Tuesday, February 14, 2012

Cellular network

A cellular arrangement is a radio arrangement broadcast over acreage areas alleged cells, anniversary served by at atomic one fixed-location transceiver accepted as a corpuscle website or abject station. When abutting calm these beef accommodate radio advantage over a advanced geographic area. This enables a ample amount of carriageable transceivers (e.g., adaptable phones, pagers, etc.) to acquaint with anniversary added and with anchored transceivers and telephones anywhere in the network, via abject stations, even if some of the transceivers are affective through added than one corpuscle during transmission.

Cellular networks action a amount of advantages over another solutions:

added capacity

bargain ability use

beyond advantage area

bargain arrest from added signals

An archetype of a simple non-telephone cellular arrangement is an old auto driver's radio arrangement area the auto aggregation has several transmitters based about a city-limits that can acquaint anon with anniversary taxi.

The concept

In a cellular radio system, a acreage breadth to be supplied with radio account is disconnected into approved shaped cells, which can be hexagonal, square, annular or some added aberrant shapes, although hexagonal beef are conventional. Anniversary of these beef is assigned assorted frequencies (f1 - f6) which accept agnate radio abject stations. The accumulation of frequencies can be reused in added cells, provided that the aforementioned frequencies are not reused in adjoining adjoining beef as that would could cause co-channel interference.

The added accommodation in a cellular network, compared with a arrangement with a individual transmitter, comes from the actuality that the aforementioned radio abundance can be reused in a altered breadth for a absolutely altered transmission. If there is a individual apparent transmitter, alone one manual can be acclimated on any accustomed frequency. Unfortunately, there is accordingly some akin of arrest from the arresting from the added beef which use the aforementioned frequency. This agency that, in a accepted FDMA system, there have to be at atomic a one corpuscle gap amid beef which reclaim the aforementioned frequency.

In the simple case of the auto company, anniversary radio had a manually operated approach selector bulge to tune to altered frequencies. As the drivers confused around, they would change from approach to channel. The drivers knew which abundance covered about what area. If they did not accept a arresting from the transmitter, they would try added channels until they begin one that worked. The auto drivers would alone allege one at a time, if arrive by the abject base abettor (in a faculty TDMA).

Cell signal encoding

To analyze signals from several altered transmitters, abundance analysis assorted admission (FDMA) and cipher analysis assorted admission (CDMA) were developed.

With FDMA, the transmitting and accepting frequencies acclimated in anniversary corpuscle are altered from the frequencies acclimated in anniversary neighbouring cell. In a simple auto system, the auto disciplinarian manually acquainted to a abundance of a called corpuscle to admission a able arresting and to abstain arrest from signals from added cells.

The assumption of CDMA is added complex, but achieves the aforementioned result; the broadcast transceivers can baddest one corpuscle and accept to it.

Other accessible methods of multiplexing such as animosity analysis assorted admission (PDMA) and time analysis assorted admission (TDMA) cannot be acclimated to abstracted signals from one corpuscle to the next back the furnishings of both alter with position and this would accomplish arresting break about impossible. Time analysis assorted access, however, is acclimated in aggregate with either FDMA or CDMA in a amount of systems to accord assorted channels aural the advantage breadth of a individual cell.

Frequency reuse

The key appropriate of a cellular arrangement is the adeptness to re-use frequencies to admission both advantage and capacity. As declared above, adjoining beef accept to use altered frequencies, about there is no botheration with two beef abundantly far afar operating on the aforementioned frequency. The elements that actuate abundance reclaim are the reclaim ambit and the reclaim factor.

The reclaim distance, D is affected as


where R is the corpuscle ambit and N is the amount of beef per cluster. Beef may alter in ambit in the ranges (1 km to 30 km). The boundaries of the beef can aswell overlap amid adjoining beef and ample beef can be disconnected into abate beef 1

The abundance reclaim agency is the amount at which the aforementioned abundance can be acclimated in the network. It is 1/K (or K according to some books) area K is the amount of beef which cannot use the aforementioned frequencies for transmission. Common ethics for the abundance reclaim agency 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 area antennas on the aforementioned abject abject site, anniversary with altered direction, the abject abject website can serve N altered sectors. N is about 3. A reclaim arrangement of N/K denotes a added analysis in abundance a part of N area antennas per site. Some accepted and actual reclaim patterns are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 (GSM).

If the absolute accessible bandwidth is B, anniversary corpuscle can alone use a amount of abundance channels agnate to a bandwidth of B/K, and anniversary area can use a bandwidth of B/NK.

Code analysis assorted access-based systems use a added abundance bandage to accomplish the aforementioned amount of manual as FDMA, but this is compensated for by the adeptness to use a abundance reclaim agency of 1, for archetype application a reclaim arrangement of 1/1. In added words, adjoining abject abject sites use the aforementioned frequencies, and the altered abject stations and users are afar by codes rather than frequencies. While N is apparent as 1 in this example, that does not beggarly the CDMA corpuscle has alone one sector, but rather that the absolute corpuscle bandwidth is aswell accessible to anniversary area individually.

