On the Uplink Throughput of Zero Forcing in Cell-Free Massive MIMO With Coarse Quantization

The recently proposed cell-free massive multiple input multiple output (MIMO) architecture is studied for the uplink. In contrast to most previous works, joint detection is performed using global channel state information (CSI). Therefore, we study strategies for transferring CSI to the central processing unit taking into account the fronthaul capacity which limits CSI quantization. Two strategies for pilot-based CSI acquisition are considered: estimate-and-quantize and quantize-and-estimate. These are analyzed using the Bussgang decomposition. For a given quantization constraint for the data and CSI the achievable rate per user with zero forcing is determined. Numerical results show that quantize-and-estimate (the simpler strategy) is similar to or better than estimate-and-quantize at low resolution, especially for 1-bit.