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FSO-based Vertical Backhaul/Fronthaul Framework for 5G+ Wireless Networks
Mohamed Alzenad, Muhammad Zeeshan Shakir, Halim Yanikomeroglu, Mohamed-Slim Alouini
TL;DR
The paper addresses the challenge of backhauling dense small-cell deployments where terrestrial solutions can be costly or infeasible. It investigates a vertical framework in which NFPs transport traffic over point-to-point FSO links, finding multi-Gbit/s rates under improved link conditions while identifying weather, cost, and platform-association challenges.
Problem
Dense small-cell deployments require backhaul/fronthaul, but terrestrial fiber or line-of-sight solutions can be costly or infeasible in challenging locations.
Method
The paper investigates a vertical backhaul/fronthaul architecture connecting small cells and the core network through NFPs and point-to-point FSO links.
Results
Multi-Gbit/s rates can be achieved by reducing the FSO divergence angle, while clouds accompanied by fog create high path loss.
Takeaways & Limitations
The vertical FSO/NFP framework is presented as a complementary solution for challenging ultra-dense small-cell deployments.
Takeaways & Limitations
The system has high total cost of ownership compared with terrestrial backhaul/fronthaul networks.
Abstract
from arXiv · showhide
The presence of a super high rate, but also cost-efficient, easy-to-deploy, and scalable, backhaul/fronthaul framework is essential in the upcoming fifth-generation (5G) wireless networks \& beyond. Motivated by the mounting interest in the unmanned flying platforms of various types including unmanned aerial vehicles (UAVs), drones, balloons, and high-altitude/medium-altitude/low-altitude platforms (HAPs/MAPs/LAPs), which we refer to as the networked flying platforms (NFPs), for providing communications services and the recent advances in free-space optics (FSO), this article investigates the feasibility of a novel vertical backhaul/fronthaul framework where the NFPs transport the backhaul/fronthaul traffic between the access and core networks via point-to-point FSO links. The performance of the proposed innovative approach is investigated under different weather conditions and a broad range of system parameters. Simulation results demonstrate that the FSO-based vertical backhaul/fronthaul framework can offer data rates higher than the baseline alternatives, and thus can be considered as a promising solution to the emerging backhaul/fronthaul requirements of the 5G+ wireless networks, particularly in the presence of ultra-dense heterogeneous small cells. The paper also presents the challenges that accompany such a novel framework and provides some key ideas towards overcoming these challenges.
I. INTRODUCTION
Dense small-cell deployment is important for high-rate wireless coverage, but connecting many small cells to the network remains costly and difficult. The paper therefore explores an FSO- and NFP-based vertical architecture for 5G+ backhaul/fronthaul.
- I. INTRODUCTION: Dense small-cell deployment supports extremely high data-rate coverage, but backhauling or fronthauling many small-cell base stations is a significant challenge.Small-cell base stations cost less than macro-cell base stations, yet serving many of them creates substantial transport difficulty and cost.
- I. INTRODUCTION: The proposed architecture connects small-cell base stations to the core network through a vertical backhaul/fronthaul using FSO and networked flying platforms.The authors use NFPs as a generic term covering UAVs, drones, balloons, and HAPs, MAPs, and LAPs.
- I. INTRODUCTION: Conventional fiber deployment may be infeasible or too expensive, while RF NLOS alternatives face interference, congestion, and higher cost.The difficulty is especially acute in ultra-dense urban areas where some small cells are hard to reach or lack line of sight.
- I. INTRODUCTION: The paper investigates whether 5G+ networks can use NFPs and FSO for vertical backhaul/fronthaul and examines the challenges of integrating them.These questions motivate a paradigm shift in backhaul/fronthaul network design.
B. Overview of Networked-Flying Platforms
The paper uses networked flying platforms as a broad class of airborne platforms and investigates their integration with FSO-based vertical backhaul/fronthaul. It also considers deployment economics, weather effects, and ways to improve link performance.
- B. Overview of Networked-Flying Platforms: NFPs encompass UAVs, drones, balloons, and quasi-stationary HAPs, MAPs, and LAPs used to provide wireless backhaul/fronthaul links.In this paper, NFPs can carry heavy FSO payloads, remain quasi-stationary, and move in response to weather, coverage, or traffic conditions.
- C. Our Contributions: The article investigates the feasibility of integrating NFPs with FSO in a vertical backhaul/fronthaul framework.This is identified as a principal contribution of the paper.
- C. Our Contributions: The study examines vertical FSO data rates under several weather conditions and compares deployment and operational costs with terrestrial solutions.The contribution list explicitly includes weather impacts and economic comparison.
