THE EFFECTS OF CONSTELLATION DENSITY ON TRELLIS-CODED MODULATION IN FADING CHANNELS

Dense constellations such as 16QAM have not seen much use in mobile communication because of their greater peak-to-average power ratio and their seemingly greater sensitivity to noise and channel interference. In this paper, though, we demonstrate that dense constellations can actually improve performance. Using a completely analytical method, we compare three constellations with the same net throughput of 2 bits/symbol: uncoded QPSK, rate 2/3 TCM 8PSK, and rate 1/2 TCM 16QAM. Comparison on the basis of average power puts TCM 16QAM 5 dB ahead of TCM 8PSK. Even comparison on the basis of peak power gives TCM 16QAM a 2.44-dB advantage over TCM 8PSK. QPSK is much poorer than either. Our results are particularly relevant now, while the new cellular and PCN systems are being designed.