Vocational Training Report

On

STUDY OF TRANSPONDERS

TRAINING UNDERTAKEN

AT

DEFENCE ELECTRONICS APPLICATIONLABORATORY RAIPUR ROAD,

DEHRADUN-248001

PREPARED BY: - UNDER THE SUPERVISION OF:-

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MOHIT KUMARB.Tech (ECE)Univ. R.NO.-0981562805NIEC, New Delhi

ASHOK KUMARScientist ‘F’Millimeter Wave GroupDEAL, Dehradun.

Defence Electronics Application LaboratoryRaipur Road, Dehradun (UK)-248001

CERTIFICATE

This is to certify that the study project work entitled “Transponders” was carried outand successfully completed by Mohit Kumar, Roll.No-0981562805, a student of B.TechECE from Northern India Engineering College, New Delhi (IP University) at MMWDivision, DEAL, Dehradun from 2nd June 2008 to 2nd July 2008.

Dated:

Ashok Kumar K.SivakumarScientist ‘F’ Scientist ‘G’MMW Systems Group Group DirectorDEAL, Dehradun MMW Systems Group

DEAL, Dehradun

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ACKNOWLEDGEMENT

I would like to express my gratitude to all those who gave me the possibility tocomplete the project. The successful completion of this report is attributed from greathelp and support I have received from various members of D.E.A.L family.

First of all, I want to thank Shri RC Aggarwal ,Director ,D.E.A.L and Mr.Deshmukh ,Director ,H.R. Department ,D.E.A.L for kindly giving me his consent formy practical training at D.E.A.L, Dehradun. I shall forever be indebted to them forproviding me with such a sterling opportunity.

I would like to extend my heartfelt gratitude to Mr.K.Sivakumar ,Scientist ‘G’,Director, Millimeter Wave’s group for giving me permission to commence this thesis inthe first instance, to do the necessary research work and to use departmental data.

I am deeply indebted to my mentor Dr. Ashok Kumar (scientist ‘F’), whoseconstant guidance, stimulating suggestions and encouragement helped me in all the timeof my training and successful completion of this project.

I have furthermore to thank Mr. Hoshiar Singh Kalsi, Technical Asst. ‘A”, Mrs.Ranjana Thakur and Mr. Rajeev who helped and encouraged me to go ahead with myproject. This magnificent team has guided me through the most demanding part of myengineering curriculum and I shall forever be indebted to them for providing me a strongfoundation to my career.

I would like to extend my cordial gratitude and regards to T.I.C (TechnicalInformation Center), D.EA.L. for providing standard text on the subject.

Last but not least, I would like to give my special thanks to all the members ofD.E.A.L family MMW group who have directly or indirectly helped me in the completionof my project.

Mohit Kumar

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Defence Research & Development Organization (DRDO) works under Departmentof Defence Research and Ministry of Defence. DRDO is dedicatedly working towardsenhancing self-reliance in Defence Systems and undertakes design & developmentleading to production of world class weapon systems and equipment in accordance withthe expressed needs and the qualitative requirements laid down by the three services.DRDO is working in various areas of military technology which include aeronautics,armaments, combat vehicles, electronics, instrumentation engineering systems, missiles,materials, naval systems, advanced computing, simulation and life sciences.

DRDO was formed in 1958 from the amalgamation of the then already functioningTechnical Development Establishment (TDEs) of the Indian Army and the Directorate ofTechnical Development & Production (DTDP) with the Defence Science Organization(DSO). DRDO was then a small organization with 10 establishments or laboratories.Over the years, it has grown multi-directionally in terms of the variety of subjectdisciplines, number of laboratories, achievements and stature. Today, DRDO is a networkof more than 50 laboratories which are deeply engaged in developing defensetechnologies covering various disciplines. Presently, the Organization is backed by over5000 scientists and about 25,000 other scientific, technical and supporting personnel.Several major projects for the development of missiles, armaments, light combataircrafts, radars, electronic warfare systems etc are on hand and significant achievementshave already been made in several such technologies.