- C. Our Contributions: The paper presents solution ideas intended to increase achievable data rate and link margin under bad weather conditions.These ideas address a central implementation challenge of the proposed system.
II. PROPOSED SYSTEM
The proposed transport plane uses connected NFPs and point-to-point FSO beams to link small cells or aggregation points with the core network. It is intended as a complementary option for difficult terrestrial deployment settings.
- II. PROPOSED SYSTEM: The vertical system is designed as a complementary transport network for dense small cells in challenging environments.Examples include terrestrial-network failures, temporary demand during social events, and locations where fiber or microwave links are unavailable.
- II. PROPOSED SYSTEM: A swarm of autonomous NFP nodes forms the vertical plane, with a mother NFP connecting the swarm to the ground core network.NFP-to-mother-NFP and mother-NFP-to-core connections use FSO links.
- II. PROPOSED SYSTEM: NFP altitude ranges from a few hundred meters to typically 20 km and is selected according to coverage, weather, and serving capability.The system assumes an altitude high enough to help guarantee a line-of-sight connection.
- II. PROPOSED SYSTEM: Point-to-point FSO beams connect small cells or aggregation points to NFPs, while nearby line-of-sight small cells can relay traffic for small cells without direct visibility.Traffic may also be distributed among small cells before reaching an aggregation hub with an NFP connection.
III. LINK BUDGET ANALYSIS OF VERTICAL BACKHAUL/FRONTHAUL FRAMEWORK
The link-budget analysis models atmospheric attenuation and scattering mechanisms that reduce FSO signal power across vertical paths. It distinguishes absorption, Rayleigh, Mie, and non-selective scattering, with fog, rain, and cloud attenuation treated using corresponding path and visibility parameters.
- FSO transmits high-bandwidth information by laser, but atmospheric attenuation and geometrical, pointing, and optical losses reduce link performance.Atmospheric attenuation includes absorption, scattering, and turbulence.
- Absorption converts photons into kinetic energy through collisions with gas molecules, with loss depending on gas type, concentration, and optical-beam frequency.Wavelength selectivity creates transmission windows where absorption loss is negligible.
- Scattering occurs when the FSO beam collides with atmospheric particles in the troposphere and stratosphere.The considered NFP altitude range extends from several hundred meters to less than 20 km.
- The Kruse model predicts Mie-scattering attenuation from scattering coefficient, affected distance, visibility range, wavelength, and visibility-dependent exponent δ.The exponent is δ = 0.585V^(1/3) for V < 6 km, 1.3 for 6 < V < 50 km, and 1.6 for V > 50 km.
- Fog attenuation uses Mie scattering and the Kruse model, with affected distance d = Δd_fog/sin(φ) determined by fog-layer thickness and elevation angle.Table I relates fog conditions, visibility, wavelength, and attenuation per kilometer.
- Rain produces non-selective scattering, and rain attenuation depends on rainfall rate and the distance over which rain affects the beam.The affected rain distance is d_rain = Δd_rain/sin(φ).
C. Turbulence Loss
The turbulence-loss analysis characterizes atmospheric fluctuations through the altitude-dependent refractive-index structure parameter and derives scintillation attenuation from it. The Hufnagel–Valley model supplies the vertical-link turbulence profile using wind speed and a model constant.
- The refractive-index structure parameter C_n^2(h) measures turbulence strength as a function of altitude.Various measurement-based models are available to predict it.
- Geometrical loss is expressed using the ratio of receiver-aperture area A_R to beam area A_B.For a circular receiver, A_R = πr^2, while beam size depends on link length and transmitter divergence angle.
- Scintillation attenuation L_sci is calculated from the turbulence parameter and path length l.The path length is an input to the scintillation-loss expression.
D. Geometrical Loss
Geometrical loss arises because beam spreading enlarges the illuminated area at the receiver, reducing the power collected by its aperture. The loss depends on receiver-aperture radius, link length, and transmitter divergence angle.
- Beam spreading reduces received power because light energy is distributed over a larger area as it travels through the atmosphere.The paper expresses this reduction as geometrical loss in decibels.
- The geometrical-loss expression uses receiver aperture and beam-spread geometry to quantify the received-power reduction.The supplied formulation identifies aperture radius, link length, and divergence angle as its governing variables.
- Geometrical loss depends on receiver-aperture radius r, communication-link length l, and transmitter divergence angle θ.These parameters determine the fraction of the expanded beam collected by the receiver.
