EEC4113Data Communication &

Multimedia SystemChapter 3: Broadband Encoding

by Muhazam Mustapha, October 2011

Learning Outcome

• By the end of this chapter, students are expected to be able to explain link level broadband encoding for transmission

Chapter Content

• Amplitude Shift Keying

• Frequency Shift Keying

• Phase Shift Keying

• Pulse Width Modulation

• Quadrature Modulation

• Spread Spectrum Technology

Amplitude Shift Keying

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Amplitude Shift Keying (ASK)

• Values represented by different amplitudes of carrier frequency

• It is similar to Amplitude Modulation (AM) in analog communication, but with only two levels of amplitude

• Usually, one amplitude is zero – i.e. presence and absence of carrier

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Amplitude Shift Keying (ASK)

• Susceptible to sudden changes in gain

• Up to 1200bps on voice grade lines

• Used over optical fiber

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Amplitude Shift Keying (ASK)

0 0 1 1 0 1 0 0 0 1 0

0binary

1binary

0

)2sin()(

tfA

ts c

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Frequency Shift Keying

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Frequency Shift Keying (FSK)

• Different frequency used to represent data

• Two types:– Binary FSK (BFSK)– Multiple FSK (MFSK)

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FSK on Voice Grade Line

Signal strength

Frequency (Hz)1170 2125

Spectrum of signal transmitted in one direction Spectrum of signal

transmitted in opposite direction

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Multiple FSK

• More than two frequencies used

• Each signaling element represents more than one bit– Example: 3 bits per signal element, 8 signal

elements, 8 different frequencies

• Advantage: More bandwidth efficient

• Disadvantage: More prone to error

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Binary Frequency Shift Keying (BFSK)

• Most common form of FSK

• Two binary values are represented by two different frequencies

• It is similar to Frequency Modulation (FM) in analog communication, but with only two frequencies

• Less susceptible to error than ASK

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Binary Frequency Shift Keying (BFSK)

• Up to 1200bps on voice grade lines

• High frequency radio transmission (3 to 30 MHz)

• Even higher frequency on LANs using coaxial cable

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Binary Frequency Shift Keying (BFSK)

0 0 1 1 0 1 0 0 0 1 0

0binary

1binary

)2sin(

)2sin()(

2

1

tfA

tfAts

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Phase Shift Keying

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Phase Shift Keying (PSK)

• Phase of carrier signal is shifted to represent data

FSK PSKASK

QAM

Broadband signaling

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Binary Phase Shift Keying (BPSK)

0 0 1 1 0 1 0 0 0 1 0

0binary

1binary

)2sin(

)2sin()(

tfA

tfAts

c

c

• Two phases represent two binary digits

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Differential Phase Shift Keying (DPSK)

0 0 1 1 0 1 0 0 0 1 0

• Binary 1: Phase change

• Binary 0: No phase change

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Pulse Width Modulation

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Pulse Width Modulation (PWM)

• The width (duration) of the pulse is used to represent data

• In analog communication, PWM needs continuous width values to represent the analog waveform at certain sampling instances

• In data (digital) communication, the pulses can have discrete values of width

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Pulse Width Modulation (PWM)

• An integrator or timing circuit can be used to decode the carried bits

10 01 11 01 00Data

Pulse

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Quadrature Modulation

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Quadrature PSK (QPSK)• More efficient use by each signal representing

more than one bit– e.g. for shifts of π/2 (90°), each element represents two

bits

10 4

2sin

00 4

32sin

01 4

32sin

11 4

2sin

)(

c

c

c

c

fA

fA

fA

fA

ts

135°:01 45°:11

315°:10

Constellation diagram for QPSK

225°:00

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Quadrature PSK (QPSK)• Alternative choice of phases:

270°:11

Constellation diagram

90°:01

180°:10 0°:00

Dibit Phase

00 0

01 90

10 180

11 270

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00 11 01 00 10

Quadrature PSK (QPSK)

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Phase Detector

Ref

Ref

Ref

Ref

Signal

Signal

Signal

Signal

phase leading

phase lagging

phase lagging

phase lagging

phase lagging

phase leading

phase leading

phase leading

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OQPSK (Offset / Orthogonal)

• QPSK sometimes causes a phase change of 180° (π).

• Phase change of 180° sometimes means discontinuity jump, and it is the largest possible jump.

• The jump causes a large amplitude of high frequency and small amplitude of low frequency in transmission, and if it is low pass filtered, then the signal will experience a large fluctuation during the phase change.

