A Tutorial on VSATs – Topic-4


Access Technologies

The primary objective and advantage of these networks is to maximize the use of common satellite and other resources amongst all VSAT sites. The method by which these networks optimize the use of the satellite capacity, and spectrum utilization in a flexible and cost effective manner are referred to as satellite access schemes. Each of the above typologies is associated with an appropriate satellite access scheme. The most commonly used satellite access schemes are:

  • Time Division Multiple Access (TDMA)
  • Frequency Division Multiple Access (FDMA)
  • Code Division Multiple Access (CDMA)
  • Demand Assigned Multiple Access (DAMA)
  • Pre-Assigned Multiple Access (PAMA)
  • Frequency-Time Division Multiple Access (FTDMA)

These technologies are explained in another article VSAT access technologies.

Space Segment Support

The ideal orbit for a communications satellite is geostationary, or motionless relative to the ground. Satellites used for communications are almost exclusively in the geostationary orbit, located at 3600 km above the equator. In line with ITU stipulations, for avoiding interference, all satellites are placed 2 degree a part. This places a maximum limit of 180 satellites operating in a geostationary orbit.

However, with a view to maximize the utilization of orbit slots, co-located satellites are being deployed. Co-located satellites are separated by 0.1 degree in space or approximately 30 kms .Signal interference from the co-located satellites is prevented by using orthogonal polarization. Hence a group station equipment can receive signals from two co-located satellites without any reorientation of the antenna. The signals can be differentiated based on their polarization.

Space segment: Space segment is available from organizations which have prospered satellites, arranged launches and conducted preliminary tests in-orbit and who then operate these satellites on commercial basis.

Transponders: Contained in the satellite body are a number of transponders ,or repeaters.These transponders perform the following functions :

  • Signal Reception-it receives the signal up linked by a VSAT and /or hub.
  • Frequency Translation-the frequency of the receive signal is translated to a different frequency, known as the downlink frequency. The frequency translation ensures that there is no positive feedback and also avoid interference related issues.
  • Amplification-the transponder also amplifies the down link signal.

The number of transponders determines the capacity of a satellite. The INSAT series of satellites have typically 12/18 transponders in various frequency bands. Each transponder typically has a bandwidth of 40 Mhz. The various frequency bands are as below-


Frequency Band Uplink                     (GHz)Earth Station to satellite Downlink                (GHz)Satellite to Earth Station
C Band 5.925 to 6.425 3.700 to 4.200
Extended C Band 6.725 to 7.025 4.500 to 4.800
K u Band 14.000 to 14.500 10.950 to 11.700

  Internationally K u-Band is a popular frequency band in use. The K u-Band by virtue of its higher frequency can support traffic with smaller antenna sizes in comparison to C / Ext-C Band. It is , however, susceptible to rain outages making it unsuitable for use in South East Asian regions. Indian service providers are presently allowed to hire space segment only on the INSAT series and operate in Ext-C band only. Ext-C band is available only on the INSAT series of satellites and is not a standard band available internationally.

Link Budgets: Ascertains that the RF equipment would cater to the requirements of the network topology and satellite modems in use. The link Budget estimates the ground station and satellite EIRP required. Equivalent isotropically radiated power (EIRP) is the power transmitted from a transmitting object. Satellite ERP can be defined as the sum of output power from the satellite’s amplifier, satellite antenna gain and losses.

 Calculations of signal levels through the system (from originating earth station to satellite to receiving earth station) to ensure the quality of service should normally be done prior to the establishment of a satellite link. This calculation of the link budget highlights the various aspects. EIRP required at the transmitting VSAT, Satellite EIRP which will be required for a desired specified gain of this receiving system. A part from the known losses due to various cables and inter – connecting devices, it is customary to keep sufficient link margin for various extraneous noise which may effect the performance. It is also a safeguard to meet eventualities of signal attenuation due to rain/snow. As mentioned earlier a satellite provides two resources, bandwidth and amplification power. In most VSAT networks the limiting resource in satellite transponder is power rather than bandwidth.

With all their advantages, VSATs are taking on an expanding role in a variety of interactive, on-line date, voice and multimedia applications. Whether it is gas station service, rural telephony, environmental monitoring, distance learning/ remote training or the Internet, VSATs are truly poised to be the Space Age Technology.


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