Adaptive carrier tracking (ACT) demodulation for QPSK mobile radio transmission

A new QPSK demodulating scheme (adaptive carrier tracking (ACT)) enabling high-speed carrier tracking has been developed for application to digital mobile communications demodulators. The ACT demodulating scheme, unlike the conventional detection tracking which extracts the carrier by averaging the phases, establishes the phase synchronization and frequency synchronization independently, without using phase-averaging techniques. This method enables high-speed tracking of the carrier phase, thereby improving demodulation characteristics of burst signals and demodulation characteristics under Rayleigh fading to the same degree as those of differential detection. In addition, by adopting the digital oscillator of the new scheme, frequency synchronization of a higher speed than coherent detection that allows avoidance of the false lock has been achieved. A new phase likelihood comparison (PLC) diversity comparing the accuracy of the data obtained from the signal reception phase has been developed, thereby improving the sensitivity and characteristics of detection with respect to co-channel interference over those of conventional coherent detection and differential detection using the reception level comparison diversity. The ACT demodulation LSI with the diversity implemented by an entirely digital circuit yields good demodulating characteristics and a low power consumption of less than 9 mW, which corresponds to about 1/20 of the conventional differential detection circuit. The foregoing results confirm the practicability of the ACT demodulation method.