Modulation Techniques Lecture 7: Modulation Techniques ASK, FSK, PSK ,BPSK and QPSK 1st semester 1439 - 2017 1 By: Elham Sunbu

OUTLINE  ASK  FSK  PSK  BPSK  QPSK

WHY DO WE NEED DIGITAL-TO-ANALOG CONVERSI2 ON?! The medium/channel is band pass, and/or Multiple users need to share the medium.

MODULATION OF DIGITAL DATA3 Modulation –process of converting digital data or a low-pass analog to band-pass (higher-frequency) analog signal Digital-to-analog modulation. Analog-to-analog modulation. Carrier Signal – aka carrier freq. or modulated signal - high freq. signal that acts as a basis for the information signal information signal is called modulating signal bandpass channel freq

MODULATION OF DIGITAL DATA (C4ONT.) Digital-to-Analog – process of changing one of the characteristic Modulation of an analog signal (typically a sinewave) based on the information in a digital signal sinewave is defined by 3 characteristics (amplitude, frequency, and phase) digital data (binary 0 & 1) can be represented by varying any of the three application: transmission of digital data over telephone wire (modem) Types of Digital-to-Analog Modulation

MODULATION OF DIGITAL DATA: A5 SK ASK –strength of carrier signal is varied to represent binary 1 or 0 both frequency & phase remain constant while amplitude changes commonly, one of the amplitudes is zero Is this picture, from the textbook, entirely correct?! demodulation: only the presence or absence of a sinusoid in a given time interval needs to be determined advantage: simplicity disadvantage: ASK is very susceptible to noise interference – noise usually (only) affects the amplitude, therefore ASK is the modulation technique most affected by noise application: ASK is used to transmit digital data over optical fiber

Modulation of Digital Data: ASK 6 Example [ ASK ] vd(t) vc(t) vASK(t) How does the frequency spectrum of vASK(t) look like!?

Modulation of Digital Data: ASK 7

MODULATION OF DIGITAL DATA:8 FSK FSK – frequency of carrier signal is varied to represent binary 1 or 0 peak amplitude & phase remain constant during each bit interval  2 demodulation: demodulator must be able to determine which of two possible frequencies is present at a given time FSK is less susceptible to errors than ASK – receiver looks for specific frequency changes over a number of intervals, so voltage (noise) spikes can be ignored advantage: disadvantage: application: FSK spectrum is 2 x ASK spectrum over voice lines, in high-freq. radio transmission, etc.

MODULATION OF DIGITAL DATA: FSK9 Example [ FSK ]

MODULATION OF DIGITAL DATA: FSK10

MODULATION OF DIGITAL DATA: P11 SK PSK – phase of carrier signal is varied to represent binary 1 or 0 peak amplitude & freq. remain constant during each bit interval example: binary 1 = 0º phase, binary 0 = 180º ( rad) phase  PSK is equivalent to multiplying carrier signal by +1 when the information is 1, and by -1 when the information is 0 demodulation: demodulator must determine the phase of received sinusoid with respect to some reference phase PSK is less susceptible to errors than ASK, while it requires/occupies the same bandwidth as ASK more efficient use of bandwidth (higher data-rate) are possible, compared to FSK !!! more complex signal detection / recovery process, than in ASK and FSK advantage: disadvantage:

MODULATION OF DIGITAL DATA: P12 SK Example [ PSK ] vd(t) vc(t) vPSK(t)

BINARY PHASE SHIFT KEYING (BPSK)  In BPSK, the symbol mapping table encodes bits (bn) 1 and 0 to transmission symbols (an) 1 and –1, respectively  Every Tb seconds the modulator transmits one of the two carrier bursts that corresponds to the information bit being a 1 or 0 0  t  Tb 18

QUADRATURE MODULATION SCHEMES  In BPSK the phase of the carrier burst is shifted 0 or 180 degrees every pulse or symbol interval depending upon the information sequence. Thus each modulated carrier pulse transmits 1 bit of information  If, on the other hand, the modulation scheme can use phase shifts of 45, 135, 225, or 315 degrees, each modulated carrier pulse transmits 2 bits of information. This technique is called Quadrature Phase Shift Keying (QPSK)  Using QPSK, we can double the data rate over the same channel bandwidth.  QPSK is one of the modulation methods in the family known as Quadrature modulation schemes which are widely used, including in cellular and cable modem applications 11

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