vsat_doc

8/7/2019 vsat_doc

http://slidepdf.com/reader/full/vsatdoc 1/24

A spirat ions of 24

VSATTECHNOLOGY

8/7/2019 vsat_doc


A spirat ions of 24

VSAT Introduction

VSAT stands for Ver y Small Aperture Terminal and refers to receive/transmitterminals installed at dispersed sites connecting to a central hub via satellite usingsmall diameter antenna dishes (0.6 to 3.8 meter). Traditionally, the satellite is a radiorelay station that receives, amplifies and redirects analog and digital signals containedwithin a carrier frequency. These signals contain data, voice, and videocommunications. VSAT systems can be configured for bi-directional or receive-onlyoperation. In bi-directional operation, the dish both sends (uplinks) and receives(downlinks) the information for use in LANs.

What is a Satellite ?Any object in the Solar system that revolves around another object that is either

static or in motion is a satellite to the latter. For e.g. Earth is a satellite to the Sun &the Moon is a satellite to Earth.

How are satellites classified?Broadly we can classify satellites into 2 types:

a) Natural Satellites: These are satellites that have been existing even beforeexistence of any living organism on earth e.g. Moon.

b) Man made satellites: These are satellites that have been placed into space byhuman being to achieve a specific purpose. These satellites are sophisticated

electronic communications relay station orbiting around the equator moving in afixed orbit at the same speed & direction of the earth. These satellites like all livingthings has a specific life time e.g. the INSAT series of satellites which have beenlaunched by India.

Why do we need man made satellites?Man made satellite is used in a variety of areas like weather forecasting,communication, navigation systems, television broadcasting etc.

What are the different kinds of man made satellite?Based on the orbit in which a satellite is placed we can classify man made satellites

as:a) LEOLEO stands for Low Earth Orbit satellite. These satellites circle the earth at a distancethat varies from 100 to 300 miles. The orbit in which these satellites are placed iscalled Polar Orbits. Leo's are also known as Polar Orbit satellites, a Polar Orbitsatellite travels from North-South direction. Since these satellites are very close toEarth and to avoid getting pulled back into the gravitational pull of the earth, theyhave to travel at speeds of 29,359 Kms/ hour and they circle the earth once in every

8/7/2019 vsat_doc


A spirat ions of 24

90 minutes. Hence they have a rather short life span, as the amount of fuel it uses tostay into orbit is very high. Polar Orbit satellites are mostly used for scanning theEarth's surface. Some of the most

popular areas where Polar Obits satellites are used are Weather Satellite & RemoteSensing satellites. Iridium is another example of LEO satellites.LEO Satellites are deployed for Mobile / Satellite Telephony applications. Goodexample is Iridium and ICO Global kind of networks.

b) MEOMEO stands for Medium Earth Orbit satellite. These satellites circle the earth at adistance that varies between 6,000 to 12,000 miles and would take approximately 5 to12 hours to circle the earth once. MEO's are most popular in GPS services.

c) GEOGEO stands for Geosyncronous Equatorial Orbit satellite. These satellites circle theearth at a distance of 22,282 miles or 36,000 Kms. These satellites move at the paceof the earth & will rotate at the same speed, as the earth. As the move at the samespeed of the Earth they appear to be stationary. A Satellite placed in the GEO Orbitwill take about 24 hrs to complete one rotation. These satellites rotate in an equatorialorbit. Since GEO's move along with the earth's rotation they will cover the same area

8/7/2019 vsat_doc


A spirat ions of 24

all their life. INSAT 2B and 2C are some examples of Geosynchronous satellites. Ittakes about 3 satellite placed in the Geosyncronous Orbit to completely cover Earth.

What is an orbit?The path, which a satellite takes to circle round its parent planet, is called Orbit.

What do these satellites consist of?Man made satellites that orbit earth & the sun are highly sophisticated tools thatinvolve complex electronics however two main components to all the satellites are thePayload and the Bus.The payload is all the equipment; a satellite needs to do its job. This can include

antennas, cameras, radar, and electronics. The payload is different for every satellite.For example, the payload for a weather satellite includes cameras to take pictures of cloud formations, while the payload for a communications satellite includes largeantennas to transmit TV or signals to Earth.The bus is the part of the satellite that carries the payload and all its equipment intospace. It holds all the satellite's parts together and provides electrical power,computers, and propulsion to the spacecraft. The bus also contains equipment thatallows the satellite to communicate with Earth.

Who uses these satellites and for what applications?Satellite services are used for a variety of applications. Satellites are used for weatherforecasting, TV Broadcast, GPS, Long Distance Telephony and DataCommunication.