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Defence Electronics Application Laboratory

The origin of Defence Electronics Applications Laboratory (DEAL) can betraced back to 1959 when the Defence Research Laboratory (DRL) was set up in thebarracks of British Military Hospital at Landour Cantt, Mussoorie as a small field unit ofthe Defence Science Center (DSC), Delhi. DRL was engaged in radio wave propagationstudies, food preservation & packaging and study of problems at high attitudes. Thereorganization of DRDO in 1962 saw the consolidation of Propagation Studies in theform of Propagation Field Research Station (PFRS), as a detachment of DLRL,Hyderabad. PFRS became an independent entity as Himalayan Radio PropagationUnit (HRPU) at Mussoorie with the strength of 84 persons on February 23, 1965.HRPU was responsible for helping the Services to set up communication links in theborder areas and providing frequency prediction services using data collected frompropagation studies. HRPU moved to Dehradun in 1968 and was temporarily located inthe old barracks of Instruments Research & Developments Establishment (IRDE). It wasrenamed as Defence Electronics Applications Laboratory (DEAL) and established inthe present location in 1976.

Shri RC Aggarwal has been appointed Director, Defence ElectronicsApplications Laboratory (DEAL), Dehradun , wef 01 December 2007.

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CONTENTS

1). A Brief Overview of Satellite Communication1.1). Abstract 1.2). Types of orbits1.3). Basic terms in satellite communication.1.4). Components of a satellite

2) Satellite payloads2.1) Abstract2.2) Basic operations at transmitting earth station.

3) Transponders3.1) Bent pipe3.2) On board processing

4) Case Study4.1 INTELSAT IV

5) Satellite Link budget5.1 Example of a link budget5.2 Various terms in budget

6) Conclusion

7) Bibliography

1 - A Brief Overview of Satellite Communication

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1.1 Abstract

Satellites have now become an integral part of the worldwide communication systems. Although long–range and long distance communication took place much before the introduction of satellite systems, they had a lot of disadvantages. Point – to – point communication systems are very difficult in the case of remote & isolated locations, which are surrounded by oceans, mountains and other obstacles created by nature.

The satellite is nothing more than a radio-relay station But, they have one potential advantage- The capability of a direct line of sight path to 98% (excluding the polar caps, which are in accessible to satellites) of the earth's surface.

One of the most important events in the history of satellite communication took place when COMSAT or communication satellite corporation, launched four satellites within 6 years that is between 1965 to 1979. The first of these series was the ‘Early Bird’, which was launched in 1965. This was the first communication station to handle worldwide commercial telephone traffic from a fixed position in space. The next series INTELSAT was a group of satellites that served 150 stations in 80 countries.

Fig 1.1 Figure to show the basic components in satellite communication.

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1.2 Types of Orbits

Different orbits serve different purposes. Each has its own advantages and disadvantages. There are several types of orbits:

1. Polar 2. Sun Synchronous 3. Geosynchronous

Polar Orbits The more correct term would be near polar orbits. These orbits have an inclination near90 degrees. This allows the satellite to see virtually every part of the Earth as the Earth rotates underneath it. It takes approximately 90 minutes for the satellite to complete one orbit. These satellites have many uses such as measuring ozone concentrations in the stratosphere or measuring temperatures in the atmosphere.

Sun Synchronous Orbits

These orbits allow a satellite to pass over a section of the Earth at the same time of day.Since there are 365 days in a year and 360 degrees in a circle, it means that the satellite has to shift its orbit by approximately one degree per day. These satellites orbit at an altitude between 700 to 800 km. These satellites use the fact since the Earth is not perfectly round (the Earth bulges in the center, the bulge near the equator will cause additional gravitational forces to act on the satellite. This causes the satellite's orbit to either proceed or recede. These orbits are used for satellites that need a constant amount of sunlight. Satellites that take pictures of the Earth would work best with bright sunlight,while satellites that measure long wave radiation would work best in complete darkness.

