Millimeter wave (mmWave) communications has been regarded as a key enablingtechnology for 5G networks. In contrast to conventionalmultiple-input-multiple-output (MIMO) systems, precoding in mmWave MIMO cannotbe performed entirely at baseband using digital precoders, as only a limitednumber of signal mixers and analog-to-digital converters (ADCs) can besupported considering their cost and power consumption. As a cost-effectivealternative, a hybrid precoding transceiver architecture, combining a digitalprecoder and an analog precoder, has recently received considerable attention.However, the optimal design of such hybrid precoders has not been fullyunderstood. In this paper, treating the hybrid precoder design as a matrixfactorization problem, effective alternating minimization (AltMin) algorithmswill be proposed for two different hybrid precoding structures, i.e., thefully-connected and partially-connected structures. In particular, for thefully-connected structure, an AltMin algorithm based on manifold optimizationis proposed to approach the performance of the fully digital precoder, which,however, has a high complexity. Thus, a low-complexity AltMin algorithm is thenproposed, by enforcing an orthogonal constraint on the digital precoder.Furthermore, for the partially-connected structure, an AltMin algorithm is alsodeveloped with the help of semidefinite relaxation. For practicalimplementation, the proposed AltMin algorithms are further extended to thebroadband setting with orthogonal frequency division multiplexing (OFDM)modulation. Simulation results will demonstrate significant performance gainsof the proposed AltMin algorithms over existing hybrid precoding algorithms.Moreover, based on the proposed algorithms, simulation comparisons between thetwo hybrid precoding structures will provide valuable design insights.
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