UEE3504: Introduction to Communication Systems Po-Ning Chen, Professor Dept. of Electrical and Computer Eng. National Chiao Tung University.

UEE3504: Introduction to Communication Systems

Po-Ning Chen, Professor

Dept. of Electrical and Computer Eng.

National Chiao Tung University

Background and Preview

To give you a basic understanding of communications

© Po-Ning [email protected] Background 3

Figure-1 Theory

The next figure is always the “Figure 1” in every book regarding communications.


Communications

What is communication (or more specifically, communication engineering)? The transmission of information from one point to

another through a succession of certain processes.


Basic Elements Regard Communications

Source of information Voice, music, picture, or computer data



Transmitter Source → Source Symbol (i.e., Source Word)

Symbolize the information from a source Source Symbol → Code Word

Encode the source symbol so that the other sources (i.e., noise and interfering signal) can hardly interfere the information transmission.

Code Word → Channel Symbol (i.e., Transmitted Signals) Modulate the code word into a form that is suitable for

transmission over the channel, which involves varying some parameter of a carrier wave in accordance with the message signal.



Noise/Interference Unwanted waves that tend to disturb the

transmission and processing of messages. Could be internal or external to the system. Could be additive or multiplicative (or both) to the

information-bearing signals.



Receiver Hard Decision

Channel Symbol → Code Bit Decode from Code Bits to Code Word Code Word → Source Symbol → Source

Soft Decision Decode from Channel Symbol to Code Word Code Word → Source Symbol → Source


Example: Basic Elements Regard Communications

Source = An alphabet “A”


Example : Basic Elements Regard Communications

Transmitter Source “A” → Binary Source Symbol (01000001)

Symbolize the information from a source

Source Symbol (01000001) → Code Word (000 111 000 000 000 000 000 111) Encode the source symbol by the three-times repetition code

so that the other sources (i.e., noise and interfering signals) can hardly interfere the information transmission.

001, 010, 011, 100, 101, 110 are not code words. Hence, their appearance is possible only when noise is introduced.



Code Word (000 111 000 000 000 000 000 111) → Channel Symbol (000 555 000 000 000 000 000 555 ) Modulate the code word into some channel-permissible

(physical-medium permissible) symbols.

Due to Channel Interference, we receive: 010 442 222 033 011 020 032 434



Receiver Hard Decision

Channel Symbol 010 442 222 033 011 020 032 434 → Code Bit (Threshold 2.5) 000 110 000 011 000 000 010 111

Decode from Code Bits to Code Word (Majority Rule) 000 111 000 111 000 000 000 111

Code Word 000 111 000 111 000 000 000 111 → Source Symbol 01010001 → Source “Q”



Receiver Soft Decision

Decode from Channel Symbol 010 442 222 033 011 020 032 434 to (channel-symbolized) Code Word 000 555 000 000 000 000 000 555 By finding the minimum distance to legitimate codewords

000 and 111. E.g., d(033, 000) = (00)2+(30)2+(30)2 = 18 d(033, 555) = (05)2+(35)2+(35)2 = 33

Code Word 000 111 000 000 000 000 000 111 → Source Symbol 01000001→ Source “A”


Basic Modes of Communications

Broadcasting Often, uni-directional. A single powerful transmitter to numerous

(inexpensive) receivers Example. Radio and TV.

Point-to-point communication Often, bi-directional. Two entities exchange information. Example. Telephone.


Feature of Communications

Statistics The source is statistical in nature. The noise and interference are naturally random. Principles of Communication Engineering:Principles of Communication Engineering: How to

design a communication system only based on the knowledge of the statistics of the source and interferences (without knowing exactly what the true source and interference are)?



Example Source

We do not know if the next source symbol is 0 or 1. But, we do know the probability of the next source

symbol being 0, and also, the probability of the next source symbol being 1.

Noise/Interference We do not know what value the noise/interference will

take? But, we do know the noise is, say, Gaussian

distributed.



This is the reason why “Probabilities” (Chapter 1) is considered an important background to communication study.


Primary Communication Resources

Primary Communication Resources are something “known” at the design stage. As aforementioned, source and noise/interference

are (often) something “unknown” at the design stage.


