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1、Cellular networkFrom Wikipedia, the free encyclopediaTop of a cellular radio towerA cellular network is a radio n etwork distributed over la nd areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station. When joined together these cells provide ra
2、dio 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 tran sceivers and teleph ones an ywhere in the n etwork, via base stati ons, eve n if some of the transceivers are moving t
3、hrough more than one cell during transmission.Cellular networks offer a number of advantages over alternative solutions: in creased capacity reduced power use larger coverage area reduced interferenee from other signalsAn example of a simple non-telephone cellular system is an old ta)i drivers radio
4、 system where the taxi company has several transmitters based around a city that can communicate directly with each taxi.Contentshide1 The concept2 Cell signal cncoding3 Frequency reuse 1 Directional anlennas5 Broadcast messages and paging6 Movement from cell to cel 1 and handover7 Example of a cell
5、ular network: the mobi1e phone network7. 1 Structure of the mobi1e phone cellular network7. 2 Cellular handover in mobile phone networks7. 3 Cellular frequency choice in mobi1e phone networks7. 4 Coverage comparison of different frequencics8 See also9 ReferenceseditiThe conceptExample of frequency r
6、euse factor or pattern 1/4In 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 frequ
7、encies （斥 fQ） which have corresp on ding radio base stati ons. The group of freque ncies can be reused in other cells, provided that the same freque ncies are not reused in adjace nt neighbori ng cells as that would cause cochannel interferenee.The in creased capacity in a cellular n etwork, compare
8、d with a network with a single tran smitter, comes from the fact that the same radio frequency can be reused in a different area for a completely different transmission. If there is a single plain transmitter, only one transmission can be used on any given frequency. Unfortunately, there is inevitab
9、ly some level of interference from the signal from the other cells which use the same frequency. This means that, in a standard FDMA system, there must be at least a one cell gap between cells which reuse the same frequency.In the simple case of the taxi company, each radio had a manually operated c
10、hannel selector knob to tune to different frequencies As the drivers moved around, they would change from channel to channel. The drivers know which freciuency covers approximately what area When they do not receive a signal from the transmitter, they will try other channels until they find one that
11、 works. The taxi drivers only speak one at a time, when invited by the base station operator (in a sense TDMA),editiCell signal encodingTo distinguish signals from several different transmitters, freauencv division multiple access (FDMA) and code division multiple access (CDMA) were developed.With F
12、DMA, the transmitting and receiving frequencies used in each cell are different from the freque ncies used in each n eighbouri ng cell .In a simple taxi system, the taxi driver manu ally tuned to a frequency of a chosen cell to obtain a strong signal and to avoid interferenee from signals from other
13、 cellsThe principle of CDMA is more complex, but achieves the same result; the distributed transceivers can select one cell and listen to it.Other available methods of multiplexing such as polarization division multiple access (PDMA) and time division multiple access (TDMA) cannot be used to separat
14、e signals from one cell to the next since the effects of both vary with position and this would make signal separation practically impossible. Time divisio n multiple access, however, is used in combi nation with either FDMA or CDMA in a number of systems to give multiple channels within the coverag
15、e area of a single cell.editiFrequency reuseThe key characteristic of a cellular n etwork is the ability to re-use freque ncies to in crease both coverage and capacity. As described above, adjace nt cells must utilize different frequencies, however there is no problem with two cells sufficiently far
16、 apart operating on the same frequency. The elements that determine frequency reuse are the reuse distanee and the reuse factor.The reuse distanee, D is calculated aswhere R is the cell radius and N is the number of cells per cluster. Cells may vary in radius in the ranges (1 km to 30 km). The boundaries of the cells can also overlap between adjacent cells and large cells can be divided into smaller cells
