Source-linked AI summary
A Survey of Millimeter Wave (mmWave) Communications for 5G: Opportunities and Challenges
Yong Niu, Yong Li, Depeng Jin, Li Su, Athanasios V. Vasilakos
TL;DR
Growing mobile data demand and spectrum shortages motivate using mmWave bands for high-capacity 5G, despite their high loss, directionality, blockage sensitivity, and mobility dynamics. The paper surveys mmWave characteristics, standards, solutions, applications, and design guidelines, concluding with open research issues for architectures and protocols. It reports that propagation conditions vary substantially across bands and environments, including strong LOS/NLOS differences and obstruction-related outages.
Problem
MmWave communications must overcome high propagation loss, directional links, blockage sensitivity, and mobility dynamics to realize their 5G capacity potential.
Method
The paper surveys mmWave characteristics, standards, existing solutions, applications, design guidelines, and open research issues for 5G architectures and protocols.
Results
Propagation measurements show 28 GHz LOS and NLOS path-loss exponents of 2.55 and 5.76, respectively, with 57% of Manhattan locations experiencing outage, mostly beyond 200 m.
Takeaways & Limitations
MmWave 5G deployment requires redesigned architectures and protocols addressing integrated circuits, spatial reuse, blockage, mobility, control, and heterogeneous networking.
Abstract
from arXiv · showhide
With the explosive growth of mobile data demand, the fifth generation (5G) mobile network would exploit the enormous amount of spectrum in the millimeter wave (mmWave) bands to greatly increase communication capacity. There are fundamental differences between mmWave communications and existing other communication systems, in terms of high propagation loss, directivity, and sensitivity to blockage. These characteristics of mmWave communications pose several challenges to fully exploit the potential of mmWave communications, including integrated circuits and system design, interference management, spatial reuse, anti-blockage, and dynamics control. To address these challenges, we carry out a survey of existing solutions and standards, and propose design guidelines in architectures and protocols for mmWave communications. We also discuss the potential applications of mmWave communications in the 5G network, including the small cell access, the cellular access, and the wireless backhaul. Finally, we discuss relevant open research issues including the new physical layer technology, software-defined network architecture, measurements of network state information, efficient control mechanisms, and heterogeneous networking, which should be further investigated to facilitate the deployment of mmWave communication systems in the future 5G networks.
I. INTRODUCTION
mmWave communications offer extensive bandwidth for 5G but differ fundamentally from microwave systems through high propagation loss, directivity, blockage sensitivity, and mobility-driven dynamics. The survey reviews standards, challenges, solutions, applications, design guidelines, and open research issues.
- Motivation: The 30–300 GHz mmWave band could provide multi-gigabit 5G services as mobile traffic growth intensifies spectrum shortages.Research focuses particularly on 28, 38, 60, and E-band frequencies.
- Challenges: High propagation loss, directional transmission, blockage sensitivity, and mobility create PHY, MAC, and routing challenges requiring new architectures and protocols.These characteristics distinguish mmWave communications from systems operating at 2.4 and 5 GHz.
- Scope and contributions: The survey covers IEEE 802.15.3c and IEEE 802.11ad, existing solutions, design guidelines, and applications spanning small-cell access, cellular access, and wireless backhaul.It also discusses integrated-circuit design, interference management, spatial reuse, anti-blockage, and mobility dynamics.
- Wireless channel characteristics: At 60 GHz, free-space loss is 28 dB higher than at 2.4 GHz, while oxygen absorption peaks at 15–30 dB/km.The paper describes directional antennas as a response to severe propagation loss.
- Wireless channel measurements: Measurements report a 28 GHz LOS path-loss exponent of 2.55, an average NLOS exponent of 5.76, and 57% outage locations in Manhattan, mostly beyond 200 m.A combined TX-RX antenna gain of 49 dBi enabled 200 m maximum coverage in a highly obstructed environment.
- Wireless channel characteristics: At 200 m, 28 and 38 GHz show low rain attenuation and oxygen absorption, whereas 60 and 73 GHz show significant attenuation and absorption; NLOS transmission adds loss in all four bands.These observations summarize the propagation characteristics compared across bands.
B. Directivity
mmWave links are inherently directional because electronically steerable antenna arrays use phase control to steer beams and concentrate gain toward selected directions.
- B. Directivity: Electronically steerable antenna arrays control each element’s signal phase to steer a beam toward any direction.The resulting pattern provides high gain in the selected direction and very low gain elsewhere.
C. Sensitivity to Blockage
MmWave links are highly sensitive to physical blockage, making reliable support for delay-sensitive applications difficult under human mobility.
