Ka-band 500 Mbps terminal investigation of QPSK and MSK modulation

QPSK and MSK modulation were compared in a Ka-band 500 Mbps data link. The test results were in close agreement with computer link simulations which predicted the superiority of MSK. The MSK testing was conducted using coherent demodulation and the QPSK using both coherent and differentially coherent demodulators. The tests were performed by transmitting the modulated signals through a 600 MHz wide Ka-band demonstration link consisting of S-band QPSK and MSK modulators, a parametric upconverter, a Ka-band avalanche diode amplifier, and a low noise Ka to S-band downconverter. Additional equipment included a differential encoder, coherent and differentially coherent demodulators, and a bit synchronizer. The primary performance criterion was the relative link bit error rate performance for a constant system bandwidth. The measured BER degradation for the total system at an error probability of 1 x 10 to the -6th was 0.8 dB from theoretical for MSK as compared to 1.9 dB for coherent QPSK and 4.0 dB for differentially coherent QPSK.