DITDar es Salaam institute of Technology (DIT)

ETU 07123

Introduction to Communication Systems

Ally, J

jumannea@gmail.com

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Course OutlinePrinciples of Communication SystemsAnalogue ModulationAngle ModulationPulse ModulationDigital Modulation

Reference:Introduction to Analogue and Digital Communication, by Simon HaykinDIT

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Principle of Communication System

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DITInformation RepresentationCommunication systems convert information into a format appropriate for the transmission medium.Channels convey electromagnetic waves (signals).

Analog communication systems convert (modulate) analog signals into modulated (analog) signals

Digital communication systems convert information in the form of bits into binary/digital signalsTypes of Information:Analog Signals: Voice, Music, Temperature readingsAnalog signals or bits: Video, ImagesBits: Text, Computer DataAnalog signals can be converted into bits by quantizing/digitizing

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DITBasic Mode of CommunicationThere are two basic modes of communication:

Broadcasting: which involves the use of a single powerful transmitter and numerous receivers that are relatively inexpensive to build. Here information-bearing signals flow only in one direction.

Point-to-point communication: in which the communication process takes place over a link between a single transmitter and a receiver. In this case, there is usually a bidirectional flow of information-bearing signals, which requires the use of a transmitter and receiver at each end of the link.

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DITPrimary Communication Resources In a communication system, two primary resources are employed: Transmitted Power and Channel Bandwidth.

The Transmitted Power: is the average power of the transmitted signalThe channel bandwidth is defined as the band of frequencies allocated for the transmission of the message signal

NB:A general system design objective is to use these two resources as efficiently as possible.

In most communication channels, one resource may be considered more important than the other.

Therefore we may classify communication channels as Power limited or Band-limited.

Example, the telephone circuit is a typical Band-limited channel, whereas a space communication link or satellite channel is typically Power limited.

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DITSource of Information The telecommunications environment is dominated by four important sources of information: speech, music, pictures, and computer data

Speech is the primary method of human communication

Music is the one originates from instruments such as the piano, violin, and flute

Pictures is the one relies on the human visual system for its perception. The picture can be dynamic, as in television, or static, as in fascimile (fax) machine

Computer data is the information transmitted or exchanged through computer or other electronic devices

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DITCommunication System Block DiagramSource encoder converts message into message signal or bits.

Transmitter converts message signal or bits into format appropriate for channel transmission (analog/digital signal).

Channel introduces distortion, noise, and interference.

Receiver decodes received signal back to message signal.

Source decoder decodes message signal back into original message.

NB: The good communication system is to produce at the destination (receiver) an acceptable replica of the source message.

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SourceEncoder

Transmitter

Channel

Receiver

SourceDecoder

TextImageVideo

Focus of this class

DITModulation and DemodulationModulationIs the process of changing ore or more properties such as amplitude, frequency, and phase of the analog carrier in proportion with the information signalPerformed in a transmitter by a circuit called a modulator

DemodulationIs the reverse process of modulation and converts the modulated carrier back to the original informationPerformed in a receiver by a circuit called a demodulator

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DITElectromagnetic Frequency Spectrum The purpose of an electronic communications system is to communicate information between two or more locations commonly called stations.Accomplished by converting the original information into electromagnetic energy and then transmitting it to one or more receive stations where it converted back to its original form.Electromagnetic energy can propagate as a voltage or current along a metallic wire, as emitted radio waves through free space, or as a light waves down an optical fiber.Electromagnetic energy is distributed throughout an almost infinite range of frequencies.

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DITThe electromagnetic Spectrum

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DITBandwidthBandwidth of an information of signal: is the difference between the highest and lowest frequencies contained in the information.Bandwidth of a communication channel: is the difference between the highest and lowest frequencies that the channel will allow to pass through it.The bandwidth of the communications channel must be equal to or greater than the bandwidth of the information.For example, voice frequencies contain signals between 300 Hz and 3000 Hz. Therefore, a voice frequency channel must have a bandwidth equal to or greater than 2700 Hz (300 Hz-3000 Hz).If a cable television transmission system has a passband from 500kHz to 5000kHz, it has a bandwidth of 4500 kHz.

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DITInformation CapacityInformation theory: is a theoretical study of the efficient use of bandwidth to propagate information through electronic communications systems.Information theory can be used to determine the information capacity of a data communications system.Information capacity: represents the number of independent symbols that can be carried through a system in a given unit of time.The most basic digital symbol used to represent information is the binary digit or bit. It is convenient to express the information capacity of a system as a bit rate.Bit rate: is the number of bits transmitted during one second and is expressed in bits per second (bps).

