Editing Cell site (section)

== Overview ==
A [[cellular network]] is a network of handheld [[mobile phone]]s (cell phones) in which each phone communicates with the [[telephone network]] by [[radio wave]]s through a local antenna at a cellular base station (cell site). 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 [[bandwidth (signal processing)|radio bandwidth]] by [[frequency reuse]]; the low power radio signals used within each cell do not travel far beyond the cell, so the [[radio channel]]s 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 [[ultrahigh frequency|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 antenna]]s. 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 ''(vertical white rectangles in pictures)'', called a [[sector antenna]], usually consists of a vertical collinear array of [[Dipole antenna|dipole]]s. 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.