Geosynchronous Orbits

Also known as geostationary orbits, satellites in these orbits circle the Earth at the same rate as the Earth spins. The Earth actually takes 23 hours, 56 minutes, and 4.09 seconds to make one full revolution. So based on Kepler's Laws of Planetary Motion, thiswould put the satellite at approximately 35,790 km above the Earth. The satellites are located near the equator since at this latitude; there is a constant force of gravity from all directions. At other latitudes, the bulge at the center of the Earth would pull on the satellite. Geosynchronous orbits allow the satellite to observe almost a full hemisphere of the Earth. These satellites are used to study large scale phenomenon such as hurricanes, or cyclones. These orbits are also used for communication satellites. The disadvantage of this type of orbit is that since these satellites are very far away, they have poor resolution. The other disadvantage is that these satellites have trouble monitoring activities near the poles..

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Fig 1.2 Figure to show the basic types of satellite orbits.

The communications satellites are placed in orbits called equatorial geostationaryorbit. The satellite placed in this orbit will appear stationery over a selected location on the earth’s surface. So, communications satellites are placed in an orbit that is directly over the equator, moving in a west to east direction at an altitude of 22,282 miles above sea level (36,000 km appor. as explained earlier) and with a forward velocity of 6874mphto complete one orbit in 24 hours. This orbit is called the Clarke orbit.

Fig 1.3 Figure to show final geostationary orbit

1.3 Basic terms in satellite communication.

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Up-link and Down-link

All of the ground equipment along with the transmission path and receiving antenna at the satellite are included in the up-link system. Basically, this includes everything before the input terminals of the satellite receiver. The down-link is described in terms of satellite transmitted output power, down link antenna gain and beam width and the ground area that the transmitted signal will cover the foot print.

Cross –link

At the attitude of the Clarke-orbit, one satellite could command a footprint area of42.2% of the earth's surface. The beam-width from the satellite for such coverage is 17.2since such a satellite is not sufficient for global coverage; we need more than one to be specific 3 satellites.

These three satellites are placed 120 apart in the Clarke orbit and would cover the earth's entire surface except for the polar caps. This makes it possible for one earth station to transmit to another station on the opposite side of the globe.

Satellite footprints

The footprint is the area on the earth covered by a satellite antenna. It may embrace up to 50% of the earth’s surface, or, by means of signal focusing, be restricted to small, regional spots.

The higher the frequency of the signal emitted, the more it can be focused and the smallerthe footprint becomes. The focusing of the satellite signal on smaller footprints can increase the energy of the signal. The smaller the footprint, the stronger the signal, and thus the smaller the receiving antennae may be.

1.4 Components of a Satellite

There are 3 major components in a satellite, they are:

(i) Transponder and antenna system

The transponder is a high – frequency radio receiver, a frequency down-converterand a power amplifier, which is used to transmit the downlink signal. The antenna system contains the antennas and the mechanism to position them correctly. Once properly in place, they will generally function trouble-free fro the life of the satellite.

(ii) Power Package

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It is a power supply to the satellite. The satellite must be powered either from a battery or a solar energy system. In case of communications satellites in the Clarke orbit, a combination of battery power and solar energy is used. A solar cell system supplies the power to run the electronics and change the batteries during the sunlight cycle and batteryfurnishes the energy during the eclipse.

(iii) Control and information system & rocket thruster system

The control and information system and the rocket thruster system are called the station keeping system. The function of the station keeping system is to keep the satellite in the correct orbit with the antennas pointed in the exact direction desired.

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2 - Satellite Payloads

2.1 Abstract

A payload is the part of the satellite that performs the purpose it was put in space for. There are many different types of satellite but communications satellites are the kind we are interested in here. The payloads on communications satellites are effectively just repeaters. They receive the signals that are transmitted to them and then retransmit them at a different frequency back to earth.