- C. Sensitivity to Blockage: Human blockage can reduce the link budget by 20–30 dB, and indoor channels were blocked about 1% or 2% of the time with one to five persons.Human mobility makes mmWave links intermittent.
- C. Sensitivity to Blockage: Maintaining reliable mmWave connections for delay-sensitive applications such as HDTV is a major communications challenge.The challenge follows from intermittent links caused by mobile obstructions.
III. STANDARDIZATION
The paper reviews mmWave standards and the challenges of deploying mmWave systems, including circuit design, antennas, propagation, and coordination.
- IEEE 802.11ad specifies 60 GHz PHY and MAC layers for multi-gigabit wireless applications.
- IEEE 802.11ad defines OFDM and single-carrier modes for high-performance and low-power, low-complexity applications.
- IEEE 802.11ad organizes channel access into beacon intervals coordinated by an AP for associated devices.
- IEEE 802.15.3c specifies 60 GHz PHY and MAC layers for indoor WPANs coordinated by a piconet controller.
- MmWave deployment faces circuit and antenna challenges, including power-amplifier distortion, phase noise, and IQ imbalance.
B. Interference Management and Spatial Reuse
MmWave directionality can reduce interference but creates deafness, while dense indoor deployments make interference management and spatial reuse central design problems.
- Directional transmission reduces interference between links but prevents third-party nodes from carrier sensing, creating the deafness problem.
- Indoor mmWave networks cannot generally assume pseudowired links because limited range and dense AP deployment increase interference.
- Power control and transmission coordination are proposed to manage interference, enabling concurrent transmissions within and across BSSs.
- REX uses exclusive-region conditions to enable concurrent transmissions and achieve significant spatial-reuse gains, but considers only two concurrent links.
- FDMAC uses centralized coordination and Greedy Coloring to exploit spatial reuse, but assumes a pseudowired WPAN interference model.
- MDMAC alleviates deafness through memory-based approximate TDM schedules but does not fully exploit spatial reuse.
C. Anti-blockage
Blockage and mobility challenge mmWave connectivity by disrupting links, channel conditions, load balance, and handovers; proposed remedies include diversity, relaying, and improved association.
- Human-body blockage can require many APs for perfect shadowing avoidance, while switching between two APs provides 98% propagation-path visibility.
- Reflections, static reflectors, relaying, and deflection routing provide physical- and network-layer approaches to maintain connectivity around obstacles.
- BFDR selects low-interference relay paths to maximize throughput, while RFDR achieves nearly the same order of improvement with lower complexity.
- Multi-AP diversity can complete transmissions through another AP after blockage, but robustness and efficiency depend on AP deployment and placement.
- Mobility changes channel state as transmitter-receiver distance varies, altering channel capacity under line-of-sight transmission.
- Small BSS coverage causes rapid load fluctuations, making intelligent association and handovers necessary; RSSI-based association may use resources inefficiently.
V. APPLICATIONS OF MMWAVE COMMUNICATIONS
The survey identifies mmWave applications in dense small-cell access, cellular access, and wireless backhaul, enabled by wide bandwidth but dependent on dense infrastructure.
- Massive small-cell densification is proposed to support a 10 000-fold increase in network capacity by 2030.
- MmWave small cells can provide multi-gigabit rates for wideband multimedia applications.
- MmWave enhanced local-area access can achieve peak data rates above 10 Gbps and edge data rates above 100 Mbps.
- MmWave cellular networks can provide high coverage and capacity when infrastructure is densely deployed.
- At 28 and 38 GHz, cellular access feasibility was demonstrated with cell sizes on the order of 200 m.
- Directional antennas produced capacity gains 20 times greater than 4G LTE in one reported cellular-network analysis.
C. Wireless Backhaul
mmWave wireless backhaul offers a cost-effective, flexible alternative to fiber for densely deployed 5G small cells, while hybrid beamforming addresses the associated propagation-loss and hardware-complexity constraints.
- Wireless backhaul: Several-Gbps mmWave wireless backhaul in the 60 GHz and E-band can be more cost-effective, flexible, and easier to deploy than fiber-based backhaul.The cited bands are 60 GHz and E-band at 71–76 GHz and 81–86 GHz.
- Wireless backhaul: In-band backhaul multiplexes access and backhaul on the same frequency band to provide a cost-effective and scalable solution.TDM scheduling was proposed for point-to-multipoint, non-line-of-sight mmWave backhaul.
- Wireless backhaul: Joint access-backhaul scheduling, including D2DMAC, can further optimize resource allocation and enable D2D transmissions for performance improvement.D2DMAC targets radio access and backhaul in 60 GHz small-cell networks.
