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Cellular (CDMA and TDMA)
Description: 

Code Division Multiple Access (CDMA) is a dynamic channel access method which is based on IS-95 developed by the ANSI-accredited Telecommunications Industry Association (TIA). In a CDMA system, spread-spectrum technology and a special coding scheme are employed to enable several users to be multiplexed over the same physical channel. The receiver knows the correct spreading code and is able to easily detect the desired signal. The signal from each user will have very low power and be seen by others as background noise. Therefore, as long as the total power of noise (i.e., multi-user interference) is less than a threshold, it is possible to detect the desired signal using the spreading code used to encode the signal at the transmitter. New users can be added simultaneously to the system at any time provided the total power does not cause multi-user interference. CDMA provides data rate of 64 kbps. One of the advantages of CDMA over TDMA (see below) is that it can use the spectrum more efficiently.

Time Division Multiple Access (TDMA) is a channel access method for shared medium networks that allows multiple users to share the same frequency channel. This can be accomplished by dividing the signal into different time slots, each of which is assigned to a single user to transmit data information. A user can send large data file within time slots of periodical frames. Data from a single user always sits in the same time slot position of a frame, so at the receiver all the information from that portion can be collected and aggregated to shape the original transmitted packet. TDMA is used in the digital 2G cellular systems, satellite systems and combat-net radio systems. In TDMA technique, the time slots are fixed and allotted to a single user for the whole period of communications. This guarantees service quality but is inefficient in utilizing the scarce spectrum especially with exponential increase in number of users. TDMA provides data rate of 64-120 kbps.

Applications: 

Currently CDMA being the technology of choice throughout the world is available in combination with 3G technology. The G.hn standard for wired network is based on the TDMA technology.

References: 

[1] Telecommunications Industry Association: http://www.tiaonline.org/

Key Technology Area: 
Integrated Communications

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