Cellular Networks

The mobile telephone service that preceded cellular service was known as Improved Mobile Telephone Service (IMTS), which operated in several frequency ranges: 35 to 44 MHz, 152 to 158 MHz, and 454 to 512 MHz. But IMTS suffered from call setup delay, poor transmission, limited frequency reuse, and lack of service areas. IMTS was supplanted by Advanced Mobile Phone Service (AMPS) that operates in the 800- to 900-MHz range. AMPS overcame the limitations of IMTS and set the stage for the explosive growth of cellular service which continues today.

Proposed by AT&T in 1971, AMPS is still the standard for analog cellular networks. It was trialed in 1978, and in the early 1980s cellular systems based on the standard were being installed throughout North America. Although AMPS was not the first system for wireless telephony, the existence of a single standard enabled the United States to dominate analog cellular. Europe suffered from a multiplicity of competing standards such as Nordic Mobile Telephone (NMT) at 450 MHz, NMT and Total Access Communications System (TACS) at 900 MHz, and an assortment of other standards in individual countries.

Analog cellular systems have been a huge success. In just 15 years they have attracted around 50 million subscribers in 60 countries worldwide. Today over two-thirds of these subscribers are on the North American AMPS standard at 800 MHz. Although the AMPS standard was originally defined for networks in North America, it is now widely implemented throughout Europe, Latin America, Australia, and New Zealand, as well as many Asian countries, including China, Hong Kong, Malaysia, and Taiwan.

Over the years, AMPS has amply proven itself in terms of being easy to implement and expand to keep pace with increasing demand for mobile phones. It supports automatic roaming so that mobile phone users can continue to use their phones as they move into an area served by a different network. Analog cellular is delivered over networks which employ large cellular hubs and base stations. Despite its success, this method of transmission has its limitations. Analog signals can be intercepted easily and suffer signal degradation from numerous sources, such as terrain, weather, and traffic volume.

A digital version of AMPS—referred to as D-AMPS—solves many of these problems, while providing increased capacity and a greater range of services. Both AMPS and D-AMPS operate in the 800-MHz band and can coexist with each other. D-AMPS can be implemented with time division multiple access (TDMA) as the underlying technology. TDMA provides 10 to 15 times more channel capacity than AMPS networks and allows the introduction of new feature-rich services such as data communications, voice mail, call waiting, call diversion, voice encryption, and calling line identification. A digital control channel supports such advanced features as a sleep mode, which increases battery life on newer cellular phones by as much as ten times over the current battery capabilities of analog phones. D-AMPS can also be implemented with code division multiple access (CDMA) technology to increase channel capacity by as much as 20 times and provide a comparable range of services and features. Unlike TDMA, which can be overlayed onto existing AMPS networks, CDMA requires an entirely new network infrastructure.

D-AMPS also allows operators to build overlay networks using small micro- and picocells, boosting network capacity still further in high-traffic areas and providing residential and business in-building coverage. Advanced software in the networks' exchanges continuously monitors call quality and makes adjustments, such as handing calls over to different cells or radio channels, when necessary. The network management system provides an early warning to the network operator if the quality of service is deteriorating so that steps can be taken to head off serious problems. Graphical displays of network configuration and performance statistics help ensure maximum service quality for subscribers.

Cellular systems, through their interconnection with the public switched telephone network, allow users to originate or receive communications with more portability, and nearly the same degree of functionality as wired telephones. This is accomplished through a hybrid system that utilizes radio technology for the link between the mobile user and the mobile telephone switching office (MTSO), traditional telephone switching technology for the interconnection between the MTSO and those using the wireline public switched telephone network (PSTN) with whom the user communicates, and computer technology to continually monitor the location of mobile users.

In the early 1980s, cellular network service providers became licensed by the Federal Communications Commission (FCC) to operate based on limited competition in each service area. One provider is usually the local telephone company (also known as the "wireline" provider because of its traditional operation of the wired telephone network), and the other licensee is a competitor to the local telephone company, also known as the "nonwireline" carrier. Because of this limited competition, carriers could feel confident that their investment in developing a network would be rewarded with a significant enough portion of the subscriber base to support continued operations. Without this arrangement, it would have been unlikely that the current network would have evolved in such a rapid manner.

