Generation of Amplitude Modulation (AM).

The device which is used to generate an amplitude modulation (AM) wave is known as amplitude modulator. The methods as amplitude modulator Generation may be broadly classified as following:-

1) Low level AM Modulation.

2)High level AM Modulation.

1)Low Level Amplitude Modulation:-

Figure shows  the block diagram of a low level AM modulation system. In a low level amplitude modulation system, the modulation is done at low power level. At low power levels, a very small power is associated with the carrier signal and the modulation signal. Because of this the output power of modulation is low. Therefore the power amplifiers are required to boost the amplitude modulated signals up to the desired output level.

 From block diagram in figure it is clear that modulation is done at low power level. After this the amplitude modulated signal is applied to a wide band power amplifier . A wide band power amplifier is used just to preserve the sidebands of the modulated signal. Amplitude modulated systems , employing modulation at low power levels are also called low level amplitude modulation transmitters.

Square-law diode modulation and switching modulation are examples of low-level modulation.

2)High level Amplitude Modulation:-

Figure  shows the block diagram of a high level AM modulation system. In a high-level amplitude -modulation system, the modulation is done at high power level. Therefore, to produce amplitude modulation at these high power levels, the base band signal and the carrier signal must be at high power levels. In block diagram of figure the modulating signal and carrier signal are first power amplified and then applied to AM  high level modulator. For modulating signal the wide band power amplifier is required just to preserve all the frequency components present in modulating signal.

On the other hand for carrier signal , the narrow band power amplifier is required because it is a fixed frequency signal. The collector modulation method is the example of high level modulation. Before we discuss low level and high level modulation methods in detail, we shall establish the fact that a non- linear resistance of non linear device can be made to produce amplitude modulation when two different frequencies are passed together through it.

Pulse Amplitude Modulation (PAM).

Pulse amplitude modulation may be defined as that type of modulation in which the amplitudes of regularly spaced rectangular pulses vary according to instantaneous value of the modulating or message signal . In fact the pulses in a PAM signal may be of flat top type or natural type or ideal type. Actually all the sampling methods which have been discussed in last sections are basically pulse amplitude modulation methods. Out of these three pulse amplitude modulation methods the flat top PAM is most popular and is widely used. 

The reason for using flat top PAM is that during the transmission the noise interferes with the top of the transmitted pulses and this noise can be easily removed if the PAM pulse has flat top. However , in case of natural samples PAM signal the pulse has varying top in accordance with the signal variation. Now when such type of pulse is received at the receiver it is always contaminated by noise. Then it becomes quite difficult to determine the shape of the top of the pulse ans thus amplitude detection of the pulse is not exact. Due to this errors are introduced in the received signal. Therefore, flat top sampled PAM is widely used signal. 

Working Principle:- 

A sample and hold circuit shown in fig. is used to produce flat top sampled PAM . The working principle of this circuit is quite easy. The sample and Hold circuit consists of two field 

effect transistors (FET) switches and a capacitor. The sampling switch is closed for a short duration by a short pulse applied to the gate G1 of the transistor . During this period the capacitor C is quickly charged up to a voltage equal to the instantaneous sample value of the incoming signal x(t) . Now the sampling switch is opened and the capacitor C holds the charge. The discharge switch is then closed by a pulse applied to gate G2 of the other transistor . Due to this the capacitor C is discharged to zero volts. The discharges switch is then opened and thus capacitor has no voltage.

Hence the output of the sample and hold circuit consists of a sequence of flat top samples .

Research Process

Research process consists of series of actions or steps necessary to carry out research and the desired sequencing of these steps:-

A brief description of these steps is as follows-

1.Formulation

2.Review of Literature

3.Formulation of hypothesis

4.Research Design

5.Determining Sample design

6.Collection of Data

7.Execution of project

8.Analysis of Hypothesis

9.Testing of Hypothesis

10.Generations and Interpretations

11.Preparation of Research Report

1.Formulation  of research problem-

the first step in any research is to formulate a research problem

There are basically 2 steps of research problem:

1.problem which relates to state of nature.

2.problem which relates to relationship between variables

At the very beginning researcher must identity the problem be wants to study.

Essentially two steps are involve in formulation a research problem

1.Understanding the problem thoroughly and the best way of it is to discuss it with ones own colleagues or the experts in that matter.

2.Rephrasing the problem in to meaningful terms from an analytical point of view.
2.Review of Literature-

 After the problem is formulated, a brief summary of it should be written down and this summary is known as synopsis an for getting phd degree it is necessary to submit it to the committee or research board for approval. At this time the researcher should undertake extensive literature survey connected

with the problem. For this purpose journals and published / unpublished bibliographies are first place to go to. The other things that can be referred are academic journals, conference proceedings, Government reports, books, internet sites etc.

3.Formulation of Hypothesis-

 After extensive literature survey, the next step is to formulate working hypothesis. Hypothesis is a tentative assumption made in order to draw out and test its logical consequences. Hypothesis should be very specific and limited to the piece of research in hand because it has to be tested. The role of hypothesis is to guide the researcher by delimiting the area of research and to keep him on the right track. It sharpens the thinking of researcher & focuses attention on more important facts of the problem.

4.Preparation of Research Design-

The next step is to prepare a conceptual structure within which research would be conducted. The function of research design is to provide for the collection of relevant information and evidence with minimum expenditure of effort , time and money.

research objective / purpose may be grouped into 4 categories..

1.Exploration

2.Description 

3.Diagnosis

4.Experimental/Hypothesis

The research design should be flexible and it should provide apportunity for considering many different aspects of a problem fit is of exploratory nature.
The following points should be considered while preparing a research design:
1.The means of obtaining information.
2.The availability and skill of researcher and his staff.
3.The time available for research.
4.The cost factor relating to research i.e finance available for research . There are number of research design such as: Experiment and non Experiment research design.
5.Determining Sample Design:-
All the items under consideration in any field of enquiry or research constitute a universe or population. If all these items are included in our study then it is known as census method of enquiry.
For example
1.Random Sampling
2.Systematic Sampling
3.Stratified Sampling
Non probability samples are those in which there is some bias on the part of researcher.
6.Collection of Data:-
For carreing out any research successfully data collected should be adequate and should be based on nature of enquiry.
There are several ways of collecting data and these differ on the basis of time cost money cost and other resources at the disposal of research.
The data to be collected is of 2 types.
1.Primary data is data which is collected for the first time by researcher himself.
2.Secondary data is data which has been collected by someone else earlier and is used by research for his research.
7.Execution of Project:-
It is a very important step in research process. The data collected would be adequate and dependable if project is executed in right manner. The research should see that project is executed in a systematic manner and within prescribed time.
If data is to be collected by interviews then steps should be taken for selection of right person and proper training should be given to him. Occasional field check should be made to ensure that interviewers are doing their assigned job efficiently and sincerely.If details to be collected by means of structure questionarie then questions as well as answers should be coded so the data can be readily matching processed.
8. Analysis of data:-
Analysis of data refers to computation of certain measures along with serearching for pattrens of relationship that exist among data groups.It requires editing classification and codification of data tabulation and then drawing statistical interference out of arranged data. After tabulation of data analysis process involves calculation of various percentages coefficient etc. By applying various statical methods and tools so as to find out whether observation support theory on not . In this case our problem will be whether the two means value differ significantly or the difference  just due to fluctuations of sampling.

 9.Testing of Hypothesis:-
After data has been analysed the research is in a position to test the hypothesis formed earlier. He has to test whether the facts support the hypothesis or not? The various test such as t test z test chi sequar test f test have been designed for this purpose . Hypothesis may be tested either by using one test or one or more test depending upon the object and significance of research study.
10.Generalization and interpretations:-
When Hypothesis is tested and up held several times then it is possible for researcher to build a theory . This is known as Generalization.
If there is no hypothesis then researcher might explain his findings on the basis of some theory . This process is known as interpretation.
11.Preparation of research report:-
The last step in research process is to prepare report of what has been done or achieved by him .It is not an easy task and proper care should be kept while preparing it.
It may stood as follows .
1.Preliminary pages-
Title page
Acknowledgment
Preface
Table of contents
List of tables and charts etc.
2.Main text
Introduction
Main report
Summary of findings
Conclusion
Suggestions
3.The End matter.
Appendics
Bibliography etc.

