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The simplest PSK technique is called binary phase-shift keying (BPSK). It uses two opposite signal phases (0 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 (0 or 1). If the phase of the wave changes by 180 degrees -- that is, if the phase reverses -- then the signal state changes (from 0 to 1, or from 1 to 0). Because there are two possible wave phases, BPSK is sometimes called biphase modulation.
More sophisticated forms of PSK exist. In In m-ary or multiple phase-shift keying (MPSK), there are more than two phases, usually four (0, +90, -90, and 180 degrees) or eight (0, +45, -45, +90, -90, +135, -135, and 180 degrees). If there are four phases (m = 4), the MPSK mode is called quadrature phase-shift keying or quaternary phase-shift keying (QPSK), and each phase shift represents two signal elements. If there are eight phases (m = 8), the MPSK mode is known as octal phase-shift keying (OPSK), and each phase shift represents three signal elements. In MPSK, data can be transmitted at a faster rate, relative to the number of phase changes per unit time, than is the case in BPSK.
Amateur-radio operators use a special form of BPSK or QPSK known as PSK31. In this mode, the data transmission rate is 31.25 baud (state changes per second), and the signal bandwidth is approximately 31 Hz. The main advantage of PSK31 is its excellent signal-to-noise ratio (S/N or SNR), which allows communication under adverse conditions such as severe fading, noise, or interference where other communications modes fail.
See also frequency-shift keying (FSK).