Active RIS-assisted Communication Systems with Discrete Phase Shifts

Reconfigurable intelligent surface (RIS) has made significant contributions to enhancing wireless coverage via the effective management of scattering radio environment. This paper centers on an active RIS-assisted communication system over Nakagami-m fading channels where the discrete phase shifter is taken into consideration. We first obtain the approximate expression of ergodic achievable rate. Analyzing the expression shows that 4-bit quantization can achieve over 99% of the performance of continuous phase shifts. We also make a comparison between active RIS and passive RIS. Our theoretical analysis illustrates that passive RIS outperforms active RIS for large numbers of RlS elements. The numerical results are consistent with our analysis results. Finally, we propose a low-complexity power allocation scheme to improve the system rate effectively.