Impact of Beam Misalignment on Hybrid Beamforming NOMA for mmWave Communications

The effect of beam misalignment on rate performance of the downlink hybrid beamforming-based nonorthogonal multiple access (HB-NOMA) systems is analyzed in this paper. First, an HB-NOMA framework is introduced in the context of multiuser millimeter wave (mmWave) communications, a sum-rate maximization problem is formulated for that, and an algorithm is introduced to design digital and analog precoders and power allocation. Then, a lower bound for the achievable rate of the perfectly aligned line-of-sight (LoS) channels is derived. Subsequently, the impact of beam misalignment is evaluated when the users experience misaligned LoS or non-LoS channels. 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. Furthermore, a lower bound for the achievable rate, and an upper bound for the rate gap expression between the aligned and misaligned HB-NOMA systems are established. The analyses reveal that a large misalignment can remarkably degrade the rate. Extensive numerical simulations are conducted to verify the findings.