Four key components make up most cellular radio systems: the cellular layout itself, a carefully engineered network of radio base stations and antennas, base station controllers which manage several base stations at a time, and a mobile switch, which gathers traffic from dozens of cells and passes it on to the public switched telephone network.
All analog and digital mobiles use a network of base stations and antennas to cover a large area. The area a base station covers is called a cell, the spot where the base station and antennas are located is called a cell site. Viewed on a diagram, the small territory covered by each base station appears like a cell in a honeycomb, hence the name cellular. Cell sizes range from sixth tenths of a mile to thirty miles in radius for cellular (1km to 50km). GSM and PCS use much smaller cells, no more than 6 miles (10km) across. A large carrier may use hundreds of cells.
Each cell site’s radio base station uses a computerized 800 or 1900 megahertz transceiver with an antenna to provide coverage. Each base station uses carefully chosen frequencies to reduce interference with neighboring cells. Narrowly directed sites cover tunnels, subways and specific roadways. The area served depends on topography, population, and traffic. In some PCS and GSM systems, a base station hierarchy exists, with pico cells covering building interiors, microcells covering selected outdoor areas, and macrocells providing more extensive coverage to wider areas. See the Ericsson diagram below.
The macro cell controls the cells overlaid beneath it. A macro cell often built first to provide coverage and smaller cells built to provide capacity.
Macario describes a business park or college campus as a typical situation. In those cases a macrocell provides overall coverage, especially to fast moving mobiles like those in cars. A microcell might provide coverage to slow moving people between large buildings and a piconet might cover an individual lobby or the floor of a convention center.
Steve Punter, of the excellent Steve’s Toronto Area Cellular/PCS Site Guide, says that typically microcells are employed along the sides of busy highways or on street corners. Steve sent in pictures of two typical microcells in the Toronto area:
Base station equipment by itself is nothing without a means to manage it. In GSM and PCS 1900 that’s done by a base station controller or BSC. As Nokia puts it, a base station controller “is a high-capacity switch which provides total overview and control of radio functions, such as handover, management of radio network resources and handling of cell configuration data. It also controls radio frequency power levels in the RBSs, and in the mobile phones. Base station controllers also set transceiver configurations and frequencies for each cell.” Depending on the complexity and capacity of a carrier’s system, there may be several base station controllers.
These BSCs react and coordinate with a mobile telecommunication switching office or MTSO, sometimes called, too, a MSC or mobile switching center. With AMPS or D-AMPs, however, the mobile switch controls the entire network. In either case, the mobile switch interacts with distant databases and the public switched telephone network or PSTN. It checks that a customer has a valid account before letting a call go through, delivers subscriber services like Caller ID, and pages the mobile when a call comes in. Among many other administrative duties. Learn more about cellular switches by checking out this small graphic. Also, if you want to see pictures of a “mobile” mobile switching center, (a Motorola EMX 100 Plus Cellular Switch) go to Michael Hart’s excellent site (external link)[Link not working right now]
How does this work out in the real world? Consider Omnipoint’s PCS network for the greater New York city area. To cover the 63,000-square-mile service area, Ericsson says Omnipoint installed over 500 cell sites, with their attendant base stations and antennas, three mobile switching centers, one home location register, and one service control point. (The latter two are network resources.) The New York Times says the entire system cost $680 million dollars, although they didn’t say if that included Omnipoint’s discounted operating license. Now that we’ve seen what makes up a cellular network, let’s discuss the idea that makes that makes those networks possible: frequency reuse.
Dual band IS-136 Ericsson RBS 884 base station
B. Frequency reuse
The heart and soul, the inner core, the sine qua non of cellular radio is frequency reuse. The same frequency sets are used and reused systematically throughout a carrier’s coverage area. If you have frequency reuse you have cellular. If you don’t, well, you don’t have cellular. Frequency reuse distinguishes cellular from conventional mobile telephone service, where only a few frequencies are used over a large area, with many customer’s competing to use the same channels. Much like a taxi dispatch operation, older style radio telephone service depended on a high powered, centrally located transmitter which paged or called mobiles on just a few frequencies.
Cellular instead relies on a distributed network of cells, each cell site with its own antenna and radio equipment, using low power to communicate with the mobile. In each cell the same frequency sets are used as in other cells. But the cells with those same frequencies are spaced many miles apart to reduce interference. Thus, in a 21 cell system a single frequency may be used several times. The lone, important exception to this are CDMA systems which we will cover later. In those, the same frequencies are used by every cell.
Each base station, in addition, controls a mobile’s power output, keeping it low enough to complete a circuit while not high enough to skip over to another cell.
- The frequency reuse concept. Each honeycomb represents a cell. Each number represents a different set of channels or paired frequencies. A cellular system separates each cell that shares the same channel set. This minimizes interference while letting the same frequencies be used in another part of the system. This is frequency reuse. Note, though, that CDMA based systems can use, in theory, all frequencies in all cells, substantially increasing capacity . For review, a channel is a pair of frequencies, one for transmitting on and one for receiving. Frequencies are described by their place in the radio spectrum, such as 900mHZ, whereas channels are described by numbers, such as channels 334 through 666. Illustration from the CDC
- (back to Cellular basics article)
C. Adding cells and cell 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 omni directional 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.
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).