Channel Estimation in Reconfigurable Intelligent Surface Assisted mmWave MIMO Systems
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Reconfigurable intelligent surfaces (RISs) can be used to establish a communication link when there is no or weak line-of-sight (LoS) path between a base station (BS) and user equipment (UE) in millimeter-wave MIMO systems. RIS comprises of many passive phase shifters, which can be controlled from the BS to focus a signal to a desired location and modify the propagation environment. Due to the passive-only elements, pilots cannot be decoded at the RIS. Thus channel estimation in RIS-assisted MIMO communication systems is challenging. Although the LoS channel between the BS and RIS can be obtained from the knowledge of their locations, the RIS-UE channel needs to be estimated. Estimating the RIS-UE channel at the BS amounts to localizing the UE. In this paper, we present a pilot-based uplink channel estimation technique to estimate the LoS channel between the RIS and UE at the BS. To do so, we assume an angular channel model. Central to the proposed algorithm is a new technique in which we observe the channel through multiple soundings with different phase shifts at the RIS. These multiple measurements allow us to resolve the underlying ambiguity in resolving the complex path gain and the angle of arrival at the RIS and to estimate the RIS-UE channel. Simulation results indicate that the performance of the proposed method is comparable to that of an oracle estimator, which assumes a partial channel state information with perfect knowledge of the locations of the BS, UE, and RIS.
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