Impact of Beam Misalignment on Hybrid Beamforming NOMA for mmWave Communications

Abstract

This paper analyzes the effect of beam misalignment on rate performance in downlink of hybrid beamforming-based non-orthogonal multiple access (HB-NOMA) systems. First an HB-NOMA framework is designed in multiuser millimeter wave (mmWave) communications. A sum-rate maximization problem is formulated for HB-NOMA, and an algorithm is introduced to design digital and analog precoders and efficient power allocation. Then, regarding perfectly aligned line-of-sight (LoS) channels, a lower bound for the achievable rate is derived. Next, when the users experience misaligned LoS or non-LoS (NLoS) channels, the impact of beam misalignment is evaluated. To this end, a misalignment factor is modeled and each misaligned effective channel is described in terms of the perfectly aligned effective channel parameters and the misalignment factor. Further, a lower bound for the achievable rate is extracted. We then derive an upper bound for the rate gap expression between the aligned and misaligned HB-NOMA systems. The analyses reveal that a large misalignment can remarkably degrade the rate. Extensive numerical simulations are conducted to verify the findings.

Radio Channel Capacity with Directivity Control of Antenna Beams in Multipath Propagation Environment

“…Antenna beam misalignment in emerging 5G systems, especially in NLOS conditions, is a significant issue from the viewpoint of effective beamforming and tracking procedures, which ensure the achievement of the maximum capacity. Numerous papers, i.a., [31][32][33], presenting both the effects and methods of reducing mismatch, testify to the importance of this topic. The direction mismatch of the transmitting and receiving beams is the reason for the increase in path loss.…”

“…Power losses have a significant impact on reducing the transmission data rate. Examples of solutions that minimize the effects of mismatches for MIMO and hybrid systems, non-orthogonal access systems based on the beamforming technique, are presented in [32,33].…”