Detection process of a 9600 bit/s serial modem for HF radio links

The paper describes a near-maximum-likelihood detection process suitable for use in a synchronous serial digital data transmission system that operates at 9600 bit/s over an HF radio link. The data signal at the input and output of the HF radio link is here a 2400 baud 16-level QAM signal with a bandwidth (measured between the 6 dB points) from about 600 to 3000 Hz. The detector is a development of a technique that operates at 2400 bit/s with a four-level QAM signal and which itself originated from a reduced-state Viterbi-algorithm detector. Results of computer simulation tests are presented, showing the tolerance to additive white Gaussian noise of a 9600 bit/s data transmission system employing the new detector and operating over a model of an I-IF radio link. The latter has two independent Rayleigh fading sky waves that introduce frequency spreads of 0.5 or 2 Hz into the data signal and have a relative transmission delay of 1 or 3 ms. It is assumed that the correct estimation of the channel is achieved at the receiver.