RFQPSK vs. SQPSK: Modulation Techniques Compared
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This article delves into the comparison between QPSK (Quadrature Phase Shift Keying) and SQPSK (Staggered Quadrature Phase Shift Keying) modulation techniques. We will explore the differences, advantages, and characteristics of each method.
Understanding QPSK Modulation
As the name suggests, Quadrature Phase Shift Keying (QPSK) involves modulating data onto a carrier signal by varying its phase. The maximum phase shift is generally limited to approximately 90 degrees.
In QPSK, the incoming bit stream is divided into two separate streams: the odd and even bit streams. Both of these streams are then simultaneously fed into mixers.
Fig 1. QPSK vs SQPSK modulator block diagram
For a deeper understanding of the power spectral density of a QPSK modulated signal, you can refer to resources comparing BPSK and QPSK.
Exploring SQPSK Modulation
SQPSK, short for Staggered QPSK, is also known as Offset QPSK or OQPSK.
Fig.2 SQPSK modulation waveforms
Standard QPSK models can be modified to generate SQPSK signals by introducing a delay of half a bit period to the Q (quadrature) signal before it enters the modulator. This staggering of the Q signal offers distinct advantages. In SQPSK, deep nulls in the signal envelope are eliminated, though the envelope itself isn’t constant.
By offsetting the I (in-phase) and Q signals by half a bit period, one of the inputs always remains constant while the other changes. This prevents the signal from transitioning through the origin, which is a characteristic observed in QPSK constellations.
QPSK vs. SQPSK: A Side-by-Side Comparison
The following table summarizes the key differences between QPSK and SQPSK modulation:
| Feature | QPSK Modulation | SQPSK Modulation |
|---|---|---|
| Phase Changes | +/- 90 and +/- 180 degrees | +/- 90 degrees |
| Amplifier Requirements | Requires a highly linear amplifier. Non-linear amplifiers can cause spectral regrowth due to the abrupt +/-180 degree transitions when both bits change simultaneously. | Less demanding on amplifier linearity. Efficient non-linear amplifiers can be used without significant spectral regrowth, as only one bit changes phase at a time, occurring twice during the symbol period with half the intensity of QPSK. |
| Null Bandwidth | 1.0 X Data rate | Same as QPSK |
| 90% Power Bandwidth | 0.8 X Data rate | Same as QPSK |
| Power Spectral Density | Falls off as inverse second power of frequency | Same as QPSK |
| 99% Power Bandwidth | 1.0 X Data rate | Same as QPSK |
| Amplitude Variations | Of the order of 30dB | Of the order of 3 dB |
