Cell site


A cell site, cell phone tower, cell base tower, or cellular base station is a cellular-enabled mobile device site where antennas and electronic communications equipment are placed to create a cell, or adjacent cells, in a cellular network. The raised structure typically supports antennae and one or more sets of transmitter/receivers transceivers, digital signal processors, control electronics, a GPS receiver for timing, primary and backup electrical power sources, and sheltering.
Some companies provide infrastructure services for cellular networks, including site acquisition, construction, and ongoing maintenance. These third-party providers can manage multiple sites and coordinate network upgrades, helping operators expand coverage efficiently. Such services are offered by companies that specialize in tower management and network support.
Multiple cellular providers often save money by mounting their antennas on a common shared mast; since separate systems use different frequencies, antennas can be located close together without interfering with each other. Some provider companies operate multiple cellular networks and similarly use colocated base stations for two or more cellular networks,.
Cell sites are sometimes required to be inconspicuous; they may be blended with the surrounding area, mounted on buildings or advertising towers. Preserved treescapes can often hide cell towers inside an artificial or preserved tree. These installations are generally referred to as concealed cell sites or stealth cell sites.

Overview

A cellular network is a network of handheld mobile phones in which each phone communicates with the telephone network by radio waves through a local antenna at a cellular base station. The coverage area in which service is provided is divided into a mosaic of small geographical areas called "cells", each served by a separate low power multichannel transceiver and antenna at a base station. All the cell phones within a cell communicate with the system through that cell's antenna, on separate frequency channels assigned by the base station from a common pool of frequencies used by the system.
The purpose of cellular organization is to conserve radio bandwidth by frequency reuse; the low power radio signals used within each cell do not travel far beyond the cell, so the radio channels can be reused in geographically separated cells. When a mobile user moves from one cell to another, their phone is automatically "handed off" to the new cell's antenna, and assigned a new set of frequencies, and subsequently communicates with this antenna. This background handoff process is imperceptible to the user and can occur in the middle of a phone call without any service interruption. Each cell phone has an automated full duplex digital transceiver and communicates with the cell antenna over two digital radio channels in the UHF or microwave band, one for each direction of the bidirectional conversation, plus a control channel which handles registering the phone with the network, dialing, and the handoff process.
Typically, a cell tower is located at the edge of one or more cells and covers multiple cells using directional antennas. A common geometry is to locate the cell site at the intersection of three adjacent cells, with three antennas at 120° angles each covering one cell. The type of antenna used for cellular base stations , called a sector antenna, usually consists of a vertical collinear array of dipoles. It has a flat, fan-shaped radiation pattern, that is tilted slightly downward to cover the cell area, avoiding radiation at higher angles that could interfere with distant cells reusing the same frequencies. The elevation angle of the antenna must be carefully adjusted, so the beam covers the entire cell without radiating too far. In modern sector antennas beam tilt can usually be adjusted electronically, to avoid the necessity of a lineman climbing the tower to mechanically tilt the antenna when adjustment is needed.

Operation

Range

The working range of a cell site is not a fixed figure. It will depend on a number of factors, including:
  • Height of antenna over surrounding terrain.
  • The frequency of signal in use.
  • The transmitter's rated power.
  • The required uplink/downlink data rate of the subscriber's device
  • The directional characteristics of the site antenna array.
  • Reflection and absorption of radio energy by buildings or vegetation.
  • Being limited by local geographical or regulatory factors and weather conditions.
  • Timing limitations in some technologies
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.
  • 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 site is limited by the base station's capacity; of -56 dBm signal there is a finite number of calls or data traffic that a base station can handle at once. This capacity limitation is commonly the factor that determines the spacing of cell mast sites. In suburban areas, masts are commonly spaced apart and in dense urban areas, masts may be as close as 400–800 m apart.
The maximum range of a mast depends on the same considerations. In any case the limiting factor is 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 may be possible to get between. When the terrain is hilly, the maximum distance can vary from as little as due to encroachment of intermediate objects into the wide center Fresnel zone of the signal. Depending on terrain and other circumstances, a GSM Tower can replace between of cabling for fixed wireless networks. In addition, some technologies, such as GSM, have an additional absolute maximum range of, which is imposed by technical limitations. CDMA and IDEN have no such limit defined by timing.

Practical example of range

  • 3G/4G/5G Mobile base station tower: it is technically possible to cover up to 50–150 km.
  • 5G Mobile base station: the distances between the 5G base-station is about 250–300 m, due to the use of millimetre waves.

    Channel reuse

The concept of "maximum" range is misleading in a cellular network. Cellular networks are designed to support many conversations with a limited number of radio channels that are licensed to an operator of a cellular service. To overcome this limitation, it is necessary to repeat and reuse the same channels at different locations. Just as a car radio changes from one local station to a completely different local station with the same frequency when traveling 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 in many cases tilted downward to limit its reach. This allows covering 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 for each sector depending on the coverage from other towers in the area.

Signal limiting factor

A cell phone may not work at times because it is too far from a mast, or because the phone is in a location where cell phone signals are attenuated by thick building walls, hills, or other structures. The signals do not need a clear line of sight but greater radio interference will degrade or eliminate reception. When many people try to use the cell mast at the same time, e.g. during 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 to send a signal from its low powered battery to the cell site. Some cell phones 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 and the intelligence of the cell phone 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.

Geolocation

Cellular geolocation is less precise than by GNSS, but it is available to devices that do not have GPS receivers and where the GNSS is not available. The precision of this system varies and is highest where advanced forward link methods are possible and is lowest where only a single cell site can be reached, in which case the location is only known to be within the coverage of that site.
An advanced forward link is where a device is within range of at least three cell sites and where the carrier has implemented timing system use.
Another method is using angle of arrival and it occurs when the device is in range of at least two cell sites, produces intermediate precision. Assisted GPS uses both satellite and cell phone signals.
In the United States, for emergency calling service using location data, it was required that at least 95% of cellular phones in use on 31 December 2005 support such service. Many carriers missed this deadline and were fined by the Federal Communications Commission.

Radio power and health

According to the U.S. Federal Communications Commission: "Measurement data obtained from various sources have consistently indicated that 'worst-case' ground-level power densities near typical cellular towers are on the order of 1 μW/cm2 or less."
Cell phones, cell towers, wi-fi, smart meters, digital enhanced cordless telecommunications phones, cordless phones, baby monitors, and other wireless devices all emit non-ionizing radio frequencies, which the World Health Organization has classified as a "potential" carcinogen, According to the U.S. National Cancer Institute, "No mechanism by which ELF-EMFs or radiofrequency radiation could cause cancer has been identified."
According to the U.S. Food and Drug Administration, "Scientific consensus shows that non-ionizing radiation is not a carcinogen and, at or below the radio frequency exposure limits set by the FCC, non-ionizing radiation has not been shown to cause any harm to people."