555 Timer.

The 555 IC  is available as an 8-pin metal can, an 8-pin mini DIP(dual-in-package) or a 14-pin DIP, This IC consists of 23 transistors , 2 diodes and 16 resistors. The explanation of terminals coming out of the timer IC is as follows. The pin number is used in the following discussion refer to the 8-pin DIP and 8-pin metal can packages.

Pin1: grounded Terminal-

All the voltages are measured w.r.t this terminal.

Pin2: Trigger Terminal-

This pin is an inverting input to a comparator that is responsible for transition of flip-flop from set or reset. The output of the timer depends on the amplitude of the external trigger pulse applied to this pin.

Pin3: Output Terminal-

Output of the timer is available at this pin. There are two ways in which a load can be connected to the output terminal either between pin3 and ground pin or between pin3 and supply pin .The load connects between pin3 and supply in is called the normally  on load and that connected between pin 3 and ground pin is called the normally off load.

Pin4: Reset Terminal-

To disable or reset the timer a negative pulse is applied to this pin due to which it is referred to as reset terminal. When this pin is not to be used for reset purpose. It should be connected to +vcc to avoid any possibility of false triggering.

Pin5: Control Voltage Terminal-

The function of this terminal is to control the threshold and trigger levels. Thus either the external voltage or a pot connected to this pin determines the pulse width of the output waveform. The external voltage applied to this pin can be used to modulate the output waveform. When this pin is not in used , it should be connected to ground through a 0.01uf to avoid any noise problem.

Pin6: Threshold Terminal-

This is the non-inverting input terminal of comparator 1, which compares the voltage applied to this terminal with s reference voltage of +2/3 vcc . The amplitude of voltage applied to this terminal is responsible for the state of flip-flop.

Pin7: Discharge Terminal-

This pin is connected internally to the collector of transistor and mostly a capacitor is connected between this terminal and ground. It is called discharge terminal because when transistor saturates ,capacitor discharges through the transistor. When transistor is cutoff, the capacitor charges at a rate determined by external resistor and capacitor.

Pin8: Supply Terminal-

 A supply voltage of +5v to   +18v is applied to this terminal with respect to ground .

The timer combines a relaxation oscillator, two comparators, an R-S flip-flop and a discharge capacitor.R-S flip -flop A pair  of cross-coupled transistors each collector drives the opposite base through resistance.On the other hand if transistor Q1 is cut off, its collector voltage , which is approximately equal to _vcc . drives the transistor Q2 into saturation. The low collector voltage of this transistor then keeps the transistor Q1 in cutoff.

Depending on which transistor is saturated, the Q output is either low or high. By adding more components to the circuit , an R-S flip-flop is obtained , R-S flip-flop is a circuit that can set the Q output to high or reset it low. The circuit latches in either two states. A high S input sets Q to high a high R input  resets Q To low. Output Q remain in given state until it is triggered into the opposite state.

Noise Analysis.

Electrical noise is defined as any undesirable electrical energy that falls within the pass band of the signal. For example in audio recording any unwanted electrical signal that fall within the audio frequency band of o hz to 15khz will interfere with the music and therefore be considered noise. The effect that noise has on an electrical signal. A sin wave without noise same signal except in the presence of noise. The grassy-looking squiggles superimposed on the sine wave are electrical noise, which contains a multitude of frequencies and amplitudes that can interfere with the quality of the signal.

Noise can be divided into general categories ; correlated and uncorrelated . Correlation implies a relationship between the signal and the noise. Therefore correlated noise exists only when a signal is present . Uncorrelated noise on the other hand is present all the time whether there is a signal or not.

Uncorrelated noise is present regardless of whether of there is a signal present or not. Uncorrelated noise can be further subdivided into two general categories: external and internal.

External noise is noise that is generated outside the device or circuit . The three primary sources of external noise are atmospheric, extraterrestrial, and man-made. Atmospheric noise is naturally occurring electrical disturbances that originate within earth s atmosphere. Atmospheric noise is commonly called static electricity and is the familiar sputtering , crackling , and so on often heard from a speaker when there is no signal present. The source of most static electricity is naturally occurring electrical conditions, such as lighting. static electricity is often in the form of impulses that spread energy throughout a wide range of frequency. The magnitude of this energy however is in inversely proportional to its frequency. Consequently at frequencies above 30 Mhz or so , atmospheric noise is relatively insignificant.

Extraterrestrial noise; Extraterrestrial noise consists of electrical signals that originate from outside earth s atmosphere and is therefore sometimes called deep-space noise. Extraterrestrial noise originates from the milky way, other galaxies, and the sun. Extraterrestrial noise is subdivided into two categories: solar and cosmic.

Solar noise is generated directly from the sun s heat. There are two parts to solar noise :  quiet condition , when a relatively constant radiation intensity exists, and high intensity , sporadic disturbances caused by sunspot activity and solar flare-ups. The magnitude of the sporadic noise caused by sunspot activity follows a cyclic pattern that repeats every 11 years.

Cosmic noise, sources are continuously distributed throughout the  galaxies. because the sources of galactic noise are located much farther away than our sun, their noise intensity is relatively small. Cosmic noise is often called black-body noise and is distributed fairly evenly throughout the sky.

Man-made noise man made noise is simply noise that is produced by mankind . The predominant sources of man made are spark producing mechanisms, such as commutators in electric motors, automobile ignition systems . ac power generating and switching equipment , and fluorescent lights.

Man made noise is impulsive in nature and contains a wide range of frequencies that are propagated through space in the same manner as radio waves. Man made noise is most intense in the more densely populated metropolitan and industrial areas and is therefore sometimes called industrial noise.

Internal noise: Internal noise is electrical interference generated within a device or circuit . There are three primary kinds of internally generated noise; shot transit time and thermal. Short noise is caused by the random arrival of carries at the output element of an electronic device ,such as a diode , field -effect transistor, or bipolar transistor . Short noise was first observed in the anode current of a vacuum-tube amplifier and was described mathematically by W.Schottky in 1918. The current carriers are not moving in a continuous, steady flow as the distance they travel varies because of their random paths of motion. Short noise is randomly varying and is superimposed onto any signal present.

When amplified short noise sounds similar to metal pellets falling on a tin roof . Short noise is sometimes called transistor noise and is additive with thermal noise.Transit-time noise and modification to a stream of carriers as they pass from the input to the output of a device produces an irregular, random variation categorized as transit-time noise . When the time it takes for a carrier to propagate through a device is an appreciable part of the time of one of the signal the noise becomes noticeable. Transit-time noise in transistor is determined by carrier mobility, bias voltage and transistor construction.

Carriers traveling from emitter to collector suffer from emitter time delays, base transit time delays and collector recombination time and propagation time delay . If transmit delays are excessive at high frequencies the device may add more noise than amplification to the signal. Thermal noise thermal noise is associated with the rapid and random movement of electrons within a conductor due to thermal agitation. The English botanist Robert Brown first noted this random movement . Brown first observed evidence for the moving particle nature of matter in pollen grains and later noticed that the same phenomenon occurred with smoke particles.

Thermal noise is present in all electronic components and communications systems. Because thermal noise is uniformly distributed across the entire electromagnetic frequency spectrum , it is often referred to as white noise. Thermal noise is a form of additive noise , meaning that it cannot be eliminated , and it increase in intensity with the number of devices in a circuit and with circuit length.

Therefore thermal noise sets the upper bound on the performance of a communication system.

Propagation of electromagnetic waves.

