VSAT TECHNOLOGY Aspirations Page 1 of 24

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1 VSAT TECHNOLOGY Page 1 of 24

2 VSAT Introduction VSAT stands for Ve r y Small Aperture Terminal and refers to receive/transmit terminals installed at dispersed sites connecting to a central hub via satellite using small diameter antenna dishes (0.6 to 3.8 meter). Traditionally, the satellite is a radio relay station that receives, amplifies and redirects analog and digital signals contained within a carrier frequency. These signals contain data, voice, and video communications. VSAT systems can be configured for bi-directional or receive-only operation. 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 before existence of any living organism on earth e.g. Moon. b) Man made satellites: These are satellites that have been placed into space by human being to achieve a specific purpose. These satellites are sophisticated electronic communications relay station orbiting around the equator moving in a fixed orbit at the same speed & direction of the earth. These satellites like all living things has a specific life time e.g. the INSAT series of satellites which have been launched 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) LEO LEO stands for Low Earth Orbit satellite. These satellites circle the earth at a distance that varies from 100 to 300 miles. The orbit in which these satellites are placed is called Polar Orbits. Leo's are also known as Polar Orbit satellites, a Polar Orbit satellite travels from North-South direction. Since these satellites are very close to Earth and to avoid getting pulled back into the gravitational pull of the earth, they have to travel at speeds of 29,359 Kms/ hour and they circle the earth once in every Page 2 of 24

3 90 minutes. Hence they have a rather short life span, as the amount of fuel it uses to stay into orbit is very high. Polar Orbit satellites are mostly used for scanning the Earth's surface. Some of the most popular areas where Polar Obits satellites are used are Weather Satellite & Remote Sensing satellites. Iridium is another example of LEO satellites. LEO Satellites are deployed for Mobile / Satellite Telephony applications. Good example is Iridium and ICO Global kind of networks. b) MEO MEO stands for Medium Earth Orbit satellite. These satellites circle the earth at a distance that varies between 6,000 to 12,000 miles and would take approximately 5 to 12 hours to circle the earth once. MEO's are most popular in GPS services. c) GEO GEO stands for Geosyncronous Equatorial Orbit satellite. These satellites circle the earth at a distance of 22,282 miles or 36,000 Kms. These satellites move at the pace of the earth & will rotate at the same speed, as the earth. As the move at the same speed of the Earth they appear to be stationary. A Satellite placed in the GEO Orbit will take about 24 hrs to complete one rotation. These satellites rotate in an equatorial orbit. Since GEO's move along with the earth's rotation they will cover the same area Page 3 of 24

4 all their life. INSAT 2B and 2C are some examples of Geosynchronous satellites. It takes 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 that involve complex electronics however two main components to all the satellites are the Payload 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 large antennas to transmit TV or signals to Earth. The bus is the part of the satellite that carries the payload and all its equipment into space. It holds all the satellite's parts together and provides electrical power, computers, and propulsion to the spacecraft. The bus also contains equipment that allows 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 weather forecasting, TV Broadcast, GPS, Long Distance Telephony and Data Communication. How has satellite technology touched the life of a common man? Television, one of mans greatest invention of all times uses satellite technology. TV Broadcasters use satellite communication methods to ensure that you're favorite program come to you on a flip of a single button on your remote. Have you ever thought that without this satellite technology how would we ever have seen any television 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 spans of territory is by using Satellite. This is not only economical but also cost effective compared 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 where satellites have not only touched the life of a common man but also improved it for the better. What are the advantages of using satellite technology? Some advantages of using satellite technology are: Page 4 of 24

5 a) Very high reliability, all onboard systems are fully redundant. b) Distance insensitive c) High bandwidth capacity d) No last mile issues e) Speedy installation f) Mobile, can be used for short term or emergency communications g) Excellent for broadcast transmission h) Bandwidth on demand What are VSATs? The term VSATs stand for Very Small Aperture Terminal, these are fixed satellite terminals that are used to provide interactive or receive -only communications. Why are VSATs used? VSATs are used for a wide variety of telecommunications applications such as Corporate networks, Rural Telecom, Distance Learning, Telemedicine, Disaster Recovery, Ship - Board communications (communication on large ships), etc. Who uses VSATs? VSATs have become increasingly popular, because they are a flexible communication platform that can be installed quickly & cost effectively to provide telecom solutions to consumers, governments & corporations. VSATs have a wide range of users starting from large corporates with large value chains 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 is not a constraint. c) A centrally managed network, which reduces a lot of logistics cost for the customer. 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 customer f) Most important One Vendor Management. What are the components that go into making a VSAT system? Antenna Power Amplifier Up - Converter Page 5 of 24

