg{(}2\sqrt{\frac{\alpha\beta
I_{a}}{g\beta+\Omega^{{}^{\prime}}}}\Bigg{)},I_{a}>0,$ (35)

where

$A\triangleq\frac{2\alpha^{\alpha/2}}{g^{1+\alpha/2}\Gamma(\alpha)}{\Bigg{(}\frac{g\beta}{g\beta+\Omega^{{}^{\prime}}}\Bigg{)}}^{\alpha+\beta/2},$
(36)

$a_{m}\triangleq\binom{\beta-1}{m-1}\frac{{(g\beta+\Omega^{{}^{\prime}})}^{1-m/2}}{(m-1)!}{\bigg{(}\frac{\Omega^{{}^{\prime}}}{g}\bigg{)}}^{m-1}{\bigg{(}\frac{\alpha}{\beta}\bigg{)}}^{m/2},$
(37)

where $g$ is the average power of the scattering component and
$\Omega^{{}^{\prime}}$ is the average power from the coherent contributions.

TABLE XII: Channel modeling for NTFPs considering RF and FSO links. RF Links |  FSO Links
---|---
LAPs Large-Scale Path Loss • LOS probability model [156] • Simplified LOS probability model [157] • Path loss exponent model [158] LAPs Small-Scale Fading • Rayleigh distribution [161, 160] • Rician distribution [162, 163, 164, 158] • Nakagami-$m$ distribution [160, 165, 166] • (Rice + Log-Normal) Loo Model [167] HAPs Large-Scale Path Loss • Free Space Path loss [11] HAPs Small-Scale Fading • Rayleigh distribution [168, 169] • Rician distribution [169, 11, 168] • Loo Model [170] |  Atmospheric Turbulence • Log-normal distribution [172] • Gamma-Gamma distribution [173] • $K$-distribution [174] • Negative exponential distribution [175] • Málaga distribution [176] Pointing Errors • Beckmann distribution [177] • Rayleigh distribution [178] • Rician distribution [179] • Hoyt distribution [180] Large-Scale Path Loss • Path loss [181] Atmospheric Attenuation • Fog and haze attenuation [182] • Rain attenuation [183] • Snow attenuation [183] • Dust attenuation [184]

####  V-C2 Pointing Errors

The pointing errors originate from a misalignment between the transmitter and
the receiver due to errors in tracking, vibrations in the system, or building
sway. We assume that a Gaussian beam, with a beamwidth denoted by $w_{z}$,
propagates in a photo-detector. We denote the radial displacement by
$r_{dep}$. Hence, the collected power at a given distance $z$ is approximated
by the following formula:

$I_{p}\approx A_{0}\exp\left(-\frac{2r_{dep}^{2}}{w_{z_{eq}}^{2}}\right),$
(38)

where we denote by $w_{z_{eq}}$ the equivalent beamwidth given by

$w_{z_{eq}}=w_{z}^{2}\frac{\sqrt{A_{0}\pi}}{2v\exp(-v^{2})},$ (39)

where $A_{0}={[\textrm{erf}(v)]}^{2}$ is the maximum fraction of the collected
power, and $v=\sqrt{\frac{a^{2}\pi}{w_{z}^{2}}}$ is the ratio between the
aperture radius denoted by $a$ and the beamwidth $w_{z}$.

The radial displacement at the receiver is expressed as
$\textrm{r}_{dep}=[x\>y]^{T}$, where $x$ and $y$ are the vertical and
horizontal displacement on the plane.

Consequently, the distribution of
$r_{dep}=|\textrm{r}_{dep}|=\sqrt{x^{2}+y^{2}}$ depends on the distribution of
$x$ and $y$. Assuming independent Gaussian displacements on the horizontal and
elevation axes, $r_{dep}$ can have several distributions depending on the
Gaussian parameters. For instance, several distributions are used in the
literature for modeling pointing errors, such as the Beckmann distribution
[177], Rayleigh distribution [178], Rician distribution [179], and Hoyt
distribution [180].

##### Beckmann distribution

When both the $x$ and $y$ displacements are nonzero mean Gaussian random
variables with $x\sim\mathcal{N}(\mu_{x},\sigma_{x})$ and
$y\sim\mathcal{N}(\mu_{y},\sigma_{y})$, then $r_{dep}$ follows a Beckmann
distribution with a PDF given by [177]

$\displaystyle f_{r_{dep}}(r_{dep})=\frac{r_{dep}}{2\pi\sigma_{x}\sigma_{y}}$
$\displaystyle\int_{0}^{2\pi}\exp\left(-\frac{(r_{dep}\cos(\theta)-\mu_{x})^{2}}{2\sigma_{x}^{2}}-\frac{(r_{dep}\sin(\theta)-\mu_{y})^{2}}{2\sigma_{y}^{2}}\right)d\theta.$
(40)

The $n$th moment of the pointing error effect, denoted by $I_{p}$, is given by

$\mathbb{E}[I_{p}^{n}]=\frac{A_{0}^{2}\xi_{x}\xi_{y}}{\sqrt{(n+\xi_{x}^{2})(n+\xi_{y}^{2})}}\exp\left(-\frac{2n}{w_{z_{eq}}^{2}}\left[\frac{\mu_{x}^{2}}{1+\frac{n}{\xi_{x}^{2}}}+\frac{\mu_{y}^{2}}{1+\frac{n}{\xi_{y}^{2}}}\right]\right),$
(41)

where $\xi_{x}=\frac{w_{z_{eq}}}{2\sigma_{x}}$ and
$\xi_{y}=\frac{w_{z_{eq}}}{2\sigma_{y}}$.

