Reducing Pilot Overhead for Channel Estimation in RIS-assisted Millimeter-wave Systems

The reconfigurable intelligent surface (RIS) is a potential technology to enhance millimeter wave (mmWave) coverage due to its low hardware cost and low power consumption. By reconfiguring the RIS elements according to the surrounding environment, it is possible to achieve high beamforming gain. However, for reliable beamforming, accurate channel state information (CSI) is required. In general, the channel estimation task is non-trivial as RIS is passive and cannot transmit, receive, or process signals. Moreover, due to a large number of RIS elements, the required pilot overhead will be very large. In this paper, we propose an algorithm to reduce the pilot overhead associated with channel estimation. Assuming symmetrical propagation conditions, we are able to show that the pilot overhead can be reduced by half. Numerical results and Monte Carlo simulations verify our analysis.