6 Down Converter Modulator Demodulator CPE Low Noise Amplifier Antenna: The antenna is responsible for transmitting, the amplified signal from the power amplifier to the satellite and also receiving the signal from the satellite in conjunction with the low noise amplifier. The Antenna is parabolic in size. Antenna Antennas are the passive equipment, which serve the purpose of directing a transmission to a specific satellite as well as receiving the relevant transmission from the same. The Antenna systems also provide the mechanical support for mounting the RF 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 depends on Frequency of operation: Antenna size varies in inverse proportion to the frequency of operation for a given set of specifications like directional gain. A C/Ext C band antenna with the same features shall be larger than a Ku band antenna. Page 6 of 24

7 Feed horn is a part of Antenna. It plays a vital role in a VSAT system. It receives signal from the Satellite reflected to Antenna and fed to the Indoor unit. Also, the signal which to be send to the Satellite is also thrown through this feed horn so that the 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 being fed 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 send the up stream signals to the Satellite. Low Noise Amplifier: The signal that travels from the satellite would have become weak due to various atmospheric issues, the signal strength is reduced to a few watts hence the signal need to pass through an equipment that will increase the signal strength from a few watts to several Kilowatts. Page 7 of 24

8 The low noise amplifier is responsible for amplifying very low power satellite signals received 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 is received from the low noise amplifier. This is then passed on to the demodulator. Up-Converter An up-converter amplifies & converts the frequency (IF to RF), that is received from the modulator. This is then passed on to the power amplifier for further amplification and transmission. Demodulator & Modulators: Demodulator is responsible for converting the IF signals into digital format. This is understood by the networking components like Routers, Switches, Telephone systems, etc. and the same is then fed into the computer. Modulators on the contrary are responsible for converting the digital data into IF signals. What is a VSAT hub? A VSAT hub is a huge earth station that is responsible for controlling & monitoring all the activities of the geographical spread of VSATs. In some cases all the remote VSATs communicate to one central site, this Central Site is connected to the hub, as the Hub is the switching element. Multiplexing Techniques A satellite link can relay signals from a single earth station. These signals must be separated to avoid interfering with each other. This separation is called multiplexing. The most common forms of multiplexing are: Page 8 of 24

9 a) FDM - Frequency Division Multiplexing A group of signals pass through the same channel but on different frequencies. b) TDM -Time Division Multiplexing A 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. This is because digital signals are precisely timed and consists of groups of short pulses with relatively long intervals between them. FDM is more convenient with analog. Multiple access is "the ability of a large numbers of remote stations to simultaneously interconnects their respective voice, data, Teletype, facsimile and television links through a satellite". The multiple-access is fundamental to satellite communication because it is the means by which the wide geographical coverage capability and broadcast nature of the satellite channel are exploited. It affects all the elements of the system, determines the system 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 acceptability Usually all the elements in this list cannot be optimized and some may have to be traded 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 at the same time. But, each transmits in its own unique frequency band. This is most commonly employed with analog modulation, where signals are present all the time. Page 9 of 24

10 b) TDMA (Time Division Multiple Access) - All the users transmit in turn in their own unique time slots. While transmitting, each occupant has exclusive use of one or more transponders. The intermittent nature of TDMA transmission makes it particularly attractive for digital modulation. c) CDMA (Code Division Multiple Access) -Many earth stations simultaneously transmit orthogonal coded spread-spectrum signals that occupy the same frequency band. Decoding ("de-spreading") systems receive the combined transmissions from many stations and recover one of them. In all the three classical multiple access schemes some resource is shared. If the proportion allocated to each earth station is fixed in advance, the system is called fixed access (FA) or Pre-assigned Access (PA). If the resource is allocated as needed in response to changing traffic conditions, the multiple access arrangement is termed Demand Access (DA). Frequency Division Multiple Access (FDMA) - FDM/FM/FDMA Frequency division multiple access with FM frequency division multiplexing is abbreviated as FDM/FM/FDMA. In it a remote station is permanently assigned a carrier frequency. The station frequency modulates all its outgoing traffic, whatever the destination, on that carrier. An originating station's traffic capacity is limited by its allocated bandwidth and the carrier to noise ratio (denoted as C/N) that it can achieve on the downlink. The carrier frequencies and bandwidth assigned to all the remote stations constitutes a satellite frequency plan. Every station that operates in an FDM/FM/FDMA network must be able to receive atleast one carrier from all the stations in the network. Thus most FDM/FM/FDMA stations have a large number of separate IF receivers & de- multiplexers. Satellite Page 10 of 24

