Mobile Communication

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1 5 Satellite Communication and Mobile Communication Satellite Communication And Mobile Communication 5.1 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 antenna 5.7 Construction and working principle of Parabolic dish and horn antenna. 5.8 Satellite application overview. 5.9 Principle, advantages and disadvantages of TDMA, FDMA, CDMA 5.10 Concepts of mobile phone Block diagram of cellular mobile phone system and description Frequency band and types of modulation used for Cellular mobile communication 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 1

2 Q.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 Uplin Down the earth in circular k Link 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 2

3 earth. 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 f 2. 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 3

4 Q.What is Transponder?State function of transponder?list and describe different types of transponder? Ans.Transponder Transponders 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 4

5 carrier processing equipment determines whether the transponder is of conventional or regenerative type Regenerative 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 Satellite Ans. 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 5

6 satellite 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 satellite Downlink 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 Frequencies Ans.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 Frequencies After 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. 6

7 Q.List the Frequency based used in Satellite communication Ans.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 Communication Frequency Band Mhz P Mhz J Mhz L Mhz S Mhz C Mhz X Ghz Ku ghz Kc K Ghz Q Ghz V Ghz W The most widely used satellite communication band is the C band. The uplink frequencies are in the to 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 Orbit Q.List and Describe the types of Orbits of the satellite Ans.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 7

8 to 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. Angles The 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 90 o 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

9 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 120 o 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 orbits Ans. 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. 9

10 The 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 LEO (Low Earth Orbit) - Altitude below 2,000 km (1,250 miles), rotation period of 90 minutes to 2 hours. MEO (Medium Earth Orbit) - Altitude of approximately 10,000 km (22,300 miles), rotation period of around 6 hours. GEO (Geosynchronous Earth Orbit) - Altitude of approximately 35,680 km (22,300 miles), rotation period 24 hours Fig.5.8.Different Satellite orbit LEO - 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 , 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 Appplication This 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 10

11 continue 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 Orbit Short 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 Global Positioning System Global Positioning System Operation This figure 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 11

12 systems 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 Angles Ans.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 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

13 The 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. Angle To 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 Elevation Angle 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 13

14 deteriorate 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 Angle Azimuth 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, 14

15 azimuth 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 perigee True 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 Concept Ans.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. 15

16 The 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 These satellite consist of five main subsystem as i. Power sub system ii. Telemetry and Control/Command sub system iii. Main and auxiliary propulsion sub system iv. Communication channel sub system v. 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. Solar Panels Sun Battery charger Nickel-Cadmium Batteries Acting as Buffers Power conditioning Voltage Regulators DC to DC converters DC to DC Inverter Low voltage DC High voltage DC AC To all other subsystems To TWTS 16

17 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 geostationary 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. 17

18 Fig.5.14.Telemetry System Fig Satellite design Concept Telemetry and Control/Command Sub-System 18

19 The 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 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 systems Command system receives instructions from ground system of satellite and decodes the instruction and sends commends to other systems as per the instruction. Tracking Tracking 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 onstation, 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 19

20 decoder. 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 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 20

21 Fig 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 The received signals so down converted to the range of 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; 21

22 large 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 Receiver 22

23 Satellite 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. 23

24 24

25 Fig.5.18.Block Diagram of Earth Station Antenna 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 functions 1. 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 Antenna Ans.An antenna is a device that is made to efficiently radiate and receive radiated electromagnetic waves. There are several important antenna characteristics that should be considered when choosing an antenna for your application as follows: 1. Antenna radiation patterns 2. Power Gain 3. Directivity 4. Polarization Antenna Radiation Patterns An antenna radiation pattern is a 3-D plot of its radiation far from the source. Antenna radiation patterns usually take two forms, the elevation 25

26 pattern 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 Gain The 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. Directivity The 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 Polarization Polarization 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 Note: Clockwise rotation of the Electromagnetic wave is right-hand polarization; counterclockwise rotation is left-hand polarization. Types of Antenna 26

27 Q.List the different types of Antennas used in satellite communcaiton Ans.There are many different types of antennas. Antennas most relevant to designs at2.4ghz that are further detailed are as follows: 1. Dipole Antennas 2. Multiple Element Dipole Antennas 3. Yagi Antennas 4. Flat Panel antennas 5. Parabolic Dish antennas 6. Slotted Antennas 7. Microstrip Antennas Q.Write short note on Horn Antenna Ans.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 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. 27

28 . Fig.5.20 Different Horn Antenna Horn 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 For dominant waveguide mode excitation, the E-plane is vertical and Hplane 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 freespace 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) 28

29 Ans.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. Working Electromagnetic 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. 29

30 Q.Describe with neat diagram Parabolic Hornfeed Cassegrain Antenna Fig Parabolic Dish Antenna Parabolic 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. Working When 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 Satellite Ans. 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 30

31 few channels for radio transmission. It means satellite has large capacity of communication. Satellite provide information regarding weather, make forecast about rains and cyclones. Satellite can give important information about the mineral deposits in earth and also deep inside the sea. It provides maritime remote sensing etc. Satellites are also used by defense departments of many countries for communication. surveillance and spying; Satellites are also used for mobile communications 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 communication Ans.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 reinstalled. 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. 31

32 9. 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 Technology Ans.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 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. 32

33 The multiple access and broadcast networks are the most common multiuser communication system. The third type of multi-user system is a storeand-forward network shown in Fig 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 33

34 assigned 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 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 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 There are N non-overlapping sub channels, as shown in Fig 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 Allocation Fig 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 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 34

35 signals. Disadvantages of FDMA a. 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 Access Q.What is TDMA( Time Division Multiple Access)?State is Advantages and disadvantages Ans.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 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 35

36 Another 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 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 TDMA a. 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 information e. 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 36

37 Disadvantages 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 TDMA Ans. 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 disadvantages Ans.CDMA is based on spread spectrum technology. Since it is suitable for encrypted transmissions, it has long been used for military purposes. 37

38 CDMA 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 thirdgeneration 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, CDMA In 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 ultrahigh-frequency (UHF) cellular telephone systems in the 800MHz and 1.9-GHz bands. CDMA Technique An 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

39 Fig.5.33 CDMA Technique At 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 CDMA 1. 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. Disadvantages 1. Reduced efficiency due to coding. 2. Synchronization and precise liming is essential 3. 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 39

40 phone 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 40

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