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Cell site

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Cell tower in Morrisville, North Carolina
An antenna array on top of an electricity pylon
A "camouflaged" monopole, called a monopalm, located in Tucson, Arizona
Cell site tower in the Philippines
A short-mast cell site on top of a mountain in Wyoming, USA.

A cell site is a term used primarily in North America for a site where antennas and electronic communications equipment are placed on a radio mast or tower to create a cell in a cellular network. A cell site is composed of a tower or other elevated structure for mounting antennas, and one or more sets of transmitter/receivers transceivers, digital signal processors, control electronics, a GPS receiver for timing (for CDMA2000 or IS-95 systems), regular and backup electrical power sources, and sheltering.[1]

A synonym for "cell site" is "cell tower", although many cell site antennas are mounted on buildings rather than as towers. In GSM networks, the technically correct term is Base Transceiver Station (BTS), and colloquial British English synonyms are "mobile phone mast" or "base station". The term "base station site" might better reflect the increasing co-location of multiple mobile operators, and therefore multiple base stations, at a single site. Depending on an operator's technology, even a site hosting just a single mobile operator may house multiple base stations, each to serve a different air interface technology (CDMA or GSM, for example). Preserved treescapes can often hide cell towers inside an artificial tree or preserved tree. These installations are generally referred to as concealed cell sites or stealth cell sites.

Cell site range

The working range of a cell site - the range within which mobile devices can connect to it reliably is not a fixed figure. It will depend on a number of factors, including

  • The frequency of signal in use (i.e. the underlying technology).
  • The transmitter's rated power.
  • The transmitter's size.
  • The array setup of panels may cause the transmitter to be directional or omni-directional.
  • It may also be limited by local geographical or regulatory factors and weather conditions.

Generally, in areas where there are enough cell sites to cover a wide area, the range of each one will be set to:

  • Ensure there is enough overlap for "handover" to/from other sites (moving the signal for a mobile device from one cell site to another, for those technologies that can handle it - e.g. making a GSM phone call while in a car or train).
  • Ensure that the overlap area is not too large, to minimize interference problems with other sites.

In practice, cell sites are grouped in areas of high population density, with the most potential users. Cell phone traffic through a single cell mast is limited by the mast's capacity; there is a finite number of calls that a mast can handle at once. This limitation is another factor affecting the spacing of cell mast sites. In suburban areas, masts are commonly spaced 1-2 miles apart and in dense urban areas, masts may be as close as ¼-½ mile apart. Cell masts always reserve part of their available bandwidth for emergency calls.

Objects intruding into the fresnel zone between radio transmitters and receivers can greatly impact signal strength.

The maximum range of a mast (where it is not limited by interference with other masts nearby) depends on the same circumstances. Some technologies, such as GSM, have a fixed maximum range of 40km (25 miles), which is imposed by technical limitations. CDMA and iDEN have no built-in limit, but the limiting factor is really the ability of a low-powered personal cell phone to transmit back to the mast. As a rough guide, based on a tall mast and flat terrain, it is possible to get between 50 to 70 km (30-45 miles). When the terrain is hilly, the maximum distance can vary from as little as 5 kilometres (3.1 mi) to 8 kilometres (5.0 mi) due to encroachment of intermediate objects into the wide center fresnel zone of the signal.[2] Depending on terrain and other circumstances, a GSM Tower can replace between 2 and 50 miles of cabling for fixed wireless networks.[3]

Communications channel reuse

The concept of "maximum" range is misleading, however, in a cellular network. Cellular networks are designed to create a mass communication solution from a limited amount of channels (slices of radio frequency spectrum necessary to make one conversation) that are licensed to an operator of a cellular service. To overcome this limitation, it is necessary to repeat and reuse the same channels. Just as a station on a car radio changes to a completely different local station when you travel to another city, the same radio channel gets reused on a cell mast only a few miles away. To do this, the signal of a cell mast is intentionally kept at low power and many cases tilting downward to limit its area. The area sometimes needs to be limited when a large number of people live, drive or work near a particular mast; the range of this mast has to be limited so that it covers an area small enough not to have to support more conversations than the available channels can carry. Due to the sectorized arrangement of antennas on a tower, it is possible to vary the strength and angle of each sector depending on the coverage of other towers in view of the sector.

A cellphone may not work at times, because it is too far from a mast, but it may also not work because the phone is in a location where there is interference to the cell phone signal from thick building walls, hills or other structures. The signals do not need a clear line of sight but the more interference will degrade or eliminate reception. Too many people may be trying to use the cell mast at the same time, e.g. a traffic jam or a sports event, then there will be a signal on the phone display but it is blocked from starting a new connection. The other limiting factor for cell phones is the ability of the cell phone to send a signal from its low powered battery to the mast. Some cellphones perform better than others under low power or low battery, typically due to the ability to send a good signal from the phone to the mast.

The base station controller (a central computer that specializes in making phone connections) and the intelligence of the cellphone keeps track of and allows the phone to switch from one mast to the next during conversation. As the user moves towards a mast it picks the strongest signal and releases the mast from which the signal has become weaker; that channel on that mast becomes available to another user.

Temporary cell sites

Cell-on-wheels, or COW

Although cell antennas are normally attached to permanent structures, cell providers maintain a fleet of temporary cell sites. When mounted on a trailer, they are called a COW or Cell On Wheels. These usually include a base station controller and a telescoping tower with antennas attached. A generator may be included when electrical power isn't available, and an additional backhaul antenna may be mounted to link the temporary tower into the network.

COWs are often used at the site of a permanent cell site. Floods, fires, terrorism, and other disasters may destroy permanent antennas or base stations controllers, and fast dispatch of COWs can maintain vital communications during an emergency. They are also used in planned outages, such as when an antenna site is unavailable due to construction or maintenance. Finally, they are often used to augment capacity when large number of additional cell phone users are expected such as at large football games or NASCAR races.

See also

References

  1. ^ International Engineering Consortium On-line Education, "Cellular Communications" undated, URL retrieved 14 August 2007.
  2. ^ Frequently Asked PCS Questions undated, URL retrieved 14 August 2007.
  3. ^ NTIA Seeks Input on Broadband Stimulus Money undated, URL retrieved 03 March 2009.
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