Cell-Free Massive MIMO-NOMA Systems with Nonreciprocal Channels: Performance Analysis and Calibration Method Design

The integration of communication technologies is regarded as a potential direction of future network. Combining cell-free massive multiple-input multiple-output (CF-mMIMO) with nonorthogonal multiple access (NOMA) can achieve substantial access and uniform coverage, but it also poses challenges on channel state information (CSI) acquisition. Although systems operating in time division duplex can approximate uplink CSI for downlink precoding design, the performance degradation resulting from nonreciprocity is still nonnegligible. This article proposes a calibration method to improve system sum rate of CF-mMIMO-NOMA with nonreciprocal channels. The proposed method exploits the impact of all access points (APs) on achievable rate to provide a prioritization and calibrates the selected APs orderly, finding an optimal compromise between pilot overhead and performance gain. The order and number of calibrated APs can be adjusted according to the parameters including coherence interval, number of users, etc. Based on the calibration method, a power allocation algorithm focusing on maximizing the minimum rate is presented. The nonconvex initial model is transformed into a standard second-order cone programming problem through slack variables for bisection solution. Simulation results validate the importance of calibration and indicate that the proposed method provides enhanced achievable rate with flexibility and adaptability.