[Paper Review] User Fairness Non-orthogonal Multiple Access (NOMA) for 5G Millimeter-Wave Communications with Analog Beamforming

Zhenyu Xiao, Lipeng Zhu+3 more|arXiv (Cornell University)|Nov 7, 2018

Advanced Wireless Communication Technologies28 references4 citations

TL;DR

This paper proposes a user fairness-aware non-orthogonal multiple access (NOMA) scheme for 5G millimeter-wave (mmWave) networks with analog beamforming, optimizing joint beamforming and power allocation to maximize the minimum user rate (max-min fairness). By exploiting angular domain sparsity and deriving a closed-form power allocation, the method achieves near-optimal spectral efficiency, significantly outperforming conventional mmWave-OMA systems in fairness and sum rate.

ABSTRACT

The integration of non-orthogonal multiple access in millimeter-Wave communications (mmWave-NOMA) can significantly improve the spectrum efficiency and increase the number of users in the fifth-generation (5G) mobile communication. In this paper we consider a downlink mmWave-NOMA cellular system, where the base station is mounted with an analog beamforming phased array, and multiple users are served in the same time-frequency resource block. To guarantee user fairness, we formulate a joint beamforming and power allocation problem to maximize the minimal achievable rate among the users, i.e., we adopt the max-min fairness. As the problem is difficult to solve due to the non-convex formulation and high dimension of the optimization variables, we propose a sub-optimal solution, which makes use of the spatial sparsity in the angle domain of the mmWave channel. In the solution, the closed-form optimal power allocation is obtained first, which reduces the joint optimization problem into an equivalent beamforming problem. Then an appropriate beamforming vector is designed. Simulation results show that the proposed solution can achieve a near-upper-bound performance in terms of achievable rate, which is significantly better than that of the conventional mmWave orthogonal multiple access (mmWave-OMA) system.

Motivation & Objective

• To address user fairness limitations in mmWave-OMA by integrating NOMA for improved spectral efficiency and user capacity.
• To jointly optimize beamforming and power allocation in a downlink mmWave-NOMA system with analog beamforming and a single RF chain.
• To maximize the minimum achievable rate among users (max-min fairness) under hardware constraints and non-convex optimization.
• To develop a sub-optimal solution that leverages channel sparsity in the angle domain to reduce computational complexity.

Proposed method

• Formulates a non-convex joint beamforming and power allocation problem to maximize the minimum user rate under total power and single RF chain constraints.
• Exploits angular domain sparsity of mmWave channels to simplify the optimization and reduce dimensionality.
• Derives a closed-form optimal power allocation that transforms the joint problem into a beamforming-only optimization.
• Designs an effective beamforming vector based on the strongest user's channel direction to enhance fairness and system performance.
• Uses successive interference cancellation (SIC) at users, with users sorted by increasing channel gain for decoding order.
• Proves that the derived power allocation achieves equal rates across all users and is optimal under the max-min fairness criterion.

Experimental results

Research questions

• RQ1 Can NOMA be effectively applied to mmWave systems with analog beamforming to improve fairness and spectral efficiency?
• RQ2 How can user fairness be maximized in a mmWave-NOMA system under single-antenna RF chain and analog beamforming constraints?
• RQ3 What is the optimal power allocation strategy that achieves max-min fairness in a mmWave-NOMA downlink with SIC?
• RQ4 How does exploiting angular domain sparsity improve the performance and tractability of beamforming and power allocation?
• RQ5 Can the proposed sub-optimal solution achieve near-optimal performance compared to the theoretical upper bound?

Key findings

• The proposed method achieves near-optimal performance in terms of achievable rate, closely approaching the theoretical upper bound.
• The closed-form power allocation ensures all users achieve equal rates, satisfying the max-min fairness criterion.
• Simulation results show significant performance gains over conventional mmWave-OMA, particularly in fairness and spectral efficiency.
• The system achieves higher sum rates and better outage performance compared to orthogonal multiple access schemes.
• The beamforming design based on the strongest user's channel direction effectively enhances fairness and system robustness.
• The solution maintains feasibility under power and hardware constraints, with the total power consumption not exceeding the budget.