Improving the Performance of Trellis Coded Modulation Over Rayleigh Fading Channel Using Locally Rotated Constellations

Abstract

In this paper, we propose a new type of modulation technique; it is called Locally-Rotated (LR) modulation. With this new technique we are looking to optimize the channel encoding block named “Trellis-Coded Modulation with a Locally-Rotated constellation” (TCM+LR). This channel encoding block uses PSK/QAM mapping technique which takes into consideration both Ungerboeck mapping and Gray code mapping (UGM). The simulation results, over Rayleigh fading channels and Proakis B, are performed for throughput 2 and 3 bits/s/Hz with 8PSK and 16 QAM modulations, and rate 2/3, 3/4 constituent TCM encoders with memory order of 3, 4 and 5. In the Rayleigh fading channel, the results show that the proposed scheme TCM+LR-UGM with three memories produces a slight improvement in comparison to TCM+LR, by increasing the number of memories to four. It is shown that TCM+LR-UGM outperforms the TCM+LR Ungerboeck mapping by a gain of 2.5 dB and we notice a gain of 5 dB compared to the 8 state TCM+LR, moreover,  a gain of 7.5 dB is achieved in comparison with the original TCM with three memories. In the selective frequency channel, the results show that the proposed scheme TCM+LR-UGM with four memories outperforms the TCM+LR Ungerboeck mapping with a gain of 1 dB and outperforms the TCM+LR with three memories with a gain of 2 dB and with a gain of 3 dB compared to the original TCM 8-states.
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