Progressive Channel Estimation and Passive Beamforming for Intelligent Reflecting Surface with Discrete Phase Shifts
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Intelligent reflecting surface (IRS) has emerged as a promising technology to engineer the radio signal propagation for catering to wireless communication, by leveraging a massive number of low-cost passive reflecting elements. Prior studies on IRS have mostly assumed perfect channel state information (CSI) available for designing the IRS passive beamforming as well as the continuously adjustable phase shift at each of its reflecting elements, which, however, have simplified two challenging issues for implementing IRS in practice, namely, its channel estimation and passive beamforming designs both under the constraint of discrete phase shifts. To address these issues, we consider in this paper an IRS-aided single-user communication system with discrete phase shifts and propose a new joint design framework for progressive IRS channel estimation and passive beamforming. Specifically, we consider the practical block-based transmission, where each block has a finite number of training symbols for channel estimation. However, different from the conventional "all-at-once" channel estimation, i.e., the channels of all IRS elements are estimated at one time which inevitably causes long delay for data transmission, we propose a novel hierarchical training reflection design for IRS such that by properly partitioning its reflecting elements into groups/subgroups and assigning each group/subgroup of elements differently-combined discrete phase shifts over multiple blocks, all IRS elements' channels can be efficiently resolved even if there is only a small (insufficient) number of training symbols per block. Based on the resolved IRS channels in each block, we further design the progressive passive beamforming at the IRS with discrete phase shifts to improve the achievable rate for data transmission over the blocks.
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