Electromagnetic waves traveling within earth s atmosphere are called terrestrial waves and communications between two or more points on earth is called terrestrial radio communications. Terrestrial waves are influenced by the atmosphere and earth itself. In terrestrial radio communications, electromagnetic waves can be propagate in several ways , depending on the type of system and the environment. As previously explained electromagnetic waves travels in straight lines except when earth and its atmosphere alter their path. Essentially there are three ways of propagating electromagnetic waves within earths  atmosphere ground wave space wave and sky wave propagation.

The three modes of propagation possible between two radio terrestrial antennas. Path 1 a direct or free-space wave, path 2 a ground - reflected wave. Direct and ground - reflected waves together are called space waves. Path 3 a surface wave which consists of the electric and magnetic fields associated with the currents induced in the ground. The magnitude of the ground current depends on the constants of the ground and the electromagnetic waves propagation. The cumulative sum of the direction ground - reflected , and surface waves is sometimes referred to as the ground wave, which is confusing because a surface wave by itself is also sometimes called a ground wave. Path 4 is called the sky wave which depends on the presence of the ionized layers above earth that returns some of the energy that otherwise would be lost in outer space.

Each of the four propagation modes exists in every radio system;however some are negligible in certain frequency ranges or over a particular type of terrain. At frequencies below approximately 2mhz , surface waves provide the best coverage because ground losses increase rapidly with frequency .Sky waves are used for high frequency applications , and apace are used for very high frequencies and above.

A surface wave propagation is an earth guided electromagnetic wave that travels over the surface of earth. As a surface wave moves over earth s surface , it is accompanied by charges induced in the earth. The charges move with the wave producing a current . Since the earth offers resistance to the flow of current , energy is dissipated in a manner very similar to those in a transmission line. Earth s surface also has dielectric losses. Therefore surface waves are attenuated as they propagate . because energy is absorbed from the surface wave , the portion of the wave in contact with earth s surface is continuously wiped out. The energy is replenished by diffraction of energy downward from the portion of the ground wave immediately above earth s surface. This phenomenon produces a slight forward tilt in the wavefront .

Attenuation of the surface wave due to absorption depends on the conductivity of earths surface and the frequency of the electromagnetic wave. Surface waves propagate best over a good conductor. It is apparent that the best surface wave transmission occurs over seawater and that the highest degree of attenuation is over jungle areas. Attenuation over all types of terrain increases rapidly with frequency . Extremely high losses make it impractical to use surface waves for long -distance transmission of high-frequency electromagnetic waves.

Ground wave must be vertically polarized because the electric field in a horizontally polarized wave woulds be parallel to earth s surface and such waves would be short circuited by the conductivity of the ground. Earth s atmosphere has a gradient density which also causes the wavefront to tilt progressively forward. Therefore the wave propagates around the earth remaining close to its surface , and if enough power is transmitted, the wavefront could propagate beyond the horizon or even around the entire circumference of the earth.

However , care must be taken when selecting the frequency and terrain over which surface waves will propagate to ensure that the wavefront does not tilt excessively and simply turn over , lie flat the ground and case to propagate.Surface wave propagation is commonly used for ship-to-ship ans ship-to-shore communications, for radio navigation and for maritime mobile communications. Surface waves are used at frequencies as low as 15khz.

Transverse electromagnetic waves.

Transverse electromagnetic (TEM) waves and also described how metallic wires can be used as a transmission medium to transfer TEM waves from one point to another. However, very often in electronic communication systems, it is impractical or impossible to interconnect two pieces of equipment is separated by large a metallic wire or cable. This is especially true when the equipment is separated by large spans of water , rugged mountains , or harsh desert terrain or when communicating with satellite transponders orbiting 22,300 miles above earth. also when the transmitters and receivers are mobile, as with two-way radio communications and cellular telephone, providing connections with metallic facilities is impossible. Therefore , earth s atmosphere is often used as a transmission medium.

Free-space propagation of electromagnetic waves is often called radio-frequency propagation or simply radio propagation. Although free space implies a vacuum propagation through earth s atmosphere is often referred to as free -space propagation and can often be treated as just that, the primary difference being that earth s atmosphere introduces losses and impairments to the signal that are not encountered in a vacuum. TEM waves will propagate through any dielectric material including air.TEM waves , however do not propagate well through lossy conductors, such as seawater, because the electric fields cause currents to flow in the material that rapidly dissipate the waves energy.

Radio waves are electromagnetic waves and like light propagate through free space in a straight line with a velocity of approximately 300,000,000 meters per second . Other forms of electromagnetic waves include infrared, ultraviolet, x rays and gamma rays. To propagate radio waves through earth s atmosphere , it is necessary that the energy be radiated from the source, then the energy must be captured at the receive end. Radiating and capturing energy are antenna functions and the properties of electromagnetic waves were explained The purpose of its behavior and optical properties of radio waves propagating through earth s atmosphere.

An electromagnetic waves is electrical energy that has escaped into free space. Electromagnetic waves travel in a straight line at approximately the speed of light and are made up of magnetic and electric fields that are at right angles to each other and at right angles to the direction of propagation. The essential properties of radio waves are frequency intensity dirction of travel, and place of polarization.

Radio waves are a form of electromagnetic radiation similar to light and heat. They deffer from these radiations in the manner in which they are generated and detected and in their frequency range . A radio waves consists of traveling electric and magnetic fields, with the energy evenly divided between the two types of fields.

The polarization of a plan electromagnetic waves is simply the orientation of the electric field vector in respect to the surface of the earth. If the polarization remains consists , it is described as linear polarization. Horizontal polarization and vertical polarization. If the electric field is propagating parallel to the earth s surface, the wave is said to be horizontally polarized. If the electric field id propagating perpendicular to the earth s surface, the wave is said to be vertically polarized. If the polarization vector rotates 360 degree as the waves moves one wave length through space and the field strength is equal to all angles of polarization, the waves is described as having circular polarization. When the field strength varies with changes in polarization, this is described as elliptical polarization . 

A rotating waves can turn in either direction. If the vector rotates in a clockwise direction, it is right handed, and if the vector rotates in a counterclockwise direction , it is considered left handed. Electromagnetic waves are invisible ; therefore , they must be analyzed by indirect methods using methods. They concepts of rays and wavefronts are  aids to the effects of electromagnetic wave propagation through free space . A ray is a drawn along the direction of electromagnetic wave propagation; however , it does not necessarily represent the propagation or a single electromagnetic wave.

Amplitude Modulation Receiver.

AM demodulation is the reverse process of AM modulation. A conventional double-sideband AM receiver simply converts a received amplitude-modulated wave back to the original source information. To do this, a receiver must be capable of receiving , amplifying, and demodulating an AM wave. It must also be capable of band limiting the total radio- frequency spectrum to a specific desired band of frequencies. The selection process is called tuning the receiver.

To completely understand the demodulation process, first it is necessary to have a basic understanding of the terminology commonly used to describe radio receivers and their characteristics. The RF section is the first stage of the receiver and is therefore often called the receiver front end. The primary functions of the RF section are detecting , band limiting and amplifying the receiver RF signals.

The mixer/converter section is the next stage. This section down converts the received RF frequencies to intermediate frequency , which are simply frequency that fall somewhere between the RF and information frequency hence the name intermediate. The primary functions of the IF section are amplification and selectivity. The AM detector demodulates the AM wave and converts it to the original information signal , and the audio section simply amplifies the recovered information.

There are several parameters commonly used to evaluate the ability of a receiver to successfully demodulate a radio signal. The most important parameters are selectivity and sitivity, which are often used to compare the quality of one radio receiver to another.

Selectivity is a receiver parameter that is used to measure the ability of the receiver to accept a given band of frequency and reject all others.For example, with the commercial AM broadcast band , each station's transmitter is allocated a 10-khz bandwidth . Therefore, for a receiver to select only those frequency assigned a single channel, the receiver must limit its bandwidth to 10khz. If the passband is greater than 10khz , more than one channel may be received and demodulated  simultaneously. If the passband of a receiver is less than 10khz ,a portion of the modulating signal information for that channel is rejected or blocked from entering the demodulator and , consequently, lost.

