Pilot assisted SOVA decoding for turbo codes

A pilot-assisted turbo coding system is proposed in which pilot bits are periodically inserted into the bit stream to be encoded by turbo encoder. In our rate 1/2 example, both the systematic bit for the pilot and a corresponding parity are punctured to keep the code rate unchanged On the decoding side, two schemes are proposed to improve the performance; the first uses the pilots to detect possible errors and scales the extrinsic information accordingly, while the second modifies the path metric. These two ideas can be combined to further improve the performance. The application of this scheme to soft output Viterbi algorithm (SOVA) is studied. In comparison with the standard SOVA decoder, our simulation results show that the use of pilot symbols to detect errors and scale the extrinsic information can result in a coding gain of 0.38 dB at an SNR of 2.5 dB. By itself, the modified metric contributes to an improvement of 0.12 dB at this same SNR, while a combination of the above two schemes achieves a 0.45 dB improvement.