  We consider the problem of slotted asynchronous coded communication, where in
each time frame (slot), the transmitter is either silent or transmits a
codeword from a given (randomly selected) codebook. The task of the decoder is
to decide whether transmission has taken place, and if so, to decode the
message. We derive the optimum detection/decoding rule in the sense of the best
trade-off among the probabilities of decoding error, false alarm, and
misdetection. For this detection/decoding rule, we then derive single-letter
characterizations of the exact exponential rates of these three probabilities
for the average code in the ensemble.
