Coherent communication over a waveform channel corrupted by thermal noise and by an unknown and arbitrary interfering signal of bounded power is considered. For a fixed encoder, a random modulator/demodulator (modem) and detector are derived. They asymptotically minimize the worst-case error probability as the blocklength of the encoder becomes large. This optimal modem is independent of the encoder, and the optimal detector is the standard correlation receiver. A simple upper bound to the performance of any encoder when used with the optimal modem and detector is presented. These results provide a benchmark with which the performance of spread-spectrum modems and robust detection rules can be compared. >