Quadrature Phase Shift Keying

In a communication system, modulation is the method by which the properties of the carrier signal are varied according to the message signals. There are two types of modulation methods based on the type of baseband signal. They are Analog modulation and Digital Modulation. In digital Modulation, the baseband signal is the digital data in the form of 0’s and 1’s. Phase Shift Keying is a method of digital modulation where the phase of the carrier is changed according to the baseband signal. There are two types of phase-shift keying methods – Binary Phase Shift Keying and Quadrature Phase Shift Keying.

What is Quadrature Phase Shift Keying?

Quadrature Phase Shift Keying is a digital modulation method. In this method, the phase of the carrier waveform is changed according to the digital baseband signal. The phase of the carrier remains the same when the input logic is the 1 but goes a phase shift when the logic is 0. In Quadrature Phase Shift Keying, two information bits are modulated at once, unlike Binary Phase Shift Keying where only one bit is passed per symbol. Here, there are four carrier phase offsets with a phase difference of ±90° for four possible combinations of two bits( 00, 01, 10, 11). Symbol duration in this modulation is twice the bit duration.

Circuit Diagram

Instead of converting bits into a digital stream, QPSK converts it into bit pairs. This method is also known as Double Side Band Suppressed Carrier modulation method. QPSK modulation circuit consists of a bit-splitter, 2-bit serial to parallel converter, two multipliers, a local oscillator, and a summer.

Quadrature-Phase-Shift-Keying-Circuit-Diagram
Quadrature-Phase-Shift-Keying-Circuit-Diagram

At the transmitter input, the message signal bits are separated as even bits and odd bits using a bit splitter. These bits are then multiplied with the same carrier waveform to generate Even QPSK and Odd QPSK signals. The Even QPSK signal is phase shifter by 90°, using a phase shifter, before modulation. Here, Local Oscillator is used for generating the carrier waveform. After separation of bits, a 2-bit serial to parallel converter is used. After multiplying with the carrier waveform, both Even QPSK and Odd QPSK are given to the summer when modulation output is obtained.

At the receiver end for demodulation, two product detectors are used. This product detectors convert the modulated QPSK signal into Even QPSK and Odd QPSK signals. Then the signals are passed through two bandpass filters and two integrators. After processing the signals are applied to the 2-bit parallel- to-series converter, whose output is the reconstructed signal.

Waveform of Quadrature Phase Shift Keying

After processing of the Even and Odd QPSK signals, they are applied to the summer where the modulated output is obtained.

Quadrature-Phase-Shift-Keying-Waveform.
Quadrature-Phase-Shift-Keying-Waveform.

Advantages and Disadvantages

  • It provides good noise immunity.
  • Compared to BPSK, bandwidth used by QPSK is reduced to half.
  • The information transmission rate of Quadrature Phase Shift Keying is higher as it transmits two bits per carrier symbol.
  • Carrier power remains constant as the variation in the QPSK amplitude is small.
  • Effective utilization of available transmission bandwidth.
  • Low error probability compared to other methods.
  • The disadvantage of QPSK compared to BPSK is the circuit complexity.

QPSK is usually preferred for applications where the high bit rates and speed transfer of data are required. Matlab code is used for simulation o this method. What is the use of summer in QPSK modulation?



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