11 FDM/FM/FDMA were patterned after the terrestrial analog telephone microwave and cable 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 TDMA systems are digital, TDMA has all the advantages over FDM/FM/FDMA that digital transmission usually has over analog. TDMA is easy to reconfigure for changing traffic demands, resists noise and interference, and mixes voice and data traffic. But, one advantage of TDMA systems is that it permits a transponder's TWT (Traveling Wave Tube) to operate at or near saturation and thus it maximizes the downlink (C/N). Since only one carrier is in the TWT at a time, there are no inter-modulation products to worry about and no back off is necessary. Many of the concepts for time division multiplexing (TDM) apply without change to TDMA. In TDM digital data streams from many sources are transmitted sequentially in assigned time slots; the slots are organized into frames that also contain synchronization information. A receiving station must first recover the transmitter carrier frequency, then recover the transmitting station clock pulses, and then identify the start of each frame so that it can recover each transmitted channel and route it on to its destination. The principal difference is that in TDM everything comes from the same transmitter. The clock and the carrier frequencies do not change. While, in TDMA each frame contains a number of independent transmission. Each TDMA station has to know when to transmit, and it must be able to recover the carrier & clock for each receive burst in time to sort out all wanted baseband channels. Page 11 of 24

12 TDMA Frame Structure & Design :In TDMA transmissions a group of earth stations, each a different distance from a satellite, must transmit individual bursts of RF energy in such a way that the bursts arrive at the satellite in a prescribed order. The stations have to adjust their transmissions to compensate for the variations in satellite range, and they must be able to enter or leave the network without disrupting its operations. These goals are accomplished by organizing TDMA transmissions into frames containing reference bursts that establish absolute time for the network. Each station transmits once per frame so that its burst begins to leave the satellite a specified time interval 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 a group 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 smaller the overhead, the more efficient a working TDMA system is, but the difficulty it may have in acquiring and maintaining sync. Code Division Multiple Access Code Division Multiple Access (CDMA) is a scheme in which, a number of users occupy all of a transponder bandwidth all of the time. Their signals are encoded so that information from an individual transmitter can be detected and recovered only by a properly synchronized receiving station that knows the code being used. This provides a decentralized satellite network, as only the pairs of stations that are communicating 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 their frequency (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 station simply modulates its transmission with the address of the intended receiver whenever it wishes to send a message to that receiver. CDMA is most suited for a military tactical communication environment where many small groups of mobile stations communicate briefly at irregular intervals than to a commercial environment where large volumes of traffic pass continuously between a small number of fixed locations. Page 12 of 24

13 Mesh and Star networks VSAT's are connected by radio frequency links via a satellite. Those links are radio frequency links with a so-called `unlink' from the station to the satellite and a socalled `downlink' from the satellite to the station. The overall link from station to station, sometimes called a Hop, consists of an uplink and downlink. A radio frequency link is a modulated carrier conveying information. Basically the satellite receives the unlinked carriers from the transmitting earth stations within he field of view of its receiving antenna, amplifies those carriers. Translates their frequency to a lower band in order to avoid possible output / input interference, and transmits the amplified carriers to the stations located within the field of view of its transmitting antenna. Present VSAT networks use Geo-stationary satellites and as a result all the VSAT's are visible from the satellite all the time, carriers can be relayed by the satellite from any e VSAT to any other VSAT in the network. These are nothing but Mesh networks. However in mesh networks one must take into account the following limitations: Typically 200 db carrier power attenuation on the uplink & the downlink as a result of 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 receiving sensitivity As a result of the above limitations, it may well be that the demodulated signals at the receiving VSAT do not match the quality requested by the user terminals. Therefore direct 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 the Hub. The Hub station has a larger antenna size than those of a VSAT, say 4 meters to 11 meters. This results in higher gain than that of a typical VSAT antenna, and it is also equipped with a more powerful transmitter. As a result of its improved capability, the hub station is able to receive adequately all the carriers transmitted by the VSATs, and to convey the desired information to all the VSATs by means of its own transmitted carriers. These are nothing but Star networks. The links from the Hub the VSAT are named `outbound links'. The ones from the VSAT to the Hub are named `inbound links'. Both inbound and outbound links consist 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. This configuration supports broadcast services from a central site where the hub is located to remote sites where the receive-only VSATs are installed. Two-way networks: where VSATs can transmit & receive. Such networks support interactive traffic. One Way connectivity happens where there is a broadcast from the central location to all the remotes. The two-way connectivity between VSAT's can be achieved in two ways: Page 13 of 24

