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Wireless Mobile Communications
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
There are a variety of wireless communication systems for transmitting voice, video, and data in local or wide areas. There are point-to-point wireless bridges, wireless local area networks, multidirectional wireless cellular systems, and satellite communication systems.
This topic discusses "mobile" wireless technologies that provide voice and data communication services to mobile users who use cell phones, PDAs, Internet terminals, and related computing devices. Refer to "Wireless Communications" for a list of related wireless topics.
The number of wireless mobile devices is increasing globally. Users equipped with portable computers, PDAs (personal digital assistants), and a variety of small wireless communication devices increasingly need to connect to corporate networks, perform database queries, exchange messages, transfer files, and even participate in collaborative computing. At the same time, wireless systems are achieving higher data rates to support Internet and other data-related applications. The newest mobile communication systems are targeting data rates as high as 2 Mbits/sec.
Cellular Systems and Topology
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.
From Analog to Digital Systems
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:
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.
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
Wireless Data Networking
While early cellular systems were focused on voice, there is now a lot of interest in supporting data transmissions. The older analog and analog/digital hybrid networks were limited in their data rates, but new standards are emerging with a focus on high data rates.
In a circuit-switched wireless network, a dedicated radio channel is allocated to a single transmission. As long as data transmissions are long and continuous (file transfers), a circuit is used efficiently. However, most data transmissions are bursty, and dedicating an entire circuit to them is usually a waste of valuable wireless bandwidth. During idle periods when no data is being sent, bandwidth is still dedicated to the user and not available for others to use.
Packet-switching schemes are best for bursty data traffic. Several packet-switching schemes may be used. In one technique, packets from many users are multiplexed over a single channel. In another technique, packets are inserted into the idle space on any available channel. The busier the network, the less bandwidth that will be available for data. An entire network may be designed just for packet data. Most wireless data systems offer minimal data rates, usually in the 10-Kbit/sec range. That's really only useful for short messaging and occasional Web page lookups. However, new wireless protocols bond multiple channels to increase data rates.
The billing methods help differentiate circuit versus packet switching methods. When you connect over a circuit-switched line, the phone company bills you for the entire duration of the call. With packet-switching systems, you are typically billed by the packet.
Several packet data schemes are outlined here:
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.