Adding cells and sectorizing
Adding cells and sectoring cells allows cellular expansion. Don't have enough circuits in a crowded cell? Too many customers? Then add to that cell by providing smaller cells like micro and pico cells, underneath and controlled by the existing and larger macro cell. As Steve Punter puts it, "By placing these short-range microcells along busy highways or at busy street corners, you effectively reduce the strain on the primary macrosites by a substantial margin.
Splitting a single cell does not mean that it is broken into smaller cells, like a dividing amoebae, but rather into sectors. A previously omnidirectional base station antenna, radiating equally in all directions, is replaced by several directional antennas on the same tower. This "sectorizing" thus divides the previously homogeneous cell into 3 or 6 distinct areas (120 and 60 degrees around the site respectively). Each sector gets its own frequencies to operate on.
As Fernando Lepe-Casillas neatly puts it, "We sector cells to reduce interference between similar cells in adjacent clusters. Cell splitting is done to increase traffic capacity." Still confused by all of this? I understand. I give another, I think somewhat clearer, explanation at this link.
According to Telephony Magazine, AT&T began splitting their macrocell based New York City network in 1994. (They use IS-136 at both 800 and 1900 MHz.) Starting in Midtown Manhattan, the $30 million-plus project added 55 microcells to the three square mile area by 1997, with 10 more on the way. Lower Manhattan got a "few dozen." Microcells in lower Manhattan sought to increase signal quality, while Midtown improvements tried to increase system capacity. An AT&T engineer said "a macrocell costs $500,000 to $1 million to build, a microcell one-third as much and you don't have to build a room around it." AT&T used Ericsson base stations, with plans to use Ericsson 884 base stations as pictured above in the future. Camouflaged antennas got placed on buildings between 25 and 50 feet above street level.
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Resources:
Keiser, Bernhard, and Eugene Strange. Digital Telephony and Network Integration. 2d ed. New York, 1995
Landler, Mark." Yipes! Invasion of the 9-inch antennas! A new form of
wireless phone service is in the works for New York City. (Omnipoint Communications to offer wireless personal communications services)" (Company Business and Marketing) New York Times v145 (August 19, 1996):C1(N), D1(L).
Luxner, Larry. "The Manhattan Project: AT&T Wireless invades the Big Apple with microcells" Telephony, Feb 24, 1997, 232(8):20. 1997