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Chapter 5 – Signal Encoding and Chapter 5 – Signal Encoding and Modulation Techniques Modulation Techniques

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Encoding and Modulation TechniquesEncoding and Modulation Techniques

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Digital Signaling Versus Analog Digital Signaling Versus Analog SignalingSignaling

Digital signaling Digital or analog data is encoded into a digital signal Encoding may be chosen to conserve bandwidth or to

minimize error

Analog Signaling Digital or analog data modulates analog carrier signal The frequency of the carrier fc is chosen to be compatible

with the transmission medium used Modulation: the amplitude, frequency or phase of the carrier

signal is varied in accordance with the modulating data signal by using different carrier frequencies, multiple data signals

(users) can share the same transmission medium

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Digital SignalingDigital Signaling

Digital data, digital signalSimplest encoding scheme: assign one voltage level to binary

one and another voltage level to binary zeroMore complex encoding schemes: are used to improve

performance (reduce transmission bandwidth and minimize errors).

Examples are NRZ-L, NRZI, Manchester, etc.

Analog data, Digital signalAnalog data, such as voice and videoOften digitized to be able to use digital transmission facilityExample: Pulse Code Modulation (PCM), which involves

sampling the analog data periodically and quantizing the samples

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Analog SignalingAnalog Signaling

Digital data, Analog SignalA modem converts digital data to an analog signal so that it

can be transmitted over an analog lineThe digital data modulates the amplitude, frequency, or

phase of a carrier analog signalExamples: Amplitude Shift Keying (ASK), Frequency Shift

Keying (FSK), Phase Shift Keying (PSK)

Analog data, Analog SignalAnalog data, such as voice and video modulate the

amplitude, frequency, or phase of a carrier signal to produce an analog signal in a different frequency band

Examples: Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM)

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Digital Data, Digital SignalDigital Data, Digital Signal

Digital signaldiscrete, discontinuous voltage pulseseach pulse is a signal elementbinary data encoded into signal elements

Periodic signalsPeriodic signals

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Data element: a single binary 1 or 0 Signal element: a voltage pulse of constant amplitude Unipolar: All signal elements have the same sign Polar: One logic state represented by positive voltage the other

by negative voltage Data rate: Rate of data (R) transmission in bits per second Duration or length of a bit: Time taken for transmitter to emit

the bit (Tb=1/R)

Modulation rate: Rate at which the signal level changes, measured in baud = signal elements per second. Depends on type of digital encoding used.

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Interpreting SignalsInterpreting Signals

Need to know timing of bits: when they start and endsignal levels: high or low

factors affecting signal interpretationData rate: increase data rate increases Bit Error Rate (BER)Signal to Noise Ratio (SNR): increase SNR decrease BERBandwidth: increase bandwidth increase data rate encoding scheme: mapping from data bits to signal elements

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Comparison of Encoding Schemes

signal spectrumLack of high frequencies reduces required bandwidth, lack of dc component allows ac coupling via transformer,

providing isolation, should concentrate power in the middle of the bandwidth

Clockingsynchronizing transmitter and receiver with a sync

mechanism based on suitable encoding

error detectionuseful if can be built in to signal encoding

signal interference and noise immunitycost and complexity: increases when increases data rate

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Encoding Schemes

Positive level (+5V)

Negative level (-5V)

Positive level (+5V)No line signal (0V)Negative level (-5V)

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Encoding Schemes

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NonReturn to Zero-Level (NRZ-L)

Two different voltages for 0 and 1 bitsVoltage constant during bit interval

no transition, i.e. no return to zero voltagemore often, negative voltage for binary one

and positive voltage for binary zero

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NonReturn to Zero INVERTED (NRZI)

Nonreturn to zero inverted on onesConstant voltage pulse for duration of bitData encoded as presence or absence of signal

transition at beginning of bit time transition (low to high or high to low) denotes binary 1no transition denotes binary 0

Example of differential encoding since have– data represented by changes rather than levels– more reliable detection of transition rather than level

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Advantages and disadvantages of NRZ-L, NRZI

Advantageseasy to engineergood use of

bandwidth

Disadvantagesdc component lack of synchronization

capability

Unattractive for signal transmission applications

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Multilevel BinaryBipolar Alternate Mark Inversion (AMI)

Use more than two levels (three levels, positive, negative and no line signal)

Bipolar-AMIzero represented by no line signalone represented by positive or negative pulseone pulses alternate in polarityno loss of sync if a long string of oneslong runs of zeros still a problemno net dc componentlower bandwidtheasy error detection

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Multilevel BinaryPseudoternary