Modern satellites do more than this. They receive the signals and then demodulatethem to access the data, the data can then be processed before being modulated and retransmitted. The data can be stored for later retransmission or modulated using a different method, even at a different data rate.

There is an uplink receiver chain and a downlink transmits chain. The central areashown as ‘Processing’ is where the frequency is translated or any demodulation, processing and modulation would take place.

Fig 2.1 Figure to show the basic steps in satellite communication.

2.2 The basic operations at transmitting earth station

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The digital data input at the transmitting end is compressed.

1. The signal is then passed to the multiplexer as usually the bandwidth of channel is much higher than the bandwidth of the original signal so many signals are combined or multiplexed together to form a block of signals called channel.

2. The ordinary analogue data is converted to digital data and is modulated onto the carrier

3. The technique used in the points 1, 2, 3 is usually termed as multiple access. The basic multiple access techniques are FDM/FM/FDMA used in satellite telephony or TDM/PSK/TDMA used in digital satellite communication.

4. The resulting baseband signal is then sent to the upconvertor.Usually more than one upconvertors are used.The signals are sent to the up converters at around 70 MHz. The 1st up converter mixes the signals with another frequency; the result is both the sum and difference of the signals. By filtering out the original and the difference frequencies the result is that the original frequencies are now the sum frequencies - higher up in the frequency spectrum. An example would be the up conversion of 70 MHz to 1 GHz which is IF to L Band. The 2nd up converter then up converts the L Band signals to a Radio Frequency (RF) of around 10 GHz. this is then ready for the HPA to transmit through the antenna.

5. The HPA (High Power Amplifier), otherwise known as a TWTA (Traveling Wave TubeAmplifier) or an SSHPA (Solid State High Power Amplifier),has one job. It amplifies aspecific band of frequencies by a large amount, sufficiently large to enable the antenna to beam them up to the satellite. These can range in power from a few watts up to over 1000 watts in power. The bigger the dish, usually the bigger the power amplifiers. The largest have to be cooled using liquid nitrogen and resemble electron microscopes. The smallest look more like a lump of metal bolted to a small heat sink.

6. The parabolic antenna is a high-gain reflector antenna. A typical parabolic antenna consists of a parabolic reflector illuminated by a small antenna.

7. Diplexer or OMT- The circulator is used to make sure that the transmit signals go out through the dish and not back into the receive chain. It also makes sure that the receive signals come from the dish into the receive chain and not into the transmit chain.This is often referred to as an Orthomode Transducer or OMT and is, these days, built into the feed assembly

8. The down converters convert signals down in frequency. The signals arrive at the dishat anything from 10 to 40 GHz and are then filtered and amplified, they now need to be moved down the frequency spectrum so that the equipment can be made cheaper and easier. The 1st down converter mixes the signals with another frequency; the

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result is both the sum and difference of the signals. By filtering out the original and the sum frequencies the result is that the original frequencies are now the difference frequencies - lower down in the frequency spectrum. An example would be the down conversion of 10 GHz to 1 GHz which is Ku band to L Band. The 2nd down converter then down converts the L Band signals to an Intermediate Frequency (IF) ofaround 70 MHz. this is then ready for the demodulator.

9. The LNA (Low Noise Amplifier), sometimes known as an LNB on receive only terminals, is a very good amplifier which has the job of amplifying the small signals picked up by the antenna without amplifying the noise.

3. Transponders

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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 microwaverepeater and antenna system that is housed onboard the operating satellite

These satellites and most of their cohorts in the geostationary orbit have bent-piperepeaters using C and Ku bands; a bent pipe repeater is simply one that receives all signals in the uplink beam, block translates them to the downlink band, and separates them into individual transponders of a fixed bandwidth

The transponder itself is simply a repeater. It takes in the signal from the uplink ata frequency f1, amplifies it and sends it back on a second frequency f2. Figure 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 transpondersdo not interact with each other.

