I believe I understand well enough how the Viterbi algorithm can reverse the effects of ISI when the combined impulse response (transmit filter, multipath, receive filter) is known. I've read How I Learned to Love the Trellis by Bernard Sklar, and I also asked about it in a previous question: Applying Viterbi algorithm to compensate for ISI in PSK31.
But how can this technique be applied when the channel distortion isn't known? From my superficial understanding of equalizing algorithms like CMA, they attempt to infer the channel distortion and compensate for it dynamically. My particular interest is HF channels, which would certainly seem to benefit from some dynamic adjustment.
However, in my study of Viterbi equalization so far, I've not found an explanation of how the unknown impulse response of the channel is found. Without that information, the best we can accomplish is to compensate for known elements like the receive and transmit filters. And in a case with no inherent ISI (like RRC filters on each end), then does Viterbi equalization have any value at all?