Two-dimensional trellis-coded modulation with a fractional number of bits per symbol

We introduce several 2-D trellis-coded modulation (TCM) schemes which transmit a fractional number of information bits per symbol interval. The coding gain is expressed with respect to hypothetical uncoded modulation which transmits the same number of bits per symbol. The number of bits per symbol in the proposed TCM schemes is not constant, and takes one of two values with a priori known probabilities. Consequently, in addition to decision errors that are common to all communication systems, also bit deletions and additions occur in the proposed modulation schemes. Transmitter and receiver implementation issues are addressed, and some techniques are discussed to reduce the probability of occurrence of bit deletions or additions. The proposed modulation schemes offer some interesting intermediate steps between the spectral efficiencies of conventional 2-D TCM schemes