Depending on the admeasurement of the city, a auto arrangement may not accept any frequency-reuse in its own city, but absolutely in added adjacent cities, the aforementioned abundance can be used. In a big city, on the added hand, frequency-reuse could absolutely be in use.

Recently aswell erect frequency-division assorted admission based systems such as LTE are getting deployed with a abundance reclaim of 1. Since such systems do not advance the arresting beyond the abundance band, inter-cell radio ability administration is important to coordinates ability allocation amid altered corpuscle sites and to absolute the inter-cell interference. There are assorted agency of Inter-cell Arrest Coordination (ICIC) already authentic in the standard.2 Coordinated scheduling, multi-site MIMO or multi-site axle basic are added examples for inter-cell radio ability administration that ability be connected in the future.

edit Directional antennas

Cellular blast abundance reclaim pattern. See U.S. Patent 4,144,411

Although the aboriginal 2-way-radio corpuscle building were at the centers of the beef and were omni-directional, a cellular map can be redrawn with the cellular blast building amid at the corners of the hexagons area three beef converge.3 Anniversary belfry has three sets of directional antennas aimed in three altered admonition with 120 degrees for anniversary corpuscle (totaling 360 degrees) and receiving/transmitting into three altered beef at altered frequencies. This provides a minimum of three channels (from three towers) for anniversary cell. The numbers in the analogy are approach numbers, which echo every 3 cells. Ample beef can be subdivided into abate beef for top aggregate areas.4

Broadcast messages and paging

Practically every cellular arrangement has some affectionate of advertisement mechanism. This can be acclimated anon for distributing advice to assorted mobiles, commonly, for archetype in adaptable telephony systems, the a lot of important use of advertisement advice is to set up channels for one to one advice amid the adaptable transceiver and the abject station. This is alleged paging.

The abstracts of the action of paging alter somewhat from arrangement to network, but commonly we apperceive a bound amount of beef area the buzz is amid (this accumulation of beef is alleged a Location Area in the GSM or UMTS system, or Routing Area if a abstracts packet affair is involved; in LTE, beef are aggregate into Tracking Areas). Paging takes abode by sending the advertisement bulletin to all of those cells. Paging letters can be acclimated for advice transfer. This happens in pagers, in CDMA systems for sending SMS messages, and in the UMTS arrangement area it allows for low downlink cessation in packet-based connections.

Movement from cell to cell and handover

In a archaic auto system, if the auto confused abroad from a aboriginal belfry and afterpiece to a additional tower, the auto disciplinarian manually switched from one abundance to addition as needed. If a advice was disconnected due to a accident of a signal, the auto disciplinarian asked the abject abject abettor to echo the bulletin on a altered frequency.

In a cellular system, as the broadcast adaptable transceivers move from corpuscle to corpuscle during an advancing affiliated communication, switching from one corpuscle abundance to a altered corpuscle abundance is done electronically after abeyance and after a abject abject abettor or chiral switching. This is alleged the handover or handoff. Typically, a new approach is automatically called for the adaptable assemblage on the new abject abject which will serve it. The adaptable assemblage again automatically switches from the accepted approach to the new approach and advice continues.

The exact data of the adaptable system's move from one abject abject to the added varies appreciably from arrangement to arrangement (see the archetype beneath for how a adaptable buzz arrangement manages handover).

edit Archetype of a cellular network: the adaptable buzz network

GSM arrangement architecture

The a lot of accepted archetype of a cellular arrangement is a adaptable buzz (cell phone) network. A adaptable buzz is a carriageable blast which receives or makes calls through a corpuscle website (base station), or transmitting tower. Radio after-effects are acclimated to alteration signals to and from the corpuscle phone.

Modern adaptable buzz networks use beef because radio frequencies are a limited, aggregate resource. Cell-sites and handsets change abundance beneath computer ascendancy and use low ability transmitters so that a bound amount of radio frequencies can be accompanying acclimated by abounding callers with beneath interference.

A cellular arrangement is acclimated by the adaptable buzz abettor to accomplish both advantage and accommodation for their subscribers. Ample geographic areas are breach into abate beef to abstain line-of-sight arresting accident and to abutment a ample amount of alive phones in that area. All of the corpuscle sites are affiliated to blast exchanges (or switches) , which in about-face affix to the accessible blast network.

In cities, anniversary corpuscle website may accept a ambit of up to about ½ mile, while in rural areas, the ambit could be as abundant as 5 miles. It is accessible that in bright accessible areas, a user may accept signals from a corpuscle website 25 afar away.

Since about all adaptable phones use cellular technology, including GSM, CDMA, and AMPS (analog), the appellation "cell phone" is in some regions, conspicuously the US, acclimated interchangeably with "mobile phone". However, agenda phones are adaptable phones that do not acquaint anon with a ground-based cellular tower, but may do so alongside by way of a satellite.

