Communication systems v1

Communications Systems

Course InformationCourse Information

• Lecturer: Dr. Muhammad Saleem Awan– Email: [email protected]

• Handouts– Slides: exam is based on slides– Presentations from students– Problem sheets

• Grading– 6 quizzes (10%), 3 Assignments (10%), Mid term (20%), End term (60%)

•You’re welcome to ask questions– You can interrupt me at any time.– Please don’t disturb others in the class

•Our responsibility is to facilitate you to learn. You have to make the effort.• Spend time reviewing lecture notes afterwards

– It isn’t to watch a movie!• If you have a question on the lecture material after a class, then

– Look up a book! Be resourceful.– Try to work it out yourself.– Ask me during the problem class or one of scheduled times of availability

Course OutlinesCourse Outlines• Week 1 – Introduction to communication systems & media bandwidth•Week 2 – Transmission impairments, channel capacity• Week 3 – Study of twisted pair & its transmission characteristics• Week 4 – Study of coaxial cable & its transmission characteristics• Week 5 – Introduction to optical fiber communication systems• Week 6 – Introduction to wireless medium, Antenna basics, polarization• Week 7 – Terrestrial microwave transmission characteristics• Week 8 – Introduction to satellite communication system• Week 9 – Satellite Communication• Week 10 – Circuit & packet switching• Week 11 – Multiplexing, types, study of FDM • Week 12 – Study of TDM• Week 13 – Spread Spectrum Communication System (FHSS, DSSS)• Week 14 – Introduction to CDMA & OFDM• Week 15 – Introduction to Mobile & cellular communications• Week 16 – Mobile & cellular communications

IndexIndex•Introduction

• Communication?• Evolution of communication?• Electromagnetic Spectrum• Historical Background?

•Communication System• Electrical Communication System• Analog Communication System• Digital Communication System

•Classification of Communication Systems• Topology?• Point-to-Point or Broadcast?• Underwater, Terrestrial, ground-space or space to space links?• Channel Distances?• Wired or Wireless?

•Important Communication Systems• Radio Communication Systems?• Optical Communication Systems?• Satellite Communication Systems?• Terrestrial Microwave Communication System?• Summary

Communication is the transfer of information from one place to another.

This should be done - as efficiently as possible - with as much fidelity/reliability as possible - as securely as possible

Communication System: Components/subsystems act together to accomplish information transfer/exchange.

OverviewOverview

Input Transducer: The message produced by a source must be converted by a

transducer to a form suitable for the particular type of communication system.

Example: In electrical communications, speech waves are converted by a

microphone to voltage variation.

Transmitter: The transmitter processes the input signal to produce a signal suits

to the characteristics of the transmission channel.

Signal processing for transmission almost always involves modulation and may

also include coding. In addition to modulation, other functions performed by the

transmitter are amplification, filtering and coupling the modulated signal to the

channel.

Remember the components of a communications system:

Channel: The channel can have different forms: The atmosphere (or free space), coaxial

cable, fiber optic, waveguide, etc.

The signal undergoes some amount of degradation from noise, interference and distortion

Receiver: The receiver’s function is to extract the desired signal from the received signal at

the channel output and to convert it to a form suitable for the output transducer.

Other functions performed by the receiver: amplification (the received signal may be

extremely weak), demodulation and filtering.

Output Transducer: Converts the electric signal at its input into the form desired by the

system user.

Example: Loudspeaker, personal computer (PC), tape recorders.

Remember the components of a communications system:

Communication Systems

What goes into the engineering of these systems?

Key Ingredients

• Software• Hardware• Communication architecture, with coding

and signal processing algorithms

• How do you communicate?

• The history of communication goes back thousands of years.

• What has this progress meant to you today?

The Evolution of Communication

• Verbal Communication• The most common form of communication.

– Spoken communication– Languages and dialects

• What dialects are spoken in your area?– In the country?


• Written Communication – Symbols, hieroglyphics, and drawings– The Chinese invent paper


Communication has a long history

• Smoke signals, telegraph, telephone…

• 1895: invention of the radio by Marconi

• 1901: trans-atlantic communication

State of affairs:Early 20th century

• Most communication systems are analog.• Engineering designs are ad-hoc, tailored for

each specific application.

50 years later….• Our communication infrastructure is going

fully digital.• Most modern communication systems are

designed according to the principles laid down by Shannon.

Internet

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Lessons for Us

• Think different• Think big• Think simple

To be transmitted, Information (Data) must be transformed to electromagnetic

signals.

Electromagnetic WavesElectromagnetic Waves

..

Electromagnetic Spectrum

http://www.edumedia-sciences.com/a185_l2-transverse-electromagnetic-wave.html

Electromagnetic Spectrum

Wave length FrequencyDesignations

TransmissionMedia

PropagationModes

RepresentativeApplications

Frequency

1 cmExtra HighFrequency (EHF) 100 GHz

10 cmSuper HighFrequency (SHF)

Satellite,Microwave relay,Earth-satellite radar.

