chapter 1 introduction students

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EE 23353Analog Communications

Chapter 1: Introduction

Dr. Rami A. WahshehCommunications Engineering Department

Chapter 1: Introduction

1.1Communication system.

1.2Analog and digital messages.

1.3 Signal-to-noise ratio, channel bandwidth, andthe rate of communication.

1.4Modulation.

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Elements of a CommunicationSystem

The Goal of a Communication System Designer: is to

a source of interest across a noisy channel to a user atthe other end of the channel.

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Human Voice

Television Picture

Teletype message

Data

Elements of a CommunicationSystem

The objective is to deliver the message signal to: . .

Subject to certain design constraints:

1. Allowable transmit power.2. Available channel bandwidth.

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. .


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1.1 Communication SystemConverts the nonelectricalsignal to electrical oneThe output is called either a

message signal

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Human Voice

Television Picture

Teletype message

Data

Modifies the baseband signalfor efficient transmission

Classification of CommunicationSystems

Classification of communication systems based on thetransmitted range of frequencies.

1.Baseband system: transmits the information signalwithout modulation (not suitable for microwave or

satellite links).2.Passband system: shifts the frequency of the

information signal to a higher frequency before

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ransm ss on.

Modulation: is the process of shifting the basebandsignal to passband range for transmission.

Demodulation: is the process of shifting the passbandsignal to baseband frequency range at the receiver.


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BasebandSpectrum of a baseband signal, energy as a function of

In telecommunications and signal processing, basebandis an adjective that describes signals and systems whoserange of frequencies is measured from close to 0 hertz toa cut-off frequency. Baseband can often be considered asa synonym to lowpass, and antonym to passband and

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bandpass.

PassbandIn telecommunications, optics,

,(not to be confused with band-pass) is the portion of the

frequency spectrum that istransmitted (with minimumrelative loss or maximum

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device. In other words, it is aband of frequencies whichpasses through some filter orset of filters.


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Band-Pass FilterA band-pass filter is a device that passes frequencies

frequencies outside that range.

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Bandwidth measured at half-power points (gain -3 dBrelative to peak) on a diagram showing magnitudetransfer function versus frequency for a band-pass filter

1.1 Communication SystemConverts the nonelectricalsignal to electrical oneThe output is called either a

message signal

Is a medium through whichthe transmitter output is sent

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Human Voice

Television Picture

Teletype message

Data

Modifies the baseband signalfor efficient transmission


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Transmission Media

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Transmission: Link Technologies

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Calculation of Decibel (dB)

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Definitions of dBm and dBW

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dB Hint

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1.1 Communication systemConverts the nonelectricalsignal to electrical oneThe output is called either a Converts the electricalbaseband signal or amessage signal

Is a medium through whichthe transmitter output is sent

signal to its originalform---the message

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Human Voice

Television Picture

Teletype message

Data

Modifies the baseband signalfor efficient transmission Undo the signal

modifications madeat the transmitterand the channel


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NoiseThe received signal looks different from the

(internal or external) and channel distortion.

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The noise is one of the factors that limits the rateof communication.

NoiseExternal Sources: interference from signalstransmitted on nearby channels, human-made noise

equipment, automobile ignition radiation, fluorescentlights or natural noise from lightning, as well aselectrical storms and solar radiation, etc.

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Crosstalk


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NoiseInternal Sources: thermal motion of electrons inconductors, random emission and diffusion or

devices.

Without Noise

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With Noise

Solutions for External Noise

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The effect of the external noise can be minimized oreliminated.


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Solutions for Internal Noise

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The effect of the internal noise can be minimized butcan never be eliminated.

Distortion

The attenuation of the signal increases with the.

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The waveform is distorted because of different

Linear Distortion

different frequency components of the signal.

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It depends on both the channels type and length.

Linear Distortion

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Linear Distortion

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Solution for Linear Distortion

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NonLinear Distortion

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The attenuation varies with the signal amplitude.

NonLinear Distortion

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1.2 Analog and Digital MessagesDigital Messages: are constructed with a finitenumber of symbols.

Example: A Morse-coded telegraph message is adigital message constructed from a set of only twosymbols-mark and space. It is a binary message(two symbols).

M-ary message is constructed of M-Symbols.

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Analog Messages: are characterized by data whosevalues vary over a continuous range and can assumean infinite number of possible values.

