Estimation of Channel State Information (CSI) in Cell-Free Massive MIMO Based on Time of Arrival (ToA)

Cell-free massive multiple input multiple output (MIMO) comprises a large number of distributed single antenna access points (APs) connected to a network controller through backhaul. Each user is served by neighbouring APs simultaneously. Cell-free Massive MIMO provides decent throughput and coverage improvement as compared to a small-cell. Yet, channel state information (CSI) estimation is still a bottleneck in massive MIMO systems. In this paper, we propose an estimation method that does not need channel statistics or any historical assumption. We use the received pilots at the APs to provide CSI with mitigated co-pilot interference. The received signal is used to estimate the time of arrival (TOA) of signals from different paths, and, subsequently, signals that travel long distances are filtered out. The results show noticeable enhancement over LS estimation if the estimate of TOA is accurate.