Heterodyne video transmission with differentially encoded quadrature phase shift keying

Heterodyne transmission experiments of multiple video channels using differentially encoded quadrature phase shift keying (QPSK) are described. Combining the QPSK microwave modulation format with the subcarrier multiplexing technique results in a total of 20 video channels at 50 Mb/s being transmitted on 10 microwave subcarriers. Using a differential encoding approach, this system provides good receiver sensitivity and bandwidth efficiency, while maintaining a simple receiver design. No carrier recovery circuit or phase-locked loop is required for demodulation, which greatly reduces the receiver complexity. It is demonstrated that only a standard delay and multiply demodulator are necessary. With optical heterodyne detection, a receiver sensitivity of -37.5 dBm is achieved for the 20 channel system. The extension of this technique to intensity modulated direct detection subcarrier systems along with migration scenarios for digital video and broadband services is discussed