Signal Encoding Techniques

부산대학교 정보컴퓨터공학부

김종덕 (kimjd at pusan.ac.kr)

강의의 목표

“Analog vs. Digital Signal”, “Analog vs. Digital Transmission” 의 차

이를 이해한다.

Digital Communication에서 Baseband Modulation과 Bandpass

Modulation의 차이를 이해한다.

다양한 Baseband Modulation 기법을 살펴보고 그 장, 단점을 평가한다.

Bandpass Modulation의 필요성을 이해한다.

Analog Modulation과 Digital Modulation의 차이를 이해한다.

주요 Bandpass Modulation 기법을 살펴본다.

교재 Chapter 5. Signal Encoding Techniques

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Analog and Digital Signals

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Signal - Means by which data are propagated

Analog

Continuously Varying Electromagnetic Wave

Various media : Wired, Fiber optic, Space (Radio, Wireless)

Less Attenuation

Digital

Sequence of Voltage Pulse

Wired Media (Baseband Channel)

Cheaper and Less Susceptible

to noise interference

Greater Attenuation

Data and Signals

Can we use Analog Signal to carry Analog Data ?

Can we use Digital Signal to carry Digital Data ?

Can we use Analog Signal to carry Digital Data ?

Modem (Modulator/Demodulator)

Modulate digital data onto an analog carrier frequency.

• PC modem over telephone lines

Can we use Digital Signal to carry Analog Data ?

Codec (Coder/Decoder)

Sample an analog signal to produce a stream of integers.

Compact Disc audio, DVD video

All 4 Combinations are possible (and necessary)

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Analog Signals Carrying Analog and Digital Data

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Digital Signals Carrying Analog and Digital Data

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Analog and Digital Transmission

Analog Transmission

Analog signal transmitted

without regard to content

Use Amplifiers to boost signal,

Also amplifies Noise

Digital Transmission

Analog and Digital signal

transmitted with regard to

content

Use Repeaters to regenerate the

original signal. Noise is not

amplified.

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Analog Data Analog Signal

Digital Data

Analog Signal

Digital Signal

Analog vs. Digital Transmission

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Analog/Digital Data, Signal, Transmission

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Analog Data Analog Signal Analog Transmission

Digital Data

Analog Signal

Digital Transmission

Digital Signal

Digital Communication

Why Digital ?

High Quality / Data integrity

Repeaters; Error Correction Technology

Integration / Flexibility

Can treat analog and digital data similarly

All analog information can be represented in digital data.

High Capacity Utilization

High degree of multiplexing easier with digital techniques

Compression Technology.

Security & Privacy

Encryption

Hurdle : Need Massive Processing Power Digital technology

Low cost LSI/VLSI technology

Problem : All or nothing.

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디지털 통신에 아날로그 신호가 왜 필요할까?

특정 매체는 아날로그 신호 전달 기술만 실용화.

대표적 예: Wireless Medium

주파수 대역의 한계와 확장

Baseband Signal

Carrier Frequency & Bandpass Signal

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Block Diagram of a typical digital communication system

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Digital Communications, 2nd Edition, - Bernald Sklar, Prentice-Hall

Can you identify the role or function of each block?

Essential Blocks

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Format Source Encode

Encrypt Channel Encode

Multiplex Pulse

Modulation Bandpass

Modulation Frequency

Spread Multiple Access

X M T

Format Source Decode

Decrypt Channel decode

De- Multiplex

Detect

De- Modulate

& Sample

Frequency Despread

Multiple Access

R C V

Synchro- nization

Information Source

Information Sink

Message Symbols

Digital Baseband waveform

Digital bandpass waveform

SIGNAL ENCODING/MODULATION

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Encoding and Modulation Techniques

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Baseband Communication

Bandpass Communication (3)

(4)

(3)

(1) (2)

(5)

Preview

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Digital Digital Digital Analog

Analog Digital Analog Analog

Digital Data, Digital Signal

Less Complex and Less expensive

Digital signal

Discrete, discontinuous voltage pulses

Each pulse is a signal element

Binary data encoded into signal elemen

ts

Key Terms

Data Element : Bits

Data Rate : Bits per sec (bps)

Signal Element

Signaling Rate / Modulation Rate :

Baud

Unipolar / Polar

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Interpreting Signals

Need to know

Timing of bits - when they start and end

Signal levels : Ex) High (0) or low (1)

Factors affecting successful interpreting of signals

Signal to noise ratio

• An increase in SNR decreases bit error rate (BER).

Data rate

• An increase in data rate increase BER.

Bandwidth

• An increase in bandwidth allows an increase in data rate.

