Analog-FM vs. Digital-PSK Transmission

This paper provides a comparison of the transmission of information in analog form by frequency modulation (FM) and in digital form by phase shift keying (PSK). The two systems are compared on the basis of relative accuracy provided, relative power and bandwidth required, and relative equipment complexity involved, with the emphasis being on spacecraft application. Although the two systems require about the same transmitter power (within 1 dB) to provide rms output signal-to-noise (S/N) ratios between 20 and 30 dB at threshold, the analog system is generally simpler, whereas the digital system is capable of greater accuracy. However, the digital system is more efficient for extremely high S/N ratios. The transmission bandwidth required by the FM system is less than that for the pulse code modulation (PCM) system for moderate S/N ratios. For S/N ratios above 35 dB, however, the PCM system requires less bandwidth, as well as less power, than the FM system. Although the analog-FM technique is usually the most straightforward approach, the information may be transmitted in digital form in order to take advantage of one or more of the following factors: 1) transmitter power savings over conventional analog-FM transmission for high S/N ratios 2) ability to regenerate the digital signal 3) solidity with which the information is known when digitized 4) ease of handling YES-NO or ON-OFF signals 5) ease of multiplexing digital signals 6) ability to encrypt the digital signal 7) practicality of processing and sending information at extremely slow rates, as from deep space probes, when in digital form.