[Paper Review] A Multi-Beam NOMA Framework for Hybrid mmWave Systems

Zhiqiang Wei, Lou Zhao+3 more|arXiv (Cornell University)|Apr 23, 2018

Millimeter-Wave Propagation and Modeling19 references3 citations

TL;DR

This paper proposes a multi-beam non-orthogonal multiple access (NOMA) framework for hybrid millimeter wave (mmWave) systems that uses beam splitting to generate multiple analog beams from a single RF chain, enabling simultaneous transmission to users with diverse angles of departure. The scheme achieves higher spectral efficiency than both single-beam NOMA and time-division multiple access (TDMA), with analytical conditions proving its superiority under high SNR and large antenna arrays.

ABSTRACT

In this paper, we propose a multi-beam non-orthogonal multiple access (NOMA) framework for hybrid millimeter wave (mmWave) systems. The proposed framework enables the use of a limited number of radio frequency (RF) chains in hybrid mmWave systems to accommodate multiple users with various angles of departures (AODs). A beam splitting technique is introduced to generate multiple analog beams to facilitate NOMA transmission. We analyze the performance of a system when there are sufficient numbers of antennas driven by a single RF chain at each transceiver. Furthermore, we derive the sufficient and necessary conditions of antenna allocation, which guarantees that the proposed multi-beam NOMA scheme outperforms the conventional time division multiple access (TDMA) scheme in terms of system sum-rate. The numerical results confirm the accuracy of the developed analysis and unveil the performance gain achieved by the proposed multi-beam NOMA scheme over the single-beam NOMA scheme.

Motivation & Objective

• Address the limited user multiplexing capability in hybrid mmWave systems due to a small number of RF chains.
• Overcome the constraints of conventional orthogonal multiple access (OMA) and single-beam NOMA in mmWave bands, where narrow beams restrict user grouping.
• Enable flexible, high-capacity multi-user access in hybrid mmWave systems by exploiting spatial multiplexing via multiple analog beams per RF chain.
• Derive analytical conditions for antenna allocation that guarantee performance gain over TDMA in terms of system sum-rate.
• Demonstrate the superiority of multi-beam NOMA over single-beam NOMA and TDMA in terms of spectral efficiency and robustness to user angular distribution.

Proposed method

• Introduce a beam splitting technique that divides the full antenna array into multiple subarrays, each generating an independent analog beam.
• Assign one RF chain per NOMA group, with each group served by multiple analog beams to accommodate users with different angles of departure (AODs).
• Use equal power allocation across users within each NOMA group and equal time allocation in the TDMA baseline for fair comparison.
• Derive asymptotic system sum-rate expressions in the large-antenna and high-SNR regimes to analyze performance under ideal conditions.
• Establish sufficient and necessary conditions for antenna allocation (via equations 22 and 30) that ensure the multi-beam NOMA scheme outperforms TDMA.
• Validate the analytical results through simulations under varying transmit power and user channel gain ratios.

Experimental results

Research questions

• RQ1 Can a multi-beam NOMA framework be designed to support multiple users with arbitrary AODs using only a single RF chain in hybrid mmWave systems?
• RQ2 What are the sufficient and necessary conditions on antenna allocation that guarantee the proposed multi-beam NOMA scheme achieves higher sum-rate than TDMA?
• RQ3 How does the performance of the proposed multi-beam NOMA scheme compare to single-beam NOMA and TDMA in terms of spectral efficiency under realistic mmWave channel conditions?
• RQ4 To what extent does the system sum-rate of the proposed scheme scale with the number of antennas per beam and transmit power?
• RQ5 How accurately can the asymptotic analysis predict the performance gain of multi-beam NOMA over TDMA in high-SNR, large-antenna regimes?

Key findings

• The proposed multi-beam NOMA scheme achieves a monotonically increasing system sum-rate with the number of antennas allocated to the strongest user ($M_1$), as confirmed by simulations and asymptotic analysis.
• The derived antenna allocation condition (equation 30) accurately predicts the threshold for $M_1$ beyond which the multi-beam NOMA outperforms TDMA in sum-rate.
• The performance gain of multi-beam NOMA over TDMA remains constant as transmit power ($p_{ ext{max}}$) increases, indicating that the gain is dominated by spatial multiplexing and channel gain disparity at high SNR.
• Compared to the baseline single-beam NOMA scheme, the proposed multi-beam NOMA achieves a significant performance gain due to its ability to serve users with diverse AODs simultaneously.
• Simulation results confirm the accuracy of the asymptotic analysis, with close match between theoretical predictions and empirical outcomes for both sum-rate and performance thresholds.
• The system sum-rate of multi-beam NOMA increases with the channel gain disparity between users, demonstrating the efficiency of power domain multiplexing in exploiting channel strength differences.