Bell Labs built the first cellular telephone in 1924. After decades of development, cellular telephones have emerged as a “must have” item among mobile professionals and consumers alike, growing in popularity every year since they became commercially available in 1983. Their widespread use for both voice and data communications has resulted from significant progress made in their functionality, portability, the availability of network services, and the declining cost for equipment and services.
There are several categories of cellular telephone. Mobile units are mounted in a vehicle. Transportable units can be easily moved from one vehicle to another. Pocket phones, weighing in at less than 4 ounces, can be conveniently carried in a jacket pocket or purse. There are even cellular telephones that can be worn. Regardless of how they are packaged, cellular telephones consist of the same basic elements.
Handset/Keypad. The handset and keypad provide the interface between the user and the system. This is the only component of the system with which, under normal operation, the user needs to be concerned. Any basic or enhanced system features are accessible via the keypad, and once a connection is established, this component provides similar handset functionality to that of any conventional telephone. Until a connection is established, however, the operation of the handset differs greatly from that of a conventional telephone.
Instead of initiating a call by first obtaining a dial tone from the network switching system, the user enters the dialed number into the unit and presses the “Send” function key. This process conserves the resources of the cellular system, since only a limited number of talk paths are available at any given time. The “Clear” key enables the user to correct misdialed digits.
Once the network has processed the call request, the user will hear conventional call-progress signals such as a busy signal or ringing. From this point on, the handset operates in the customary manner. To disconnect a call, the “End” function key is pressed on the keypad. The handset contains a small illuminated display that shows dialed digits and provides a navigational aid to other features. The keypad enables storage of numbers for future use and provides access to other enhanced features, which may vary according to manufacturer.
Logic/Control. The logic/control functions of the phone include the numeric assignment module (NAM) for programmable assignment of the unit’s telephone number by the service provider and the electronic serial number of the unit, which is a fixed number unique to each telephone. When a customer signs up for service, the carrier makes a record of both numbers. When the unit is in service, the cellular network interrogates the phone for both of these numbers in order to validate that the calling/called cellular telephone is that of an authentic subscriber.
The logic/control component of the phone also serves to interact with the cellular network protocols. Among other things, these protocols determine what control channel the unit should monitor for paging signals and what voice channels the unit should use for a specific connection. The logic/control component is also used to monitor the control signals of cell sites so that the phone and network can coordinate transitions to adjacent cells as conditions warrant.
Transmitter/Receiver. The transmitter/receiver component of the cell phone is under the command of the logic/control unit. Powerful 3-watt telephones are typically of the vehiclemounted or transportable type, and their transmitters are understandably larger and heavier than those contained within lighter-weight handheld cellular units.
These more powerful transmitters require significantly more input wattage than hand-held units that transmit at power levels of only a fraction of a watt, and they use the main battery within a vehicle or a relatively heavy rechargeable battery to do so. Special circuitry within the phone enables the transmitter and receiver to use a single antenna for full-duplex communication.
Antenna. The antenna for a cellular telephone can consist of a flexible rubber antenna mounted on a hand-held phone, an extendible antenna on a pocket phone, or the familiar curly stub seen attached to the rear window of many automobiles. Antennas and the cables used to connect them to radio transmitters must have electrical performance characteristics that are matched to the transmitting circuitry, frequency, and power levels. Use of antennas and cables that are not optimized for use by these phones can result in poor performance. Improper cable, damaged cable, or faulty connections can render the cell phone inoperative.
Power Sources. Cell phones are powered by a rechargeable battery. Nickel-cadmium (NiCd) batteries are the oldest and cheapest power source available for cellular phones. Newer nickel–metal hydride (NiMH) batteries provide extend talk time compared to lower-cost conventional NiCd units. They provide the same voltage as NiCd batteries but offer at least 30 percent more talk time than NiCd batteries and take approximately 20 percent longer to charge.
Lithium ion batteries offer increased power capacity and are lighter in weight than similar-size NiCd and NiMH batteries. These batteries are optimized for the particular model of cellular phone, which helps ensure maximum charging capability and long life.
Newer cellular phones may operate with optional highenergy AA alkaline batteries that can provide up to 3 hours of talk time or 30 hours of standby time. These batteries take advantage of lithium–iron disulfide technology, which results in 34 percent lighter weight than standard AA 1.5- volt batteries (15 versus 23 grams per battery) and 10-year storage life—double that of standard AA alkaline batteries.
