How do Wireless Telephone Systems Work?
Wireless telephone systems work on a different concept from radio transmission systems most people are familiar with, such as television and radio stations. In most types of radio transmission systems, the object is to transmit your signal as far as possible, in order to maximize the amount of listeners or viewers you may have.
In cellular systems, the object is to transmit a controlled signal. This is done to maximize the amount of channels that are given to each cellular provider for use. Cellular systems are assigned a set number of channels for a given area, usually around 400. In order to maximize the amount of calls/channels per given geographic area, they break the coverage area into a series of cells. Each cell can cover anywhere from a one mile radius from the base station in the city and urban areas to a 10 mile radius in the countryside and rural areas. Usually an arrangement of seven repeating cells is used, with 50 or 60 channels used per cell. As you move a mobile phone between these cells, the mobile phone is 'handed off' between the cell sites and channels, being controlled by the mobile telephone switching office (MTSO), or mobile switching center (MSC). This makes use of the most important part of the cellular system, frequency re-use. There are only a limited amount of channels available in any cellular system, and this system makes the same channel available in different geographic parts of the system, to different users.
Signals are sent back and forth between the MTSO and the cell site over high capacity circuits (DS3) or microwave links. This is known as backhaul, or the process of bringing the signal from the switch to and from the individual cell site.
There are two main frequencies used for wireless telephone communications in the United States. These are 1.9 Ghz (1900 Mhz) and 800 Mhz. All systems using 1.9 Ghz utilize digital technology and are referred to as PCS (Personal Communications Services). These systems do not have as great a coverage area from the cell site, so more cell towers are needed for these systems, vs. conventional 800 mhz systems. This is because higher frequency signals are attenuated (reduced) quicker that lower frequency signals.
Therefore, providers which use these 1.9Ghz frequencies (AT&T Wireless, SprintPCS, T-Mobile, etc...) must place more tower sites to ensure adequate coverage. This is not always as apparent in densely populated areas, but as you get further into the suburbs and less densely populated areas, there will be more sites for these carriers to provide the same signal strength as their lower frequency competitors.
There are four methods for accessing wireless networks in the United States. (FDMA)Frequency Division Multiple Access, which is the oldest system in use, essentially separates each user to a different channel.
Digital systems which encode the signal and place more than one call on a channel include: (TDMA) Time Division Multiple Access, (CDMA) Code Division Multiple Access, and (GSM) Global System for Mobile. GSM is a system based on TDMA technology, however it is a completely uniform (compatible) system based on a set standard, where one vendors equipment will work with another vendors piece of equipment.
Before the more modern digital systems, there was AMPS, Advanced Mobile Phone System. This system worked using FDMA, exclusively in the 800 Mhz band. The analog AMPS had many problems, which led to the development of the modern digital systems. The analog system was plagued with security problems, as well as severe limitations in capacity. This system separated individual users phone conversations by transmitting each user on a separate frequency (FDMA). Engineers realized the limitations in the analog system, and began developing digital systems.
Cellular phone systems evolved to the digital systems to eliminate the multiple problems associated with the analog systems. In most cellular systems, analog frequencies have been converted over to digital, with backward compatibility (meaning that if you still have an old analog phone, it will still work on the cellular system). This allows the system to switch back to an analog signal if that type of phone tries to access the system.
Digital systems convert your voice from an analog signal to a digital signal using a voice encoder (vocoder). Once your voice has been digitized, it can be transmitted in digital format over the airwaves. Depending on the type of digital system used, this can happen in a number of different ways.
Digital systems also utilize power much more efficiently, resulting in much greater battery life for even smaller battery capacities. This is one of the reasons for the tiny handsets in use today.
Wireless System Definitions
Backhaul Method of transmitting signal from Base station to Mobile Switching Center.
Base Station A fixed station in a mobile radio system used for radio communication with mobile stations. Ease stations are located at the center or on the edge of a coverage region and consist of radio channels and transmitter and receiver antennas mounted on a tower.
COW Portable cell tower used in emergecies/large events/ and set up of cellular/PCS systems. Known as a 'Cell on Wheels'.
Control Channel Radio channels used for transmission of call setup, call request, call initiation, and other beacon or control purposes.
Forward Channel Radio channel used for transmission of information from the base station to the mobile.
Handoff The process of transferring a mobile station from one channel or base station to another.
Mobile Station A station in the cellular radio service intended for use while in motion at unspecified locations. Mobile stations may be hand-held personal units (portables) or installed in vehicles (mobiles).
Mobile Switching Center
Switching center which coordinates the routing of calls in a large service area. In a cellular radio system, the MSC connects the cellular base stations and the mobiles to the PSTN. An MSC is also called a mobile telephone switching office (MTSO).
Page A brief message which is broadcast over the entire service area, usually in a simulcast fashion by many base stations at the same time.
Reverse Channel Radio channel used for transmission of information from the mobile to base station.
Roamer A mobile station which operates in a service area (market) other than that from which service has been subscribed.
Subscriber A user who pays subscription charges for using a mobile communications system.
Transceiver A device capable of simultaneously transmitting and receiving radio signals.
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