Exploiting protocol information for transmission over unknown fading channels

We consider the transmission of two independent sources with different priorities over unknown fading channels. One source (protocol information) has a low information rate and a delay constraint. The other source has a high information rate and no delay constraints. We study a receiver structure in which the protocol information is first decoded incoherently and then fed back to facilitate coherent decoding for the high-rate infor- mation. We derive lower bounds on the achievable rate for the high- rate source by this decision directed structure. These bounds are given as functions of the low-rate message error probability y, which characterizes the impact of y on the performance of the system. It is shown that, in terms of the achievable rate for the high-rate information, the decision directed scheme achieves the performance of the known-training scheme as y goes to zero.