Quadrature Phase Shift Keying (QPSK) is the digital modulation technique.Quadrature Phase Shift Keying (QPSK) is a form of Phase Shift Keying in which two bits are modulated at once, selecting one of four possible carrier phase shifts (0, Π/2, Π, and 3Π/2).
QPSK perform by changing the phase of the In-phase (I) carrier from 0° to 180° and the Quadrature-phase (Q) carrier between 90° and 270°. This is used to indicate the four states of a 2-bit binary code. Each state of these carriers is referred to as a Symbol.
Quadrature Phase-shift Keying (QPSK) is a widely used method of transferring digital data by changing or modulating the phase of a carrier signal. In QPSK digital data is represented by 4 points around a circle which correspond to 4 phases of the carrier signal. These points are called symbols. Fig 4.1 shows this mapping.
Fig: QPSK diagram showing how four different binary codes can be transmitted
Fig: Block diagram of a QPSK transmitter
In fig shows a block diagram of a typical QPSK transmitter. The unipolar binary message (data) first converted into a bipolar non-return-to-zero (NRZ) sequence using a unipolar to bipolar converter. The bit stream is then split into two bit streams I(in-phase) and Q(Quadrature) .The bit stream in-phase(I) is called the “even” stream and quadrature(Q) is called “Odd” stream.
The input data go to the Serial to Parallel Converter then it is split up into two. The two bit stream fed to the Low pass filter (LPF).Then the two bit stream after filtering fed to the modulator. The filter at the output of the modulator confines the power spectrum of the QPSK signal within the allocated band.The two modulator bit stream are summed and fed to the band pass filter (BPF) and produce the QPSK output.
QPSK is used extensively. Some of the popular applications of QPSK include:
- CDMA Systems
- Iridium(voice/data) Satellite Communication System
- Digital Video Broadcasting Satellite (DVB-S)
- QPSK is used for satellite transmission of MPEG-2 video
- Cable modems
- Videoconferencing
- Cellular phone systems and other forms of digital communication over an RF carrier.
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