Before it can be transmitted or received in a satellite system, a data stream must be encoded onto a carrier signal that will propagate by means of an electromagnetic wave. This process, called modulation, applies to both conventional radio-frequency and newer optical techniques. In either case, the data stream is converted to a waveform that is compatible with the transmission technology.
A typical spectrum of a digital signal consists of a main lobe, centered on the carrier frequency, where the majority of the signal power is contained, and sidelobes, where the remaining signal power is contained.
Carrier signals can be completely described by three parameters: amplitude, frequency, and phase. Any one of these three can be manipulated to suit the data transmission requirements. Amplitude-shift keying, for example, modifies the amplitude of the carrier wave. Frequency-shift keying adjusts the frequency, and phase-shift keying manipulates the phase.
Amplitude-shift keying is used extensively for commercial terrestrial applications, but its usefulness for satellite applications is limited. Space systems typically employ saturated power amplifiers, which function at the maximum operating point to maximize power efficiency. Around this maximum point, however, amplifier performance is nonlinear, meaning the output is no longer directly proportional to the input. When an amplitude-shifted keying signal is passed through such a nonlinear amplifier, sidelobes can grow large enough to interfere with the adjacent signals. As a result, the amount of bandwidth or power needed for signal transmission increases.
Frequency-shift keying has some attractive characteristics, but it generally does not use bandwidth very efficiently and is not suitable for applications where bandwidth efficiency is crucial.
Phase-shift keying does not suffer the same degradation through a saturated power amplifier as amplitude-shift keying and generally uses bandwidth more efficiently than frequency-shift keying. Some common formats include binary and quarternary phase-shift keying. Binary phase-shift keying takes each input bit individually and chooses one of two possible phases to represent that bit value—so for example, to send a "0," a phase of 0 degrees might be chosen, and to send a "1," a phase of 180 degrees might be chosen.
Quarternary phase-shift keying takes two bits at a time and chooses one of four possible phases to represent them—so, to send a "00," a phase of 0 degrees might be chosen; to send a "01," a phase of 90 degrees might be chosen; to send a "11," a phase of 180 degrees might be chosen; and to send a "10," a phase of 270 degrees might be chosen.
This method can be extended to take three input bits, choosing one of eight (or 23) phases, or four bits, choosing one of sixteen (or 24) phases. This pattern can be extended to a generalized M phase-shift keying signal, and all these waveforms would have the same power spectral density. But the improvement in bandwidth efficiency comes at the cost of decreased power efficiency. The receiver must decide which phase, out of a possible M choices, was transmitted. Those M phases are only separated by 360/M degrees. The phase separation for binary phase-shift keying, for example, is 360/2 or 180 degrees. A higher M value would have a correspondingly smaller phase separation. This smaller separation means it takes less noise to corrupt the signal and cause the receiver to make an error. To combat this, more power must be transmitted.
Another approach is to shape the waveform spectra coming out of the modulator. This can be accomplished by filtering the waveform to narrow the main lobe and reduce sidelobes. The downside to filtered waveforms is that the filtering process distorts the amplitude of the signal, resulting in the same distortion through a saturated power amplifier found in amplitude-shift keying.
Another option is to use continuous phase modulation. Waveforms produced in this manner exhibit smooth phase transitions rather than the abrupt phase transitions produced through binary or quarternary phase-shift keying. This is important because smooth phase transitions require less bandwidth for signal transmission. Gaussian minimum shift keying, the waveform that Aerospace proposed for the Advanced Extremely High Frequency program, is a form of continuous phase modulation.