Source-linked AI summary
Multi-Gigabits Millimetre Wave Wireless Communications for 5G: From Fixed Access to Cellular Networks
Peng Wang, Yonghui Li, Lingyang Song, Branka Vucetic
TL;DR
Growing demand for higher throughput is pushing wireless communications toward higher-frequency spectrum for capacity increase. The paper investigates E-band feasibility, propagation, fixed and mobile applications, and proposes an E-band mobile broadband system while discussing channelization and frame structure.
Problem
Ever-increasing data-throughput requirements are straining the conventional microwave band below 10 GHz, motivating exploration of higher-frequency spectrum for additional capacity.
Method
The paper reviews E-band spectrum and propagation, investigates fixed and mobile applications, analyzes multiplexing gain in fixed point-to-point links, and proposes an E-band mobile broadband system.
Results
The paper discusses E-band transmissions for mobile broadband, including interference suppression and techniques that can address coverage while providing good link qualities.
Takeaways & Limitations
A hybrid EMB and 4G system may provide a tradeoff between coverage and data rate, supporting continued exploration of E-band mobile broadband.
Abstract
from arXiv · showhide
With the formidable growth of various booming wireless communication services that require ever-increasing data throughputs, the conventional microwave band below 10 GHz, which is currently used by almost all mobile communication systems, is going to reach its saturation point within just a few years. Therefore, the attention of radio system designers has been pushed towards ever-higher segments of the frequency spectrum in a quest for capacity increase. In this article, we investigate the feasibility, advantages and challenges of future wireless communications over the E-band frequencies. We start from a brief review of the history of E-band spectrum and its light licensing policy as well as benefits/challenges. Then we introduce the propagation characteristics of E-band signals, based on which some potential fixed and mobile applications at the E-band are investigated. In particular, we analyze the achievability of non-trivial multiplexing gain in fixed point-to-point E-band links and propose an E-band mobile broadband (EMB) system as a candidate for the next generation mobile communication networks. The channelization and frame structure of the EMB system are discussed in details.
I. INTRODUCTION
Growing demand is straining heavily used sub-10-GHz cellular bands, motivating investigation of E-band millimetre-wave communications for fixed and mobile broadband. The paper reviews E-band properties and applications, analyzes multiplexing gain, and proposes an EMB system with defined channelization and frame architecture.
- Motivation: Existing cellular systems below 10 GHz are heavily utilized, leaving little room to further increase transmission rates.This saturation is driven by growing demand from services including video, gaming, conferencing, education, and health applications.
- MMW and E-band opportunity: The millimetre-wave band from 30 to 300 GHz offers large spectrum swathes for future wireless communications.A possible 100 GHz of new spectrum has been forecast, although it is discretely distributed and subject to differing service restrictions.
- MMW and E-band opportunity: The E-band spans 71–76 GHz and 81–86 GHz, providing a 10 GHz low-attenuation window with 5 GHz bandwidth per channel.The paper associates this bandwidth with multi-gigabits-per-second and higher data rates, reduced latency, and long-distance transmission.
- Challenges and feasibility: E-band communications face increased phase noise, limited amplifier gain, and transmission-line modelling requirements, while electronics development improves hardware feasibility.The paper highlights cost-effective CMOS technology and high-gain, steerable antennas at mobile devices and base stations.
- Paper scope: The paper reviews E-band history, licensing, propagation, benefits, challenges, and fixed and mobile applications.It specifically analyzes non-trivial multiplexing gain in fixed point-to-point links and proposes EMB as a candidate next-generation mobile system.
- Paper scope: The proposed EMB system is discussed through its channelization and frame architecture as part of the paper’s exploration of E-band mobile broadband.The conclusion also discusses interference suppression and significant overlap techniques for coverage and data-rate challenges, including a possible hybrid EMB and 4G tradeoff.
II. E-BAND SPECTRUM
The E-band comprises broad 71–76 GHz and 81–86 GHz allocations, offering 10 GHz of largely unpartitioned spectrum for high-capacity wireless transmission. Its channel structure supports Gbps rates with relatively simple radio architectures.
