Secure Transmission in RIS-Assisted Cell-free Massive MIMO system with Low Resolution ADCs/DACs

This paper investigates the secure communication in reconfigurable intelligent surface aided cell-free massive MIMO system in the presence of active eavesdropping, where the access points (APs) are equipped with low resolution analog-to-digital/digital-to-analog converters (ADCs/DACs). Specifically, the additive quantization noise model (AQNM) is explored to capture the impacts of coarse ADCs/DACs. To reduce the channel estimation overhead, all APs employ a minimum mean-squared-error channel estimator which estimates the aggregated channel including both the direct and indirect channels. With the available imperfect channel state information (CSI), conjugate beamforming and random beamforming are applied at the APs and the RISs for downlink data transmission. Moreover, the closed-form expression of the achievable ergodic secrecy rate is derived which can be used to evaluate the impact of the number of the APs, the number of the RISs and the ADCs/DACs resolution on the systems performance. Finally, numerical and simulation results are presented to verify the analytical results.