Binary one represented by absence of line signal

Binary zero represented by alternating positive and negative pulses

No advantage or disadvantage over bipolar-AMI

Each used in some applications

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Multilevel Binary Issues

Advantages:No loss of synchronization if a long string of 1’s occurs, each

introduce a transition, and the receiver can resynchronize on that transition

No net dc component, as the 1 signal alternate in voltage from negative to positive

Less bandwidth than NRZPulse alternating provides a simple mean for error detection

Disadvantagesreceiver distinguishes between three levels: +A, -A, 0a 3 level system could represent log23 = 1.58 bits

requires approx. 3dB more signal power for same probability of bit error

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Theoretical Bit Error Rate (BER) For Various Encoding Schemes

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Manchester Encoding

has transition in middle of each bit periodlow to high represents binary onetransition serves as clock and datahigh to low represents binary zeroused by IEEE 802.3 (Ethernet) LAN standard

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Differential Manchester Encoding

midbit transition is clocking onlytransition at start of bit period representing binary 0no transition at start of bit period representing binary 1used by IEEE 802.5 token ring LAN

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Advantages and disadvantages of Manchester Encoding

Disadvantagesat least one transition per bit time and possibly twomaximum modulation rate is twice NRZ

requires more bandwidth

Advantagessynchronization on mid bit transition (self clocking codes)has no dc componenthas error detection capability (the absence of an expected transition can be

used to detect errors)

elementssignalperbitsofnumberL

bpsRateDataR

baudrateModulationDL

RD

:

][,:

][,:

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Modulation Rate versus Data RateData rate (expressed in bps)

Data rate or bit rate R=1/Tb=1/1μs=1Mbps

Modulation Rate (expressed in baud) is the rate at which signal elements are generatedMaximum modulation rate

for Manchester is D=1/(0.5Tb)=2/1μs=2Mbaud

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Scrambling

Use scrambling to replace sequences that would produce constant voltage

These filling sequences mustproduce enough transitions to maintain synchronizationbe recognized by receiver & replaced with originalbe same length as original

Design goalshave no dc componenthave no long sequences of zero level line signalhave no reduction in data rategive error detection capability

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B8ZS and HDB3

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Bipolar with 8-Zero Substitution (B8ZS)

To overcome the drawback of the AMI code that a long string of zeros may result in loss of synchronization, the encoding is amended with the following rules: If 8 zeros occurs and the last voltage pulse was positive,

then the 8 zeros are encoded as 000+–0–+ If zeros occurs and the last voltage pulse was negative,

then the 8 zeros are encoded as 000–+0+–

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High Density Bipolar-3 zeros (HDB3)

The scheme replaces strings with 4 zeros by sequences containing one or two pulses

In each case, the fourth zero is replaced with a code violation (V) successive violations are of alternate polarity

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Digital Data, Analog Signal

Main use is public telephone systemhas freq range of 300Hz to 3400Hzuse modem (modulator-demodulator)

The digital data modulates the amplitude A, frequency fc , or phase θ of a carrier signal

Modulation techniquesAmplitude Shift Keying (ASK)Frequency Shift Keying (FSK)Phase Shift Keying (PSK)

)2cos( tfA c

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Modulation Techniques

Amplitude Shift Keying (ASK)

Binary Frequency Shift Keying (BFSK)

Binary Phase Shift Keying (BPSK)

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Amplitude Shift Keying (ASK)

In ASK, the two binary values are represented by to different amplitudes of the carrier frequency

The resulting modulated signal for one bit time is

Susceptible to noiseInefficient modulation techniqueused for

up to 1200bps on voice grade linesvery high speeds over optical fiber

0,0

1),2cos()(

binary

binarytfAts c

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Binary Frequency Shift Keying (BFSK)

The most common form of FSK is Binary FSK (BFSK)Two binary values represented by two different frequencies ( f1 and

f2 )

less susceptible to noise than ASKused for

up to 1200bps on voice grade lineshigh frequency radio (3 to 30MHz)even higher frequency on LANs using coaxial cable

0),2cos(

1),2cos()(

2

1

binarytfA

binarytfAts

0 0 1 1 0 1 0 0 0 1 0

f2 f2 f1 f1 f2 f1 f2 f2 f2 f1 f2

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Full-Duplex BFSK Transmission on a Voice-Grade line

Voice grade lines will pass voice frequencies in the range 300 to 3400Hz

Full duplex means that signals are transmitted in both directions at the same time

f1 f 3 f4f2

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Multiple FSK (MFSK)

