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QPSK vs DQPSK-difference between QPSK and DQPSK modulations

This page QPSK vs DQPSK covers difference between QPSK and DQPSK modulation techniques.

As we know QPSK stands for Quadrature Phase Shift Keying while DQPSK stands for Differential Quadrature Phase Shift Keying.

QPSK-Quadrature Phase Shift Keying


QPSK OQPSK modulator block diagram
Fig:1 QPSK Block Diagram

As shown in the figure-1, two bits are mapped to one symbol in QPSK. This will reduce the bandwidth to half in comparison with BPSK modulation. In BPSK, one bit is mapped to one symbol.

Before mapping, binary data stream is converted from serial to parallel so that two bits are incident simultaneously to two mixer inputs. Later on these two outputs are summed to have one single symbol output. Hence for every two bits, one complex symbol is generated as output.


QPSK

The figure-2 above depicts complex data output for serial bit stream of (01001011).

Refer QPSK constellation➤ and QPSK Spectrum➤ and QPSK Block Diagram➤.

DQPSK-Differential Quadrature Phase Shift Keying


DQPSK constellation and BER curve
Fig:2 DQPSK constellation and BER performance curve

DQPSK is the differential QPSK which uses differential modulation format. Here bits for given symbol is determined based on phase change from previous symbol.

There are four possible states 0, π, +π/2, -π/2. Hence each symbol represents two information bits. Splitting of binary pattern is same as QPSK here except one chain of bits are shifted in phase to about π/4 or π/2 depending upon the design requirement.

The figure depicts DQPSK constellation diagram for π/4 shifted version. It also mentions BER performance curve for QPSK and DQPSK. From figure it is imperative that QPSK performs better compare to DQPSK.

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