Abstract — Turbo coded Quadrature Amplitude Modulated

(QAM) systems have been adopted by standards such as Code Division Multiple Access (CDMA) 2000 and Long Term Evolution (LTE) to achieve high data rates. Although several techniques have been developed to improve the performance of Turbo coded QAM systems, combinations of these techniques to produce hybrids with better performances, have not been fully exploited. This paper investigates the performance of LTE Turbo codes with Joint Source Channel Decoding (JSCD), adaptive Sign Division Ratio (SDR) based scaling and prioritised QAM constellation mapping. JSCD exploits a-priori source statistics at the decoder side and SDR based scaling provides a scale factor for the extrinsic information as well as a stopping criterion. Additionally, prioritised constellation mapping exploits the inherent Unequal Error Protection (UEP) characteristic of the QAM constellation and provides greater protection to the systematic bits of the Turbo encoder. Simulation results show that with 16-QAM at Bit Error Rates (BERs) above 10-1, the combination of these three techniques achieves an average gain of 1.7 dB over a conventional LTE Turbo coded 16-QAM system. For BERs below 10-1, the combination of prioritized constellation mapping, JSCD and SDR scaling provides an average gain of 0.6 dB. The proposed scheme with 64-QAM gives a performance gain of 3 dB on average over the conventional LTE Turbo coded 64-QAM system over the whole Eb/N0 range. Keywords- Turbo Code; QAM; JSCD; SDR; Priortised Mapping. I