Recursive method for the equivalent baseband channel for hybrid beamforming in massive MU-MIMO mmWave systems

Communications using the millimeter wave (mmWave) spectrum are a mainstream technology of the next generation systems due to its huge available bandwidth. However, the mmWave band will experience much more propagation loss than a low-frequency band. Conventional precoding techniques are impractical at mmWave scenarios due to manufacturing costs and power consumption. Hybrid beamforming, which combines large dimensional analog processing with lower-dimensional digital processing, is the most promising approach for reducing the hardware cost in massive mmWave MU-MIMO systems. The digital processing part can be designed from traditional MIMO methods, however, the analog processing part has to be carefully selected to produce a workable equivalent baseband channel. In this paper, we propose three hybrid processing designs based on MMSE, whose difference relies on their analog part. The considered strategies for designing the analog part are through singular value decomposition, an iterative method, and a recursive algorithm, where the last one highlights due to its robustness. Numerical results in terms of BER evidence that the proposed recursive algorithm reaches the best performance regardless of the scenarios and data detectors.