Experimental radio link with fast carrier recovery for universal digital portable communications

To investigate the viability of time-division multiple access (TDMA) in a fading environment, an experimental radio link has been designed and implemented. It consists of a 500-kb/s differentially encoded four-level quadrature amplitude modulation (QAM) burst transmitter and a coherent receiver with a fast-carrier-recovery circuit. The demodulator is a modified Costas carrier tracking loop with a dynamic loop (bandwidth switching) filter using hard-decision detection. A bandwidth switching technique, coupled with another technique, i.e. freezing the loop filter between two consecutive bursts so that at the start of a burst only phase must be reacquired, results in efficient burst coherent detection. A carrier-acquisition preamble length equivalent to seven symbols (28 μs) has been achieved. Bandwidth switching also helps recover from a deep fade faster than when a constant narrow bandwidth is used. There is only a small degradation in the radio link performance with increasing user speed (Doppler frequency) in a fading environment