A cell in a cellular system is a roughly circular area with a central transmitter/receiver base station as shown in Figure W-6 (although the base station may be located off-center to conform to local topology). The station is raised up on a tower or placed on top of a building. Some are located on church steeples. The station has a 360-degree omnidirectional antenna (except when directional transmissions are required) that is tuned to create a cellular area of a specific size. Cells are usually pictured as hexagonal in shape and arranged in a honeycomb pattern. Cell size varies depending on the area. In a city, there are many small cells, while rural area may have very large cells.

Cellular topology provides a way to maintain an adequate number of call channels even though the actual number of channels available to the entire service area is small. This is possible through frequency reuse. Each cell is assigned a set of channel frequencies, and no adjoining cells may use those frequencies. However, cells further away may use those frequencies because the distance between cells provides a buffer zone that prevents frequency interference.

The system is scalable, even though it has a finite number of channels. If channel demand increases in a specific area (such as a metro area), the service provider can divide cells into a number of smaller cells. Transmitter power is turned down to fit the new smaller cell size and channel frequencies are allocated so that no adjoining cells use the same channels. However, channel reuse is possible in cells that are at least one cell apart. Thus, frequency reuse and smaller cell size allow the system to scale. Metro areas may have many small cells while rural area may have large cells. The cell size is designed to accommodate the number of people in the area.

When a user turns a phone on, its phone number and serial number are broadcast within the local cell. The base station picks up these signals and informs the switching office that the particular device is located within its area. This information is recorded by the switching office for future reference. An actual call takes place when the user enters a phone number and hits the Send button. The cellular system selects a channel for the user to use during the duration of the call.

As users travel, they may move from one cell to another, necessitating a handoff and the selection of a new channel. While in the vicinity of a cell, mobile phone users are under the control of the transmitter/receiver in that cell. A handoff takes place when the base station in one cell transfers control for a user's call to a base station in another cell. When a base station begins to lose a user's signal, it notifies base stations in all the surrounding cells that the user may be moving into their cells. As the user moves into a new cell, the base station in that cell takes over the call. The frequency of the call is changed to a frequency used in the new cell during the transition. This is because adjoining cells cannot use the same frequencies.

Mobile wireless analog communication systems have been around since the 1950s. The early systems were single channel "over-and-out" systems. Instead of a cellular configuration, a single radio tower serviced a metropolitan area, which severely limited the scalability of the systems. Service quality varied depending on the location of the caller. Later systems added multiple two-way channels but still had limited capacity.

Analog cellular services were introduced by AT&T in the 1970s and became widespread in the 1980s. The primary analog service in the United States is called AMPS (Advanced Mobile Phone Service). There are similar systems around the world that go by different names. The equivalent system in England is called TACS (Total Access Communications System).

The AMPS system is a circuit-oriented communication system that operates in the 824-MHz to 894-MHz frequency range. This range is divided into a pool of 832 full-duplex channel pairs (1 send, 1 receive). Any one of these channels may be assigned to a user. A channel is like physical circuit, except that it occupies a specific radiofrequency range and has a bandwidth of 30 kHz. The circuit remains dedicated to a subscriber call until it is disconnected, even if voice or data is not being transmitted.

Cellular systems are described in multiple generations, with third- and fourth-generation (3G and 4G) systems just emerging:

• 1G systems    These are the analog systems such as AMPS that grew rapidly in the 1980s and are still available today. Many metropolitan areas have a mix of 1G and 2G systems, as well as emerging 3G systems. The systems use frequency division multiplexing to divide the bandwidth into specific frequencies that are assigned to individual calls.


• 2G systems    These second-generation systems are digital, and use either TDMA (Time Division Multiple Access) or CDMA (Code Division Multiple Access) access methods. The European GSM (Global System for Mobile communications) is a 2G digital system with its own TDMA access methods. The 2G digital services began appearing in the late 1980s, providing expanded capacity and unique services such as caller ID, call forwarding, and short messaging. A critical feature was seamless roaming, which lets subscribers move across provider boundaries.


• 3G systems    3G has become an umbrella term to describe cellular data communications with a target data rate of 2 Mbits/sec. The ITU originally attempted to define 3G in its IMT-2000 (International Mobile Communications-2000) specification, which specified global wireless frequency ranges, data rates, and availability dates. However, a global standard was difficult to implement due to different frequency allocations around the world and conflicting input. So, three operating modes were specified. According to Nokia, a 3G device will be a personal, mobile, multimedia communications device that supports speech, color pictures, and video, and various kinds of information content. Nokia's Web site (http://www.Nokia.com) provides interesting information about 3G systems. There is some doubt that 3G systems will ever be able to deliver the bandwidth to support these features because bandwidth is shared. However, 3G systems will certainly support more phone calls per cell.


• 4G Systems    On the horizon are 4G systems that may become available even before 3G matures (3G is a confusing mix of standards). While 3G is important in boosting the number of wireless calls, 4G will offer true high-speed data services. 4G data rates will be in the 2-Mbit/sec to 156-Mbit/sec range, and possibly higher. 4G will also fully support IP. High data rates are due to advances in signal processors, new modulation techniques, and smart antennas that can focus signals directly at users. OFDM (orthogonal frequency division multiplexing) is one scheme that can provide very high wireless data rates. OFDM is described under its own heading.

The move to digital technologies opened up the wireless world. It improved capacity, reduced equipment costs, and allowed for the addition of new features. Reduced handset costs meant more people were vying for services and taxing systems. 3G systems add more capacity. In addition, packet technologies were developed that use bandwidth more efficiently. The primary 1G and 2G digital systems are listed here.

• Analog cellular    These are the traditional analog systems such as AMPS and TACS that use frequency division multiplexing. AMPS operates in the 800-MHz range, while TACS operates in the 900-MHz frequency range.


• Hybrid analog/digital cellular  (usually called digital cellular)     These systems are analog AMPS systems in which digitized voice and digital data is modulated onto the analog sine wave of the channel being used. They operate in the same 800-MHz range as analog AMPS and even use the same topology and equipment configuration (cells, towers, etc.). The access method may be either TDMA or CDMA, as discussed in the next section.


• GSM (Global System for Mobile Communications)    This is a second-generation mobile system designed from the ground up without trying to be backward compatible with older analog systems. GSM is popular in Europe and Asia, where it provides superior roaming ability among countries. It uses TDMA, but Europe is moving from this system into 3G systems based on a wideband form of CDMA.

When digital cellular services were being designed in the early 1980s, the choice was to design a system that was backward compatible with existing analog systems (and used the same frequency allocation) or to design a whole new system. The European community had about seven incompatible analog services, so it created the GSM system from scratch to operate in the 900-MHz range (and later in the 1,800-MHz range).

In the U.S., the digital cellular systems were developed using the AMPS frequency allocation and the TDMA and CDMA access methods. See "CDMA (Code Division Multiple Access)" and "TDMA (Time Division Multiple Access)." In addition, the FCC allocated new bandwidth in the 1,900-MHz frequency range to accommodate what was called PCS (Personal Communication Services). PCS refers to the 1,900-MHz frequency allocation and to mobile systems that provide services beyond voice (such as digital services that support caller ID, messaging, and other features).

Keeping track of the analog and digital cellular standards can be difficult. Table W-2 lists the most common standards.

Table W-2: Wireless mobile standards