Transponder complexity varies from the simple "bent pipe" approach to on-board processing (OBP) and on-board switching (OBS) transponders. Common elements include receivers, mixers, oscillators, channel amplifiers, and RF switches. OBP transponders may include additional elements of demodulators, demultiplexers, remodulators, and baseband switches.

The basic types of transponders are

Bent Pipe On board processing.

3.1 Bent Pipe or conventional transponders

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The bent pipe transponders are so called because it takes a band of signals and bents it back to the earth just like a bent pipe which changes the direction of flowing water.

Fig 3.1 – Fig to show the bent-pipe architecture of satellite

An onboard oscillator and mixer are used to translate the uplink band to a different downlink band.

The translation is done in order to separate the uplink and downlink signals .This is done in order to prevent the antenna receive the same signal that is being transmitted by it. The uplink frequency is always greater than downlink frequency as the antenna size at ground terminal can have larger size while the size of antenna on the satellite is fixed as the gain is higher in upper frequencies.

Fig 3.2 Figure to show block diagram of a bent-pipe architecture .

Amplifier used may be linear or non- linear. Linear amplifiers are used to minimize the crosstalk. In order to keep the amplifier in linear region a we use an

LNA X

LO

TWTAFILTER

UPLINK 6 GHz in C-Band14 GHz in Ku- band

DOWNLINK4 GHz in C-Band12 GHz in Ku- band

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Network 1

Network 2

AGC or automatic gain control. If the amplifier operates in non-linear mode it increases the cross talk caused due to intermodulation interference.

The major characteristics of Bent pipe architecture is Simpler satellites Complex ground stations Controlled by a ground station Longer propoagation delay Strong feeder links puts gound processor virtualy onboard. Limited means of sharing resources. Fixed interconnectivity

3.2 Regenerative / On board processing

In regenerative or onboard processing the signal is processed on the satellite and then transmitted towards the destination. In this case the destination may be a different network or any onter satellite. In this type of model inter-satellite links or cross links is possible.

Fig 3.3 Figure to show the basic components of OBP satellite communication.

3.3 Classes of OBP

The three main classes of OBP are:

Network 2Network 1

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1. Baseband processing and switching (routing) -- two subclasses: autonomous and ground-controlled,

2. IF or RF switching (frequency or time domains), and3. Support processing.

OBP can provide greatly increased efficiency and performance in communications satellites with trade-offs in increased cost and complexity. The increased efficiency can be used for significant mass reduction or for increased capacity. With the current trends toward decreased launch costs/unit mass, the increased capacity appears to be the logical benefit of choice.

ISL, Ku- or Ka-band receivers and transmitters, digital modulation and coding, and multiple access techniques.

Satellite-switched networking can be implemented via two primary approaches:

(1) fully processed by the satellite, and

(2) support by terrestrial control.

However, response time and throughput efficiency are compromised.

Class 1: Baseband Processing and Switching.

Baseband processing and switching involves the demodulation and demultiplexing of the received signal, performing error detection and correction, removing routing and control information (if not transmitted in a common channel signalling mode), routing the data, pointing directed antennas, buffering the data, multiplexing the data, tra nsmitting the data. The data could be of three types: circuit switched, message switched, or packet switched. Required technologies include multiple beam antennas, signal processing, microprocessors, time and/or space switches, ISLs, protocol processor s, and stored- program switches. LEO systems require sophisticated position and pointing capabilities, satellite-to-satellite handover control, and beam-to-beam handover control.

Class 2: RF or IF On-Board Switching.

On-board RF/IF switchi ng involves electronically controlled RF/IF switches which can be reconfigured on a near-real- time basis via ground control. OBP for carrier switching has become fairly common in recent years, the INTELSAT spacecraft being the common example. On-boards ignal regeneration (demod-remod) is also now being used fairly frequently to gain the signal to noise (thus low BERs). Baseband processing with message and packet switching is much less common and is generally used for special-purpose spacecraft only. H owever, with the rapidly increasing speed, power, and reliability of microprocessors, the more significant baseband processing and switching is expected to move forward rapidly.