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Comparison of Encoding Schemes

Signal Spectrum Lack of high frequencies reduces required bandwidth Lack of dc component allows ac coupling via transformer, providing isolation Concentrate power in the middle of the bandwidth

Clocking Synchronizing transmitter and receiver External clock Sync mechanism based on signal

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 Techniques

NRZ

Nonreturn to Zero-Level (NRZ-L)

Nonreturn to Zero Inverted (NRZI)

Multilevel Binary

Bipolar -AMI

Pseudoternary

Biphase

Manchester

Differential Manchester

Scrambling Techniques

B8ZS

HDB3

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Spectral Density of Various Signal Schemes

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Nonreturn to Zero

Nonreturn to Zero-Level (NRZ-L)

Two different voltages for 0 and 1 bits, Voltage constant during bit interval

• no transition, i.e. no return to zero voltage

• e.g. Absence of voltage for zero, constant positive voltage for one

• More often, negative voltage for one value and positive for the other

Nonreturn to Zero-Inverted (NRZ-I)

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

Constant voltage pulse for duration of bit

An example of differential encoding

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Differential Encoding

Data represented by changes rather than levels

More reliable to detect a transition in the presence of

noise than to compare a value to a level threshold

In complex transmission layouts it is easy to lose sense

of polarity

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NRZ pros and cons

Pros

Easy to engineer

Make good use of bandwidth

Cons

DC component

Lack of synchronization capability

Not often used for signal transmission

Used for magnetic recording

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Multilevel Binary

Use more than two levels

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Multilevel Binary

Bipolar-AMI

zero represented by no line signal

one represented by positive or ne

gative pulse

one pulses alternate in polarity

Advantages

No net dc component

No loss of sync if a long string of o

nes (zeros still a problem)

Lower bandwidth

Easy error detection

Pseudo Ternary

One represented by absence of line signal

Zero represented by alternating positive and negative

No advantage or disadvantage over bipolar-AMI

Tradeoff for Multilevel Binary

Not as efficient as NRZ Each signal element only represents on

e bit, In a 3 level system could represent log23 = 1.58 bits

Receiver must distinguish between three levels (+A, -A, 0), Requires approx. 3dB more signal power for same probability of bit error

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Biphase

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Biphase

Manchester

Transition in middle of each bit period

• Low to high represents one;

• High to low represents zero

Transition serves as clock and data

Used by IEEE 802.3

Differential Manchester

Midbit transition is clocking only

Transition at start of a bit period represents zero

No transition at start of a bit period represents one

Note: this is a differential encoding scheme

Used by IEEE 802.5

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Biphase Pros and Cons

Cons

At least one transition per bit time and possibly two

Maximum modulation rate is twice NRZ

Requires more bandwidth

Pros

Synchronization on mid bit transition (self clocking)

No DC component

Error detection

• Absence of expected transition

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Scrambling

Use scrambling to replace sequences that would produce constant v

oltage

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Scrambling

Filling sequence

Must produce enough transitions to sync

Must be recognized by receiver and replace with original

Same length as original

Benefit

No dc component

No long sequences of zero level line signal

No reduction in data rate

Error detection capability

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B8ZS and HDB3

Bipolar With 8 Zeros Substitution

Based on bipolar-AMI

If octet of 8 zeros, the sequence is

replaced with 000VB0VB. Where

B=Bipolar Bit, and V=Violation Bit

• If octet of all zeros and last voltage

pulse preceding was positive encode

as 000+-0-+

• If octet of all zeros and last voltage

pulse preceding was negative encode

as 000-+0+-

Receiver detects and interprets as

octet of all zeros

Causes two violations of AMI code

Unlikely to occur as a result of noise

High Density Bipolar 3 Zeros

Based on bipolar-AMI

String of four zeros replaced with one

or two pulses

• If number of ones since the last

substitution is odd, then 4 zeros are

replaced with 000V

• If number of ones since the last

substitution is even, then 4 zeros are

replaced with B00V

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Analog Data, Analog Signals

Why modulate analog signals?

Higher frequency can give more

efficient transmission

Permits frequency division

multiplexing

Types of modulation

Amplitude

Frequency

Phase

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Analog Modulation

Amplitude Modulation

𝑠 𝑡 = 1 + 𝑛𝑎 ⋅ 𝑚 𝑡 ⋅ cos2𝜋𝑓𝑐𝑡

𝑛𝑎 : Modulation Index

Angle Modulation (Phase Modulation, Frequency Modulation)

𝑠 𝑡 = 𝐴𝑐 ⋅ cos 2𝜋𝑓𝑐𝑡 + 𝜙 𝑡

PM : 𝜙 𝑡 = 𝑛𝑝 ⋅ 𝑚(𝑡)

FM : 𝜙′ 𝑡 = 𝑛𝑓 ⋅ 𝑚(𝑡) Instantaneous frequency

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Digital Data, Analog Signal; Digital Modulation

Application

Transmitting Digital data through the public telephone network.

