ISI-reduced Modulation Over a Fading Multipat h channel1

In this work, the idea of using the channel eigenvectors as the basis for a block based signaling scheme over a fading multipath channel is introduced. This basis minimizes the product of the average fading attenuations along different dimensions. The IS1 from the preceding blocks (intra-block ISI) is modeled by an additive Gaussian noise. To reduce the effect of the intra-block ISI, a number of zeros are transmitted between successive blocks. The number of zeros is optimized to minimize the average probability of error. As the transmission of zeros reduces the bandwidth efficiency, this optimization procedure is more useful for lower bit rates. By applying Quadrature Amplitude Modulation (QAM) to each dimension, we obtain a set of two-dimensional subchannels with unequal fadings. A coherent M-PSK constellation is employed over each QAM subchannel. We propose two methods t o distribute the rate and energy between the subchannels. In both methods, we impose the restriction that the average error probability for all the subchannels is the same. In the optimum method, the energy is distributed equally between the nonempty subchannels and the rate is distributed to obtain equal average error probabilities. In a second method, the rate is distributed equally and the energy is distributed to obtain equal average error probabilities. The second method allows us t o use the same modulator/demodulator for all the subchannels and thereby reduces the complexity. Numerical results are presented for the second method. The results over a space of moderate dimensionality show substantial performance improvement with a small increase in the complexity.