offset quadrature phase shift keying (Modulation) (OQPSK)
Offset Quadrature PhaseShift Keying (OQPSK) is a special form of quadrature phase shift keying( QPSK) in which there is an offset of one bit between the reference phase and the quadrature bit stream. As with quadrature phase shift keying, four different level values are assigned to four phase positions. Thus, at the phase position of 45 ° the value corresponds to the digital value 11, at 135 ° to the digital value 01.
With quadrature phase shift keying (QPSK), phase changes of 180 degrees can occur, namely when, for example, the dibit changes from 00, which has a phase angle of 45 degrees, to digital value 10, with a phase angle of 225 degrees. This causes the transmitter amplitude to drop to zero in amplitude for the time of the zero crossing, which can affect the synchronization of the receiving equipment. This is what OQPSK modulation prevents.
In OQPSK modulation, the even and odd bits get a time offset of half a bit period. This means that there is no longer a phase change of 180 degrees, but always only between two adjacent modulation points, i.e. always of 90 degrees. The signals are therefore no longer distorted by high phase jumps.
The OQPSK modulation technique is used with ZigBee or 802.15.4 in the 2.4 GHz range. It is similar to minimum shift keying( MSK).