# Beyond Line of Sight Defense Communication Systems: Recent Advances and
Future Challenges

Ruhul Amin Khalil, , Muhammad Haris, and Nasir Saeed Ruhul Amin Khalil and
Nasir Saeed are with the Department of Electrical and Communication
Engineering, UAE University, Al-Ain 15551, UAE e-mail<EMAIL_ADDRESS><EMAIL_ADDRESS>Haris is with the Department of Electrical
Engineering, Hanyang University, Ansan 15588, South Korea (e-mail:
haris737@hanyang.ac.kr).

###### Abstract

Beyond Line of Sight (BLOS) communication stands as an indispensable element
within defense communication strategies, facilitating information exchange in
scenarios where traditional Line of Sight (LOS) methodologies encounter
obstruction. This article delves into the forefront of technologies driving
BLOS communication, emphasizing advanced systems like phantom networks,
nanonetworks, aerial relays, and satellite-based defense communication.
Moreover, we present a practical use case of UAV path planning using
optimization techniques amidst radar-threat war zones that add concrete
relevance, underscoring the tangible applications of BLOS defense
communication systems. Additionally, we present several future research
directions for BLOS communication in defense systems, such as resilience
enhancement, the integration of heterogeneous networks, management of
contested spectrums, advancements in multimedia communication, adaptive
methodologies, and the burgeoning domain of the Internet of Military Things
(IoMT). This exploration of BLOS technologies and their applications lays the
groundwork for synergistic collaboration between industry and academia,
fostering innovation in defense communication paradigms.

###### Index Terms:

Defense Communication systems, beyond line of sight, aerial relays, phantom
networks, Internet of Military Things

##  I Introduction

The global defense communication system market is poised for robust expansion,
forecasted to reach a valuation of US $46.7 billion in 2023 and expected to
soar to an impressive US $120.62 billion by 2033, exhibiting a substantial
Compound Annual Growth Rate (CAGR) of 9.9% [1]. Comprising nearly a quarter of
the global aerospace and defense market, this sector experiences heightened
demand, owing to its reliance on commercial wireless technologies and an
escalating need for satellite communication equipment. Regionally, North
America commands market dominance with a 36.5% share in 2022, driven by
substantial national defense expenditures. Europe, notably the United Kingdom,
France, and Germany, account for a significant 24.8% market share. The Asia
Pacific region, contributing 28% in 2022, displays promising growth due to
substantial government investments in military and defense equipment [1].
While exhibiting slower growth, the Middle East and Africa are anticipated to
show improvement by the forecast’s end. Noteworthy efforts in modernizing
defense communication systems include the United States’ substantial
investments in new handheld man-pack radios. With its robust industrial base,
Germany stands prominently in Europe’s military communication market.
Simultaneously, China’s marine communication systems industry foresees
considerable expansion, fueled by coastal security demands and the growth of
marine commerce operations [1]. In a categorical breakdown, the sale of
defense communication satellites exhibits a notable uptick, propelled by
escalating tensions along international borders. The market’s future
trajectory hinges on strategic collaborations, continuous technological
innovations, and the integration of the Internet of Things (IoT) [2].
Manufacturers have substantial opportunities for scaling their businesses by
harnessing advanced communication systems, low-latency wireless links, and IoT
integration, presenting a dynamic landscape for growth in the defense
communication sector.

Communication systems are a linchpin within military operations, enabling
seamless information exchange among diverse units within the armed forces.
Effective communication transcends mere necessity in the intricate threatening
of defense; it assumes the mantle of a strategic imperative, wielding profound
influence over the success of tactical missions and the overarching
operational readiness [3]. This specialized communication domain grapples with
challenges far beyond those encountered in civilian networks, confronting
distinctive obstacles such as Electronic Warfare (EW), Command and Control
Warfare (C2W), and the difficulties inherent in beyond-line-of-sight (BLOS)
communication, as depicted in Figure 1. The complexities inherent in defense
communication amplify its strategic significance as the core enabling
coordinated military action in the face of evolving threats and operational
landscapes. Moreover, the multifaceted nature of military forces, spanning
Ground, Water, and Air domains, introduces an additional stratum of complexity
to defense communication. Serving as the sinew-binding coordinated military
action, it equips forces to adapt, strategize, and prevail in the face of
diverse, ever-evolving threats [4]. Defense communication, thus, emerges as
the quintessential backbone, empowering troops to navigate the complexities of
modern warfare and emerge triumphant amidst the dynamism of global defense
landscapes.

