Joint source/channel coding for wireless channels

Shannon's fundamental theorem showing that source coding and channel coding can be separated without any loss of optimality does not apply to general time-varying channels [1, 2]. Since distortion by the source encoder decreases with data rate, while channel errors increase with data rate, the joint source/channel coding problem reduces to allocating bits in an optimal way between the source and channel encoders as the source and channel vary. We introduce two additional degrees of freedom by allowing both the transmit power and the data rate to vary, subject to an average power constraint. Under these varying power and rate conditions, we first obtain an expression to minimize end-to-end distortion of general joint source/channel codes for fading channels. We then propose an adaptive joint source/channel coded modulation technique. The channel code adapts both transmission rate and power using variable-rate coded MQAM. We analytically derive the minimum end-to-end distortion of our joint coding scheme. The solution cannot be obtained in closed form, and therefore requires computer search methods. We also obtain a simple upper bound on distortion by holding the channel error rate constant. Numerical results for this distortion upper bound as a function of channel coding grain and error rate are obtained. The optimal power control which achieve this bound is also determined.