Signal Encoding.pdf

8/10/2019 Signal Encoding.pdf

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COE312 Signal Encoding Techniques

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Data Communications &Computer Networks

Lecture 5

Signal Encoding Techniques

Fall 2007

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Agenda

Digital Data, Digital Signals

Digital Data, Analog Signals

Home Exercises


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Encoding Techniques

There are a number of transmission optionsavailable today, depending on the encodingtechnique

There are four possible combinations ofencoding techniques

Digital data, digital signal

Digital data, analog signal

Analog data, digital signal

Analog data, analog signal

We shall examine only the first two techniques

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Digital Data

Digital Signals


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1. Digital Data, Digital Signals

Digital signalDiscrete, discontinuous voltage pulses

Each pulse is a signal element

Binary data encoded into signal elements

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Terms (1)

UnipolarAll signal elements have same sign, i.e. all positive or

all negative

PolarOne logic state represented by positive voltage the

other by negative voltage

Data rate

Rate of data transmission in bits per second Duration or length of a bit

Time taken for transmitter to emit the bit

eg. For a data rate R, the bit duration is 1/R


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Terms (2)

Modulation rateRate at which the signal level changes

Modulation rate is measured in baud = signalelements per second

Mark and Space

Mark is Binary 1, Space is Binary 0

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Interpreting Signals

Receiver needs to know

Timing of bits - when they start and end

Signal levels

What factors determine how successful thereceiver will be interpreting the incoming signal?

Signal to noise ratio

Data rateBandwidth

Encoding Scheme


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Encoding Schemes

considerations (1)

Signal SpectrumLack of high frequencies reduces required bandwidth

Lack of dc component also desirable since it allows accoupling via transformer, providing electrical isolation

Concentrate tx power in the middle of tx bandwidth

Clocking

Synchronizing transmitter and receiver

External clock

Sync mechanism based on tx signal with suitable

encoding

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Encoding Schemesconsiderations (2)

Error detection

Can be built in to signal encoding

Signal interference and noise immunity

Some codes are better than others

Cost and complexity

Higher signal rate (& thus data rate) lead to higher

costsSome codes require signal rate greater than data rate


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Encoding Schemes

Return to Zero (RZ) Nonreturn to Zero-Level (NRZ-L)

Nonreturn to Zero Inverted (NRZI)

Bipolar - AMI

Pseudoternary

Manchester

Differential Manchester

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Return to zero (RZ)

Signal amplitude varies between a positivevoltage, i.e. unipolar

Binary 1: a constant positive voltage

Binary 0: Absence of voltage (i.e. 0 Volts orGround)

Example:1 0 1 1 0 0 0 1

+V

0 Volts


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Non-return to Zero-Level (NRZ-L)

Two different voltages for 0 and 1 bits Negative voltage for one value and positive for

the other, eg

Binary 0 : Positive

Binary 1 : Negative

Voltage constant during bit interval

no transition i.e. no return to zero voltage

Example:

0 1 0 0 1 1 1 0+V0 Volts-V

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Non-return to Zero Inverted(NRZI)

Non-return to zero inverted on ones

Constant voltage pulse for duration of bit time

Data encoded as presence or absence of signaltransition at beginning of bit time

Transition (low-to-high or high-to-low)denotes a binary 1

No transition denotes binary 0

NRZI is an example of differential encoding Example:

or

0 1 1 0 1 0 0 1+V0 Volts-V


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NRZ-L and NRZI format

examples

0V

0V

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Differential Encoding

Data represented by changes rather than levels

Benefits

More reliable detection of transition in the presenceof noise rather than to compare a value to athreshold level

In complex transmission layouts it is easy to loosesense of polarity of the signal


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NRZ pros and cons

AdvantagesEasy to engineer

Make efficient use of bandwidth

DisadvantagesDC component

Lack of synchronization capability

Used for magnetic recording

Not often used for signal transmission

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Multilevel Binary

Uses more than two levels

Bipolar-AMI (Alternate MarkInversion)

zero represented by no line signal

one represented by positive or negative pulse

Binary 1 pulses alternate in polarity

Benefits with respect to NRZ No loss of sync if a long string of ones (zeros still a problem)

No net DC component

Lower bandwidth

Easy error detection


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Pseudoternary

Binary 1 is represented by absence of line signal

Binary 0 is represented by alternating positiveand negative pulses

No advantage or disadvantage over bipolar-AMI

No loss of sync if a long string of zeros (ones still aproblem)

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Bipolar-AMI and Pseudoternary

0 1 0 0 1 1 0 0 0 1 1

0V

0V


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Disadvantages of Multilevel

Binary

Not as efficient as NRZ Each signal element only represents one bit

The line signal may take on one of 3 levels

Each signal element, which could represent log23 = 1.58 bitsbears only one bit of information

Receiver must distinguish between three levels(+A, 0, -A) instead of two in NRZ

Requires approximately 3dB more signal power for sameprobability of bit error

bit error for NRZ at a given SNR is much less than that formultilevel binary

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Biphase

Another set of coding techniques thatovercomes NRZ limitations

Biphase

Manchester

Differential Manchester


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Manchester Encoding

Transition occurs at the middle of each bit period

Transition serves as clock and data

Low to high represents binary 1

High to low represents binary 0

Used by IEEE 802.3 (Ethernet)

