Carrier recovery for digital transmission in land mobile cellular communication channels.

摘要

In recent years, technologies such as cellular radio, integrated circuits, and microprocessors have made long range communications from mobile stations practical and affordable. The availability of technology has resulted in a demand for mobile radio communication services that exceeds the capacity of our present systems. To ease demand, the available system capacity must be increased. This may be provided by either allocating more frequency channels to the service or by making more efficient use of those currently allocated.; Spectrally efficient digital modulations, such as higher ordered quadrature amplitude modulation (M-ary QAM), are among the most promising techniques to improve system capacity. Whereas, these modulations are spectrally efficient, they require coherent demodulation which requires accurate representation of the signal carrier. Mobile signals, especially at UHF, are subject to multipath propagation that produces rapid variations in signal amplitude and phase. These variations prohibit the use of a stable carrier reference for demodulation; a reference that varies with channel conditions is required. This research is to investigate a method for recovering the dynamically changing carrier from modulated signals. The methods investigated are applicable to the narrowband land mobile channel over which the fading may be considered non-dispersive.; The background for this work is composed of research characterizing land mobile communication channels, and research in developing carrier recovery techniques. Previous work to characterize the channel has been concerned with change of the signal level over time, and the period of time the signal falls below an acceptable level. This work is now expanded to characterize the dynamic channel in terms of rates at which amplitude and phase may change, providing an understanding of channel effects at the demodulator. A carrier recovery technique is proposed to compensate for amplitude and phase distortion of the incoming signal resulting in correct recovery of data. Data is recovered using differential decision directed demodulation using a digital gain/phase-locked loop (D{dollar}sp5{dollar}GPLL) technique. A DSP demodulator is constructed and used to demonstrate the technique using recorded measurements of channel multipath to provide the channel environment.