10 GHz

1 mUltra HighFrequency (UHF)

Wireless comm.service,Cellular, pagers, UHFTV

1 GHz

10mVery HighFrequency (VHF)

Mobile, Aeronautical,VHF TV and FM,mobile radio 100 MHz

100mHigh Frequency(HF)

Amateur radio, CivilDefense 10 MHz

1 kmMedium HighFrequency (MF)

AM broadcasting1 MHz

10 kmLow Frequency(LF) 100 kHz

100kmVery LowFrequency (VLF)

Wave guide

Coaxial Cable

Wire pairs

Line-of-sight radio

Sky wave radio

Ground waveradio

Aeronautical,Submarine cable,Navigation,Transoceanic radio

10 kHz

There are many kinds of information sources, which can be categorized into two distinct message categories, analog and digital.

an analog communication system should deliver this waveform with a specified degree of fidelity.

a digital communication system should deliver data with a specified degree of accuracy in a specified amount of time.

Analog and Digital Communication Systems

Basic analog communications system

Modulator

Demodulator

Transmission Channel

Input transducer

Transmitter

Receiver

Output transducer

Carrier

EM waves (modulated signal)

EM waves (modulated signal)

Baseband signal (electrical signal)

Baseband signal (electrical signal)

Types of Analog Modulation

Amplitude Modulation (AM)

Amplitude modulation is the process of varying the amplitude of a

carrier wave in proportion to the amplitude of a baseband signal. The

frequency of the carrier remains constant

Frequency Modulation (FM)

Frequency modulation is the process of varying the frequency of a

carrier wave in proportion to the amplitude of a baseband signal. The

amplitude of the carrier remains constant

Phase Modulation (PM)

Another form of analog modulation technique which we will not discuss

Amplitude Modulation

Carrier wave

Baseband signal

Modulated waveAmplitude varying-frequency constant

Frequency Modulation

Carrier wave

Baseband signal

Modulated waveFrequency varying-amplitude constant

Large amplitude: high frequency

Small amplitude: low frequency

AM vs. FM• AM requires a simple circuit, and is very easy to generate.

• It is simple to tune, and is used in almost all short wave broadcasting.

• The area of coverage of AM is greater than FM (longer wavelengths (lower frequencies)

are utilized-remember property of HF waves?)

• However, it is quite inefficient, and is susceptible to static and other forms of electrical

noise.

• The main advantage of FM is its audio quality and immunity to noise. Most forms of

static and electrical noise are naturally AM, and an FM receiver will not respond to AM

signals.

• The audio quality of a FM signal increases as the frequency deviation increases

(deviation from the center frequency), which is why FM broadcast stations use such

large deviation.

• The main disadvantage of FM is the larger bandwidth it requires

Information Representation

• Communication system converts information into electrical electromagnetic/optical signals appropriate for the transmission medium.

• Analog systems convert analog message into signals that can propagate through the channel.

• Digital systems convert bits (digits, symbols) into signals

– Computers naturally generate information as characters/bits– Most information can be converted into bits– Analog signals converted to bits by sampling and quantizing (A/D

conversion)

Goals in Communication System Design

• To maximize transmission rate, R• To maximize system utilization, U• To minimize bit error rate, Pe

• To minimize required systems bandwidth, W• To minimize system complexity, Cx

• To minimize required power, Eb/No

Signal Nomenclature

• Information Source– Discrete output values e.g. Keyboard – Analog signal source e.g. output of a microphone

• Character– Member of an alphanumeric/symbol (A to Z, 0 to 9)– Characters can be mapped into a sequence of binary digits using

one of the standardized codes such as• ASCII: American Standard Code for Information Interchange• EBCDIC: Extended Binary Coded Decimal Interchange Code

Signal Nomenclature

• Digital Message– Messages constructed from a finite number of symbols; e.g., printed

language consists of 26 letters, 10 numbers, “space” and several punctuation marks. Hence a text is a digital message constructed from about 50 symbols

– Morse-coded telegraph message is a digital message constructed from two symbols “Mark” and “Space”

• M - ary– A digital message constructed with M symbols

• Digital Waveform– Current or voltage waveform that represents a digital symbol

• Bit Rate– Actual rate at which information is transmitted per second

Signal Nomenclature

• Baud Rate– Refers to the rate at which the signaling elements are

transmitted, i.e. number of signaling elements per second.

