Unitary isotropically distributed inputs are not capacity-achieving for large-MIMO fading channels

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Unitary isotropically distributed inputs are not capacity-achieving for large-MIMO fading channels

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Creation Date: Jul 01, 2012

Published In: Jul 2012

Paper Type: Conference Paper

Book Title: Proc. of IEEE International Symposium on Information Theory

Address: Cambridge, MA USA

Abstract:

We analyze the capacity of Rayleigh block-fading multiple-input multiple-output (MIMOchannels in the noncoherent setting and prove that unitary space-time modulation (USTM) is not capacity-achieving when the total number of antennas exceeds the coherence time of the fading channel. This situation is relevant for MIMO systems with large antenna arrays (large-MIMO systems). Our result settles a conjecture by Zheng & Tse (2002) in the affirmative. The capacity-achieving inputsignal, which we refer to as Beta-variate space-time modulation (BSTM), turns out to be the product of a unitary isotropically distributed random matrix, and a diagonal matrix whose nonzero entries aredistributed as the square-root of the eigenvalues of a Beta-distributed random matrix of appropriate size. Numerical results illustrate that using BSTM instead of USTM in large-MIMO systems yields a rate gain as large as 13% for SNR values of practical interest.

IEEE Explore link: http://dx.doi.org/10.1109/ISIT.2012.6283571

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