Prof.Manoj Kavedia-9324258878-9773552051-9860174297...

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMIT

Satellite Communication And Mobile Communication5.1 Block diagram of Satellite communication system.5.2 Brief introduction to Communication and orbits.(Elevation and Azimuth angles of satellite) 5.3 Uplink model, Transponder and Downlink model and the frequencies used.5.4 Frequency band used in Satellite communication.5.5 Functions of a satellite.5.6 Concept of antenna5.7 Construction and working principle of Parabolic dish and horn antenna.5.8 Satellite application overview.5.9 Principle, advantages and disadvantages of TDMA, FDMA, CDMA5.10 Concepts of mobile phone.5.11 Block diagram of cellular mobile phone system and description.5.12 Frequency band and types of modulation used for Cellular mobile communication.5.13 Call processing, Frequency reuse and cell splitting Forward and reverse direction (handset to handset) and (Handset to Landline)5.14 Hand Off procedure.

Satellite Communication

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55Satellite Communication Satellite Communication

andandMobile CommunicationMobile Communication

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITQ.Draw the block diagram of Satellite Communication.Describe its different sections

Ans.Satellite is man made system which is kept in continuous rotation around the earth in a specific orbit at a specific height above the earth and with a specific height above the earth and with a specific around speed

Fig.5.1. Satellite Communication System

Fig.5.2. Uplink and Down Link

Frequencies

Geostationary satellite is a satellite which revolves around the earth in circular orbit at height of 36,000 km above the earth with the same angular speed as the earth rotates around itself i.e. it completes one revolution around

earth in 24 hour. Therefore it looks like stationary over certain spot of the Website- www.kavediasir.yolasite.com 2

Uplink

Down Link

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITearth. Hence the name Geo Satellite is given. It is synchronous with earth’s speed around itself. hence it is called synchronous satellite. They are also called communication satellite.

Fig.5.1 and Fig.5.2. shows the basic operation of a communication satellite. An earth station transmits information to the satellite. The satellite contains a receiver which picks up the transmitted signal, amplifies it, and translates it to another frequency. This new frequency is then retransmitted to the receiving stations bath to earth. The original signal being transmitted from the earth station to the satellite is called uplink, and the retransmitted signal from the satellite to the receiving stations is called downlink.

Usually the downlink frequency is lower than the uplink frequency. A typical uplink frequency is 6 GHz and a downlink frequency is 4GHz.

The transmitter receiver combination in the satellite is known as a transponder. The basic function of transponder is amplification and frequency translation. A simplified block diagram of transponder is shown in Fig: The signal amplified by low noise amplifier and then the frequency is uncovered by using and oscillator.

A transponder is a broadband RF channel used to amplify one or more carriers on the downlink side of a geostationary communications satellite. It is part of the microwave repeater and antenna system that is housed onboard the operating satellite. Examples of these satellites include AMC 4 and Telstar 5, located at 101 and 97 degrees west longitude, respectively

The transponder itself is simply a repeater. It takes in the signal from the uplink at a frequency f1, amplifies it and sends it back on a second frequency f2. Figure 5.3 shows a typical frequency plan with 24-channel transponder. The uplink frequency is at 6 GHz, and the downlink frequency is at 4 GHz. The 24 channels are separated by 40 MHz and have a 36 MHz useful bandwidth. The guard band of 4 MHz assures that the transponders do not interact with each other.

Figure.5.3. Basic 24 channel C-band transponder frequency plan


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITQ.What is Transponder?State function of transponder ?List and describe different types of transponder?Ans.TransponderTransponders are used in communications satellites and on board space vehicles. They receive incoming signals over a range, or band, of frequencies, and retransmit the signals on a different band at the same time.

• Satellite TV and radio channels are transmitted back to earth via a transponder on a satellite.

o A communications satellite must have at least one transponder, but may have as many as 125 transponders.

o Because of the various functions it provides in the communication link, the transponder is considered the active component on a communications satellite.

• Major transponder functions include: o Receiving the incoming signal (uplink). o Amplifying the signal. o Converting the signal frequency within a given bandwidth. o Retransmitting the frequency converted signal (downlink).

• A satellite transponder can carry any of the following: o Telephone information o Data up to 155 Mbps o Video conference o TV broadcast o Radio broadcast

Fig.5.4.Transponder

Types of Transponders 1. Bend pipe type

Transponder 2. Regenerative type

Transponder. Bend pipe type transponders are also called conventional type transponders.

Diplexer (acting as a two-way microwave gate) is the device which is responsible or used by the satellite for both receiving the uplink signal and transmitting the downlink signal. The frequency down conversion is done in the carrier processor. Amplification of the weak received signal is done in the front end. The downlink frequency is brought to a sufficient power level by amplification by the power amplifier such as Traveling Wave tube. The


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITcarrier processing equipment determines whether the transponder is of conventional or regenerative typeRegenerative Transponders: The regenerative transponder is one where there is provision for detection and demodulation process. The main advantages for these kind of transponders are:

1. The signal to noise ratio is improved.2. These are simpler and more flexible to implement.3. At low baseband frequency the amplification is easier to obtain in case

of regenerative type.Types of multi channel transponder systems:

1. Broadband system 2. Dual channelized system.

Q.List the different region of SatelliteAns. different region of Satellite-POR , AOR , IOR

The signal covers the vast area of continents and oceans. One satellite Fig.5.4 covers 1/3 part of earth’s surface. Therefore to set up communication over entire globe three satellites placed in space at a distance of 35,887 km above equator at

1200 from each other are required. They cover the total world as shown in Fig.5.4.

Fig.5.5 Range covered by satellite

Three regions of the earth are formed:

1. Atlantic Ocean Region (AOR)2. Pacific Ocean Region (POR)3. Indian Ocean Region (

One satellite is placed in space over each of these region are called as POR satellite, AOR


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITsatellite and IOR satellite respectively shown in fig.5.6. Signals which are to be transmitted are multiplexed. It includes many telephone channels, telegraph, telex, teleprinter, computer data and few television channels. These signals modulate carrier signal of very high frequency. Frequency modulation is used. There modulated Signals are fed to highly directive parabolic dish antenna, which is directed towards the satellite. Parabolic dish antenna converts RF signals into electromagnetic waves, which travels from antenna to satellite. These electromagnetic wave contains all the information and called as uplink.

Each geostationary satellite is a repeater station. The repeater station receives signals sent by earth station. These signals becomes yew weak after travelling a distance of about 36,000 km. They are first received by antenna of satellite. Electromagnetic waves are converted into electrical signal of radio frequency. They are amplified by LNA (Low Noise Amplifier), then their carrier frequency is lowered down by down converter. They are sent to other earth stations in the form of electromagnetic waves. This is called as down link.

Fig.5.6.Different Region of the satelliteDownlink signals are received by dish antenna of another station. This

dish antenna is also highly directive and it is always directed towards satellite. Received signals are very weak signals, they are amplified by WA, carrier is converted to Intermediate Frequency (IF) through mixer, the demodulated by FM demodulator, demultiplexed to obtain base band signals and then sent to their destinations through land network.

Q.Define the terms Uplink And Downlink FrequenciesAns.Uplink Frequencies

The information like television signal, telephone signal, weather information signal which is transmitted through ground station are first upconverted using upconverter and then retransmitted through the antenna towards the satellite.

A transponder which is transmitter responder accepts this frequency and after modification retransmits it to the earth station. The frequency link from ground station antenna to the transponder is known as uplink frequency see Fig.

Downlink FrequenciesAfter desired processing of uplink frequency, the differential frequency

components related to desired information are retransmitted towards earth. Thus a frequency link from satellite to earth station in downward direction is known as downlink.



Q.List the Frequency based used in Satellite communicationAns.Most communication satellite operate in the microwave frequency spectrum. The microwave spectrum is divided up into frequency bands which have been allocated to satellite as well as other communication services. These frequency bands are generally designed by a letter of alphabet. The table shows various frequency bands used in satellite communication.

Frequency Bands used in Satellite CommunicationFrequency Band225-390 Mhz P350-530Mhz J1530-2700Mhz L2500-2700Mhz S3400-6425Mhz C7250-8400Mhz X10.95-14.5Ghz Ku17.7-21.2ghz Kc27.5-31.0 K36.0-46.0Ghz Q46.0-56.0Ghz V56.0-100.0Ghz W

The most widely used satellite communication band is the C band. The uplink frequencies are in the 5.925 to 6.425 GHz. In general, the uplink frequencies in C band are 6 GHz. The downlink frequencies are 3.7 to 4.2 GHz. In general, the downlink frequencies in C band are 4 (3Hz. Generally C band is written as 6 GHz 4 GHz where the uplink frequency is given first.Satellite OrbitQ.List and Describe the types of Orbits of the satelliteAns.There is only one main force acting on a satellite when it is in orbit, and that is the gravitational force exerted on the satellite by the Earth. This force is constantly pulling the satellite towards the centre of the Earth. A satellite doesn't fall straight down to the Earth because of its velocity. Throughout a satellites orbit there is a perfect balance between the gravitational force due to the Earth, and the centripetal force necessary to maintain the orbit of the satellite.

If a satellite simply launched vertically from the earth and then

released, it will fall back to earth because of gravity. In order for the satellite


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITto go into orbit around the earth, it must have some forward motion. Therefore, the satellite is launched when both vertical and forward motion. A satellite revolves around the earth in either a circular or elliptic path. The speed of satellite is measured in either miles per hour or kilometer per hour.

When the satellite is revolving around the earth in circular orbit then it is called geostationary satellite. This satellite completes one revolution around earth in 24 hours. Therefore it looks like stationary over certain spots of the earth. It is synchronous with earth’s speed around itself, hence it is called synchronous satellite. They are used for communication hence they are also called as communication satellites.

AnglesThe orbit is defined by the angle with respect to the earths equator ie

the plane which has 0o angle. There are three types of angles namely polar angle inclined angle and equatorial angles.

Polar Orbits In this orbit the angle of the orbital plane is 90o and satellite rotates mainly over north and south poles. The polar orbit is mainly used for the low altitude satellite, spy satellites as satellite will be in the view of every point on the earth. Polar - circles the North and South Poles (90 degrees incline). Used for weather, scientific and military.Shown on fig.5.7.



Fig.5.7. Different orbits

Inclined Orbit(Elliptical)In this orbit the satellite orbit plane is inclined with the equator. It is used in communication satellite. The satellite covers more of the central part of the globe. Inclined - circles the earth at a certain angle other than 0 to 90 degrees incline. Used for commercial communications, military, and scientific. Shown on fig.5.7.

A satellite in elliptical orbit follows an oval-shaped path. One part of the orbit is closest to the centre of Earth (perigee) and another part is farthest away (apogee). A satellite in this type of orbit generally has an inclination angle of 64 degrees and takes about 12 hours to circle the planet. This type of orbit covers regions of high latitude for a large fraction of its orbital period.

