What is OQPSK Modulation?

Figure 1: OQPSK Constellation Diagram

Offset Quadrature Phase Shift Keying (OQPSK) is a variant of the QPSK modulation scheme where the phase or timing of either the in-phase or Quadrature component is shifted relative to each other by a one bit-period or half a symbol-period Ts as compared to QPSK. As a result, this limits the phase of the total OQPSK signal to no more than 90˚ at a time. In a traditional QPSK scheme, taking four different phase values to generate the signal can allow the phase of the QPSK signal to shift by as much as 180˚ at a time. The below Figures 2 and 3 illustrate the point.

Figure 2: QPSK Signal Indicating Phase Shifts

Figure 3: QPSK Signal

When such a signal is received at the receiver and low-pass filtered, the response exhibits undesirable and high amplitude fluctuations due to the phase shift, thereby degrading the performance of communication systems. In an OQPSK signal, the phase or timing of the in-phase and Quadrature component is shifted by half a symbol-period relative to each other as shown below.

Figure 4: Offset-QPSK Signal

When comparing both Figures 3 and 4, it can be seen that the quadrature component of the QPSK signal is shifted by half a symbol-period relative to the in-phase component. These two signals namely the in-phase and phase/time-shifted Quadrature component are then added at the mixer to obtain the OQPSK signal. In the signal shown in Figure 4, it can be observed that the phase of the OQPSK signal does not change more than 90˚ at a time. This results in relatively lower amplitude fluctuations than the non-offset QPSK and therefore, enables decoding of the received bits with a relatively lower bit error rate (BER). In practice, the OQPSK modulation scheme is sometimes preferred as it contributes to lower amplitude fluctuations and distortions compared to a QPSK signal. The phase shift response of an OQPSK signal is shown below.

Figure 5: Phase Shift response of an OQPSK signal

In the above figure, it can be observed that the phase shift of the OQPSK signal is not greater than 90˚. This reduced phase shift is due to the time delay that is introduced to the Quadrature component of the QPSK signal. Therefore, this reduces the amplitude fluctuations in the OQPSK signal. Due to this, the Bit Error Rate (BER) of OQPSK signal is relatively lower compared to traditional QPSK signal.

The amplitude fluctuations or variations in a QPSK signal are generally on the order of 30 dB and in an offset-QPSK signal; the variations are on the order of only 3 dB. This makes it possible to achieve a relatively higher Signal to Noise Ratio (SNR) and a lower BER while maintaining the same signal bandwidth as that of QPSK.

Owing to this improved characteristic, even non-linear amplifiers can be employed as they do not cause spectral regrowth unlike the case of QPSK. However, linear amplifiers can still be used to further improve the performance of communication systems.

This makes it ideally suitable for various wireless applications such as WLANs, RFID, Wi-Fi, and Bluetooth communication systems.