High speed serial data modem with modified Viterbi decoders

The HF transmission channel is characterized by severe distortions and very fast variations; in order to allow high speed data communications, very complex demodulators are required, employing both fast adaptive algorithms and sophisticated decoders. This paper reports the results of a study for the realization of 8PSK adaptive serial modem for communication in the HF frequency range. The modem operates with 2400 Baud, PSK modulation, over a 300-3400 Hz voiceband channel. Two problems have to be solved to allow efficient use of the available voiceband: channel identification, that is adaptive estimation of the channel impulse response, and optimum data decoding, this latter not being separated from some kind of adaptive equalization of the received signal. The choice for the decoding algorithm has fallen on a modified Viterbi algorithm decoder due to its superior performance with respect to more simple decoders. The decoder is preceeded by an adaptive equalizer. Both the decoder and the equalizer are driven by an adaptive estimator and the overall structure of the modem is shown