An Experimental Equipment to Reduce Teleprinter Errors in the Presence of Multipath

Multipath propagation between radio terminals employing binary signalling elements produces a catastrophic rise in teleprinter error rate when the difference in path delay time becomes appreciable compared to the duration of a bit. A commonly used four-channel multiplex system has a bit length of 6.7 milliseconds and is thus seriously affected by differences in path delay greater than approximately 3 milliseconds. This paper describes a method of preventing the rise in errors when multipath propagation is present, by synchronously shifting the frequency of the transmitter and receiver following transmission of each bit. The receiver is thus responsive to the signalling element propagated by the shortest path and rejects the long path signals by filter selectivity. Spectrum occupancy and receiver bandwidth considerations which determine the magnitude and number of frequency changes are discussed, and a practical system is described. The general features of equipment used in testing this device are illustrated and discussed. The compatability of the anti-multipath equipment with existing equipment and the ability to reduce the error rate substantially, under conditions of ground-scatter multipath propagated by the layer, have been successfully demonstrated.