QPSK Modulator| QPSK Demodulator |Advantages of QPSK | Disadvantages of QPSK |

QPSK Modulator| QPSK Demodulator  |Advantages of QPSK | Disadvantages of QPSK |

Quadrature Phase Shift Keying 

QPSK Modulator  

This is the phase shift keying technique, in which the sine wave carrier takes four phase reversals such as 0°, 90°, 180°, and 270°.

If this kind of techniques are further extended, PSK can be done by eight or sixteen values also, depending upon the requirement.

The Quadrature Phase Shift Keying QPSK is a variation of BPSK, and it is also a Double Side Band Suppressed Carrier DSBSC modulation scheme, which sends two bits of digital information at a time, called as bigits.

Instead of the conversion of digital bits into a series of digital stream, it converts them into bit pairs. This decreases the data bit rate to half, which allows space for the other users.

QPSK Modulator

The QPSK Modulator uses a bit-splitter, two multipliers with local oscillator, a 2-bit serial to parallel converter, and a summer circuit. Following is the block diagram for the same.

At the modulator’s input, the message signal’s even bits (i.e., 2nd bit, 4th bit, 6th bit, etc.) and odd bits (i.e., 1st bit, 3rd bit, 5th bit, etc.) are separated by the bits splitter and are multiplied with the same carrier to generate odd BPSK (called as PSKI) and even BPSK (called as PSKQ). The PSKQ signal is anyhow phase shifted by 90° before being modulated.

The QPSK waveform for two-bits input is as follows, which shows the modulated result for different instances of binary inputs.

QPSK Demodulator

The QPSK Demodulator uses two product demodulator circuits with local oscillator, two band pass filters, two integrator circuits, and a 2-bit parallel to serial converter. Following is the diagram for the same.

The two product detectors at the input of demodulator simultaneously demodulate the two BPSK signals. The pair of bits are recovered here from the original data. These signals after processing, are passed to the parallel to serial converter.

Following are the benefits or advantages of QPSK:

➨In QPSK modulation, two bit are carried by one single analog carrier. Hence bandwidth is twice efficient in comparison to BPSK modulation. Here symbol rate is half of the raw bit rate.
➨BPSK is bandwidth efficient modulation technique as two bits are carried by single carrier.

Following are the disadvantages of QPSK:

➨In QPSK modulation technique, one complex symbol represents two binary bits. Due to this, QPSK receiver is more complex compare to BPSK receiver due to four states needed to recover binary data information.
➨Due to above reason, QPSK is not power efficient modulation technique compare to other modulation types as more power is required to transmit two bits.

QPSK Modulation Applications

1.  Similar to BPSK, QPSK is used in various cellular wireless standards such as GSM, CDMA, LTE, 802.11 WLAN, 802.16 fixed and mobile WiMAX, Satellite and CABLE TV applications.
2.  As difference between any two constellation point is 90 degree maximum, QPSK modulation type has many benefits. It is robust compare to other modulation techniques except BPSK.
3.  It has double data rate carrying capacity compare to BPSK as two bits are mapped on each constellation points. In BPSK only 1 bit is mapped to each constellation point.