System Components | Voice Communications

Whether conventional cellular or PCS, the mobile units come in a variety of form factors: those that are permanently mounted in a vehicle, transportable units that can be easily moved from one vehicle to another, or pocket phones weighing in at less than four ounces. Regardless of form factor, mobile phones consist of the same basic elements:

• A handset with keypad
• A logic/control unit
• A transmitter/receiver
• An antenna
• A power source

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 telephone. Until a connection is established, however, the operation of the handset differs greatly from that of a conventional telephone.

Rather than 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. This conserves the resources of the cellular system since only a limited number of talk paths are available. 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 forward throughout the conversation, the handset operates in a customary manner. To end a call, an END function key exists on the keypad. In addition to these functions, the handset typically contains a display that shows dialed digits as well as other features, a CLEAR key that enables the user to correct misdialed digits, functions that enable storage of numbers for future use, and other enhanced features that can vary greatly from one phone to the next.

Logic/control

The logic/control functions of the phone include the numeric assignment module, or NAM, for programmable assignment of the unit's telephone number by the user's carrier of choice, and the electronic serial number of the unit, which is a fixed number unique to each telephone. When signing up for service, the selected 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. This component of the phone also serves to interact with the cellular network protocols that determine what control channel the unit should monitor for paging signals to indicate the network's desire to connect a call coming into the phone, to determine and select the voice channels that the unit should utilize for a specific connection, and to monitor the received control signals of cell sites when the phone is in either standby or an in-use mode so that the phone and network can coordinate transitions to adjacent cells as conditions warrant.

Transmitter/receiver

The transmitter/receiver unit of the telephone is the heart of the radio communications component of the system, under the command of the logic/control unit. Powerful three-watt telephones are typically of the vehicle-mounted 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 handheld units that only transmit at power levels of a fraction of a watt, and they utilize the main battery within a vehicle or a relatively heavy rechargeable battery to do so. A diplexer unit within the phone enables the transmitter and receiver to utilize a single antenna while simultaneously transmitting and receiving.

Antenna

The antenna system, comprising the antenna and connecting cable, determines whether the full power produced by the transmitter is effectively coupled to free space and also whether the minute electromagnetic impulses received from the airwaves can be delivered intact to the receiver circuitry of the telephone. The antenna for a cellular telephone can consist of a flexible rubber antenna mounted on a handheld phone, an extendible antenna on a pocket phone, or the familiar curly stub seen attached to the rear window of many automobiles. The antenna and connecting cable are selected specifically for functionality in the 800-MHz frequency band. 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 telephone completely inoperative.

Power source

Cellular phones are typically 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 extended talk time as compared to lower cost NiCd units. They provide the same voltage as NiCd batteries, but offer at least 30 percent more talk time than NiCd batteries. Unfortunately, NiMH batteries take approximately 20 percent longer to charge than NiCd units.

Newer cellular phones may operate with optional high-energy AA alkaline batteries which provide up to 3 hours of talk time or 30 hours of standby time. These batteries take advantage of the new lithium/iron disulfide technology, which results in 34 percent lighter weight than standard AA 1.5-V batteries (15 vs. 23 grams/battery) and 10-year storage life—double that of standard AA alkaline batteries.

Vehicle mounted and handheld portable cell phones can be optionally powered via the vehicle's 12-V DC battery by using an adapter plugged 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. The adapter 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 unit away from the vehicle. The phone and battery are usually carried in a vinyl pouch.

The latest type of battery uses lithium ion (Li-Ion) to offer longer life and lighter weight than similar sized NiCd and NiMH batteries. Among the many advantages of Li-Ion batteries is that one cell is roughly equivalent to three NiCd or NiMH battery cells in terms of voltage. Li-Ion batteries also provide approximately twice the energy density of NiCd and NiMH batteries by weight. This means that a Li-Ion battery providing similar energy to a conventional NiCd or NiMH battery will weigh one-half as much.