Equatorial Orbit When orbit inclination is 0o i.e. plane of satellite orbit and equatorial

plane are same. This leads to the geostationary orbits. From single geostationary satellite about 40% of the earth can be covered. Thus these geostationary satellites provide almost entire (95% approximately) coverage of earth’s surface. Some part of the polar region. Fig.5.6. shows three such geostationary satellites, kept away from each other at 120o Used for communication. Allows continuous communication from point-to-point on the earth's surface. DIRECTV uses an Equatorial orbit.

Q.List and Describe the Types of Satellite orbitsAns. When a satellite is launched, it is placed in orbit around the Earth. The Earth's gravity holds the satellite in a certain path as it goes around the Earth, and that path is called an "orbit." There are literally an infinite number of possible orbits for an earth satellite.


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITThe altitude of the satellite determines how long it takes for the

satellite to circle the earth. There are several kinds of orbits. As shown in Fig.5.8.

1. LEO (Low Earth Orbit) - Altitude below 2,000 km (1,250 miles), rotation period of 90 minutes to 2 hours.

2. MEO (Medium Earth Orbit) - Altitude of approximately 10,000 km (22,300 miles), rotation period of around 6 hours.

3. GEO (Geosynchronous Earth Orbit) - Altitude of approximately 35,680 km (22,300 miles), rotation period 24 hours

Fig.5.8.Different Satellite orbitLEO - Low Earth Orbit

Short for low earth orbit, a satellite system used in telecommunications. LEO satellites orbit the earth between 400 and 1,250 miles above the earth's surface. LEOs are mostly used for data communication such as e-mail, paging and videoconferencing. Because LEOs are not fixed in space in relation to the rotation of the earth, they move at very high speeds and therefore data being transmitted via LEOs must be handed off from one satellite to the next as the satellites move in and out of range of the earth-bound transmitting stations that are sending the signals into space. Because of the low orbit, the transmitting stations do not have to be as powerful as those that transmit to satellites orbiting at greater distances from the earth's surface. LEO telecommunication systems are a promising technology because they provide the ability for underdeveloped territories to acquire satellite telephone service in areas where it is either too costly or not geographically possible to lay land lines.

Fig.5.9.LEO AppplicationThis Figure.5.9.

shows a LEO satellite system. In this diagram, a portable satellite telephone is communicating with a

landline telephone. The satellite telephone communicates with the closest LEO satellite. Because LEO satellites fly very close to the surface of the earth, they go across the visible horizon in approximately 10 minutes in reference to a mobile satellite customer's location. When the first satellite moves out to the horizon, another LEO satellite becomes available to


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITcontinue the call. However, robust network communications need to be in place to maintain calls (especially data transmission) within this period. Some systems will use satellite diversity to allow talking through more than one satellite at a time, avoiding call "dropouts" from signal blockage.

MEO – Medium Earth OrbitShort for medium, or middle, earth orbit, a satellite system used in telecommunications. MEO satellites orbit the earth between 6,000 and 22,300 miles above the earth's surface. MEOs are mainly used in geographical positioning systems and are not stationary in relation to the rotation of the earth.

Fig.5.10. Global Positioning SystemGlobal Positioning System Operation

This figure.5.10. shows a global positioning satellite (GPS) system. This diagram shows how a GPS receive receives and compares the signals from

orbiting GPS satellites to determine its geographic position. Using the precise timing signal based on a very accurate clock, the GPS receiver compares these signals from 3 or 4 satellites. Each satellite transmits its exact location along with a timed reference signal. The GPS receiver can use these signals to determine its distance from each of the satellites. Once the position and distance of each satellite is known, the GPS receiver can calculate the position where all these distances cross at the same point. This is the location. This information can be displayed in latitude and longitude form or a computer device can use this information to display the position on a map on a computer display.

Geostationary / Geosynchronous Earth Orbit geo = Earth + synchronous = moving at the same rate

Short for geosynchronous, or geostationary, earth orbit, a satellite system used in telecommunications. GEOs orbit the earth at 22,300 miles above the earth's surface. They are tied to the earth's rotation and are therefore in a fixed position in space in relation to the earth's surface. The satellite goes around once in its orbit for every rotation of the earth. The advantage of a GEO system is that the transmission station on earth needs to point to only one place in space in order to transmit the signal to the GEO satellite. GEO


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITsystems are used for transmissions of high-speed data, television signals and other wideband applications.

TV Broadcasting System One of the best known uses of communications satellites is for the transmission of video and television.

1. High bandwidth can easily be provided using satellites which allow easy sending of television broadcasts.

2. Developments in broadcast technology allow different types of transmissions to be sent, taking advantage of sharing the same satellite signal.

3. Using receivers, amplifiers and transmitters and the electronic techniques of multiplying, these communication satellites can simultaneously relay many telephone and television signals.

In order for a satellite to be used for television transmission, it must "hang" over one spot above the Earth.

1. Satellite television satellites are in a geosynchronous orbit that exactly matches the speed that the Earth spins.

2. The satellite appears to be stationary when viewed from the ground. 3. The satellite must be directly over the equator and about 22,300 miles from the

Earth.

Q.Related satellite communication describe the terms Footprint, Altitude and AnglesAns.Footprint

It is observed that as height and orbit of satellite change the area of coverage changes. The satellite with higher height has more lines of sight-of earth’s surface. The wider beam width gives rise to more interfering signals pick-tip satellite uplink receiver and desired signal strength decreases while transmitting downlink antenna broadcast to areas other than intended earth’s station, hence wasting power.

Fig..5.11. Satellite Footprint

The footprint of a satellite is defined as the earth’s area that the satellite can receive from or it can transmit to.

Satellite footprint In the downlink mode, the satellite signal illuminates the portion of the earth’s surface. This coverage is called footprint.Shown in fig.5.11.


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITThe satellite footprint can range from several hundred to several

thousand miles in width depending on application. e.g. the satellite used for point to point applications has a narrow footprint while satellite used for television broadcasting has a wide footprint.

The focused antenna pattern of satellite the footprint can be Limited.

Altitude The satellite revolves around the earth man orbit. The orbit may be

low of few tens of miles to few thousands of miles (high). This orbit mainly determines the time of revolution, e.g. a satellite at 100 miles requires 90 minutes for one revolution while orbit at 22,300 miles(36000 approximately) requires 24 hours. This 24 hour orbit when combined with specific elevation angle it results int6 geostationary or gyro synchronous orbit.

Generally lower orbit requires small transmitter and satellite power while the power delivered to the ground station by the satellite is high. This is due to the small - distance of communication. These low orbits create hacking problem and need antenna to be moved physically. For more details refer LEO , MEO , GEO.

AngleTo optimize the performance of a satellite communications system, the

direction of maxi mum gain of an earth station antenna (sometimes referred to as the foresight) must be pointed directly at the satellite. To ensure that the earth station antenna is aligned, two angles must be determined:

1. The azimuth and 2. The elevation angle.

Azimuth angle and elevation angle are join referred to as the antenna look angles. With geosynchronous satellites, the look angles of earth station antennas need to be adjusted only once, as the satellite will it- main in a given position permanently, except for occasional minor variations.

Angle of ElevationAngle of elevation (sometimes called elevation angle) is the vertical

angle formed between the direction of travel of an electromagnetic wave radiated from an earth station antenna pointing directly toward a satellite and the horizontal plane. The smaller the angle of elevation, the greater the distance a propagated wave must pass through Earth’s atmosphere. As with any wave propagated through Earth’s atmosphere, it suffers absorption and may also be severely contaminated by noise.

Consequently, if the angle of elevation is too small and the distance the wave travels through Earth’s atmosphere is too long, the wave may Website- www.kavediasir.yolasite.com 13

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITdeteriorate to the extent that it no longer provides acceptable transmission quality. The angle of elevation affects the signal strength of a propagated

electromagnetic wave due to normal atmospheric absorption, absorption due to thick fog, and absorption due to heavy rainfall.Fig.5.12.Azimuth and Elevation Angle

Azimuth AngleAzimuth is the horizontal angular distance from a reference direction,

either the southern or northern most point of the horizon. Azimuth angle is defined as the horizontal pointing angle of an earth station antenna. For navigation purposes, azimuth angle is usually measured in a clockwise direction in degrees from true north. How ever, for satellite earth stations in the Northern Hemisphere and satellite vehicles in geosynchronous orbits,


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITazimuth angle is generally referenced to true south (i.e., 180°). Figure5.12.a illustrates the azimuth angle referenced to due north (0°) and due south (180°), and Figure .5.12.c shows elevation angle and azimuth of an earth station antenna relative to a satellite.

Angle of elevation and azimuth angle both depend on the latitude of the earth station and the longitude of both the earth station and the orbiting satellite. For a geosynchronous satellite in an equatorial orbit, the procedure for determining angle of elevation and azimuth is as follows: From a good map, determine the longitude and latitude of the earth station.

Q.Definitions And Related Terms Of Earth-Orbiting Satellites Ans.Apogee: The point farthest from earth.Perigee: The point of closest approach to earth.Line of apsides : The line joining the perigee and apogee through the center of the earth.Ascending node: The point where the orbit crosses the equatorial plane going from south to north.Descending node: The point where the orbit crosses the equatorial plane going from north to south.Line of nodes: The line joining the ascending and descending nodes through the center of the earth.Inclination: The angle between the orbital plane and the earth's equatorial plane.Prograde orbit: An orbit in which the satellite moves in the same direction as the earths rotation. Retrograte orbit: An orbit in which the satellite moves in a direction counter to the earth's rotation.Argument of perigee: The angle from ascending node to perigee, measured in the orbital plane at the earth's center in direction of satellite motion. Mean anomaly: Mean anomaly M gives an average value of the angular position of the satellite with reference to the perigeeTrue anomaly: The true anomaly is the angle from perigee to the satellite position, measured at the earth's center. this gives the true angular position of the satellite in the orbit as a function of time.

Q.Describe the Satellite Design ConceptAns.In Satellite design various parameter of importance are its weight , size , reliability , available DC power , receiver’s sensitivity , transmitter power requirement , antenna size , beamwidth etc , it is also important to design the ground station for some modification with the telemetry.


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITThe satellite’ as a whole consists of many sub-system functions which

are integrated into a single system. The block diagram of the key satellite functional block is as shown in fig 5.13.These satellite consist of five main subsystem as

i. Power sub systemii. Telemetry and Control/Command sub system

iii. Main and auxiliary propulsion sub systemiv. Communication channel sub systemv. Antennas

Power Sub-System This section provides electric power for the operation of various mechanical, electrical and electronic systems incorporated in a satellite. It can be divided into three parts: (a)Solar-array (b)Battery, (c)Power supply control unit.

On the surface of the satellite thousands of minute solar cells are placed. If this surface area is inadequate then separate projections are used. These solar panels, convert sun energy into electrical energy. Sunlight is not available to the solar panels for all the 24 hours. Thus, a battery is floated in parallel with the solar cells

During the day time i.e. when the satellite solar cells are exposed to sunlight, these cells generate sufficient energy as to recharge the battery and also provide for the satellite working. The charged batteries supply the necessary electrical power for proper control of this power. For protection purposes, power supply control circuit is provided.