How has satellite technology touched the life of a common man?Television, one of mans greatest invention of all times uses satellite technology. TVBroadcasters use satellite communication methods to ensure that you're favoriteprogram come to you on a flip of a single button on your remote. Have you everthought that without this satellite technology how would we ever have seen anytelevision channel at all? Why Satellite? Is another question that comes to mind.The only way the TV Broadcaster can reach millions of people covering large spansof territory is by using Satellite. This is not only economical but also cost effectivecompared to any other medium.Satellites again play a very important role in long distance telephony, rural telephony.Farmers depend on weather reports for their crop. These are a few areas of life wheresatellites have not only touched the life of a common man but also improved it for thebetter.

What are the advantages of using satellite technology?Some advantages of using satellite technology are:

8/7/2019 vsat_doc


A spirat ions of 24

a) Very high reliability, all onboard systems are fully redundant.b) Distance insensitivec) High bandwidth capacityd) No last mile issuese) Speedy installation

f) Mobile, can be used for short term or emergency communicationsg) Excellent for broadcast transmissionh) Bandwidth on demand

What are VSATs?

The term VSATs stand for Very Small Aperture Terminal, these are fixed satelliteterminals that are used to provide interactive or receive onlycommunications.

Why are VSATs used?VSATs are used for a wide variety of telecommunications applications such asCorporate networks, Rural Telecom, Distance Learning, Telemedicine, DisasterRecovery, Ship - Board communications (communication on large ships), etc.

Who uses VSATs?VSATs have become increasingly popular, because they are a flexiblecommunication platform that can be installed quickly & cost effectively to providetelecom solutions to consumers, governments & corporations.VSATs have a wide range of users starting from large corporates with large valuechains having a wide geographical spread to smaller organizations which have office

in different locations, the defense establishments, stock exchanges, manufacturing &FMCG companies are the typical users of VSAT's.

What are the advantages of using VSATs?Some of the advantages of using VSATs are:a) VSATs are highly reliable & boasts of uptimes as high as 99.5%b) Since VSATs use a satellite to communicate geographical boundaries or terrain isnot a constraint.c) A centrally managed network, which reduces a lot of logistics cost for thecustomer.d) In case of a failure the Mean Time to Repair is in the order of a few Hours.e) No last Miles for the customerf) Most important One Vendor Management.

What are the components that go into making a VSAT system?

AntennaPower AmplifierUp - Converter

8/7/2019 vsat_doc


A spirat ions of 24

Down ConverterModulatorDemodulatorCPELow Noise Amplifier

Antenna:

The antenna is responsible for transmitting, the amplified signal from the poweramplifier to the satellite and also receiving the signal from the satellite in conjunctionwith the low noise amplifier. The Antenna is parabolic in size.

Antenna

Antennas are the passive equipment, which serve the purpose of directing atransmission to a specific satellite as well as receiving the relevant transmission fromthe same. The Antenna systems also provide the mechanical support for mounting theRF units as well as the rest of the VSAT equipment configured for outdoor mounting.Antennae are specified for the frequency band of operation, directional gain, aperture

efficiency levels and the accuracy of orientation in the specific frequency.The antenna sizing for VSATs is one key aspect of Link design. The sizing dependson Frequency of operation: Antenna size varies in inverse proportion to the frequencyof operation for a given set of specifications like directional gain. A C/Ext C bandantenna with the same features shall be larger than a Ku band antenna.

8/7/2019 vsat_doc


A spirat ions of 24

Feed horn is a part of Antenna. It plays a vital role in a VSAT system. It receivessignal from the Satellite reflected to Antenna and fed to the Indoor unit. Also, thesignal which to be send to the Satellite is also thrown through this feed horn so thatthe signal reflecting to the reflector should reach Satellite.

Working of feed horn and reflector

Power Amplifier:The Power Amplifier is used for amplifying the Up converter RF signal before beingfed into the Antenna system. The Amplifier can be either Mounted on the Antenna

system or could be placed in the Indoor Rack. The amplification is required to sendthe up stream signals to the Satellite.

Low Noise Amplifier:The signal that travels from the satellite would have become weak due to variousatmospheric issues, the signal strength is reduced to a few watts hence the signal needto pass through an equipment that will increase the signal strength from a few watts toseveral Kilowatts.

8/7/2019 vsat_doc


A spirat ions of 24

The low noise amplifier is responsible for amplifying very low power satellite signalsreceived at the antenna to a higher signal strength before it is fed into the down converter.