Figure 1: Illustraion of a BLOS defense communication system.

Recently, five significant trends have reshaped the landscape of defense
communications. First, space technologies are elevated in defense operations,
where space-based connectivity is pivotal for various defense applications [5,
6]. The military’s investments prioritize purpose-built and resilient space
systems functioning across diverse transports and orbits, envisioning space as
a potential frontline in emerging conflicts. Secondly, the U.S. Department of
Defense (DoD) increasingly embraces multi-constellation, multi-transport, and
software-defined networks, with a focus on low earth orbit (LEO) systems for
their resilience and low latency [7]. A future outlook envisions a fusion of
LEO and geostationary satellites orchestrated via software-defined networking.
Thirdly, pivotal efforts center around developing Joint All-Domain Command and
Control (JADC), Project Convergence, and Project Overmatch. The objective is
establishing a unified command and control system spanning diverse military
branches, emphasizing collaboration with industry stakeholders. The fourth
trend underscores the escalating reliance on commercial technology within
defense networks, integrating commercial innovations and off-the-shelf
products into space-based networks to bolster resilience [7]. Finally,
standalone 5G deployments tailored for the DoD offer high-speed, secure, and
private wireless communication at facilities, featuring a zero-trust security
architecture and the capability to run edge applications employing artificial
intelligence and machine learning. These trends align with strategic
objectives to address end-user needs, foster collaboration, and explore
commercial technologies to enhance defense communication capabilities.

While defense communication has attracted considerable scholarly focus,
investigations span cognitive radios [8], blockchain-based privacy
preservation [9], dynamic spectrum anti-jamming communication [10], Internet
of Radars, joint Radar communication [11], and BLOS phantom networks [12].
However, to the best of the author’s knowledge, a comprehensive examination of
BLOS communication within defense communication systems is absent in existing
literature. Therefore, this paper aims to bridge this gap by delving into
state-of-the-art technologies and strategies explicitly tailored for BLOS
scenarios in military contexts.

The remainder of this paper is organized as follows. First, we present state-
of-the-art technologies in BLOS defense communication, encompassing phantom
networks, nanonetworks, satellite networks, and aerial relays. Following this,
a comprehensive case study illustrating UAVs for BLOS route planning in
defense communication is presented. Subsequently, emerging challenges and
future research directions are delineated, culminating in the conclusive
section of this article.

##  II Cutting Edge Technologies for BLOS Defense Communication Systems

Defense communication encompasses critical components such as military radios,
C2W, EW, secure transmission, and alert measurement systems. Direct wave
propagation, reliant on an unobstructed line of sight (LOS) between the Source
(Transmitter) and Destination (Receiver), stands as an optimal solution for
information exchange where frequencies below approximately 50 MHz favor
surface or ground wave propagation [13]. Nevertheless, higher frequencies face
significant attenuation, necessitating ducting for signal propagation across
substantial distances within two Troposphere layers. Moreover, LOS
communication effectiveness can be hindered by obstacles like the Earth’s
curvature and natural or man-made impediments, requiring the adoption of BLOS
communication techniques [3]. Military domains extensively rely on various
BLOS techniques, including phantom networks, nanonetworks, satellite networks,
and aerial relays. Phantom networks provide an on-demand aerial communication
method among out-of-range devices, while nanonetworks comprise nano nodes
within atomized aqueous mists, forming the basis of phantom networks.
Historically, balloons, drones, and unmanned aerial vehicles (UAVs) facilitate
BLOS communication. Despite their utility, these BLOS techniques encounter
challenges. Table I offers a comparative overview of BLOS techniques,
highlighting key features and associated challenges.