Each carrier, wireline and nonwireline, has been assigned separate radio frequencies under which their license permit them to operate. This allows the competitors to coexist within the same physical operating area without interfering with each other's systems. Cellular telephones are manufactured with the inherent capability to operate on either carrier's network, since they have the capability to transmit and receive on either group of frequencies or channels.

The primary wireless communications link established with the cellular telephone is to the nearest cell site. The cellular carrier's network consists of a number of cell sites, each typically covering a radius of approximately one to ten miles, which are in turn connected to an MTSO either via cable or microwave radio links (Figure 1). The system is engineered so that the cell sites are located in close enough proximity to one another to provide seamless networking capability.

Figure 1: A typical cellular system.

The coverage areas for adjacent cells actually overlap in order to allow continuous coverage for a user in motion across the network as well as to allow for some load balancing of network traffic. Three hundred and twelve radio channels are available for use by each carrier for voice communications between telephones and the cell site, and the channels used by one cell can be reused by other nonadjacent cells since the transmitted power levels are relatively low.

The radio frequencies used for cellular communication between the mobile user and the cell site are in the range of 825 to 890 megahertz (MHz). Separate channels are utilized for transmitting and receiving voice communications, and the telephone equipment allows transmit and receive channels to be utilized simultaneously so that the parties communicating with each other experience a full-duplex conversation not unlike that of a conventional wireline telephone.

Additional radio communications between the telephone and the cell site takes place over control channels that exchange data between the telephone and the cellular network as to the active phones operating within a particular service area. These control channels also provide functions critical to the establishment of calls and the management of the voice communications channels. From the moment the telephone is turned on, even when idle, communication periodically takes place between the telephone and the nearest cell site. The phone and the cellular network repeatedly exchange information via control channel protocols as to the location and status of the phone and the relative strength of the radio signal between them. This allows the network to find the optimal cell site through which it should route incoming calls to the cellular telephone, to determine when the network should "hand off" an established connection from one cell site to another in order to maintain a strong radio connection, and to allow the phone and the network to synchronize their dynamic use of the many available communications frequencies.

A mobile unit operating outside its local service area is considered to be "roaming." The user's account is established with a local provider, but other providers will allow visitors to their network to use the service. Billing is through the home service provider. A service provider's coverage area might be statewide or might represent a particular area code. Billing to the user represents all on-air use or airtime, whether for outgoing or incoming calls, plus any long-distance charges. Most carriers offer an arrangement such that basic airtime charges, on a per-minute basis, are the only usage cost for an extended calling area. Calls to locations that might incur toll charges within the carrier's service area if made by conventional phone might not incur those charges for a cellular call, but they would be billed based on a flat airtime basis, usually in the area of $0.20 to $0.45 per minute. Calls made while roaming outside this area would come at a higher per-minute rate and/or with additional per-call surcharges. At this writing, only Nextel provides business customers with wireless communication nationwide in the United States without roaming charges.

Since a cellular telephone is so dependent upon a radio link to establish and maintain communications, most of the factors that affect their operation are related to aspects of radio technology. Some of these factors are outside the control of the end user and are specific to the engineering of the carrier's network. The location of cell sites, proximity of adjacent cells, transmitter power, receiver sensitivity, and antenna location can all have a significant impact on the quality of communications. In many locations, service quality between providers is virtually indistinguishable. It is quite likely that each service provider will have areas in which strengths and weaknesses exist, especially pertaining to signal coverage in any specific location.

Service providers are not always able to place their cell sites and antennas in the locations that their engineers might find to be ideal, but they do continually test and tune their network to attempt to provide the best level of service possible. An additional factor somewhat beyond the user's control is that of network traffic loading. Service can suffer even on the best of networks merely due to the congestion that results when too many users attempt to access the network at once. Newer cellular network technologies enable a greater number of channels to be derived from existing frequencies (i.e., frequency reuse) and permit the creation of smaller cell coverage areas or microcells to increase overall network capacity.