Down-Converter:A down converter amplifies and converts the frequency (RF to IF), which isreceived from the low noise amplifier. This is then passed on to the demodulator.

Up-ConverterAn up-converter amplifies & converts the frequency (IF to RF), that is received fromthe modulator. This is then passed on to the power amplifier for further amplificationand transmission.

Demodulator & Modulators:Demodulator is responsible for converting the IF signals into digital format. This isunderstood by the networking components like Routers, Switches, Telephonesystems, etc. and the same is then fed into the computer.Modulators on the contrary are responsible for converting the digital data into IFsignals.

What is a VSAT hub?A VSAT hub is a huge earth station that is responsible for controlling & monitoringall

the activities of the geographical spread of VSATs.In some cases all the remote VSATs communicate to one central site, this CentralSite is connected to the hub, as the Hub is the switching element.

Multiplexing TechniquesA satellite link can relay signals from a single earth station. These signals must beseparated to avoid interfering with each other. This separation is called multiplexing.The most common forms of multiplexing are:

8/7/2019 vsat_doc


A spirat ions of 24

a) FDM - Frequency Division MultiplexingA group of signals pass through the same channel but on different frequencies.b) TDM -Time Division MultiplexingA group of signals take turns in different time intervals to use a channel.Theoretically either multiplexing technique can be used with analog or digital

modulation. But, TDM is more easier to implement when the content is digital. Thisis because digital signals are precisely timed and consists of groups of short pulseswith relatively long intervals between them. FDM is more convenient with analog.

Multiple access is "the ability of a large numbers of remote stations to simultaneouslyinterconnects their respective voice, data, Teletype, facsimile and television linksthrough a satellite".The multiple-access is fundamental to satellite communication because it is the meansby which the wide geographical coverage capability and broadcast nature of thesatellite channel are exploited. It affects all the elements of the system, determines thesystem capacity and flexibility, and has a strong influence on costs. It involves how to

permit a changing group of remote stations to share a satellite in a way that optimizes(1) satellite capacity(2) spectrum utilization(3) satellite power(4) interconnectivity(5) flexibility(6) Adaptability to different traffic mixes(7) Cost(8) user acceptabilityUsually all the elements in this list cannot be optimized and some may have to betraded off against others.Classically there are three main multiple access techniques, they are:

a) FDMA (Frequency Division Multiple Access) - All the users share the satellite atthe same time. But, each transmits in its own unique frequency band. This ismost commonly employed with analog modulation, where signals are present allthe time.

8/7/2019 vsat_doc


A spirat ions of 24

b) TDMA (Time Division Multiple Access) - All the users transmit in turn in theirown

unique time slots. While transmitting, each occupant has exclusive use of one ormore transponders. The intermittent nature of TDMA transmission makes itparticularly attractive for digital modulation.

c) CDMA (Code Division Multiple Access) -Many earth stations simultaneouslytransmit orthogonal coded spread-spectrum signals that occupy the same frequencyband. Decoding ("de-spreading") systems receive the combined transmissions frommany stations and recover one of them.In all the three classical multiple access schemes some resource is shared. If theproportion allocated to each earth station is fixed in advance, the system is calledfixed access (FA) or Pre-assigned Access (PA). If the resource is allocated as neededin response to changing traffic conditions, the multiple access arrangement is termedDemand Access (DA).Frequency Division Multiple Access (FDMA) - FDM/FM/FDMAFrequency division multiple access with FM frequency division multiplexing isabbreviated as FDM/FM/FDMA. In it a remote station is permanently assigned acarrier frequency.The station frequency modulates all its outgoing traffic, whatever the destination, onthat carrier. An originating station's traffic capacity is limited by its allocatedbandwidth and the carrier to noise ratio (denoted as C/N) that it can achieve on thedownlink. The carrier frequencies and bandwidth assigned to all the remote stationsconstitutes a satellite frequency plan.Every station that operates in an FDM/FM/FDMA network must be able to receiveatleast one carrier from all the stations in the network. Thus most FDM/FM/FDMAstations have a large number of separate IF receivers & de-multiplexers. Satellite

8/7/2019 vsat_doc


A spirat ions of 24

FDM/FM/FDMA were patterned after the terrestrial analog telephone microwave andcable transmission systems used in the early days of the Intelsat system.Some common example of the FDMA system is Our PAMA & DAMA services.Time Division Multiple Access:In time division multiple access (TDMA) a number or earth stations take turns

transmitting bursts through a common transponder. Since all practical TDMAsystems are digital, TDMA has all the advantages over FDM/FM/FDMA that digitaltransmission usually has over analog.TDMA is easy to reconfigure for changing traffic demands, resists noise andinterference, and mixes voice and data traffic. But, one advantage of TDMA systemsis that it permits a transponder's TWT (Traveling Wave Tube) to operate at or nearsaturation and thus it maximizes the downlink (C/N).Since only one carrier is in the TWT at a time, there are no inter-modulation productsto worry about and no back off is necessary. Many of the concepts for time divisionmultiplexing (TDM) apply without change to TDMA.

