Cell-Free Massive MIMO With Mixed-Resolution ADCs and I/Q Imbalance Over Rician Spatially Correlated Channels

The practical deployment of cell-free massive multiple-input multiple-output (CF mMIMO), a promising technology empowering the sixth-generation with multitudinous access points (APs). This paper considers a CF mMIMO system over Rician spatially correlated fading channels with the single-antenna user equipments (UEs) and multi-antenna APs, where the phase-shift induced by the UEs' movement exists at the line-of-sight path. Though the low-resolution analog-to-digital converters (ADCs) can effectively address the expensive hardware cost and power-hungry problems, complicated data processing algorithms are of vital importance to be well designed to compensate for the performance gap compared with the real scenarios since the hardware impairments. However, the novel mixed-ADC architecture could solve the above challenges with better performance. So, we investigate the effect of mixed-resolution ADCs deploying at APs and the in-phase and quadrature-phase imbalance (IQI) on the CF mMIMO systems. Furthermore, the achievable uplink spectral efficiency (SE) is analyzed considering IQI and quantization loss based on a two-layer decoding scheme. Moreover, the novel closed-form SE expression with the maximum ratio (MR) combining is derived. And we find the relationship between the SE performance and different combining schemes, quantization bits, and IQI parameters. Finally, the numerical results reveal the mixed-resolution ADCs show a great advantage in boosting the SE performance, and increasing the number of APs is an effective means to promote the system performance.