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An Efficient Hybrid Beamforming and Channel Acquisition for Wideband mm-Wave Massive MIMO Channels

In this paper, an efficient hybrid beamforming architecture together with a novel spatio-temporal receiver processing is proposed for single-carrier (SC) mm-wave wideband massive MIMO channels in time-domain duplex (TDD) mode. The design of two-stage beamformers is realized by using a virtual sectorization via second-order channel statistics based user grouping. The novel feature of the proposed architecture is that the effect of both inter-group-interference (due to non-orthogonality of virtual angular sectors) and the inter-symbol-interference (due to SC wideband transmission) are taken into account. While designing the analog beamformer, we examine the dimension reduction problem and proper subspace (beamspace) construction (by exploiting the joint angle-delay sparsity map and power profile of the multi-user channel) based on which a highly efficient spatio-temporal digital receiver processing is proposed. Furthermore, a least square type channel estimator, based on a proper subspace projection via radio frequency (RF) processing, is proposed. It is shown to achieve a remarkable robustness to pilot contamination with a significant reduction in pilot length. Moreover, the effect of utilizing inaccurate estimates for channel covariance matrices (CCMs) on achievable information rate (AIR) is analyzed. The sensitivity of proposed techniques with respect to estimation accuracy of central angle and the angular spread (AS) of angle-of-arrival (AoA) values is investigated.