Hierarchical optimization: A hybrid processing for downlink massive MU‐MIMO mmWave systems

Abstract

Abstract Communications using the millimeter wave (mmWave) spectrum are a mainstream technology of the next generation systems due to their huge available bandwidth. However, the mmWave band will experience much more propagation loss than a low‐frequency band. Conventional precoding techniques are impractical in mmWave scenarios due to manufacturing costs and power consumption. Hybrid alternatives have been considered as a promising technology to provide a compromise between hardware complexity and system performance by reducing the number of radio‐frequency (RF) chains. In this paper, a hybrid processing design for downlink in mmWave scenarios is proposed, where the number of RF chains is reduced to the number of data streams. The precodification process is made by a three‐stage‐based hybrid precoder, whereas in the terminals, the equalization process only requires a low complex analog combiner. The three stages of the hybrid precoder are designed by a hierarchical strategy. The first stage addresses the phase shifters aiming to maximize the sum‐rate system, whereas the second and third stages are dedicated to mitigating the inter‐user interference effects. The proposed methodology gets a robust signal processing that can greatly mitigate the side effects of hardware reduction. Numerical results in terms of bit error rate (BER) and sum‐rate evidence that the proposed method not only overcomes other hybrid precoders/combiners but also reaches the same performance as a fully digital technique in some scenarios.