DC to DC converters

VoltageRegulators

Power conditioning

DC to DC Inverter

Low voltageDC

High voltageDC

To all othersubsystems

To TWTS

AC

Nickel-CadmiumBatteries

Acting asBuffers

Batterycharger

Solar PanelsSun

DC to DC converters

VoltageRegulators

Power conditioning

DC to DC Inverter

DC to DC converters

VoltageRegulators

Power conditioning

DC to DC Inverter

Low voltageDC

High voltageDC


To TWTS

ACLow voltageDC

High voltageDC


To TWTS

AC


Acting asBuffers


Acting asBuffers

Acting asBuffers

BatterychargerBatterycharger

Solar PanelsSolar PanelsSolar PanelsSunSun


Fig.5.13.Power Supply SubSystem

The power supply requirement of the satellite is in the range of ± 5 V to ± 15 V for low logic and amplifier circuits. On the other hand high power is required for power amplifiers. The high power is generated with the help of dc to dc converter using primary battery power and converting it to high frequency ac. This ac power is stepped up to required value and then converted to dc. The power converter efficiency achieved at highest is about 80 to 90%.

The primary electrical power for operating electronic equipment is obtained from solar cells. Individual cells can generate small amounts of power, and therefore array of cells in series-parallel connection are required .

Cylindrical solar arrays are used with spinning satellites,(The gyroscopic effect of the spin is used for mechanical orientational stability) Thus the array are only partially in sunshine at any given time.

Another type of solar panel is the rectangular array or solar sail. solar sail must be folded during the launch phase and extended when in geo-stationary orbit. Since the full component of solar cells are exposed to sun light ,and since the Sail rotate to track, the sun , they capable of greater power output than cylindrical arrays having a comparable number of cells.

To maintain service during an eclipse, storage batteries must be provided .



Fig.5.14.Telemetry System

Fig.5.15. Satellite design Concept

Telemetry and Control/Command Sub-System


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITThe telemetry and control sub-system provides means to ground station to control and monitor the satellite. It uses separate communication channel than the channel used for the uplink and downlink. The telemetry includes status telemetry and command telemetry. As shown in fig.5.14.

The telemetry, tracking, and command (TT&C) subsystem performs several routine functions abroad a spacecraft. the telemetry or "telemetering" function could be interpreted as "measurement at a distance". specifically, it refers to the over all operation of generating an electrical signal proportional to the quantity being measured, and encoding and transmitting this to a distant station, which for satellite is one of the earth stations, which for the satellite is one of the earth stations. Data that are transmitted as telemetry signals include attribute information such as obtained from sun earth sensors; environmental information such as magnetic field intensity and direction; the frequency of meteorite impact and so on ;and spacecraft information such as temperatures and power supply voltages, and stored fuel pressure.

Command systemsCommand system receives instructions from ground system of satellite

and decodes the instruction and sends commends to other systems as per the instruction.

TrackingTracking of the satellite is accomplished by having the satellite is

accomplished by having the satellite transmit beacon signals which are received at the TT&C earth stations. Tracking is obviously important during the transmitter and drift orbital phases of the satellite launch. When on-station, a geo-stationary satellite will tend to shifted as a result of the various distributing forces, as described previously. Therefore it is necessary to be able to track the satellites movements and send correction signals as required. Satellite range is also required for time to time. This can be determined by measurement of propagation delay of signals specially transmitted for ranging purposes.

The telemetry sub-system helps satellite to achieve proper orbit and maintain correct position. Error signals generated from the sensors are fed to a telemetry encoder block. This block encodes the error signals in a predetermined fashion and feeds it to the telemetry transmitter block. This stage suitably amplifies the signal and beams it back to earth through a diplexer and an antenna. The ground control station analyses this signal and a suitable command is transmitted which is received by the satellite antenna and is fed to a command receiver. This receiver amplifies the telecontrol command signal and feed it to the block labelled command- Website- www.kavediasir.yolasite.com 19

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITdecoder. The output of command decoder is used to control the altitude, the orbit and the antenna.

Main and Auxiliary Propulsion Sub-System The launching of satellite into an approximate desired orbit is carried

out with the help of large rocket. This rocket may carry several satellites at a time. After it is launched into an approximate orbit the satellite uses its own main propulsion system to go into the final orbit. The satellite should attain its correct orbit in not more than two attempts as there is limited fuel available. Once the satellite has achieved the correct orbit, auxiliary rocket engines are used to turn the satellite so that its position between sun and earth is as per desired value. In case of satellite the earth, moon, sun influences the satellite and hence such changes are required to be monitored continuously. This is achieved with the help of thruster with ground station.Shown in fig.5.15.

This can be done with the main five sub-systems as (a) sensor, (b) propulsion system, (c) altitude control, (d).antenna (e)-orbit-control.

Altitude control and orbit control are used to control the orbit of the satellite beside helping to maintain stabilization and its position. The control can be made from the satellite itself (automatically) and from ground control.The sensors sense the various changes in parameters of the orbit of the satellite and the error signals generated by the sensors are fed to altitude control, orbit control and antenna control, to suitably adjust these three. The three parameters can also be controlled by ground control using telecontrol. The sensor signals can also be transmitted to the ground with the help of telemetry telecontrol section.

Communication Channel Subsystem



Fig.5.16. Block diagram of Satellite Transponder

The signal beam from the ground station is intercepted by the a and is fed to a receiver. This receiver has a tunnel diode front end RF amplify the received signal Signals from earth stations are received over the frequency range from 5.93 to 6.42 GHz in parallel Transponder. The output received is fed to a frequency converter which is usually a down converter shown in fig.5.16. The received signals so down converted to the range of 3.705 to 4.19 GHz and is fed to TWT power amplifier. The .overall amplification achieved lies in the range of 125 to 150 dB. The amplified output is beamed back to the earth using high-gain antennas.

The transponder uses demodulation before downlink to avoid any interference. It also provides security. After demodulation the signals are encoded at transponder to provide privacy and secrecy.

Antenna

Antennas The satellite uses separate uplink and downlink antenna as their frequency and directions are different. The satellite uplink and downlink antenna should provide high gain and proper coverage i.e. beam width. The high antenna provides narrow focus beam and hence reduces the interference

but it requires higher accuracy in position or coverage.The satellite antenna may use different types of microwave antennas like linear dipole, helix horn, parabolic reflector etc. The use of array of small dipoles provides low gain. The parabolic reflector or horn antenna provides high gain. The antenna is so designed that it provides high directive gain; Website- www.kavediasir.yolasite.com 21

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITlarge bandwidth and minimum side lobes. Various sub-systems of satellites with their functions and parameters can be summarized below.

Earth station satellite are basic element of satellite communication system. Fig.Shows block diagram of earth station/ It is the link between land and network satellite. It collects various signals from land network, process them and transmit them to satellite through antenna in the form of uplink. Similarly it receives signal, separate them and distribute them to land network to make them to each respective destination.

Q.Draw and Describe the block diagram of Earth station of satellite communication.Ans.Transmitter

Bandwidth of each base band channel like voice, fax , telex , teleprinter etc is 4khz. These analog signal are converted into digital signal using encoder 24 such channel are multiplexed together by using TDMA .Many such group of 24 channels are formed. It depends on number of transponder and capacity of each transponder.

Each group modulate carrier signal of certain frequency. The type of modulation used is frequency modulation. Various carrier frequencies are allotted to each group .these modulated signal of various frequencies are FDM signal. It multiplex computer data, control signal, also frequency modulate carrier signal of certain frequencies. As frequencies of those FM signal are different, they are multiplexed by using FDMA technique. Control signal are used for tracking the satellite and for monitoring various parameters of satellite. Carrier frequency is increased to uplink frequency by using up converter. Microwaves are amplified first by driver stage and then by HPA. Multicavity Klystron or Traveling Wave Tube are used to amplify wide band microwaves to high power level. Transmit power per 40 MHz channel is typically of the order of 5 to 10 W. High power microwave signals are carried HPA to polarized diplexer through waveguide. Diplexer connects HP transmitting signals to antenna but it does not allow to leak these signals to LNA. Similarly received signals are directed LNA but does not allow to leak them to HPA Diplexer enables to use only one antenna for transmission and reception without interference of transmitting .signals with LNA and receiving signals with HPA.

Transmitting signals are fed to parabolic dish antenna through diplexer. Cassegrain horn feed parabolic dish antenna is very commonly used. Antenna converts transmitting signals into parallel beam of electromagnetic waves called uplink

ReceiverWebsite- www.kavediasir.yolasite.com 22

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITSatellite directs electromagnetic waves to another earth station, if it is

meant for point to point communication, otherwise it will spread signals all over its foot print Parabolic dish antenna of another earth station is also highly directed towards satellite. Due to its large diameter, its gain is high, it collects large amount of signal, and concentrates them on horn feed. They are coupled to diplexer through waveguide which directs them only to LNA of receiver, but not allowed to reach to HPA transmitter.

LNA (Low Noise Amplifier)It is wide band, small signal, class A, low noise tuned multistage

amplifier. It is tunned frequencies. It is enclosed in constant temperature enclosure which is maintained at low temperature for low noise. It is mounted on antenna or very close to antenna to minimise attenuation of received signals before amplification. LNA is front end amplifier, therefore any noise introduced by it will get amplified by all succeeding amplifiers and finally S/N ratio becomes less than required level. Their carrier frequency is reduced by down converter. They are demultiplexed by using FDMA demultiplexer. Demultiplexed signals are FM signals which are demodulated by FM demodulator. We get base band signals. To get base band voice signals demultiplexed and demodulated signals are decoded by decoder. All base band signals like voice, fax, telex, teleprinter, videophone, T.V., computer data etc. are sent to their respective destination through land network or terrestrial network.





Fig.5.18.Block Diagram of Earth StationAntenna

An antenna, or aerial, is a device to send or receive signals. Antenna comes from a Latin word meaning “sail yard,” and it has two plurals: antennae and antennas. There are many antenna types and many ways of categorizing them.

The major categorical division of antenna types is between those that transmit signals, known as a transmitting antennas, and those that receive signals, called receiving antennas. It is also possible to have antennas that are made to both transmit and receive. Usually, transmitting antennas handle a good deal more electrical energy than receiving antennas.

Definition and functions of antenna• Antenna is a met object, often a wire or collection of wire which is

used to perform the following functions1. It couples the transmitter output to the free space, or the received

input to the receiver.2. It must be capable of radiating or receiving the electromagnetic waves.3. It converts the high frequency current into electromagnetic waves and

vice versa.. Even though the functions to be performed arc different, the transmitting and receiving antennas basically share the same principle and a identical.

• If RF current flows in a wire conductor ,it is found that not all the energy applied at one end finds its way to the other end Some of the energy escapes’ i.e. is radiated.

• It is possible to calculate the amount of energy escaped, its direction or directions of propagation etc. using the Maxwell’s equations.

