Cellular Access Technologies

Cellular Access Technologies

Introduction:

There are three common technologies used by cell-phone networks for transmitting information:
• Frequency division multiple access (FDMA)
• Time division multiple access (TDMA)
• Code division multiple access (CDMA)
Although these technologies sound very intimidating, you can get a good sense of how they work just by breaking down the title of each one.
The first word tells you what the access method is. The second word, division, lets you know that it splits calls based on that access method.
• FDMA puts each call on a separate frequency.
• TDMA assigns each call a certain portion of time on a designated frequency.
• CDMA gives a unique code to each call and spreads it over the available frequencies.
The last part of each name is multiple access. This simply means that more than one user can utilize each cell.

Code-Division Multiple Access, a digital cellular technology that uses spread-spectrum techniques. Unlike competing systems, such as GSM, that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence.
GSM stands for Global System for Mobile communications. The GSM is built around the TDMA, or the Time Division Multiple Access concept.
Basically, the two technologies address differently the same fundamental problem of mobile communication, how to divide the finite frequency of airwaves between multiple users at the same time, or, how to make more than one person to carry on a
conversation on the same frequency without causing mutual interference.

CDMA takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it out over the entire available bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. CDMA is a form of spread spectrum, which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range.


for more info visit.
http://www.enjineer.com/forum