Dynamic characteristics of narrowband land mobile communication channels

The growing demand for land mobile communication systems has opened new requirements for digital modulations with high spectral efficiency. Although efficient modulation types, namely higher ordered m-ary QAMs, have found use in terrestrial applications, the land mobile environment is considered too arduous for their use. The mobile signal at UHF is subject to multipath that results in a channel characterized by rapid variations of received amplitude and phase. We review the dynamic measures that characterize the effects of multipath on a land mobile communication signal and introduce a new measure, fractional power change rate. This is the rate at which multipath causes a change to the received power level. Just as the phase change rate defines the low-end modulation frequency response supportable by the channel, fractional power change rate defines the low-end rate at which the channel will support level changes in PAM or QAM. Characterization of these measures will provide the basic rates at which adaptive carrier recovery techniques compensate for effects of the varying channel. A model based directly on the multipath and receiver architecture is used to derive the distribution of the fractional power change rate, power change rate, and phase change rate. The model statistics are then compared to actual measurements to verify their applicability to the environment