Robust Efficient Hybrid Pre-Coding Scheme for mmWave Cell-Free and User-Centric Massive MIMO Communications

Millimeter-wave cell-free and user-centric massive multiple input multiple output (MIMO) systems are promising, due to the reliable ultra-high data rate services. Thus, it is a significant issue to design low-complexity hybrid precoders. In this paper, a simplified sum-mean-square-error (SMSE) is used to design hybrid precoders, based on the array gain and spatial macro-diversity. Then, a heuristic hybrid pre-coding scheme is proposed, with two analog design algorithms that are with or without the knowledge of statistical information. The influence of the imperfect state information of beams is theoretically analyzed, from the instantaneous and ergodic aspects, verifying the robustness of the proposed scheme. Simulation results show that our proposed low-complexity hybrid precoder can be easily realized with acceptable bit error rate (BER) and sum-rate performances, especially when the total transmit power is small.