Q.List and describe the Characteristics of AntennaAns.An antenna is a device that is made to efficiently radiate and receive radiatedelectromagnetic waves. There are several important antenna characteristics that should beconsidered when choosing an antenna for your application as follows:

1. Antenna radiation patterns2. Power Gain3. Directivity4. Polarization

Antenna Radiation PatternsAn antenna radiation pattern is a 3-D plot of its radiation far from the

source. Antenna radiation patterns usually take two forms, the elevation Website- www.kavediasir.yolasite.com 25

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITpattern and the azimuth pattern. The elevation pattern is a graph of the energy radiated from the antenna looking at it from the side as can be seen in Figure 1a. The azimuth pattern is a graph of the energy radiated from the antenna as if you were looking at it from directly above the antenna as illustrated in Figure 1b. When you combine the two graphs you have a 3-D representation of how energy is radiated from the antenna (Figure 1c.).

Power GainThe power gain of an

antenna is a ratio of the power input to the antenna to the power output from the antenna. This gain is most often referred to with the units of dBi, which is logarithmic gain relative to an isotropic antenna. An isotropic antenna has a perfect spherical radiation pattern and a linear gain of one.

DirectivityThe directive gain of an

antenna is a measure of the concentration of the radiated power in a particular direction. It may be regarded as the ability of the antenna to direct radiated power in a given direction. It is usually a ratio of radiation intensity in a given direction to the average radiation intensity.

Fig.5.19.Directivity and PolarizationPolarization

Polarization is the orientation of electromagnetic waves far from the source. There are several types of polarization that apply to antennas. They are Linear, which comprises, Vertical, Horizontal and Oblique, and circular, which comprises, Circular Right Hand (RHCP); Circular Left Hand (LHCP), Elliptical Right Hand and Elliptical Left Hand. Polarization is most important if you are trying to get the maximum performance from the antennas. For best performance you will need to match up the polarization of the transmitting antenna and the receiving antenna. Shown in fig.5.19.Note: Clockwise rotation of the Electromagnetic wave is right-hand polarization; counterclockwise rotation is left-hand polarization.Types of Antenna


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITQ.List the different types of Antennas used in satellite communcaitonAns.There are many different types of antennas. Antennas most relevant to designs at2.4GHz that are further detailed are as follows:

1. Dipole Antennas2. Multiple Element Dipole Antennas3. Yagi Antennas4. Flat Panel antennas5. Parabolic Dish antennas6. Slotted Antennas7. Microstrip Antennas

Q.Write short note on Horn AntennaAns.A horn Antenna is capable of radiating energy into open space, if it suitably excited at one end and open at the other. This radiation is much greater than that obtained from the two wire transmission line. The end of the waveguide represents an abrupt transition from the characteristics impedance of the waveguide into that of a free space. This state affairs can be improved by flaring out the end of the waveguide to form a horn like structure- A gradual transition can thus take place as the wave, passes from the mouth of the horn.Narrow mounted horns with long, flare sections produce sharper beams than shallow wide mounted horn.

Three types of horns are shown in Fig.5.20. 1. The first is the sectoral horn, which is, flared in only one plane. 2. The second is the pyramidal horn, which is flared in both planes. 3. The third type is conical horn which is used for circular

waveguides. The horn antenna is not nearly as directive as an antenna with

parabolic reflector, but it has good directivity, an adequate band width and simple

construction; Horn antennas are used when the frequencies are in microwave range.



.

Fig.5.20 Different Horn AntennaHorn antennas are used typically in the microwave region (gigahertz range) where waveguides are the standard feed method, since horn antennas essentially consist of a waveguide whose end walls are flared outwards to form a megaphone like structure.

Horns provide high gain, low VSWR, relatively wide bandwidth, low weight, and are easy to construct. The aperture of the horn can be rectangular, circular or elliptical. However, rectangular horns are widely used. The three basic types of horn antennas that utilize a rectangular geometry are shown in Figure . These horns are fed by a rectangular waveguide which have a broad horizontal wall as shown in the figure.5.20. For dominant waveguide mode excitation, the E-plane is vertical and H-plane horizontal.

If the broad wall dimension of the horn is flared with the narrow wall of the waveguide being left as it is, then it is called an H-plane sectoral horn antenna as shown in the figure. If the flaring occurs only in the E-plane dimension, it is called an E-plane sectoral horn antenna. A pyramidal horn antenna is obtained when flaring occurs along both the dimensions. The horn basically acts as a transition from the waveguide mode to the free-space mode and this transition reduces the reflected waves and emphasizes the traveling waves which lead to low VSWR and wide bandwidth. The horn is widely used as a feed element for large radio astronomy, satellite tracking, and communication dishes.

Q.Describe with neat sketch Paraboloidal Reflector Antenna (Dish Antenna)


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITAns.The most widely used antenna for microwave applications is the paraboloidal reflector antenna, which consists of primary antenna such as dipole or horn situated at the focal point of paraboloidal reflector. The parabolic reflector is known as secondary antenna.

Radiation from Parabolic Reflectors with Focal Feed Point:A parabolic reflector is used to concentrate the radiation from an

antenna located at the focus in the same way as a search light reflector produces a sharply defined beam of light. The parabolic reflector does this by converting the spherical waves originated by the radiator wave across the mouth or aperture of the parabola. The ratio of focal length to operate size (D) i.e. ‘D ratio is an important characteristics of parabolic reflector. By the geometry of parabola, all the waves originating from the radiator kept at the focus irrespective of any inclination with axis of parabola will be reflected parallel to the parabolic axis Thus, all the waves reaching at the aperture place are in phase and hence a very strong and concentrated beam of radiation along the axis at the parabola is produced.

WorkingElectromagnetic waves are radiated from focal point when antenna

transmits signal energy from HPA output. Reflecting surface of paraboloid reflector is illuminated. It reflects and focuses energy into narrow beam. When antenna work in - receiving mode, parallel rays coming from satellite falls on reflector then they are the reflected and concentrated on feed which is placed at the Focus. Signal are then send to input of OMT(Ortho Mode Transducer) through waveguide from feed point to OMT. It directs these signal to LNA but not to HPA(High Power Amplifier). This kind of paraboloid is easily steered and offers very high gain.

In order to increase power gain and reduce beam width large antennas are used, Antenna of 32 m diameter is used in 6/4 GHz system. Diameter of antenna used in Vikram satellite earth station are also 32 m. Its power gain for transmission is 64dB and for reception about 60 db. Diameter of antenna can not be increased beyond certain limit otherwise it will interfere with adjacent satellite.

In order to increase capacity and efficiency of satellite, various techniques such as TDMA, FDMA, SCPC, CSSB, analog FM/FDM, CDMA, DA-FDMA (Demand assignment), DA-TDMA, CDMA, spectrum techniques, CSMA etc. are used.



Q.Describe with neat diagram Parabolic Hornfeed Cassegrain Antenna

Fig.5.21. Parabolic Dish AntennaParabolic dish antenna shown in fig.5.21.It uses two reflectors

(1) Paraboloid main reflector and (2) subreflector,

the arrangement is shown in Fig. Focal point of main reflector and virtual focal point of sub reflector coincides. Feed is mounted in such a way that its phase centre coincides with real focal point sub reflector.

WorkingWhen antenna is in transmit mode, antenna, emits energy from HPA

Feed emits E-M waves and illuminates convex reflecting surface of paraboloid main reflector, from where they are reflected in space in form of almost parallel beam with very less beam width. In case the antenna is in receive mode, the signal energy captured by the main reflector is directed towards its focal point. But there is sub reflector between, which reflects rays back to concentrate them at the phase centre of feed. Feed receives this energy which is then routed to the input of LNA(Low Noise Amplifier) through OMT(Ortho Mode Transducer) .

Q.List the Applications of SatelliteAns. The Applications of Satellite are

1. Satellites are used for telecommunication, radio, T.V. transmission. One satellite can provide more than 25,000 voice channels, two T.V. channels, few number of wide band, computer data channels etc. and


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITfew channels for radio transmission. It means satellite has large capacity of communication.

2. Satellite provide information regarding weather, make forecast about rains and cyclones.

3. Satellite can give important information about the mineral deposits in earth and also deep inside the sea.

4. It provides maritime remote sensing etc.5. Satellites are also used by defense departments of many countries for

communication. surveillance and spying;6. Satellites are also used for mobile communications7. satellite have proved to be very useful in search and rescue

operations of affected ships in sea, aircraft, helicopters, navigations etc.

Q.List advantage and disadvantages of satellite communicationAns.Advantages of Satellite Communication

Before the advent of communication satellites, SW (3-30Mhz) cascaded relays were used for long distance communication. Similarly coaxial cables, microwave links and optical fiber links are used for distant communication. Following are some of the advantages of satellite communication over terrestrial and cable communication system:

1. Satellite systems are wide range broad cast systems because point to point and point to multipoint communication is possible, whereas only point to point communication is possible through other systems.

2. Capacity of satellite communication is large about 1500 voice channels, sortie data channels and one or two TV channels.

3. Once the satellite is in position, the earth stations can be installed and communication may be established in days or even in hours. Thus a station may be removed relatively quickly from one location and re-installed.

4. Mobile communication can be easily achieved by satellite communications.

5. Many type of communications are possible through satellite i.e. voice, telegraph, computer data, fax, video conferencing, TV etc.

6. For search rescue and navigation efforts satellites offer the advantages which no other system can offer.

7. Degradation of signal with increase in distance of receiving station from transmitting station does not take place.

8. For remote areas as well as hilly areas like Himachal Pradesh, Ladakh, Sikkim, Assam etc. and communication between the islands and the mainlands, satellite communication is the only cost effective option.


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMIT9. Satellite communication economical for long distance communication

as compared to other communication system.

Disadvantages of Satellite Communication:1. Launching and positioning of satellite is costly elaborate and needs

high technology.2. Repair is nearly impossible after launching the satellite.3. Communication path between the terrestrial transmitter and receiver

is approximately 75,000 km/s, there is delay of ¼ second between transmission and reception of a signal. Thus there is an elapse of ½ second between talks. The delay causes an echo. Therefore echo suppressors ate used in satellite communications. It - reduces the efficiency o satellite in data transmission.

Access Technology

Q.What is Access Technologies ?List different types of Access TechnologyAns.As one of the major problems facing the development of telecommunications, bandwidth demand has driven the search for protocols that could be used to maximize bandwidth efficiency. Multiple accesses ("multiplexing" for short) enable multiple signals to occupy a single communications channel.

The multiple access methods are used in the satellite networks, cellular and mobile communication networks and underwater acoustic networks. One type of multiple access system in which a large number of users sharing the same communication channel is shown in Fig.5.23.

The common channel can be the uplink in a satellite communication system or it can be a cable or it can be a frequency band.

For example in a mobile communication system, the user of the network will be the transmitters and the receiver is in the base station.

The second type of multi-user communication system is as shown in Fig.5.24

In the second type multi-user system of Fig. a single transmitter (satellite) sends information to multiple receivers. The other examples of such system are the radio and TV broadcast systems.Website- www.kavediasir.yolasite.com 32

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITThe multiple access and broadcast networks are the most common multi-user communication system. The third type of multi-user system is a store-and-forward network shown in Fig.5.25.