- The allocation history includes ITU establishment for fixed services, FCC opening of the band in 2002, and subsequent technical and band-plan actions in North America and Europe.
- The E-band consists of 71–76 GHz and 81–86 GHz bands, providing a combined 10 GHz allocation.
- Unlike microwave bands divided into sub-channels no wider than 50 MHz, E-band uses two unpartitioned 5 GHz channels.
- Each E-band channel is 100 times larger than the largest microwave channel, enabling Gbps data rates with simple architectures and modulation schemes.
C. Light Licensing for E-band
E-band light licensing was introduced to simplify coordination and reduce costs while preserving the protections of traditional link licensing. The band also offers high-capacity, long-distance connectivity with directional antennas and compact hardware.
- E-band licenses can be obtained in minutes at costs of a few tens of dollars per year, unlike traditional microwave licenses requiring months or years.
- Directional pencil-beam antennas enable dense link configurations, high frequency reuse, and simplified interference analysis.
- Light licensing retains link registration, first-come-first-served rights, and full interference protection.
- Small antennas can provide high gain, while FCC rules permit up to 3 W output power, helping offset rain fading and foliage losses.
- E-band supports Gbps-level transmission over distances up to 12 miles and remains robust against fog, dust, turbulence, and other atmospheric impairments affecting optical links.
- Compared with buried or leased fiber, E-band systems can offer lower costs and payback periods measured in months.
E. Technical Research Challenges on the E-band Communications
E-band communications face severe propagation loss, uncertain channel models, complex large-array transceivers, and hardware constraints. The paper discusses beamforming, sparse channels, and hybrid designs as ways to address these challenges.
- The main technical challenges are severe propagation loss, unclear high-frequency channel modeling, large-MIMO transceiver complexity, and E-band hardware constraints.
- E-band propagation loss can be exploited to reduce interference, increase frequency reuse, and prevent eavesdropping.
- A. Free Space Propagation: Given fixed antenna gains and distance, free-space path gain is proportional to λ2, making 75 GHz propagation 30 dB worse than at 2.4 GHz.
- A. Free Space Propagation: At 75 GHz, equal received power requires 30 dB more transmitted power than at 2.4 GHz, making single omnidirectional antennas impractical.
- A. Free Space Propagation: Massive antenna arrays compensate loss through beamforming, and E-band’s short wavelength allows compact, electronically steerable directional arrays.
- A. Free Space Propagation: Doubling operating frequency can quadruple the number of elements in a fixed aperture and provide about 6 dB beamforming gain at each link end.
B. Blockage, Multi-Path and Scattering
E-band signals are readily blocked and do not diffract like lower-frequency signals, making reflection and scattering important in urban environments. Measurements indicate that multipath and directional processing can support viable links despite added loss.
- Urban buildings, vehicles, and people can block or bend line-of-sight transmission, while buildings and streets also create reflected and scattered paths.
- Because of millimetre-scale wavelengths, E-band transmissions are effectively blocked by wooden boards and brick walls and are not prone to diffraction.
- Reflected E-band links generally have lower received power than line-of-sight links because of absorption, diffusion, and reduced specular reflection.
- Despite blockage, NLoS links can provide substantial connectivity and coverage extension when line-of-sight transmission is unavailable.
- NYU WIRELESS measurements in dense urban New York found an NLoS path loss exponent of 5.88 and a shadow factor of 14.19 dB.
- Compared with 28 GHz, E-band urban paths show slightly higher path loss and fewer multipath components, yielding sparse channels that can reduce channel-estimation and transceiver-design complexity.
- Combining paths using their departure and arrival angles can improve path loss exponents and link margins through beamforming and beam combining.
- These mechanisms make an E-band mobile network with reasonable base-station coverage and acceptable outage performance feasible.
