Aboveground Wireless Links Overview

• Aboveground wireless links are communication channels using free-space propagation across various frequency bands to connect aerial and ground nodes.
• They rely on propagation models such as free-space path loss, two-ray reflection, and empirical shadow fading to capture environmental effects and guide network design.
• Advanced aerial architectures utilizing UAVs, HAPs, and integrated multi-plane networks enhance throughput, reliability, and spatial coverage in dynamic settings.

Aboveground wireless links refer to communication channels established through free-space propagation between nodes that are physically located above ground level. This encompasses radio, microwave, millimeter-wave (mmWave), terahertz (THz), and optical wireless links between various combinations of ground terminals, aerial nodes (e.g., unmanned aerial vehicles [UAVs], high-altitude platforms [HAPs], or balloons), rooftop/lamppost fixed stations, and—within indoor environments—elevated transceivers such as those found in data centers. Aboveground links are distinguished by their complete or partial exposure to the environment, resulting in propagation mechanisms and impairments distinct from both buried (underground) and in-building links.

1. Fundamental Propagation Models for Aboveground Links

Aboveground propagation is dominated by direct line-of-sight (LoS), diffraction, reflection (notably ground and building reflections), and, at higher frequencies, blockage and scattering. The canonical models include:

• Free-Space Path Loss (FSPL): For unobstructed scenarios:

$$PL_{FSPL}(d) = 20\log_{10}\left(\frac{4\pi d f_c}{c}\right)$$

where $d$ is slant distance, $f_c$ is carrier frequency, and $c$ is the speed of light (Shayegan, 2024, Baldansa et al., 2022).

• Two-Ray Ground Reflection Model: Critical for low-altitude UAV or mast-mounted links, describing interplay between LoS and the ground-reflected ray. The received field is a vector sum, and in the far-region ($d \gg d_{bp}$),

$$P_r(d) \approx P_t G_t G_r \left(\frac{h_t h_r}{d^2}\right)^2$$

where $h_t, h_r$ are antenna heights above ground (Baldansa et al., 2022).

• Empirical Power-Law and Shadow Fading: Path loss is modeled as

$PL(d) = PL_0 + 10 n \log_{10}(d/d_0) + X_\sigma$

where $X_\sigma$ is log-normal shadow fading (Du et al., 2018, Du et al., 2021).

• Blockage and Obstacle Attenuation: Obstacles induce excess loss $\Delta PL_{obs}$ via penetration/diffraction. Simulation (ns-3 HybridBuildingsPropagationLossModel) aggregates losses by wall, roof, material, and angle, crucial in urban AG links (Shayegan, 2024).
• Deterministic and Stochastic Geometry: Deployments (PPP of drone-BS, Manhattan Poisson Line Process for urban grids) yield area-wide coverage/statistics (Azari et al., 2017, Hriba et al., 2021).

2. Air-to-Ground and Aerial Communication Architectures

Aerial Platforms:

• Low-Altitude UAVs are modeled as nodes at altitude $h$, typically forming a homogeneous PPP with beamforming antennas pointing down. Coverage probability expressions account for LoS probability as a function of elevation, shadowing, and aggregate interference (Azari et al., 2017).
• High-Altitude Platforms (HAPs): Support relay-aided X-network architectures with decode-and-forward (DF) balloon relays, enabling interference alignment without CSIT and maximizing network DoF (Sudheesh et al., 2017).
• Integrated Data-Control-Sensing-Compute (LAWN): A reconfigurable, 3D multistratum mesh with tightly coupled planes, providing mission-data, control, sensing, and AI inference. Applications span emergency response, precision agriculture, UAM/logistics, and environmental monitoring (Yuan et al., 14 Jun 2025).

Antenna and Link Configuration:

• Downward-pointing conical or sector beams are employed, with strict constraints on beamwidth to mitigate co-channel interference versus coverage (Azari et al., 2017, Du et al., 2018).
• UAV-to-ground connectivity is bounded by maximum LoS range, strongly modulated by urban block geometry, UAV/vehicle heights, and block/building distributions (Hriba et al., 2021).

Obstacles and Placement Optimization:

• LoS/NLoS probability is elevation- and environment-dependent; empirical fits or geometric counts of discrete intersections are used (e.g., $P_{LoS}(h,r) = \frac{1}{1+a \exp(-b(\theta-a))}$) (Shayegan, 2024).
• Placement optimization maximizes sum throughput or coverage under explicit LoS, SNR, and altitude constraints (Shayegan, 2024). The optimal altitude $h^*$ balances building-clearance (LoS gain) against reduced projected range and increased path loss (Hriba et al., 2021, Azari et al., 2017).

3. Channel Measurements and Modeling Across Frequency Bands

Sub-6 GHz and Microwave:

• Two-ray and empirical path-loss models dominate, with severe impacts from ground reflections and vegetation (additive tens of dB loss even in visual LoS) (Baldansa et al., 2022, Du et al., 2018).

