Millimeter Wave Cellular Wireless Networks: Potentials and Challenges (1401.2560v1)

Abstract: Millimeter wave (mmW) frequencies between 30 and 300 GHz are a new frontier for cellular communication that offers the promise of orders of magnitude greater bandwidths combined with further gains via beamforming and spatial multiplexing from multi-element antenna arrays. This paper surveys measurements and capacity studies to assess this technology with a focus on small cell deployments in urban environments. The conclusions are extremely encouraging; measurements in New York City at 28 and 73 GHz demonstrate that, even in an urban canyon environment, significant non-line-of-sight (NLOS) outdoor, street-level coverage is possible up to approximately 200 m from a potential low power micro- or picocell base station. In addition, based on statistical channel models from these measurements, it is shown that mmW systems can offer more than an order of magnitude increase in capacity over current state-of-the-art 4G cellular networks at current cell densities. Cellular systems, however, will need to be significantly redesigned to fully achieve these gains. Specifically, the requirement of highly directional and adaptive transmissions, directional isolation between links and significant possibilities of outage have strong implications on multiple access, channel structure, synchronization and receiver design. To address these challenges, the paper discusses how various technologies including adaptive beamforming, multihop relaying, heterogeneous network architectures and carrier aggregation can be leveraged in the mmW context.

• The paper shows that mmWave cellular networks can increase capacity over LTE by more than an order of magnitude through advanced beamforming and small cell deployment.
• It finds that mmWave frequencies offer robust non-line-of-sight outdoor coverage up to 200 meters despite high path loss, with directional antennas reducing interference.
• The study highlights the need to redesign cellular systems for mmWave by addressing synchronization, random access, and shadowing challenges through adaptive techniques.

Overview of "Millimeter Wave Cellular Wireless Networks: Potentials and Challenges"

The paper "Millimeter Wave Cellular Wireless Networks: Potentials and Challenges" by Sundeep Rangan, Theodore S. Rappaport, and Elza Erkip presents an in-depth exploration of the viability of mmWave frequencies (30-300 GHz) for cellular communication. The authors assess mmWave technologies through extensive channel measurements and capacity studies, focusing mainly on small cell deployments in urban environments such as New York City.

Key Findings

The paper provides several key insights:

1. Signal Coverage:
   • Despite concerns about high path loss at mmWave frequencies, the measurements demonstrate significant non-line-of-sight (NLOS) outdoor, street-level coverage up to approximately 200 meters from a low power micro- or picocell base station.
2. Capacity Gains:
   • Using statistical channel models from measurements, mmWave systems were shown to offer more than an order of magnitude increase in capacity compared to current 4G LTE networks at similar cell densities.
3. Directional Communication:
   • The gains in capacity and coverage are largely attributed to beamforming and spatial multiplexing via multi-element antenna arrays. However, these gains necessitate highly directional and adaptive transmissions and have implications for network design and receiver architecture.
4. Redesign of Cellular Systems:
   • Current cellular systems would require substantial redesigns to leverage mmWave frequencies fully. Challenges include synchronization, random access, and opportunistic communication due to the fast-changing nature of mmWave channels.
5. Interference Management:
   • Directional isolation between links can significantly reduce interference, implying that interference mitigation techniques employed in LTE may have limited relevance in mmWave systems.
6. Robustness to Shadowing:
   • The paper acknowledges the severe vulnerability of mmWave signals to shadowing by obstacles but suggests that multihop relaying, heterogeneous network architectures, and carrier aggregation may mitigate these issues.

Numerical Results and Claims

The paper provides compelling numerical evidence to support its claims:

• In an experimental simulation with cell radii around 200m and ISD of 200m, mmWave networks showed cell capacities exceeding 1 Gbps and cell edge rates over 10 Mbps. In comparison, existing LTE systems demonstrated typical capacities around 50 Mbps per cell.
• The 5% cell edge rates for mmWave networks, though significantly enhanced, still reflect the challenge posed by potential outage and shadowing effects.

Implications and Future Developments

Practical Implications:

1. Enhanced Data Rates:
   • The increase in average cell throughput by a factor of 20 suggests practical implications for ultra-high-speed mobile internet, HD video streaming, and future applications like AR/VR.
2. Network Topology:
   • With the potential drop in interference due to directional isolation, network design could move towards deployment models where backhaul and access links are differentiated by frequency bands.
3. Cost and Coverage:
   • Deployment of mmWave cells would likely benefit densely populated urban areas first, given the necessity for a base station every 100 to 200m and the reliance on LOS or near-LOS conditions.

Theoretical Implications:

1. Channel Models:
   • The clustering nature of mmWave propagation paths requires new statistical models for channel behavior that go beyond traditional Rayleigh or Rician fading models.
2. Signal Processing:
   • Techniques for compressed sensing and sparse recovery in millimeter-wave channels can be integral to managing the expected high Doppler spreads and intermittent connectivity.

Future Developments:

• Research Directions:
  • Future advancements in adaptive beamforming, phased array antennas, and front-end RF designs are critical. CMOS RF circuit advancements will play an essential role in reducing power consumption for multi-antenna mmWave devices.
• Standardization:
  • Positioning mmWave technology within future 5G and Beyond 5G standards will involve collaboration across industry and academia, driving initial use cases in dense urban environments towards broader applications.

In conclusion, the paper establishes a solid foundation for mmWave-based cellular networks, underpinning their potential to significantly revolutionize data rates and network capacities. However, this transformation comes with substantial redesign challenges, necessitating future research in adaptive systems, robust against high path loss and shadowing characteristic of the mmWave spectrum.