In TDM digital data streams from many sources are transmitted sequentially inassigned time slots; the slots are organized into frames that also containsynchronization information. A receiving station must first recover the transmittercarrier frequency, then recover the transmitting station clock pulses, and then identifythe start of each frame so that it can recover each transmitted channel and route it onto its destination. The principal difference is that in TDM everything comes from thesame transmitter. The clock and the carrier frequencies do not change.While, in TDMA each frame contains a number of independent transmission. EachTDMA station has to know when to transmit, and it must be able to recover thecarrier & clock for each receive burst in time to sort out all wanted basebandchannels.

8/7/2019 vsat_doc


A spirat ions of 24

TDMA Frame Structure & Design :-In TDMA transmissions a group of earth stations, each a different distance from asatellite, must transmit individual bursts of RF energy in such a way that the burstsarrive at the satellite in a prescribed order. The stations have to adjust theirtransmissions to compensate for the variations in satellite range, and they must beable to enter or leave the network without disrupting its operations.These goals are accomplished by organizing TDMA transmissions into framescontaining reference bursts that establish absolute time for the network. Each stationtransmits once per frame so that its burst begins to leave the satellite a specified timeinterval before or after the start of a reference burst.Each frame contains one (or two for redundancy) reference burst and a series of

traffic bursts. Each traffic burst contains a preamble, which provides synchronization(sync) and signaling information and identifies the transmitting station, followed by agroup of traffic bits. The traffic bits are the revenue-producing portion of the frame,and the reference bursts and the preamble constitutes system overhead. The smallerthe overhead, the more efficient a working TDMA system is, but the difficulty it mayhave in acquiring and maintaining sync.

Code Division Multiple AccessCode Division Multiple Access (CDMA) is a scheme in which, a number of usersoccupy all of a transponder bandwidth all of the time. Their signals are encoded sothat information from an individual transmitter can be detected and recovered only bya properly synchronized receiving station that knows the code being used. Thisprovides a decentralized satellite network, as only the pairs of stations that arecommunicating need to coordinate their transmissions.

Subject to transponder limitations and the practical constraints of the codes in use,stations having traffic can access a transponder on demand without coordinating theirfrequency (as in FDMA) or their time slot (as in TDMA) with any central authority.Each receiving station has its own code called its "address", and a transmitting stationsimply modulates its transmission with the address of the intended receiver wheneverit wishes to send a message to that receiver.CDMA is most suited for a military tactical communication environment where manysmall groups of mobile stations communicate briefly at irregular intervals than to acommercial environment where large volumes of traffic pass continuously between asmall number of fixed locations.

8/7/2019 vsat_doc


A spirat ions of 24

Mesh and Star networksVSAT's are connected by radio frequency links via a satellite. Those links are radiofrequency links with a so-called ùnlink' from the station to the satellite and a so-called `downlink' from the satellite to the station. The overall link from station tostation, sometimes called a Hop, consists of an uplink and downlink. A radio

frequency link is a modulated carrier conveying information. Basically the satellitereceives the unlinked carriers from the transmitting earth stations within he field of view of its receiving antenna, amplifies those carriers. Translates their frequency to alower band in order to avoid possible output / input interference, and transmits theamplified carriers to the stations located within the field of view of its transmittingantenna.Present VSAT networks use Geo-stationary satellites and as a result all the VSAT'sare visible from the satellite all the time, carriers can be relayed by the satellite fromany e VSAT to any other VSAT in the network. These are nothing but Meshnetworks. However in mesh networks one must take into account the followinglimitations:

· Typically 200 dB carrier power attenuation on the uplink & the downlink as a resultof the distance to and from a GEO-stationary satellite· Limited satellite radio frequency power, typically few tens of watts· Small size of the VSAT, which limits its transmitting power and its receivingsensitivityAs a result of the above limitations, it may well be that the demodulated signals at thereceiving VSAT do not match the quality requested by the user terminals. Thereforedirect links form VSAT to VSAT may not be acceptable.The solution then is to install in the network a station larger than a VSAT, called theHub. The Hub station has a larger antenna size than those of a VSAT, say 4 meters to11 meters. This results in higher gain than that of a typical VSAT antenna, and it isalso equipped with a more powerful transmitter. As a result of its improvedcapability, the hub station is able to receive adequately all the carriers transmitted bythe VSATs, and to convey the desired information to all the VSATs by means of itsown transmitted carriers. These are nothing but Star networks.The links from the Hub the VSAT are named òutbound links'. The ones from theVSAT to the Hub are named ìnbound links'. Both inbound and outbound linksconsist of two links, uplink and downlink to & from the satellite.

