Spatial Spectrum-Based Channel Estimation for Wideband mmWave System With Beam Squint

Channel estimation for millimeter wave (mmWave) systems is challenging due to their large antenna arrays. Owing to the sparse scattering nature of the mmWave channel, channel estimation can be performed by estimating the directions and the gains of paths. Most existing schemes assume that a wideband mmWave channel exhibits a common sparsity in the frequency domain. Unfortunately, they ignore the beam squint effect caused by the wide bandwidth, resulting in severe performance loss. In this paper, we investigate the wideband channel estimation problem with beam squint. Specifically, by utilizing measurement signals at all subcarriers, we propose a spatial spectrum-based scheme for a subarray architecture that requires only a single training slot. We first prove that the scheme can accurately obtain the spatial spectrum from a theoretical perspective. Then, we design the beamforming weights of the subarray to avoid pseudo peaks and analyze the inherent spectrum ambiguity phenomenon under the subarray architecture. Finally, to cope with beam squint, we divide the entire bandwidth into multiple subbands and design a combination criterion for the spatial spectrum of each subband. During this process, we prove that the spectrum ambiguity is eliminated, and the joint estimation of the path directions can be obtained. Simulation results demonstrate that the proposed scheme has better estimation accuracy than other methods and significantly reduces the required number of training slots.