• Bit Error Rate– The probability that one of the bits is in error or simply

the probability of error

1 Ground Wave Propagation

Follows contour of the earth Can Propagate considerable distances

Frequencies up to 2 MHz Example : AM radio

Radio Wave Communication Systems

2 Sky Wave Propagation

Signal reflected from ionized layer

of atmosphere. Signal can travel

a number of hops, back and forth

Examples SW radio

3 Line-of-Sight Propagation

Transmitting and receiving antennas

must be within line of sight

example

Satellite communication

Ground communication

Figure Comparison of analog and digital signals

Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital

signals can have only a limited number of values.

PERIODIC ANALOG SIGNALSPERIODIC ANALOG SIGNALS

Periodic analog signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A compositeperiodic analog signal is composed of multiple sine waves.

Sine WaveWavelengthTime and Frequency DomainComposite SignalsBandwidth

Topics discussed in this section:Topics discussed in this section:

In communication systems, we commonly use periodic analog signals and nonperiodic digital signals.

Figure A sine wave

Figure Two signals with the same phase and frequency, but different amplitudes

Frequency and period are the inverse of each other.

• Frequency is the rate of change with respect to time.

• Change in a short span of time means high frequency.

• Change over a long span of time means low frequency.

If a signal does not change at all, its frequency is zero.If a signal changes instantaneously, its frequency is infinite.

Figure Two signals with the same amplitude and phase, but different frequencies

Table Units of period and frequency

The period of a signal is 100 ms. What is its frequency in kilohertz?

Example

SolutionFirst we change 100 ms to seconds, and then we calculate the frequency from the period (1 Hz = 10−3 kHz).

Figure Three sine waves with the same amplitude and frequency, but different phases

Phase describes the position of the waveform relative to time 0.

A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians?

Example

SolutionWe know that 1 complete cycle is 360°. Therefore, 1/6 cycle is

Figure Wavelength and period

Figure The time-domain and frequency-domain plots of a sine wave

Time and frequency domains

Time and frequency domains (continued)

A complete sine wave in the time domain can be represented by one single spike in the frequency domain.

The frequency domain is more compact and useful when we are dealing with more than one sine wave. For example, Next Figure shows three sine waves, each with different amplitude and frequency. All can be represented by three spikes in the frequency domain.

Example

Figure The time domain and frequency domain of three sine waves

Example Amplitude modulation

Figure AM band allocation

Each AM radio station is assigned a 10-kHz bandwidth. The total bandwidth dedicated to AM radio ranges from 530 to 1700 kHz.

Example

Figure Frequency modulation

Figure FM band allocation

Each FM radio station is assigned a 200-kHz bandwidth. The total bandwidth dedicated to FM radio ranges from 88 to 108 MHz.

Example

According to Fourier analysis, any composite signal is a combination of

simple sine waves with different frequencies, amplitudes, and phases.

A single-frequency sine wave is not useful in communication systems;we need to send a composite signal, a signal made of many simple sine

waves.

If the composite signal is periodic, the decomposition gives a series of signals

with discrete frequencies; if the composite signal is nonperiodic,

the decomposition gives a combination of sine waves with continuous

frequencies.

Figure A composite periodic signal

Above Figure shows a periodic composite signal with frequency f. This type of signal is not typical of those found in data communications. We can consider it to be three alarm systems, each with a different frequency. The analysis of this signal can give us a good understanding of how to decompose signals.

Figure Decomposition of a composite periodic signal in the time and frequency domains

Three harmonics

Square wave

Adding first three harmonics

Frequency spectrum comparison

A digital signal

A digital signal is a composite signal with an infinite bandwidth.

Figure The time and frequency domains of a nonperiodic signal

Above Figure shows a non-periodic composite signal. It can be the signal created by a microphone or a telephone set when a word or two is pronounced. In this case, the composite signal cannot be periodic, because that implies that we are repeating the same word or words with exactly the same tone.

The bandwidth of a composite signal is the difference between the

highest and the lowest frequencies contained in that signal.

Figure The bandwidth of periodic and nonperiodic composite signals

If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is its bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of 10 V.SolutionLet fh be the highest frequency, fl the lowest frequency, and B the bandwidth. Then

Example

The spectrum has only five spikes, at 100, 300, 500, 700, and 900 Hz (see next Figure).

Figure The bandwidth for Example

A periodic signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency? Draw the spectrum if the signal contains all frequencies of the same amplitude.SolutionLet fh be the highest frequency, fl the lowest frequency, and B the bandwidth. Then

Example

The spectrum contains all integer frequencies. We show this by a series of spikes (see next Figure).

A nonperiodic composite signal has a bandwidth of 200 kHz, with a middle frequency of 140 kHz and peak amplitude of 20 V. The two extreme frequencies have an amplitude of 0. Draw the frequency domain of the signal.

Example

SolutionThe lowest frequency must be at 40 kHz and the highest at 240 kHz. Next Figure shows the frequency domain and the bandwidth.

Analog signals of bandwidth W can be represented by 2W samples/sChannels of bandwidth W support transmission of 2W symbols/s

Communication systems v1

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