Paper detail

On the Distortion SNR Exponent of Some Layered Transmission Schemes

We consider the problem of joint source-channel coding for transmitting K samples of a complex Gaussian source over T = bK uses of a block-fading multiple input multiple output (MIMO) channel with M transmit and N receive antennas. We consider the case when we are allowed to code over L blocks. The channel gain is assumed to be constant over a block and channel gains for different blocks are assumed to be independent. The performance measure of interest is the rate of decay of the expected mean squared error with the signal-to-noise ratio (SNR), called the distortion SNR exponent. We first show that using a broadcast strategy of Gunduz and Erkip, but with a different power and rate allocation policy, the optimal distortion SNR exponent can be achieved for bandwidth efficiencies 0 < b < (|N-M|+1)/min(M,N). This is the first time the optimal exponent is characterized for 1/min(M,N) < b < (|N-M |+ 1)/ min(M, N). Also, for b > MNL^2, we show that the broadcast scheme achieves the optimal exponent of MNL. Special cases of this result have been derived for the L=1 case and for M=N=1 by Gunduz and Erkip. We then propose a digital layered transmission scheme that uses both time layering and superposition. This includes many previously known schemes as special cases. The proposed scheme is at least as good as the currently best known schemes for the entire range of bandwidth efficiencies, whereas at least for some M, N, and b, it is strictly better than the currently best known schemes.