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Class 3: Support Function On-Board Switching.

On-board support processi ng encompasses several functional areas. They include control of waveguide switching parameters, ephemeris calculations for small beamwidth, electronically scanned antennas, communications network protocol processing, special processing for such function s as handover for LEO spacecraft, error detection and correction, and elastic buffering and control.

The major techniques used in On board processing are Antenna beam switching Adaptive antennas Demodulation-Remodulation

Antenna beam switchingThis type of On board processing is applied in case of use of multiple antennas

and is done to increase the capacity of the satellite. As we know the link capacity varies inversely with the square of diameter of the beam on earth so we use small spot beams which are pencil thin to cover a smaller area on the earth.This technique is reffered to as Spot- beams technique and is quite effective.

Fig 3.4 Figure to show spot beams.

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Spot beams

interference

This technique called frequency reuse allows satellites to communicate with a number of ground stations using the same frequency, by transmitting in narrow beams pointed toward each of the stations. Beam widths can be adjusted to cover areas – footprints – as large as the entire country or as small as a island. Two stations far enough apart can receive different messages transmitted on the same frequency. Satellite antennashave been designed to transmit several beams in different directions, using the same reflector.

Adaptive antennasAn adaptive antenna is type of smart antenna. It's "smart" because it improves on

the traditional antenna by adjusting for traffic patterns at a given time to increase signal strength and quality. To adjust for frequency and channel use, the adaptive antenna uses multiple antennas and an algorithm in order to maximize the strength of the signals being sent and received while eliminating, or at least reducing, interference.

Demodulation-Remodulation:

The technique called demodulation and remodulation is one of the most powerful on board processing techniques

In order to convert the satellite signals back into digital signals for transport across the onboard processor, the transponder must demodulate the signal and then remodulate it before sending back down to earth. This remodulation significantly increases the power of the signal, allowing satellite terminals to be similar in cost to normal VSAT terminals while providing significantly increased performance. In this scheme uplink is demodulated into a bit stream.This bit stream is then processed by a digital switching subsystem that can route and reformat the streams and finally remodulate them onto one or more downlinks.

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Fig 3.5 Figure to show the basic components in OBP demodulaton - remodulation.

The basic characteristics of OBP are: Users in different antenna beams can be interconnected Uplink and downlink signals can be independently optimized It has a complex satellite but a relatively simpler and fewer ground station Works well with crosslinks Power sharing advantage

Permits normalization of downlink power sharing Amount of power devoted to each downlink can be adjusted Downlink power is thus not wasted

DEMODULATOR

PROCESSING REMODULATOR

FILTER

LNA

TWTA

LO

UPLINK 6 GHz in C-Band14 GHz in Ku- band

DOWNLINK4 GHz in C-Band12 GHz in Ku- band

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4) CASE STUDY

4.1) INTELSAT IV TRANSPONDER

Abstract-INTELSAT –IV is intended for broadband and multi-carrier operation.

Fig 4.1 Figure to show the block diagram of transponder in INTELSAT - IV.

Receiver 1

Receiver 2

Receiver 3

Receiver 4

3 dBHybrid

redundancyGlobal Rx

2

1

switch

TWTA

Odd channel

Even channel

Global transmitter

Spot transmitter

Input mux assembly

output mux assembly

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Basic elements of transponder

Wideband front end receiver 6 GHz antenna and a receiver section which translates the frequency to 4 GHz.the two sets of receiver is called redundancy as in case of some failure in the first set of receiver the second sets automatically takes control of the operation.The 3 dB hybrid circuit divides the input into two channels even and odd.The first set of channel is polarized in one form either horizontal or vertical and the other channel i.e the odd channel is polarized into the other form.This is done to achieve frequency reuse in oder to efficiently utilize the channel bandwidth.

