Abstract

Bandwidth-efficient modulation techniques are important to reduce the required spectrum for digital video transmission. One approach is to employ a four-level code instead of a binary format. Previous direct-detection subcarrier-multiplexed (SCM) experiments have demonstrated the feasibility of quaternary phase-shift-keying (QPSK) transmission.^1,2 With heterodyne systems, wide-deviation quaternary frequency-shift-keying (FSK) systems have achieved good sensitivity,^3,4 and a homodyne QPSK system using He-Ne lasers has been reported.⁵ In this paper we present the results of the first heterodyne SCM-QPSK system that uses differential encoding. With the differential encoding scheme, only a simple delay-and-multiply demodulator is necessary, thereby avoiding the complexity of implementing a carrier recovery circuit (CRC) and a phase-locked loop. Specifically, a CRC involves high-frequency signal processing to strip away the QPSK data with a × 4 operation. In addition to bandwidth efficiency, QPSK modulation also provides very good receiver sensitivity. Ten video channels (50 Mb/s/channel) are transmitted on five microwave subcarriers, obtaining a bit error rate (BER) of 10⁻⁹ with a detected signal power of −41 dBm.

© 1991 Optical Society of America