14 Either direct links from VSAT to VSAT via the central Hub, hence making it a double hop scenario, with a first hop from VSAT to hub and then a second hop using the hub as a relay to destination VSAT. The second way is by single hop links via satellite 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 conjunction with power limitation of satellites. Meshed networks are considered whenever such limitations 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 for double 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 Access PAMA is an access scheme where in when 2 VSATs want to communicate with Each other a bandwidth is assigned to them exclusively. This assigned bandwidth will Be available the VSAT's on a permanently basis. This link can either be a symmetric and asymmetric link. It is nothing but a point to point connectivity. Page 14 of 24

15 Point to Point connectivity, Leased Line in the sky The PAMA service interconnects high data traffic sites within an organization. It is a cost-effective alternative to terrestrial leased lines, providing high reliability links to support mission critical applications. DAMA: Demand Assigned Multiple Access The DAMA scheme is very similar to a telephone connection. Whenever, there is a need to talk to someone, you dial a number. The call lands at the telephone exchange, and the telephone exchange connects you to the dialed number. The role of the telephone exchange is to connect you to the desired number. This is exactly how a DAMA network operates. The HUB plays the role of a telephone exchange, between any two VSAT's. The DAMA service addresses point to point voice, fax, and data communication requirements of remote sites. It provides a cost effective and reliable solution to business having a high internal voice/ fax communication requirements. Additionally it enables organizations with operations in remote areas, to establish a reliable communications network. Typical DAMA / PAMA Network TDM/TDMA: Time Division Multiplexed/Time Division Multiple Access The TDMA network operates in a Star topology. All the remote VSATs communicate to the central hub station, on a Time Division Multiple Access Modes. At the hub the signal is re -transmitted to the destination VSAT using TDM technology after amplification. The Access mechanism of TDMA operates on a technology called Slotted Aloha. All the remote VSAT's contend for a time divisional slot to transmit their packets to the Hub. The channel used by the remotes to communicate to the Hub is 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. Page 15 of 24

16 The communication channel from the Hub to the remote is also called the Outbound or Outlink. The outbound or outlink from operates at 256 Kbps. In TDM/TDMA the implementation topology is a Hub & Spoke architecture here all the remote sites communicate to the central site via the HUB. The Hub is connected to the central site on an SCPC connection. Typical applications on TDMA Interactive Data Enterprise Resource Planning solutions like SAP, BPCS, BAAN, JD Edwards, to name a few have been implemented on TDMA VSAT network. These solutions require interactive data communication between remote sites and the central host site. The network is also suited to carry intra-office traffic from cc: Mail, MSMail amongst others. The VSATs support multiple protocols enabling them to interface with existing customer networks. Data Broadcast Continuous data broadcast to a large number of locations as in a stock exchange application or occasional file transfer from a central location to multiple remote locations is supported on the TDMA VSAT network. This is supported using the Validate system, which is part of the TDMA VSAT system. The Validate system provides both confirmed data broadcast, as required in file transfer applications and also unconfirmed data broadcast to meet continuous data feed transmission. Occasional Voice on TDMA networks The TDMA systems also offer voice communication support. This is suitable to interconnect remote locations. Various encoding rates are offered ranging from 4.8 to 16 Kbps. This gives the customer a choice to choose the appropriate voice quality as per the requirement. Page 16 of 24