There are two alternatives to star shaped VSAT networks:· One-way networks: where the hub transmits carriers to receive only VSATs. Thisconfiguration supports broadcast services from a central site where the hub is locatedto remote sites where the receive-only VSATs are installed.· Two-way networks: where VSATs can transmit & receive. Such networks supportinteractive traffic.One Way connectivity happens where there is a broadcast from the central location toall the remotes.The two-way connectivity between VSAT's can be achieved in two ways:

8/7/2019 vsat_doc


A spirat ions of 24

Either direct links from VSAT to VSAT via the central Hub, hence making it adouble hop scenario, with a first hop from VSAT to hub and then a second hop usingthe hub as a relay to destination VSAT. The second way is by single hop links viasatellite in a star shaped network In conclusion, star shaped networks is imposed by power requirements resulting from

the reduced size and hence the low costs of the VSAT earth station in conjunctionwith power limitation of satellites. Meshed networks are considered whenever suchlimitations do not hold, or are unacceptable. Meshed networks have the advantage of reduced propagation delay (single hop delay is 0.25 sec's instead of 0.5 sec's fordouble hop) which is especially of interest for telephony services.

What are the different access methods used in VSAT communication?

Various Access methods used in VSATs to communicate with each other are:1. SCPC Single Channel Per Carrier ( In simple terms this is nothing but lease lines in the sky).SCPC Channels can be either PAMA or DAMA.2. TDMA

PAMA: Pre Assigned Multiple AccessPAMA is an access scheme where in when 2 VSATs want to communicate with Eachother a bandwidth is assigned to them exclusively. This assigned bandwidth will Beavailable the VSAT's on a permanently basis. This link can either be a symmetric andasymmetric link. It is nothing but a point to point connectivity.

8/7/2019 vsat_doc


A spirat ions of 24

Point to Point connectivity, Leased Line in the sky

The PAMA service interconnects high data traffic sites within an organization. It is acost-effective alternative to terrestrial leased lines, providing high reliability links tosupport mission critical applications. DAMA: Demand Assigned Multiple Access The

DAMA scheme is very similar to a telephone connection. Whenever, there is a needto talk to someone, you dial a number. The call lands at the telephone exchange, andthe telephone exchange connects you to the dialed number.The role of the telephone exchange is to connect you to the desired number. This isexactly how a DAMA network operates. The HUB plays the role of a telephoneexchange, between any two VSAT's. The DAMA service addresses point to pointvoice, fax, and data communication requirements of remote sites. It provides a costeffective and reliable solution to business having a high internal voice/ faxcommunication requirements. Additionally it enables organizations with operations inremote areas, to establish a reliable communications network.

Typical DAMA / PAMA Network

TDM/TDMA: Time Division Multiplexed/Time Division Multiple Access TheTDMA network operates in a Star topology. All the remote VSATs communicate tothe central hub station, on a Time Division Multiple Access Modes. At the hub thesignal is re -transmitted to the destination VSAT using TDM technology afteramplification. The Access mechanism of TDMA operates on a technology calledSlotted Aloha. All the remote VSAT's contend for a time divisional slot to transmittheir packets to the Hub. The channel used by the remotes to communicate to the Hubis called the Return Link. Each of these return channels operates at a maximum of 128 Kbps.The Hub communicates to all the destination remotes using the TDM technology.

8/7/2019 vsat_doc


A spirat ions of 24

The communication channel from the Hub to the remote is also called the Outboundor Outlink. The outbound or outlink from operates at 256 Kbps.In TDM/TDMA the implementation topology is a Hub & Spoke architecture here allthe remote sites communicate to the central site via the HUB. The Hub is connectedto the central site on an SCPC connection.

Typical applications on TDMA· Interactive DataEnterprise Resource Planning solutions like SAP, BPCS, BAAN, JD Edwards, toname afew have been implemented on TDMA VSAT network. These solutions requireinteractive data communication between remote sites and the central host site. Thenetwork is also suited to carry intra-office e-mail traffic from cc: Mail, MSMailamongst others.The VSATs support multiple protocols enabling them to interface with existingcustomer networks.

