Editing Phase-shift keying (section)

====Offset QPSK (OQPSK)====
[[File:Pi-by-O-QPSK Gray Coded.svg|thumb|Signal doesn't pass through the origin, because only one bit of the symbol is changed at a time.]]

''Offset quadrature phase-shift keying'' (''OQPSK'') is a variant of phase-shift keying modulation using four different values of the phase to transmit.  It is sometimes called ''staggered quadrature phase-shift keying'' (''SQPSK'').

[[File:Oqpsk phase plot.svg|thumb|Difference of the phase between QPSK and OQPSK]]

Taking four values of the phase (two [[bit]]s) at a time to construct a QPSK symbol can allow the phase of the signal to jump by as much as 180° at a time. When the signal is low-pass filtered (as is typical in a transmitter), these phase-shifts result in large amplitude fluctuations, an undesirable quality in communication systems. By offsetting the timing of the odd and even bits by one bit-period, or half a symbol-period, the in-phase and quadrature components will never change at the same time. In the constellation diagram shown on the right, it can be seen that this will limit the phase-shift to no more than 90° at a time. This yields much lower amplitude fluctuations than non-offset QPSK and is sometimes preferred in practice.

The picture on the right shows the difference in the behavior of the phase between ordinary QPSK and OQPSK. It can be seen that in the first plot the phase can change by 180° at once, while in OQPSK the changes are never greater than 90°.

The modulated signal is shown below for a short segment of a random binary data-stream. Note the half symbol-period offset between the two component waves. The sudden phase-shifts occur about twice as often as for OQPSK (since the signals no longer change together), but they are less severe. In other words, the magnitude of jumps is smaller in OQPSK when compared to QPSK.

[[File:OQPSK timing diagram.png|frame|center|Timing diagram for offset-QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown the top and the total, combined signal at the bottom. Note the half-period offset between the two signal components.]]