0V

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Differential ManchesterEncoding

Midbit transition occurs always and is used for clocking only

Transition at start of a bit period represents binary 0

No transition at start of a bit period represents binary 1

Note: this is a differential encoding scheme

Used by IEEE 802.5 (token ring LAN)

0V0V


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Biphase Pros and Cons

AdvantagesSynchronization on mid bit transition (self clocking)

No dc component

Error detection

Absence of expected transition can be used to detect errors

Disadvantages

At least one transition per bit time and possibly two

Maximum modulation rate is twice as that of NRZ

Requires more bandwidth

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Modulation Rate (1)

Data rate or bit rate is 1/Tb, where Tb = bit duration

Modulation rate is the rate at which signal elements aregenerated

where

D = modulation rate in baud

R = Data rate in bps

M = number of different signal elements = 2L

L = number of bits per signal element

M

R

L

RD

2log==Tb

Tb


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Modulation Rate (2)

For ManchesterEncoding, theminimum size signalelement is a pulse of the duration of abit interval.

For a string of allbinary 0s or all 1s, acontinuous stream ofsuch pulses isgenerated.

Hence maximumModulation rate is2/Tb

Bit rate = 1/Tb

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Digital Data

Analog Signals


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2. Digital Data, Analog Signals

Transmission of digital data with analog signals

Example: Public telephone system (PSTN)

Voice frequency range of 300Hz to 3400Hz

Digital devices are attached to the network via a modem(modulator-demodulator), which converts digital data to analogsignals and vice-versa

Residence

Modem

Corporate Network

Server

ModemAccess Router

PSTNnetwork

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Modulation techniques

We will examine three basic modulationtechniques

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)


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Modulation Techniques

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Amplitude Shift Keying (ASK)

Values represented by different amplitudes of carrier

Usually, one amplitude is zero

i.e. presence and absence of carrier is used

s(t) = A cos(2fct) binary 1

s(t) = 0 binary 0where fc is the carrier frequency

Susceptible to sudden gain changes Inefficient

Up to 1200bps on voice grade lines

Used over optical fiber


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Binary Frequency Shift Keying

Most common form is binary FSK (BFSK) Two binary values represented by two different

frequencies (near carrier)

s(t) = A cos(2f1t) binary 1

s(t) = A cos(2f2t) binary 0where f1, f2 are offset from carrier frequency fc by equal but opposite amounts

Less susceptible to errors than ASK

Up to 1200bps on voice grade lines

High frequency (HF) radio (3-30MHz)

Even higher frequency on LANs using co-axialcable

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Multiple FSK

More than two frequencies used

More bandwidth efficient

More prone to error

Each signalling element represents more than one bit

si(t)=A cos(2fit), 1


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DPSK

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

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Quadrature PSK (QPSK)

Quadrature means a 4-level scheme

More efficient use by each signal elementrepresenting more than one bit

e.g. shifts of /2 (90o)

Each element represents two bits

Can use 8 phase angles and have more than oneamplitude

9600bps modem use 12 angles, four of which havetwo amplitudes


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QPSK equation

Each signal element represents two bits ratherthan one

s(t)=A cos(2fct+/4) 11

s(t)=A cos(2fct+3/4) 01

s(t)=A cos(2fct-3/4) 00

s(t)=A cos(2fct-/4) 10

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Quadrature AmplitudeModulation

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

Combination of ASK and PSK

Can also be considered a logical extension of QPSK

Send two different signals simultaneously on samecarrier frequency

Use two copies of carrier, one shifted by 90 with respect to theother

Each carrier is ASK modulated

Two independent signals over same medium

At the receiver the two signals are demodulated and combinedto produce the original binary signal


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QAM Levels

Two-level ASKEach of two streams in one of two states

Four state system

Essentially QPSK

Four-level ASK, i.e. 4 different amplitudelevelsCombined stream in one of 16 states

64 and 256 state systems have beenimplemented

Improved data rate for given bandwidthIncreased potential error rate

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Required Reading

Stallings chapter 5


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Home Exercises

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Review Questions List and briefly define important factors that can be used

in evaluating or comparing the various digital-to-digitalencoding techniques

What is differential encoding?

Contrast all digital encoding schemes listed in thislecture (NRZL, NRZI, Bipolar AMI, Pseudoternary,Manchester, Differential Manchester), outlining theiradvantages and disadvantages

Define the modulation rate and write an expressionwhich relates it with the bit rate.

Explain the difference between ASK, FSK and PSKmodulation techniques

What is the difference between Binary PSK, DPSK andQPSK?


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Exercises (1)

1. For the bit stream 01001110, sketch thewaveforms for the following codesa) NRZ-L

b) NRZI

c) Bipolar-AMI

d) Pseudoternary

e) Manchester

f) Differential Manchester

Assume that: the most recent preceding 1 bit (AMI) has a negative voltage

the most recent preceding 0 bit (pseudoternary) has a negativevoltage.

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Exercises (2)

2. The bipolar-AMI waveform representing the binary sequence0100101011 is transmitted over a noisy channel. The receivedwaveform, which contains a single error, is shown in the followingfigure. Locate the position of this error and explain your answer.

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Signal Encoding.pdf