# QPSK vs DP-QPSK | difference between QPSK and DP-QPSK modulation

This page compares **QPSK vs DP-QPSK** and mentions **difference between QPSK and DP-QPSK** modulation types.

The terms QPSK stands for Quadrature Phase Shift Keying and DP-QPSK stands for Dual Polarization Quadrature Phase Shift Keying. Both of these are modulation techniques used to convert bits into symbols. QPSK is widely used as modulation technique in RF and Wireless communication. DP-QPSK is used in optical communication to represent laser output into symbols for transmission in order to reduce the bandwidth in the transmission of information. For example in order to transmit 100 Gbps, we require only about 25 Gsymbols per second. This is due to the fact that DP-QPSK represents 4 bits per symbol.

## QPSK modulation | Quadrature Phase Shift Keying modulation

Figure-1: QPSK modulation Block Diagram

As shown in the figure-1, QPSK modulator maps 2 bits per symbol.

As shown input binary data is divided into two streams using S/P converter. One stream is known as in-phase (I) and the other stream is known as quadrature phase (Q). LO (Local Oscillator) generates cos(wt) and 90 degree phase shift of this gives sin(wt). I signal is represented by cos(wt) signal while Q signal is represented by sin(wt) signal. Both I signal and Q signal are combined to produce QPSK signal as shown. Hence QPSK signal is formed by both cosine and sine waveforms. Following mathematical equations are useful to understand QPSK modulation technique.

I signal = cos(wt)... for binary 1

= -cos(wt)...for binary 0

Q signal = sin(wt) ...for binary 1

= -sin(wt) for binary 0

Here w is angular frequency equal to 2*π*f , where f is frequency.

Figure-2: QPSK Time Waveforms and QPSK constellation

The combined QPSK signal which is I+Q combination is represented as follows.

QPSK signal = cos(wt)+sin(wt) for (1,1)= sqrt(2)*sin(wt+π/4)

= -cos(wt)+sin(wt) for (0,1)= sqrt(2)*sin(wt+3*π/4)

= cos(wt)-sin(wt) for (1,0)= sqrt(2)*sin(wt+7*π/4)

= -cos(wt)-sin(wt) for (0,0)= sqrt(2)*sin(wt+5*π/4)

The figure-2 represents the time waveforms of I, Q and combined QPSK signal (I+Q) as well as constellation diagram.

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

## DP-QPSK modulation | Dual Polarization Quadrature Phase Shift Keying

Figure-3: DP-QPSK modulator block diagram

DP-QPSK is the short form of Dual Polarization Quadrature Phase Shift Keying.
The figure-3 depicts DP-QPSK modulator block diagram. **DP-QPSK modulation** uses two polarizations with the same original QPSK
constellation to represent bits. It uses horizontal polarization and vertical polarization along with QPSK to
represent information bits. In DP-QPSK, one symbol represents 4 bits in DP-QPSK compare to 2 bits in QPSK.

Laser beam in optical communication can be split into two orthogonal polarizations i.e. horizontal and vertical. DP-QPSK is digital modulation technique used in optical domain. It uses two orthogonal polarizations i.e. vertical and horizontal of the laser beam with QPSK modulation on each of these polarizations. Refer Horizontal vs Vertical Polarization>>.

Let us understand how **DP-QPSK modulator** works as shown in the figure-3.
As mentioned here laser source is linearly polarized i.e. it has only one polarization.
Let us assume it is horizontally polarized. As shown power of laser source is splitted
using beam splitter. Beam splitter produces two signals having same polarization and
equal power. One is given to upper QPSK modulator part while the other is
given to lower QPSK modulator part. In the upper part QPSK signal polarization is rotated to make
vertical polarizated signal. This vertical polarized QPSK signal is combined with horizontal polarized QPSK
signal from the lower part to obtain DP-QPSK modulation signal.

## Advantages of DP-QPSK modulation over QPSK

Following are the **advantages** of DP-QPSK over QPSK are also mentioned.

• The transmission capacity of DP-QPSK per second is double than QPSK as it represents
2 bits more than QPSK per symbol.

• The electronics hardware requirement is cost effective as for 100 Gbps transmission,
DP-QPSK modulator requires about 25 GHz processing instead of 100 GHz which is simple and cheaper.

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