Cell-Free Massive MIMO with Low-Resolution ADCs and I/Q Imbalance Over Spatially Correlated Channels

In this paper, we investigate a cell-free massive multiple-input multiple-output (CF mMIMO) system with both multi-antenna user equipments (UEs) and access points (APs) over spatially correlated Rayleigh fading channels. In practi-cal CF mMIMO systems, the in-phase and quadrature-phase imbalance (IQI) and low-resolution analog-to-digital converters (ADCs) at the APs are critical for the system performance. Taking these factors into account, the achievable uplink spectral efficiency (SE) is analyzed based on a two-layer decoding scheme. In particular, the maximum ratio (MR) and local minimum mean-square error (L-MMSE) combining are adopted at the APs while the large-scale fading decoding (LSFD) is implemented at the central processing unit (CPU). Furthermore, we derive novel closed-form SE expressions with the MR combining and investigate the SE performance for different combining schemes, quantization bits, and IQI parameters. Numerical results reveal the performance degradations caused by both the low-resolution ADCs and IQI. Additionally, increasing the number of APs is an effective means to promote the system performance.