TABLE I: Various BLOS communication networks and challenges. BLOS Networks |  Applications |  Challenges
---|---|---
Phantom Networks |  Facilitates seamless communication between military units in urban environments with complex structures to navigate obstacles in BLOS scenarios |  Dynamic topology management with rapidly changing terrains and scenarios, such as urban environments, where network topology needs continuous adjustments for optimal communication
Nanonetworks |  Provides covert surveillance in hostile BLOS environments, enabling discreet information gathering without compromising operational security |  Limited energy to sustain continuous communication in resource-limited BLOS environments
Satellite-based Networks |  Supports worldwide military operations by ensuring continuous communication and data exchange, even in remote or contested areas |  High time delay due to large transmission distance, impacting real-time communication in critical military operations
Aerial Relays |  Provides rapid deployment scenarios for quick and flexible communication setups and provides BLOS connectivity during fast-paced operations |  Suffers from communication disruptions caused by atmospheric interference or potential vulnerability due to electronic warfare

###  II-A Phantom Networks

Phantom networks represent an innovative cornerstone in BLOS communication
strategies for defense and military applications, signaling a paradigm shift
with on-demand, intangible aerial connectivity crucial where traditional LOS
communication falls short [12]. Their adaptability and responsiveness prove
indispensable in dynamic and challenging military environments, fortifying
communication effectiveness and enhancing the versatility of military radio
systems. For instance, phantom MESH combines state-of-the-art communication
and advanced jamming technologies, employing MESH radios, a token passing-
based mobile ad-hoc network [14]. This self-healing network architecture
establishes robust RF paths in challenging environments, crucial for
operations beyond direct visual or radio contact. The iMESH suite within this
solution, including iMESH KRIP and iHIVE, ensures flexibility, security, and
rapid deployability. Other phantom technologies, like the Eagle 108 Drone
Jammer and EW1600 military tactical satellite communication (SATCOM) jammer,
neutralize threats from unauthorized drones, ensuring safety and security in
BLOS scenarios. Blu Wireless introduces PhantomBlu, a groundbreaking V-Band
radio node optimized for BLOS military communications, featuring dynamic mesh
networking capabilities with multi-gigabit data rates suitable for diverse
BLOS scenarios [15]. This system efficiently processes high-bandwidth sensor
data and video at the tactical edge, providing covert and resilient
communication for ground forces operating in BLOS environments. PhantomBlu’s
ongoing advancements aim to include W-band transceivers, aligning it with IEEE
6G standards and positioning it as a versatile BLOS solution for near-peer
tactical operations.

###  II-B Nanonetworks

Nanonetworks constitute a pioneering technological foundation in BLOS
communication for defense and military applications [16]. Operating at the
nanoscale, these networks employ nano nodes suspended within atomized aqueous
mists, providing essential sophistication for robust BLOS communication. Their
intricacies offer a nuanced solution for surmounting environmental obstacles
in dynamic operational theatres [17]. Their nanoscale components significantly
enhance flexibility and adaptability, enabling effective communication in
scenarios where traditional methods falter [18]. Moreover, nanotechnology’s
potential impact on BLOS military applications extends to medical care and
materials advancements. Specifically tailored for BLOS scenarios, military
nanotechnology focuses on improving body Armor effectiveness. Technologies
such as Silica $S_{i}$, Titanium dioxide $T_{i}O_{2}$, and Silicon dioxide
$S_{i}O_{2}$ nanoparticles. These nanoparticles form the nanodevices that aim
to offer soldiers lightweight yet resilient protection in situations where
direct visual or radio contact is obstructed [19]. Furthermore, nanotechnology
advances sensors, presenting smaller, more sensitive alternatives critical for
BLOS operations. Examples include highly sensitive infrared thermal sensors,
compact accelerometers with GPS for motion and position sensing, miniature
high-performance camera systems, and biochemical sensors, illustrating the
adaptability of nanomaterials in augmenting military sensor capabilities in
BLOS environments. Integrating nanotechnology into BLOS robotic systems and
control mechanisms further enhances efficiency, emphasizing nanotechnology’s
transformative influence on BLOS military technologies.