More than two frequencies (M frequencies) are usedMore bandwidth efficient compared to BFSKMore susceptible to noise compared to BFSKMFSK signal:

elementsignalperbitsofnumberL

elementssignaldifferentofnumberM

frequencydifferencethef

frequencycarrierthef

fMiff

where

MitfAts

L

d

c

dci

ii

2

)12(

1),2cos()(

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Multiple FSK (MFSK)

MFSK signal:

Period of signal element

Minimum frequency separation

MFSK signal bandwidth:

elementsignalperbitsofnumberL

elementssignaldifferentofnumberM

fMiff

where

MitfAts

L

dci

ii

2

)12(

1),2cos()(

ddd MffMW 2)2(

periodbitTperiodelementsignalTLTT bsbs ::,

)(2/12)/(12/1 ratebitLfTfLTfT dbdbds

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Example With fc=250KHz, fd=25KHz, and M=8 (L=3 bits), we have the following frequency assignment for each of the 8 possible 3-bit data combinations:

This scheme can support a data rate of:

KHzMfWbandwidth

KHzf

KHzf

KHzf

KHzf

KHzf

KHzf

KHzf

KHzf

ds 4002

425111

375110

325101

275100

225011

175010

125001

75000

8

7

6

5

4

3

2

1

KbpsHzbitsLfT db 150)25)(3(22/1

dci fMiff )12(

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Example

The following figure shows an example of MFSK with M=4. An input bit stream of 20 bits is encoded 2bits at a time, with each of the possible 2-bit combinations transmitted as a different frequency.

dc

dc

dc

dc

dci

fffi

fffi

fffi

fffi

fMiff

3411

310

201

3100

)12(

4

3

2

1

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Phase Shift Keying (PSK)

Phase of carrier signal is shifted to represent dataBinary PSK (BPSK): two phases represent two

binary digits

0 0 1 1 0 1 0 0 0 1 0

π π 0 0 π 0 π π π 0 π

1)(),2cos()(

0),2cos(

1),2cos(

0),2cos(

1),2cos()(

tdtftAd

binarytfA

binarytfA

binarytfA

binarytfAts

c

c

c

c

c

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Differential PSK (DPSK)

In DPSK, the phase shift is with reference to the previous bit transmitted rather than to some constant reference signal

Binary 0:signal burst with the same phase as the previous one Binary 1:signal burst of opposite phase to the preceding one

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Four-level PSK: Quadrature PSK (QPSK)

10)4

2cos(

00)4

32cos(

01)4

32cos(

11)4

2cos(

)(

tfA

tfA

tfA

tfA

ts

c

c

c

c

More efficient use of bandwidth if each signal element represents more than one bit eg. shifts of /2 (90o) each signal element represents two bits split input data stream in two & modulate onto the phase of the carrier

can use 8 phase angles & more than one amplitude 9600bps modem uses 12 phase angles, four of which have two

amplitudes: this gives a total of 16 different signal elements

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QPSK and Offset QPSK (OQPSK) Modulators

)2sin()(2

1)2cos()(

2

1)(:

)2sin()(2

1)2cos()(

2

1)(:

tfTtQtftItsOQPSK

tftQtftItsQPSK

cbc

cc

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Example of QPSK and OQPSK Waveforms

41101

4

31100

4

31110

41111

:

QPSKfor

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Performance of ASK, FSK, MFSK, PSK and MPSK

Bandwidth Efficiency

ASK/PSK:

MPSK:

MFSK:

10,1

1

r

rB

R

bandwidthontransmissi

ratedata

T

elementssignaldifferentofnumberMr

M

B

R

T

:,1

log2

Mr

M

B

R

T )1(

log2

Bit Error Rate (BER)bit error rate of PSK and QPSK are about 3dB superior to

ASK and FSK (see Fig. 5.4) for MFSK & MPSK have tradeoff between bandwidth

efficiency and error performance

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Performance of MFSK and MPSK

MFSK: increasing M decreases BER and decreases bandwidth Efficiency MPSK: Increasing M increases BER and increases bandwidth efficiency

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Quadrature Amplitude Modulation (QAM)

QAM used on asymmetric digital subscriber line (ADSL) and some wireless standards

combination of ASK and PSKlogical extension of QPSKsend two different signals simultaneously on

same carrier frequencyuse two copies of carrier, one shifted by 90°

each carrier is ASK modulated

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QAM modulator

ASK

c

ASK

c tftdtftdtsQAM )2sin()()2cos()()(: 21

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QAM Variants

Two level ASK (two different amplitude levels)each of two streams in one of two statesfour state systemessentially QPSK

Four level ASK (four different amplitude levels)combined stream in one of 16 states

Have 64 and 256 state systems Improved data rate for given bandwidth

but increased potential error rate