1、Cellular networkFrom Wikipedia, the free encyclopediaTop of a cellular radio towerA 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 or base station. When joined together these cells provide radi
2、o 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 m
3、ore than one cell during transmission.Cellular networks offer a number of advantages over alternative solutions: increased capacity reduced power use larger coverage area reduced interference from other signalsAn example of a simple non-telephone cellular system is an old taxi drivers radio system w
4、here the taxi company has several transmitters based around a city that can communicate directly with each taxi.Contentshide 1 The concept 2 Cell signal encoding 3 Frequency reuse 4 Directional antennas 5 Broadcast messages and paging 6 Movement from cell to cell and handover 7 Example of a cellular
5、 network: the mobile phone networko 7.1 Structure of the mobile phone cellular networko 7.2 Cellular handover in mobile phone networkso 7.3 Cellular frequency choice in mobile phone networkso 7.4 Coverage comparison of different frequencies 8 See also 9 ReferenceseditThe conceptExample of frequency
6、reuse 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 freq
7、uencies (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.The increased capacity in a cellular network, compared wi
8、th a network with a single transmitter, 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 inevitably so
9、me 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 channe
10、l selector knob to tune to different frequencies. As the drivers moved around, they would change from channel to channel. The drivers know which frequency covers approximately what area. When they do not receive a signal from the transmitter, they will try other channels until they find one that wor
11、ks. The taxi drivers only speak one at a time, when invited by the base station operator (in a sense TDMA).editCell signal encodingTo distinguish signals from several different transmitters, frequency division multiple access (FDMA) and code division multiple access (CDMA) were developed.With FDMA,
12、the transmitting and receiving frequencies used in each cell are different from the frequencies used in each neighbouring cell. In a simple taxi system, the taxi driver manually tuned to a frequency of a chosen cell to obtain a strong signal and to avoid interference from signals from other cells.Th
13、e 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 separate signal
14、s from one cell to the next since the effects of both vary with position and this would make signal separation practically impossible.Time division multiple access, however, is used in combination with either FDMA or CDMA in a number of systems to give multiple channels within the coverage area of a
15、 single cell.editFrequency reuseThe key characteristic of a cellular network is the ability to re-use frequencies to increase both coverage and capacity. As described above, adjacent cells must utilize different frequencies, however there is no problem with two cells sufficiently far apart operating
16、 on the same frequency. The elements that determine frequency reuse are the reuse distance and the reuse factor.The reuse distance, 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 cel
17、ls can also overlap between adjacent cells and large cells can be divided into smaller cells 1The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies f
18、or transmission. Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation).In case of N sector antennas on the same base station site, each with different direction, the base station site can serve N different sectors. N is typically 3.
19、 A reuse pattern of N/K denotes a further division in frequency among N sector antennas per site. Some current and historical reuse patterns are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 (GSM).If the total available bandwidth is B, each cell can only utilize a number of frequency chan
20、nels corresponding to a bandwidth of B/K, and each sector can use a bandwidth of B/NK.Code division multiple access-based systems use a wider frequency band to achieve the same rate of transmission as FDMA, but this is compensated for by the ability to use a frequency reuse factor of 1, for example
21、using a reuse pattern of 1/1. In other words, adjacent base station sites use the same frequencies, and the different base stations and users are separated by codes rather than frequencies. While N is shown as 1 in this example, that does not mean the CDMA cell has only one sector, but rather that t
22、he entire cell bandwidth is also available to each sector individually.Depending on the size of the city, a taxi system may not have any frequency-reuse in its own city, but certainly in other nearby cities, the same frequency can be used. In a big city, on the other hand, frequency-reuse could cert
23、ainly be in use.Recently also orthogonal frequency-division multiple access based systems such as LTE are being deployed with a frequency reuse of 1. Since such systems do not spread the signal across the frequency band, inter-cell radio resource management is important to coordinates resource alloc
24、ation between different cell sites and to limit the inter-cell interference. There are various means of Inter-cell Interference Coordination (ICIC) already defined in the standard 2. Coordinated scheduling, multi-site MIMO or multi-site beam forming are other examples for inter-cell radio resource m
25、anagement that might be standardized in the future.editDirectional antennasCellular telephone frequency reuse pattern. See U.S. Patent 4,144,411Although the original 2-way-radio cell towers were at the centers of the cells and were omni-directional, a cellular map can be redrawn with the cellular te
26、lephone towers located at the corners of the hexagons where three cells converge.3 Each tower has three sets of directional antennas aimed in three different directions with 120 degrees for each cell (totaling 360 degrees) and receiving/transmitting into three different cells at different frequencie
27、s. This provides a minimum of three channels (from three towers) for each cell. The numbers in the illustration are channel numbers, which repeat every 3 cells. Large cells can be subdivided into smaller cells for high volume areas.4editBroadcast messages and pagingPractically every cellular system
28、has some kind of broadcast mechanism. This can be used directly for distributing information to multiple mobiles, commonly, for example in mobile telephony systems, the most important use of broadcast information is to set up channels for one to one communication between the mobile transceiver and t