###  II-C Satellites-based BLOS Communication

Safeguarding communication across vast and challenging terrains, satellite-
based defense communication stands as a linchpin in BLOS operations, ensuring
continuous connectivity during critical military missions and protecting
sensitive data through encrypted transmission [20, 21]. However, persistent
challenges like signal latency and susceptibility to electronic warfare
underscore the need for ongoing technological advancements. Key innovations,
including emerging LEO constellations and adaptive communication protocols,
are poised to augment the resilience and effectiveness of modern military
communication strategies [22]. The Defense Satellite Communication System
(DSCS), serving as a critical component of global satellite communications for
the U.S. military, comprises high-capacity military satellites supporting
diverse defense entities with its six-channel transponder system [23]. This
constellation, known for its secure, nuclear-hardened, anti-jam, and high-
data-rate communication, is integral for long-haul communications in contested
environments. Other satellite-based defense communication networks such as
SATCOMBw in Germany, Syracuse IV in France, and Skynet in the U.K. serve as
critical communication relays in BLOS operations, ensuring uninterrupted
connectivity in military scenarios [24]. These systems employ encrypted data
transmission protocols, ensuring the confidentiality and integrity of defense
communications. Ongoing advancements are targeted to address challenges such
as signal latency and susceptibility to electronic warfare, aiming to bolster
the effectiveness and resilience of modern military communication strategies.

###  II-D Aerial Relays

This BLOS communication technology involves both manned aircraft and UAVs
serving as airborne relays, establishing and maintaining communication links
over extended distances [25]. Manned aircraft with advanced communication
systems act as crucial nodes in military communication networks, enhancing
versatility and coverage in BLOS communication. Meanwhile, UAVs present a
dynamic and adaptable solution, adept at navigating challenging environments
and swiftly deploying to establish communication links in contested airspace
[26]. Using tethered drones, tethered balloons, and floating balloons further
diversifies the aerial relay landscape, offering unique solutions for
sustained communication in diverse operational scenarios [27]. These aerial
relays strategically overcome LOS limitations, ensuring continuous and
resilient communication support for military operations. The following
discusses prospective aerial relays for BLOS communication in defense
communication systems.

####  II-D1 Manned Aircrafts:

Manned aircraft are pivotal and multifaceted in BLOS communication within
defense and military applications. Equipped with advanced communication
systems, these aircraft act as airborne relays, forming crucial links over
extended distances. Their versatility goes beyond conventional communication,
enabling strategic deployment across diverse operational scenarios. These
aircraft offer unparalleled coverage and adaptability, functioning as vital
nodes within military communication networks. Beyond their role as relays,
they contribute significantly to intelligence, surveillance, and
reconnaissance (ISR) efforts, elevating the overall situational awareness of
military operations. The human-operated nature of manned aircraft introduces
dynamic decision-making, allowing real-time adjustments to communication
strategies in response to the evolving needs of complex military environments.
Manned aircraft are effective conduits for BLOS communication and stand as
cornerstones in comprehensively integrating communication, surveillance, and
strategic capabilities within defense operations.

####  II-D2 Unmanned Aerial Vehicles (UAVs):

Heralded as transformative assets in modern military strategies, UAVs
epitomize adaptability and innovation in BLOS communication. These unmanned
systems, purpose-built for navigating challenging environments, contribute
significantly to the military’s communication capabilities where traditional
methods prove impractical. UAVs bring unparalleled flexibility to BLOS
scenarios, rapidly deploying to establish communication links in contested
airspace, inaccessible terrains, or high-risk zones [28, 29]. Beyond their
role as communication relays, UAVs contribute extensively to intelligence
gathering and surveillance, enhancing situational awareness in dynamic
military landscapes. The autonomous nature of UAVs allows for swift and agile
responses to changing operational demands, showcasing their pivotal role in
real-time decision-making processes. As integral components of military BLOS
strategies, UAVs bolster communication resilience and embody a transformative
force in enhancing the agility and effectiveness of defense operations in an
evolving and complex security landscape.

