Current satellite launch technology limits the weight of satellites that can be placed in orbit in turn this restricts the power generation capability of the satellite. As a consequence satellite communications are power limited requiring their transmitting devices to be as power efficient as possible. For this reason Travelling Wave Tube Amplifiers (TWTAs) are often used as the amplifying device in satellite transponders. A disadvantage of TWTAs is that their input-output characteristic curve is non-linear. If they are operated near saturation this can result in: (i) intermodulation products if multiple signals are fed into the same transponder, and (ii) amplitude compression of the signal.
These are not insurmountable problems. Television transmission requires wide bandwidths so it is often the case that there will be only one television signal per transponder. The second major disadvantage of TWTAs can also be overcome if modulation methods that do not rely on amplitude linearity are chosen. Generally satellite transmissions of analogue television signals will employ frequency modulation. (As opposed to terrestrial transmission which uses a form of amplitude modulation known as Vestigial Sideband modulation). Digital signals will use different forms of phase modulation. Phase modulation systems are less sensitive to amplitude distortion effects than other commonly used digital modulation systems. Quaternary (4 level) Phase Shift Keying (QPSK, also known as 4-PSK) is the most commonly proposed system.
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