Quaser Tech

Archive for March, 2009

A satellite in the geostationary orbit of the earth (36000 km above the earth) broadcasts a few watts of microwave signal over a wide footprint area the broadcast signal gets attenuated by approximately 200 dB and becomes very weak at the stationary reception centers on the earth. This weak signal has to amplified by means of sophisticated electronics for healthy signal reproduction. It is important to ensure that the amplifications do not contribute any significant amount of noise amplification; otherwise it degrades the signal of interest. Low Noise Amplifier (LNA) and Low Noise Block Converter (LNBC) are used for amplification and reproduction of weak signals.

Contrary to this, in terrestrial wireless communication, both the transmitter and receiver are mobile. The base station used in terrestrial mobile communication broadcasts signal in all directions simultaneously. This transmitted signal has high power. The signal transmitted is so powerful that it has capability of suturing ‘C’ band satellite receiving systems. That is, Low Noise Amplifier (LNA) and Low Noise Block Converter (LNBC) are saturated by high power signals (having same frequency). The probability of saturation of LNA and LNBC is nearly 100% due to their inherent characteristics. Saturation of receiving system will lead to total collapse of the ‘C’ band satellite network services.

Source: Science Reporter, January 2009 issue.

‘C’ band RF frequency (standard ‘C’ band and extended ‘C’ band) was the first frequency band allocated for commercial satellite communication. According to literature, there are more than 160 geostationary satellites orbiting around the globe and operating in ‘C’ band. Two out of three satellites being manufactured by companies use the ‘C’ band spectrum. Throughout the world itself, most of the essential services like TV transmission, distance learning programs by universities, national television network and disaster recovery etc. are provided through ‘C’ band satellite link.

The upgradation in terms of speed and range of these services would lead to significant interference with ‘C’ band satellite communication services because WiMAX wireless services being developed lately are using the same frequency spectrum as allotted to ‘C’ band satellite link.

The development and usage of broadband wireless communication and WiMAX in extended ‘C’ band frequencies from 3.4 GHz to 3.7 GHz will provide significant interference for ‘C’ band satellite link. Several national administrations have designated portions of the frequency band 3.4 to 4.2 GHz for terrestrial wireless applications such as broadband wireless access (BWA) and future mobile service for 3G and 4G systems.

Source: Science Reporter, January 2009 issue.

With the world becoming more and more mobile consumers are demanding real-time conversation (Mobile and Video broadcasting) for their business and entertainment purposes. This requirement can be fulfilled by very high capacity digital networks having sufficient bandwidth allocation. This is what WiMAX and Broadband communication seek to provide.

WiMAX stands for Worldwide Interoperability for Microwave Access. It is not a technology but rather a certification mark given to the equipment that meets certain conformity and interoperability tests for IEEE 802.16 family of standards. Similarly the term Wi-Fi is also a certification mar for equipments that meet IEEE 802.11 standard. Neither WiMAX nor Wi-Fi is a technology but their names have been adopted in popular usage to denote the technologies behind them.

WiMAX basically supports point to multi point broadband wireless access. In this topology, a base station is connected to several public networks. These public networks are fixed subscriber stations, which can be mounted on rooftops. These subscriber stations will allocate uplink and downlink bandwidth to the subscribers as per their individual requirements. The development of WiMAX came about as a complement to Bluetooth and Wi-Fi technologies. The characteristics of these technologies are summarized in the table below.

According to the communication theory, one can have either high bandwidth or long range, but not both simultaneously. However, as can be seen from the above table, one can get fairly long distance with high data rate using WiMAX technology equipments. In practical, using WiMAX technology a subscriber can get symmetrical speed of 10 mbps at 10 km in Line of Sight (LOS) environment.

WiMAX technology offers the following advantages to terrestrial communication providers:

(a) Very high speed

(b) Fairly long distance for communication

(c)WiMAX changes the last mile problem for broadband in the same way as Wi-Fi changed the last hundred feet of networking

(d)Available bandwidth is shared between the users

(e)Subscriber stations are small and can be mounted on rooftops

(f)The data is transmitted over air, thus reducing cabling cost.

Source: Science Reporter, January 2009 issue.

Although the development and implementation of the next generation broadband wireless services (WiMAX & Wi-Fi) is imperative as they would provide solutions for high-speed communications, WiMAX technology could interfere with ‘C’ band satellite communication and hence needs to be tackled with care.

The ‘C’ band is the most popular RF spectrum for terrestrial and satellite communication. This band offers several advantages to communication links such as:

a. Less rain fade.

b. Less atmospheric attenuation

c. Wider coverage area.

d. Small antenna (approximately 2.5 to 3.5 meter, diameter).

‘C’ Band Variants Used Around the World

The ‘C’ band spectrum is extensively used in satellite communication links because it provides several advantages to the links such as:

a. Many satellites available across the world having ‘C’ band transponder

b. Receivers available as off-the-shelf items

c. Well established and inexpensive technology

d. ‘C’ band satellites have wide coverage area across the globe so that it has capability to provide service in remote locations and sparsely populated areas.

Source: Science Reporter, January 2009 issue.