There are several acceptable ways to describe the selectivity of a radio receiver. One common way is to simply given the bandwidth of the receiver at the -3-db points. This bandwidth, however , is not necessarily a good means of determining how well the receiver band width at two levels of attenuation, for example, -3db and -60db. The ratio of these two bandwidths is called the shape factor.

In today's overcrowded radio frequency spectrum the FCC makes adjacent channel assignments as close together as possible, with only 10khz separating commercial broad cast band AM channels. Spacing for adjacent commercial broadcast- band FM channels is 200khz, and commercial television channels are separating by 6 mhz . A radio receiver must be capable of separating the desired channel's signal without allowing interference from an adjacent channel to spill over into the desired channel's passband.

If the bandwidth can be reduced , the noise will also be reduced by the same proportion, thus increasing the signal-to -noise power ratio , improving system performance . There is . of course , a system performance limitation as to how much the bandwidth can be reduced .The bottom line is that the circuit bandwidth must exceed the bandwidth of the information signal; otherwise , the information power and/ or the frequency content of the information signal will be reduced , effectively  degrading system performance. 

When a signal propagates from the antenna through the RF section , mixer converter section. and if section the bandwidth is reduced , thus reducing the noise. The theoretical problem is hoe much the bandwidth should be reduced, and the practical problem is in the difficulty of constructing stable narrow- band filters.

The input signal-to -noise ratio is calculated at a receiver input using the RF bandwidth for the noise power measurement. However the RF bandwidth is generally wider than the bandwidth of the rest of the receiver. Reducing the bandwidth is effectively equivalent to reducing the noise .

Meaning of Research.

Research in common parlance refers to a , " Search for Knowledge". One can also define research as a scientific and systematic search for pertinent information on a specific topic. In fact, research is an art of scientific investigation. The Advanced learner's Dictionary of current English lays down the meaning of research as "a carefull investigation or inquiry specially through research for new facts in any branch of knowledge . Redman and Mory define reaserch as a "systematized effort to gain new knowledge . Some people consider research as  a movement , a movement  from the known to the unknown. It is actually a voyage of discovery. We all posses the vital instinct of inquisitiveness for , when the unknown confronts us, we wonder and our inquisitiveness makes us probe and attain full and understanding of the unknown . This inquisitiveness is the mother of all knowledge and the method, which man employs for obtaining the knowledge of whatever the unknown, can be termed as research.

Research is an academic activity and as such the term should be used in a technical sense. According to Clifford Woody research comprises defining and re defining problems, formulating hypothesis or suggested solutions, collecting, organising and evaluating data; making deductions and reaching conclusions; and at last carefully testing the conclusions to determine whether they fit the formulating hypothesis. D. Slesinger and M. Stephenson in the Encyclopedia of social sciences define research as "the manipulation of things , concepts or symbols for the purpose of generalising to extend , correct or verify knowledge , whether that knowledge aids in construction of theory or in the practice of an art. Research is thus an original contribution to the existing stock of knowledge making for its advancement. It is the pursuit of truth with the help of study , observation, comparison and experiment.

In short, the search for knowledge through objective and systematic method of finding solution to a problem is research. The systematic approach concerning , generalisation and the formulation of a theory is also research. As such the term research refers to the systematic methods consisting of enunciating the problem formulating hypothesis, collecting the facts or data, analysing the facts and reaching certain conclusions either the form of solutions towards the concerned problem or in certain generalisations for some theroretical formulation.

Approaches to research-there are two basic types of approaches to research viz.., Quantitative approach and the Qualitative approach.

Quantitative Approach-Quantitative Research involves the generation of data in quantitative form which can be subjected to quantitative analysis in a formal and rigid fashion. This approach can be further sub-divided into inferential, experimental and simulation approaches to research.

1.Inferential approach- The aim of inferential research is to form a data base to infer relationships of population, e.g survey method. It is done to conclude that  sample of population posses same characteristics as that of population.

2.Experimental approach- The experimental approach is conducted under a controlled research environment and some variables are manipulated to observe their effect on other variables.

3.Simulation Approach-Simulation approach involves construction of an artificial environment within which relevant information and data can be generated. It permits an observation of the continuous or dyamic behaviour of a system or its subsystem under controlled conditions.

Qualitative Approach- Qualitative approach involves study of subjectives assessment of attitude behaviour amd  opinions. i.e, non - Quantitative data.

In this case , research is a function of researcher's insight and impressions. It involes use of techniques such as depth interview , group interview , projective techniques etc. This approach involves study of non-quantitative data i,e characteristics or attributes, attitude opinion or behaviour . In this case , research is a function of researcher's insight and impressions.

Research has its importance in each and every field because it includes scientific and inductive thinking and promotes the development of logical habits of thinking P.M Cook explains the significance of research as that it is an honest. exhaustic and intelligent searching for facts and their meanings or implicaaaations with reference to a given problem.

Antenna

An antenna is a metallic conductor system capable of radiating and capturing electro-magnetic energy. Antennas are used to interface  transmission lines to the atmosphere, the atmosphere to transmission lines , or both. In essence, a transmission line couples energy from a transmitter to an antenna to a receiver. The antenna ,in turn, couples energy from a transmitter to an anteena or from an antenna to a receiver. the antenna in turn couples energy received from a transmission line to the atmosphere and energy received from the atmosphere to a transmission line.

At the transmit end of a free - space radio communications system, an antenna converts electrical energy traveling along a transmission line into electromagnetic waves that are emitted into space.At the receive end , an antenna coverts electromagnetic waves in space into electrical energy on a transmission line. A waveguide is a special type of  transmission line that consists of a conducting metallic tube through which high frequency electromagnetic energy is propagated . A waveguide is used to efficient interconnect high frequency electromagnetic waves between an antenna and a transceiver.

Radio waves are electrical energy that has escaped into free space in the form of transverse electromagnetic waves. The escaped radio waves travel at approximately the velocity of light and are comprised of magnetic and electric fields that are at right angles to each other and at right angles to the direction of travel. The plane parallel to the mutually perpendicular lines of the electrical and magnetic fields is called the waveform. The wave always travels in a direction at right angles to the waveform and may go forward or backward, depending on the relative direction of the lines of magnetic and electric flux. If the direction of either the magnetic or the electric flux reverses, the direction of travel is reversed. However , reversing both sets of flux has no effect on the direction of propagation.

All electrical circuits that carry alternating current radiate a certain amount of electrical energy in the form of electromagnetic waves. However the amount of energy radiated is negligible unless the physical dimensions of the circuit approach the dimensions of a wavelength of the wave .For example, a power line carrying 60-hz current with 20 feet of separation between conductor radiates virtually no energy because a wavelength at 60hz is over 3000 miles long , and 20 feet is insignificant in comparison. In comparison an inductor 1cm long carrying a 6ghz signal will radiate a considerable amount of energy because 1cm is comparable with the 5-cm wavelength.

It is apparent that the size of an antenna is inversely proportional to frequency. A relatively samll antenna can efficient radiate high frequency electromagnetic waves, while low- frequency waves require relatively large antennas. Every antenna has directional characteristics and radiate more energy in certain directions relative to other directions. Directional characteristics of antennas are used to concentrate radiation in a desired direction or capture energy arriving from a particular direction.

For an antenna to efficiently receive radio signals it must abstract energy from the radio wave as it passes by the receiving point. Electromagnetic waves reception occurs in an antenna because the electromagnetic flux of the wave cuts across the antenna conductor , including a voltage into the conductor that varies with time in exactly the same manner as the current flowing in the antenna that radiated the wave. The induced voltage along with the antenna absorbs from the passing waves.

