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

QPSK OQPSK modulator block diagram
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.

QPSK time diagram or waveforms
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

DP-QPSK modulator used for DP-QPSK modulation
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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