Improving DVB-S2 performance through constellation shaping and iterative demapping

Because it is widely supported by commercial-off-the-shelf (COTS) technology, the DVB-S2 waveform standard has become an attractive solution for military communication links. The waveform uses a combination of amplitude-phase-shift keying (APSK) modulation and low-density parity-check (LDPC) codes. Typical DVB-S2 implementations select signals from the APSK signal set with uniform probability. However, information-theoretic results suggest that performance may be improved by selecting lower-energy signals more frequently than higher-energy signals. In this paper, we propose and analyze a DVB-S2-compatible system that shapes the APSK constellation by selecting signals with a nonuniform probability. The receiver iterates between the APSK demapper, the shaping decoder, and the LDPC decoder. Using 32-APSK and a rate of 3 data bits per symbol, the system described in this paper achieves a gain of over 1 dB relative to a standard DVB-S2 system (i.e. one that does not use shaping or iterative demodulation) in additive white Gaussian noise (AWGN) at a bit-error rate of 10^-5. About 0.7 dB of the gain can be attributed to shaping, and rest of the gain can be attributed to iterative demodulation and decoding. Lesser gains can be achieved over Rayleigh fading channels.