QPSK vs DP-QPSK: Modulation Techniques Compared

This page compares QPSK and DP-QPSK modulation types, highlighting the differences between them. QPSK stands for Quadrature Phase Shift Keying, while DP-QPSK stands for Dual Polarization Quadrature Phase Shift Keying. Both are modulation techniques used to convert bits into symbols for transmission.

QPSK is widely used as a modulation technique in RF and wireless communication. DP-QPSK, on the other hand, is used in optical communication to represent laser output into symbols for transmission, aiming to reduce bandwidth in information transmission. For example, to transmit 100 Gbps, DP-QPSK requires only about 25 Gsymbols per second because it represents 4 bits per symbol.

QPSK Modulation | Quadrature Phase Shift Keying

Figure-1: QPSK modulation Block Diagram

As shown in Figure-1, a QPSK modulator maps 2 bits per symbol. The input binary data is divided into two streams using a serial-to-parallel (S/P) converter. One stream is known as the in-phase (I) component, and the other is the quadrature phase (Q) component.

A local oscillator (LO) generates cos(ωt), and a 90-degree phase shift of this signal produces sin(ωt). The I signal is represented by the cos(ωt) signal, while the Q signal is represented by the sin(ωt) signal. Both I and Q signals are combined to produce the QPSK signal.

Hence, the QPSK signal is formed by both cosine and sine waveforms. The following mathematical equations are useful to understand the QPSK modulation technique:

I signal = cos(ωt) … for binary 1 = -cos(ωt) … for binary 0

Q signal = sin(ωt) … for binary 1 = -sin(ωt) … for binary 0

Here, ω is the angular frequency equal to 2πf, where f is the frequency.

Figure-2: QPSK Time Waveforms and QPSK constellation

The combined QPSK signal, which is the I+Q combination, can be represented as follows:

QPSK signal = cos(ωt) + sin(ωt) for (1,1) = √2 _ sin(ωt + π/4) = -cos(ωt) + sin(ωt) for (0,1) = √2 _ sin(ωt + 3π/4) = cos(ωt) - sin(ωt) for (1,0) = √2 _ sin(ωt + 7π/4) = -cos(ωt) - sin(ωt) for (0,0) = √2 _ sin(ωt + 5π/4)

Figure-2 represents the time waveforms of I, Q, and the combined QPSK signal (I+Q), as well as the constellation 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. Figure-3 depicts the DP-QPSK modulator block diagram. DP-QPSK modulation uses two polarizations with the same original QPSK constellation to represent bits. It utilizes horizontal and vertical polarization along with QPSK to represent information bits.

In DP-QPSK, one symbol represents 4 bits compared to 2 bits in QPSK. A laser beam in optical communication can be split into two orthogonal polarizations, i.e., horizontal and vertical. DP-QPSK is a digital modulation technique used in the optical domain. It employs two orthogonal polarizations (vertical and horizontal) of the laser beam with QPSK modulation on each of these polarizations.

Let’s understand how the DP-QPSK modulator works, as shown in Figure-3. As mentioned, the laser source is linearly polarized, meaning it has only one polarization. Let’s assume it is horizontally polarized. The power of the laser source is split using a beam splitter. The beam splitter produces two signals having the same polarization and equal power. One is given to the upper QPSK modulator part, while the other is given to the lower QPSK modulator part. In the upper part, the QPSK signal polarization is rotated to create a vertically polarized signal. This vertically polarized QPSK signal is combined with the horizontally polarized QPSK signal from the lower part to obtain the DP-QPSK modulation signal.

Advantages of DP-QPSK Modulation over QPSK

Following are the advantages of DP-QPSK over QPSK:

• The transmission capacity of DP-QPSK per second is double that of QPSK, as it represents 2 bits more than QPSK per symbol.
• The electronics hardware requirement is cost-effective because, for 100 Gbps transmission, the DP-QPSK modulator requires about 25 GHz processing instead of 100 GHz, which is simpler and cheaper.