Basic antenna operation is best understood by looking at the voltage standing - wave patterns on a transmission line . The transmission line is terminated in an open circuit , which represents an abrupt  discontinuity to the incident voltage wave in the form of a phase reversal. The phase reversal results in some of the incident voltage being radiated, not reflected back toward the source. The radiated energy propagates away from the antenna in the form of transverse electromagnetic waves. The radiation efficiency of an open transmission line is extremely low. Radiation efficiency is the ratio of radiated to reflected energy. To radiate more energy, simply spread the conductor farther apart.Such an antenna is called a dipole.

History of Satellites

The simplest type of satellite is a passive reflector which is a device that simply bounces signals from one place to another . A passive satellite reflects signals back to earth as there are no gain devices on board to amplify or modify the signals. The moon is a natural satellite of earth , visible by reflection of sunlight and having a slightly elliptical orbit. Consequently , the moon became the first passive satellite in1954, when the U.S Navy successfully transmitted the first message over this earth to moon to earth communications systems . In 1956, a relay service was established between Washington, D.C  and Hawaii and, unit 1962 , offered reliable long distance radio communications service limited only by the availability of the moon. Over time, however , the moon proved to be an inconvenient and unreliable communications satellite, as it is above the horizon only half the time and its position relative to earth is constantly changing.

An obvious advantage of passive satellite is that they o not required sophisticated electronic equipment on board, although they are not necessarily void of power. Some passive satellite required radio beacon transmitters for tracking and ranging purposes. A beatcon is a continuously transmitted un modulated carrier that an earth station can lock on to and use to determine the exact location of a satellite so the earth station can align its antennas . Another disadvantages of passive satellite is thier inefficient use of transmitted power . 

In 1957, Russia launched Sputnik I the first active earth satellite. An active satellite is capable of receiving , amplifying ,reshaping, regenerating, and retransmitting information. Sputnik I transmitted telemetry information for 21 days, later in the same year the United State launched Explore I , which transmitted telementry information for nearly five months. In 1958, NASA launched Score a 150 pound conical -shaped satellite . With an on board tape recording ,score rebroadcast President Eisenhower's 1958 Christmas message.

In 1960, NASA in conjunction with Bell Telephone Laboratories and the Jet Propulsion Laboratory lunched Echo, a 100-foot-diameter plastic balloon  with an aluminum coating. Echo, passively reflected radio signals it received from large earth station antennas. Eacho was simple and realiable but required extremely high power transmitters at the earth stations . The forst transatlantic transmission using a satellite was accomplished using Echo. The Syncom project s, a number of using geosynchronous satellites.

Since the Syncom projects a number of nations and private corporations have successfully launched satellite that are currently being used to provide national as well as regional and international global communications . Today's , there are several hundred satellite communications systems operations operating in virtually every corner of the world . There companies provide worldwide , fixed common- carrier telephone and data ciruits: point to point television broadcasting ; network television distribution; music broadcasting ; mobile telephone servies; navigation services; and private communications networks for large corporations, governemt agencies, and military applications.

Intelset I was the first commercial telecommunications satellite. It was launched from Cape Kennedy in 1965 and used two transponders and a 25-mhz band width to simultaneously carry one television signal and 480 voice channels. Intelsat stands for international Telecommunications Satellite Organization. Intelsat is a commercial global satellite network that manifested in 1964 from within the United Nations. Intelsat is a consortium of over 120 nations with the commitment to provide worldwide, non discriminatory satellite communications using four basic service categories; international public switched telephony , broadcasting, private- line business networks and domestic communications. Between 1966 and 1987 Intelsat launched a series of satellite designated Intelsat II ,III,IV,V, ans VI . Intelsat VI has a capacity of 80,000 voice channels. Intelsat s most recent satellite launches include the 500,600,700, and 800 series space vehicles.

The former Soviet Union launched the first set of domestic satellites in 1966 and called them Molniya , meaning lightning. Domsats are satellites that are owned operated and used by a single country. In 1972 Canada launched its first commercial satellite designated Anik , which is an Inuit word meaning 'little brother". Western Union launched their first westar satellite in 1974 and radio corporation of America launched its first Satcom satellites in 1975.

 In the United States today a publicly owned company called communications satellite corporation regulates the use and the operation of U.S  satellite and also sets their tariffs. Although a company or government may own a satellite , its utilities are generally made available to anyone willing to pay for them. The United States currently utilizes the largest share of available worldwide satellite time ; Great Britain is second with 13percent followed by France with 6 percent.

Multiple Acessing techniques

Satellite multiple accessing implies that more than one user has access to a one or more radio channels within a satellite communications channel. Transponders are typically leased by a company or a common carrier for the purpose of providing voice or data transmission to a multitude of users. The method by which a satellite transponder's bandwidth is used or accessed depends on the multiple accessing method utilized.

The three most commonly used multiple- accessing arrangements: frequency division multiple accessing , time division multiple accessing and code division multiple accessing . With FDMA each earth stations  transmission are assigned specific uplink and downlink frequency bands within an allotted satellite bandwidth they may be preassigned or demand assigned . FDMA transmission are separated in the frequency domain and, therefore, must share to total available transponder banswidth as the total transponder power. With TDMA , each earth station transmits a short burst of information during a specific time slot within a TDMA frame. The burst must be synchronized so that each stations burst arrives at the satellite at a different time.

TDMA transmissions are separated in the time domain , and with TDMA the entire transponder bandwidth and power are used for each transmission but for only a prescribed interval of time. With CDMA , all earth stations transmit within the same frequency band and , for all practical purposes, have no limitations on when they may transmit or on which carrier frequency.Thus, with CDMA, the entire satellite transponder bandwidth is used by all stations on a continuous basis. Signal separation is a accomplishes with envelop encryption/decryption techniques.

TDMA, Time division multiple access is the predominant multiple-access method used to-day. It provides the most efficient method of transmitting digitally modulated carriers(PSK) . TDMA is a method of time division multiplexing digitally modulated carriers between participating earth stations within a satellite networks through a common satellite transponder . With TDMA , each earth stations a short burst of a digitally modulated carrier during a precise time slot within a TDMA frame. Each station's burst is synchronized so that it arrives at the satellite transponder at any given time , thus avoiding a collision with another station's carrier. The trans[ponder is an RF=RF  repeater that simply receives the earth station transmissions, amplifies them and then retransmits them in a downlink beam that is received by all the participating earth stations.

Each earth station receives the burst from all other earth stations and must select from them the traffic destined only for itself. Transmissions from all earth stations are synchronized to a reference burst. The reference burst as a separate transmission but it may be the preamble that precedes a reference stations transmission of data. also , there may be more than one synchronizing reference burst. The reference burst contain a carrier recovery sequence (CRS) , from which all receiving stations recover a frequency and phase coherent carrier for PSK  demodulation.

Also included in the reference burst is a binary sequence for bit timing recovery. At the end of each reference burst, a unique word is transmitted.The unique word is used to establish a precise time reference that each of the stations uses to synchronized the transmission of its burst. The UW is typically a string of 20 successive binary 1s terminated with a binary0. Each earth station receiver demodulate and integrator and threshold detector are designed so that the threshold voltage is reached precisely when the last bit of the UW sequence is integrated. This generates a correlation spike at the output of the threshold detector at the exact time the UW sequence ends.

Each earth station synchronizies  the transmission of its carrier to the occurrence of the UW correlation spike . Each station waits a different length of time before it begin transmitting. No two stations will transmit the carrier at the same time. Note the guard time transmissions from successive stations. This is analogous to a guard band in a frequency division multiplexing system. Each stations precedes the transmission of data with a preamble. The preamble is logically equivalent to the reference burst. Because each stations transmission must be received by all other earth stations all stations must recover carrier and clocking information prior to demodulating the data. If demand assignment is used a common signaling channel also must be include in the preamble.

FDMA

Frequency-Division-Multiple-Access(FDMA) is a method of multiple accessing where a given RF bandwidth is divided into smaller frequency bands called subdivisions. Each subdivision has its own IF carrier frequency. A control mechanism is used to ensure that two or more earth stations do not transmit in the same in the same subdivision at the same time  essentially, the control mechanism designates a receiver station for each of the subdivisions. In demand-assignment systems, the control mechanism is also used to establish or terminates the voice-band links between the source and destinations earth stations. Consequently, any of the subdivision may be used by any of the participating earth stations at any given time.

