Introduction
A mobile, or cell, phone is a communications device that transmits voice and data signals using wireless means. The term cell phone comes from the method in which the devices work. In order to transmit a signal, a series of base stations are set up, each one covering a predetermined area - called a cell [see figure 1.0]. Typically analogue cells are approximately 6-12 miles in diameter, whereas digital cells are 1-6 miles in diameter.
Each cell phone company is assigned a range of frequencies to use to transmit the information between the cell phone and the base stations. Due to the way in which the cells are formed, each frequency can be used more than once, however, to avoid interference between communications in two cells, no two adjacent cells can use the same frequency, as demonstrated in figure 1.0. Each cell has more than 50 voice channels assigned to it, so more than 50 phones can use the same cell at any one time.
figure 1.0 - Cells over an area, the green coloured cells are using the same frequency.
As soon as the cell phone is turned on, it listens for a System Identification Code (SID) that is broadcast on the control channel. This sets up the frequencies that will be used for voice, data and how to handle a move between cells. During the same time, the cell phone will send a Registration Request which is sent through the base stations until it reaches the Mobile Telephone Switch Office (MTSO). This is the main connection between all the base stations and the landlines. When the registration request has been processed, the MTSO stores the location of the cell phone by recording which cells base station it was communicating with. This way if a call comes through for that phone, the MTSO will know which frequency and cell to send it to.
Switching Cells
It is important to note here, that each cell overlaps its surrounding cells. (this is not apparent in the figure). When on the move, the phone may be required to us a number of cells. Switching to another cells frequency is known as a Handoff. This happens as when the cell phone moves further away from the base station of the cell its currently in the signal becomes weaker, when the signal for an adjacent cells is stronger the MTSO and base stations switch to the stronger frequency and the MTSO records the new location for the phone.
Analogue Transmissions
Analogue cell phone systems use a method known as Frequency Division Multiple Access (FDMA). This method basically uses the MTSO to assign duplex channels to the system. Duplex channels are two separate frequencies used in the same channel, this was two people can talk at the same time, one on each frequency. In contrast, Simplex is the method of using on one frequency to transmit, using simplex, each person has to wait for the other to finish before they can speak. CB radios are an example of simplex devices. Each analogue system generally uses 95 Duplex channels. The voice pattern is attached to a carrier signal, when the signal reaches its destination the carrier is stripped away and the remaining voice pattern is amplified and played in the earpiece.
Digital Transmissions
Whereas the analogue system uses FDMA for transmission. Digital uses one of two methods. Either, Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA).
Time Division Multiple Access (TDMA)
TDMA divides the carrier signals into lots of small chunks of time. Each conversation takes turns in using the time pieces on the same frequency. The chunks of time switch so quickly that it appears to the user that they are using one unbroken signal.
Code Division Multiple Access (CDMA)
Also known as Spread Spectrum, CDMA assigns a code to each chunk of data that is being transmitted as part of the conversation. These chunks, or “packets” are spread among all the available frequencies and transmitted to every device in use. The receiving device uses the code in the packet to determine whether it is part of the same conversation, if so, it reassembles the packets to provide separate, unbroken signals.