Downlink Precoding for Massive MIMO Systems Exploiting Virtual Channel Model Sparsity

摘要

In this paper, the problem of designing a forward link linear precoder forMassive Multiple-Input Multiple-Output (MIMO) systems in conjunction withQuadrature Amplitude Modulation (QAM) is addressed. First, we employ a noveland efficient methodology that allows for a sparse representation of multipleusers and groups in a fashion similar to Joint Spatial Division andMultiplexing. Then, the method is generalized to include Orthogonal FrequencyDivision Multiplexing (OFDM) for frequency selective channels, resulting inCombined Frequency and Spatial Division and Multiplexing, a configuration thatoffers high flexibility in Massive MIMO systems. A challenge in such systemdesign is to consider finite alphabet inputs, especially with largerconstellation sizes such as $M\geq 16$. The proposed methodology is nextapplied jointly with the complexity-reducing Per-Group Processing (PGP)technique, on a per user group basis, in conjunction with QAM modulation and insimulations, for constellation size up to $M=64$. We show by numerical resultsthat the precoders developed offer significantly better performance than theconfiguration with no precoder or the plain beamformer and with $M\geq 16$.