Iterative demodulation and decoding of coded GMSK

This paper considers iterative demodulation and decoding of the concatenation of a convolutional code and partial response Gaussian-filtered minimum-shift keying (GMSK) modulation. Demodulation is simplified through the approximation of GMSK as a superposition of pulse amplitude modulated signals, leading to a representation as the inner convolutional code of a serially concatenated convolutional code (SCCC). The modulation has both recursive and non-recursive implementations. Iterative decoding of the recursive implementation of GMSK yields a gain over 2 dB for a bit error rate of 10^-5 compared to the nonrecursive implementation. However, modulating a portion of each block with recursive GMSK and the remainder with non-recursive GMSK provides an additional gain of up to 0.7 dB. The partial-block recursive technique may also be adapted to SCCC schemes independent of the modulation.