Asynchronous Reception Effects on Distributed Massive MIMO-OFDM System

In a distributed massive multiple-input and multiple-output (DM-MIMO) orthogonal frequency division multiplexing (OFDM) system, each of the multiple receivers receives the OFDM signals simultaneously from all transmitters intended for itself and other receivers. Due to the distributed nature of the network and the differences in propagation delays, these OFDM signals received by any receiver are most likely not synchronized. We have analyzed analytically and numerically the asynchronous reception effects on a DM-MIMO OFDM system in this paper. It is shown that the desired signal power reduction due to phase shifts caused by timing offsets is the dominant performance degrading factor. Surprisingly, the increase of various interfering power (including multiuser, inter-carrier, and inter-symbol interference) due to timing offsets is negligible when compared with the desired signal power reduction. Two per-user rate lower bounds have been developed by averaging over small-scale fading. With the aids of these two lower bounds, averaged per-user rate over small- and large-scale fading have been simulated. It is found that around 50% reduction in per-user downlink rate could occur to the majority of users due to asynchronous reception in DM-MIMO OFDM systems. This important factor needs to be considered in implementing any future DM-MIMO system.