 

If each subdivision carriers only one 4khz voice band channel, this is known as a single channel per carrier system . When  several voice band channels are frequency division multiplexed together to from a composite baseband signal comparied of groups, supergroups, or even mastergroups, a wider subdivision is assigned . This is referred to as multiple-channel per carrier. Carrier frequencies ans bandwidths for FDM/FM satellite systems using multiple-channel-per-carrier formats are generally assigned and remain fixed for a long period of time.

 

This is referred to as fixed assignment multiple access(FDM/FM/FAMA). An alternate channel allocation scheme is  demand-assignment, multiple access(DAM). Demand assignment allows all users continuous and equal access of the entire transponder bandwidth by assigning carrier frequencies on a temporary basis using a statistical assignment process. The first FDMA demand-assignment system for satellite was developed by com-sat for use on the Intelsat series IVA and V satellites.

 

Spared DAMA  satellite system. SPADE is an acronym for single-channel-per-carrier PCM multiple-access demand -assignment equipment. With SPADE 800 PCM encoded voice -band channels separately QPSK modulate an IF carrier signal. Each 4khz voice band channel is sampled at an 8khz rate and converted to an eight-bit PCM code. The produces a 64kbps PCM code for each voice band channel. The PCM code from each voice band channel QPSK modulates a different IF carrier frequency . With QPSK the minimum required bandwidth is equal to one half the input bit rate. The output of each QPSK modulator requires a minimum bandwidth is a 13khz guard band between pairs of frequency -division-multiplexed channels.
 

The IF carrier frequency being at 52.0225mhz and increase in 45 kHz steps to 87.9775 MHz .

The entire 36mhz band is divided in half, producing two 400 channel bands . For full-duplex operation, 40045-khz channels are used for one direction of transmission, and 400 are used for the opposite direction. Also channel 1,2, and 400 from each band are left permanently vacant. This reduces the number of usable full-duplex voice band channels to 397. The 6-ghz c-band extends from 5.725 Ghz to 6.425 ghz . This allows for apporoximately 1936-mhz RF channels per system. Each RF channel has a capacity of 397 full duplex voice band channels.

 

Evalution of SS7

When telephone networks and network switching hierarchies were first engineered, their creators gave little thought about future technological advancements. Early telephone systems were based on transferring analog voice signals using analog equipment over analog transmission media. As a result , early telephone systems were not well suited for modern day digital services, such as data, digitalized video transmission. There fore, when digital services were first offered in the early 1960s, the telephone networks were ill prepared to handle them, and need for an intelligent all-digital networks rapidly became evident.

The ITU commissioned the Comitee Consultatif International Telephonique et Telegraphique to study the possibility of developing an intelligent all-digital telecommunications networks. In the mid -1960s, the ITU - TS developed a digital signaling standard known as Signaling system N0.6(SS6) that modernized the telephone industry. Signaling refers to the exchange of information between call components required to provide and maintain service SS6, based on a proprietary , high-speed data communications networks, evolved into signaling system.no.7(SS7) , which is now the telephone industry standard for most of the people in the world. High-speed packet data and out-of-data signaling characterize SS7. Out -of-band signaling is signaling that does not take place over the same path as the conversation.

The protocol used with SS7 uses a message structure, similar to x.25 and other message based protocol , to request services from other networks. The message propagate from one network to another in small bundles of data called packets that are independent of the subscriber voice or data signals they pertain to. In the early 1960s, the ITU - TS developed common channel signaling known as common channel interoffice signaling system no. 6 (SS6). The basic concept of the common channel signaling is to use a facility for transferring control and signaling information between telephone offices.

SS7 in an architecture for performing out-of-band signaling in support of common telephone system functions of the ,such as call establishing , billing , call routing , and information exchanging functions of the PSTN .SS7 identifiers functions and enables protocols performed by a telephone signaling networks . The major advantages of SS7 include better monitoring maintenance , and network administration. The major disadvantages is its complex coding .

Because SS7 evolved from SS6 there are many similarities between the two systems. SS7 uses variable - length signal units with a maximum length, therefore making it more versatile and flexible than SS6. In addition, SS7 uses 56-kbps data links which provide a much faster and efficient signaling network. In the future, data rates of 1.544 Mbps nationally and 2.048 mbps internationally are expected. The solution involved adding a second number to every 800 number that is used by the switching equipment to actually route a call through the voice network.

The second number is placed in a common, centralized database accessible to all central offices. When an 800 number is called, switching equipment uses a data link to access the database and retrieve the actual routing number. This process is of course , transparent to the user. Once the routing number is known , the switching equipment can route the call using standard signaling methods.

Today , SS7 is being used throughout the Bell Operating Companies telephone network and most of the independent telephone companies. This itself makes SS7 the worlds largest data communications networks, as it links wireline telephone companies, cellular telephone companies , and long-distance telephone companies together with a common signaling system. Because SS7 has the ability to transfer all types of digital information, it support most of the new telephone features and applications and is used with ATM, ISDN, and cellular telephone.

 



Cables and Connecters.

Cabling of fibers is necessary to protect them from deterioration during transportation and installation. Cable design depends on the type of application. For some applications it may be enough to buffer the fiber by placing it inside a plastic jacket. For others the cables must be made mechanically strong by using strengthening elements such as steel rods.

A light-duty is made by surrounding the fiber by a buffer jacket of hard plastic. A tight jacket can be provided by applying a buffer plastic coating of 0.5.1mm thickness on top of the primary coating applied during the drawing process. In an alternative approach the fiber lies loosely inside a plastic tube. Microbending losses are nearly eliminated in this loose-tube construction, since the fiber can adjust itself within the tube. This construction can also be used to make multifiber cables by using a slotted tube with a different slot for each fiber.

Heavy-duty cables use steel or a strong polymer such as Kevlar to provide the mechanical strength. In the loose - tube construction, fiberglass rods embedded in polyurethane and a Kevlar jacket provide the necessary mechanical strength . The same design can be extended to multifiber cables by placing several loose-tube fibers around a central steel core. When a large number of fibers need to be placed inside a single cable , a ribbon cable is used .

The ribbon is manufactured by packaging typically 12 fibers between two polyester tapes. Several ribbon are then stacked into a rectangular array which is placed inside a polyethylene tube. The mechanical strength is provided by using steel rods in the two outermost polyethylene jackets.The outer diameter of such fiber cables is about 1.1.5cm.

Connectors are needed to use optical fibers in an actual communication system. They can be divided in to two categories. A permanent joint between two fibers is known as a fiber splice, and a detachable connection between them is realized by using a fiber connector. Connectors are used to link fiber cable with the transmitter ,while splices are used to join fiber segments.

 The main issue in the use of splices and connectors is related to the loss. Some power is always lost, as the two fiber ends are never perfectly aligned in practice. Splice losses below 0.1db are routinely realized by using the technique of fusion splicing. Connector losses are generally larger. State-of-the-art connectors provide an average loss of about 0.3db.

Scattering based Non - Linesr - Effects.

In scattering based non-linear effects energy gets transferred from an light wave to another wave at a longer wavelength . The lost energy is absorbed by the molecular  vibrations, or phonons , in the medium. This second wave if called the stokes wave. The first wave can be thought of as being a pump wave that causes amplification of the stoke's wave. As the pump propagates in the fiber, it loses power and the stokes wave gains power.

In the case of SBS, the pump wave is the signal wave and the Stoke's wave is the unwanted wave that generated due to the scattering process. In the case of SRS. the pump wave is a high-power wave, and the stoke's wave is the signal wave that gets amplified at the pump of the pump wave.

