[Paper Review] Random Beamforming in Millimeter-Wave NOMA Networks

Z. Ding, P. Fan+1 more|arXiv (Cornell University)|Jul 21, 2016

Millimeter-Wave Propagation and Modeling13 citations

TL;DR

This paper proposes random beamforming schemes for millimeter-wave non-orthogonal multiple access (mmWave-NOMA) networks to reduce feedback overhead by avoiding full channel state information at the base station. By leveraging directional mmWave propagation and stochastic geometry, the schemes achieve high spectral efficiency and outage performance, with one-bit feedback enabling significant overhead reduction while maintaining full diversity gain through optimal threshold design.

ABSTRACT

This paper investigates the coexistence between two key enabling technologies for the fifth generation (5G) mobile networks, non-orthogonal multiple access (NOMA) and millimeter-wave (mmWave) communications. Particularly, the application of random beamforming to the addressed mmWave-NOMA scenario is considered in this paper, in order to avoid the requirement that the base station knows all the users' channel state information. Stochastic geometry is used to characterize the performance of the proposed mmWave-NOMA transmission scheme, by using the key features of mmWave systems, e.g., mmWave transmission is highly directional and potential blockages will thin the user distribution. Two random beamforming approaches which can further reduce the system overhead are also proposed to the addressed mmWave-NOMA communication scenario, where their performance is studied by developing analytical results about sum rates and outage probabilities. Simulation results are also provided to demonstrate the performance of the proposed mmWave-NOMA transmission schemes and verify the accuracy of the developed analytical results.

Motivation & Objective

• Address the high feedback overhead in mmWave-NOMA systems where full channel state information (CSI) is typically required.
• Leverage the directional nature and blockage effects of mmWave propagation to reduce the number of users needing feedback.
• Develop low-feedback beamforming strategies—partial CSI and one-bit feedback—to minimize system overhead while maintaining performance.
• Characterize sum rates and outage probabilities analytically using stochastic geometry to model user distribution and interference.
• Demonstrate that limited-feedback schemes can outperform perfect-CSI schemes due to favorable user pairing under partial CSI.

Proposed method

• Apply stochastic geometry to model user distribution in mmWave networks, incorporating path loss, shadowing, and blockage effects.
• Use random beamforming with a single beam to avoid full CSI feedback, relying on user distance or one-bit feedback for scheduling.
• Propose a partial CSI scheme where users feed back only distance-based path loss, enabling ordering by path loss instead of effective channel gain.
• Design a one-bit feedback scheme where users report only whether their channel quality exceeds a threshold, reducing feedback to one bit per user.
• Analyze the impact of feedback threshold on user pairing and diversity gain, showing that optimal threshold selection preserves full diversity for the strong user.
• Model inter-beam interference using angular separation and beam pattern directivity, showing suppression via narrow beamwidth (Δ = 0.01, ~4° beamwidth).

Experimental results

Research questions

• RQ1 Can random beamforming reduce feedback overhead in mmWave-NOMA without sacrificing spectral efficiency or outage performance?
• RQ2 How does partial CSI based on path loss (distance) affect system performance compared to full CSI in mmWave-NOMA?
• RQ3 What is the impact of one-bit feedback on user pairing and diversity gain in mmWave-NOMA, and can full diversity be preserved?
• RQ4 How does the choice of feedback threshold influence the performance and reliability of the one-bit feedback scheme?
• RQ5 Can limited-feedback schemes outperform perfect-CSI schemes in mmWave-NOMA due to better user pairing under partial CSI?

Key findings

• The diversity gain of the strong user in the one-bit feedback scheme is an increasing function of the number of users K, while the weak user’s diversity gain remains constant at one.
• With a properly designed threshold, the one-bit feedback scheme achieves full diversity gain for the strong user, ensuring reliable performance.
• The sum rate of the one-bit feedback scheme can exceed that of the perfect-CSI scheme when the worst and second-worst users are paired, due to favorable user pairing under partial CSI.
• Inter-beam interference is significantly suppressed due to the directional nature of mmWave beams, especially with narrow beamwidth (Δ = 0.01), enabling effective multiple-beam operation.
• mmWave-NOMA outperforms mmWave-OMA in both sum rate and outage probability, with NOMA achieving zero outage probability for the strong user at high transmit power, unlike OMA.
• Simulation results confirm the analytical accuracy of the stochastic geometry-based performance models, validating the derived expressions for sum rate and outage probability.