QPSK vs SQPSK modulation-Difference between QPSK and SQPSK modulation
This page compares QPSK vs SQPSK modulation and describes difference between QPSK and SQPSK modulation types. The advantages of SQPSK modulation are also mentioned.
QPSK | Quadrature Phase Shift Keying
Fig1. QPSK vs SQPSK modulator block diagram
As mentioned Quadrature Phase Shift Keying is referred as QPSK. Here maximum phase shift is limited to about 90 degree.
In QPSK, first input bit stream is split into two bit streams referred as odd and even. These streams are applied simultaneously to the mixers.
Refer BPSK vs QPSK page for power spectrum density of QPSK modulated spectrum.
SQPSK modulation | Staggered Quadrature Phase Shift Keying
Fig.2 SQPSK modulation waveforms
Standard QPSK models are used to generate SQPSK by delaying Q signal by half a bit period prior to modulator input. SQPSK stands for Staggered QPSK. It is also known as Offset QPSK or OQPSK. In SQPSK envelope deep nulls are eliminated and moreover envelope is not constant.
If I and Q are offset by half bit period, one input always remains constant while the other input will change its value. Due to this, transition through origin can be eliminated as observed in QPSK modulation constellation.
The table below summarizes comparison between QPSK and SQPSK modulation types.
| QPSK modulation | SQPSK modulation |
|---|---|
| phase changes of +/- 90 and +/-180 degrees | phase changes of +/- 90 exist |
| Requirements of linear amplifier as non linear amplifier cause spectral regrowth because of abrupt +/-180 degree transitions of the both bits change the phase at the same time. | less demands of linear amplifiers, efficient non linear amplifier can be employed and they do not cause much spectral regrowth, as one of the bits changes the phase at a time and occurs twice during the symbol period with half the intensity of QPSK |
| Null Bandwidth is 1.0 X Data rate | Same as QPSK |
| Bandwidth containing 90% of power is in 0.8 X Data rate | Same as QPSK |
| Power spectral density falls of as inverse second power of frequency | Same as QPSK |
| 99% of power is contained in 1.0 X data rate | Same as QPSK |
| Amplitude variations are of the order of 30dB | Amplitude variation are of the order of 3 dB |
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