Primary Communication Resources Examples of Primary Communication

Resources Transmitted Power

Specifically, averaged power of the transmitted signals. A more useful measure than the absolute transmitted

power is the signal-to noise power ratio (SNR), defined as the ratio of the average signal power to the average noise power. This quantity is often expressed in dB, 10 log10(SNR).

Channel Bandwidth The band of frequencies for use of transmitting

messages.


Primary Communication Resources

Design principle of a communication system How to efficiently use (usually in a tradeoff

fashion) the communication resources!


Sources of Information

“Sources” can sometimes be viewed as one kind of Communication Resources. For example, there are systems designed

specifically for “exchanging voices.” Such a system may not be apt to transmit computer

data. This introduces the subjects of “Source-Specific

Communication.” Next, we brief several sources commonly seen in

the literature.


Sources of Information: (1) Speech

Features Voice spectrum extends well beyond 10kHz. Most of the average power is concentrated in the

range of 100 to 600 Hz.

A band of 300 to 3100 Hz gives good articulation. The sound wave propagates through the air at a

speed of 300 meter/second.

Do Re Mi Fa So La Si Do

261.6 293.7 329.6 349.2 392.0 440 493.9 523Freq (Hz)

Pitch Name

Schematic representation of the vocal system



Sources of Information: (1) Speech

The speech-production process may be viewed as a form of filtering: A sound source excites a vocal tract filter.

D

D

a1

a9

a10

+

Excitation Speech

+

+

+

LipsLips

Vocal TractVocal Tract

Glottal VolumeGlottal Volume


Sources of Information: Speech

As the sound propagates along the vocal tract, the spectrum (i.e., frequency content) is shaped by the frequency selectivity of the vocal tract —a resonance phenomenon observed in organ pipe.

So the hearing mechanism is (and should be) sensitive to frequency.


Source of Information: Music

Originate from musical instruments, such as piano, violin, and flute.

It consists of: Melody: A time sequence of sounds. Harmony: A set of simultaneous sounds.

Different from speech, the spectrum of a music source may extend up to about 15 KHz. Accordingly, a much wider bandwidth resource is

demanded.


Source of Information: Pictures

Two dimensional information. Classifications

Dynamic pictures – Video, such as North American Audio TV (NAA-TV)

Still pictures – Facsimile. To transmit still picture over a telephone channel.


Source of Information: NAA-TV

North American Analog TV 525 horizontal lines, decomposed into two 262.5

line interlaced fields (See the next slide.) Completion of each interlaced field takes 1/60

second Horizontal line-scanning frequency is 262.5/(1/60) =

15.75 KHz. Hence, 30 still pictures are shown per second. The human “persistence of vision” phenomenon

will perceive these still pictures to be moving pictures.



Interlaced raster scan.



In the NTSC (National Television System Committee) system, a total of 4.2 MHz bandwidth is demanded for TV transmission.


Source of Information: Computer Data

The first code developed specifically for computer communication (1967) – ASCII (American Standard Code for Information Interchange).





ASCII (American Standard Code for

Information Interchange) 7-bit code for alphabetic numerical characters Bit 8 is sometimes used as parity-check bit or used

to form the extended ASCII code Even parity: Total number of 1’s is even. Odd parity: Total number of 1’s is odd.

Extended ASCII code can be displayed but cannot necessarily be printed out.

Bit originates from “Binary Digit.”



Since ASCII is defined for communication, it also includes some symbols for communication purpose such as ENQ (enquiry) – 05X ETB (end of transmission block) – 17X



RS (Recommended Standard) -232 Transmission Synchronous Asynchronous



Asynchronous Serial Data No clock or timing signal required. ST : start bit S : stop bit P : parity bit D6~D0 : data bits (often, exact one ASCII character) Usually, 10 bit frame with even-parity/7-data-bit or

no-parity/8-data-bit.

S ST D0 D1 D2 D3 D4 D5 D6 P S ST D0 D1 D2 D3 D4 D5 D6 P S

frame



Synchronous Serial Data No start and stop bits required. P : parity bit D6~D0 : data bits (ASCII) Clock : Timing signal. Note that it requires sync character (after a certain

number of frames) to avoid losing synchronization. If two sync characters are used. it is called bi-sync.