C. Other Attenuation Factors at E-band
E-band atmospheric and weather losses vary substantially by condition, but characterized propagation enables links spanning several miles under clear conditions. Fixed E-band applications also address backhaul, access, recovery, campus networking, and storage connectivity.
- Propagation attenuation: 0.5dB/km total atmospheric attenuation makes E-band favorable for transmissions over many miles under clear conditions.The passage compares this loss with higher attenuation at 60GHz and above 100GHz.
- Propagation attenuation: Over 10dB/km attenuation can occur during 25mm/hour heavy rain, increasing to 30dB/km for tropical rainfall at 100mm/hour.Rain outage tends to be short and is mainly problematic for longer-distance transmissions; adaptive power control can compensate for known conditions.
- Fixed applications: E-band fixed links provide Gbps backhaul, temporary fiber-break restoration, gigabit campus LANs, last-mile access, and secure storage connectivity.These links are generally fixed and use guaranteed LoS paths, with impairments dominated by adjacent-link interference, rain, and antenna perturbation.
- Fixed LoS MIMO: Maximum multiplexing gain is achievable in fixed E-band channels when aligned ULAs satisfy the Rayleigh-distance criterion.The channel then has min{Nt, Nr} equal-gain eigenmodes and can support many spatially independent streams; beyond the Rayleigh distance, weak modes reduce practical efficiency.
- Fixed LoS MIMO: The farthest distance supporting m spatially independent streams is determined mainly by the product of the transmit and receive ULA aperture sizes.This distance is termed the effective multiplexing distance of EDOF-m.
V. E-BAND MOBILE BROADBAND (EMB) COMMUNICATIONS
The proposed E-band mobile broadband architecture separates indoor and outdoor networks and combines dense deployment, beamforming, and weather-aware techniques to support outdoor mobility. Indoor users can avoid weather impairment through room-level access points.
- Architecture: E-band mobile broadband is investigated as a candidate system for outdoor mobile communications despite rain and foliage attenuation.The authors state that the outdoor EMB network can overcome these issues using potential techniques.
- Architecture: Brick walls can effectively isolate EMB networks into indoor and outdoor domains because E-band signals do not penetrate solid materials well.Indoor environments commonly support NLoS diffuse links through reflecting materials.
- Architecture: Indoor users can access room-level networks without weather impairment, and restricted indoor mobility limits Doppler concerns.Access points are installed in each room, while handoff between indoor and outdoor networks is assumed at building entrances.
A. Dense EMB BS Deployment
Dense EMB base-station deployment uses short cells and adaptive placement to address severe path attenuation and blocked LoS paths. Narrow beams reduce adjacent-cell interference and permit overlapping coverage.
- Dense EMB BS Deployment: Dense base-station deployment combats severe E-band path attenuation and LoS blockage from surrounding buildings and obstacles.Stations may be placed on building surfaces, lampposts, and street corners according to local topography and architecture.
- Dense EMB BS Deployment: 2dB attenuation occurs for heavy rainfall at 25mm/hr, supporting EMB cell sizes of about 200 meters in urban environments.This distance is described as sufficient to guarantee qualified LoS links under the stated conditions.
- Dense EMB BS Deployment: E-band narrow beams significantly suppress interference among adjacent EMB base stations, allowing their coverage areas to overlap substantially.Propagation measurements in dense New York City environments found negligible atmospheric attenuation for inter-site distances up to 200 meters.
B. Adaptive Beamforming
Adaptive beamforming and combining enhance LoS and reflected NLoS links while exploiting E-band channel sparsity to reduce CSI-estimation overhead. The approach depends on channel-state information and remains an active research area.
- Adaptive Beamforming: At both link ends, phase, amplitude, and combining weights are selected to create desired patterns and maximize received power or SNR.When LoS exists, mutually aligned beamforming and combining patterns can significantly enhance link quality.
- Adaptive Beamforming: Adaptive beamforming directs transmission toward strong reflected NLoS paths when direct LoS links are unavailable, while preserving link quality with power control.Buildings with smooth glass or marble surfaces may provide stronger reflections and less diffusion.