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1 3 5 7 9

Examples: QPSK & OQPSK Waveforms

2 4 6 8 10bit number

1 1 −1 −1 1−1 1 −1 1 1value

I I I I IQ Q Q Q Q

input signal

I(t)

Q(t)

1

2

3

4

5

6

7

8

9

10

phase of output signal −π/4 π/4 −3π/4 3π/4 π/4

Q(t−Tb)

phase of output signal −π/4 π/4 −3π/4 3π/4 π/4−π/4 3π/4 −3π/4 π/4

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OQPSK

• OQPSK was designed to reduce this fluctuation by delaying one of the signal combined in the modulator.

• Revisit the example: Transition between bit 3-4 & 5-6 causes 180° in QPSK, but OQPSK signal made a gradual 2 steps of 90°. This reduces the fluctuation.

• The signal would be strobed at the right time to get the right binary combination.

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QPSK and OQPSK Modulators• Offset QPSK (orthogonal QPSK)

– Delay in Q stream

π/2

Σ2 bit

serial to parallelconversion

DelayTb

binary input

bTR

1

R/2 bps

R/2 bps

I(t)an = ±1

Q(t)bn = ±1

carrier

frequency

phase shift

output

s(n)

OQPSK only

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input signal

I(t)

Q(t)

1

2

3

4

5

6

7

8

9

10

phase of output signal −π/4 π/4 −3π/4 3π/4 π/4

Q(t−Tb)

phase of output signal −π/4 π/4 −3π/4 3π/4 π/4−π/4 3π/4 −3π/4 π/4

180° transition

two 90° transitions

OQPSK

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Multilevel PSK• More levels taking more than 2 bits at a time

– e.g. Transmit 3 bits at a time by using 8 phase angles (8-PSK)

– Further, each angle can have more than one amplitude (QAM)

Constellation diagram

010Tribit Phase

000 0

001 45

010 90

011 135

100 180

101 225

110 270

111 315

011

100

101

110

111

000

001

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Quadrature Amplitude Modulation (QAM)• QAM used on Asymmetric Digital Subscriber Line

(ADSL) and some wireless standards• Combination of ASK and PSK• Logical extension of QPSK• Send different signals simultaneously on same

carrier frequency– Signals are distinguished by phase and amplitude

difference

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QAM Constellations

010

011

100101

110

111

000 001

01

10 11

00

4-QAM1 amplitude 4 phases

8-QAM2 amplitudes 4

phases

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QAM Constellations

3 amplitudes 12 phases

16-QAM

16-QAM

4 amplitudes 8 phases2 amplitudes 8 phases

16-QAM

Standard 9600 bps modem use 12 angles, four of which have twoamplitudes for a total of 16 different signal elements

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Spread Spectrum Technology

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Spread Spectrum Technology

• Spread spectrum technology is a method whereby a signal is being transmitted over a bandwidth much wider than the required bandwidth to transmit the intended information.

• The main reason for the technology is security.– The signal that has been scattered over a large

bandwidth can easily be scrambled and become hard to detect or interpret

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Frequency Hopping Spread Spectrum (FHSS)

• The communication is done over many carrier frequencies that is changed randomly

• Sequence:– Initiating side sends a request via a predefined

frequency– The receiving side sends a number, known as seed– The initiating side uses the number to calculate a

sequence of frequencies to be used– The initiating side sends a synchronization signal

through the first frequency in the sequence– Both sides would then continue communication via

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Signal strength

Frequency (Hz)

Frequency Hopping Spread Spectrum (FHSS)

Carrier frequency hops from channel to channel

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Carrier Frequency

Time

Frequency Hopping Spread Spectrum (FHSS) Carrier frequencies are

chosen at random over times

1

2

3

4

5

6

7

8

1 2 3 4 5 6 7 8 9 10 11 12 13 14

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Direct Sequence Spread Spectrum (DSSS)

• The communication is done over many phase modulations that is applied randomly

• The original modulated signal is further phase modulated at random phases – called chips– The rate of chips are much higher than the data rate

• The receiver should know the same sequence of chips– The original data can then be retrieved by removing the

chips

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Signal strength

Frequency (Hz)

Narrow Band Spectrum

Spread Spectrum

Noise Level

Direct Sequence Spread Spectrum (DSSS)

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Other Spread Spectrum Technologies

• Time Hopping Spread Spectrum (THSS)– Data is burst at random times

Time

bursts

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Other Spread Spectrum Technologies

• Chirp Spread Spectrum (CSS)– A chirp is sinusoidal signal that increases or decreases

over some period of time– CSS data is transmitted over a pulse that is modulated

by chirp (time varying carrier frequency)

Frequency (Hz)

data spectrum at start of pulse

data spectrum at end of pulse

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