D0 D1 D2 D3 D4 D5 D6 P D0 D1 D2 D3 D4Data

Clock



Windows 98 Baud rate : 110 baud~921600

baud (The # is different for different computers)

(E)ven parity, (O)dd parity, (N)one-parity, Mark, Space

4~8 Data-bit 1, 1.5, 2 Stop-bit

The name of “mark” and “space” for 1 and 0 comes from the days of telegraphy.







The computer data stream so formed is then applied to a device called a modem (modulator-demodulator).

Unlike source traffic from speech or video, the computer data is often bursty rather than continuous.


Missed Part of Figure-1 in Textbook

Source before entering the transmitter is often compressed (in order to save time or space).

This part is missed in Figure 1 of the textbook.


With a source encoder, a digital communication system (rather an analog communication system) is formed.


Data Compression

Lossless Data Compression (or Data Compaction) Completely reversible (or asymptotically

reversible). E.g., Lempel-Ziv algorithm (PKZIP, compress,

etc), which will be introduced in Chapter 9. Lossy Data Compression

Non-reversible with loss of information in a controlled manner.

E.g., JPEG, MPEG, etc.


Lossy Data Compression for Images

JPEG (Joint Photographic Experts Group) An image coding standard Pixels are grouped in 8-by-8 block. DCT (discrete cosine transform) is then applied to

each block. Quantize each of the 64 DCT coefficients according

to a pre-specified table. Huffman-encode (introduced in Chapter 9) the

quantization results.


Lossy Data Compression for Images

DCT

7

0

7

0

7

0

7

0

16

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16

)12(cos),()()(

4

1),(

16

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)12(cos),()()(

4

1),(

u v

x y

vyuxvuFvCuCyxf

vyuxyxfvCuCvuF

where

otherwise1

0for ,2

1)(

uuC


Lossy Data Compression for Video

MPEG-1 (Motion Photographic Experts Group) video coding standard A video coding standard primarily for 30 fps

(frames per second) video Result in a bit-stream rate of 1.5 megabits per

second



Design objective : To reduce four kinds of redundancies: Interframe (temporal) redundancy

Its reduction is achieved through the use of prediction to estimate each frame from its neighbors.

The resulting prediction error is transmitted for motion estimation and compensation.

Interpixel redundancy within a frame Psychovisual redundancy Entropic coding redundancy



As with JPEG, the last three redundancies are reduced through the combined use of DCT, quantization and lossless entropic coding.


Lossy Data Compression for Audio

MPEG-1 audio coding standard A perceptual (waveform) coder, as contrary to a

vocoder The amplitude-time waveform of the decoded audio

signal closely approximates that of the original audio signal.



Encoding process Time-Frequency Mapping (sub-band decomposition) Psychoacoustic modeling (operates according to the

psychoacoustic behavior of the human auditory system)

Quantization and coding Frame-packing (format the quantized audio samples

into a decodable bit stream)



Why Psychoacoustic modeling? Human ears have a perceptual phenomenon known as

auditory masking. Specifically, the human ear does not perceive

quantization noise in a given frequency band if the average noise power lies below the masking threshold

The masking threshold varies with frequency across the band.

Hence, a perceptual weighting filter is applied to waveforms before quantization.


OSI

OSI (Open System Interconnection) model; the acronym DLC in the middle of the figure stands for data link control.


Communication Networks

OSI reference model was developed by ISO (International Organization for Standardization) in 1977.

Figure 1 only concerns PHY layer. Now we take a quick look of its relation with

higher layers, such as Network layers.



Network Layer : Routers



Routing mechanisms Circuit Switching

Uninterrupted, exclusively use of links E.g., Telephone.

Packet Switching Shared-on-demand links



Why OSI reference model? Each layer can perform its related subset of

primitive functions without knowing the implementation details of the next lower layer.

The adjacent layers communicate through well-defined interfaces, which defines the services offered by the lower layer to the upper layer.



The entities that comprise the corresponding layers on different systems are referred to as peer processes.

Two peer entities then communicate through a well-defined set of rules of procedures, named Protocol.

Again, this text/course primarily considers PHY layer.


Internet

Internet – A special communication network, as contrary to an Intranet.

Features of Internet Applications are carried out independently of the

technology employed to construct the network. The network technology is capable of evolving

without affecting the applications.


