Very Small Aperture Terminal (VSAT) - Seminar Paper

Introduction

VSAT is a electronic device used to transmit and Receive information via satellite using a small Diameter dish antena (0.6m-3.8m).VSAT may be transmit data, voice, fax and video conferencing can be done with help of this.

A Very Small Aperture Terminal (VSAT) is a micro-Earth station that uses the latest innovations in the field of satellite communications to allow user's access to reliable satellite communications. VSATs provide users with services comparable to large gateways and terrestrial networks, at a fraction of the cost. A typical VSAT consists of communications equipment and a small antenna with a diameter less than 3.5 meters.

VSAT networks provide users with simple equipment that requires minimal installation and repair. They are easy to operate and simple to troubleshoot. VSAT installations do not require staff with extensive expertise.

A typical VSAT installation consists of an antenna, an outdoor unit (ODU), the inter facility link cable (IFL), and an indoor unit (IDU). The antenna and ODU provide the radio frequency conversion and amplification for the satellite uplink and downlink. The ODU is often called the transceiver because it includes the up converters (U/Cs); the Solid State Power Amplifier (SSPA); the Low Noise Amplifier (LNA), and the down converter (D/C). The IDU provides the baseband interfacing required to carry the user’s services.

The power requirement for each VSAT is low and in some cases solar cells supply the power. Because of its simplicity, a VSAT installation takes only a few hours and the terminals are ready for service.

VSAT NETWORK CONFIGURATIONS

VSATs are connected by radio frequency (RF) links via a satellite, with a so-called uplink from the station to the satellite and a so-called downlink from the satellite to the station. The overall link from station to station, sometimes called hop, consists of an uplink and a downlink. A radio frequency link is a modulated carrier conveying information. Basically the satellite receives the uplinked carriers from the transmitting earth stations within the 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 geostationary satellites, which are satellites orbiting in the equatorial plane of the earth at an altitude above the earth surface of 35 786 km.

ARCHITECTURE OF VSAT

Its architecture divided in to 2 parts

• Indoor unit
• Outdoor unit

INDOOR UNIT

The indoor unit is placed inside so it can interface with the user’s communications de-vice.it can be a small desktop box that contains the receiver and transmitter boards and an interface to the user’s equipment. 

OUTDOOR UNIT

Outdoor unit is placed outside for a line of sight to the satellite. It consists of small antenna and electronic equipment for signal reception and transmission.

A typical VSAT site consists of a parabolic-shaped antenna mounted on the roof of a                             building, connected by a cable to a chassis inside the building. Operators install these antennas at customer sites and buy transmission capacity on satellites. It contains a modem for translating satellite transmissions back into data (and vice versa) and terrestrial interfaces for connecting customer equipment. A block diagram of a complete VSAT link is shown in figure 4.

COMPONENETS

• Antenna
• LNB
• BUC
• Local oscillator
• Power amplifier   
• Horn feed
• IFL cables
• iDirect Satellite Router

ANTENA

Antenna is required for reception and transmission of the signal to and from the satellite. The usually antenna used in VSAT is a parabolic dish antenna. A parabolic antenna shown in Fig 5 Below

LNB

(LOW NOISE BLOCK DOWN CONVERTER)

The LNB (Low Noise Block Down-Converter) is part of the receive chain of your VSAT. Located on the feed horn, the LNB converts the satellite signal that was reflected off of the satellite antenna’s reflector from C-Band into an L-Band signal. The L-Band signal is in the frequency range of 950 to 1750 MHz and is considered more manageable. This is partially due to the fact that the transmission of the lower frequency signal can be more reliable when using a coaxial cable than is the case when higher frequency C-Band signals are transmitted of on this type of cable. Virtually all new satellite routers today use L-Band inputs.

BUC

 (Block up converter)

The BUC (Block Up-Converter) is part of the transmit chain of your VSAT. It is often located on the feed horn, but if it is a large BUC, it may be located at the base of the antenna and connected with RF conduits (waveguides). The BUC converts the modem's L-Band transmit signal into higher frequency C-Band signals, then amplifies it before it is reflected off the satellite antenna towards the satellite. In order to perform both of its functions, the BUC is composed of two individual components: the Local Oscillator and the Power Amplifier. The Local Oscillator performs the frequency conversion between the L-Band and the satellite frequency, such as C-Band. The resulting satellite frequency is calculated by adding the L-Band frequency to a number known as the Local Oscillator Frequency that will be stamped onto the BUC. A Local Oscillator Frequency of 4900 MHz is used for a non inverted spectrum, and a Local Oscillator Frequency of 7375 MHz is used for an inverted spectrum. A typical system will require a 2-watt BUC or higher, depending on the application. Although BUCs are available with very powerful amplifiers, it is unlikely that a VSAT installation will require more than 10W, even in less than ideal circumstances. 

