14-11-2012, 04:21 PM
Cellular Networks
cellular network bitendra.ppt (Size: 9.69 MB / Downloads: 101)
A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site orbase station. When joined together these cells provide radio coverage over a wide geographic area. This enables a large number of portable transceivers (e.g., mobile phones, pagers, etc.) to communicate with each other and with fixed transceivers and telephones anywhere in the network, via base stations, even if some of the transceivers are moving through more than one cell during transmission.
The concept
In a cellular radio system, a land area to be supplied with radio service is divided into regular shaped cells, which can be hexagonal, square, circular or some other irregular shapes, although hexagonal cells are conventional. Each of these cells is assigned multiple frequencies (f1 - f6) which have corresponding radio base stations. The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent neighboring cells as that would cause co-channel interference.
Global System for Mobile communications (GSM)
900/1800 MHz band (US: 850/1900 MHz)
For 900 MHz band
Uplink: 890-915
Downlink: 935-960
25 MHz bandwidth - 124 carrier frequency channels, spaced 200KHz apart
Time Division Multiplexing for 8 full rate speech channels per frequency channel.
Circuit Switched Data with data rate of 9.6 kbps
Handset transmission power limited to 2 W in GSM850/900 and 1 W in GSM 1800/1900.
The Base Station Subsystem (BSS)
Base Transceiver Station BTS - transceivers serve different frequencies.
Frequency hopping by handsets and transceivers
Sectorization using directional antennas
Base Station Controller (BSC) controls several (tens to hundreds) of BTSs
allocation of radio channels
handovers between BTSs
concentrator of traffic
databases with information such as carrier frequencies, frequency hopping lists, power reduction levels, etc. for each cell site