(A)Stimulated Brillouin Scattering (SBS):-

In the case of SBS, the phonons involved in the scattering interaction are acoustic phonons, and the intersection occurs over a very narrow line width. The stoke's and pump waves propagate in opposite directions. Thus SBS does not cause any interaction between different wavelengths, as long as the wavelength spacing is much greater than 20 MHz, which is typically the case SBS can however, create significant distortion with a single channel.

SBS produces gain in the direction opposite to the direction of propagation of the signal, in other words, back toward the source. Thus it depletes the transmitted signal as well as generates a potentially strong signals back toward the transmitter, which must be shielded by an isolator . The SBS coefficient is approximately 4x10 m/w , independent of the wavelength.

(B) Stimulated Raman Scattering (SRS):-

In SRS , if two or more signals at different wavelengths are injected into a fiber, SRS causes power to be transferred from the lower-wavelength channels to the higher - wavelength channels. This coupling of energy from a lower-wavelength signal to a higher-wavelength signal is a fundamental effect that is also the basis of optical amplification and lasers. The energy of a photon at a wavelength is where h is planck's constant

Unlike SBS,SRS is a broadband effect. Its gain coefficient as a function of wavelength spacing. The peak gain coefficient gr is approximately 6x10m/w at 2.55um, which is much smaller then the gain coefficient for SBS .Also SRS causes coupling in both the direction of propagation and the reverse directions.

Optical Fiber Comparisom.

1)Single-Mode step-index fiber:-

Advantages include the following:-

1.Minimum dispersion -All rays propagating down the fiber take approximately the same path; thus take approximately the same length of time to travel down the cable. Consequently , a pulse of light entering the cable can be reproduced at the receiving end very accurately.

2.Because of the accuracy in reproducing transmitted pulse at the receive end , wider bandwidths and higher information transmission rates are possible with single - mode step-index fibers than with the other types of fibers.

Disadvantages include the following:-

1. Because the central core is very small it is difficult to couple light into and out of this type of fiber. The source-to-fiber aperture is the smallest of all the fiber types.

2.Again, because of the small central core, a highly directive light source, such as a laser , is required to couple light into a single - mode step-index fiber.

3.Single-Mode Step index fibers are expensive and difficult to manufacture.

2)Multi mode step-index fiber:-

Advantages include the following:-

1.Multi mode step-index fibers are relatively inexpensive and simple to manufacture.

2.It is easier to couple light into and out of multimode step index fibers because they have a relatively large source to fiber aperture.

Disadvantages include the following:-

1. Light rays take many different paths down the fiber, which results in large differences in propagation times. because of this rays traveling down this type of fiber have a tendency to spread out. 

2.The bandwidths and rate of information transfer rates possible with this type of cable are less then that possible with the other types of fiber cables.

3)Multimode gradded-index fiber:-

There are no outstanding advantages or disadvantages of this type of fiber. Multi mode gradded-index fibers are easier to couple light into out of than single mode step-index fibers but are more difficult than multimode step-index fibers but less than in multimode step index fibers. This multimode gradded-index fiber is considered an intermediate fiber compared to the other fiber types.

Global System for Mobile Communications.

 In the early 1980, analog cellular telephone systems were experiencing a period of rapid growth in western Europe, particularly in Scandinavia and the United Kingdom and to a lesser extent in France and Germany . Each country subsequently developed its own cellular telephone system.Which was incompatible with everyone else 's system operated at different frequencies , and all were analog .

 In 1982, the Conference of European posts and Telegraphs formed a study group called Groupe Special Mobile(GSM) to study the development of a pan- European public land mobile telephone system using ISDN. In 1989, the responsibility of GSM was transferred to the European Telecommunications Standards Institute (ETSI) , and phase I of the GSM  specifications was published in 1990.

GSM has the advantage of being designed from scratch with little or no concern for being backward compatible with any existing analog cellular telephone system. GSM provides its subscribers with good quality , privacy , and security. GSM is sometimes refferred to as the Pan -European celluar system.

The first GSM system developed was GSM -900 , which operates in the 900-MHZ band for voice only. Phase 2 was introduced in 1995 , which included facsimile, video, and data communications services. After implementing PCS frequencies in 1997 GSM -1800 and GSM-1900 were create GSM is a second-generation cellular telephone system initially developed to solve the fragmentation problems inherent in first- generation cellular telephone systems in Europe.

GSM telephone services can be broadly classified into three categories: bearer services , teleservices, and supplementary services.Probably the most basic bearer service provided by GSM is telephony.

With GSM analog speech signals are digitally encoded and then transmitted through the network as a digital data stream.There is also an emergency service where the closest emergency service provider is notified by dialing three digits similar to 911 services in the United States.

A wide variety of data services is offered through GSM, where users can send and receive data at rates up to 9600 bps tp subscribers in POTS, ISDN networks , packet switched public data networks , and circuit switched public data networks using a wider variety of access methods and protocals such as X.25. In addition, since GSM is a digital networks , a modem is not required between the user and the GSM network.

Optical Communication Systems.

Introduction:-
Optical Communication is a form of telecommunication that use light as the transmission medium.
It consists of a transmitter, which encodes a message into an optical signals, a channel, which carriers the signal it its destination, and a receiver , which reproduces the message from the received optical signal. In optical fiber communication optical fibers are use as the channel for the transmission of signals.

Classification:-
Optical fiber communication systems employ optical fibers as the transmission channel and use other optical communication supporting equipments at the transmitter and receiver end. Optical communication systems can be broadly classified into two categories. these are:

1. Analog optical fiber optical communication systems.

2.Digital optical fiber optical communication systems.

1)Analog Optical Fiber Communication System:-
The information source provides an electrical signal to the transmitter which consists of an electrical stage which drives an optical source to give modulation of the light wave carrier. The optical source which provides the electrical optical conversion may be a semiconductor laser or an ELD. The transmission medium consists of an optical fiber cable and the receiver is an optical detector which drives a further electrical stage and hence provides demodulation of the optical carrier. Photodiodes and in some instances photo transistors and photoconductors any be utilized for the detection of the optical signal and its conversion to electrical signal. Thus electrical interfacing at either end of the optical link is needed.

2.Digital optical fiber optical communication :-
The input signal from the information sources is suitably encoded for electrical optical transmission. The laser drive circuit directly modulates the intensity of the semiconductor laser with encoded digital signal. Hence a digital signal is lunched into the optical fiber cable. The Avalanche Photodiode detector is followed by a front end amplifier and equalizer or filter to provide gain as well as liner signal processing and noise bandwidth reduction. Finally , the signal obtained is decoded to give original digital information.

Besides this , optical communication systems can be classified as :
1.Guided optical communication systems.
2.Unguided optical communication systems.

In guided optical communication systems, the optical beam emitted by the transmitter remain spatially confined. However , in the case of unguided optical communication systems, the optical beam emitted by the transmitter spreads in space, similar to the spreading of microwaves.

CDMA

With IS-95, each mobile user within a given cell, and mobile subscribers in adjacent cells use the same radio-frequency channels. Tn essence , frequency reuse is available in all cells. This is made possible because IS-95 specifies a direct-sequence , spread-spectrum CDMA system and does not follow the channelization principles of traditional cellular radio communications systems.

Rather than dividing the allocated frequency spectrum into narrow bandwith channels, one for each user, information is transmitted over a very wide frequency spectrum with as many as 20 mobile subscriber units simultaneousaly using the same carrier frequency within the same frequency band. Interference is incorporated into the system so that there is no limit to the number of subscribers that CDMA can support.

With CDMA unlike other cellular telephone standards, subscriber data change in real time , depending on the voice activity and requirements of the networks and other users of the network. IS-95 also specifies a different modulation and spreading technique for the forward and reverse channels. on the forward channel, the base station simultaneously transmits user data from all current mobile units in that cell by using different spreading sequences for each users transmissions.

A pilot code is transmitted with the user data at a higher power level , thus allowing all mobile units to use coherent detection. On the reverse link, all mobile units respond is an asynchronous manner with a constant signal level controlled by the base station.

