05 signal encoding

ACOE312 Signal Encoding Techniques 1

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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 options available today, depending on the encoding technique

• There are four possible combinations of encoding 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 signal

—Discrete, discontinuous voltage pulses

—Each pulse is a signal element

—Binary data encoded into signal elements

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Terms (1)

• Unipolar—All signal elements have same sign, i.e. all positive or all negative

• Polar —One 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 rate

—Rate at which the signal level changes

—Modulation rate is measured in baud = signal elements 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 the receiver will be interpreting the incoming signal?

—Signal to noise ratio

—Data rate

—Bandwidth

—Encoding Scheme


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Encoding Schemes

considerations (1)

• Signal Spectrum

—Lack of high frequencies reduces required bandwidth

—Lack of dc component also desirable since it allows ac coupling 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 Schemes

considerations (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 costs

—Some 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 positive voltage, i.e. unipolar

• Binary 1: a constant positive voltage

• Binary 0: Absence of voltage (i.e. 0 Volts or Ground)

• 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 signal transition 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 presence of noise rather than to compare a value to a threshold level

—In complex transmission layouts it is easy to loose sense of polarity of the signal


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NRZ pros and cons

• Advantages—Easy to engineer

—Make efficient use of bandwidth

• Disadvantages—DC 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 Mark Inversion)

— 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 positive and negative pulses

• No advantage or disadvantage over bipolar-AMI

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

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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 bits bears 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 same probability of bit error — bit error for NRZ at a given SNR is much less than that for

multilevel binary

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Biphase

• Another set of coding techniques that overcomes 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 Manchester

Encoding

—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

• Advantages

—Synchronization 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 are generated

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==T

bTb


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Modulation Rate (2)

For Manchester Encoding, the minimum size signal element is a pulse of ½ the duration of a bit interval.

For a string of all

binary 0s or all 1s, a continuous stream of such pulses is generated.

Hence maximum Modulation rate is 2/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 analog signals and vice-versa

Residence

Modem

Corporate Network

Server

ModemAccess Router

PSTN

network

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Modulation techniques

• We will examine three basic modulation techniques

—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(2πfct) 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(2πf1t) binary 1

s(t) = A cos(2πf2t) 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-axial cable

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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(2πfit), 1<i<M

where, fi=fc+(2i-1-M)fdfc = carrier frequency

fd = difference frequency

M = number of different signal elements = 2L

L = number of bits per signal element


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BFSK example on Voice Grade

Line

1170 Hz 2125 Hz

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Phase Shift Keying (PSK)

• Phase of carrier signal is shifted to represent data

• Binary PSK

—Two phases represent two binary digits

s(t) = A cos(2πfct) binary 1

s(t) = A cos(2πfct+π) = -A cos(2πfct) binary 0

• Differential PSK (DPSK)

—Phase shifted relative to previous transmission rather than some reference signal


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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 element representing more than one bit

—e.g. shifts of π/2 (90o)

—Each element represents two bits

—Can use 8 phase angles and have more than one amplitude

—9600bps modem use 12 angles, four of which have two amplitudes


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QPSK equation

• Each signal element represents two bits rather than one

s(t)=A cos(2πfct+π/4) 11

s(t)=A cos(2πfct+3π/4) 01

s(t)=A cos(2πfct-3π/4) 00

s(t)=A cos(2πfct-π/4) 10

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Quadrature Amplitude

Modulation

• 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 same carrier frequency

—Use two copies of carrier, one shifted by 90°with respect to the other

— Each carrier is ASK modulated

— Two independent signals over same medium

— At the receiver the two signals are demodulated and combined to produce the original binary signal


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QAM Levels

• Two-level ASK—Each of two streams in one of two states

—Four state system

—Essentially QPSK

• Four-level ASK, i.e. 4 different amplitude levels—Combined stream in one of 16 states

• 64 and 256 state systems have been implemented

• Improved data rate for given bandwidth—Increased 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-digital encoding techniques

• What is differential encoding?

• Contrast all digital encoding schemes listed in this lecture (NRZL, NRZI, Bipolar AMI, Pseudoternary, Manchester, Differential Manchester), outlining their advantages and disadvantages

• Define the modulation rate and write an expression which relates it with the bit rate.

• Explain the difference between ASK, FSK and PSK modulation techniques

• What is the difference between Binary PSK, DPSK and QPSK?


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Exercises (1)

1. For the bit stream 01001110, sketch the waveforms 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 negative voltage.

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Exercises (2)

2. The bipolar-AMI waveform representing the binary sequence 0100101011 is transmitted over a noisy channel. The received waveform, which contains a single error, is shown in the following figure. Locate the position of this error and explain your answer.

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05 signal encoding