##### Rayleigh distribution

If both $x$ and $y$ have zero mean and the same variance, that is,
$\mu_{x}=\mu_{y}=0$ and $\sigma_{x}=\sigma_{y}=\sigma$, then $r_{dep}$ follows
a Rayleigh distribution with the PDF given by [178]

$f_{r_{dep}}(r_{dep})=\frac{r_{dep}}{\sigma^{2}}\exp\left(-\frac{r_{dep}^{2}}{2\sigma^{2}}\right),$
(42)

and the PDF of the pointing error $I_{p}$ given by

$f_{I_{p}}(I_{p})=\frac{\xi^{2}}{A_{0}^{\xi^{2}}}I_{p}^{\xi^{2}-1},$ (43)

where $\xi=\frac{w_{z_{eq}}}{2\sigma^{2}}$.

##### Rician distribution

In case both displacements have different non-zero means and the same
variance, that is, $\mu_{x}\neq\mu_{y}$ and $\sigma_{x}=\sigma_{y}=\sigma$,
then $r_{dep}$ follows a Rician distribution with the PDF given by [179]

$f_{r_{dep}}(r_{dep})=\frac{r_{dep}}{\sigma^{2}}\exp\left(-\frac{(r_{dep}^{2}+s^{2})}{2\sigma^{2}}\right)I_{0}\left(\frac{r_{dep}s}{\sigma^{2}}\right),$
(44)

where $s=\sqrt{\mu_{x}^{2}+\mu_{y}^{2}}$. The PDF of the pointing error
$I_{p}$ is given by

$f_{I_{p}}(I_{p})=\frac{\xi^{2}\exp(\frac{-s^{2}}{2\sigma^{2}})}{A_{0}^{\xi^{2}}}I_{p}^{\xi^{2}-1}I_{0}\left(\frac{s}{\sqrt{2}\sigma^{2}}\sqrt{-w_{z_{eq}}\log\left(\frac{I_{p}}{A_{0}}\right)}\right).$
(45)

##### Hoyt distribution

When $x$ and $y$ have zero mean and different variances, that is,
$\mu_{x}=\mu_{y}=0$ and $\sigma_{x}\neq\sigma_{y}$, then $r_{dep}$ follows a
Hoyt distribution with the PDF given by [180]

$f_{r_{dep}}(r_{dep})=\frac{r_{dep}}{q\sigma_{y}^{2}}\exp\left(-\frac{r_{dep}^{2}(1+q^{2})}{4q^{2}\sigma_{y}^{2}}\right)I_{0}\left(\frac{r_{dep}^{2}(1-q^{2})}{4q^{2}\sigma_{y}^{2}}\right),$
(46)

where $q=\frac{\sigma_{x}}{\sigma_{y}}=\frac{\xi_{x}}{\xi_{y}}$. The PDF of
the pointing error $I_{p}$ is given by

$f_{I_{p}}(I_{p})=\frac{\xi_{x}\xi_{y}}{A_{0}}{\bigg{(}\frac{I_{p}}{A_{0}}\bigg{)}}^{\frac{\xi_{x}^{2}(1+q^{2})}{2}-1}I_{0}\bigg{(}\frac{\xi_{x}^{2}(1-q^{2})}{2}\log\left(\frac{I_{p}}{A_{0}}\right)\bigg{)},$
(47)

where $0\leq I_{p}\leq A_{0}$.

####  V-C3 Atmospheric Attenuation

The FSO signal is affected by path loss, which depends on the distance between
the transmitter and receiver, but also on the atmospheric conditions given by
[181]

$I_{Att}=\frac{A_{r}}{(\Theta d)^{2}}\textrm{Att},$ (48)

where $A_{r}$ is the receiver effective area and $\Theta$ is the beam
divergence. The parameter
$\textrm{Att}\in\\{\textrm{Att}_{\textrm{fog}},\textrm{Att}_{\textrm{rain}},\textrm{Att}_{\textrm{snow}},\textrm{Att}_{\textrm{dust}}\\}$
represents the atmospheric attenuation, and it depends on atmospheric
conditions, such as fog, rain, snow, and dust, denoted by
$\textrm{Att}_{\textrm{fog}}$ [182], $\textrm{Att}_{\textrm{rain}}$ [183],
$\textrm{Att}_{\textrm{snow}}$ [183], and $\textrm{Att}_{\textrm{dust}}$
[184], respectively.

##  VI Conclusion

In this survey, we provided an extensive and comprehensive overview of NTFPs.
We provided a general overview of this solution for all readers interested
irrespective of their background by reviewing all the existing types of NTFPs,
including their components, their characteristics, their applications,
advantages, challenges, and regulations. We also have detailed some case
studies that have been used and the major companies related to NTFPs. Then, we
addressed platforms from a wireless communications perspective. We carried out
different type of analysis: geometrical analysis, performance, and economic
analysis. Finally, we proved comprehensive channel modeling for NTFPs and
free-flying platforms for different altitudes and type of links. In this
survey, we highlighted the effectiveness of NTFPs as a future solution
solution for 6G.

##  VII Acknowledgment

Figure 21 and Figure 23 were created by Heno Hwang, scientific illustrator at
King Abdullah University of Science and Technology (KAUST).

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