The speech coder used with IS-95 is the Qualcomm 9600-bps Code-Excited Linear Predictive coder. The vocoders converts an 8-kbps compressed data stream to a 9.6-kbps data stream. the vocoders original design detects voice activity and automatically reduces the data rate to 1200bps during silent periods. Intermediate mobile user data rates of 2400 bps and 4800 bps are also used for special purposes. In 1995 , Qualcomm introduced a 14,400 bps vocoder that transmits 13.4 kbps of compressed digital voice information.

Comparison between the WDN and FDM

WDM:-

1.WDM stands for wavelength division multiplexing .

2.WDM is used in optical communication.

3.In WDM the signal frequencies used are of the orde 10-10hz.

4.WDM generally employes optical fiber as communication channel.

5.WDM network setup costs are high.

6.WDM provides bit rates of the order of Gbps.

7. WDM provides the concepts of virtual fibers.

FDM:-

1.FDM stands for frequency division multiplexing.

2.FDM is generally used in radio communication.

3.In FDM the signal frequencies used are of the order of 10hz.

4.FDM generally employes copper cables as communication channel.

5.FDM networks set up cost are comparitively lower.

6.FDM provides bit rates of the order of Mbps.

7.FDM dose not provides the concept of virtual fibers.

Direct - Sequence Encoding.

The new components needed for CDM systems are the encoders and decoders located at the transmitter and receiver ends , respectively. The encoder spreads the signal spectrum over a much wider region than the minimum bandwidth necessary for transmission. Spectral spreading is accomplishing by means of a unique code that is independent of the signal itself. The decoder uses the same code for compressing the signal spectrum and recovering the data. 

The spectrum - spreading code is called is called a Signature Sequence. An advantage of the spread spectrum method is that it is difficult ti jam or intercept the signal because of its coded nature. The CDM technique is thus especially useful when security of the data is of concern.

Several methods can be used for data coding including direct sequence encoding , time hopping , and frequency hopping . The direct sequence coding for optical CDM systems. Each bit of data is coded using A signature  sequence consisting of a large number , say M of shorter bits, called time"chips'. borrowing the terminology used for wireless. The effective bit rate increases by the factor of M because of coding. 

The signal spectrum is spread over a much wider region related to the bandwidth of individual chips.The resulting pulse train is split into several branches and optical delay lines are used to encode the channel. At the receiver end , the decoder consists of the delay lines in the reverse order such that it produced a peak in the correlation output whenever the user s code matches with a sequences of time chips in the received signal.Chip patterns of this peak is lower than the autocorrelation peak produced when the chip pattern matches precisely. The CDM pulse trains consisting of 0 and 1 chips suffer from two problems;

1.First , only unipolar codes can be used simply because optical intensity or power cannot be negative. The number of such codes in a family of orthogonal codes is often not very large until the code length is increased to beyond 100 chips.

2.Second , the crosscorrelation function of the unipolar codes is relatively high, making the probability of an error also large.

Both of these problems can be solved if the optical phase is used for coding in place of the amplitude.

Such schemes are being pursued and are called coherent CDMA techniques. An advantages of coherent CDM is that many families of bipolar orthogonal codes , can be employed in the optical domain. When a CW laser source is used in combination with a phase modulator . another CW laser is required at the receiver for coherent detection.

Multiwavelength SCM Systems.

The combination of  WDM and SCM provides the potential of designing broadband passive optical networks capable of providing integrated services (audio , video , data , etc) to a large number of subscribers. In this scheme , multiple optical carriers are launched into the same optical fiber through the WDM technique. Each optical carrier carries multiple SCM channels using several microwave subcarriers. 

One can mix  analog and digital signals using different subcarriers or different optical carriers. such networks are extremely flexible and easy to upgrade as the demand grows.

The limiting factor for multi wavelength SCM networks is inter channel crosstalk resulting from both the liner and nonlinear processes.The nonlinear effects that produce interchannel crosstalk are as SRS and XPM (cross phase modulation). The probe power varies with time because of SRS and XPM , and the crosstalk is defined as the ratio of radio frequency powers in the two channels. The XPM induced crosstalk increases and the Raman induced crosstalk decreases with the modulation frequency but each has the same magnitude.

The linear crosstalk results from the phenomenon of optical beat interference. It occurs when two or more users transmit simultaneously on the same optical channel using differnt subcarrier frequencies.

As the optical carrier frequencies are then slightly different , their beating produces a beat note in the photocurrent. If the beat note frequency overlaps an active subcarrier channel, an interference signal would limit the detection process in a way similar to IMD.

Multiwavelength SCM systems are quite useful for LAN and MAN applications. They can provide multiple services with only one optical transmitter and one optical receiver per user because different services can use different microwave subcarriers. This approach lowers the cost of terminal equipment in access networks. Different services can be offered without requiring synchronization, and microwave subcarriers can be processed using commercial electronic components.

The main advantage of multiwavelength SCM is that the network can serve NM users , where N is the number of optical wavelengths and M is the number of microwave carriers by using only N distinct transmitter wavelengths. The optical wavelengths can be relatively far apart technology is used to provide broadband integrated services to the subscriber.

Advantages and Disadvantages of LED.

Advantages:-

1.Efficiency:-
LEDs emit more lumens per watt than incandescent light bulbs. The efficiency of LED lighting fixtures is not affected by shape and size , unlike fluorescent light bulbs or tubes.
2.Color:-
LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
3.Size:-
LEDs can be very small and are easily attached to printed circuit boards.
4.On/off TIME:-
LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under a microsecond. LEDs used in communications devices can have even faster response times.
5.Cycling:-
LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps that fail faster when cycled often, or HID lamps that require a long time before restarting.
6.Dimming:-
LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current. This pulse-width modulation is why LED lights viewed on camera, particularly headlights on cars, appear to be flashing or flickering. This is a type of stroboscopic effect.
7.Cool Light:-
In contrast to most light sources , LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
8.Slow Failure:-
LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.
9.Shock Resistance:-
LEDs being solid-state components , are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.
10.Focus:-
The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For large LED packages total internal reflection (TIR) lenses are often used to the same effect.

Disadvantages:-

1.High Initial Price:-
LEDs are currently more expensive , price per lumen, on an initial capital cost basis , than most conventional lighting technologies. The additional expense partially systems from the relatively low lumen output and the drive circuitry and power supplies needed.
2.Temperature Dependence:-
LED performance largely depends on the ambient temperature of the operating environment or thermal management properties. Over driving an LED in high ambient temperatures may result in overheating the LED package , eventually leading to device failure.
3.Voltage Sensitivity:-
LEDs must be supplied with the voltage above the threshold and a current below the rating .This can involve series resistors or current regulated power supplies.
4.light Quality:-
Most cool white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. However , the color rendering properties of common fluorescent lamps are often inferior to what is now available in state of art white LEDs.
5.Area Light Source:-
Single LEDs do not approximate a point source of light giving a spherical light distribution , but rather a lambertian distribution. So LEDs are difficult to apply to uses needing a spherical light field , however different fields of light can be manipulated by the application of different optics or lenses.
6.Electrical polarity:-
Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity LEDs will only light with correct electrical polarity.
7.Blue Pollution:-
Because cool white LEDs with high color temperature emit proportionally more blue light than conventional outdoor light sources such as high- pressure sodium vapor lamps, the strong wavelength dependence of Rayleigh scattering means that cool white LEDs can cause more light pollution than other light sources.
8.Blue Hazard:-
There is concern that blue LEDs and cool white LEDs are now capable of exceeding safe limits of the so called blue - light hazard as defined in eye safety specification such as ANSI/IESNA RP-27.1-05: recommended practice for Photobiological safety for lamp and lamp Systems.
9.Efficiency Droop:-
The luminous efficacy of LEDs decreases as the electrical current increases. Heating also increases with higher currents which compromises the lifetime of the LED. These effects put practical limits on the current through an LED in high power applications.