In this system the communication takes place in all the possible directions.

Fig.5.24.Access Technology Fig.5.25.Access Technology

Fig.5.25.a.Bidirectional Communication

There are three basic types of division-based protocols used to do this: 1. Frequency division multiple access (FDMA), 2. Time division multiple access (TDMA) and 3. Code division multiple access (CDMA).

FDMA-Frequency Division Multiple Access

Q.What is FDMA( Frequency Division Multiple Access) ?State its disadvantages?Ans.Is the most common analog system. It is a technique whereby spectrum is divided up into frequencies and then


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITassigned to users. With FDMA, only one subscriber at any given time is assigned to a channel. The channel therefore is closed to other conversations until the initial call is finished, or until it is handed-off to a different channel. A “full-duplex” FDMA transmission requires two channels, one for transmitting and the other for receiving. FDMA has been used for first generation analog systems shown in fig.5.26.

With Frequency Division Multiple Access (FDMA), different signals are assigned frequency channels. A channel is a frequency. FDMA is a basic technology in the analog Advanced Mobile Phone System (AMPS). With FDMA, each channel can be assigned to only one user at a time. FDMA is also used in the Total Access Communication System (TACS).

Fig.5.26. Frequency Division Multiple Access FDMA Technique

One of the simplest multiple access methods is the frequency division multiple access (FDMA). In this method. the channel bandwidth is subdivided into a number of sub channels as shown in Fig.5.27. There are N non-overlapping sub channels, as shown in Fig.5.27. This method is therefore called as frequency division multiple access and it is commonly used for the voice and data transmission.

Fig.5.27.Freequency Band AllocationFig.5.28. Guard Band

The overall channel bandwidth is being shared by the multiple users. Therefore a number of use’s can transmit their information simultaneously.

The adjacent frequency bands in the FDMA spectrum are likely to interfere with each other. Therefore it is necessary to include the guard bands between the adjacent frequency bands as shown in Fig.5.28.

No code words and synchronization is not required. Power efficiency is reduced. FDMA is an old and proven system and is used for the analog


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITsignals.

Disadvantages of FDMAa. It is suitable only for analog signals and does not lend itself

well to implement in software.b. Due to simultaneous transmission of a large number of

frequencies there is a possibilities of inter modulation distortion at the transponder.

c. Large bandwidth requirement for transponders, also bandwidth is wasted in guard band

d. Storage, enhancement of signals is not possible.e. The station must be power controlled. If there is too much

power in main band , it will automatically fill power in the side band spilling over into adjacent channel and causing interference.

TDMA - Time Division Multiple AccessQ.What is TDMA( Time Division Multiple Access) ?State is Advantages and disadvantagesAns.Improves spectrum capacity by splitting each frequency into time slots.

TDMA allows each user to access the entire radio frequency channel for the short period of a call. Other users share this same frequency channel at different time slots. The base station continually switches from user to user on the channel. TDMA is the dominant technology for the second generation mobile cellular networks. Shown in fig.5.29.

Fig.5.29.Time Division Multiple Access

TDMA is utilized by Digital-Advanced Mobile Phone System (D-AMPS) and Global System for Mobile communications (GSM). However, each of these systems implements TDMA in a somewhat different and incompatible way.

TDMA Technique


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITAnother method of creating multiple subchannels for multiple access

is by subdividing the time duration T called as the frame duration into N non-overlapping subintervals each of duration T/N.

After that each user who want to transmit information is assigned a particular slot, within each frame. This method is known as time division multiple access TDMA.

Fig.5.30.Time Slice

TDMA is used for the transmission of data and digital voice signals.It is necessary to include guard times’’ between the adjacent channels

as shown in Fig. 5.30.Synchronization is necessary in TDMA. Power efficiency of TDMA is

better than that of the FDMA.TDMA is a method of time division multiplexing the digitally modulated carriers between various earth stations in a satellite network through a common satellite transponder.

Each earth station transmits a short burst of digitally modulated carrier during the lime slot assigned to it in the TDMA flame. Such a time slot is called as epoch. The burst of each earth station is synchronized so that at any instant of time, only one earth station’s carrier is present in the transponder.

The transponder receives this carrier, amplifies it and relay it back to all the earth stations. Thus each earth station receives the bursts from all other stations.

Advantages of TDMAa. It can easily adapt to transmission of data as well as voice communication. b.c. At any instant of time, the carrier from only one station is

present at the transponder. This reduces the inter modulation distortion

d. TDMA is suitable for transmission of digital informatione. It is possible to store the digital information, change the rate etc.

in TDMA.f. This allows the operator to do services like fax, voice band data, and SMS

as well as bandwidth-intensive application such as multimedia and videoconferencing.

g. Since TDMA technology separates users according to time, it ensures that there will be no interference from simultaneous transmissions


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITDisadvantages

1. Precise synchronization is required.2. Bit and frame timing must be achieved and maintained.3. One major disadvantage using TDMA technology is that the users

has a predefined time slot. When moving from one cell site to other, if all the time slots in this cell are full the user might be disconnected. Likewise, if all the time slots in the cell in which the user is currently in are already occupied, the user will not receive a dial tone.

4. Another problem in TDMA is that it is subjected to multipath distortion. To overcome this distortion, a time limit can be used on the system. Once the time limit is expired the signal is ignored.

Q.Describe the Problems associated with FDMA and TDMAAns. The Problems associated with FDMA and TDMA are

1. The problem with the FDMA and TDMA system is that, the channel is basically partitioned into independent single user subchannels.

2. That means each subchannel in the FDMA is allotted to a single user and each time slot in TDMA has been allotted to a separate single user

3. The FDMA and TDMA systems however prove to be inefficient when the data from the users is bursty in nature as shown in Fig.

4. This type of data has low value of duty cycle, i.e. the time for which data is being transmitted is much shorter than the silent time.

5. Under such, circumstances where the transmission from different users is bursty and low duty cycle, the FDMA and TDMA system will be inefficient, This is because a large percentage of the available time or frequency slots do not convey any information.

Fig.5.31.Data Burst

6. Such a type of data is observed in computer communication networks and to some extent, in the mobile cellular communication systems earning digitized voice.

CDMA - Code Division Multiple Access

Q.What is CDMA( Code Division Multiple Access )?State the advantages and disadvantagesAns.CDMA is based on “spread” spectrum technology. Since it is suitable for encrypted transmissions, it has long been used for military purposes.


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITCDMA increases spectrum capacity by allowing all users to occupy all channels at the same time. Transmissions are spread over the whole radio band, and each voice or data call are assigned a unique code to differentiate from the other calls carried over the same spectrum. CDMA allows for a “soft hand-off”, which means that terminals can communicate with several base stations at the same time. The dominant radio interface for third-generation mobile, or IMT-2000, will be a wideband version of CDMA with three modes (IMT-DS, IMT-MC and IMT-TC).

Fig.5.32.FDMA , TDMA , CDMAIn Code Division Multiple

Access (CDMA), each user is assigned a different pseudorandom binary sequence that modulates the carrier, spreading the spectrum of the waveform and giving each user a unique code pattern.This technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.

CDMA TechniqueAn alternative to FDMA and TDMA is an another system called code

division multiple access (CDMA). The most important feature of CDMA is as follows. In ( more than one user is allowed to sham a channel or subchannel with the help of direct-sequence spread spectrum signals. In CDMA each user is given a unique code sequence or signature sequence. This sequence allows the user to spread the information signal across the assigned frequency band.shown in fig.5.33.Website- www.kavediasir.yolasite.com 38


Fig.5.33 CDMA TechniqueAt the receiver the signal is recovered by using the same code

sequence. At the receiver, the signals received from various users ale separated by checking the cross-correlation of the received signal with each possible user signature sequence. In CDMA the users access the channel in a random manner. Hence the signals transmitted by multiple uses will completely overlap both in time and in frequency. The CDMA signals are spread in frequency. Therefore the demodulation and separation of these signals at the receiver can be achieved by using the pseudorandom code sequence. (DMA is sometime also called as spread spectrum multiple access (SSMA).

CDMA as he bandwidth as well as time of the channel is being shared by the users, it is necessary to introduce the guard times and guard hands as shown in Fig. CDMA does not need any synchronization, but the code sequences or signature waveforms are required Full power efficiency is possible to attain even when the data is of bursty nature with a low duty cycle. This is the most important advantage of CDMA over FDMA and TDMA.

Advantages of CDMA1. Entire bandwidth can be used for every station.2. Immunity to interference (Jamming).3. One of the main advantages of CDMA is that dropouts occur only

when the phone is at least twice as far from the base station. Thus it is used in the rural areas where GSM cannot cover.

4. Another advantage is its capacity; it has a very high spectral capacity that it can accommodate more users per MHz of bandwidth. It uses a vocoder EVR C for noise reduction where the background noise is reduced. This is exclusively available in CDMA technology only.

Disadvantages1. Reduced efficiency due to coding.2. Synchronization and precise liming is essential3. One major problem in CDMA technology is channel pollution, where

signals from too many cell sites are present in the subscriber’s phone but none of them is dominant. When this situation arises the quality of the audio degrades.

4. Another disadvantage in this technology when compared to GSM is the lack of international roaming capabilities. The ability to upgrade or change to another handset is not easy with this technology because the network service information for the phone is put in the actual


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITphone unlike GSM which uses SIM card for this. One another disadvantage is the limited variety of the handset, because at present the major mobile companies use GSM technology.

Q.Comparison of FDMA , TDMA ,CDMA

Q.Different access Technology in Digital Communication

Mobile and Cellular

Introduction to Mobile and Cellular technology


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITMobile telephone services began in the l940s and were called MTSs (mobile telephone systems or sometimes manual telephone systems, as all calls were handled by an operator). MTS systems utilized frequency modulation and were generally assigned a single carrier frequency in the 35-MHz to 45-M range that was used by both the mobile unit and the base station. The

mobile unit used a push-to-talk (PTT) switch to activate the transceiver Depressing the PTT button turned the transmitter on and the receiver off, whereas releasing the

Fig.5.35. Different Mode of Mobile CommunicationPTT turned the receiver on and the transmitter off. Placing a call from

a MTS mobile telephone was similar to making a call through a manual switchboard in the public telephone network. When the PTT switch was depressed, the transmitter turned on and sent a carrier frequency to the base station, illuminating a lamp on a switchboard.


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITAn operator answered the call by plugging a headset into a jack on the

switchboard, After the calling party verbally told the operator the telephone number they wished to call, the operator connected the mo bile unit with a patchcord to a trunk circuit connected to the appropriate public telephone network destination office. Because there was only one carrier frequency, the conversation was limited to half-duplex operation and only one conversation could take place at a time. The MTS system was comparable to a party line, as all subscribers with their mobile telephones turned on could hear any conversation. Mobile units called other mobile units by signaling the operator who rang the destination mobile unit. Once the destination mobile unit answered, the operator disconnected from the conversation, and the two mobile units communicated directly with one another through the airways using a single carrier frequency.

