Phase-shift keying (PSK) is a digital modulation technique that conveys data by changing the phase of a carrier signal. There are three major classes of digital modulation: amplitude-shift keying, frequency-shift keying, and phase-shift keying. Quadrature phase-shift keying (QPSK) is a type of PSK that can either double the data rate compared to binary phase-shift keying (BPSK) while maintaining bandwidth, or maintain the BPSK data rate while halving the required bandwidth. QPSK works by splitting the binary data stream into in-phase and quadrature-phase components at the transmitter, and using matched filters or correlates to detect symbols at the receiver.

1. Nishita Gogoi

2.  INTRODUCTION
Phase-shift keying (PSK) is a digital modulation scheme that conveys
data by changing, or modulating, the phase of a reference signal (the
carrier wave).
.
PSK is a modulation technique used by modems in
which different phase angles in the carrier signal are used to represent
the binary states of 0 and 1.

3. Instead of operating with respect to a constant reference wave, the
broadcast can operate with respect to itself. Changes in phase of a single
broadcast waveform can be considered the significant items. In this
system, the demodulator determines the changes in the phase of the
received signal rather than the phase (relative to a reference wave)
itself.

4. There are three major classes of digital modulation techniques
used for transmission of digitally represented data: 
Amplitude-shift keying
(ASK)
Frequency-shift keying
(FSK)
Phase-shift keying (PSK)

5. Phase Shift
Keying
Binary phase-
shift keying
(BPSK)
Binary phase-
shift keying
(BPSK)
Higher-order
PSK
Differential
phase-shift
keying (DPSK)
Offset QPSK
(OQPSK)
dual-
polarization
QPSK
shaped-offset
QPSK
Quadrature phase-
shift keying (QPSK)
Offset QPSK
(OQPSK)

6. Quadrature phase-shift keying (QPSK)
Sometimes this is known as quadriphase PSK, 4-PSK, or 4-QAM.
The mathematical analysis shows that QPSK can be used either to
double the data rate compared with a BPSK system while maintaining
the same bandwidth of the signal, or to maintain the data-rate of
BPSK but halving the bandwidth needed.

7. Conceptual transmitter structure for QPSK. The binary data stream is
split into the in-phase and quadrature-phase components

8. Receiver structure for QPSK. The matched filters can be replaced with
correlates. Each detection device uses a reference threshold value to
determine whether a 1 or 0 is detected.
Receiver structure:-