1

Communication System 1

September 2007Prepared and Lectured by Assoc Prof. Thuong Le-Tien

Slides with references from HUT Finland, La Hore uni.,

Mc. Graw Hill Co., and A.B. Carlson’s Communication Systems book

2

Communications Communications = Information

transfer This course is about communications

Limited to information in electrical form We will not consider delivering newspapers

We will primarily cover information transfer at systems level

We will not deal [too much] with circuits, chips, signal processing, microprocessors, protocols, and networks

3

What exactly is information? Information is a word that is too

generic for our purposes We will use the word message

A physical manifestation of information What do communication systems

have to do with messages? Communication systems are

responsible for producing an “acceptable” replica of message at the destination

4

Is Signal = Message? Just like information, signal is also a

generic word Derived directly from information

Scientists and Engineers use signal to denote information in electrical form We will use signal and message

interchangeably

5

Can we classify signals? Messages or signals can be classified: Analog

A physical quantity that varies with “time”, usually in a smooth or continuous fashion

Fidelity describes how close is the received signal to the original signal. Fidelity defines acceptability

Digital An ordered sequence of symbols selected from a

finite set of discrete elements When digital signals are sent through a

communication system, degree of accuracy within a given time defines the acceptability

6

Examples Analog Signals

Values are taken from an infinite set

Digital Signals Values are taken from

a discrete set

Binary Signals Digital signals with

just two discrete values

t

t

t

1

0 0 0

1 1

0

7

Elements of Communication Systems Transmitter

Modulation Coding

Channel Attenuation Noise Distortion Interference

Receiver Detection (Demodulation+Decoding) Filtering (Equalization)

8

Elements of Communication Systems

Encoder: Message Message Signal or bits Transmitter: Message signal Transmitted signal Channel: Introduces noise, distortion, interference Receiver: Received Signal Message Signal Decoder: Message Signal Original MessageExample: Microphone ---------------> Speaker

TextImages

VideoAudio

)(ts )(ˆ ts)(ˆ

...ˆˆ 21

tm

bb

)(

...21

tm

bb

SourceEncoder

SourceDecoder

Channel ReceiverTransmitter

9

Modulation converts message signal or bits into amplitude, phase, or frequency of a sinusoidal carrier (Am, FM, QPSK)

Modulation may make the transmitted signal robust to channel impairments

Channel introduces noise, distortion, and interference Demodulator tries to mitigate the channel impairments

Analog or Digital“Modulator”

)(ts )()(ˆ tnts )(ˆ...ˆˆ21

tmbb

)(...21

tmbb

Transmitter

Channel

h(t)

+Analog or Digital“Demodulator”

Receiver

n(t)Analog or Digital

“Modulator”

Transmitter

10

Fundamental Limitations If practical implementation is not a concern and we

don’t worry about feasibility, is there something else that limits acceptable communications?

Bandwidth Channel must be able to allow signal to pass through Channels usually have limited bandwidth Can we reduce signal bandwidth? Do “something” at

source Noise

Can we reduce it? Can we reduce its effects? Do something at the transmitter and receiver

Signal to Noise Ratio

11

Block Diagram

(Modulator)Analog

or DigitalDemodulator

m(t) s(t)h(t)

n(t)

Transmitter Channel Receiver

)(tm

12

Performance Criterion How a “good” communication system can be

differentiated from a “sloppy” one? For analog communications

How close is to ? Fidelity! SNR is typically used as a performance metric

For digital communications Data rate and probability of error No channel impairments, no error With noise, error probability depends upon

data rate, signal and noise powers, modulation scheme

m(t))(tm

13

Limits on data rates Shannon obtained formulas that

provide fundamental limits on data rates (1948)

Without channel impairments, an infinite data rate is achievable with probability of error approaching zero

For bandlimited AWGN channels, the “capacity” of a channel is:C = B log(1+ SNR)

14

Example: PSTN Public Switched Telephone Network Components

Phone set (analog signal is generated) Local exchange (A/D conversion) Long-haul exchange

Characteristics Circuit-switched network Designed for voice communications (analog???) Faxes and modems use PSTN for transmission

of digital data in analog form

15

Example:PSTN

Localexchange

Internationalexchange

Long distanceexchange

Localexchange

Local line

Long distance line

Long distance line

International line

16

Example: CellularIslamabad

MTSO

PSTNMTSO

Lahore

MTSO: Mobile TelephoneSwitching Office

17

Example: Cellular Cellular Communication System

A cell is assigned some number of channels Typically one channel is allocated to a user Users communicate with a base station Base station is connected to MTSO/PSTN AMPS is an analog system

Uses FM and frequency-division multiple access Digital systems use digital modulation

18

Example: Radio broadcast Two modes are used

AM Amplitude modulation 535-1605kHz 10kHz channels

FM Frequency modulation 88-108MHz Channels centered at 200kHz intervals starting at

88.1MHz

19

Example: Wireless LANs Various standards IEEE 802.11a/b/g popular IEEE 802.11b

11Mb/s data rate 2.4-2.4835GHz band Modulation: Direct sequence spread spectrum (DSSS)

IEEE 802.11a 55Mb/s data rate 5.725-5.825GHz band (in U.S.) Uses orthogonal frequency division multiplexing

(OFDM)

20

Example: LANs and WANs Local Area Networks (LANs)

Connect “closely” located computers Data bits are transmitted in chunks

(packets) for efficiency/feasibility reasons Various LAN protocols are used in practice

Wide Area Networks (WANs) A wide area backbone network connects

different LANs A standard protocol is needed for such

communication (TCP/IP)

21

Example: Ad Hoc Networks Various devices connected to each

other without using an infrastructure Sensor Networks

Similar to ad hoc Networks (may be considered a special case of ad hoc networks)

Have power constraints (Use non-rechargeable battery)

Mesh Networks Another example of ad hoc networks Used for provide communications to remote areas

22

A Generic Communication System

(Modulator)Analog

or DigitalDemodulator

m(t) s(t)h(t)

n(t)

Transmitter Channel Receiver

)(tm

23

Elements of Communication Systems Transmitter

Modulation Coding

Channel Attenuation Noise Distortion Interference

Receiver Detection (Demodulation+Decoding) Filtering (Equalization)

24

Transmitter What does modulation do?

Encodes messages (analog) or bits (digital) into amplitude, frequency, or phase of a carrier signal

Also makes transmitted signal robust against channel impairments

Coding Source coding – remove redundancy Channel coding – add redundancy

25

Channel Channel introduces impairments

Noise Thermal noise is the most significant Additive white Gaussian noise (AWGN)

Distortion Inter-symbol interference

Attenuation and fading Constant attenuation Variable attenuation

Interference Crosstalk

26

Receiver What does demodulator do?

Extracts messages or bits from the received signal

Mitigates channel impairments by making use of equalizers

Decodes the signal, especially if channel coding was performed at the transmitter

27

Performance Criterion How a “good” communication system can be

differentiated from a “sloppy” one? For analog communications

How close is to ? Fidelity! SNR is typically used as a performance metric

For digital communications Data rate and probability of error No channel impairments, no error With noise, error probability depends upon

data rate, signal and noise powers, modulation scheme

m(t))(tm

28