RF Impairments and Low-Resolution ADCs for Nonideal Uplink Cell-Free Massive MIMO Systems

In this article, a generalized uplink cell-free massive multi-input multi-output (MIMO) system with nonideal radio frequency (RF) chains and low-resolution analog-to-digital converters (ADCs) is investigated. Specifically, the RF impairments and ADC imperfections not only hinder the accuracy of channel estimation but also deteriorate the strength of received signal-to-interference-plus-noise ratio. Achievable closed-form rate expressions are first derived by leveraging on the Gaussian RF impairment model and linear additive quantization noise model, after that, we explore the uplink achievable rate surrounding the key design parameters, i.e., the quality of RF chains, the precision of low-resolution ADCs, and the number of total antenna arrays. Considering channel estimation, we find employing poor RF chains and ADCs could fetch finite ceilings on estimation accuracy. Besides, the RF scaling law is also established, which suggests that the access points (APs) can tolerate lower receiver quality as the number of total antenna arrays increases, and the receiver quality can be cut down at most by 1/√{MN}, where M and N are the number of APs and the number of antennas of each AP, respectively. Moreover, we observe 5-bit ADCs are enough to substitute infinite-bit ADCs with scarce performance degradation. Finally, the compensation between RF impairments and ADC imperfections is presented. The resultant analysis provides insights for designing a more economical cell-free massive MIMO system while keeping the achievable rate at an acceptable level.