Effect of multipath propagation on performance of narrowband frequency-modulation radiotelegraph systems

An explanation is given of the high error rate and poor diversity action experienced with narrowband frequency-modulation radiotelegraph systems with fading multipath signals, even in the absence of noise. The mechanism of occurrence of these residual errors is examined in detail, and they are shown to be due to a combined effect of the frequency modulation and spurious amplitude modulation of the signal, which together cause severe phase aberrations during the signal transitions at troughs in the fading. The spurious amplitude modulation is due to the removal of the higher-order frequency-modulation sidebands by the normal channel filters, and it is shown that a useful reduction in the residual-error rate and improved diversity operation may be achieved if the second-order sidebands reach the output of the channel-receiving filter at their correct level. A method of establishing this condition in a multichannel system with practical filters is described, but it is concluded that some increase in channel spacing in relation to modulation rate would be necessary to restrict interchannel interference.