1. In digital transmission, angle modulation in which the phase of the carrier is discretely varied in relation either to a reference phase or to the phase of the immediately preceding signal element[?], in accordance with data being transmitted.
2. In a communications system, the representing of characters, such as bits or quaternary digits, by a shift in the phase of an electromagnetic carrier wave with respect to a reference, by an amount corresponding to the symbol being encoded.
Note 1: For example, when encoding bits, the phase shift could be 0° for encoding a "0," and 180° for encoding a "1," or the phase shift could be -90° for "0" and +90° for a "1," thus making the representations for "0" and "1" a total of 180° apart.
Note 2: In PSK systems designed so that the carrier can assume only two different phase angles, each change of phase carries one bit of information, i.e., the bit rate[?] equals the modulation rate. If the number of recognizable phase angles is increased to 4, then 2 bits of information can be encoded into each signal element[?]; likewise, 8 phase angles can encode 3 bits in each signal element. Synonyms biphase modulation, phase-shift signaling.
Source: from Federal Standard 1037C and from MIL-STD-188
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