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VSAT Tutorial


Refer VSAT tutorial covering following sub topics:
RF Terminology   Satellite Terminology  Access types  SCPC  MCPC  RF Transceiver  C band RFT  Microstrip  RF upconverter design   Power Amplifier  what is modulation  satellite Modem  Link Budget 


Introduction

VSAT is the short form of Very Small Aperture Terminal. VSAT is used for many applications which include Internet access,distance education,telemedicine,telephony,banking,video conferencing and more. It makes use of already launced satellites to provide all these facilities. The interface to satellite is through disc antenna and various bands are in use. The most common frequency bands which VSAT use are C band, Ku band and Ka band.

VSAT system overview

VSAT tutorial-VSAT system overview As shown in the figure, VSAT system composed of two main parts ODU(Outdoor Unit) and IDU(Indoor Unit). ODU and IDU are interfaced using cable. ODU consists of LNA,PA,Transceiver,Antenna and OMT. IDU consists of Modem,MUX-DEMUX and interface to telephone/PC/EPABX.

Read VSAT system overview article for more details.

VSAT subsystems

RF transceiver: RF Transceiver is basically a frequency converter. It consists of two parts frequency up converter and frequency down converter. Up converter converts modulator frequency usually in either 70/140 MHz to satellite frequency bands (C,Ku,Ka) as per satellite in use for particular location/application. Down converter does reverse of the up converter. In addition to frequency conversion both provide gain to the signal as per VSAT link budget and the same need to be adjusted as per need of the link using attenuation settings available in RF transceiver in both up/down link. Refer following links for RF trasceiver basics and for Rf transceiver design and development.

RF Transceiver basics   Design of RF frequency converter
C band RF Transceiver Design and Development   BUC-Block Up Converter basics  

RF power amplifier: RF Power amplifier is the unit which provide power amplification without any frequency change before signal is transmitted to the antenna and consecutively to the air. Refer following link to know more on RF Power Amplifier basics,various stages,classes,specifications and design of rf amplifier with example.

RF Power Amplifier

RF LNA: RF LNA is the low noise amplifier used in VSAT. As the signal is received by antenna is composed of noise as well as useful information signal part. In addition it is very low in the power level. LNA's job is to boost this low level of signal to the sufficient level considering less amplification of noise part, so that adequate C/N is maintained. For more on LNA and LNB refer following links.

LNA basics   LNB basics  

Satellite modem: Satellite modem provides two major functionalities in the VSAT. The first one is it makes link reliable by adding forward error correction functionality using various techniques such as convolution coding/turbo coding and so on. The second one is that it does task of modulation and demodulation. There are various modulation techniques used in modem, the most popular among them are QPSK/8PSK. To know what is modulation,various modulation techniques and satellite modem design refer following links.

what is modulation   satellite Modem design  

Antenna: Antenna is basically a electro-magnetic frequency transducer. It sends and receives EM waves of various frequency bands. Antenna diameter and aperture vary band to band. Hence C band antenna design is different than Ku band. The signal to be transmitted will be provided to antenna by Power Amplifier. The signal received by Antenna is fed to LNA/LNB through OMT as shown in the figure. For more on Antenna refer our Antenna tutorial.

VSAT network architecture

VSAT network architecture is the way Hub station and/or VSATs are interfaced with satellite to provide the service. There are five main topologies exist, viz. broadcast,point to point, point to multipoint(star),mesh,hybrid.Let us understand each of this VSAT topology.

In Broadcast type, there is a single broadcasting station interfaced with satellite and satellite will relay signals to all the VSATs. Here broadcasting station-satellite-all VSATs link exist.

In point to point type of topology, two VSATs communicate via satellite using dedicated assigned channel. So here VSAT1-Satellite-VSAT2 dedicated link exist.

In Star topology, there are three entities hub station(usually with larger antenna), VSATs and Satellite. All the communications between VSATs happen through Hub station. hence here if VSAT1 and VSAT2 need to communicate then,link is VSAT1-satellite-Hub-Satellite-VSAT2. Hence two hop communication is needed to communicate between any two VSATs in the network.

In Mesh type of topology, VSATs can communicate with one another directly and no Hub station is needed . But each VSAT need to be complex owing to more functionalities required similar to the Hub station. Also antenna specifications need to be different than star type of topology.

In Hybrid type is the combination of both star and mesh type. Here few of the VSATs communicate via Hub and few can communicate directly with one another.

VSAT Resource Access Techniques

The access techniques used in VSAT mainly are SCPC,MCPC,DAMA,FDMA,TDMA. Refer following links to know more on TDMA versus FDMA and SCPC versus MCPC. DAMA stands for Demand Assigned Multiple Access, here frequency allocation is done based on the demand from VSATs.

TDMA-Basics and TDMA types   FDMA versus TDMA versus CDMA
SCPC- Single Channel Per Carrier  MCPC- Multiple Channels Per Carrier  SCPC vs MCPC 

VSAT NMS basics and NMS Protocol

To knoe more on NMS(Network Management System) basics for star configuration VSAT including NMS protocol used for local and remote NMS. Refer following link.
VSAT NMS protocol

VSAT Link Budget

Refer following link for VSAT link budget example for Ku band VSAT system.

RF Link Budget
Ku band satellite system RF Link Budget

Useful RF terminologies for VSAT testing

Following are useful links for RF,VSAT and satellite related terms.

RF Terminology   Satellite Terminology

VSAT Vendors

Refer following links for VSAT system providers,antenna suppliers,RF amplifier manufacturers,RF transceiver manufacturers,LNA/LNB/BUC manufacturers,waveguide component manufacturers,satellite modem manufacturers and EPABX manufacturers.

VSAT Providers  RF antenna  RF Amplifier  RF Transceiver  LNA  LNB  BUC  Waveguide components  satellite modem  epabx 


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