Editing Cellular network (section)

== Mobile phone network ==
[[File:GSM ArchitecturePL.svg|thumb|300px|WCDMA network architecture|alt=3G network]]
The 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 wave]]s are used to transfer signals to and from the cell phone.

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 the usually limited number of radio frequencies can be simultaneously used by many callers with less interference.

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 connected to [[telephone exchange]]s (or switches), which in turn connect to the [[public telephone network]].

In cities, each cell site may have a range of up to approximately {{convert|1/2|mi}}, while in rural areas, the range could be as much as {{convert|5|mi}}. It is possible that in clear open areas, a user may receive signals from a cell site {{convert|25|mi}} away.  In rural areas with low-band coverage and tall towers, basic voice and messaging service may reach {{convert|50|mi}}, with limitations on bandwidth and number of simultaneous calls. {{citation needed|date=August 2022}}
 
Since almost all mobile phones use [[cellular technology]], including [[GSM]], [[CDMA]], and [[Advanced Mobile Phone System|AMPS]] (analog), the term "cell phone" is in some regions, notably the US, used interchangeably with "mobile phone". However, [[satellite phone]]s are mobile phones that do not communicate directly 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), [[cdmaOne]], [[CDMA2000]], [[Evolution-Data Optimized]] (EV-DO), [[Enhanced Data Rates for GSM Evolution]] (EDGE), [[Universal Mobile Telecommunications System]] (UMTS), [[Digital Enhanced Cordless Telecommunications]] (DECT), [[Digital AMPS]] (IS-136/TDMA), and [[Integrated Digital Enhanced Network]] (iDEN). The transition from existing analog to the digital standard followed a very different path in Europe and the [[United States|US]].<ref>Paetsch, Michael (1993): The evolution of mobile communications in the US and Europe. Regulation, technology, and markets. Boston, London: Artech House (The Artech House mobile communications library).
</ref> As a consequence, multiple digital standards surfaced in the US, while [[Europe]] and many countries converged towards the [[GSM]] standard.

=== Structure of the mobile phone cellular network ===
A simple view of the cellular mobile-radio network consists of the following:

* A network of radio [[base station]]s forming the [[base station subsystem]].
* The [[Network switching subsystem|core circuit switched network]] for handling voice calls and text
* A [[GPRS|packet switched network]] for handling mobile data
* The [[public switched telephone network]] to connect subscribers to the wider telephony network

This 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, and [[handoff|handover]].

Any phone connects to the network via an RBS ([[Radio Base Station]]) at a corner of the corresponding cell which in turn connects to the [[Mobile switching center]] (MSC). The MSC provides a connection to the [[public switched telephone network]] (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 medium through the use of the following multiplexing and access schemes: [[frequency-division multiple access]] (FDMA), [[time-division multiple access]] (TDMA), [[code-division multiple access]] (CDMA), and [[space-division multiple access]] (SDMA).

=== Small cells ===
{{Main|Small cell}}
Small cells, which have a smaller coverage area than base stations, are categorised as follows:
* [[Microcell]] -> less than 2 kilometres,
* [[Picocell]]  -> less than 200 metres,
* [[Femtocell]] -> around 10 metres,
* [[Attocell]]  -> 1–4 metres

=== Cellular handover in mobile phone networks ===
{{Main|Handover}}
As the phone user moves from one cell area to another cell while a call is in progress, the mobile 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 channel. The signals are separated by using a [[pseudorandom noise|pseudonoise]] code (PN code) that is 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 traditional [[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 [[Nordic Mobile Telephone|NMT]] it will typically be impossible to test the target channel directly while communicating. In this case, other techniques have to be used <!--how does NMT handover work exactly?? --> 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 or the communication can be interrupted, it is possible for the mobile unit to spontaneously move from one cell to another and then notify the base station with the strongest signal.

=== Cellular frequency choice in mobile phone networks ===
{{Main|Cellular frequencies}}
The effect of frequency on cell coverage means that different frequencies serve better for different uses. Low frequencies, such as 450&nbsp; MHz NMT, serve very well for countryside coverage. [[GSM]] 900 (900&nbsp;MHz) is suitable for light urban coverage.  [[GSM]] 1800 (1.8&nbsp;GHz) starts to be limited by structural walls.  [[UMTS]], at 2.1&nbsp;GHz is quite similar in coverage to [[GSM]] 1800.

Higher frequencies are a disadvantage when it comes to coverage, but it is a decided advantage when it comes to capacity. Picocells, covering e.g. one floor of a building, become possible, and the same frequency can be used for cells which are practically neighbors.

Cell service area may also vary due to interference from transmitting systems, both within and around that cell.  This is true especially in CDMA based systems.  The receiver requires a certain [[signal-to-noise ratio]], and the transmitter should not send with too high transmission power in view to not cause interference with other transmitters. As the receiver moves away from the transmitter, the power received decreases, so the [[power control]] algorithm of the transmitter increases the power it transmits to restore the level of received power. As the interference (noise) rises above the received power from the transmitter, and the power of the transmitter cannot be increased anymore, the signal becomes corrupted and eventually unusable. In [[CDMA-based systems]], the effect of interference from other mobile transmitters in the same cell on coverage area is very marked and has a special name, ''[[cell breathing (telephony)|cell breathing]]''.

One can see examples of cell coverage by studying some of the coverage maps provided by real operators on their web sites or by looking at independently crowdsourced maps such as [[Opensignal]] or [[CellMapper]]. In certain cases they may mark the site of the transmitter; in others, it can be calculated by working out the point of strongest coverage.

A [[cellular repeater]] is used to extend cell coverage into larger areas. They range from wideband repeaters for consumer use in homes and offices to smart or digital repeaters for industrial needs.

=== Cell size ===
The following table shows the dependency of the coverage area of one cell on the frequency of a [[CDMA2000]] network:<ref>{{cite web|url=https://www.itu.int/ITU-D/tech/events/2003/slovenia2003/Presentations/Day%203/3.3.1_Chandler.pdf|title=CDMA 2000 and CDMA 450|author=Colin Chandler|page=17|date=3 December 2003|access-date=28 January 2024}}</ref>
{| class="wikitable"
|-
! Frequency (MHz)
! Cell radius (km)
! Cell area (km<sup>2</sup>)
! Relative cell count
|-
| 450
| 48.9
| 7521
| 1
|-
| 950
| 26.9
| 2269
| 3.3
|-
| 1800
| 14.0
| 618
| 12.2
|-
| 2100
| 12.0
| 449
| 16.2
|}