Band Limitation and Error Rate in Digital UHF-FM Transmission

The literature on systems design for the transmission of high speed digital data in the "megabits per second" range over UHF links is scarce and somewhat contradictory. Essentially, the designer is confronted with an optimization problem of finding the best compromise between radiated bandwidth, power balance, and output SNR, the latter being directly related to the error rate. Confining the analysis to frequency modulation, the available parameters are: transmitter frequency deviation and transmitter band limitation in video and RF stages, receiver predetection band characteristic and noise bandwidth, degree of amplitude limiting and finally receiver postdetection band limitation. The mutual relationship between these parameters and their optimization is analyzed. The effects of intersymbol interference, FM transient characteristics, and FM improvement threshold are discussed, and a simple mathematical expression for strong signal conditions developed by assuming quasi-static conditions in which the carrier is kept at a constant off-tune position. The results are discussed and possible ways of improvements are indicated.