RIS-Aided Fronthaul Constrained Cell-Free Massive MIMO Systems with Hardware Impairments

This paper investigates the impact of hardware impairments in terms of low-cost radio frequency (RF) chains and dynamic-resolution analog-to-digital/digital-to-analog converters (ADCsIDACs) over a reconfigurable intelligent surface (RIS) enhanced cell-free massive multiple-input multiple-output system. We aim to model a practical scenario by considering phase noise and spatial correlation at the RIS. We utilize the well-known linear minimum mean square error estimation to obtain the channel estimates and present a closed-form expression for the downlink spectral efficiency (SE). To reduce the fronthaul load, we implement an access point (AP) selection strategy that assigns each user to a specific set of APs rather than all the APs. Our analysis reveals that i) the SE with RIS-aided links is significant in scenarios with low transmit power, especially when the direct link is weak, ii) the effect of ADCslDACs imperfection is more acute than RF impairments at higher transmit power levels, iii) the energy efficiency shows minimal impact at lower impairment levels but becomes significantly affected as the impairment level increases, and iv) dynamic-resolution ADCslDACs playa vital role on the system performance.