17 What are the different bands available on a satellite? Any Satellite has different frequency bands available on it the table below shows what all bands are available with its operation frequencies in which the satellite up links & down links. SL.NO BAND C BAND UP LINK RANGE (GHZ) DOWNLINK RANGE (GHZ) EX C BAND KU BAND KA BAND What are the advantages & disadvantages of each band? BAND ADVANTAGE DISADVANTAGE C-BAND Broad footprint Little rain fade Interference Large Antenna and Amplifier EX-C -BAND Broad footprint Little rain fade Less Interference Weak signals Large Antenna size Large Amplifier KU -BAND KA-BAND Focused footprint Less terrestrial Interference Smaller Antenna Smaller Amplifiers Focused footprint Less terrestrial Interference Smaller Antenna Smaller Amplifiers Interference due to rain Interference due to rain Page 17 of 24

18 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 Indian sub-continent What is Rain Fade? Rain Fade is an interruption of Wireless communication signals. As a result of rain or snow droplets whose separation approximates the signal wavelengths. This phenomenon 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 winter storms at northern latitudes, fadeouts can persist for hours at a time. The phenomenon occurs with all types of satellite systems. Coding and Modulation Modulation Modulation is a technique where in baseband data is superimposed on a carrier for transmission. There are different modulation techniques that have evolved over the Page 18 of 24

19 years. However in digital data transmission the most primitive modulation technique is called PSK (Phase Shift Keying). The reason why its called phase shift keying, is because in digital data, the data are in 0's or 1's. This data is represented by the phase relationship 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 years for digital data transmission. They are: a) BPSK: Binary Phase Shift Keying In the BPSK modulation technique the zeros and ones are represented by two phases of the RF carrier signal, which differ by 180 degrees. b) QPSK: Quadrature Phase Shift Keying In 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 Keying In 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 Keying In 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 Keying In M-PSK modulation, zeros & ones are represented by multiple phases of the RF carrier. The difference would vary in accordance to the output required. As the difference in the phase shifts increase, the probability of increase in the error becomes higher. Due to this reason the most widely used modulation techniques are BPSK & QPSK modulation. In QPSK modulation, two information bits are encoded at one time. This means that when transmitting the data in QPSK, the phase of the RF carrier must change at only half the rate. Both BPSK & QPSK are extremely efficient modulation techniques. With careful filtering techniques, bit error rate (BER) performance of 1 to 2 db of the theoretical limit may be achieved. To achieve this low error rate, one approach is to filter the baseband or digital data before modulation with a Nyquist filter. Such a filter not only allows optimum performance to be achieved, but also constraints the PSK signal to the minimum possible bandwidth. Advantages of each approach An analysis of PSK modulation shows that the theoretical performance of BPSK and QPSK modulation is identical in a channel dominated by Gaussian noise, such as a Page 19 of 24

20 satellite channel. This allows the choice to be made between BPSK and QPSK based on other considerations. With QPSK, the transmitted spectrum occupies only half the bandwidth of BPSK, and would therefore be a good choice in an environment where bandwidth efficiency is required. The prime advantage of BPSK is that it is much more tolerant to phase noise than QPSK. If the system is designed from the beginning with BPSK in mind, then lower cost microwave equipment can be used in the up and down conversion process, without compromising performance. Likewise, in a burst mode system, BPSK has a second advantage over QPSK as the burst demodulator takes shorter acquisition time. This allows the frame overhead to be kept to a minimum leading to increased efficiency 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 when designing 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 the transponder. Hardware costs System Goals Forward Error Correction (FEC) In forward error correction a few coding bits are added to the actual information data stream. The added bits have an in built mechanism to identify & rectify errors at the receiving end. This is done to achieve good bit error rates and low carrier to noise ratio. 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 denotes information 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 bit is added to every 5 information Bits. Bit and Symbol Error Rates Bit Error Rate (BER) is also called the Bit Error Probability (PB). Mathematically this 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 large number o f transmitted bits will be received incorrectly. Like a probability, it is usually stated as a single number - Page 20 of 24

21 for example 1 x 10-4 or.001. The BER plays the same role as an indicator of quality in a 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 in the link noise has corrupted the transmitted symbol so that the decision circuitry at the receiver cannot identify it correctly. For example, the carrier phase may have been transmitted as +90 degrees but additive noise may have changed the received carrier phase to -90 degrees. VSATs in the Indian context What is ISRO's role been in the Indian satellite industry as well as in the VSAT industry? ISRO stands for Indian Space Research Organization, setup in June 1972 under the Department o f Space program. The primary objective of ISRO is to develop satellites, launch vehicles, rockets and associated ground systems. Some of the achievements of ISRO over the years has been launch of satellites Aryabhata, Bhaskara, Rohini, INSAT series, IRS, etc. ISRO has also been the key organization behind the development of satellite launch vehicles like the PSLV (Polar Satellite Launch Vehicle), GSLV (Geosyncronous Satellite Launch Vehicle) etc. This in 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 India TRAI is an autonomous governing body that lays down guidelines & recommendations to DOT on policy making. However weather these inputs are followed by DOT is at the discretion of DOT. However TRAI is not a body that is directly involved in governing any service providers. b) DOT: Department Of Telecommunication Page 21 of 24