In 1964, the Improved Mobile Telephone System (IMTS) was introduced, which used several carrier frequencies and could, therefore, handle several simultaneous mobile conversations at the same time. IMTS subscribers were assigned a regular PSTN telephone number; therefore, callers could reach an IMTS mobile phone by dialing the PSTN directly, eliminating the need for an operator. IMTS and MTS base station transmitters outputted powers in the 100-W to 200-W range, and mobile units transmitted between SW and 25W Therefore, IMTS and MTS mobile telephone systems typically covered a wide area using only one base station transmitter.

Q.Write short note on Mobile CommunicationAns.Telephony can be used successfully for the long distance voice communication This communication based system has many limitation such as service capability poor service performance , under utilization of the frequency’ spectrum, feasibility, service reliability, spectrum efficiency etc. On the other hand mobile phone provides good service quality, effective use of frequency channel sharing, reuse of frequency channel etc.Shown in fig.5.36.

Cellular mobile telephony is designed to provide stable communication between two moving devices or between one mobile unit and one stationary (land) unit. service provider must be able to locate and track a caller, assign a channel to the call and transfer the signal from channel to channel as caller moves out off range of the one channel and into the range of another.

Fig.5.36. Bands and Channels in mobile communication


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITFrequency Band and Type of Modulation use for Mobile Communication

The FCC(Federal Communication Commission) allocated 800MHz band with 40MHz system for mobile radio communication. Generally traditional cellular band transmission is analog. To minimize noise, frequency modulation (FM) is used for communication between the telephone itself and the cell office. The FCC has assigned the two bands namely between 824 - 849MHz carries those communications that initiates from mobile phones. The band between 869 - 894MHz carries those communication that initiates from land phones.. Carrier frequency are spaced every 30 KHz allowing each band to support 833 carrier. However two carriers are required for full duplex communication which doubles the required width of each channel to 60 KHz and leave only 416 channels available for each band as cellular band.

Each band hence is divided into 416 FM channels. Of these, some are reserved. for control and. set up data rather then voice communication. In Addition to prevent interference, the channels are distributed among the cells in such a way that adjacent cells do not use the same channels. This restriction means that each cell normally has access to 40 channels only.

In 1970, the FCC decided to allocate 75 MHz for a wire-line common carrier. In 1980, FCC reconsidered its one-system-per market strategy and brought in two licensed carriers per service area. This two licensed carrier per service, area resulted into decrease in trunking efficiency. The advantage of this system was the frequencies were assigned in 20MHz groups identified as Block A and Block B or Band A and Band B as given in Table1.

The Band A was served for the wire line companies i.e. telephone. Each company divides the area into geographic area ie cell. Each of this cell operates in its own bands. The band width specified per channel is 30 KHz. Each band consists of 333 channels.

Q.Draw the block Diagram of Cellular Phone System ?Describe itAns.The basic cellular system is made up of three sub systems namely mobile unit a cell site, and a mobile telephone switching office (MTSO). The Fig.5.37.shows the inter connection of these sub systems.



Fig.5.37.Mobile Phone system

Mobile Unit A mobile telephone

unit is a portable handset unit consists of control unit, Transreceiver and antenna system. The power level can be controlled by the base station so that strength of the signal is minimum required which help in avoiding interferences. These mobile units can be used for wireless data connections and wireless internet access. They can also be used as two way digital massaging units.

Cell Sites To make tracking possible each cellular service area is divided into

small region called cell. Each cell contains antenna and is controlled by a small office called cell office. This sub system provides interface between MTSO and the mobile unit. It has control unit, radio cabinets, antenna, a power plant and data terminals.

Mobile telephone switching office (MTSO). This is heart of the cellular mobile system. It consists of switching

office the central coordinating element for all cell sites, cellular processor and cellular switch. This unit helps to interface with telephone company zone offices, it controls call processing and dose billing activities.

The processor of MTSO is used for central coordination and cellular administration. MTSO is computerized centre that is responsible for connecting calls as well as recording call information and billing.

The cellular switch used may be analog type or digital type. It helps to connect the one mobile subscriber to other mobile subscriber, and also nationwide telephone network, It uses data links for control and supervision between processor and the, switch and between the cell site


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITand the processor. The radio link carriers the voice and signaling between the mobile unit and the cell site. Factors controlled by MSTO

1. Control of transmitter output power2. Monitoring of the received signal strength3. Frequency division ratio

Connections The radio and high speed data links connects the three sub systems. Each mobile unit can use only one channel for its communication at a time. The channel may be any one in entire band assigned by the serving area.

Q.Draw Block Diagram of Cellular Mobile Unit and describe itFig. shows the general block diagram of a cellular phone system.The five major parts of this system are

1. Control unit2. Logic unit3. Transmitter4. Receiver5. Frequency synthesizer.

Fig.5.38.Block diagram of Mobile Unit The system consists of five major sections , shown in fig.5.38.1. Transmitter

Transmit the original information through antenna to the receiver.

2. ReceiverThe receiver is dual-conversion super heterodyne. The transmitted signal is demodulated here and is sent to the cell cite so that MTSO can monitor the received signal from the cell and make decisions about switching to another cell.

3. Frequency synthesizerThe frequency synthesizer section develops all the signals used by transmitter and receiver. This sets the transmit and receive channel frequencies.

4. Logic unitWebsite- www.kavediasir.yolasite.com 45

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITThe logic unit contains the master control circuitry for cellular mobile.

It is made up of a microprocessor with both RAM and ROM puts additional circuitry used for interpreting signals from the MTSO/cell site and generating control signals for the transmitter and receiver.

5. Control unit The control unit contains the handset with speaker and microphone.

The control unit operated by a separate microprocessor that drives the LCD display and other indicators. It also implement all manual control functions.

Q.Descrine in detail Block Diagram of Analog Cellular Phone SystemAns.The block diagram is shown in Fig.5.39. It is seen that single antenna is used for the transmitting and signal. To achieve this successfully special duplex filter is used which connects the transmitter circuitry, receiver circuitry and antenna. It directs the flow of signal power so that the receiver is not overloaded by high power transmitter OR as well as it directs the received signal from the antenna.

The link between the voice. Transceiver circuitry and cellular processor controlling circuit occurs at three different points namely, first, the point where controller extracts base station messages from The received signal and also sends out



Fig.5.39. Cellular Phone SystemIts status and identification signal, secondly the point at where the frequency synthesizer sets the receiver and transmit channels and them if required and finally at point where the output power. is controlled, as shown Fig.5.39. The FCC uses frequencies in the range 825-890Mhz.The cellular phone uses frequency band of 825 to 845 MHz for transmission while reception is in the range of 870 to 890 MHz. This shows that 45 MHz separation is used between transmit and receive channel pair. eg. Channel transmit/receive is 825.015/ 870.015 MHz.

The voice signal i.e. in the range of 300 Hz to 3000 Hz is transmitted as narrow band FM with ± 12 KHz frequency deviation having 100% modulation. The signal bandwidth required is 30Khz as calculated as twice the sum of carrier deviation plus signal bandwidth i.e. 2 x (12 KHz + 3khz) = 30 Khz. In cellular transmission due to the high demand of channel no guard band between channel is used, while channels. are assigned every 30 KHz. The cell phone receiver has selectivity of 50 dB to avoid adjacent channel interference while its sensitivity is about 1mv. The total 665 channels to transmit and 665 channels to receive, are assigned in the allocated frequency spectrum.

Mobile Phone

Q.Describe the Concept of Mobile Phone


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITAns.In privately owned radio system there is no automatic - connection to the telephone network. -It requires an attendant or the dispatcher i.e. -operator. In modem common carrier mobile telephone system the user is provided either rotary dial or push button instrument and thus can function as a land line -telephone user. Some mobile system use operator and switch board to provide a manual interface. All the mobile units which are not involved in the call, keep finding through the various available - channels and lock onto one which system logic has - currently marked with an -idle tone. ALE the incoming and outgoing signals are routed through this marked channel. If this marked channel is seized for the use by one of the subscriber the idle tone automatically moves to the another available

channel to be ready for use for the call The signaling at beginning of each call, using tone pulses recognizes

the Calling or the called mobile unit. If called the mobile then rings on the given dial tone as normal telephone. In mobile telephony the end of the call is clearly, marked with disconnect tone to prevent incorrect billing

The Federal Communications Commission(FCC) has allocated portion of frequency spectrum in 2 - 35 MHz, 150 - 450 Mhz and 850 MHz bands to be used for communication with moving or temporarily stationary vehicles on the land. This allocation is divided as shown in TableThe concept of mobile system takes into account various parameters like Effective Radiated Power (ERP). It is expressed in dBW. It may be given by transmitter output power minus losses external to the transmitter minus loss in transmission -line plus- gain in transmitting antenna.

QDefine the terms Sensitivity ,Site noise Ans.Sensitivity

It is measured at 12dB for modulated signal or at 20 dB for no modulated signal.

Site Noise In most locations external noise caused by audio ignition, power. line,

co-channel interference etc. limits the receivers sensitivity. The mobile


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITcommunication then uses the typical Levels of RF signal powers required for adequate audio signal for the narrow band FM as given in Table.

The local environment like building and hills and mountains adds to the losses.

Q.Draw and describe block Diagram of Mobile System and describe its OperationAns.

The basic block diagram for the mobile phone system consists of the transmitter, the antenna. Practically all equipment built for the service satisfies the EIA (Electronic industries Association) Standards.Transmitter The transmitter used for Land line mobile radio service are based on FM specifications. It consists of stable crystal oscillator, an audio section, a phase modulator, the multiplier section and power amplifier whose output is given to the antenna. The block diagram is as shown in fig.5.40, which consists of transmitter, receiver and antenna.

The crystal oscillator provides reference frequency. The frequency synthesizer provides various frequencies from stable source. To provide the matching between the input which is obtained from microphone, to the transmitter is carried out by shaping and limiter circuit Frequency multipliers are used for multiplying and amplifying the frequency. The power amplifier Class ‘C’ amplifier.


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITFig.5.40.Transmitter of mobile radio service

Receiver In most receiver the synthesizer output which is fed to the transmitter

modulator is multiplied and used in the mixer to demodulate the receiver signal at first IF at frequency about 5 MHz i.e. separation between the land transmit and mobile transmit channel assignment while in mobile telephony typically at 850 MHz separate frequency synthesizers are used. .

After first IF amplifier signal is given to second IF amplifier stage, the output second IF amplifier is given to the filter circuit The frequency discriminator and de-emphasis network provides separate voice and supervisory signals. The sensitivity of the circuit is about 5mV, with 90 dB adjacent channel selectivity.

AntennaThe mobile antenna may be a quarter wave whip which can be placed at the top of the roof of the vehicle or half wave coaxial antenna mounted on trunk. A single mobile antenna is generally used for the transmitting and receiving which is decoupled by diplexer.

The 450-850 MHz plane which Uses two frequencies for two directions with spectrum conversion the base station can work as repeater for mobile to mobile calling with diplexer as shown in Fig.5.40.

