A Comparison of Frequency-Shift Anti-Multipath Signaling Techniques for Digital Communication Systems

An important result of multipath propagation is the lengthening of the received signal due to a single transmitted pulse. The received pulse duration may be increased by the difference in propagation times between the longest and shortest of the active paths. This time difference represents the minimum pulse repetition period which can be used if errors due to inter-symbol interference are to be avoided. This minimum pulse repetition period places a limit on the available data rate per channel. The total data rate can be increased by frequency division multiplexing. Several recent papers have described a technique for synchronous frequency-division multiplexing for use under the multipath conditions described above. This synchronous system is analyzed to show its bandwidth, data rate, and signal-to-noise properties, and these parameters are compared with those of a conventional FSK system. A second refinement of conventional FSK is the use of more than two possible frequencies for each pulse. The bandwidth, data rate, and signal-to-noise properties of multilevel synchronous FSK are presented, and some circumstances under which it may be used to advantage are described.