In this work, we propose a novel formulation for a precoder design considering practical rate assignment based on the modulation and coding scheme of the long‐term evolution table, beam selection, and power optimization, which exploits the geometric sparsity of the multiuser massive multiple‐input–multiple‐output channel. We consider two different channel models and provide an optimal solution for the joint beam selection and power optimization as well as a heuristic using Lagrangian relaxation. Assuming knowledge of the beamspace channel, the beamspace precoder consists in selecting and optimizing the power of the beams steered to the user equipments (UEs) in order to maximize the signal‐to‐interference‐plus‐noise ratio at the UE. Furthermore, we propose three additional simple heuristics with low complexity. For these three heuristics, we solve the problem in two steps: (1) selection of beams based on the maximal ratio transmission principle and (2) power allocation per UE. Simulation results show that our optimal solution can achieve a better performance than the zero‐forcing beamforming scheme in the high‐sparsity scenarios. Besides, compared to the linear maximal ratio transmission precoder, the proposed low‐complexity heuristics improve the performance under a scenario with channel sparsity.
Resumo-Neste artigo, propõe-se um esquema de transmissão MIMO (multiple-input multiple-output) de baixa complexidade com quatro antenas transmissoras e taxa de transmissão espacial unitária. Um pré-processador é projetado baseado na realimentac ¸ão de fase provinda do receptor. O pré-processamento permite que o esquema proposto alcance um grau de diversidade completo bem como um ganho de codificac ¸ão. Realiza-se uma análise de SNR (signal-to-noise ratio) do sistema proposto a qual é utilizada para se encontrar a informac ¸ão de fase ótima, no sentido de maximizar a SNR instantânea. Considera-se também uma análise do canal de realimentac ¸ão quantizado supondo que o canal de comunicac ¸ão é do tipo Rayleigh, plano e quase-estático. Realizam-se simulac ¸ões do tipo Monte Carlo para o esquema proposto e para outros esquemas de comunicac ¸ão MIMO com quatro antenas transmissoras e taxa de transmissão unitária. Resultados mostram que o esquema proposto alcanc ¸a ordem de diversidade completa e supera outras boas propostas em termos de ganho de codificac ¸ão e atraso de decodificac ¸ão.
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