A Tutorial on VSATs – Topic-2

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What is a VSAT?

The term Very Small Aperture Terminal (VSAT) refers to a small fixed earth station. VSATs provide the vital communication link required to set up a satellite based communication network. VSATs can support any communication requirement be it voice, data, or video conferencing.

The VSAT comprises of two modules – an outdoor unit and an indoor unit .The outdoor unit consists of an Antenna and Radio Frequency Transceiver. (RFT). The antenna size is typically 1.8 metre or 2.4 metre in diameter, although smaller antennas are also in use. The indoor unit functions as a modem and also interfaces with the end user equipment like stand alone PCs, LANs, Telephones or an EPABX. VSAT can typically be divided into two parts – an outdoor unit and an indoor unit. The outdoor unit is generally ground or even wall mounted and the indoor unit which is the size of a desktop computer is normally located near existing computer equipment in your office.

Outdoor Unit

The antenna system comprises of a reflector, feedhorn and a mount. The size of a VSAT antenna varies from 1.8 metres to 3.8 metres. The feedhorn is mounted on the antenna frame at its focal point by support arms. The FEED HORN directs the transmitted power towards the antenna dish or collects the received power from it. It consists of an array of microwave passive components. Antenna size is used to describe the ability of the antenna to amplify the signal strength.

The RFT is mounted on the antenna frame and is interconnected to the feed horn. Also termed as outdoor electronics, RFT, in turn, consists of different subsystems.

These include low noise Amplifiers (LNA) and down converters for amplification and down conversion of the received signal respectively. LNAs are designed to minimize the noise added to the single during this first stage of the converter as the noise performance of this stage determines the overall noise performance of the converter unit. The noise temperature is the parameter used to describe the performance of a LNA.

Up converters and High Powered Amplifiers (HPA) are also parts of the RFT and are used for up converting and amplifying the signal before transmitting to the feedhorn.The Up/Down converters convert frequencies between intermediate frequency (Usually IF level 70 MHz) and radio frequency. For Extended C band, the down converter receives the signal at 4.500 to 4.800 GHz and the upconverter converts it to 6.725 to 7.025 GHz. The HPA ratings for VSATs range between 1 to 40 watts.

Interlink Facility

The outdoor unit is connected through a low loss coaxial cable to the indoor unit. The typical limit of an IFL cable is about 300 feet.

Indoor Unit

The IDU consists of modulators which superimpose the user traffic signal on a carrier signal. This is then sent to the RFT for up conversion, amplification and transmission. It also consists of demodulators which receive the signal from the RFT in the IF range and demodulates the same to segregate the user traffic signal from the carrier. The IDU also determines the access schemes under which the VSAT would operate. The IDU also interfaces which various end user equipment ranging from stand alone computers, LAN’s, routers, multiplexes, telephone instruments, EPABX as per the requirement .It performs the  necessary protocol conversion on the input data from the customer end equipment prior to modulation and transmission to the RFT. An IDU is specified by the access technique, protocols handled and number of interface ports supported.

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