Cell-Free Massive MIMO-OFDM: Asynchronous Reception and Uplink Performance Analysis

Cell-free (CF) massive multiple-input multiple-output (mMIMO) is a promising technology for enhancing the spectrum efficiency in future wireless systems. It leverages distributed access points (APs) to jointly serve multiple users over the same time-frequency resource. In this paper, we study the uplink performance of CF mMIMO orthogonal frequency division multiplexing (OFDM) wireless systems under asynchronous signal reception. First, we provide a complete and detailed frequency-domain uplink asynchronous signal reception model for CF mMIMO-OFDM systems, which shows that asynchronous reception introduces phase shift, inter-carrier interference (ICI), and inter-symbol interference (ISI) at the receivers. Then, we design the minimum mean square error channel estimation under asynchronous reception. The results indicate that the phase shift, ICI, and ISI destroy the orthogonality of pilot sequences at APs, leading to significant channel estimation error. Furthermore, the FDM pilot sequences prove to be more effective in mitigating the impact of asynchronous reception on channel estimation compared to the CDM pilot sequences. Given the imperfect channel state information caused by pilot contamination and asynchronous reception, we characterize the uplink achievable spectral efficiency under asynchronous reception. The results indicate that severe uplink rate loss mainly arises from phase shift caused by asynchronous reception. Numerical simulation results corroborate our analyses.