Millimeter Wave (28-60 GHz) and THz:

• Aboveground links at 28 GHz in suburban settings exhibit fitted exponents $n=4.06$ (same-street, vLOS), pronounced azimuth gain reduction due to scattering, and 1 Gbps coverage at 100 m for 90% of outdoor links (Du et al., 2018).
• 60 GHz rooftop links (Terragraph radios) demonstrate near-free-space behavior ($n \approx 2.1$), but require ±2.8° alignment precision for main lobe signal; edge-diffraction provides secondary diversity in aerial settings (Du et al., 2021).
• Outdoor THz backhaul/kiosk/aerial links critically depend on LoS, with path loss dominated by spreading and molecular absorption. Typical sustainable ranges are 10–200 m (backhaul), 1–10 m (kiosk), with required beamwidths of 1–10° and gains of 30–50 dBi (Singh et al., 2019).

Optical Wireless Links:

• Indoor aboveground (e.g., data center ToR-to-ceiling) IR uplinks leverage angle-diverse transmitters and wide-FOV receivers, providing 2.8–7 Gb/s at SNR ≥ 15.6 dB over 1–7 m (Alhazmi et al., 2020). Outdoor/rooftop extensions demand narrower beams and active pointing.

4. Network Design, Deployment, and Optimization Strategies

Aspect	Key Parameters/Constraints	Principle Outcomes
Coverage & Optimums	$(\lambda, h, \theta)$ for drones; $h^*$ for UAVs	Unimodal $P_{cov}$; optimums
LoS Planning	Elevation statistics, building-height distributions	Probabilistic LoS mapping
Tower/Antenna Height	$h_t$, $h_r$ vs $d_{bp}$, Fresnel clearance	Fade margin, breakpoint, SNR
Frequency Selection	$f_c$, absorption, interference	Per-link frequency assignment
Backhaul Economics	CLOS/NLOS trade-offs, 3D terrain & viewshed modeling	9–45% capex savings, hybrid

Design and operation involve deterministic viewshed analysis, stochastic simulations (for urban deployments), and integrated cost/performance optimization. Hybrid CLOS/NLOS approaches leverage single-knife-edge diffraction for short NLOS hops, enabling cost-efficient coverage in challenging terrain, with capex reductions up to 45% over CLOS-only deployments (Oughton et al., 2021).

5. Performance, Reliability, and Practical Considerations

• Throughput, Delay, and Reliability: Obstacles and blockages cause significant drops in throughput (∼20%) and increased delay (∼60%) even if PDR remains high; recovery is possible by repositioning to an LoS configuration (Shayegan, 2024).
• Antenna Alignment and Stability: Narrow beam antennas (HPBW ~ 2–10°) require rigorous mechanical/electronic alignment, with ±2° error causing >10 dB SNR loss at mmWave/THz (Du et al., 2021, Singh et al., 2019). Mixed beamwidths trade coverage for robustness.
• Gain Reduction due to Scattering: Realized azimuth gain is log-normally distributed; scattering from vegetation/buildings can diminish gain by 4–6 dB for 10% of links at 28 GHz (Du et al., 2018).
• Environmental and Deployment Constraints: Height above ground, vegetation/obstruction avoidance, and Fresnel clearance directly dictate performance—in suburban FWA, 3 m elevation for CPE provides substantial gains (Du et al., 2018).

6. Interference, Security, and Cross-Layer Challenges

• Aggregate Interference Modeling: For Poisson aerial networks, mean interference is tractably approximated via integral expressions; co-channel interference among main lobes dominates (Azari et al., 2017).
• Spectrum Coexistence and 3D Scheduling: Aerial nodes’ operation across overlapping frequency bands necessitates altitude-aware scheduling, beamforming, and spectrum management to avoid cross-link interference (notably with 5G terrestrial) (Yuan et al., 14 Jun 2025).
• Energy and Security Constraints: Battery limitations promote use of semantic communication, integrated sensing-communication (ISAC), and on-board AI for efficiency; security relies on signed beacons, physical-layer noise injection, and adversarial training for control (Yuan et al., 14 Jun 2025).

7. Future Directions and Roadmap

Advances in aboveground wireless links are expected to emerge from:

• Unified 3D channel models (joint LoS/NLoS, blockage, Doppler, altitude)
• Real-time, AI-based multi-plane (data, control, sensing, compute) optimization frameworks
• Standardization for UTM and 3GPP extensions incorporating cross-layer architectures
• SWaP-optimized hardware with ISAC/semantic capabilities
• End-to-end lightweight cryptographic and physical-layer security across all functional planes
• Large-scale, multi-drone field deployments to empirically validate full-stack solutions (Yuan et al., 14 Jun 2025).

These research avenues aim to underpin robust, low-latency, high-capacity aboveground links across highly dynamic terrestrial and aerial network architectures.