Joint Power Allocation and Beamforming for Non-Orthogonal Multiple Access (NOMA) in 5G Millimeter-Wave Communications (1711.01380v1)

Abstract: In this paper we explore non-orthogonal multiple access (NOMA) in millimeter-wave (mmWave) communications (mmWave-NOMA). In particular, we consider a typical problem, i.e., maximization of the sum rate of a 2-user mmWave-NOMA system. In this problem, we need to find the beamforming vector to steer towards the two users simultaneously subject to an analog beamforming structure, while allocating appropriate power to them. As the problem is non-convex and may not be converted to a convex problem with simple manipulations, we propose a suboptimal solution to this problem. The basic idea is to decompose the original joint beamforming and power allocation problem into two sub-problems which are relatively easy to solve: one is a power and beam gain allocation problem, and the other is a beamforming problem under a constant-modulus constraint. Extension of the proposed solution from 2-user mmWave-NOMA to more-user mmWave-NOMA is also discussed. Extensive performance evaluations are conducted to verify the rational of the proposed solution, and the results also show that the proposed sub-optimal solution achieve close-to-bound sum-rate performance, which is significantly better than that of time-division multiple access (TDMA).

• The paper introduces a suboptimal joint optimization approach for beamforming and power allocation in 5G mmWave NOMA to maximize the sum rate in a 2-user system.
• It decomposes the non-convex problem into two sub-problems: optimizing power/beam gain allocation and designing beamforming under constant modulus constraints.
• Numerical evaluations demonstrate that the proposed method outperforms TDMA by achieving near-upper-bound performance in spectrum efficiency and user capacity.

Joint Power Allocation and Beamforming for NOMA in 5G Millimeter-Wave Communications

The paper under consideration addresses a significant challenge in 5G millimeter-wave (mmWave) communications, specifically focusing on the optimal design of beamforming and power allocation strategies for non-orthogonal multiple access (NOMA) techniques. NOMA has been identified as a crucial approach for enhancing the spectrum efficiency and user capacity in next-generation wireless systems, especially in scenarios characterized by a limited number of resource blocks.

Core Contributions

The primary focus of this paper is on maximizing the sum rate in a typical 2-user mmWave-NOMA system. Given the constraints posed by an analog beamforming structure, the problem requires a joint optimization of the beamforming vector and power allocation strategy. The complication arises due to the non-convex nature of the problem, which, as the authors point out, cannot be straightforwardly addressed using conventional convex optimization techniques. The paper introduces a suboptimal solution, which involves decomposing the problem into two interconnected sub-problems:

1. Power and Beam Gain Allocation: This sub-problem is tackled to determine the optimal power distribution and beam gains for the users. The authors employ a novel theoretical framework that ensures the sum of the squared beam gains weighted by the channel coefficients equals the number of antennas, maintaining a balance between gain and allocation efficiency.
2. Beamforming with Constant Modulus Constraint: The second sub-problem focuses on the design of a beamforming vector under a constant-modulus constraint, inherent to the mmWave communication hardware. The vector needs to satisfy certain gain requirements for the users, making this step critical for maintaining the link quality and system performance.

Numerical Insights

The authors provide comprehensive numerical evaluations to demonstrate the efficacy of their proposed solution. These evaluations reveal that the sum-rate performance achieved by their method approaches the theoretical performance upper bound set by ideal conditions. Specifically, the proposed solution outperforms traditional time-division multiple access (TDMA) approaches, showcasing significant improvements in both individual user rates and overall sum rate.

Practical and Theoretical Implications

From a practical standpoint, the proposed solution has significant implications for the deployment of 5G systems, where beamforming and power allocation need to be dynamically optimized to cater to varying user demands and channel conditions. The findings also underscore the potential of NOMA schemes in mmWave scenarios, where the high directionality and frequency reuse characteristics can be exploited for enhanced performance.

Theoretically, the decomposition of the original non-convex problem into two manageable sub-problems offers a promising approach for other complex optimization issues in wireless communications. The authors further explore extensions of their method to more extensive user systems, signifying the scalability and adaptability of their approach.

Future Directions

This work sets a foundation for future research on multi-user scenarios in mmWave-NOMA systems. The adaptation of the proposed method to accommodate more users or alternative antenna array configurations is highlighted as a potential avenue for further investigation. Moreover, integration with hybrid beamforming structures could further enhance user capacity and system robustness. The continued exploration of analytic and algorithmic improvements will be vital in fine-tuning the balance between computation complexity and performance gains in future wireless systems.