Engineering Made Easy: Quadrature Phase Shift Keying Modulation (QPSK) Basics, Waveform and Benefits

18 Jun 2018

What is Quadrature Phase Shift Keying (QPSK) Modulation

In the last posts we have discussed ASK, FSK and BPSK i.e. Amplitude Shift Keying, Frequency Shift Keying and Binary Phase Shift Keying. In this post, we are going to discuss Quadrature Phase Shift Keying (QPSK). It is also a kind of phase shift keying but it is different from the binary phase shift keying.

Watch the complete Video Here-

So here we will understand what we mean by QPSK and why it is called as Quadrature Phase Shift Keying? what is 'Quadrature' here? and what is 'binary' in binary phase shift keying. We will also see the waveform of QPSK in this post and how it is formed. Here you will also get an idea about the benefits of QPSK over other digital modulation techniques.

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How QPSK is different from ASK, FSK and BPSK

Actually we have seen in other posts on ASK, FSK, and BPSK that in these digital modulation techniques, the carrier wave assumes one of two possible states (symbols). Actually each discrete state of the carrier is called as symbol.

It means that if we talk about ASK, then we have two states (symbols) only of carrier wave's amplitude either 'No transmission of the carrier' or 'Transmission of the carrier'. In ASK, no carrier wave is transmitted, when we want to transmit binary '0' and when we want to transmit binary 1 then a continuous carrier wave is transmitted.
In the same way, in FSK, we have two levels of frequency- 'Low frequency' and 'High Frequency'. Low frequency corresponding to the transmission of binary '0' while high frequency corresponds to the transmission of binary '1'.
The same thing is with BPSK, here binary '0' and binary '1' corresponds to phase shift of '0 degree' or '180 degree', so there are two phases in binary phase shift keying, that's why it is known as binary phase shift scheme.

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So you can see that in ASK, FSK and PSK, 1 bit per symbol is transmitted. But in case of QPSK, two bits per symbol are transmitted, i.e. in each symbol we have 2 bits.
Note that, with 2 bits we can have 4 possible combinations, therefore 4 phases. These 4 combinations with 2 bits (0 and 1) can be-
00, 01, 10 and 11
The 'Quadrature' word in Quadrature phase shift keying means that we have four phases here, Unlike the BPSK, where we have only two phases (0 degree and 180 degree)
Therefore in QPSK the total 360 degree phase is divided into four phases.
360/4 =90 degrees.
So here we have separation of 90° in phases. So in QPSK the 4 phases that are used are-

45°
45°+ 90° = 135°
135° + 90° = 225°
225° + 90° = 315°

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So these all phases are at a separation of 90 degrees. Using the separation in phases helps in distinguishing easily at the receiving end and it can easily be demodulated without any mixing. That's why it is called as quadrature phase shift keying unlike to binary phase shift keying, where we use only 0 degree and 180 degrees. If they all were in the same phase then it would have been difficult to distinguish these messages.

QPSK Waveform Generation

Now look at the image given below-

Quadrature Phase Shift Keying (QPSK) Waveform

This image shows two waveforms-

The first waveform is a complete cycle of a sinusoidal wave, showing angles (in degrees) on the x-axis,

While the second waveform is the QPSK waveform. This waveform shows only the phases (angles) that are used in quadrature phase shift keying.

We know that quadrature phase shift keying has the following phases-

45 degrees, 135 degrees, 225 degrees and 315 degrees. So this QPSK waveform shows only these four phases.

Note- Watch carefully both the waveforms and observe these four phases (angles) used in QPSK; in the first waveform of sinusoidal wave.

By observing the first waveform, you will clearly understand how we have phase shifted in QPSK.

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