DEVIATION MONITORING AND FEEDBACK TO IMPROVE COVERAGE

Publishing Venue

Motorola

Related People

Abstract

For a Frequency Modulated (FM) signal the recovered message signal amplitude is directly related to the modulation index or deviation of the transmitted signal. Thus, keeping the modulation signal at the maxi- mum designed deviation value would result in the highest received signal to noise (S/N) ratio. When a transmitter is at the threshold range of a communications system maximum deviation is desirable to provide the most range or coverage. However, when the transmitter carrier is 60 dB or higher than the noise (C/N), adjacent channel noise is reduced with a reduction in the FM deviation. The optimum system would decrease transmitter devi- ation to the minimum for strong C/N conditions (C/N >60 dB), and increase transmitter deviation to the max- imum for weak C/N conditions.

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MOTOROLA INC. Technical Developments Volume 16 August 1992

DEVIATION MONITORING AND FEEDBACK TO IMPROVE COVERAGE

by Roger A. Davenport and Robert E. Stengel

  For a Frequency Modulated (FM) signal the recovered message signal amplitude is directly related to the modulation index or deviation of the transmitted signal. Thus, keeping the modulation signal at the maxi- mum designed deviation value would result in the highest received signal to noise (S/N) ratio. When a transmitter is at the threshold range of a communications system maximum deviation is desirable to provide the most range or coverage. However, when the transmitter carrier is 60 dB or higher than the noise (C/N), adjacent channel noise is reduced with a reduction in the FM deviation. The optimum system would decrease transmitter devi- ation to the minimum for strong C/N conditions (C/N >60 dB), and increase transmitter deviation to the max- imum for weak C/N conditions.

  Figure 1 is a measurement of receiver Sinad versus a tnmking transmitter signal deviation for three carrier levels close to the noise sensitivity liiit (Sinad = 12 dB).

Sinad = signal+noise+distortion

noise+distortion

S/N can be determined from Sinad using the following relation: The S/N will change by 6 dB for a doubling of the deviation, shown in Figure 1 for C = -115 dBm and deviation of 2.5 and 5.0 KHz.

noise~s~~rtion= S/N = Sinad - 1

  The system repeater would measure the transmitter received signal strength and the deviation to provide devi- ation feedback. The measurement of the deviation would be applied to digital signalliig transmissions such as the inbound trunked signaIling word (ISW), Motorola dig- ital code (MDC), or private line (PL) signalling. These arc signals where the deviation is constant across the message and defmed within predetermined limits. Devi- ation measurement accuracy can be improved by solv- ing for the a...