Effect of Spatial Correlation in Massive MIMO Transmission

This work presents a study on single-carrier and orthogonal frequency-division multiplexing massive MIMO transmission systems for the frequency selective Gaussian multi-user downlink and uplink channels. It compares the spatially uncorrelated channels with the correlated ones in terms of the user achievable sum-rate. The correlation is considered to be among antenna elements at the base station (the transmitter side in the downlink direction and the receiver side in the uplink direction) in a channel with $K$ single-antenna users receiving and sending information from and to $M$ antennas at the base station. This work provides a general expression for the achievable rate by users in the channel and it shows for low-SNR regions in the channel, the channel matched filter technique outperforms all the other considered techniques, when the channel is uncorrelated over space. In a highly-correlated massive MIMO channel, conventional channel matched filter technique does not perform as expected. We show the failure of channel matched filter technique in the presence of two different correlation patterns among antennas at the base station and suggest better schemes which lead to a better achievable sum-rate. It has been shown as the result of computations that the suggested techniques (precoders in the downlink direction and equalizers in the uplink direction) perform better than the conventional channel matched filter technique in terms of achievable sum-rate