I'm working on a BPSK demodulator, which I guess is a kind of PAM. The signal has some inherent ISI so I'm using Viterbi to decode it. It's working well in idealized conditions, but I'm a bit lost on how to get the amplitude right. It seems important that the amplitude is consistent (modulo noise) for the Viterbi metrics to be accurate.
If that doesn't make sense, I'd think QAM has a similar problem: if the amplitude is wrong then the wrong symbols are decoded.
What's the best way to do this? Is there something better than the basic agc_ff block? I know each sample should land on one of 6 places -- perhaps some kind of least-squares fit?
I've noticed when the input to agc_ff has low amplitude, say -40dB relative to a normalized +/- 1 signal, it is slower to respond. and likewise, at +40dB it's faster to respond, and it can start introducing noise as the gain changes significantly from sample to sample. Not sure if that's intended behavior, but it does present another challenge since input amplitude could vary over many orders of magnitude.