####  II-D3 Tethered Drones:

Tethered Drones emerge as a distinctive and versatile solution within BLOS
communication for defense and military applications. These drones, tethered to
a fixed point, present a unique capability for sustained communication, making
them suitable for prolonged surveillance and communication relay tasks.
Tethering enables prolonged flight duration and ensures a stable platform,
contributing to persistent and reliable communication support. In military
scenarios, tethered drones offer adaptability in various operational contexts,
providing a dynamic vantage point for surveillance and communication even in
challenging environments. Their ability to hover at specific altitudes for
extended periods makes them valuable assets for maintaining continuous
communication links in complex, dynamic military theatres. Beyond their role
in communication, tethered drones also contribute to intelligence gathering,
surveillance, and monitoring, showcasing their multifaceted utility in
enhancing military capabilities beyond BLOS communication resilience. The
strategic use of tethered drones underscores their significance as agile and
enduring tools in modern defense communication strategies.

####  II-D4 Tethered Balloons:

Tethered Balloons present a distinctive and strategically valuable solution
within BLOS communication for defense and military applications. Tethered
securely to the ground, these balloons serve as stable, elevated platforms for
improved signal propagation. Tethering provides a cost-effective solution and
ensures enduring and consistent communication links over expansive areas. In
military applications, tethered balloons can be strategically positioned to
enhance coverage and connectivity in specific operational zones, offering a
dynamic and adaptable means of establishing communication in challenging
terrains. Their stability, adaptability, and capacity to operate at various
altitudes make them valuable assets for maintaining communication resilience
in diverse and dynamic military environments. Beyond their role in
communication, tethered balloons contribute to intelligence, surveillance, and
reconnaissance efforts, showcasing their versatility as integral components of
military operations. With their enduring presence and strategic positioning,
the tethered balloons exemplify a nuanced approach to enhancing BLOS
communication capabilities, providing a reliable and comprehensive solution in
modern defense scenarios.

####  II-D5 Floating Balloons:

Floating Balloons, equipped with communication payloads, represent an
innovative and dynamic solution within the realm of BLOS communication for
defense and military applications. These balloons traverse the skies at high
altitudes, offering a mobile and adaptable platform for maintaining
communication links in challenging terrains. Their mobility and ability to
operate at varying altitudes render floating balloons effective tools for
ensuring continuous communication coverage across diverse operational
scenarios. In military applications, these balloons contribute significantly
to establishing resilient and expansive BLOS communication networks,
particularly in remote or inaccessible areas where traditional methods may
fall short. The innovative approach of floating balloons aligns with the
evolving demands of modern warfare, offering a strategic advantage by
providing flexible and robust communication capabilities. Beyond their role in
communication, floating balloons contribute to intelligence gathering,
reconnaissance, and surveillance efforts, showcasing their multifaceted
utility in enhancing military capabilities beyond BLOS communication
resilience. The strategic deployment of floating balloons underscores their
significance as forward-looking and versatile assets in the ever-evolving
landscape of defense communication strategies.

##  III Case Study: UAVs for BLOS Route Planning in Defense Communication
Systems

As discussed in the previous section, in defense communication systems, UAVs
play a crucial role in addressing BLOS challenges where various scenarios are
marked by obstructed direct visual or radio contact, demanding innovative
solutions for establishing robust communication links. As agile and adaptable
platforms, UAVs navigate complex terrains and operate amidst adversarial
threats, overcoming hurdles in BLOS environments [30]. Strategic deployment of
UAVs for BLOS route planning becomes essential, ensuring optimized paths
through intricate operational environments to enhance communication
reliability and mission success. Integrating cutting-edge technologies like
path optimization techniques into UAV-based BLOS route planning can further
enhance the efficacy [31]. Path planning for UAVs in challenging environments
presents a real-world optimization challenge, requiring an optimal planner for
efficient navigation. It involves conceptualizing path-planning missions as
optimization problems, where evolutionary computation emerges as an efficient
method for addressing such challenges. Nature-inspired optimization techniques
such as particle swarm optimization (PSO) can dynamically optimize UAV flight
paths, which is crucial in navigating radar-laden war zones. This integration
fortifies defense communication systems, enhancing agility, responsiveness,
and security, enabling seamless communication links across challenging
distances, significantly contributing to mission success [32]. This section
focuses on UAV path planning in radar-threat war zones, framing it as an
optimization problem.