· Data BroadcastContinuous data broadcast to a large number of locations as in a stock exchangeapplication or occasional file transfer from a central location to multiple remotelocations is supported on the TDMA VSAT network. This is supported using theValidate system, which is part of the TDMA VSAT system. The Validate systemprovides both confirmed data broadcast, as required in file transfer applications andalso unconfirmed data broadcast to meet continuous data feed transmission.

· Occasional Voice on TDMA networksThe TDMA systems also offer voice communication support. This is suitable tointerconnect remote locations. Various encoding rates are offered ranging from 4.8to 16 Kbps. This gives the customer a choice to choose the appropriate voice qualityas per the requirement.

8/7/2019 vsat_doc


A spirat ions of 24

What are the different bands available on a satellite?Any Satellite has different frequency bands available on it the table below showswhat all bands are available with its operation frequencies in which the satellite uplinks & down links.

SL.NO BAND UP LINK RANGE(GHZ)

DOWNLINKRANGE (GHZ)

1 C –BAND 5.925-6.425 3.700-4.200

2 EX C BAND 6.725-7.025 4.500-4.800

3 KU BAND 14.00-14.50 10.95-11.70

4 KA BAND 30.00 20.00

What are the advantages & disadvantages of each band?

BAND ADVANTAGE DISADVANTAGE

C-BANDBroad footprintLittle rain fade

InterferenceLarge Antenna and Amplifier

EX-C -BAND

Broad footprintLittle rain fade

Less Interference

Weak signalsLarge Antenna size

Large Amplifier

KU -BAND

Focused footprintLess terrestrial Interference

Smaller AntennaSmaller Amplifiers

Interference due to rain

KA-BAND

Focused footprintLess terrestrial Interference

Smaller AntennaSmaller Amplifiers

Interference due to rain

8/7/2019 vsat_doc


A spirat ions of 24

What is a footprint?The area that is covered by the beam of a satellite is called a footprint. For e.g. in the

figure above the marked area is a footprint of a satellite.

Different kinds of footprints are:· Global Beam: coverage of entire surface of the earth that is visible by the satellite.· Hemisphere Beam: coverage only of the hemisphere region.· Spot Beam: coverage only on a particular region, e.g. Coverage only of the Indiansub-continent

What is Rain Fade?Rain Fade is an interruption of Wireless communication signals. As a result of rain orsnow droplets whose separation approximates the signal wavelengths. Thisphenomenon can effect satellite connectivity and all satellite based communication.Rain fade usually does not last long. Once a heavy shower or squall has passed,

normal communication returns. However, during tropical storms or severe winterstorms at northern latitudes, fadeouts can persist for hours at a time. The phenomenonoccurs with all types of satellite systems.

Coding and ModulationModulationModulation is a technique where in baseband data is superimposed on a carrier fortransmission. There are different modulation techniques that have evolved over the

8/7/2019 vsat_doc


A spirat ions of 24

years. However in digital data transmission the most primitive modulation techniqueis called PSK (Phase Shift Keying). The reason why its called phase shift keying, isbecause in digital data, the data are in 0's or 1's. This data is represented by the phaserelationship of the RF carrier to itself or to a reference.For example the phase offset of the carrier in one direction may represent one type of

data, then, a phase offset in the opposite direction may represent another type of data.There are different types of modulation techniques that have evolved over the yearsfor digital data transmission. They are:

a) BPSK: Binary Phase Shift KeyingIn the BPSK modulation technique the zeros and ones are represented by two phasesof the RF carrier signal, which differ by 180 degrees.

b) QPSK: Quadrature Phase Shift KeyingIn QPSK modulation, zeros & ones are represented by four phases of the RF carrier,each differing by 90 degrees from the next.

c) 8 PSK: 8 Phase Shift KeyingIn 8 PSK modulation, zeros & ones are represented by 8 phases of the RF carrier,each differing by 45 degrees from the next.

d) 16 PSK: 16 Phase Shift KeyingIn 16 PSK modulation, zeros & ones are represented by 16 phases of the RF carrier,differing by 22.5 degrees from the next.

e) M-PSK: Multi Phase Shift KeyingIn M-PSK modulation, zeros & ones are represented by multiple phases of the RFcarrier. The difference would vary in accordance to the output required.As the difference in the phase shifts increase, the probability of increase in the errorbecomes higher. Due to this reason the most widely used modulation techniques areBPSK & QPSK modulation.

