viterbi with qpsk input
Just consider bit pairs in QPSK, not individual bits as in BPSK
I pick the first pair from your example (0.2 -0.9)
Find the euclidean distance from all possible transmitted values ±1, ±1. Since each transition in the trellis corresponds to one of these 4 encoder output values, you can find the branch metrics.
Of course, there is the assumption that you know which the first and which is the second bit of each pair at the decoder, which is reasonable assumtpion for QPSK.
You cant do a straight decoding with an odd number of bits, but QPSK always produces bit pairs. Hope it is clear now.
-b