Low-Complexity Hybrid Precoding Based on PAST for Millimeter Wave Massive MIMO System

Millimeter wave (mmWave) massive Multiple-Input Multiple-Output (MIMO) systems generally adopt hybrid precoding com-bining digital and analog precoder as an alternative to full digital precoding to reduce RF chains and energy consumption. In order to balance the rela-tionship between spectral efficiency, energy efficiency and hardware com-plexity, the hybrid-connected system structure should be adopted, and then the solution process of hybrid precoding can be simplified by decompos-ing the total achievable rate into several sub-rates. However, the singular value decomposition (SVD) incurs high complexity in calculating the opti-mal unconstrained hybrid precoder for each sub-rate. Therefore, this paper proposes PAST, a low complexity hybrid precoding algorithm based on projection approximate subspace tracking. The optimal unconstrained hy-brid precoder of each sub-rate is estimated with the PAST algorithm, which avoids the high complexity process of calculating the left and right singular vectors and singular value matrix by SVD. Simulations demonstrate that PAST matches the spectral efficiency of SVD-based hybrid precoding in full-connected (FC), hybrid-connected (HC) and sub-connected (SC) sys-tem structure. Moreover, the superiority of PAST over SVD-based hybrid precoding in terms of complexity and increases with the number of trans-mitting antennas.