RIS-Assisted Downlink Transmission under Unequal Coherence Intervals and CSI Feedback

Multi-user downlink systems with reconfigurable intelligent surfaces (RIS) require considerable overhead for training the channels and RIS states. This is made even more difficult when the links have non-identical coherence times, a frequently occurring condition due to the variations in node mobility and scattering environment. In the absence of RIS, previous studies have shown that pilot reuse can reduce the pilot overhead under coherence disparity and achieve gains in both rate and degrees of freedom (DoF). This work explores the impact of pilot reuse on RIS-assisted downlink channels having unequal link coherence times and shows how overlapping pilot and data transmission can reduce the training overhead and achieve rate gains. Additionally, we jointly design the transmit beamforming and RIS reflection coefficient vectors to maximize the sum-rate achieved through the proposed transmission scheme. We employ fractional programming to solve the optimization problem and determine the optimal vectors for the beamforming and RIS precoding. Numerical results are presented to corroborate our findings.