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5.1 Example of a link budgetItem Link Parameters Value unit computations

Link budget analysis for the downlink at 4 GHz(C-Band)

1 Transmit power 10 dBW assumption2 Transmit waveguide losses 1.5 dB assumption3 Transmit antenna gain 27 dBi assumption4 satellite EIRP 35.5 dBW 1-2+35 Free space loss 196 dB6 Atmospheric absorption 0.1 dB Typical7 Receive antenna gain 40.2 dBi8 Receive waveguide loss 0.5 dB9 Received carrier power -121.7 dBW 4-5-6+7-8

10System noise temperature (140K) 21.5 dBK

11 Earth station G/T 18.2 dB/K 7-(8+10)

Link budget analysis for the uplink at frequency 6 GHz (C- Band)

12 Boltzmann's Constant -228.6 dBW/Hz/K13 Bandwidth (25 MHz) 74 dB Hz14 Noise Power -133.1 dBW 10+12+1315 Carrier to noise ratio 11.4 dB (9-14)16 Transmit power 29.3 dBW17 Transmit waveguide losses 2 dB18 Transmit antenna gain (7 m) 50.6 dBi19 uplink EIRP 77.9 dBW 16-17+1820 spreading loss 162.2 dB(m2)21 Atmospheric attenuation 0.1 dB22 flux density at spacecraft -84.4 dBW/m2 19-20-2123 Free space loss 200.4 dB24 receive antenna gain 26.3 dBi25 Receive waveguide loss 0.5 dB

26System noise temperature (450K) 26.5 dB(K)

27 spacecraft G/T -0.7 dB/K 24-25-26

Combining the uplink and downlink to estimate the overall link performance

28 Received G/T -122.9 dBW/K 19-23-21+2729 Boltzmann's Constant -228.6 dBW/Hz/K30 Bandwidth (25 MHz) 74 dB Hz31 Carrier to noise ratio 31.7 dB 28-29-30

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32 uplink C/N (31.7) 1479.1 ratio 3133 N/C (uplink) 0.000676 ratio34 downlink C/N (11.4) 13.8 ratio 1535 N/C (downlink) 0.0724 ratio36 Total thermal noise (Nth/C) 0.0731 ratio 33+3537 Total thermal C/Nth 13.7 ratio38 Total thermal C/Nth 11.4 dB39 Interference C/I 63.1 ratio assumption40 I/C 0.015848 ratio41 Total noise (Nth+1)/C 0.0889 ratio 36+4042 Total C/(Nth+1) 11.2 ratio43 Total C/(Nth+1) 10.5 dB44 Required C/N 8 dB

The above table demonstrates the example of link budget.

5.2 Link budgets and their interpretation

The link between the satellite and earth is governed by the basic microwave radio link equation given by

Pr =PtGtGrC2 / (4 )R 2f2 …………1

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Where, Pr power received by the receiving antennaPt is the power transmitted by the transmitting antennaGt is the gain of the transmitting antenna (true ration)Gr is the gain of receiving antenna (true ratio)C is the speed of light in m/sf is the frequency in hertz

The same formulae when converted into decibels have the form of a power balance.

pr=pt + gt+ gr + - 20 log (f.R) +147.6 ……….(2)

the received power is in the form of dBW.

The last two terms represent the free space path loss.

We can correct the path loss for other frequencies and path lengths using the formula:

Ao = 183.5 + 20log(f) + 20log(R/35788) ………….(3)

where Ao is the free space path loss in decibels, f is the frequency in GHz and D is the path length in Km.

The link power balance in the above equation considers only the free space path loss and ignores the effects of the different layers of earth's atmosphere. The following listing provides the dominant contributors that introduce additional path loss, which can vary with time. Some are due to air and water content of the troposphere, while others result from charged particles in the ionosphere.

Terms in the link budget.

