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5. Data Encoding5. Data Encoding

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Prof. Sang-Jo Yoo

ContentsContents

Introduction Data and Signals Encoding( modulation) techniques Spread Spectrum

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Prof. Sang-Jo Yoo

IntroductionIntroduction

Data and signals

Digital Data Digital Signal

Analog SignalAnalog Data

Less complex, less expensive than digital-analog modulation equipment

Use of modern digital transmission And switching equipment

Some transmission media will onlypropagate analog signals

Efficient use of transmission channel : FDM

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Prof. Sang-Jo Yoo

IntroductionIntroduction Encoding and modulation techniques

Encoder Decoderg(t)

digitalor

analog

x(t)digital

x(t)

t

g(t)

Encoding onto a digital signal

Modulator Demodulatorm(t)digital

oranalog

s(t)analog

m(t)

S(f)

f

fc fcModulation onto an analog signal

m(t) = baseband signal or modulating signal fc = carrier signal s(t) = modulated signal

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Prof. Sang-Jo Yoo

Digital Data, Digital SignalsDigital Data, Digital Signals digital data : an abstraction of information in digital form digital signal: an sequence of discrete, discontinuous volta

ge pulses. Each pulse is a signal element Key data transmission terms

mark : binary digit 1 / space : binary digit 0

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Prof. Sang-Jo Yoo

Evaluation of various encoding techniques signal spectrum : less bandwidth, no dc component, shape of the

spectrum(powers should be in the middle of transmitted bandwidth)

clocking : synchronization based on transmitted signal

---> self-clocking error detection : useful to have error-detection capability in

physical encoding scheme signal inference and noise immunity : low bit error rate cost and complexity

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Prof. Sang-Jo Yoo

Various encoding techniques

0 1 0 0 1 1 0 0 0 1 1 NRZ-L

NRZI

Bipolar-AMI

Pseudoternary

Manchester

Differentialmanchester

Digital signal encoding formats

MultilevelBinary

Biphase

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Prof. Sang-Jo Yoo

Return to Zero(RZ) 0 : no pulse 1 : positive pulse signal returns to zero after each encoded bit

Nonreturn to Zero Level(NRZ-L) 0 : positive voltage during bit interval 1 : negative voltage during bit interval

Nonreturn to Zero,invert on ones(NRZI) 1 : transition at the beginning of bit interval 0 : no transition differential encoding advantage

more reliable to detect a transition in the presence of noise less bandwidth

+V 0-V

1 0 0 1

50% RZ

+V -V

1 0 0 1

+V -V

1 0 0 1

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Prof. Sang-Jo Yoo

Limitation of NRZ presence of dc component lack of synchronization capability (long 0s or 1s and drift)

Multilevel Binary bipolar-AMI(alternate mark inversion)

three voltage level 0 : no line signal 1 : positive or negative voltage during bit interval binary 1 (mark) must alternate in polarity benefit : no loss of sync. capability if a long string of 1 occurs

no net dc component , less bandwidth than NRZ, simple

means of error detection

pseudoternary same as bipolar-AMI except that the representations of 1 and 0 are int

erchanged

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Prof. Sang-Jo Yoo

Biphase Always a transition in the middle of each bit interval Manchester code

0 : positive to negative transition 1 : negative to positive transition

Differential Manchester code 0 : transition at the beginnig of a bit interval 1 : no transition at the beginnig of a bit interval

Advantage synchronization, no dc component, error detection

Modulation Rate date rate(bits per second) modulation rate( baud : signal elements per second)

L

R

b

RD

2log D : modulation rate(baud)

R : data rate(bps)L : number of different signal elementsb : number of bits per signal elements

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Prof. Sang-Jo Yoo

Scrambling technique biphase encoding : although used in LAN(10Mbps), but not used i

n long-distance application because of high signaling rate relative to the data rate.

Conditions of codes for long-distance application constant voltage level on the line should be replaced by sufficient tra

nsitions for the receiver’s clock synchronization no dc component, no long sequences of zero-level line signals

no reduction in data rate, error-detection capability

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Scrambling technique B8ZS(bipolar with 8-zeros substitution)

Based on a bipolar-AMI Drawback of AMI: long string of zeros (loss of synchronization). 8 consecutive zeros are encoded as either 000+-0-+ or 000-+0+-

(Positive 1) + 8 zeros = 000+-0-+ (negative 1) +8 zeros = 000-+0+-

receiver recognizes the pattern

HDB3(high-density bipolar-3 zeros) 4 consecutive zeros are encoded as either 000- , 000+, +00+, -00- Table 5.4 (stalling) fourth zeros are always a code violation and successive violations are

of alternate polarity( no dc component) In Europe and Japan.