- New physical layer technology: Hybrid beamforming combines digital precoding and analog beamforming to obtain high beamforming gain while reducing the number of dedicated RF chains.The structure is motivated by high mmWave propagation loss and the expense and complexity of one RF chain per antenna.
- New physical layer technology: Existing mmWave MIMO work develops sparse-reconstruction and hybrid analog/digital precoding methods, but multi-user mmWave MIMO remains insufficiently investigated.The cited work targets low-cost hardware and approaches digital-solution performance under analog hardware constraints.
2) Full-duplex:
Full-duplex mmWave backhaul could exploit directional beamforming to suppress interference, but practical multi-gigahertz implementations and spatial-reuse trade-offs remain open challenges.
- Full-duplex: Practical mmWave full-duplex systems must extend current roughly 40 MHz designs to bandwidths of several GHz.The cited passage identifies implementation and demonstration at several-GHz bandwidth as an open problem.
- Full-duplex: Directional beamforming can enable relay full-duplex transmission in line-type backhaul when transmit and receive beams point in opposite directions.The described arrangement places transmitters outside the receive range of other receivers, with negligible interference assumed from AP C to AP A.
- Full-duplex: The relationship between spatial reuse and full-duplex gain requires further investigation for mmWave full-duplex design.This trade-off is identified alongside the directional relay scenario.
- Network control: The survey identifies coordination, centralized control, and additional research as necessary for robust and efficient mmWave network operation.The broader control discussion links coordination with interference management, spatial reuse, connectivity, load balance, and QoS.
C. Control Mechanisms
MmWave control must coordinate interference, scheduling, mobility, beamforming, and anti-blockage across dynamic network states; centralized and adaptive mechanisms are emphasized as open research directions.
- Control mechanisms: Effective mmWave control requires mechanisms for interference management, transmission scheduling, mobility and handover, beamforming, and anti-blockage.These mechanisms are identified as needing further investigation for high network performance.
- Control mechanisms: Current MAC protocols each have advantages and shortcomings, motivating more efficient and robust protocols that exploit spatial reuse and overcome blockage.Centralized protocols are preferred, while much existing work focuses on a single BSS.
- Anti-blockage: Anti-blockage methods include beam switching, relaying, handovers, spatial diversity, and multipath routing, but each is effective only under particular conditions.Handovers require a direct path to another AP; relaying depends on high-quality relay links; spatial diversity increases complexity and cannot guarantee QoS.
- Dynamic adaptation: Dynamic adaptation must respond to diverse traffic patterns and time-varying channels by intelligently switching or combining protocols and selecting MCSs.Open problems include combining TDMA with CSMA/CA and managing user mobility.
- Network state measurement: Network-state measurement remains incomplete because most work stays within one BSS, while overlapping BSSs require neighboring link, beamforming, and interference information.Measurements must also finish quickly enough to support real-time control.
E. Heterogeneous Networking
MmWave networks are inherently heterogeneous because their limited coverage requires coexistence with wider-coverage systems such as LTE and WiFi. Cooperation across network types can support offloading, handover, interference management, and mobility control.
- Heterogeneous networking: Limited mmWave coverage necessitates coexistence with other bands, producing heterogeneous networks that combine 60 GHz picocells with macrocells and microcells.Microwave-band cells have larger coverage areas than 60 GHz BSSs.
- Heterogeneous networking: Interaction and cooperation among different network types are central to addressing mobility management, vertical handover, data offloading, and inter-cell interference.The survey frames cooperation as a way to explore heterogeneous networking's potential.
- Heterogeneous networking: A hybrid HetNet uses 60 GHz and 70–80 GHz propagation characteristics and bandwidth to reduce interference in heterogeneous networks.The cited paradigm is termed hybrid HetNet.
- Heterogeneous networking: An mmWave+4G architecture can use 4G for control while mmWave capacity offloads macrocell traffic and supports high-throughput demands.Handovers between macrocell base stations and mmWave APs can address blockage, mobility, and load balancing.
VII. CONCLUSIONS
mmWave communications are presented as a promising 5G candidate because they could provide orders of magnitude greater capacity. The survey reviews solutions, applications, and open research issues intended to support mmWave development in 5G.
- Orders of magnitude greater capacity over current communication systems makes mmWave communications a promising 5G candidate.
- The survey compares existing solutions for mmWave challenges in terms of effectiveness, efficiency, and complexity.
- Potential 5G applications of mmWave communications are discussed.
- Open research issues include new physical technology, software-defined architecture, network-state measurements, efficient control mechanisms, and heterogeneous networking.
- These open issues are discussed to promote the development of mmWave communications in 5G.