Mobile Phones - Principle of operation
A cellular/mobile system provides standard telephone operation by full-duplex two-way radio at remote locations. It provides a wireless connection to the Public Switched Telephone Network (PSTN) from any user location within the radio range of the system.
The basic concept behind the cellular radio system is that rather than serving a given geographical area within a single transmitter and receiver, the system divides the service area into many small areas known as cells, as shown in Fig. below. The typical cell covers only several square kilometers and contains its own receiver and low-power transmitter. The cell area shown in Fig. below is ideal hexagon. However, in reality they will have circular or other geometric shapes. These areas may overlap, and cells may be of different sizes.
Basic cellular system consists of mobile stations, base stations and a mobile switching center (MSC). The MSC is also known as Mobile Telephone Switching Office (MTSO). The MTSO controls '\11 the cells and provides the interface between each cell and the main telephone office. Each mobile communicates via radio with one of the base stations and may be handed off (switched from one cell to another) to any other base station throughout the duration of the call.
Each mobile station consists of a transceiver, an antenna and control circuitry. The base station consists of several transmitters and receivers which simultaneously handle full duplex communication and generally have towers which support several transmitting and receiving antennas. The base station serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls via telephone lines or microwave link to the MSC. The MSC co-ordinates the activities of all the base stations and connects the entire cellular system to the PSTN, most of the cellular system also provide a service known as roaming.
The cellular system operates in the 800-900 MHz range. The newer digital cellular systems have even greater capacity. Some of these systems operate in 1.7-1.8 GHz bands.
Cellular Telephone Unit
The Fig. below shows the block diagram of a cellular mobile radio unit. The unit consists of five major sections:
Transmitter, receiver, synthesizer, logic unit, and control unit. The mobile unit contains built-in rechargeable batteries to Provide operating power. The transmitter and receiver in the unit share the common antenna.
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