Downlink transmission and channel estimation for cell-free massive MIMO-OFDM with DSDs

Abstract

In a cell-free massive MIMO system, multiple users arrive at multiple access points at separate times, while in an OFDM system, different delays can be equivalent to delay spread differences (DSDs). Since DSDs are not all the same, in the downlink transmission process, it is necessary to consider its impact on transmission techniques, such as channel estimation and downlink precoding. In this paper, aiming at the performance loss caused by DSDs in cell-free massive MIMO-OFDM system, we propose a multi-RB precoding optimization algorithm that maximizes the downlink sum rate. We derive the sum rate maximization problem into an iterative second-order cone programming form to achieve convex approximation. Then, considering the impact of DSDs on the accuracy of cell-free massive MIMO-OFDM channel estimation, we propose a downlink channel estimation method, which jointly uses channel state information reference signal and demodulation reference signal. The simulation results show that the proposed multi-RB optimal precoding can effectively improve the downlink sum rate, and the proposed downlink channel estimation can obtain accurate multi-RB frequency-domain channel parameters.

Toward 6G TK$μ$ Extreme Connectivity: Architecture, Key Technologies and Experiments

“…It will further increase the complexity of downlink precoding of wideband OFDM systems. Therefore, we derived an optimized precoding vector within multiple resource blocks (RBs), which can obtain better performance than the traditional precoding [42]. For multi-user receiver, multi-RB interference suppression can be used first to eliminate interference between users, and then a single user receiver with interference rejection combining can be used to obtain data streams of each user.…”