Internet

Architecture of Internet

Direct Data ExchangeDirect Data Exchange

Cross-Router Data ExchangeCross-Router Data Exchange


Internet Protocol (IP)


Internet Service

Internet Service is “Best Effort” in nature. As a consequence, no guarantees of timely

transmission, and even delivery.


Communication Channels

Channels, where the noise/interference resides, can be roughly divided into two groups: Guided propagation channels

E.g., telephone channels, coaxial cables, and optical fibers

Free propagation channels E.g., broadcast channels, mobile radio channels, and

satellite channels


Communication Channels: (i) Telephone Channel

Features of telephone channel A channel performs “voice → electrical signal →

sound” Band-limited channel

A speech signal (male or female) is essentially limited to a band from 300 to 3100 Hz.



Measures used in characterizing channel Insertion loss = 10 log10 (P0/PL)

PL = power delivered to a load from a source via the channel

P0 = power delivered to the same source notnot via the channel

PL

P0

Channel

Channel



Envelope delay The negative of the derivative of the phase response with

respect to the angular frequency = 2f. Example. Envelope delay = a for the next channel.

fajefH 2)( )(tg )( atg

)].(exp[|)(|)( fjfHfH The phase response of a channel filter H(f) is (f), where



Insertion Loss Envelope Delay


Communication Channels: (ii) Coaxial Cable

A coaxial cable offers a greater degree of immunity to EMI, and a much higher bandwidth than twisted pair telephone lines.

Example of its applications Local area network in an office environment. Cable television


Communication Channels: (iii) Optical Fiber

Features Enormous potential bandwidth

The bandwidth is roughly equal to 10% of the carrier frequency (2 1014 Hz).

Notably, the transmission attainable limit (for additive white Gaussian noise with SNR=10dB) is around

secondper Gigabit 86.6918

secondper bit 1091886.6

)101(log)102(

)1(log

13

10/102

13

2

dB

SNRBC


Communication Channels: (iii) Optical Fiber

Low transmission loss 0.1dB/km

Immunity to electromagnetic interference Small size and weight (thinner than human hair) Ruggedness and flexibility

Possibility of being bent or twisted without damage.


Communication Channels: (iv) Wireless Broadcast Channels

Transmission Up-convert the modulated baseband information-

bearing signal to Radio Frequency (RF) passband signal

Transmit the RF passband signal via antenna Reception

Pick up the radiated waves by an antenna. Down-convert the received passband signal to

baseband signal (perhaps through an intermediate step called the intermediate frequency (IF) band).


Communication Channels: (v) Mobile Radio Channels

The main difference between this channel and the previous channel is the consideration of mobility. Due to mobility, there is no “line-of-sight” path for

communication; rather, radio propagation takes place mainly by way

of scattering from the surfaces of the surrounding buildings and by diffraction over and around them.

This results in a multipath fading transmission.


Communication Channels: (v) Mobile Radio Channels

Transmitter Receiver

),( 11

),( 22

),( 33

)(

)(

)(

)(

33

22

11

tn

ts

ts

ts

)(ts

Notably, j and j can also be functions of time.


Communication Channels: (vi) Satellite Channels

Satellite communications The satellite is placed in geostationary orbit.

Geostationary orbit1. The satellite orbits the Earth in exactly 24 hours

(geosynchronous).

2. The satellite is placed in orbit directly above the equator on an eastward heading.

It acts as a powerful repeater in the sky. It often uses 6 GHz for the uplink and 4 GHz for

the downlink.



The 6/4-GHz band offers the following attributes:1. Relatively inexpensive microwave equipment.

2. Low attenuation due to rainfall Rainfall is a primary atmospheric cause of signal loss.

3. Insignificant sky background noise The sky background noise due to random noise emissions

from galactic, solar and terrestrial sources reaches its lowest level between 1 and 10 GHz.



A typical satellite in the 6/4-GHz band is assigned a 500 MHz bandwidth, which is divided among 12 transponders. Each transponder can carry at least one color television

signal, 1200 voice circuits, or digital data at a rate of 50 Mb/s.


Classifications of Communication Channels (according to the natures or resources)

Linear or non-linear A wireless radio channel is linear whereas a satellite

channel is usually non-linear Time invariant or time varying

An optical fiber is time invariant, whereas a mobile radio channel is typically time varying.

Band limited or power limited A telephone channel is band limited, whereas an

optical fiber link and a satellite channel are both power limited.


