Wireless Communications
By Yogesh Baweja
 

Today's age is the era of information technology. The fast and easy movement of information has brought  the people together and made the world a global village. Telephone, telex and telegram were the  pioneering innovations that set the ball rolling for the things to come. Communication satellites in the geo-stationary orbits proved to be a giant leap in this effort. During early years of the satellites, the uplinking and downlinking stations played pivotal roles. However, the number of down linking earth stations was every low and the signals had to be invariably distributed through wires and cables to the destination. It was possible and convenient for the cities and other plain areas but was not viable for the far-flung areas spread over hills, forests, islands and difficult terrains. 

Then came VSATs - Very Small Aperture Terminals, which are proving to be the final wireless frontier. VSATs have provided a cost-effective means of communication to consumers who sought independent wireless communication. Through the VSAT technology, a large number of geographically dispersed sites can be connected into a network. It is a cost-effective technology. VSAT networks offer value-added satellite-based services, capable of supporting the Internet, data, LAN and voice / fax communication.

A VSAT system typically consists of a satellite transponder, central hub or a Master earth station and geographically dispersed remote VSATs. The transponder of a communication satellite is used for the purpose. Such satellites revolve in geo-stationary orbits, located 36,000 km away from the earth. A number of countries have launched their own communication satellites. Some multilateral agencies or individuals have also ventured into the area and one can easily hire or lease the transponder.

The VSAT terminal has the capability to receive as well as transmit signals via the satellite to other VSATs in the network. Depending upon the technology used, the signals are either sent via satellite to a central hub, which is also a monitoring centre, or the signals are sent directly to VSATs, with the hub being used for monitoring and control. For centralised data applications, a big central earth station called the hub is used where an antenna of size 6-11 metre diameter is installed. 

This hub station controls, monitors and communicates with a large number of dispersed VSATs. However, for telephony application, a group of VSATs communicates directly with any other VSAT in the network, without going through a central hub. In such situations, a hub station performs only the monitoring and control functions. A hybrid of above two systems can also be adopted.

The VSAT consists of an outdoor and an indoor unit. The outdoor unit consists of an antenna of size 1.8 metre to 3.8 metre. Larger the size of antenna, more will be its ability to amplify the original strength. The antenna consists of a reflector, feed horn and a mount. The feedhorn is mounted at the focal point of the antenna frame by support arms. The feedhorn directs the transmitted . power towards the antenna dish or collects the received power from it. The reflector is mounted on the antenna frame and is interconnected to the feedhorn. The reflector consists of different subsystems, including low noise amplifiers for amplification and down converters for down conversion of the received signal. Through the converters and amplifiers, the frequency of incoming and outgoing frequencies are regulated. The outdoor unit of VSAT is connected through a coaxial cable to the indoor unit. The distance between the indoor and outdoor units should not be more than 300 feet under normal circumstances. 

The indoor unit consists of modulators and demodulators. The modulators help in superimposing the user traffic waves on carrier waves which are then sent to reflector for upconversion, amplification and transmission, thereby doing uplinking. The demodulator receives downlinked signals from reflectors and segregates the traffic signal from the carrier wave. The indoor unit also interfaces with various end user equipments, like computers, telephone instruments, router, EPABX, etc.

The VSAT networks maximize the usage of common satellite and other resources, thereby providing cost- effective means of communications.  VSAT network is cheaper in terms of cost, offering significant savings over the leased line network in 2 to 3 years timeframe. No capital-intensive investments are required. If the cost of opportunity and downlink is also taken into account, it becomes even more cost-effective. 

Unlike other land based communication systems, where the communications have to be routed through telephone exchanges or ISP servers, VSATs are independently placed and ensure seamless communication. Their reliability (upto 99.5 percent) is far in excess of leased lines (approx. 80-85%). The deployment of VSATs does not take more than 4-6 weeks as compared to 4 to 6 months for wired communication. Moreover, network monitoring and control of the entire VSAT network is much simpler than a network of leased lines.

The VSAT network is more flexible, with enormous expansion capabilities. Additional VSATs can be rapidly installed to support the network expansion to any site, no matter however remote. Further, for leased lines, one gets dedicated circuits in multiples of 64 kbps while for VSATs, actual bandwidth as per the requirement is provided. Transmission speeds range from 9.6 kbps to 2.0 mbps. By using satellite transmission speeds range from 9.6 to 2.0 kbps. By using satellite transmission, VSAT is highly reliable, there is 1 error per 100 million bits for each terminal. Faults are easy to locate since a computerised control unit is able to direct, isolate and restore a fault.

VSAT has revolutionized the communication scenario worldwide. International VSAT technology allows multimedia access to areas that are inaccessible to fixed lines. For example, oil exploration companies, geological research expeditions, shipping and mining applications. In India, VSATs are being used for multiple applications, particularly data-transfer, real-time communications and accessing remote areas. 

It is being used to establish private telecommunications network for businesses requiring communication with multiple branch offices or business associates located over wide geographical. locations worldwide. 

The telecommunication services like real-time, high quality transmission of fax, data, video- conferencing, images and telephony, without interruption, has made them highly popular.

The Indian geo-stationary satellites offer transponders for VSAT operations. V SAT operators had till now been allotted seven transponders on INSAT 2B and 2C but with INSAT - 3B, they have been offered Ku band frequencies for the 7500-strong VSAT networks and ISPs. 

(By Arrangement with Kaleidoscope)