## Technical Terms and Definitions

 Phase-Shift Keying - Telecommunications Phase-shift keying (PSK) is a method of transmitting and receiving digital signals in which the phase of a transmitted signal is varied to convey information. There are several schemes that can be used to accomplish PSK. The simplest method uses only two signal phases: 0 degrees and 180 degrees. The digital signal is broken up timewise into individual bits (binary digits). The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, then the signal state stays the same (low or high). If the phase of the wave changes by 180 degrees -- that is, if the phase reverses -- then the signal state changes (from low to high, or from high to low). Because there are two possible wave phases, this form of PSK is sometimes called biphase modulation. More complex forms of PSK employ four or eight wave phases. This allows binary data to be transmitted at a faster rate per phase change than is possible with biphase modulation. In four-phase modulation, the possible phase angles are 0, +90, -90, and 180 degrees; each phase shift can represent two signal elements. In eight-phase modulation, the possible phase angles are 0, +45, -45, +90, -90, +135, -135, and 180 degrees; each phase shift can represent four signal elements.