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Prof. Sang-Jo Yoo

Encoding rules for B8ZS and HDB3

1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0

Bipolar-AMI

B8ZS

HDB3

0 0 0 V B 0 V B

0 0 0 V B 0 0 V B 0 0 V

B = Valid bipolar signal, V = Bipolar violation

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Prof. Sang-Jo Yoo

Spectral density of various signal encoding schemes

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Prof. Sang-Jo Yoo

Digital Data, Analog SignalsDigital Data, Analog Signals

Encoding of digital data by continuous sinusoidal carrier signal ex) MODEM : transmitting digital data through PSTN

Encoding techniques : ASK, FSK, PSK

ASK

FSK

PSK

Digital Data

A cos(2fct )

0

A cos(2f1t )

A cos(2f2t )

A cos(2fct+ )

A cos(2fct )

Binary 1

Binary 0

Binary 1

Binary 0

Binary 1

Binary 0

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Prof. Sang-Jo Yoo

QPSK(Quadrature PSK) more efficient use of bandwidth

PSK using 12 phase angles and two amplitude used in standard 9600 bps modem( modulation rate=2400 bauds)

cos(2fct + 45)cos(2fct + 135)cos(2fct + 225)cos(2fct + 315)

: Binary 11 : Binary 10 : Binary 00 : Binary 01

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Prof. Sang-Jo Yoo

Analog Data, Digital SignalsAnalog Data, Digital Signals

Digitization process Digital data NRZ-L Digital data a code other than NRZ-L Digital data analog signal (modulation)

Digitizer Modulator

Analog Data(Voice)

Digital Data(NRZ-L) Analog Signal

(ASK)

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Prof. Sang-Jo Yoo

Analog Data, Digital SignalsAnalog Data, Digital Signals

PCM(Pulse Code Modulation) based on the sampling theorem

If a signal f(t) is sampled at regular intervals of time and at a rate higher than twice the highest significant signal frequency, then the samples contain all the information of the original signal. The function f(t) may be reconstructed from these samples by the use of a low-pass filter

Appendix 5A

8000 samples * 8 bits per sample = 64 Kbps For 4 KHz voice channel

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PAM pulses(Discrete Signal): discrete-time, continuous-amplitude

PCM pulses(Digital Signal): discrete-time, discrete-amplitude

PCM outputs

Origianl signalPAM sampler

Quantizer

Encoder

011001110001011110100

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Quantization noise : S/N=6n-1.76 dB To reduce quantization noise

large n nonlinear encoding companding(compressing-expanding)

Effect of nonlinear coding

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Prof. Sang-Jo Yoo

DM(Delta Modulation) an analog input is approximated by a staircase function that moves

up or down by one quantization level( ) at each sampling interval(Ts)

simpler hardware implementation than PCM

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Delta Modulation

Comparator

One timeunit delay

+

1 = +0 = -

Analoginput Binary

output

+

One timeunit delay

Binaryinput

Reconstructedwaveform

Transmission

Reception

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Prof. Sang-Jo Yoo

Analog Data, Analog SignalsAnalog Data, Analog Signals

Motivation it is impossible to transmit baseband signals : higher frequency is

needed (especially, for unguided transmission) modulation permits FDM(frequency-division multiplexing)

DSBTC(double-sideband transmitted carrier) spectrum of an AM signal

tftxnts ca 2cos)](1[)(

Spectrum ofmodulating signal

M(f)

0 fB 0

Discrete carrier termS(f)

ffcfc-B

Uppersideband

Lowersideband

Spectrum ofAM signal with carrier at fc

Fc+B-fc-fc-B -fc+B

Uppersideband

Lowersideband

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Prof. Sang-Jo Yoo

na: modulation index

“1” : to prevents loss of information

Power ? example of DSBTC

tftxnts ca 2cos)](1[)(

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Prof. Sang-Jo Yoo

DSBSC(double-sideband supressed carrier) saves power but uses as much bandwidth as DSBTC

SSB(single-sideband) send only one of the sidebands, eliminating the other sideband and

carrier only half of the bandwidth is required less power is required

tftxAts cc 2cos)()(

S(f)

ffcfc-B

UppersidebandLower

sideband

Spectrum ofDSBSC signal with carrier at fc

Fc+B-fc-fc-B -fc+B

Uppersideband

Lowersideband

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Prof. Sang-Jo Yoo

Angle Modulation(FM and PM)

: PM(phase modulation) : FM(frequency modulation)

)](2cos[)( ttfAts cc

)()( tmnt p)()(' tmnt f

carrier

Modulating sine-wavesignal

DSBTC wave

PM wave

FM wave

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Prof. Sang-Jo Yoo

Spread SpectrumSpread Spectrum

Developed initially for military and intelligence requirements

spread the information over a wider bandwidth in order to make jamming and interception more difficult

Two types FH(frequency hopping) and DS(direct sequence)

General model of spread spectrum digital communication system

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Prof. Sang-Jo Yoo

Frequency-Hopping signal is transmitted over a seemingly random series of radio

frequencies, hopping from frequency to frequency at split-second interval

transmitter

receiver

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Prof. Sang-Jo Yoo

Direct Sequence each bit in the original signal is represented by multiple bits in the

transmitted signal, known as a chipping code the chipping code spreads the signal over a wider frequency band

in direct proportion to the number of bits used.