We focus on path planning for a UAV within a challenging radar-limited
environment, where deterministic search algorithms prove impractical due to
vast search space complexities. The presence of regularly positioned circular
radar threats, confined within predetermined global limits, further
complicates the optimization process. The approach prioritizes optimizing UAV
performance within a predefined 3D terrain environment. Efficient path
planning for UAVs in military operations, between start and target points, is
crucial for reduced time and enhanced mission effectiveness. The planning
process involves three interlinked phases: (1) establishing cost and
constraint functions, (2) aligning the digital map with the mission
environment, and (3) optimizing and refining the flight route. In combat
scenarios, UAV navigation encounters challenges from diverse danger sources
such as topography, radars, artillery, and no-fly zones. Considering the UAV’s
physical constraints, the planned route must navigate these obstacles to
ensure safe traversal and collision avoidance. Addressing the multitude of
objectives and constraints arising from various risks and limitations presents
challenges in UAV route planning [33]. Developing an effective planner is
crucial to handling the complexities of multi-objective limited UAV route
planning, particularly in intricate situations, as shown in Fig. 2.

Figure 2: An Illustration of UAVs path planning in military operations.

In UAV path planning, PSO significantly optimizes each particle’s path within
predefined constraints and initial velocities. Despite their inclination to
move towards their velocity vectors, particles in PSO abide by swarm
constraints, exchanging information and retaining their previous best position
alongside their current position and velocity. The swarm’s overall best
particle, known as the global best, emerges in PSO iterations. PSO’s iterative
process refines particle positions by updating their location and velocity
[34]. During path optimization, particles use vectors like their previous
velocity, the global best position, and the personal best position to
determine optimal routes. In the context of UAV path planning, this
optimization method guides UAVs through complex environments, facilitating
efficient navigation towards mission objectives. Leveraging PSO’s ability to
find optimal solutions within challenging and dynamic scenarios, this approach
assists UAVs in identifying safe and effective routes amidst diverse obstacles
and constraints.

Figure 3:  UAV route planning in different scenarios (a) low complexity (b)
medium complexity (c) high complexity.

Utilizing the PSO algorithm, we formulated optimal flight paths for UAVs
navigating through combat zones, particularly in the presence of radar systems
and ground-based artillery. Radar systems are critical in locating and
tracking airspace objects, making evading radar detection imperative for
mission success and security in hostile environments. Additionally, ground-
based artillery poses a significant threat to UAVs. Addressing these ground-
based threats requires navigation strategies considering not just altitude but
also the dynamic nature of the threats, demanding agile and adaptable flight
paths. Effectively evading both hostile radar detection and ground-based
threats, like artillery, requires a comprehensive strategy that recognizes the
distinct challenges posed by each threat source. Our proposed UAV’s trajectory
routing is designed to maintain a consistent altitude above sea level,
simplifying the navigation model and establishing a reliable benchmark for the
UAV’s flight trajectory. In contrast, ground-based navigation often involves
adaptations to the terrain and on-ground obstacles, necessitating deviations
from the UAV’s consistent altitude approach. The UAV’s route planning based on
the PSO algorithm is depicted in Fig. 3, where a small red circle represents
the initial UAV position, while a blue circle denotes the target location.
Large circles indicate radar threats, while small circles represent artillery
threats. The black dotted line illustrates the path generated by the PSO
algorithm. The scenarios varied in complexity, giving unique navigation
problems. We assume the UAV has prior knowledge of the target location and
aims to navigate while evading various threats to reach its designated
objective. Three scenarios with varying complexity levels have been
introduced, each presenting different configurations of radar and artillery
threats. Higher complexity levels entail more intricate radar and artillery
threats distributed sparsely along the trajectory from the starting point to
the goal. We also illustrate the convergence rate concerning the path length
for all three scenarios in Fig. 4.

Figure 4: Convergence curve of 3 different scenarios

##  IV Challenges and Future Research Directions

The evolving landscape of modern warfare demands communication systems adept
at maneuvering through diverse networks, managing contested electromagnetic
spectrums, and meeting escalating requirements for multimedia-intensive
communications. Integrating unmanned autonomous systems and diverse sensors in
military operations elevates the importance of adaptive communication
strategies, particularly in BLOS scenarios. This section delves into the
cutting-edge challenges unique to BLOS contexts, offering valuable insights
into the future of military communication technologies and their pivotal role
in shaping successful defense operations, especially under conditions where
direct visual or radio contact is limited.