In QPSK modulation, two information bits are encoded at one time. This means thatwhen transmitting the data in QPSK, the phase of the RF carrier must change at onlyhalf the rate.Both BPSK & QPSK are extremely efficient modulation techniques. With carefulfiltering techniques, bit error rate (BER) performance of 1 to 2 dB of the theoreticallimit may be achieved. To achieve this low error rate, one approach is to filter thebaseband or digital data before modulation with a Nyquist filter. Such a filter not onlyallows optimum performance to be achieved, but also constraints the PSK signal tothe minimum possible bandwidth.

Advantages of each approachAn analysis of PSK modulation shows that the theoretical performance of BPSK andQPSK modulation is identical in a channel dominated by Gaussian noise, such as a

8/7/2019 vsat_doc


A spirat ions of 24

satellite channel. This allows the choice to be made between BPSK and QPSK basedon other considerations.With QPSK, the transmitted spectrum occupies only half the bandwidth of BPSK, andwould therefore be a good choice in an environment where bandwidth efficiency isrequired.

The prime advantage of BPSK is that it is much more tolerant to phase noise thanQPSK. If the system is designed from the beginning with BPSK in mind, then lowercost microwave equipment can be used in the up and down conversion process,without compromising performance. Likewise, in a burst mode system, BPSK has asecond advantage over QPSK as the burst demodulator takes shorter acquisition time.This allows the frame overhead to be kept to a minimum leading to increasedefficiency while utilizing a lower cost transponder.Hence when designing the overall system, the designer tries to make optimum use of the satellite characteristics. There are three factors that have to be borne in mind whendesigning a particular system and choosing the modulation scheme:· Satellite limitations

· The total power of all the desired carriers must not exceed a certain power level.· The total bandwidth of all the desired carriers must not exceed the bandwidth of thetransponder.· Hardware costs· System Goals

Forward Error Correction (FEC)In forward error correction a few coding bits are added to the actual information datastream. The added bits have an in built mechanism to identify & rectify errors at thereceiving end. This is done to achieve good bit error rates and low carrier to noiseratio.There are different techniques used in FEC starting from ½, ¾, 5/6, 7/8, etc.Here the numerator denotes the actual information bit & the denominator denotesinformation bit + coding bit. When ½ FEC is used it means that to every 1 bit of actual information 1 coding bit is used. Similarly when 5/6 FEC is used 1 coding bitis added to every 5 information Bits.

Bit and Symbol Error RatesBit Error Rate (BER) is also called the Bit Error Probability (PB). Mathematicallythis is the probability that a bit sent over the link would be received incorrectly (i.e.that a 1 will be read as a 0 or vice versa) or, alternatively, the fraction of a largenumber o f transmitted bits will be received incorrectly. Like a probability, it isusually statedas a single number

8/7/2019 vsat_doc


A spirat ions of 24

for example 1 x 10 -4 or .001. The BER plays the same role as an indicator of quality ina digital communication system that the signal-to-noise ratio plays in an analog link.Physically a bit error occurs because a symbol error has occurred. At some point inthe link noise has corrupted the transmitted symbol so that the decision circuitry at thereceiver cannot identify it correctly. For example, the carrier phase may have been

transmitted as +90 degrees but additive noise may have changed the received carrierphase to 90 degrees.

VSATs in the Indian contextWhat is ISRO's role been in the Indian satellite industry as well as in the VSATindustry?ISRO stands for Indian Space Research Organization, setup in June 1972 under theDepartment o f Space program. The primary objective of ISRO is to developsatellites, launch vehicles, rockets and associated ground systems.Some of the achievements of ISRO over the years has been launch of satellitesAryabhata, Bhaskara, Rohini, INSAT series, IRS, etc. ISRO has also been the key

organization behind the development of satellite launch vehicles like the PSLV (PolarSatellite Launch Vehicle), GSLV (Geosyncronous Satellite Launch Vehicle) etc. Thisin a nutshell has been ISRO's role in the Indian Satellite industry.

What are the regulatory bodies that govern VSAT Service providers?The various bodies that govern the VSAT Service providers and lays down rules &norms to be followed by them are:

a) TRAI: Telecom Regulatory Authority of IndiaTRAI is an autonomous governing body that lays down guidelines &recommendations to DOT on policy making. However weather these inputs arefollowed by DOT is at the discretion of DOT. However TRAI is not a body that isdirectly involved in governing any service providers.

b) DOT: Department Of Telecommunication

8/7/2019 vsat_doc


A spirat ions of 24

DOT has a role in policy making, licensing and co ordination of matters related totelegraphs, telephones, wireless, data, facsimile and telematic services and otherforms of communication. In addition DOT is responsible for frequency managementin the field of radio communication in close coordination with international bodies. Italso enforces wireless regulatory measures for wireless transmissions by wireless

users in the country.

