QPSK vs. OQPSK vs. π/4-QPSK: Modulation Scheme Comparison

QPSK (Quadrature Phase Shift Keying), OQPSK (Offset QPSK), and pi/4QPSK are all modulation schemes used in wireless digital communication systems. OQPSK and pi/4 QPSK are variations of the basic QPSK modulation technique. They are widely used due to their unique phase transition functionalities. This page explains these modulation types and highlights the differences between QPSK, OQPSK, and pi/4QPSK.

QPSK (Quadrature Phase Shift Keying)

As mentioned, Quadrature Phase Shift Keying is referred to as QPSK. Here, the maximum phase shift is limited to about 90 degrees. In QPSK, the input bit stream is split into two bit streams referred to as odd and even. These streams are applied simultaneously to the mixers.

Fig 1. QPSK-OQPSK block diagram

OQPSK (Offset Quadrature Phase Shift Keying)

Offset Quadrature Phase Shift Keying is referred to as OQPSK. Here, the maximum phase shift is about +/- 90 degrees. In OQPSK, after splitting the bit stream into odd and even components, one bit stream is offset by 1 bit period with respect to the other. After this, the direct and shifted bit streams are fed to the mixers. The power spectral density (PSD) of the OQPSK modulated spectrum is the same as QPSK. Note that the offset in the bit stream will not affect the PSD.

Fig.2 QPSK-OQPSK time domain signal waveforms

As shown in Fig. 2, the transitions are offset for both the odd and even bit streams at any given time; only one of the bit streams can change its value. As shown above in the constellation diagram, the phase transition goes through the origin, resulting in a sudden phase reversal in the time domain signal envelope. When such a signal goes through nonlinear amplification, it will result in spectrum widening. To overcome the regeneration of sidelobes and spectrum widening, a linear amplifier is employed. However, linear amplifiers are less efficient. Hence, OQPSK has been developed. OQPSK prevents phase transition from the origin by shifting one stream by a bit period and allowing only one bit to change between the transitions.

pi/4 QPSK

The pi/4 QPSK modulation scheme is obtained by adding an additional π/4 phase shift to the phase of the carriers of the symbols.

The table mentions the phase output values for different input bit combinations for both the QPSK and pi/4QPSK modulations.

Fig.3 pi/4 QPSK constellation diagram

The π/4 shifted version of QPSK is referred to as pi/4 QPSK. With this modulation, a compromise between QPSK and OQPSK is achieved, and the maximum phase transition of about 135 degrees is achieved. In the presence of multipath spread and fading conditions, pi/4 QPSK performs better than OQPSK. This signal is demodulated in coherent and non-coherent fashion, and hence the design of the receiver is simple. Figure depicts all the possible states of the π/4QPSK constellation.

The table below summarizes QPSK, OQPSK, and pi/4 QPSK modulation types.