Radio communication

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Cellular Telecommunication and Radio communication

All radio systems exhibit several fundamental characteristics.

The geographic area served by a cellular radio system is broken up into smaller geographic areas, or cells. Uniform hexagons most frequently are employed to represent these cells on maps and diagrams; in practice, though, radio waves do not confine themselves to hexagonal areas, so that the actual cells have irregular shapes.

All communication with a mobile or portable instrument within a given cell is made to the base station that serves the cell.

Because of the low transmitting power of battery-operated portable instruments, specific sending and receiving frequencies assigned to a cell may be reused in other cells within the larger geographic area. Thus, the spectral efficiency of a cellular system (that is, the uses to which it can put its portion of the radio spectrum) is increased by a factor equal to the number of times a frequency may be reused within its service area.

As a mobile instrument proceeds from one cell to another during the course of a call, a central controller automatically reroutes the call from the old cell to the new cell without a noticeable interruption in the signal recep­tion. This process is known as handoff. The central controller, or mobile telephone switching office, thus acts as an intelligent central office switch that keeps track of the movement of the mobile subscriber.

As demand for the radio channels within a given cell increases beyond the capacity of that cell (as measured by the number of calls that may be sup­ported simultaneously), the overloaded cell is "split" into smaller cells, each with its own base station and central controller. The radio-frequency alloca­tions of the original cellular system are then rearranged to account for the greater number of smaller cells. Frequency reuse between discontinuous cells and the splitting of cells as demand increases are the concepts that distinguish cellular systems from other radiotelephone systems. They allow cellular providers to serve large metropolitan areas that may contain hundreds of thousands of customers.

Radio communication

Radio communication is the transfer of high-frequency energy from the transmitter to the receiver. Accordingly, the main components of radio com­munication of any kind are a transmitter and a receiver.

The function of a transmitter is to generate electrical oscillations at radio frequency which is called the carrier frequency. The main components of a transmitter are an oscillator, amplifiers, and a transducer.

Their functions are as follows.

The oscillator of a transmitter converts electric power into oscillations of definite radio frequency.

As for amplifiers, they increase the intensity of oscillations produced by the oscillator and retain the definite frequency. The transducer converts the information to be transmitted into a varying electrical voltage. In case of sound transmission a microphone serves as a transducer. And in case of pic­ture transmission a photoelectric device serves as the transducer.

Other important components of radio transmitters are the modulator and the aerial. As for the modulators, they use proportional voltages for con­trolling the variations of oscillation intensity. And the aerial is that part of a radio system from which energy is transmitted into or received from the space (or the atmosphere).

The aerial must be insulated from the ground and may occupy vertical or horizontal position. In case the transmitter aerial occupies vertical position, the receiver aerial must be also vertical and vice versa. It applies to all radio frequencies except short waves. In usual amplitude-modulated broadcasting the receiver aerial may consist of a wire wound on a core.

Дата добавления: 2015-11-14