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DITInformation Capacity (2)In 1928, R. Hartley (Bell Telephone Laboratories) developed a useful relationship among bandwidth (B), transmission time (t), and information capacity (I). Simply stated, Hartleys law is I B x tIn 1948, mathematician Claude E. shannon (Bell Telephone Laboratories) published a paper relating the information capacity of a communication channel to bandwidth and signal-to-noise power ratio (S/N).Mathematically stated, the shannon limit for information capacity is

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DITTwo Basic of Electronic Communication System

An analog communication system

Is a system in which energy is transmitted and received in analog form (a continuously varying signal such as sine wave)

Both the information and the carriers are analog signal

The digital communication system

Covers a broad range of communication techniques, including digital transmission and digital radio

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DITTwo Basic of Electronic Communication System(2)Digital transmission - Is a true digital system where digital pulses are transferred between two or more point a communication system - There is no analog carrier, and the original source may be in digital or analog form - Require physical transmission medium such as metallic cable or optical fiber

Digital Radio - Is the transmitted of digitally modulated carrier between two or more points in a communication system - The modulating signal and the demodulated signal are digital pulses - Digital pulse modulate an analog carrier - Transmission medium may be a physical facility or free space (i.e. The Earths atmosphere)

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DITAdvantage of using Digital transmission compared to Analog transmissionIncreased immunity to channel noise and external interference

Flexible operation of the system

A common format for the transmission of different kinds of message signals (e.g. voice signals, video signals, computer data)

Improved security of communication through the use of encryption

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DITSummary of various modulation techniqueAnalog Modulation TypesAmplitude Modulation (AM): is the one if the information signal is analog and the amplitude (V) of the carrier is varied proportional to the information signal Frequency Modulation (FM): is the one if the frequency (f) of the carrier is varied proportional to the information signalPhase modulation (PM): is the one if the phase () of the carrier is varied proportional to the information signal

Digital Modulation TypesAmplitude Shift Keying (ASK): is the one if the information signal is digital and the amplitude (V) of the carrier is varied proportional to the information signalFrequency Shift Keying (FSK): is the one if the frequency (f) of the carrier is varied proportional to the information signalPhase Shift Keying (PSK): is the one if the phase () of the carrier is varied proportional to the information signal Quadrature Amplitude Modulation (QAM): is the one if both the amplitude (V) and the phase () of the carrier are varied proportional to the information signal

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DITWhy Modulation is necessaryIt is extremely difficult to radiate low frequency signals from an antenna in the form of electromagnetic energyIt is possible to combine a number of baseband (information) signal and send them through the medium, provided different carrier frequencies are used for different baseband signalsTransmitting signals over large distance, because low frequency signals have poor radiation characteristics

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DITAnalog vs. Digital SystemsAnalog signalsThe amplitude changes continuously with respect to time with no discontinuities Digital signalsThe one which are discrete and their amplitudes maintains a constant level for prescribed period of time and then it changes to another level

Binary signalsHas at most 2 valuesUsed to represent bit valuesBit time T needed to send 1 bitData rate R=1/T bits per secondtx(t)tx(t)1000110TDigital systems more robust

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DITFourier Transforms

Let the frequency spacing approach zero, and the index n approach infinity such that the product approaches a continuous frequency variable f. Then

The bracketed term is the Fourier transform of v(t) symbolized by or and defined as:

The time function v(t) is recovered from V(f) by the inverse Fourier transform

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DITFrequency Translation and ModulationBesides generating new transform pairs, duality can be used to generate transform theorems. In particular, a dual of the time-delay theorem is

Since is not a real time function and cannot occur as a communication signal. However, signals of the form are common-in fact, they are the basis of carrier modulation-and by direct extension of the equation above we have the following modulation theorem:

The theorem is easily proved with the aid of Eulers theorem

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DITSpecial FunctionsDirac delta function

Exponentials

Sinusoids

Delta Function Train

d(t)0fcAd(f-fc)Aej2pfct Acos(2pfct) fc.5Ad(f-fc)-fc.5Ad(f+fc)

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*Special Functions The dirac delta function is a key function in Fourier analysis, as it described the impulse response of a filter. An exponential in time becomes a delta function in the frequency domain at the frequency of the exponential. Sinusoids become two delta functions at the positive and negative frequencies of the sinusoid. A delta function train in the time domain becomes a delta function train in the frequency domain.*