A high speed digital modem for experimental work on the Communications Technology Satellite

The Communications Technology Satellite (CTS), launched on 17 January, 1976 from Cape Canaveral, provides a very powerful and versatile vehicle for the experimental testing of new communications technology. The Communications Research Laboratory (CRL) of McMaster University has since 1972 been conducting a comparative study of digital modulation techniques for experimental work using the CTS system. This study which has been conducted in three phases, under contract to the Communications Research Centre, Ottawa is the subject of the present paper. The first phase consisted of the comparative evaluation by means of computer simulation of a variety of digital modulation techniques using a satellite type of channel. The results of this show that the best performance in terms of digital error-rate as a function of signal to noise ratio is obtained using the so-called fast-frequency shift keying (FFSK) modulation type. The second phase of the study consisted of the design and construction of a fast frequency-shift keying modem (modulator/demodulator) at a nominal data rate of 60 Mbits/sec. The details of this design and the results of preliminary testing of the modem are presented in the paper, and show that when the modulator and demodulator are operating back-to-back there is a performance degradation from theoretical of 1 dB in signal to noise ratio at a bit error-rate of 10−4. The third phase of the study consisted of the detailed laboratory testing of the modem under a variety of channel conditions preparatory to actual satellite testing. Some of the results of this testing are presented in the paper, and indicate that the FFSK is a very viable modulation type for use in high rate data communications via satellite.