Measurement of the Frequency Correlation Function of Fading Channels Using AM Stations as Signal Sources

Measurements of the statistical characteristics of a proposed HF channel are usually required before such a channel is designed. These measurements require an elaborate test setup with test transmitters at the proposed transmitting sites. This work is therefore both time consuming and expensive, particularly when alternative locations are considered. This paper describes a method which allows the signals radiated from an AM transmitter to be used for determination of the frequency correlation function for the channel. The method is based on the fact that the frequency functions of the upper sideband (USB) and lower sideband (LSB) of an AM signal are complex conjugates. The received signal is filtered into its USB, carrier, and LSB components. The USB and LSB signals are synchronously demodulated. Since the two signals are transmitted on different frequencies, they can be used for frequency correlation determinations. The modulating signal can, in most cases, be considered as random noise. Its effects are removed by normalization.