A Simplified Approach of Correlated Rician Fading Channel Estimation in Multi-User Ultra-Massive MIMO RIS-assisted Narrow Band Wireless Systems

Since 6G communication systems operate at millimeter-wave bands, the signals are susceptible to blockages. Towards addressing this issue, Reconfigurable Intelligent Surface (RIS) is envisioned as one of the key technologies in 6G communications. Estimation of RIS-assisted channels is very challenging since they are highly dependent on system architecture, channel configuration, and the algorithms to compute channel state information. It demands perfect channel state information (CSI) of every individual channel between gNodeB and the UE through RIS, and it is very challenging to obtain perfect CSI in high-speed networks due to their high-mobility scenarios. Also, the existing channel estimation methods perform uniform time-slots assignment during all the stages and cause more errors in the estimation. In this paper, we propose a two-level uplink transmission method to characterize the quasi-static Rician fading channel at gNodeB by exploiting the channel correlation properties in a multi-user ultramMIMO RIS-assisted narrow band radio system using minimum pilot sequence. In the proposed method, early stages of channel estimation are assigned with higher number of timeslots so that the error propagation to the next stage will be reduced. Also, the channel correlation properties utilized in the proposed method minimizes the number of unknowns to be computed. This led to the minimal channel estimation time and complexity. The interference cancellation is performed for the received signal at gNodeB during stage 1 and 2 so that error propagation is minimized. The normalized mean square error (NMSE) is evaluated and compared with the traditional method by exploring the relation between gNodeB antennas and RIS elements to prove the efficiency of the proposed method.