COMMUNICATION TECHNOLOGIES

Archive for June, 2009

To optimize the performance of a satellite communications system, the directions of maximum gain of an earth station antenna must be pointed directly at the satellite. To ensure that the earth station antenna is aligned, two angles must be determined: the azimuth and the elevation angle. Azimuth angle and elevation angle are jointly referred to as the antenna look angles. With geosynchronous satellite, the look angles of earth station antennas need to be adjusted only once as the satellite will remain in a given position permanently, except for occasional minor variations.

The location of a satellite is generally in terms of latitude and longitude similar to the way the location of a point on earth is described. However, because a satellite is orbiting many miles above the earth’s surface, it has no latitude or longitude. Therefore, it location is identified by a point on the surface of earth directly below the satellite. This point is called the sub satellite point (SSP). Angle of elevation (sometimes called elevation angle) is the vertical angle formed between the direction of travels of an electromagnetic wave radiated from an earth station antenna pointing directly toward a satellite and the horizontal angle.

The smaller angle of elevation, the greater the distance a propagated wave must pass through earth’s atmosphere. As with any wave propagated through earth’s atmosphere, it suffers absorption and may also be severely contaminated by noise. Azimuth angle is the horizontal angular distance from a reference direction, either the solution or northern most point of the horizon. Azimuth angle is defined as the horizontal pointing angle of an earth station antenna. For navigation proposes, azimuth angle is usually measured in a clockwise direction in degrees from true north.

However, for satellite earth stations in the northern hemisphere and satellite vehicles in geosynchronous orbits, azimuth angle is generally referenced to true south elevation angle and azimuth angle of the earth station antenna relative to a satellite. Another factor that could possibly cause collisions with CSMA/CD is propagation delay. Propagation delay is the time it takes a signal to travel from a source to a destination. Because of propagation delay, it is possible for the line to appear idle when, in fact, another station is transmitting a signal that has not yet reached the monitoring station.

Principles:

The key principles of cellular telephone were uncovered in 1947 by researchers at bell telephone laboratories and other telecommunications companies throughout the world when they developed the basic concepts and theory of cellular telephone. It was determined that by subdividing a relatively large geographic market area. Called a coverage zone, into small screens, called cells, the concept of frequency reuse could be employed to dramatically increase the capacity of a mobile telephone channel. In essence, cellular telephone systems allow a large number of users to share the limited number of common-usage radio channels available in a region. In addition, integrated-circuit technology, microprocessors and microcontroller chips, and the implantation of signaling system SS7 have recently enabled complex radio and logic circuits to be used in electronic switching machines to store programs that provide faster and more efficient call processing.

Fundamental Concepts:

The fundamental concepts of cellular telephone are quite simple. The FCC originally defined geographic cellular radio coverage areas on the basis of modified 1980 census figures. With the cellular concept, each area is further divided into hexagonal-shaped cells that fit together to from a honeycomb pattern. The hexagon shape was chosen because it provides the most effective transmission by approximating a circular pattern while eliminating gaps inherently present between adjacent circles. A cell is defined by its physical size and, more importantly, by the size of its population and traffic patterns.

The number of cell per system and the size of the cell are not specifically defined by the FCC and have been left to the providers to establish in accordance with anticipated traffic patterns. Each geographical area is allocated a fixed number of cellular voice channels. The physical size of a cell varies, depending on user density and calling patterns. Microcells are used most often in high-density areas such as found in large cities and inside building, by virtue of their low effective working radius, microcells exhibit milder propagation impairments, such as reflection and signal delays.

Microcell may overlay clusters of microcells with slow-moving mobile units using the micro-cells and faster-moving units the macrocells. The mobile unit is able to identity itself as either fast or slow moving, thus allowing it to do fewer cell transfers and location updates. Cell transfers algorithms can be modified to allow for the small distances between a mobile unit and the microcellular base station it is communicating .hexagonal cell grid might look like when superimposed over a metropolitan city.