Call ProcessingQ.Describe How call processing is done in mobile communicationAns.Call ProcessingInitialization

When the mobile station is turn on by the user, the initialization work starts. It includes tuning of strongest dedicated channel available and it also detects it is in enable or disable status. After initialization and tuning to strongest dedicated channel the mobile station tunes to strongest paging channel within 3 sec. In. the idle stage the mobile station executes tasks like responds to overhead information, page match, order and rescan access channels. This is carried out after very 46.3 msec.

Call Initiation When the system access task is started, an access timer is set for,

various times to carry out origination, page response, an order response and registration activity. The service request is placed on the strongest or the second strongest channel.


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITSeize Reverse Control Channel

If the status of BIS = 1, It implies that mobile station is ready for sending. If the status of BIS =0 it implies ‘busy’ condition. After this the maximum of 10 attempts are observed. It is necessary that once the call is initiated the update overhead information should be r by the mobile station within 1.5 sec.

The delay after failure is checked at every 1.5 sec with access timer. The service request message is sent to the land station If there is no response after request is sent for 5 sec, the call is terminated

Outgoing Call

Fig.5.41.OutGoing Call

Step in call Processing – outGoing CallStep-1 :MS sends dialled number to BSS Step-2 :BSS sends dialled number to MSCStep -3,4 : MSC c hecks VLR if MS is allowed the requested service.If

so,MSC asks BSS to allocate resources for call.Step-5 :MSC routes the call to GMSCStep-6 :GMSC routes the call to local exchange of called userStep- 7, 8,9,10 :

Answer back(ring back) tone is routed from called user to MS via GMSC,MSC,BSS

Incoming call

Fig.5.42.Incoming call

Step in Call processing – Incoming CallWebsite- www.kavediasir.yolasite.com 51

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMIT1. Calling a GSM subscribers2. Forwarding call to GSMC3. Signal Setup to HLR4. 5. Request MSRN from VLR6. Forward responsible MSC to GMSC7. Forward Call to current MSC8. 9. Get current status of MS10. 11. Paging of MS12. 13. MS answers14. 15. Security checks16. 17. Set up connection

Limitation of Mobile Phone1. In conventional mobile telephone, one or more channel can be used

out of frequency allocation which covers a limited geographic area. 2. The power transmitted is required to be high. 3. The user who initiates the call in one zone is required to re-initiate the

call when he is moved into the other zone. If re-initiation is not done then the call may be dropped.

4. The number of active users are limited to the number of channels. 5. The service performance was found to be poor. 6. The system’s frequency spectrum utilization is not efficient. The

bandwidth is not adequate. As the user is mobile the small object also works as a scatters.

7. Multiple path propagation provide reflection 8. Co-channel and -adjacent channel interference are sources of the

noise.9.

Hands Off

Q.What is Handoff ?Describe the concept of Hands OFFAns.Mobility is the most important feature of a wireless cellular communication system. Usually, continuous service is achieved by supporting handoff (or handover) from one cell to another. Handoff is the process of changing the channel (frequency, time slot, spreading code, or combination of them) associated with the current connection while a call is in progress. It is often initiated either by crossing a cell boundary or by a deterioration in quality of the signal in the current channel. Handoff is divided into two broad categories—hard and soft handoffs.

They are also characterized by “break before make” and “make before break.” In hard handoffs, current resources are released before new


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITresources are used; In soft handoffs, both existing and new resources are used during the handoff process. Poorly designed handoff schemes tend to generate very heavy signaling traffic and, thereby, a dramatic decrease in quality of service (QoS). (In this chapter, a handoff is assumed to occur only at the cell boundary.) The reason why handoffs are critical in cellular communication systems is that neighboring cells are always using a disjoint subset of frequency bands, so negotiations must take place between the mobile station (MS), the current serving base station (BS), and the next potential BS. Other related issues, such as decision making and priority strategies during overloading, might influence the overall performance.

Concept of Hand-offThe hand off is the mobile system is that which allows cellular phone

system effectively in practice. In cellular system during the call the two parties are on the voice channel.

When the mobile unit moves beyond the coverage area of a particular cell site, the received signal becomes weak. In such a case the present cell site requests a hand-off. That is the system switches the call from current cell to the new frequency channel in new cell site without interrupting the call or altering the user. The user continues as he is talking. The hand-off is always implemented on voice channel. The value of the hand-of is determined depending on size of the cell.

The hand-off is required in two main conditions 1. when the cell site receives weak signals from the mobile unit

either at the cell boundary or mobile unit is reaching the signals from the mobile unit either at the cell boundary or

2. mobile unit is reaching signal strength hole or gaps are as shown in fig.5.43.

Thus hands off is nothing but handling the signal off from old channel to new channel by MTSO.

Fig.5.43.Hands-off

Q.List and Describe different types of Hands OffAns.Types of Hand-off

There are two types of hand-off 1. The first which is based

on strength of the signal and


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMIT2. The second which is based on carrier to interference ratio.

In case of hand-off based on signal strength, the signal strength threshold for hand-off is —100 dBm in noise limited systems whereas threshold level used as standard is -95 dBm in interference - limited systems. In case of carrier - to interference ratio type hand-off the value of C/I at the cell boundary for hand-off is about 18 dB in order to have quality voice. In some cases lower values maybe used.

Handoffs are broadly classified into two categories—hard and soft handoffs. Usually, the hard handoff can be further divided into two different types—intra- and intercell handoffs. The soft handoff can also be divided into two different types—multiway soft handoffs and softer handoffs. In this chapter, we focus primarily on the hard handoff.

A hard handoff is essentially a “break before make” connection. Under the control of the MSC, the BS hands off the MS’s call to another cell and then drops the call. In a hard handoff, the link to the prior BS is terminated before or as the user is transferred to the new cell’s BS; the MS is linked to no more than one BS at any given time. Hard handoff is primarily used in FDMA (frequency division multiple access) and TDMA (time division multiple access), where different frequency ranges are used in adjacent channels in order to minimizechannel interference. So when the MS moves from one BS to another BS, it becomes impossible for it to communicate with both BSs (since different frequencies are used). Figure illustrates hard handoff between the MS and the BSs.

Force Hands Off The forced Hands off is defined as the hand off which might normally occur but is prevented from occuring or happening.Shown in Fig.5.44.

Fig.5.44.Hard Hands off

Initiation of Hand-offThe MTSO monitors the level of the signal every few seconds. if-the

strength of the signal diminishes, the MTSO seeks a new cell that can Website- www.kavediasir.yolasite.com 54

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITaccommodate the communication better. An. intelligent decision can also be made at the cell site as to whether the hand-off should have taken place earlier or latter. In such cases the handoff is required quite early then it is unnecessary hand-off while if failure hand then it is a decision that is made too late.

Creating a Hand-offIn this case the cell site does not request a hand-off but MTSO finds

that some cells are too crowded or congested as compared to other cells. In such cases MTSO can request cell sites to create early hand-offs having congested cells. This means cell sites has to observe and follow MTSO’s order. The cell site increases the hand-off threshold and pushes the mobile units at the new boundary to hand-off earlier.

Controlling a Hand-offThe cell site can assign a low hand-off threshold in cell so that mobile unit can be kept in cell for longer. On the other hand it can assign high hand-off threshold to request the hand-off earlier. The MTSO can also control the hand-off by making hand-off earlier or later after receiving the hand-off request from the cell site.

Q.Describe the Concept of Delayed Hand-off?State its advantagesIn earlier section we have seen that hand-off is necessary as signal

level diminishes. Most of the time hand-off is implemented with the two hand-off level algorithm also known as delaying hand-off.

In this algorithm the hand-off is created in two request hand-off levels. The purpose of such delaying hand-off is to provide more opportunity for the successful hand-off. The hand-off may also be delayed if no available cell can take the call.

Fig.5.46. Two level Hands off


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITThe two level hand-off scheme is as shown in Fig. It is plot of received signal strength and time.

Fig.5.46. The two levels are L1 and L2 shown with threshold level.When the received signal strength drops below the first hand-off level L1 the hand-off request is initiated. If the mobile unit is in a weak spot in a cell i.e. hole or the neighboring cell is busy then the hand-off is requested after every 5 sec. At first hand-off level hand-.off takes place , if new signal is stronger. However when the received signal strength falls below second hand-off level L2 the call wilt be directly hand-off with no condition.

The MTSO always attains the hand-off calls first and the originating calls later, if there is no cell available after the second hand-off level is reached, the call continues until the signal strength decreases below the threshold level, then the call is dropped. In such conditions if the supervisory audio tone (SAT) is not sent back to the cell site by mobile unit within duration of 5 sec, the cell site turns off the transmitter.

Advantages of Delayed Hand-offsThe delayed hand-off handles hand-off s as. well as if help in

processing. Let us consider the mobile unit is moving randomly arid the contour is not even. In such conditions the strength of the signal received at mobile unit fluctuates up and down. If the mobile unit is in the weak spot in cell for less than 5 sec then the delayed hand-off.

In case of the busy cells the delayed hand-off may takes. That is during the busy hours if the cell traffic is very heavy then the switching processor is loaded, under such circumstances lesser the hand-offs, the processor can handle the call better. It is possible that after second land-off level is reached the call may be dropped.

Q.Different different handoff techniquesAns.The hand-off techniques is distinct feature of mobile communication and attempts are made to make it more and more powerful. This results in other algorithms for hand-off as discussed below.

Power Difference Hand-offsThis hand-off is based on the power difference between the mobile

signal received by two cell sites namely the home (current cell), and hand-off. This power difference may be positive or negative. Depending on the preset value of the power difference the hand-off takes place.. This provides better facility for the switch processor capacity.

Mobile Assisted Hand-off (MAHO)


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITGenerally depending on the signal strength of received signal hand-off

is requested. In digital cellular system the mobile receiver has capacity to monitor the signal strength during service of the call. It is digital system hand-off. This is due to system implementation for e.g. TDMA. in. such system division of time slot is carried out, during the call one of slot is used while rest of the slots can be used to control Or monitor the signal strength of the channel. This TDMA unit thus can provide the signal strength as well as information about neighboring cells for hand-off. the. switching office. This MAHO helps to take more accurate neighboring cell to hand off to.

In a mobile-assisted handoff process, the MS makes measurements and the network makes the decision. In the circuit-switched GSM (global system mobile), the BS controller (BSC) is in charge of the radio interface management. This mainly means allocation and release of radio channels and handoff management. The handoff time between handoff decision and execution in such a circuit-switched GSM is approximately 1 second.

Soft Hand-offThis is also applies to digital system. It is applied to digital cellular

system - CDMA. In this system all cells can use same carrier frequency. This lead to the frequency reuse factor tends to unity. As frequency of carrier for all the cells is same there is no peed to change from one frequency to another frequency, but in this system the code is changed. That is there is no hard hand-off. Such type of hand-off is hence called as soft hand-off.

Cell Site Hand-off OnlyIn this type of hand-off the frequency on hand-off do not change. The

cell site needs to know the frequency of mobile unit and new cell tunes itself to it. Such hand- off supports only low volume of traffic. These type .of hand-offs are mainly implemented in military systems.