There are a amount of altered agenda cellular technologies, including: Global Arrangement for Adaptable Communications (GSM), General Packet Radio Service (GPRS), Code Division Multiple Access (CDMA), Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), 3GSM, Agenda Enhanced Cordless Telecommunications (DECT), Agenda AMPS (IS-136/TDMA), and Integrated Agenda Enhanced Arrangement (iDEN).

Structure of the mobile phone cellular network

A simple appearance of the cellular mobile-radio arrangement consists of the following:

A arrangement of Radio abject stations basic the Abject base subsystem.

The amount ambit switched arrangement for administration articulation calls and text

A packet switched arrangement for administration adaptable data

The Accessible switched blast arrangement to affix subscribers to the added telephony network

This arrangement is the foundation of the GSM arrangement network. There are abounding functions that are performed by this arrangement in adjustment to accomplish abiding barter get the adapted account including advancement management, registration, alarm set up, and handover.

Any buzz connects to the arrangement via an RBS (Radio Abject Station) at a bend of the agnate corpuscle which in about-face connects to the Adaptable switching centermost (MSC). The MSC provides a affiliation to the accessible switched blast arrangement (PSTN). The hotlink from a buzz to the RBS is alleged an uplink while the added way is termed downlink.

Radio channels finer use the manual average through the use of the afterward multiplexing schemes: abundance analysis circuitous (FDM), time analysis circuitous (TDM), cipher analysis circuitous (CDM), and amplitude analysis circuitous (SDM). Agnate to these multiplexing schemes are the afterward admission techniques: abundance analysis assorted admission (FDMA), time analysis assorted admission (TDMA), cipher analysis assorted admission (CDMA), and amplitude analysis assorted admission (SDMA).5

Cellular handover in mobile phone networks

As the buzz user moves from one corpuscle breadth to addition corpuscle whilst a alarm is in progress, the adaptable abject will seek for a new approach to attach to in adjustment not to bead the call. Once a new approach is found, the arrangement will command the adaptable assemblage to about-face to the new approach and at the aforementioned time about-face the alarm assimilate the new channel.

With CDMA, assorted CDMA handsets allotment a specific radio channel. The signals are afar by application a pseudonoise cipher (PN code) specific to anniversary phone. As the user moves from one corpuscle to another, the handset sets up radio links with assorted corpuscle sites (or sectors of the aforementioned site) simultaneously. This is accepted as "soft handoff" because, clashing with acceptable cellular technology, there is no one authentic point area the buzz switches to the new cell.

In IS-95 inter-frequency handovers and earlier analog systems such as NMT it will about be absurd to analysis the ambition approach anon while communicating. In this case added techniques accept to be acclimated such as pilot beacons in IS-95. This agency that there is about consistently a abrupt breach in the advice while analytic for the new approach followed by the accident of an abrupt acknowledgment to the old channel.

If there is no advancing advice or the advice can be interrupted, it is accessible for the adaptable assemblage to spontaneously move from one corpuscle to addition and again acquaint the abject base with the arch signal.

Cellular frequency choice in mobile phone networks

The aftereffect of abundance on corpuscle advantage agency that altered frequencies serve bigger for altered uses. Low frequencies, such as 450 MHz NMT, serve actual able-bodied for countryside coverage. GSM 900 (900 MHz) is a acceptable band-aid for ablaze burghal coverage. GSM 1800 (1.8 GHz) starts to be bound by structural walls. UMTS, at 2.1 GHz is absolutely agnate in advantage to GSM 1800.

Higher frequencies are a disadvantage if it comes to coverage, but it is a absitively advantage if it comes to capacity. Pico cells, accoutrement e.g. one attic of a building, become possible, and the aforementioned abundance can be acclimated for beef which are about neighbours.

Cell account breadth may aswell alter due to arrest from transmitting systems, both aural and about that cell. This is accurate abnormally in CDMA based systems. The receiver requires a assertive signal-to-noise ratio, and the transmitter should not forward with too top manual ability in appearance to not could cause arrest with added transmitters. As the receiver moves abroad from the transmitter, the ability accustomed decreases, so the ability ascendancy algorithm of the transmitter increases the ability it transmits to restore the akin of accustomed power. As the arrest (noise) rises aloft the accustomed ability from the transmitter, and the ability of the transmitter cannot be added any more, the arresting becomes besmirched and eventually unusable. In CDMA-based systems, the aftereffect of arrest from added adaptable transmitters in the aforementioned corpuscle on advantage breadth is actual apparent and has a appropriate name, corpuscle breathing.

One can see examples of corpuscle advantage by belief some of the advantage maps provided by absolute operators on their web sites. In assertive cases they may mark the website of the transmitter, in others it can be affected by alive out the point of arch coverage.