Hierarchical Distributed Distortion Aware Beamforming with Dynamic User Association & BS Clustering for Cell-Free Massive-MIMO Systems
Massive multiple-input multiple-output (CF-mMIMO) systems have been identified as a promising technique for wireless communication, in terms of the enhancement of spectral efficiency, network coverage and user fairness. The key issues to be addressed for practical deployment are impairments in the hardware, inter-cell interference, high computational complexity, and high number of backhaul signalling, all of which impact system performance. To tackle these problems, in this paper, a Hierarchical Cluster-Based Distortion-Aware User Association (HC-DA-UA) framework for the distributed CFmMIMO system under hardware impairments is proposed to solve these problems. The proposed method first clusters the distributed base stations based on the channel energy in a dynamic way to minimize the coordination overhead. Then a hierarchical distributed beamforming method is used to optimise the precoding in each cluster to minimise distortion caused by non-ideal hardware components. In addition, a distortion-aware user association mechanism is used to select the best serving base station for each user, taking into account the signal received and the distortion induced by the hardware and the interference. A second beamforming module called an interference estimation module further enhances the beamforming process to yield the most optimal achievable beamforming signal-to-interference-plus-noise-and-distortion ratio (SINDR). The simulation results reveal that the proposed HC-DA-UA framework always outperforms the existing counterparts in terms of sum-rate, whilst substantially decreasing the computational complexity and signalling overhead, thus it is an efficient and scalable solution for next-generation hardware-impaired distributed wireless communication networks.