Unequal protection with turbo decoding for high-order modulated signaling

Although binary turbo codes can achieve a significant amount of coding gain, they generally do not have good bandwidth efficiency unless they are heavily punctured. The simplest way to attain both power and bandwidth efficiency is to adopt nonbinary modulation schemes, such as 8PSK and 16QAM, in conjunction with a turbo code. This approach requires bit logarithmic likelihood ratios (LLRs) from the nonbinary demodulator for binary turbo decoding. When these bit LLRs are formed, different bits are unequally protected against channel corruption. Because the systematic and parity-check bits of a turbo codeword play different roles in the process of decoding, mapping them with different levels of protection to signal constellation results in different bit-error rate (BER) performances. In this paper, it is shown that the threshold BER can be improved when systematic bits are more protected while the decoding error floor can be lowered when parity-check bits are more protected. This provides an additional dimension of waveform design by mapping systematic or parity-check bits to different bit positions in the non-binary signal constellation.