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Abstract: We consider a time division duplex TDD $n t \times n r$ multiple-inputmultiple-output MIMO system with channel state information CSI at both thetransmitter and receiver. We propose X- and Y-Codes to achieve highmultiplexing and diversity gains at low complexity. The proposed precodingschemes are based upon the singular value decomposition SVD of the channelmatrix which transforms the MIMO channel into parallel subchannels. Then X- andY-Codes are used to improve the diversity gain by pairing the subchannels,prior to SVD precoding. In particular, the subchannels with good diversity arepaired with those having low diversity gains. Hence, a pair of channels isjointly encoded using a $2 \times 2$ real matrix, which is fixed {\em a priori}and does not change with each channel realization. For X-Codes these matricesare 2-dimensional rotation matrices parameterized by a single angle, while forY-Codes, these matrices are 2-dimensional upper left triangular matrices. Thecomplexity of the maximum likelihood decoding MLD for both X- and Y-Codes islow. Specifically, the decoding complexity of Y-Codes is the same as that of ascalar channel. Moreover, we propose X-, Y-Precoders with the same structure asX-, Y-Codes, but the encoding matrices adapt to each channel realization. Theoptimal encoding matrices for X-, Y-Codes-Precoders are derived analytically.Finally, it is observed that X-Codes-Precoders perform better forwell-conditioned channels, while Y-Codes-Precoders perform better forill-conditioned channels, when compared to other precoding schemes in theliterature.



Author: Saif Khan Mohammed, Emanuele Viterbo, Yi Hong, Ananthanarayanan Chockalingam

Source: https://arxiv.org/







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