FEED

It is the part of both receives and transmits chain of VSAT. The VSAT Feed is composed of the

• Feed Horn
• OMT (Orthomode Transducer)
• Waveguide
• Circular Tube

Transmit Reject Filter is either built-in or needs to be added on the receive end of the OMT. The parts of FEED is shown in Fig.8

 Circular Tube is used for circular polarization requirements. There are three main polarization positions used for Sky Vision Services.

• Linear Cross Polarization
• Linear Co Polarization
• Linear Cross Polarization

Linear Cross Polarization

In order to transmit and receive in opposite polarities the linear polarization is used. In this we will need to assemble the feed so that the receive part of the OMT (which the LNB is attached to) is perpendicular to the ground and the wide face of the waveguide is parallel to the ground.

Linear Co Polarization

In order to transmit and receive in same polarities the linear co-polarization is used. In this we will need to assemble the feed so that the receive part of the OMT (which the LNB is attached to) is perpendicular to the ground and the narrow face of the waveguide is parallel to the ground.

Linear Cross Polarization

In order to transmit and receive in opposite circular polarities we will need to add the circular tube between the feed horn and the OMT. Assemble the feed so that the receive part of the OMT (which the LNB is attached to) is perpendicular to the ground and the wide face of the waveguide is parallel to the ground. Make sure that the receive part of the OMT is aligned to the desired reception polarity, either LHCP or RHCP which is displayed on the mouth of the circular tube.

IFL CABLES 

(Inter Facility link Cables)

IFL cables are the basically Co-axial cables that are used to connect the I-direct satellite router to BUC and LNB. The quality of the Signal depends on the length and Quality of the IFL Cable used.

The maximum length of the cable should be 30m for sky vision.

The coaxial should have 75 ohm impendence and typically should be either an RG-6 or an       RG-11.

RG-6 – Costless, Suffers from Signal Reduction.

RG-11 – Cost more, provides High Signal Transmission.

I-direct Satellite Router

It is used to give the final output from the device.  The co-axial cable connects to the device to give the output. It has several ports to connect the IFL Cables. And this can be connect in STAR or MESH topologies. A typical I-Direct satellite Router is shown in Fig.12

TOPOLOGIES

• STAR
• MESH

STAR

This is the most popular topology. In this we have a big, central earth station known as the hub. Generally the hub antenna is in the range of 6-11metre in diameter. Hub station controls, monitors and communicates with a large number of dispersed VSATs.

MESH

In a mesh topology, a group of VSATs communicate directly with any other VSAT in the network without going through a central hub. A hub station in a mesh network performs only the monitoring and control functions.(more suitable for telephony applications). These have also been adopted to deploy point-to-point high speed links. 

STAR OR MESH?

The answer depends on three factors:

The structure of information flow within the network;

The requested link quality and capacity;

The transmission delay.

Broadcasting: a central site distributes information to many remote sites with no back flow of information. Hence a star shaped one-way network supports the service at the lowest cost.

Corporate network: most often companies have a centralized structure with administration and management performed at a central site, and manufacturing or sales performed at sites scattered over a geographical area. Information from the remote sites needs to be gathered at the central site for decision making, and information from the central site (for example, relating to task sharing) has to be distributed to the remote ones. Such an information flow can be supported partially by a star-shaped one-way VSAT network, for instance for information distribution, or Supported totally by a two-way star-shaped VSAT network. In the first case, VSATs need to be receive-only and are less expensive than in the latter case where interactivity is required, as this implies VSATs equipped with both transmit and receive equipment. Typically the cost of the transmitting equipment is two-thirds that of an interactive VSAT.

Interactivity between distributed sites: other companies or organizations with a decentralized structure are more likely to comprise many sites interacting with one another. A meshed VSAT network using direct single hop connections from VSAT to VSAT is hence most desirable. The other option is a two-way star-shaped network with double hop connections from VSAT to VSAT via the hub.

FREQUENCY BAND

C-band

Ku band

C-BAND- The C band is a name given to certain portions of the electromagnetic spectrum, including wavelengths of microwaves that are used for long-distance radio telecommunications. The IEEE C-band - and its slight variations - contains frequency ranges that are used for many satellite communications transmissions, some Wi-Fi devices, some cordless telephones, and some weather radar systems. For satellite communications, the microwave frequencies of the C-band perform better under adverse weather conditions in comparison with Ku band (11.2 GHz to 14.5 GHz) microwave frequencies, which are used by another large set of communication satellites. The adverse weather conditions, collectively referred to as rain fade, all have to do with moisture in the air, including rain and snow.

