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Massive multiple-input multiple-output (MIMO) avails of simple transceiverdesign which can tackle many drawbacks of relay systems in terms of complicatedsignal processing, latency, and noise amplification. However, the cost andcircuit complexity of having one radio frequency (RF) chain dedicated to eachantenna element are prohibitive in practice. In this paper, we address thiscritical issue in amplify-and-forward (AF) relay systems using a hybrid analogand digital (A/D) transceiver structure. More specifically, leveraging thechannel long-term properties, we design the analog beamformer which aims tominimize the channel estimation error and remain invariant over a longtimescale. Then, the beamforming is completed by simple digital signalprocessing, i.e., maximum ratio combining/maximum ratio transmission (MRC/MRT)or zero-forcing (ZF) in the baseband domain. We present analytical bounds onthe achievable spectral efficiency taking into account the spatial correlationand imperfect channel state information at the relay station. Our analyticalresults reveal that the hybrid A/D structure with ZF digital processor exploitsspatial correlation and offers a higher spectral efficiency compared to thehybrid A/D structure with MRC/MRT scheme. Our numerical results showcase thatthe hybrid A/D beamforming design captures nearly 95% of the spectralefficiency of a fully digital AF relaying topology even by removing half of theRF chains. It is also shown that the hybrid A/D structure is robust to coarsequantization, and even with 2-bit resolution, the system can achieve more than93% of the spectral efficiency offered by the same hybrid A/D topology withinfinite resolution phase shifters.

Hybrid Processing Design for Multipair Massive MIMO Relaying with Channel Spatial Correlation