A Turbo-Coded, Low-Rate HF Radio Modem

In 1993, DSTO presented a uniquely designed, low-rate HF modem called the chirp modem [2] to enable military radio communications over a broad range of adverse channel conditions. The chirp modem operates at the data rate of 75 bits/sec. It utilises a frequency-swept waveform sweeping over a voice band HF frequency from 300 Hz to 3 kHz to represent a symbol. Information is modulated onto the waveform using differential QPSK, and errors are controlled by a rate 1/2, memory length 10 convolutional code with soft-decision Viterbi decoding. To improve the performance of the chirp modem over HF channels, it is necessary to use a more powerful error-correcting code. However, the memory length of the convolutional code is constrained by the computational complexity of the decoder.

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