This paper proposes a new user grouping algorithm and three-dimensional (3D) angular-based hybrid precoding (AB-HP) scheme for massive multi-user multiple-input multiple-output (MU-MIMO) systems using uniform rectangular arrays (URA). At first, the users clustered in multiple spots are efficiently grouped according to the proposed user grouping algorithm, which only utilizes the user angle-of-departure (AoD) information and does not require prior knowledge of the number of user groups. By employing the AoD support of the user groups, the RF-beamformer of AB-HP is designed to reduce the inter-group interference, the channel state information (CSI) overhead, and the number of RF chains. Then, the digital baseband precoder of AB-HP is constructed via regularized zero-forcing (RZF) using the effective channel seen from baseband to simultaneously serve the users clustered in multiple groups, by considering three approaches: joint-group-processing (JGP), per-group-processing (PGP) and common-group-processing (CGP). For each approach, the signal-to-interference-plus-noise ratio (SINR) expressions as well as their tight deterministic approximations are derived. To further reduce the number of RF chains, we also propose a new transfer block design, which reduces the number of RF chains down to the number of independent data streams without penalizing the sum-rate performance. Illustrative results reveal that the proposed AB-HP schemes with the relaxed CSI estimation overhead and reduced hardware cost/complexity can closely approach to the sum-rate performance of the single-stage fully-digital precoding (FDP). Furthermore, AB-HP has considerably higher energy efficiency performance compared to FDP due to the reduced number of RF chains. We show through simulation that the proposed AB-HP can offer significantly better performance than existing HP techniques. The computational complexity of AB-HP is also analyzed.INDEX TERMS Massive MIMO, 3D hybrid precoding, user grouping, angle of departure (AoD), uniform rectangular array (URA), RF chain reduction.

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Operation of full-duplex systems requires efficient mitigation of the self-interference signal caused by the simultaneous transmission/reception. In this paper, we propose a maximum-likelihood (ML) approach to jointly estimate the selfinterference and intended channels by exploiting its own known transmitted symbols and both the known pilot and unknown data symbols from the other intended transceiver. The ML solution is obtained by maximizing the ML function under the assumption of Gaussian received symbols. A closed-form solution is first derived, and subsequently an iterative procedure is developed to further improve the estimation performance at moderate to high signal-to-noise ratio (SNR). We establish the initial condition to guarantee the convergence of the iterative algorithm to the ML solution. In the presence of considerable phase noise from the oscillators, a phase noise estimation method is proposed and combined with the ML channel estimator to mitigate the effects of the phase noise. Illustrative results show that the proposed methods offer good cancellation performance close to the Cramer-Rao bound (CRB) .Index Terms-Full duplex communication, channel estimation, maximum likelihood, iterative method, self-interference suppression, MIMO. 0018-9545 (c)

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