Vehicle-mounted cell phones can be optionally powered via the vehicle’s 12-volt dc battery by using a battery eliminator that plugs into the dashboard’s cigarette lighter. This saves useful battery life by drawing power from the vehicle’s battery and comes in handy when the phone’s battery has run down. Abattery eliminator will not recharge the phone’s battery, however. Recharging the battery can only be done with a special charger.
Lead-acid batteries are used to power transportable cellular phones when the user wishes to operate the phone away from a vehicle. The phone and battery are usually carried in a vinyl pouch.
Features and Options
Cellular telephones offer many features and options, including:
- Voice activation - Sometimes called “hands-free operation,” this feature allows the user to establish and answer calls by issuing verbal commands. This safety feature enables a driver to control the unit without becoming visually distracted.
- Memory functions - These allow storage of frequently called numbers to simplify dialing. Units may offer as few as 10 memory locations or in excess of 100, depending on model and manufacturer.
- Multimode - This allows the phone to be used with multiple carriers. The phone can be used to access digital service where it is available and then switch to an analog service of another wireless carrier when roaming.
- Multiband - This allows the phone to be used with multiple networks using different frequency bands. For example, the cell phone can be used to access the 1900-MHz band when it is available and then switch to the 800- MHz band when roaming.
- Visual status display - This conveys information on numbers dialed, state of battery charge, call duration, roaming indication, and signal strength. Cell phones differ widely in the number of characters and lines of alphanumeric information they can display. The use of icons enhances ease of use by visually identifying the phone’s features.
- Programmable ring tones - Some cellular phones allow the user to select the phone’s ring tone. Multiple ring tones can be selected, each assigned to a different caller. Avariety of ring tones may be downloaded from the Web.
- Silent call alert - Features include visual or vibrating notification in lieu of an audible ring tone. This can be particularly useful in locations where the sound of a ringing phone would constitute an annoyance.
- Security features - These include password access via the keypad to prevent unauthorized use of the cell phone as well as features to help prevent access to the phone’s telephone number in the event of theft.
- Voice messaging - This allows the phone to act as an answering machine. Alimited amount of recording time (about 4 minutes) is available on some cell phones. However, carriers also offer voice-messaging services that are not dependent on the phone’s memory capacity. While the phone is in standby mode, callers can leave messages on the integral answering device. While the phone is off, callers can leave messages on the carrier’s voice-mail system. Users are not billed for airtime charges when retrieving their messages.
- Call restriction - This enables the user to allow use of the phone by others to call selected numbers, local numbers, or emergency numbers without permitting them to dial the world at large and rack up airtime charges.
- Call timers - These provide the user with information as to the length of the current call and a running total of airtime for all calls. These features make it easier for users to keep track of call charges.
- User-defined ring tones - These offer users the option to compose or download ring tones of their choice to replace the standard ring tone that comes with the cell phone.
- Data transfer kit - For cell phones that are equipped with a serial interface, there is software for the desktop PC that allows users to enter directory information via keyboard rather than the cell phone keypad. The information is transferred via the kit’s serial cable. Through the software and cable connection, information can be synchronized between the PC and cell phone, ensuring that both devices have the most recent copy of the same information.
Mobile phone companies are under orders from the FCC to incorporate location-reporting technology into cellular phones. Dubbed E-911, or enhanced 911, the initiative is meant to provide law enforcement and emergency services personnel with a way to find people calling 911 from mobile phones when callers do not know where they are or are unable to say.
Since no carrier was able to make an October 2001 deadline to fully implement E-911, the FCC issued waivers permitting carriers to add location-detection services to new phones over time so that 95 percent of all mobile phones will be compliant with E-911 rules by 2005. One way manufacturers can address this requirement is by providing cell phones with a Global Positioning System (GPS) capability in which cell phone towers help GPS satellites fix a cell phone caller’s position.
Special software installed in the base station hardware serves location information to cell phones, which is picked up at the public safety answering point (PSAP). However, subscribers would need to purchase a new GPS-equipped handset, since this method would not allow legacy handsets to use the location-determination system.
Another location-determination technique is called “Time Difference of Arrival” (TDOA), which works by measuring the exact time of arrival of a handset radio signal at three or more separate cell sites. Because radio waves travel at a fixed known rate (the speed of light), by calculating the difference in arrival time at pairs of cell sites, it is possible to calculate hyperbolas on which the transmitting device is located.
The TDOAtechnique makes use of existing receive antennas at the cell sites. This location technique works with any handset, including legacy units, and only requires modifications to the network.