This is a repository copy of Joint Relay Selection and Resource Allocation for Relay-Assisted D2D Underlay Communications.

The hybrid precoding problem in point-to-point millimeter wave (mmWave) multiple input multiple output (MIMO) for narrowband channel is established as a Frobenius norm minimization problem. It is translated into trace minimization problem, and two algorithms are proposed to solve it. In the first method based on alternating minimization, we alternately determine the analog precoder and digital precoder, keeping the other constant to minimize the trace. The analog precoding subproblem with the fixed digital precoder is converted into a semi-definite programming (SDP) problem and solved by block coordinate descent (BCD) algorithm with suitable modifications. In the second method, we segregate the analog precoding and digital precoding subproblems by considering orthogonality of analog precoder. The analog precoding is further rephrased as a trace maximization problem and solved by an iterative power method by enforcing orthogonality constraint on the analog precoder. The adapted form of modified Gramm-Schmidt orthogonalization procedure is employed to impose orthogonality on the analog precoder. The proposed methods are extended for wideband channel by considering orthogonal frequency division multiplexing (OFDM). The proposed methods not only exhibit a good performance but also come with lower computational complexity when compared to existing methods with comparable performances.INDEX TERMS Millimeter wave (mmWave) communication, hybrid precoding, trace optimization, alternating minimization, block coordinate descent, power method.

It is difficult to implement a fully digital precoding in millimeter wave (mmWave) massive multiple-input multipleoutput (MIMO) systems, owing to the huge cost and power consumption of radio frequency (RF) chains. Hybrid precoding that uses the combination of analog beamforming, together with digital precoding, offers a feasible solution to this problem where far lesser number of RF chains are employed compared to the number of antennas. We consider a downlink multiuser massive MIMO scenario and propose a hybrid precoding scheme that maximizes signal-to-leakage-and-noise ratio (SLNR). Each mobile station (MS) determines its analog combiner first and then the base station (BS) constructs the analog beamformer and the digital precoder of the hybrid precoder in two separate stages to maximize the SLNR for each user. We show, through simulations, that the proposed hybrid SLNR-based precoder exhibits a performance close to the fully digital SLNR-based precoder despite using only a few number of RF chains.

In an attempt to alleviate the cost and power consumption in millimeter wave (mmWave) multiple input multiple output (MIMO) systems, hybrid precoding has been put forth as a possible solution. The hybrid precoding replaces the conventional fully digital precoding with a combination of analog and digital precoding to achieve good performance using a significantly smaller number of RF chains compared to the number of antennas. In this paper, we consider a narrowband downlink multiple user multiple-input single-output (MU-MISO) system and propose a hybrid precoding method in order to minimize the mean squared error (MSE) for all users. The analog and digital precoding design subproblems are isolated from each other. Analog precoder design is cast as a trace maximization problem, and solved by an iterative procedure based on truncated singular value decomposition (SVD). The digital precoder is determined subsequently after fixing the analog precoder, using Lagrange's method. The proposed hybrid precoder produces spectral and bit error rate (BER) performances quite close to fully digital precoder, and almost the same as existing high performance hybrid precoders, albeit at a much lower complexity. The proposed precoding method is extended to operate in wideband channel, where it exhibits equally good performance with less complexity.

This is a repository copy of An Improved Q-Learning Based Handover Scheme in Cellular-Connected UAV Network.