An Efficient Viterbi Algorithm for Combined MSK Demodulation and Gold Code Despreading

In this paper, we present an efficient Viterbi algorithm that achieves minimum shift keying (MSK) demodulation combined with despreading and provides soft decisions to the forward error correction module. To our knowledge, this combination of demodulation and despreading with the added capability to deliver soft decisions using Viterbi algorithm has not been reported in the literature before. Specifically, we have used Gold codes for the spreading and turbo code as the forward error correction code. The Viterbi algorithm used for MSK demodulation is first modified to incorporate despreading as part of the demodulation process. This reduces the computational complexity compared to doing the two operations separately. We provide a scheme to select the Gold codes that allows the software implementation at the receiver to be more efficient. Finally, the receiver is made more robust by making it capable of compensating residual errors in the estimation of the phase and carrier frequency offsets.