Performance of V.F. F.M. teleprinter circuits operating over a tropospheric-scatter link

The paper describes an investigation undertaken to derive data on which to base estimates of the telegraph performance to be expected over tropospheric-scatter radio links. H. B. Law has shown how the telegraph error rate depends on the signal/noise ratio at the input to the telegraph equipment for the case of a steady signal, and also for the case of one subject to Rayleigh fading. As the signal received over a tropospheric-scatter link has a short-term amplitude distribution following the Rayleigh law, it might be expected that Law´s results could be applied directly. The problem is complicated, however, by the fact that most operational scatter links employ frequency modulation of the radio carrier, with the result that the receiver exhibits threshold effects. When the fading signal falls below the receiver threshold, which it may frequently do, the output signal/noise ratio is no longer linearly dependent on the input carrier/noise ratio. It is shown, however, that, if the output/input characteristic of the radio receiver is known, the telegraph error rate can still be calculated in terms of the mean carrier/noise ratio. An empirical law for the output/input characteristic of an f.m. radio receiver is proposed, and, using this law, the telegraph performance of circuits carried by tropospheric-scatter links is then calculated for the cases of non-diversity and dual and quadruple selector-type diversity receiving systems. It was considered desirable to make experimental checks, and an analogue method was employed in which the effects of the radio path were recorded and used to operate a fading machine, so as to reproduce, in the laboratory, the conditions which had existed over an actual radio path. A comparison is made of predicted values of telegraph error rate with experimental results obtained using the fading machine.