Voice under Data: A Method for Transmitting Hybrid Information over Microwave Communication Links

In the Voice Under Data system relatively high capacity digital signals are transmitted together with relatively low capacity analog le. g. FDMI signals. In present digital microwave links the system can be used e.g. for service channel transmission while in future nearly-all-digital networks its significance will probably grow. In the VUD system QPSK is used for the digital modulation and FM for the anlog one. QPSK demodulation is implemented coherently, while FM demodulation via the carrier synchrony zing loop is applied. The main properties of the VUD system such as digital - to - analog intermodulation noise, error rate due to the frequency modulation, effect of the total noise are investigated. Both NRZ and biphase signalling are considered. It is shown that in some cases the latter is superior even from the spectrum-utilization point of view.