###  IV-A Resilience in Harsh Infrastructure Environments

Defense communication systems, operating across diverse and dispersed
environments, encounter infrastructure challenges distinct from those in
established commercial networks [35]. The adaptability of defense
communication systems becomes paramount for ensuring reliable and secure
connectivity in unpredictable operational scenarios, spanning various
terrains, from dense urban landscapes to remote, austere regions. Effectively
addressing these BLOS infrastructure challenges underscores the need for
strategic approaches to optimize network performance and enhance resilience
across dynamic landscapes. An emerging frontier for BLOS research involves
exploring the evolving role of space-based systems within critical
infrastructure frameworks, offering pivotal opportunities for future military
operations [36]. Understanding and harnessing the potential of BLOS space-
based systems could revolutionize the planning and execution of military
endeavors. The reliability of these systems, integral to most BLOS military
operations, highlights the interdependence between BLOS satellite control and
underground systems [36]. Future research should explore the intricate
relationship between national security and safeguarding critical
infrastructure. Considering the persistent evolution of threats driven by
technological advancements, innovative approaches, including adaptations to
federal regulations and proactive anticipation of potential crisis scenarios,
should be explored to fortify this essential connection.

###  IV-B Heterogeneous Networks for BLOS Communication

Heterogeneous networks are crucial in BLOS defense communication systems,
offering a unified infrastructure that seamlessly integrates diverse
technologies, standards, and devices [37]. This approach is instrumental in
tackling challenges arising from temporal and spatial variability during
military operations, providing adaptive solutions that dynamically adjust to
changing conditions, and ensuring continuous communication [38, 39]. The
increasing integration of Commercial Off-The-Shelf (COTS) technologies in
military communications, driven by advantages like simplified procurement,
rapid deployment, and cost-effectiveness, necessitates meticulous
customization to adhere to stringent security and reliability standards.
Tactical communications, marked by the coexistence of COTS-derived solutions
and military standards, require precise planning and integration testing for
heterogeneous networks in BLOS communication scenarios. For instance, the
Italian Navy’s research center, Selex ES, and ELMAN performed a collaborative
effort to develop a modular testbed that facilitates swift integration of
diverse technologies, providing a customizable environment for various test
campaigns, particularly relevant to maritime and littoral operations [40].
These initiatives highlight the broad applications of heterogeneous networks
in military contexts, addressing challenges in experimentation, security,
routing architectures, energy efficiency in sensor networks, and optimization
of UAV coalition-based networks for surveillance and spectrum access [41].
Together, these efforts underscore the pivotal role of heterogeneous networks
in meeting the intricate communication requirements of modern military
operations in BLOS scenarios.

###  IV-C Contested Electromagnetic Spectrum

Military communications encounter the challenges of navigating contested
electromagnetic environments, where adaptability remains pivotal for
sustaining BLOS link performance and resilience against vulnerabilities [42,
43]. BLOS considerations gain significance as military operations extend
beyond direct visual or radio contact. This strategic shift aims to secure
freedom of action in the contested electromagnetic spectrum while recognizing
the limitations of BLOS in seeking global supremacy through rapid
technological advancements. In the contemporary global landscape,
characterized by widespread access to high-end electronics technology, the
Defense Science Board conducted a comprehensive investigation into the U.S.
military’s operational capabilities within complex electromagnetic
environments [44]. This investigation revealed significant deficiencies in
operational support across diverse mission areas, highlighting potential
limitations against near-peer and regional powers in terms of sensing,
communication, networking, and synchronizing operations. The electromagnetic
spectrum has emerged as a strategic domain, interlinking land, air, naval,
space, and cyber domains. While Western armed forces historically maintained
electromagnetic superiority, the evolving landscape challenges this dominance
with its increasingly complex and congested spectrum. Competitors such as
Russia, China, and non-state actors leverage advancing technology, threatening
Western sovereignty in this domain. Addressing this challenge requires a
pragmatic approach emphasizing issues like agile spectrum management, effects
concentration, and subsidiarity at the tactical level to establish
electromagnetic local superiority within specific space-time frameworks.