29、he base station. This is calledpaging.The details of the process of paging vary somewhat from network to network, but normally we know a limited number of cells where the phone is located (this group of cells is called a Location Area in the GSM or UMTS system, or Routing Area if a data packet sessi
30、on is involved). Paging takes place by sending the broadcast message to all of those cells. Paging messages can be used for information transfer. This happens in pagers, in CDMA systems for sending SMS messages, and in the UMTS system where it allows for low downlink latency in packet-based connecti
31、ons.editMovement from cell to cell and handoverIn a primitive taxi system, when the taxi moved away from a first tower and closer to a second tower, the taxi driver manually switched from one frequency to another as needed. If a communication was interrupted due to a loss of a signal, the taxi drive
32、r asked the base station operator to repeat the message on a different frequency.In a cellular system, as the distributed mobile transceivers move from cell to cell during an ongoing continuous communication, switching from one cell frequency to a different cell frequency is done electronically with
33、out interruption and without a base station operator or manual switching. This is called the handover or handoff. Typically, a new channel is automatically selected for the mobile unit on the new base station which will serve it. The mobile unit then automatically switches from the current channel t
34、o the new channel and communication continues.The exact details of the mobile systems move from one base station to the other varies considerably from system to system (see the example below for how a mobile phone network manages handover).editExample of a cellular network: the mobile phone networkG
35、SM network architectureThe most common example of a cellular network is a mobile phone (cell phone) network. A mobile phone is a portable telephone which receives or makes calls through a cell site (base station), or transmitting tower. Radio waves are used to transfer signals to and from the cell p
36、hone.Modern mobile phone networks use cells because radio frequencies are a limited, shared resource. Cell-sites and handsets change frequency under computer control and use low power transmitters so that a limited number of radio frequencies can be simultaneously used by many callers with less inte
37、rference.A cellular network is used by the mobile phone operator to achieve both coverage and capacity for their subscribers. Large geographic areas are split into smaller cells to avoid line-of-sight signal loss and to support a large number of active phones in that area. All of the cell sites are
38、connected to telephone exchanges (or switches) , which in turn connect to the public telephone network.In cities, each cell site may have a range of up to approximately mile, while in rural areas, the range could be as much as 5 miles. It is possible that in clear open areas, a user may receive sign
39、als from a cell site 25 miles away.Since almost all mobile phones use cellular technology, including GSM, CDMA, and AMPS (analog), the term “cell phone“ is in some regions, notably the US, used interchangeably with “mobile phone“. However, satellite phones are mobile phones that do not communicate d
40、irectly with a ground-based cellular tower, but may do so indirectly by way of a satellite.There are a number of different digital cellular technologies, including: Global System for Mobile Communications (GSM), General Packet Radio Service(GPRS), Code Division Multiple Access (CDMA), Evolution-Data
41、 Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), 3GSM, Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN).editStructure of the mobile phone cellular networkMain article: GSMStructure of a 2G cellular networ
42、kA simple view of the cellular mobile-radio network consists of the following: A network of Radio base stations forming the Base station subsystem. The core circuit switched network for handling voice calls and text A packet switched network for handling mobile data The Public switched telephone net
43、work to connect subscribers to the wider telephony networkThis network is the foundation of the GSM system network. There are many functions that are performed by this network in order to make sure customers get the desired service including mobility management, registration, call set up, andhandove
44、r.Any phone connects to the network via an RBS in the corresponding cell which in turn connects to the MSC. The MSC allows the onward connection to the PSTN. The link from a phone to the RBS is called an uplink while the other way is termed downlink.Radio channels effectively use the transmission me
45、dium through the use of the following multiplexing schemes: frequency division multiplex (FDM), time division multiplex (TDM), code division multiplex (CDM), and space division multiplex (SDM). Corresponding to these multiplexing schemes are the following access techniques: frequency division multip
46、le access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), andspace division multiple access (SDMA).5editCellular handover in mobile phone networksMain article: HandoffAs the phone user moves from one cell area to another cell whilst a call is in progress, the mobi
47、le station will search for a new channel to attach to in order not to drop the call. Once a new channel is found, the network will command the mobile unit to switch to the new channel and at the same time switch the call onto the new channel.With CDMA, multiple CDMA handsets share a specific radio c
48、hannel. The signals are separated by using a pseudonoise code (PN code) specific to each phone. As the user moves from one cell to another, the handset sets up radio links with multiple cell sites (or sectors of the same site) simultaneously. This is known as “soft handoff“ because, unlike with trad
49、itional cellular technology, there is no one defined point where the phone switches to the new cell.In IS-95 inter-frequency handovers and older analog systems such as NMT it will typically be impossible to test the target channel directly while communicating. In this case other techniques have to be used such as pilot beacons in IS-95. This means that there is almost always a brief break in the communication while searching for the new channel followed by the risk of an unexpected return to the old channel.If there is no ongoing communication