E. Pointing and Optical Losses
Pointing and optical imperfections constrain vertical FSO links, while the framework evaluates data rate and link margin across weather, altitude, and divergence-angle conditions. The results show strong sensitivity to clouds and beam divergence.
- Pointing and Optical Losses: High-altitude NFP motion and turbulence can increase pointing loss, potentially causing link failure or significantly reducing received signal power.Tracking systems using electro-optic or acousto-optic devices are identified as a compensation approach for fast-moving platforms.
- Pointing and Optical Losses: Optical loss results from imperfect transceiver elements and is quantified through transmitter and receiver optical efficiencies η_t and η_r.The paper gives L_opt = 10log(η_tη_r).
- Pointing and Optical Losses: A single vertical FSO link can support one or multiple SBSs, with supported aggregation determined by link data rate and estimated small-cell traffic.Multiple SBSs can forward traffic to an aggregation point through high-capacity wireless or wired links.
- Pointing and Optical Losses: The achievable FSO data rate accounts for transmit power, optical efficiencies, pointing loss, atmospheric attenuation, and photon energy.Atmospheric attenuation includes rain, fog, cloud, and scintillation losses.
- Pointing and Optical Losses: At 20 km, data rates are 198 kbits/s, 0.5 Mbits/s, and 42 Gbits/s for clouds and fog, heavy rain, and clear sky, respectively.The data rate decreases with altitude because geometrical loss increases; clouds most strongly affect the vertical link among the stated conditions.
- Pointing and Optical Losses: At 20 km in cloudy and foggy conditions, reducing divergence from 10 µrad to 1 µrad raises data rate from 1.6 Gbits/s to 46 Gbits/s and link margin from -2.6 dB to 12 dB.The 10 µrad case has link failure, while the 1 µrad case retains positive margin.
B. Economics of Vertical System
The economic comparison includes CAPEX and OPEX across several backhaul/fronthaul technologies in a modeled urban HetNet. Fiber has the highest deployment cost, while the vertical system has the highest one-year TCO.
- TCO combines CAPEX for equipment, planning, and installation with OPEX for spectrum, maintenance, power, and fuel.
- The modeled HetNet contains 100 macro cells and 1000 small cells across a 5 km by 5 km urban area.
- Fiber optics has the highest deployment cost because urban digging and trenching are expensive.
- RF NLOS PtM is the most cost-effective deployment option but suffers interference and low data rate from spectrum sharing.
- Around $120 million, the vertical system has the highest one-year TCO, versus approximately $14, $44, and $55 million for RF NLOS PtM, terrestrial FSO, and optical fiber.
A. Implementation
Implementation is constrained by weather sensitivity, costly heavy-payload long-endurance NFPs, and evolving commercial-flight regulations. The paper identifies adaptive link control, optimized placement, hybrid mm-wave use, and alternative platforms as possible responses.
- Weather strongly affects vertical FSO links, motivating adaptive transmit power and divergence-angle control.
- A system optimization algorithm could place NFPs below clouds over regions with negligible turbulence.
- NFPs must carry heavy FSO payloads with long endurance, while suitable UAVs remain expensive to operate.
- Unmanned balloons could reduce operating costs because they are often solar-powered and quasi-stationary.
- Commercial NFP deployment remains bounded by regulations covering flight licenses, operator certificates, safety equipment, and populated or beyond-LOS operations.
D. NFP-Small Cell Association
NFP–small cell association is constrained by payload, transceiver, and NFP-backhaul capacity, while security and privacy requirements add operational safeguards. The framework’s evaluation covers link budget, achievable data rate, and weather effects, with multi-Gbit/s rates possible under improved optical alignment.
- Association constraints: Each NFP can serve only a limited number of small cells because payload capacity restricts its available FSO transceivers.The association problem must account for the NFP’s payload constraint and transceiver count.
- Association constraints: NFP-backhaul capacity can further limit the number of served small cells even when sufficient FSO transceivers are available.Association therefore depends on both access-side transceiver capacity and links between NFP nodes or the ground station.
- Security and privacy: Security and privacy are fundamental requirements because hijacking, sabotage, or inadequate protection could disrupt cellular services or expose connected entities.Suggested safeguards include advanced control mechanisms and restrictions on NFP flight paths or payloads.
- Evaluation and trade-offs: The proposed system is evaluated using link budget and achievable data rate under different weather conditions, with multi-Gbit/s rates possible by reducing divergence angle.Clouds accompanied by fog produce high path loss, while the vertical system has higher TCO than terrestrial alternatives.