staggered quadrature phase-shift keying

Staggered quadrature phase-shift keying (SQPSK), also known as offset quadrature phase-shift keying (OQPSK), is a method of phase-shift keying (PSK) in which the signal carrier-wave phase transition is always 90 degrees or 1/4 cycle at a time. A phase shift of 90 degrees is known as phase quadrature.

In SQPSK, the data is placed alternately on two channels or streams called the I channel (for "in phase") and the Q channel ("phase quadrature"). A single phase transition can never exceed 90 degrees. This property contrasts SQPSK with conventional quadrature phase-shift keying (QPSK), in which the phase can sometimes change by 180 degrees (two 90-degree shifts in a single transition). The average magnitude of the phase transitions is smaller with SQPSK than with conventional QPSK. The result of the smaller average phase "jump" is an improved signal-to-noise ratio (SNR) and a reduced error rate.

SQPSK and QPSK are not the only methods of PSK in common use. In binary phase-shift keying (BPSK) there are two possible states for the signal phase: 0 and 180 degrees. In QPSK and SQPSK there are four possible states: 0, +90, -90 and 180 degrees. In multiple phase-shift keying (MPSK) there can be more than four possible states for the signal phase. An example is the use of eight phase states: 0, +45, -45, +90, -90, +135, -135 and 180 degrees. More than eight phase states are rarely used because that takes the signal complexity past the point of diminishing returns and the error rate actually increases.

This was last updated in June 2007

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