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Phase Shift Keying & π/4 -Quadrature Phase Shift Keying

Date post:14-Feb-2017
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Phase Shift Keying & Quadrature Phase Shift Keying

Phase Shift Keying & /4 -Quadrature Phase Shift Keying Presentation by:Naveen Jakhar, ITS



Topics covered in this presentation: Some basic definitions & concepts of digital communicationWhat is Phase Shift Keying(PSK) ?Binary Phase Shift Keying BPSKBPSK transmitter & receiverAdvantages & Disadvantages of BPSKPi/4 QPSKPi/4 QPSK transmitter & receiverAdvantages of Pi/4- QPSK


Some basic concepts of Digital Communication:


M-ary coding:


Baud and Minimum bandwidth:


Baud and Minimum bandwidth: continued .


Phase Shift Keying (PSK): IntroductionPSK is a digital modulation scheme which conveys data by changing/modulating the phase of the carrier signalPhase of carrier signal is varied in proportional to the information signalThe carrier signal is also called reference signalThe modulation is done by varying sine and cosine inputs at a precise timePSK is often called angle modulated constant amplitude digital modulationSimplest form of PSK is Binary phase shift keying (BPSK)


Binary Phase Shift Keying (BPSK):BPSK has values of N=1 and M=2, so two phases for the carrier are possibleOne phase represents a logic 1 and the other phase represents a logic 0. As the input digital signal changes state (i.e., 1 -> 0 or 0 -> 1), the phase of the output carrier shifts between two angles that are separated by 180


Binary Phase Shift Keying (BPSK): continued ..


Binary Phase Shift Keying (BPSK): continued ..


BPSK Constellation Diagram


BPSK transmitter: Other names for BPSK are phase reversal keying (PRK) and biphase modulation BPSK is a form of square-wave modulation of a continuous wave (CW) signalImportant components of a BPSK transmitter are : Balanced modulator, level converter, Band pass filter and Reference Carrier OscillatorBalanced modulator acts as a phase reversing switchAnother name of Balanced modulator is Balanced Ring modulator


BPSK transmitter diagram:


BPSK Balanced Ring Modulator:

The balanced modulator has two inputs: (1) a carrier which is in phase with the reference oscillator and (2) the binary digital dataFor the balanced modulator to operate properly, the digital input voltage must be much greater than the peak carrier voltage


BPSK Balanced Ring Modulator Function: When the binary input logic is 1 When the binary input logic is 0


Output of a BPSK waveform:

Logic 1 input produces an analog output signal with a 0phase angle, and a logic 0 input produces an analog output signal with a 180 phase angle 16

BPSK Receiver:


BPSK Receiver output:


Advantages and Disadvantages of BPSK: Advantages DisadvantagesCostly due to use of Costas square loop or Costas PLL in coherent demodulationThe abrupt change of phase in time domain is an impulse function which requires infinite bandwidth for transmission in frequency domain19

Applications of BPSK:

BPSK is widely used for wireless LANs, RFID and Bluetooth communicationBPSK is used in radio communications due to robust BER20

Other types of Phase Shift Keying:

QPSK - Quadrature Phase Shift Keying /4-QPSK - Quadrature Phase Shift KeyingO-QPSK - Offset Quadrature Phase Shift Keying8 PSK - 8 Point Phase Shift Keying16 PSK - 16 Point Phase Shift KeyingQAM - Quadrature Amplitude Modulation16 QAM - 16 Point Quadrature Amplitude Modulation64 QAM - 64 Point Quadrature Amplitude Modulation21

QPSK- Quadrature Phase Shift Keying22

QPSK Constellation Diagram23

Now we have two basic functionsEs = 2 Eb since 2 bits are transmitted per symbolI = in-phase component from sI(t).Q = quadrature component that is sQ(t).

QPSK Bit Error Rate: 24

BER is related to the distance between constellation points

/4 -QPSK :25

In /4 QPSK, the maximum phase change is limited to 135o , as compared to 180o for QPSKHence the signal preserves the constant envelop property better than the band limited QPSKThis can be demodulated in a coherent or non-coherent fashion thereby, simplifying the receiver design greatlyIn presence of multipath spread and fading, /4 QPSK is found to perform better

Constellation Diagram for /4 QPSK: 26

QPSK Transmission Technique: 27

/4 QPSK phase components: 28

/4 QPSK mathematical analysis: 29

/4 QPSK FM Discriminator Detection30

Advantages of /4- QPSK:

Among all MPSK schemes, QPSK is the most-often-used scheme since it does not suffer from BER degradation while the bandwidth efficiency is increasedIn the presence of the multipath spread and fading conditions, pi/4 QPSK performs the bestSignal is demodulated in coherent and non-coherent fashion and hence the design of the receiver is simple


Thank YouAn efficient Telecommunications network is the foundation upon which an information society is built


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