Time varying channel estimation for RIS assisted network with outdated CSI: Looking beyond coherence time

The channel estimation (CE) overhead for unstructured multipath-rich channels increases linearly with the number of reflective elements of reconfigurable intelligent surface (RIS). This results in a significant portion of the channel coherence time being spent on CE, reducing data communication time. Furthermore, due to the mobility of the user equipment (UE) and the time consumed during CE, the estimated channel state information (CSI) may become outdated during actual data communication. In recent studies, the timing for CE has been primarily determined based on the coherence time interval, which is dependent on the velocity of the UE. However, the effect of the current channel condition and pathloss of the UEs can also be utilized to control the duration between successive CE to reduce the overhead while still maintaining the quality of service. Furthermore, for muti-user systems, the appropriate coherence time intervals of different users may be different depending on their velocities. Therefore CE carried out ignoring the difference in coherence time of different UEs may result in the estimated CSI being detrimentally outdated for some users. In contrast, others may not have sufficient time for data communication. To this end, based on the throughput analysis on outdated CSI, an algorithm has been designed to dynamically predict the next time instant for CE after the current CSI acquisition. In the first step, optimal RIS phase shifts to maximise channel gain is computed. Based on this and the amount of degradation of SINR due to outdated CSI, transmit powers are allocated for the UEs and finally the next time instant for CE is predicted such that the aggregated throughput is maximized. Simulation results confirm that our proposed algorithm outperforms the coherence time-based strategies.