A class of switching turbo decoders against severe SNR mismatch

In mobile communication applications, the fading channel characteristics may vary very rapidly, resulting in a severe SNR mismatch in the receiver. On one hand, the performance of Log-MAP-based turbo decoder degrades significantly in the presence of large SNR underestimation errors. On the other hand, the very robust Max-Log-MAP-based turbo decoder has a SNR loss of about 0.5 dB relative to the former decoder in the absence of SNR mismatch. In this work, we propose a class of switching turbo decoders each specified by a parameter S, which can be selected to compromise the performance without SNR mismatch for the robustness against severe SNR mismatch. Our simulation results demonstrate that by choosing a proper value of S, it is possible to obtain a switching turbo decoder which is not only robust against severe SNR mismatch but also performs reasonably well compared to the Log-MAP-based turbo decoder without SNR mismatch. In addition, the switching operation can be easily implemented by turning off all the correction term related operations in the Log-MAP component decoder. In order to further increase the robustness of the decoder, an auto-switching decoder is also proposed, which will automatically switch from Log-MAP to MAX-Log-MAP. Finally, it is anticipated that the proposed scheme can be fruitfully applied to the serial concatenated convolutional codes, which are even more sensitive to SNR mismatch than the turbo codes.