"Turbo" decoding with unequal error protection applied to GSM speech coding

Two-dimensional, systematic and parallel concatenated convolutional codes are compatibly punctured in order to provide unequal error protection (UEP). They are iteratively decoded by the MAP algorithm and other soft-in/soft-out decoders. In a case study we replace the current GSM speech channel coding scheme by this new "turbo" scheme leaving all the interleaver interfaces and the total delays unchanged. It is shown that for independent and frequency-hopped Rayleigh channels and other standard GSM channels, the frame error rate and the residual BER of the class 1a bits drop by a factor of 1.4 and 2.0 respectively. Likewise the carrier to interference ratio (CIR) can be lowered by 0.8 dB at the same performance level. Contrary to the common belief that "turbo" codes gain only with large interleavers, we have shown that together with UEP they allow gains also with the modest interleaver sizes of the GSM scheme. All this offers a potential for a future evolutionary improvement of the GSM channel coding scheme.