Information AboutOqpsk |
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Offset quadrature phase-shift keying ('''OQPSK''') is a variant of Phase-shift Keying modulation using 4 different values of the phase to transmit. It is sometimes called ''''Staggered quadrature phase-shift keying''' ('''SQPSK'''). 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. This produces large amplitude fluctuations in the signal; 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 left, 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 often 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 QPSK (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. Note that the signals depicted here are only for a particular implementation of OQPSK. If another set of basis functions are used, or if a different pattern of constellation points are used, the waveforms would look different. In this case, any abrupt transitions pass through 0. It is possible to design OQPSK systems so that these transitions do not pass through 0. Sometimes this is preferable to improve the signal reception at the receiver. See also |
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