QPSK vs DQPSK: Key Differences Explained

This article clarifies the differences between QPSK (Quadrature Phase Shift Keying) and DQPSK (Differential Quadrature Phase Shift Keying) modulation techniques.

QPSK (Quadrature Phase Shift Keying)

QPSK, which stands for Quadrature Phase Shift Keying, is a modulation scheme where two bits are mapped to a single symbol. This effectively halves the bandwidth required compared to Binary Phase Shift Keying (BPSK), where one bit is mapped to one symbol.

Fig:1 QPSK Block Diagram

As shown in Figure 1, the incoming binary data stream is first converted from serial to parallel. This ensures that two bits are simultaneously fed into two mixer inputs. The outputs of these mixers are then summed to produce a single symbol output. Therefore, for every two input bits, a single complex symbol is generated.

Fig:2 QPSK

Figure 2 illustrates the complex data output for a serial bit stream of (01001011).

DQPSK (Differential Quadrature Phase Shift Keying)

DQPSK, or Differential Quadrature Phase Shift Keying, employs a differential modulation format. In this scheme, the bits for a given symbol are determined based on the change in phase from the previous symbol. There are four possible phase states: 0, π, +π/2, and -π/2. Consequently, each symbol represents two information bits.

Fig:2 DQPSK constellation and BER performance curve

Similar to QPSK, DQPSK also splits the incoming binary pattern into two streams. However, in DQPSK, one of these bit streams is phase-shifted by π/4 or π/2, depending on the design requirements.

The figure above showcases the DQPSK constellation diagram for a π/4 shifted version. It also presents the Bit Error Rate (BER) performance curves for both QPSK and DQPSK. Notably, the curves indicate that QPSK generally exhibits better performance than DQPSK in terms of BER.