c) WPC: Wireless Planning & Coordination wingWPC is a part of DOT created in 1952 under the Ministry of Communications, is thenational radio regulatory authority responsible for frequency spectrum management,including licensing and caters to the need of all wireless users in the countrygovernment or private, security or non-security. It is also the national nodal agencyfor all matters related to ITU (International Telecommunications Union) and the AsiaPacific Telecomm unity (APT) and is responsible for treaty obligations on behalf of the Government of India. It also exercises the statutory functions of the CentralGovernment and issues licenses to establish, maintain and operate wireless stations as

well as possess, develop and deal in wireless equipment in the country.d) SACFA: Standing Advisory Committee on Frequency AllocationThe Standing Advisory Committee on Frequency Allocations (SACFA) is a highlevel committee chaired by secretary (DOT)/Chairman, Telecom Commission. Headsof major wireless users/administrative ministries of the govt. of India, Member(Technology), Telecom Commission, and Wireless Adviser to the govt. of India, JointSecretary, DOT are its members. WPC wing of the ministry of communicationsprovides secretariat help to the committee. Joint Wireless Adviser, WPC wing is themember-secretary of the committee.

The main functions of the committee are to make recommendations on:· Major frequency allocation issues,· Formulation of National Frequency Allocation Plan,· Making recommendations on various issues related to InternationalTelecommunications Union (ITU)

· Asia Pacific Telecomm unity (APT),· To sort out the problems referred to the committee by various wireless users, sitingclearance of all wireless installations in the country, etc. SACFA clearances areissued after getting 'no objection' from various SACFA members who have to carryout detailed technical evaluation including field surveys, etc. at times they have toobtain evaluations from their field units. The technical evaluation is done primarilyfor:a) Aviation hazards.b) Obstruction to line of site of existing/planned networksc) Interference (Electro Magnetic Interference (EMI)/Electro Magnetic Compatibility(EMC)) to existing and proposed networks.

8/7/2019 vsat_doc


A spirat ions of 24

TYPICAL VSAT INSTALLATION

Site Survey

Before installation of any VSAT we need to know some parameters which are verycritical. To get these parameter we have to do survey of the site where VSAT to beinstalled. The general parameters are :-

1. Look angle of the Antenna - VSATs will send and receive RF signals fromrespective satellite. Hence, we have to find out the coordinates of the site. Thecoordinates indicates latitude and longitude of the site. To get this Lat andLong information site survey engineers use GPS equipment. GPS is GlobalPositioning System which gather information from satellites the lat and longinformation. Calculating the coordinates of the site and coordinates of thesatellite we can find out the look angle of the VSAT. This look angle consists

elevation ( vertical ) and azimuth ( horizontal ) angles.2. Line of sight ( LOS ) – There must not be any obstruction from VSAT to

respective satellite. Hence , clear LOS is essential.

8/7/2019 vsat_doc


3. Base – VSAT Antenna system placed with the support of ballast. Ballasts arenothing but stones or bricks which are placed to keep the Antenna stable

against wind, thrust etc. Physically stability is very much required for a stablenetwork connectivity. Hence, the base ( roof top or ground ) must be flat andstrong to withstand the load of the VSAT Antenna. If the base is slanting thenthere is a risk slippage or if the base is not strong enough then damage of thebase may happen. At the time of site survey all these information is very muchrequired. A 3.8 meter PAMA outdoor unit may weight 3000 kg and underwind pressure at the time of thunder storm may rise subsequently.

4. Length of the IF cable from Outdoor Unit to Indoor Unit – The shortest lengthis better. We have to found the route of the cable so that the cable should besafe and should travel shortest path from ODU to IDU

5. Electrical Interference – The RF signal is prone to interfere with other signals.High voltage electrical system or cable should not be around VSAT system.The survey must find out best place to keep ODU and IDU free from electricalinterference.

6. Working space – some time it has been found that the best place to keep aVSAT ODU is such that there is not space to work for servicing orinstallation. Safety of the engineers may be an issue for such case. We mustkeep this into mind at the time of survey.

7. Electrical Earthing – All communication equipment are prone to damageagainst floating current. If the Earthing is not proper then this floating currentmay cause damage to the electronic parts of the VSAT system. In generalEarth to Neutral voltage must be below 2 volts, so that negligible current willflow. Besides, Neutral should be strong, so that though Earthing may beperfect but due to weak neutral floating current may generate and cause of damage to the equipments.

vsat_doc

Documents