Spectral and Energy Efficiency of Cell-Free Massive MIMO Systems with Hardware Impairments

Cell-free massive multiple-input multiple-output (MIMO), with a large number of distributed access points (APs) that jointly serve the user equipments (UEs), is a promising network architecture for future wireless communications. To reduce the cost and power consumption of such systems, it is important to utilize low-quality transceiver hardware at the APs. However, the impact of hardware impairments on cell-free massive MIMO has thus far not been studied. In this paper, we take a first look at this important topic by utilizing well-established models of hardware distortion and deriving new closed-form expressions for the spectral and energy efficiency. These expressions provide important insights into the practical impact of hardware impairments and also how to efficiently deploy cell-free systems. Furthermore, a novel hardware-quality scaling law is presented. It proves that the impact of hardware impairments at the APs vanish as the number of APs grows. Numerical results validate that cell-free massive MIMO systems are inherently resilient to hardware impairments.Comment: 15 pages, 5 figures, to appear in WCSP 201

Downlink Spectral Efficiency of Cell-Free Massive MIMO Systems with Multi-antenna Users

This paper studies a cell-free massive multiple-input multiple-output (MIMO) system where its access points (APs) and users are equipped with multiple antennas. Two transmission protocols are considered. In the first transmission protocol, there are no downlink pilots, while in the second transmission protocol, downlink pilots are proposed in order to improve the system performance. In both transmission protocols, the users use the minimum mean-squared error-based successive interference cancellation (MMSE-SIC) scheme to detect the desired signals. For the analysis, we first derive a general spectral efficiency formula with arbitrary side information at the users. Then analytical expressions for the spectral efficiency of different transmission protocols are derived. To improve the spectral efficiency (SE) of the system, max-min fairness power control (PC) is applied for the first protocol by using the closed-form expression of its SE. Due to the computation complexity of deriving the closed-form performance expression of SE for the second protocol, we apply the optimal power coefficients of the first protocol to the second protocol. Numerical results show that two protocols combining with multi-antenna users are prerequisites to achieve the suboptimal SE regardless of the number of user in the system