Null-Space Expansion and Transmission Power Control for Cell-Free Massive MIMO Systems

Cell-Free Massive MIMO (CF-mMIMO) has emerged as a promising architecture to overcome limitations in traditional cellular networks, particularly the degradation of communication quality at cell edges. However, downlink multi-user MIMO systems generally suffer from significant performance degradation in mobile environments due to time-varying channels. To address this issue, Null Space Expansion (NSE) has been proposed as a prior technique for robust interference suppression. This paper applies NSE to the CF-mMIMO framework to improve its resilience against channel variation. In addition to interference, power imbalance among users, especially those around former cell boundaries, remains a major challenge in CF-mMIMO systems. Therefore, this study aims to maximize overall system performance by jointly addressing both interference and transmission power control (TPC) problems. We formulate a power optimization problem under the assumption that NSE is applied and attempt to solve it using an efficient numerical approach. Simulation results confirm that the combined use of NSE and optimized power control enhances multiuser spatial multiplexing capability in CF-mMIMO systems.