1. The transponder onboard the satellite has a power output of 10 W equivalents to 10 dBW.

2. The microwave transmission line between the satellite power amplifier output andthe spacecraft antenna absorbs about 40% of the output radiated as heat. The loss is represented as positive number and then subtracted.

3. The satellite is made to cover a particular area of earth called the coverage area or footprint which determines the gain of antenna, there being an inverse relationship.

4. The EIRP specifies the maximum radiated power per transponder in the direction of a specific location on earth. This is the product of actual power given to

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transmitting antenna and antenna power gain of transmitting antenna.That the equivalent isotropic power (EIRP) may be defined as

EIRP=PTGT …………(4)EIRP is often expressed in decibels relative to 1 watt, or dBW. Let Pt be in Watts then

EIRP = [PT]+[GT] dBW …...(5)

5. Free space loss is the primary loss in the satellite link, amounting to 183 to 213dBfor frequencies between 1 to 30 GHz for a GEO satellite.

6. The atmospheric path loss is given by

Ao = 92.5+20 log(fD) ………(6)Where Ao = free space lossf= frequency in GHzD= distance in Km

7. The receiving antenna has the diameter of 3.2 m. The antenna gain is given by

GT=10 log (p2D2h/(3/f)2) ………(7)

8. Waveguide or cable loss between the antenna feed and low noise amplifier reduces the received signal and increases link noise. Thus we have assumed a small value accounting for that loss.

9. Received carrier power is calculated directly by the power is calculated directly by the power balance method. The computed value includes all the gains and losses inthe link.

10. The noise that exists in all the receiving systems is the main cause of degradation.

11. The earth station G/T is the difference in decibels between the net antenna gain and the system noise temperature converted to decibels i.e.

G/T (dB/K) = Receiver Antenna gain – 10 log(system noise temp) …(8)

12.-14 –The noise power that reaches the receiver is equal to the product

N= kTB ….(9)Where K= Boltzmann’s constantT= equivalent noise temperatureB= bandwidth

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15.The difference in decibels between received carrier power and the noise power is the carrier to noise a ratio.

C/No(dB Hz) = EIRP - path losses + G/T +228.6. ………(10)

16. The earth station high power amplifier provides the power to transmit the signal to satellite.

17. An allocation of 2 dB is made to account for the loss between the HPA and the earth station antenna feed.

18.A 7 m earth station antenna provides a 50.6 dBi gain.

19. Uplink EIRP must be sufficient to saturate the satellite transponder.

20. The spreading loss allows us to convert from earth station EIRP to the corresponding value of flux density at the face of the satellite receiver.

21. assumed value same as in downlink.

22. The SFD causes the transponder to transmit the maximum EIRP in the downlink.

Uplink EIRP = spreading loss + atmospheric loss –SFD ………(11)

23. The atmospheric path loss is given by (6).

24.The space craft antenna is designed to cover a specific area.

25.An allocation of 0.5 dB for loss between the spacecraft antenna and receiver.

26.The typical C- band satellite system has a noise temperature of 450 K.

27.The receiving system figure of merit given by G/T.

28.The value of c/t is given by

C/T = EIRP-Ao+ G/T. ………(12)

29.-31 These values are considered in the same manner as in 12.

32-43 The C/N is calculated as in eq 9 and the calculation is done to calculate whole C/N.

44. The required value of C/N is specified for receiver digital modulator.

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CONCLUSION

I have completed my project with the brief study of the transponders like Bent pipe or regenerative and components in the transponders. I have gained both the practical and theoretical knowledge of my project titled .Moreover I have also learned to calculatethe satellite link and various parameters such as free space path loss and G/T etc. that come in role during the communication of two stations via a satellite.

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BIBLIOGRAPHY

1) Satellite communication systems by: - G. Maral ,M.Bousquet

2) A handbook on satellite communication- compiled by K .Miya

3) Communication systems encyclopaedia – John Proakis

4) wikipedia.com

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