22 DOT has a role in policy making, licensing and co ordination of matters related to telegraphs, telephones, wireless, data, facsimile and telematic services and other forms of communication. In addition DOT is responsible for frequency management in the field of radio communication in close coordination with international bodies. It also enforces wireless regulatory measures for wireless transmissions by wireless users in the country. c) WPC: Wireless Planning & Coordination wing WPC is a part of DOT created in 1952 under the Ministry of Communications, is the national radio regulatory authority responsible for frequency spectrum management, including licensing and caters to the need of all wireless users in the country government or private, security or non-security. It is also the national nodal agency for all matters related to ITU (International Telecommunications Union) and the Asia Pacific Telecomm unity (APT) and is responsible for treaty obligations on behalf of the Government of India. It also exercises the statutory functions of the Central Government 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 Allocation The Standing Advisory Committee on Frequency Allocations (SACFA) is a high level committee chaired by secretary (DOT)/Chairman, Telecom Commission. Heads of major wireless users/administrative ministries of the govt. of India, Member (Technology), Telecom Commission, and Wireless Adviser to the govt. of India, Joint Secretary, DOT are its members. WPC wing of the ministry of communications provides secretariat help to the committee. Joint Wireless Adviser, WPC wing is the member-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 International Telecommunications Union (ITU) Asia Pacific Telecomm unity (APT), To sort out the problems referred to the committee by various wireless users, siting clearance of all wireless installations in the country, etc. SACFA clearances are issued after getting 'no objection' from various SACFA members who have to carry out detailed technical evaluation including field surveys, etc. at times they have to obtain evaluations from their field units. The technical evaluation is done primarily for: a) Aviation hazards. b) Obstruction to line of site of existing/planned networks c) Interference (Electro Magnetic Interference (EMI)/Electro Magnetic Compatibility (EMC)) to existing and proposed networks. Page 22 of 24

23 TYPICAL VSAT INSTALLATION Site Survey Before installation of any VSAT we need to know some parameters which are very critical. To get these parameter we have to do survey of the site where VSAT to be installed. The general parameters are :1. Look angle of the Antenna - VSATs will send and receive RF signals from respective satellite. Hence, we have to find out the coordinates of the site. The coordinates indicates latitude and longitude of the site. To get this Lat and Long information site survey engineers use GPS equipment. GPS is Global Positioning System which gather information from satellites the lat and long information. Calculating the coordinates of the site and coordinates of the satellite 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. Page 23 of 24

24 3. Base VSAT Antenna system placed with the support of ballast. Ballasts are nothing but stones or bricks which are placed to keep the Antenna stable against wind, thrust etc. Physically stability is very much required for a stable network connectivity. Hence, the base ( roof top or ground ) must be flat and strong to withstand the load of the VSAT Antenna. If the base is slanting then there is a risk slippage or if the base is not strong enough then damage of the base may happen. At the time of site survey all these information is very much required. A 3.8 meter PAMA outdoor unit may weight 3000 kg and under wind pressure at the time of thunder storm may rise subsequently. 4. Length of the IF cable from Outdoor Unit to Indoor Unit The shortest length is better. We have to found the route of the cable so that the cable should be safe 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 electrical interference. 6. Working space some time it has been found that the best place to keep a VSAT ODU is such that there is not space to work for servicing or installation. Safety of the engineers may be an issue for such case. We must keep this into mind at the time of survey. 7. Electrical Earthing All communication equipment are prone to damage against floating current. If the Earthing is not proper then this floating current may cause damage to the electronic parts of the VSAT system. In general Earth to Neutral voltage must be below 2 volts, so that negligible current will flow. Besides, Neutral should be strong, so that though Earthing may be perfect but due to weak neutral floating current may generate and cause of damage to the equipments. Page 24 of 24