Classification of Modulation Process

Continuous-wave modulation A sinusoidal wave is used as the carrier. It can be further classified as:

Amplitude modulation (AM) : Amplitude of the carrier is varied in accordance with the message.

Frequency modulation (FM) : Frequency of the carrier is varied in accordance with the message.

Phase modulation (PM) : Phase of the carrier is varied in accordance with the message.



Pulse modulation The carrier consists of a sequence of rectangular

pulses. It can be sub-divided to:

Analog pulse modulation Digital pulse modulation



Analog pulse modulation Pulse-amplitude modulation (PAM), pulse-duration

modulation (PDM), pulse-position modulation (PPM) The amplitude, duration, position of the pulses varies

in accordance with the message signals.

Digital pulse modulation Pulse-code modulation (PCM)


Example of PAM (Telephone System)

Sampling the voice according to some clocks.


Example of PCM

Originate from PAM, but with the following modifications. Convert the (sampled) pulse into bits, e.g., 8 bits. All 8 bits of the input PCM signal are gated to the

output port in parallel. The gate can now be designed using “truth table”

which facilitates system integration or multiplexing.


What is multiplexing?

To combine (several modulated) signals for their simultaneous (or concurrent) transmission. Frequency-division multiplexing (FDM) Time-division multiplexing (TDM) Code-division multiplexing (CDM) Wavelength-division multiplexing (WDM),

specifically for use of optical fibers. Some treats WDM as a special case of FDM, since c =

f .


Shannon’s Information Capacity Theorem

The underlying limit for digital communications


Transmission Rate = Source code bit per second (Information bit per second)



Reliable transmission rate (for pre-specified modulator, channel and demodulator). The rate for which a proper design of channel

encoder-decode pair can fulfill arbitrarily small error requirement.

Shannon finds the general formula for the largest reliable transmission rate, which he baptized as “(coding) channel capacity.”



For additive white Gaussian noise asdemodulator output = modulator input + Gaussian

the channel capacity is equal toC = B log2(1+SNR) bit/second, where B is the bandwidth.

It took 45 years (1948~1993) of research to reach this “capacity!”


An Exemplified Ideal Digital Communication Problem – Phase Shift Keying

ChannelEncoder

…0110Modulator

…,m(t), m(t), m(t), m(t)

m(t)

T

Carrier waveAccos(2fct)

s(t)

w(t)

x(t)

Local carriercos(2fct)

T

dt0

correlator

yT>< 0

0110…

No IF here because this is an ideal system.



Assume that the local carrier (at the receiver end) is exactly the same as the transmitter carrier.

Assume that the correlator is completely synchronized with the transmitter. So the integration inside correlator covers a

complete message signal m(t). In other words, it will not happen that the integration inside correlator covers 80% of the current m(t) but 20% of the previous m(t).

T

cc

T

c

T

ccc

T

c

Tc

c

T

c

T

cc

T

ccc

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c

T

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dttftwdttfATA

dttftwdttf

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dttftwtfA

dttftwts

dttftxy

0

00

00

00

2

0

0

0

)2cos()(2

1

)2cos()()4cos(2

1

2

1

)2cos()(2

)4cos(1

)2cos()()2(cos

)2cos()]()2cos([

)2cos()]()([

)2cos()(


(By assuming that fc is a multiple of 1/T.)



Some interesting issues to consider: What if the local carrier does not equal the

transmitter carrier.

What if fc is not a multiple of 1/T.

What if the receiver does not synchronize with the transmitter?

What is the BER of this system?

T

tcrccT dttftwtfAy0

)2cos()]()2cos([



Is the correlator receiver optimal in the sense of BER?

Is the “sign-decision” optimal in the sense of BER? Is the above combination optimal in the sense of

BER? Is the BER robust for imperfect system, such as

timing and carrier mismatch? Is the rectangular m(t) a fine choice? Moreover, is

PSK a fine choice? If affirmative, in what sense? ….



All these problems will be hopefully answered in this course (and subsequent courses).

UEE3504: Introduction to Communication Systems Po-Ning Chen, Professor Dept. of Electrical and Computer Eng. National Chiao Tung University.

Documents

source source symbol

source word

code word channel symbol

binary source symbol

code bits

code words

code word majority rule

times repetition code