Scalable Reciprocity OTA Calibration for Cell-Free Massive MIMO System

With application of a novel user-centric architecture, cell-free massive MIMO (CF-mMIMO) significantly improves the user experienced rate, thereby addressing the issue of constrained interference in dense cell networks. In this technology, high-precision joint over-the-air (OTA) calibration plays an essential role in ensuring the performance of CF-mMIMO networks. However, the large number of remote antenna units (RAUs) in CF-mMIMO, coupled with their spatial independence, introduces considerable complexity for scalable calibration solutions. To address this challenge, this paper proposes a dynamic RAUs clustering and self-healing calibration topology construction algorithm, designed to operate within the constraints of the minimum required signal-to-noise ratio (SNR) and coherence time. Furthermore, to minimize the time overhead of calibration, a calibration timing mechanism for both intra-cluster and inter-cluster of RAUs is devised. Finally, a RIS-aided and scalable OTA calibration algorithm is developed, which can achieve phase alignment for an arbitrary number of RAUs. Simulation results show that the proposed calibration algorithm exhibits low calibration errors and demonstrates excellent performance, resulting in a notable 9.5% increase in spectral efficiency.