###  IV-D Increasing Need for Multimedia-Intensive Communication

In contemporary military operations, the scope of BLOS communication surpasses
traditional voice and data transmission to encompass highly multimedia-
intensive capabilities, including video and broadband. This evolution mirrors
the dynamic nature of 21st-century military competencies, where soldiers are
required to embody essential attributes (Be), possess knowledge (Know), and
execute actions (Do) for success in diverse operational environments [45].
Addressing the challenges of flexible, rapidly deployable communication
systems within the broader spectrum of BLOS military communication also
acknowledges the need for multimedia-intensive capabilities. The Defense
Advanced Research Projects Agency (DARPA) underscores the imperative of
untethered communications, advocating for research and development efforts to
meet the Department of Defense’s requirements for security, interoperability,
and flexibility [46]. 5G and beyond technologies can be investigated to
provide real-time decision support in the future defense landscape, enabling
hyper-converged connectivity and secure data networks. Moreover, it can
facilitate the transfer of massive amounts of data, providing instant
situational awareness and enhancing training and battlefield capabilities.

###  IV-E Adapting to Changing Requirements

Integrating unmanned autonomous systems and diverse sensors poses dynamic and
evolving requirements, demanding high agility and adaptability from military
communication systems. These evolving needs are pivotal for ensuring effective
connectivity in BLOS communication amid rapidly changing operational
landscapes. While military establishments were once a smaller part of the
wireless communications market, the current commercial sector significantly
shapes technology development [47]. However, crucial disparities persist,
including the necessity for BLOS communication in areas lacking well-
maintained infrastructure, the integration of multi-hop networks, and the
operation of multiple heterogeneous networks within the same area. Bridging
these diverse networks, especially in contested electromagnetic environments,
remains a formidable challenge [48]. As the military grapples with evolving
operational landscapes, fostering a new ethic for analysis and analysts and
emphasizing cognitive engineering principles becomes paramount in ensuring the
adaptability and effectiveness of BLOS military communication strategies.

###  IV-F Towards the Internet of Military Things (IoMT)

In the realm of defense systems, the future lies in a unified network powered
by the Internet of Military Things (IoMT). This interconnected system brings
together various assets—ships, aircraft, ground vehicles, drones, and
personnel—enabled by edge computing, AI, and 5G technologies. It enhances
command structures, allowing instant decisions in real-time scenarios [49].
BLOS communication is pivotal in shaping the future of IoMT, which unifies
three key domains: physical data generation from sensors and observers,
information transmission and storage, and cognitive data processing. This
interconnected network supports various military functions like troop command,
weapon systems management, surveillance, and medical care. However, ensuring
robust device and network security remains a significant challenge, along with
addressing the battlefield’s connectivity, interoperability, and power
requirements. Overcoming these hurdles is crucial for the future of military
operations and underscores the importance of BLOS communication.

##  V Conclusion

This paper explores BLOS communication’s crucial role in defense, particularly
where traditional Line-of-Sight methods fall short. To underscore BLOS
capabilities, we’ve examined advanced technologies like phantom networks,
nanonetworks, aerial relays, and satellite defense communication. The
practical UAV path planning case study in artillery environments solidifies
these concepts, demonstrating the tangible impact of BLOS defense systems in
real scenarios. Moreover, the paper also highlights several future research
directions, including resilience enhancement, the integration of heterogeneous
networks, management of contested spectrums, advancements in multimedia
communication, adaptive methodologies, and the burgeoning domain of the
Internet of Military Things (IoMT). In short, this research holds significance
for academia and industry, opening paths for optimizing BLOS communication
systems and guiding technological advancements with a focus on security and
cyber resilience. It showcases practical BLOS applications in military
operations, encouraging collaborations among defense agencies, tech
developers, and academia. This resonates with the Sustainable Development
Goals, fostering peace, security, and technological innovation while bridging
academic and industrial frontiers.

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