Unguided media

Modulation Techniques

Amplitude shift keying (ASK)

Frequency shift keying (FSK)

Phase shift keying (PSK)

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Amplitude Shift Keying

Values represented by different amplitudes of carrier

Usually, one amplitude is zero

i.e. presence and absence of carrier is used

Susceptible to sudden gain changes

Inefficient

Up to 1200bps on voice grade lines

Used over optical fiber

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𝑠 𝑡 = �𝐴 ⋅ cos (2𝜋𝑓𝑐𝑡) 𝑏𝑏𝑛𝑏𝑏𝑏 10 𝑏𝑏𝑛𝑏𝑏𝑏 0

Frequency Shift Keying

Binary FSK (BFSK)

Most common form

Two binary values represented by two different frequencies (near carrier)

Less susceptible to error than ASK

High frequency radio

Even higher frequency on LANs using co-ax

Multiple FSK (MFSK)

More than two frequencies used; More bandwidth efficient

More prone to error

Each signaling element represents more than one bit

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𝑠 𝑡 = �𝐴 ⋅ cos (2𝜋𝑓1𝑡) 𝑏𝑏𝑛𝑏𝑏𝑏 1𝐴 ⋅ cos (2𝜋𝑓2𝑡) 𝑏𝑏𝑛𝑏𝑏𝑏 0

Phase Shift Keying

Phase of carrier signal is shifted to represent data

Binary PSK

Two phases represent two binary digits

Differential PSK

Phase shifted relative to previous transmission rather than some reference

signal

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𝑠 𝑡 = � 𝐴 ⋅ cos 2𝜋𝑓𝑐𝑡𝐴 ⋅ cos 2𝜋𝑓𝑐𝑡 + 𝜋 = � 𝐴 ⋅ cos (2𝜋𝑓𝑐𝑡) 𝑏𝑏𝑛𝑏𝑏𝑏 1

−𝐴 ⋅ cos (2𝜋𝑓𝑐𝑡) 𝑏𝑏𝑛𝑏𝑏𝑏 0

Quadrature PSK

Multiple PSK

More efficient use by each signal

element representing more than

one bit

QPSK

Phase Shifts of multiples of 𝜋/2

Each element represents two bits

Offset QPSK

Delay in Q stream

𝑠 𝑡 =

𝐴 ⋅ cos 2𝜋𝑓𝑐𝑡 + 𝜋/4 11𝐴 ⋅ cos 2𝜋𝑓𝑐𝑡 + 3𝜋/4 01𝐴 ⋅ cos 2𝜋𝑓𝑐𝑡 − 3𝜋/4 00𝐴 ⋅ cos 2𝜋𝑓𝑐𝑡 − 𝜋/4 10

𝑠 𝑡 =12𝐼 𝑡 cos 2𝜋𝑓𝑐𝑡 −

12𝑄 𝑡 sin(2𝜋𝑓𝑐𝑡)

𝑠 𝑡 =12𝐼 𝑡 cos 2𝜋𝑓𝑐𝑡 −

12𝑄 𝑡 − 𝑇𝑏 sin(2𝜋𝑓𝑐𝑡)

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QPSK and OQPSK Modulators

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Examples of QPSK and OQPSK Waveforms

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π/4 π /4 (1) (2) (3)

1

-1

1

-1

Performance of Digital to Analog Modulation Schemes

Bandwidth

ASK

𝐵𝑇 = 1 + 𝑏 𝑅

FSK

𝐵𝑇 = 2Δ𝐹 + 1 + 𝑏 𝑅

MFSK

𝐵𝑇 =1 + 𝑏 𝑀log2 𝑀

𝑅

MPSK

𝐵𝑇 =1 + 𝑏

log2 𝑀𝑅

In the presence of noise, bit error

rate of PSK and QPSK are about

3dB superior to ASK and FSK

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Quadrature Amplitude Modulation

Combination of ASK and PSK

Send two different signals simultaneously on same carrier

frequency

Use two copies of carrier, one shifted 90°

Each carrier is ASK modulated

Two independent signals over same medium

Demodulate and combine for original binary output

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

wireless

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QAM Modulator

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𝑠 𝑡 = 𝑑1 𝑡 ⋅ cos 2𝜋𝑓𝑐𝑡 + 𝑑2 𝑡 ⋅ sin(2𝜋𝑓𝑐𝑡)

QAM Levels

Two level ASK

Each of two streams in one of two states

Four state system

Essentially QPSK

Four level ASK

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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