Exploiting Low-rate Information for Transmission over Unknown Fading Channels

We consider the transmission over an unknown frequency-selective channel of two independent sources with different characteristics: one source (such as voice) has a low information rate with a strict delay constraint; the other (such data) has a high rate but without any delay constraints. We assume that, due to the delay requirements, the low-rate source must be decoded first and below a certain error probability. We study a communication system in which pilot symbols are not present and the lowrate information is decoded non-coherently. The decoded low-rate codewords are then used for channel estimation to facilitate coherent decoding of the high-rate source. For a fixed detection error probability of the low-rate source, we derive achievable rate expressions for the high-rate source. We demonstrate a convergence behavior of the achievable rate of the high-rate source as the decision error probability of the low-rate source goes to zero. Numerical results show that the achievable rate of the high-rate source converges to that achievable by a training-based scheme at moderate decision error levels.