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It is demonstrated that trellis coded signalling schemes can be improved by varying the signal constellation to a smaller constellation periodically according to a pre-selected pattern. Numerical results presented for 2-PAM/4-PAM trellis coded schemes indicate that changing the signal constellation periodically from 4-PAM to 2-PAM during selected intervals improves performance. Moreover, the proposed scheme reduces coding complexity, and simplifies symbol timing recovery and equalisation. The cost is a moderate increase in the path memory length of the decoder.