Multiplexing two information sources over fading channels: A cross-layer design perspective

We consider the transmission over an unknown frequency-selective channel of two independent sources with different application-layer characteristics: one source (such as voice) has a low information rate with a strict delay constraint; the other (such as data) has a high rate but without any delay constraints. We propose a system structure that jointly considers the different decoding requirements of the application layer and the unknown fading nature of the physical channel. In the proposed communication system, pilot symbols are not present and the low-rate information is decoded noncoherently first. 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.