An extremely robust turbo coded HF modem

We propose an extremely robust, low rate HF modem for military radio communications. The modem design is based on the existing DSTO chirp modem which uses a half rate, memory length ten convolutional code together with a soft decision Viterbi decoder. The proposed modem replaces the current convolutional codec with a turbo codec. The problems of frame synchronisation, noise variance estimation and metric optimisation have been solved in order to implement the modem. The turbo coded chirp modem outperforms the convolutionally-coded chirp modem by more than one dB on the AWGN channel and more than two dB on HF test channels.