Time synchronisation of an HF radio modem

In the serial transmission of a digital data signal over a voiceband channel involving an HF radio link, the impulse response of the channel may undergo large variations with time. When a near-maximum-likelihood detection process is used at the receiver, with no adaptive filtering of the received signal, the detector requires an estimate of the sampled impulse response of the channel, and this estimate may be used also for the time synchronisation of the receiver. Where a QAM signal is transmitted and the received demodulated signal waveform is sampled at close to the Nyquist rate, the phase of the sampling instants does not critically affect the tolerance of the system to additive white Gaussian noise, so long as the sampling phase remains approximately constant, or at least does not vary too rapidly or over too wide a range. Thus it is only necessary here to control the sampling rate rather than to synchronise to a particular sampling phase. The paper describes a new technique for achieving the required time synchronisation of the receiver under the conditions just described. Results of computer simulation tests are also presented, showing the performance of the technique in a synchronous serial QAM system operating at 9600 bit/s over a model of an HF radio link with two independent Rayleigh fading sky waves.