The IEEE C-band

The IEEE C-band is a portion of the electromagnetic spectrum in the microwave range of frequencies ranging from 4.0 to 8.0 gigahertz (GHz).but this definition is the one that is followed by radar manufacturers and users, but not necessarily by microwave radio telecommunications users.

The communications C-band was the first frequency band that was allocated for commercial telecommunications via satellites. The same frequencies were already in use for terrestrial microwave radio relay chains. Nearly all C-band communication satellites use the band of frequencies from 3.7 to 4.2 GHz for their downlinks, and the band of frequencies from 5.925 GHz to 6.425 GHz for their uplinks. Note that by using the band from 3.7 to 4.0 GHz, this C-band overlaps somewhat into the IEEE S-band for radars.

The satellite communications portion of the C-band is highly associated with television receive-only satellite reception systems, commonly called "big dish" systems, since small receiving antennas are not optimal for C-band systems. Typical antenna sizes on C-band capable systems ranges from 7.5 to 12 feet (2.5 to 3.5 meters) on consumer satellite dishes, although larger ones also can be used.

The C-band frequencies of 5.4 GHz band [5.15 to 5.35 GHz, or 5.47 to 5.725 GHz, or 5.725 to 5.875 GHz, depending on the region of the world] is used for IEEE 802.11a Wi-Fi and cordless telephone applications, leading to occasional interference with some weather radars that are also allocated to the C-band.

C-band variations

Slight variations in the assignments of C-band frequencies have been approved for use in various parts of the world, depending on their locations in the three International Telecommunications Union radio regions. Note that one region includes all of the Americas; a second includes all of Europe and Africa, plus all of Russia, and the third region includes all of Asia outside of Russia, plus Australia and New Zealand. This latter region is the most populous one, since it includes the People's Republic of China, India, Pakistan, Japan, and Southeast Asia

Ku band

The Ku band is a portion of the electromagnetic spectrum in the microwave range of frequencies. This symbol refers to "K-under" (originally German: Kurz-unten) in other words, the band directly below the K-band. In radar applications, it ranges from 10.95-14.5 GHz according to the formal definition of radar frequency band nomenclature in IEEE Standard.

 Ku band is primarily used for satellite communications, most notably for fixed and broadcast services, and for specific applications such as NASA's Tracking Data Relay Satellite used for both space shuttle and ISS communications. Ku band satellites are also used for backhauls and particularly for satellite from remote locations back to a television network's studio for editing and broadcasting. The band is split into multiple segments that vary by geographical region by the International Telecommunication Union (ITU). NBC was the first television network to uplink a majority of its affiliate feeds via Ku band in 1983.

Some frequencies in this radio band are used for vehicle speed detection by law enforcement, especially in Europe

APPLICATIONS

• ATM  machines

• Internet Banking

• wide range of data, voice, and video applications

• Computer communications

• Credit checks and credit card verification 

• Reservation systems 

• Database enquiries 

• In distance learning education 

Advantages

Point-to-multipoint and point-to-point communications-

    A VSAT network offers communications between remote terminals. As a result of the power limitation resulting from the imposed small size and low cost of the remote station, a VSAT network is most often star-shaped with remotes linked to a larger station called a hub. This star configuration often well reflects the structure of information flow within most large organizations which have a point of central control where the hub can be installed. The star configuration itself is not a severe limitation to the effectiveness of a VSAT network as point-to-point communications, which would conveniently be supported by a meshed network, can still be achieved via a double hop, using the hub as a central switch to the network.

Asymmetry of data transfer

As a result of its asymmetric configuration, a star-shaped network displays different capacities on the inbound link and on the outbound link. This may be an advantage considering the customer need for asymmetric capacities in most of his applications. Should he use leased terrestrial lines which are inherently symmetric, i.e. offering equal capacity in both directions, the customer would have to pay for unused capacity. 

Flexibility 

A VSAT network inherently provides a quick response time for network additions and reconfigurations (one or two days) as a result of the easy displacement and installation of a remote station.

Private corporate networks

A VSAT network offers its operator end-to-end control over transmission quality and reliability. It also protects him from possible and unexpected tariff fluctuations, by offering price stability and the possibility to forecast its communication expenses. Therefore it is an adequate support to private corporate networks.

Low bit error rate

The bit error rate usually encountered on VSAT links is typically 10−7. 

Distance-insensitive cost

The cost of a link in a VSAT network is not sensitive to distance.

Hence, cost savings are expected if the network displays a large number of sites and a high geographical dispersion.

CONCLUSION

VSAT networks have had a lot of success for specific, mainly data, services but the demand is there for a wider range of service provision and multimedia communications. We have focused on recent work on the development of adaptive, dynamic protocols that will result in the most efficient allocation of the space segment.

VSAT networks remain competitive and more effective than terrestrial solutions.