DIGITAL COMMUNICATION METHODS

Amplitude Shift-Keying (ASK)

ASK is a simple version of amplitude modulation used for digital modulation

Two binary values (1 and 0) are represented by two different amplitudes of the carrier frequency (normally, ‘on’ and ‘off’)

Frequency Shift-Keying (FSK)

FSK is the simplest (binary) form of frequency modulation used for digital modulation

The two binary values are represented by two different frequencies near the carrier frequency

Normally, the carrier is shifted high for a 1 and low for a 0

Phase Shift-Keying (PSK)

PSK is a form of modulation in which the phase of the carrier signal is shifted to represent digital data

Binary Phase Shift-Keying (BPSK) is PSK between two phase states, normally 180° apart

Quadrature Phase Shift-Keying (QPSK) is a form of PSK using four phase states, normally 90° apart

Quadrature Amplitude Modulation (QAM) is a modulation technique in which both the phase and amplitude of a carrier are varied by the symbols of the message

M-ary Encoding

M represents a digit that corresponds to the no. of conditions, levels or combinations possible for a given no. of binary variables

For example, an M-ary system with M=4 is a digital signal with four possible conditions

No. of bits necessary to produce a given no. of conditions is

Where N = no. of bits M = no. of conditions/levels/combinations

MN 2log

No. of conditions possible:

For example, With one bit, only 21 = 2 conditions

possible With two bits, 22 = 4 conditions possible With three bits, 23 = 8 conditions

possibleand so on…

MN 2

Amplitude Shift-Keying (ASK)

Uses logic levels in the data to control the amplitude of the carrier wave Data = 1 Amplitude HIGH (switch ON) Data = 0 Amplitude LOW (switch OFF)

ASK modulator block diagram:

Carrier input

Modulation input

Output

Carrier sine wave

Data stream

ASK waveform

Example ASK waveform:

ASK Receiver

Rectifier

Low pass filter

Voltage comparat

or

Voltage reference

Binary data

ASK waveform

ASK Receiver

Rectifier Rectifies the input ASK waveform to contain

only positive signal However, signal still contains unwanted

carrier wave components, and waveform is too rounded and of unreliable amplitudes

Low pass filter Remove remnants of carrier wave

Voltage comparator Signal passes through a voltage level to

output a true copy of the original data stream

Frequency Shift-Keying (FSK)

Uses logic levels in data to control the frequency of the carrier wave Data = 1 frequency HIGH Data = 0 frequency LOW

Example FSK waveform:

There are many ways to generate an FSK waveform. One way is to treat it as combining two different ASK waves

FSK modulator block diagram

Carrier input

Carrier input

Modulation input

Modulation input

Output

Output

Carrier sine wave

Carrier sine wave

Inverted data stream

Data stream

Summing amplifier

FSK waveform

Generating FSK waveform

Advantage of FSK over ASK: Higher reliability in terms of data

accuracy at the receiver Disadvantage of FSK over ASK:

FSK uses 2 different frequencies, hence larger bandwidth

Phase Shift-Keying (PSK)

PSK uses levels in data to control the phase of the carrier wave

Since sine wave is symmetrical, it is not possible for receiver to know whether signal is in inverted form or not. So the demodulator will create two different possibilities for the received signal, one is the inverse of the other

NRZ (non return-to-zero) code is used to detect the logic levels Data = 1 change in phase Data = 0 no change in phase

PSK waveform:

PSK modulator

Carrier input

Modulation input

Output

Carrier sine wave

Bipolar data stream

PSK waveformUnipolar

-bipolar convert

er

Unipolar data stream

PSK Receiver block diagram

PSK demodulat

or

Low pass filter

Voltage comparat

or

PSK input signal

Differential bit

decoder

NRZ data output

Voltage reference

•PSK demodulator demodulates PSK input, resulting in a waveform containing the wanted dc level and ripple at the carrier frequency

•Low pass filter smoothens the ripple, resulting in the rounded version of the data

•Voltage comparator cleans up the wave shape

•Differential bit decoder extracts the NRZ data

Binary Phase Shift Keying (BPSK)

Two phases are possible for the carrier

One phase represents a logic 1, the other represents logic 0

As input digital signal changes state (from 1 to 0, or 0 to 1), the phase of the output carrier shifts 180°

BPSK truth table, phasor diagram and constellation diagram

Binary input

Output phase

Logic 0 180°

Logic 1 0°

Truth table: (+90°)cos ωc t

(-90°)-cos ωc t

(0°)sin ωc tLogic 1

(180°)-sin ωc tLogic 0

180°Logic 0

0°Logic 1

-cos ωc t

cos ωc t

Phasor diagram:

Constellation diagram:

Quadrature Phase Shift Keying (QPSK)

QPSK is an M-ary encoding scheme Four output phases are possible for a

single carrier frequency There must be four different input

conditions: 00, 01, 10, 11 Binary input data are combined into

groups of two bits called dibits In the modulator, each dibit code

generates one of the four possible output phase (+45°, +135°, -45° and -135°)

QPSK truth table, phasor diagram and constellation diagram

Binary input

Output phase

Q I

0 0 -135°

0 1 -45°

1 0 +135°

1 1 +45°

Truth table:

cos ωc t

-cos ωc t

sin ωc t-sin ωc t

Phasor diagram:

10

00

11

01

QPSK truth table, phasor diagram and constellation diagram

cos ωc t

-cos ωc t

sin ωc t-sin ωc t

10

00

11

01

Constellation diagram:

Quadrature Amplitude Modulation (QAM)

QAM is a form of digital modulation that is similar to PSK, except that the digital information is contained in both the amplitude and phase of the transmitted carrier

Amplitude and phase-shift keying are combined

Reduces probability of error

For example, 8-QAM is an M-ary coding technique where M = 8

The phasor diagram for 8-QAM is shown below: cos ωc t

-cos ωc t

sin ωc t-sin ωc t

101

000

011

100

110

111

001

010

Constellation diagram for 8-QAM:

cos ωc t

-cos ωc t

sin ωc t-sin ωc t

101

000

011

100

110

111

001

010