Impact of Intra-Cluster Angular Spread on the Performance of NLoS Millimeter Wave Links With Imperfect Beam Alignment

Beam misalignment is an unavoidable occurrence in millimeter wave (mmWave) wireless communication systems which causes a drop in achievable spectral efficiency (SE) and adversely affects the link-level performance. Spatial parameters of the mmWave propagation channel, especially angle distributions of multi-path components are vital in assessing the loss in achievable capacity due to imperfect beam alignment in such systems. In this paper, we present a tractable approach to quantify the achievable SE of beamformed Non-Line-of-Sight (NLoS) links in mmWave fading channels with intra-cluster angular spread. We derive the expression for achievable SE of a point-to-point mmWave NLoS link after accounting for random beam misalignment errors at the transmitter as well as receiver end. Using analytic approximations, we consider the impact of antenna beam pattern parameters, and use a comprehensive mmWave channel model for performance evaluation. The analysis provides realistic bounds on achievable SE since the intra-cluster angular spread of rays at both ends is taken into account. We provide insights into the interplay between antenna half power beam width, the degree of misalignment, channel fading coefficient, channel coherence parameter and the angular spread of rays to demonstrate the decisive impact of intra-cluster spread on the performance of mmWave point-to-point NLoS links.