Inter System Hand-offThis type of the hand-off is observed between two different MTSOs. This inter system hand-off takes place when the mobile unit enters from one system to other system and to continue the call hand-off is required; This hand-off requires modification in the MTSO.

Q.Describe the Concept of Queuing of Hand-offAns.The two level hand-off mechanism is advantageous but has its own limitations. The technique of queuing of hand-off is more effective than the Website- www.kavediasir.yolasite.com 57

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITtwo threshold level hand-offs. In heavy traffic situations the MTSO may queue the requests of hand-off calls instead of rejecting or dropping them if the new cell sites are not free.

The queuing scheme is effective only when the MTSO receives the hand request in batches. On the other hand if the requests are received uniformly then queuing is not necessary.It is observed that in queuing of hand-off calls and not originating calls, the blocking probability reduces. Adding queuing in hand-off calls it does not affect the blocking of originating call. The queuing hand-offs helps to overcome call dropping.

Due to queuing the non uniform traffic pattern is regulated and the call volume is moderated To avail the queuing, the switching system should provide memory or buffers to queue -the calls when channel is busy. The queuing space are generally not large. The. limit is imposed on the queuing space beyond which queuing is diminished.The queuing feature of the hand-off calls increases the traffic capacity also.

Q.How a Mobile Originated Call , describe the process?Ans.The mobile originate call forms one of the step of the cellular system operation. In practice from the use? s point of. view the cellular mobile system can b dividedinto four distinct parts as

1. Mobile unit initialization,.2. Mobile originated call,3. Network originated call, and4. Call termination.

Mobile Unit Initialization It is a first step, in which the user just activates the mobile unit. By doing so, the receiver scans the 21 set up channels in 416 channels. It selects the strongest and closest channel and locks it. This is achieved during idle stage itself. This locking is called self location it is generally carried out after every 60 sec.

Mobile Originated Call In this- stage user has to place the called number digits into the

originating register of the mobile unit. It is checked and if the number is correct the signal is given to the button. After this the request of service is sent on the selected channel with the help of self location scheme. The cell site when receives the request as a response it select the best directive antenna for the purpose of the call. The cell site also, sends the request to Website- www.kavediasir.yolasite.com 58

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITthe mobile telephone switching office ‘i.e. MTSO through a high speed data -link. This appropriate’ voice channel is selected by the MTSO. This gives the link between the mobile unit and the cell antenna. In mobile originated call, the MTSO also connects the wire line partly through telephone office.In cellular system it is observed that almost 80 percent of calls originate from the mobile unit white 20 percent of calls originate from land line.

Network Originated Call In the mobile originated call the MTSO also., connect to the telephone

office’. Similarly if the land line partly dials the mobile Unit number. The telephone office recognize the number as mobile number and hence- directs the call to the MTSO. The MTSO sends the paging message to the various cells. The mobile unit recognizes its identification and locks itself to strongest channel.

Call Termination When the conversation is over the mobile user turns off the

transmitter this sends the signal tone to the current cell site which makes free both the side voice channels.Frequency Reuse

Q.Describe the Concept of Frequency Reuse ChannelAns.It is important to serve many users efficiently without interference. The frequency resources are limited and hence mobile system design uses the concept of the frequency reuse channel.

For the full duplex operation i.e. simultaneous communication in the both the directions a radio channel uses a pair of frequency. It uses one frequency for. Each transmission direction.



Fig.5.47.Frequency Reuse ConceptTo understand the concept of the frequency reuse channel in the

mobile radio system. Let us consider a channel corresponding to frequency f1 let it is used for the cell C1, having the coverage radius r. Then this channel can be used in another cell C2 situated at distance d with same coverage radians r. Thus channel f1 is used by the two cells. This frequency reuse allows the same frequency can be used simultaneously by the two different user’s indifferent geographic location as shown in Fig.5.47.

Fig.5.48.Cell Frequency Reuse PatternThe frequency reuse system is advantageous as it increases the

spectrum efficiency. The design of such system is very crucial as if system is not properly designed then it gives rise to interference due to the common use of the same channel. Such interference is called as co-channel interference.

The co-channel interference is the major limitation of the frequency reuse system. The concept of frequency reuse can be employed in time domain as well as space domain.

The frequency reuse in the time domain means the same frequency f is occupied in various time slots, which results in lime division multiplexing. The frequency reuse in the space domain can be implemented by assigning same frequency in two different regions for AM or FM radio stations. The Website- www.kavediasir.yolasite.com 60

Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITfrequency reuse as discussed earlier requires distance ‘d’ of separation between the cells. The distance d depends on various factors like type of geographic contour height, power transmitted, number of co-channel cells etc.The minimum distance which allows frequency reuse can be expressed as

D = √3K .rWhere

r – radius of the coverageK – number of cells in frequency reuse patternK=4 – number of calls in frequency reuse pattern

The number of frequency response cell K depend on carrier to interfere ratio, it can be given as

K =√(2/3).C/IIf high C/I is required to be high then the more frequency resuse cell as required.

Q.Write short note on Frequency Spectrum UtilizationAns.The radio frequency is expensive parameter and it is finite in nature in mobile system, the designer should use the radio frequency very efficiently.

The concept of frequency reuse increase the frequency spectrum utilization which highly depends on the location of the cell. The proper frequency management involving frequency assignment of channels in various cells can also increase frequency spectrum efficiency. Frequency spectrum utilization can be improved by narrowing the frequency bonds, queuing, call redirect etc.The techniques for increasing the frequency spectrum can be classified into five main types as:

1. By increasing the number of radio channels using narrowing of frequency band or time division.

2. Using concept of frequency reuse.3. By using proper frequency, management.4. By improving spectrum efficiency in time.5. By reducing the load of the invalid calls. This can be achieved by

(a) Off-air call set up(b) Call forwarding(c) Call waiting in case of busy-call conditions(d) Queuing.

The spectrum efficiency in case of the mobile system is nothing but the number of channels per cell. On the other hand, generally for most of radio


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITsystem, spectrum efficiency is same as channel efficiency. This shows that for mobile system spectrum efficiency differs from channel efficiency

Q.List the Service Provided By GSM Ans.Following are the services provided by GSM

Tele-services Bearer or Data Services Supplementary services

Tele Service• Telecommunication services that enable voice communication via

mobile phones • Offered services

- Mobile telephony- Emergency calling

Bearer Service Include various data services for information transfer between GSM and other

networks like PSTN, ISDN etc at rates from 300 to 9600 bps Short Message Service (SMS)

– up to 160 character alphanumeric data transmission to/from the mobile terminal

Unified Messaging Services(UMS) Group 3 fax Voice mailbox Electronic mail

Supplementary ServicesCall related services :

• Call Waiting- Notification of an incoming call while on the handset• Call Hold- Put a caller on hold to take another call• Call Barring- All calls, outgoing calls, or incoming calls• Call Forwarding- Calls can be sent to various numbers defined by the user• Multi Party Call Conferencing - Link multiple calls together• CLIP – Caller line identification presentation• CLIR – Caller line identification restriction• CUG – Closed user group

Q. What is GSM ?


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITGSM (Global System for Mobile Communications) is a second-generation

digital mobile telephone standard using a variation of Time Division Multiple Access (TDMA). It is the most widely used of the three digital wireless telephone technologies - CDMA (Code Division Multiple Access), GSM and TDMA. GSM digitizes and compresses voice data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900, 1800 or 1,900MHz frequency bands. GSM was initially developed as a pan-European collaboration, intended to enable mobile roaming between member countries. As at March 2003, GSM digital wireless services were offered in some form in over 193 countries. In June 2002, about 69% of all digital mobile subscriptions in the world used GSM phones on GSM networks.

Board Exam Question Year Wise

Test Paper -2Q1. List frequency bands used in satellite communication.Q2. Define Baud rate and bit rate.Q3. Explain effect of changes in atmospheric conditions on sky

wave propagation.Q4. Define azimuth angle and elevation angle with sketch.

Sample paper-1Q1. Describe working principle of TDMA with diagram.Q2. List applications of satellite and describe any one.Q3. Define azimuth angle and elevation angle with sketch.Q4. How frequency reuse and cell splitting take place in mobile

communication?Q5. Draw and explain block diagram of cellular mobile phone

system.Q6. Draw block diagram of satellite communication system.Q7. Describe forward and reverse call processing.

Sample Paper-2Q1. what is the difference between simplex, half duplex and duplex

systems, also give its examples. Give the Importance of communication

Q2. Describe working principle of FDMA with diagram.Q3. What is mean by terms Uplink ,Downlink , transponder ,

Geosynchronous satellite


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITQ4. Why handoffs are needed ? Explain the handoff procedure.? List

different types of Hands-offQ5. Draw and explain block diagram of satellite communication Q6. system.Q7. What is access technology? List Different type of access

technology? Describe any one in detailsQ8. Related to Antenna Describe the following characteristics

i. Antenna radiation patternsii. Power Gainiii. Directivityiv. Polarization

Winter 2008Q1. Define:

i) Baud rate ii) Bit rate iii) Channel capacityiv) Channel Bandwidth

Q2. Compare TDMA and FDMA (4 points).Q3. Define elevation and Azimuth angles of satellite with neat

diagram.Q4. Why handoffs are needed ? Explain the handoff procedure.Q5. Draw and explain the• block diagram of cellular mobile

phone system.Q6. State the advantages and disadvantages of CDMA.Q7. Explain construction and working principle of parabolic

dish antenna.Q8. Explain the concept of

i) frequency reuse ii) cell splitting

Summer 2009a. Why uplink frequency is higher than downlink frequency?b. List functions of satellite (any four).c. Describe handset to handset call processing.d. compare TDMA, FDMA and CDMA. Describe basic concept of CDMA.e. Draw and Describe the block diagram of mobile cellular system

Winter 2009Q1. Describe the working principle of CDMA with neat diagram.Q2. Draw the block diagram of satellite communication system and

explain h it works.Q3. Describe working principle of Parabolic dish Antenna.


Prof.Manoj Kavedia-9324258878-9773552051-9860174297-SHMITQ4. How hand off mechanism and cell splitting take place in mobile

communication.Q5. Describe the uplink model and downlink model used by the

satellite communication with neat diagram.Q6. Draw and explain the block diagram of cellular mobile phone

system.Q7. Define

(i) Data Rate (ii) Bit Rate (iii) Baud Rate (iv) Channel Bandwidth

Summer 2010Q1. Draw neat block diagram of satellite communication system.

State function of up convertor.Q2. What is hand off ? Explain hand off procedure.Q3. Define the following terms

(i) Channel capacity (ii) Bit rate(iii) Band rate(iv) S/N ratioQ4. What is multiple access ? State its types. State advantages of

CDMA.Q5. Draw and explain block diagram of cellular mobile phone system.Q6. Describe antennas used in satellite communication system.Q7. Draw block diagram of digital communication system.

Q8. Describe forward and reverse call processing.


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