Example: A speech waveform has amplitudes thatvary over a continuous range.

Noise Immunity of Digital SignalsTwo symbols are encoded as rectangular pulses of amplitudes A/2 and A/2

The only decision at the receiver is the selection between two possiblepulses received, not the details of the pulse shape.

The data can be recovered correctly as long as the distortion and thenoise are within limits.

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Noise Immunity of Digital Signals

,is important, and even a slight distortion orinterference in the waveform will cause an errorin the received signal.

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withstand noise and distortion.

Noise and Distortion

Amplification of the received signal to make up for theattenuation is of little help because the noise will beamplified in the same proportion, and the SNR remainsunchanged.

32 The incoming pulses are detected and new clean pulsesare transmitted to the next repeater station.


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1.3 SNR and Channel BandwidthThe fundamental parameters that control therate and quality of information transmission are:

1.The channel bandwidth B

2.The signal power S

The bandwidth of a channel is the range of

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fidelity. If a channel of bandwidth B can transmitN pulses per second, then to transmit KN pulsesper second, we need a channel of a bandwidth KB .

Bandwidth B

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Signal-to-Noise Ratio SNRA high signal-to-noise ratio (SNR) is required for ahigh quality communication

SNR = power of signal / power of noise

In a realistic channel

the noise is accumulating along the channel'spath

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therefore the SNR is continuously decreasing along

the channel.

Signal Power S Increasing the signal power reduces the effect ofchannel noise and allows transmission over a longer

.

The limitations imposed on communication by the

channel bandwidth and SNR is highlighted by theinformation capacity theorem (Shannons Theorem):

C=B Log2(1+SNR) bit/s

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C: is the information capacity of the channel (numberof bits that can be transmitted per second withouterror)

B: is the channel bandwidth

SNR: is the received signal-to-noise-ratio


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1.4 ModulationIn modulation, one characteristic or more of a signal

changed based on the information signal that we wishto transmit.

The characteristics of the carrier signal that can bechanged are the amplitude, phase, or frequency, whichresult in Amplitude modulation, Phase modulation, or

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Frequency modulation.

Modulation Types1.Continuous-Wave Modulation:

Some parameters of the sinusoidal carrier wave isvaried continuously in accordance with the messagesignal.

A. Amplitude Modulation (AM): in which the amplitude of asinusoidal carrier is varied in accordance with the

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B.Angle Modulation: in which the instantaneousfrequency or phase of the sinusoidal carrier is varied inaccordance with the message signal.


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Modulation

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Modulation Types Cont2. Pulse Modulation:

Some parameters of a pulse train is varied in accordancewith the message signal.

A. Analog Pulse Modulation: a periodic pulse train is usedas the carrier wave, and some characteristic feature ofeach pulse (e.g., amplitude, duration, or position) is

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corresponding sample value of the message signal.

B.Digital Pulse Modulation: the message signal isrepresented in a form that is discrete in both time andamplitude, thereby permitting its transmission in digitalform as a sequence of coded pulses.


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PAM

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Pulse Time Modulation Types

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Reasons for Modulation1.Ease of Radiation: For efficient radiation ofelectromagnetic energy, the radiating antenna

-wavelength of the signal radiated. For manybaseband signals, the wavelengths are too large forreasonable antenna dimensions.

For example: the frequency of a human voice is in therange of 100-3000 Hz. The corresponding wavelength

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( =c/f=3x10 8 [m/s]/Frequency) is 100-3000 Km. Theantenna size required would be impractical. That iswhy we modulate a high-frequency carrier, thustranslating the signal spectrum to the region ofcarrier frequencies that corresponds to a muchsmaller wavelength.

Reasons for Modulation

.broadcasting the audio of several radio stationsdirectly without modulation will result ininterference.

Modulation translates each signal to a differentfre uenc ran e and conse uentl the

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interference will not happen.


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Signal Power S Transmitting several signals of different frequencyrange simultaneously is known as Frequency-

- . ,can use a tunable bandpass filter to select thedesired signal.

Another method of Multiplexing several signals is

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- -transmission time is shared by a number of signalsby interleaving the pulse trains of various signals ina specific order. At the receiver, the pulse trainscorresponding to various signals are separated.

Reasons for Modulation

3. Effecting the Exchange of SNR with B: theamount of modulation used controls the exchange ofSNR and the transmission bandwidth B .

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