Double trellis-coded CPM

The method of multiple trellis-coded modulation is applied to continuous phase modulation (CPM). Double trellis-coded quaternary CPM schemes using convolutional codes with an effective rate of 2/4 are considered. Tables of optimum codes and minimum Euclidean distances are given for several frequency impulses and ratios of trellis-branches-per-symbol-interval, which have to be processed by a Viterbi-decoder. The results are clearly superior to single trellis-coded schemes with 1/2-rate codes. These improvements are accomplished by a proper selection of double-channel symbols. This method also offers improved receiver implementation properties. For a further reduction of complexity, a modified decision-feedback-sequence estimation is proposed.<>