EEE 309Communication Theory

Lecture 2Dr. Md. Forkan Uddin

Assistant Professor

Dept. of EEE, BUET, Dhaka 10000.

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Communication System (Lathi 2.1)

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Basic elements:

Input transducer Transmitter Channel ReceiverOutput transducer

Input messages:

VoiceTelevision pictureEmailMessage DataImage

3Communication System

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Input Transducer:

Converts the message to electrical signal (baseband signal)

Microphone, keyboard, camera etc. Bandwidth of the base band signal depends on the type of input message

System design depends on the type of input messageBandwidth of an information signal is the difference between the highest and the lowest frequency contained in

that signal

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Communication System

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Transmitter:

Process the baseband signal to a suitable form for transmission over a channel

Consists of several sub-systems: A/D converter, modulator, encoder etc.

Consists of oscillators, amplifiers, tuned circuits and filters, modulators, and other circuits

Bandwidth of the transmitted signal depends on the process in the transmitter

Communication System

Channel:

Transmission medium that conveys the transmitted electrical/electromagnetic signal to receiver

Channel types: wired or wireless

Wired: twisted copper wire (telephone, DSL), coaxial cable (television, internet), optical fiber (backbone)

Wireless: Microwave (Satellite and cellular), RF wave (Cellular, WiFi, WiMax, LTE)

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6Communication SystemChannel:

How much information can be sent in 1 s through a channel?

Capacity depends on the bandwidth of the channel

Copper wire: 1 MHz, Coaxial cable: 100 MHz, Microwave/RF: GHz, Optical fiber: THz

Attenuation, distortion, and noise are the main impairments

Bandwidth of a communication channel is a difference between the highest and the lowest frequency that the

channel will allow to pass through it

Bandwidth of a communication channel must be equal or greater than the bandwidth of the information.

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Spectrum

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8Communication System

Receiver:

Process the received signal such that the input signal can be recovered

Consists of several reversed sub-systems of transmitter: D/A converter, demodulator, decoder etc.

Consists of oscillators, amplifiers, tuned circuits and filters, demodulators, and other circuits

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Communication System

Output transducer:

Convert the demodulated signal into output message

(Voice, video, image, data, email etc.)

Headphone, television, computer etc. are the output transducer

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Challenges in Communication (Lathi 1.2)

Channel impairments: attenuation, distortion, noise

Receiver background noise

The magnitude of the channel impairments depends on the type of channel

Attenuation:

Signal attenuation or degradation exists in all media

Increases with distance

Wireless medium has the highest attenuation -exponential decay

Optical fibers have less attenuation, eg, 0.3 dB/km

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Challenges in Communication

Distortion:

Signals distorted during travel through medium (why??)

Wire: frequency dependent attenuation

Optical fiber: Delay differences in different modes, frequency dependent attenuation

Wireless: Delay differences due to multi-path propagation, time dependent randomness of particles,

frequency dependent attenuation

Optical fibers suffer from highest dispersion

Wires have the lowest distortion

Inter-symbol interference due to distortion

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Challenges in Communication

Noise:

Channel noise/ External noise

Random, undesirable electronic energy that enters the communication system via the communicating medium

and interferes with the transmitted message

Interference from nearby channels, human made noise (automobile ignition radiation, microwave oven), natural

noise (lightning)

External noise can be minimized with proper design

Receiver background noise/Internal noise

Thermal noise and random emission in electronic devices

One of the main problems in communication12

Capacity of a Transmission Link (Lathi 1.3)

Shannon's limit, C = B log2 (1 + SNR) bits/second

C = capacity, B = channel bandwidth

SNR = signal-to-noise ratio=Received Signal Power/

Noise Power

Capacity increases linearly with bandwidth, but only logarithmically with signal strength

Shannon's limit tells us what we can achieve it tells us nothing about how to do it

Two primary resources in communications

Transmitted power (should be green)

Channel bandwidth (very expensive in the commercial market)

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Calculation of Power and SNR

SNR (dB)= 10 log10(Pr/N)Pr = received signal power in watt

N=noise power in watt

Unit of power: watt or dBm

dBm is used for low power

Power in dBm = 10 log10 (Power in watt * 1e3)

Power in watt= 10^(Power in dBm/10)*1e-3

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Layering Architecture (Haykin ch 1)

Concept of layering is used in our daily lifeCommunication through air mail

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Why Layering?

Layer architecture simplifies the communication network design

It is easy to debug network applications in a layered architecture network

The communication system management is easier

Research/work on a layer can be done independently

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OSI Model

International standard organization (ISO)established a committee in 1977 to develop anarchitecture for computer communication

Open Systems Interconnection (OSI) model is theresult of this effort

In 1984, OSI model is approved as referencemodel

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OSI Model

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Physical layer

Provides physical interface for transmission of

information through a medium (wired/wireless)

Covers all - mechanical, electrical, functional and procedural - aspects for physical communication

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Data Link Layer

Who will transmit, when to transmit,whom to transmit to

Attempts to provide reliablecommunication over the physical layerinterface in one hop distance

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Network Layer

The network layer is responsible for the delivery of individual packets from the source host to the

destination host

Determine the route for the packets

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Upper LayersTransport Layer Controls congestion Provides transmission reliability between source and destinationSession Layer The session layer is responsible for dialog

control and synchronization

Presentation Layer Translate, encrypt and compress dataApplication LayerAllows access to network resourceMake applications into data format

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Interaction among the layers