Frequency-hop spread spectrum with quadrature amplitude modulation and error-control coding

As compared with binary or quaternary phase-shift keying, M-ary quadrature amplitude modulation (M-QAM) provides a greater spectral efficiency at a cost of a larger required signal-to-noise ratio to meet a specified error rate. For frequency-hop spread-spectrum systems, proper use of error-control coding can decrease the signal-to-noise ratio requirement and also provide protection against partial-band interference. We consider two classes of turbo codes for use in M-QAM frequency-hop spread spectrum: serially concatenated convolutional codes with iterative decoding and block product codes with iterative decoding. Performance comparisons are given for 16-QAM with the two types of turbo codes and soft-decision metrics based on the log likelihood ratio. The performance results for the turbo codes are compared with the performance of the IEEE 802.11 standard convolutional coding and Viterbi decoding for 16-QAM. An alternative soft-decision metric for Gray-coded M-QAM is developed and evaluated for each of the three coding systems.