- System challenge: Massive antenna arrays provide power gain against E-band propagation loss but increase CSI estimation and feedback overhead.The receiver estimates CSI and feeds it back to the transmitter in the EMB network.
- Channel Sparsity: E-band channels are sparse because they generally contain fewer propagation paths than antennas, even in dense urban environments.This sparsity follows from the higher signal frequencies and can be exploited after conversion to the beam-space domain.
- Channel Sparsity: Beam-space sparse channel estimation can quickly estimate path AoDs, AoAs, and fading coefficients using bi-section searching, reducing CSI overhead.More efficient and advanced channel-estimation approaches remain under investigation.
D. Hybrid Transceiver Design
The EMB design uses hybrid digital-and-analogue beamforming to manage E-band hardware constraints while supporting adaptive links and user cooperation for blocked paths.
- Hybrid Transceiver Design: Adaptive beamforming and combining vectors must track mobile-user movement so transmitter and receiver beams remain aligned.The design is hardware-constrained compared with fully digital microwave beamforming.
- Hybrid Transceiver Design: Hybrid digital-and-analogue precoding combines high-dimensional RF phase-shifter precoding with low-dimensional digital precoding.This addresses the difficulty of driving many antennas with relatively few RF chains because RF hardware is costly and power-hungry.
- User Cooperation: User cooperation provides an indirect path when a mobile user's line-of-sight link to a base station is blocked.Vacant users with good base-station links can relay traffic for users lacking direct links.
- User Cooperation: Users are classified as directly served when at least one base-station link exceeds a threshold, and indirectly served when all such links remain below it.Directly served users select the base station with the best link quality, while indirectly served users seek the best two-hop route through a vacant directly served user.
- User Cooperation: Vacant directly served users broadcast level-2 pilots and can relay indirectly served users' data through their serving base stations.The indirect user evaluates candidate two-hop links and requests the vacant directly served user offering the best overall link quality.
F. Hybrid EMB and 4G System
The paper combines EMB with 4G coverage and control support because E-band propagation can leave mobile dead spots. It proposes a Europe-based OFDM frame structure while retaining E-band gigabit data transmission.
- Hybrid EMB and 4G System: E-band propagation can leave dead spots where EMB users cannot be served, motivating emergency support when E-band communication fails.The proposed hybrid architecture retains 4G coverage support for such cases.
- Hybrid EMB and 4G System: A hybrid EMB and 4G infrastructure is proposed to improve coverage and seamless user experience while preserving E-band gigabit transmissions.E-band primarily carries data, while 4G carries system information, control, and feedback channels.
- Channelization: Europe divides each 4.75 GHz E-band allocation into nineteen 250 MHz channels and permits aggregation of one to nineteen channels.The channels may support either TDD or FDD, individually or alongside other bands.
- Frame Structure: The proposed EMB frame uses OFDM, with 10 ms frames and 1 ms sub-frames aligned with LTE durations.The structure is based on the Europe channelization plan and is also applicable to the compatible UK and Australian plan.
- Frame Structure: The frame divides each sub-frame into 32 slots of 31.25 µs, each containing 14 OFDM symbols.The cyclic-prefix design produces about 6.7% overhead and provides margin for delay spread and synchronization error.
VII. CONCLUSIONS
The paper highlights E-band transmissions as a candidate for mobile broadband, while identifying directional-beamforming coverage—especially without LoS—as a central challenge. It discusses techniques including hybrid EMB–4G operation to address this tradeoff.
- E-band mobile broadband is discussed as a potential future mobile-communications technology.
- Directional beamforming can suppress interference among adjacent E-band base stations and allow substantial coverage-area overlap.
- Good overall network coverage remains challenging when mobile users lack LoS links to surrounding base stations.
- The paper discusses techniques that can potentially address the E-band coverage problem.
- A hybrid EMB and 4G system may provide a tradeoff between coverage and data rate.