26-04-2012, 04:51 PM
Structural Mechanism and Applications of CDMA
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Modern wireless telecommunications:
A new era in wireless telephony began with the advent of a new technology that used smaller transmission areas called “cells”. The cellular concept employs variable low power levels, which allow cells to be sized according to the subscriber density and demand of a given area. As the population grows, cells can be added to accommodate that growth. Frequencies used in one cell cluster can be reused in other cells. Conversations can be handed off from cell to cell to maintain consultant phone service as the user moves between cells.
The cellular radio equipment can communicate with mobiles as long as they are within range. Radio energy dissipates over distance, so the mobiles must be within the operating range of the base station. Like the early mobile radio system, the base station communicates with mobiles via a channel. The channel is made of two frequencies, one for transmitting to the base station and one to receive information from the base station.
Multiple Access Technologies:
• Multiple Accesses is the simultaneous use of communications systems by more than one user.
• Each user’s signal must be kept uniquely distinguishable from other user’s signals, to allow private communication on demand.
• Users can be separated many ways:
1. Physically on separate wires
2. By arbitrarily defined ‘channels’ established in frequency, time, or any other variable imaginable
There are two types of access technologies, narrowband and wideband, the most desired technology is wide band technology.
Wideband Technology:
In wideband technologies, the entire bandwidth is made available to each user. This bandwidth is many times larger than the bandwidth required to transmit information. The main advantage of wideband technology is its ability to tolerate a fair amount of signal interference compared to FDMA and TDMA, which typically cannot tolerate any such interference. Because of the interference tolerance of wideband systems, the problems of frequency band assignment and adjacent cell interference are greatly simplified.
CDMA:
In CDMA each user’s signal is a continuous unique code pattern buried within a shared signal, mingled with other users’ code patterns. If a user’s code pattern is known, the presence or absence of their signal can be detected, thus conveying information. CDMA operates by using CODING to discriminate between users.
In Code Division Multiple Access (CDMA) systems all users transmit in the same bandwidth simultaneously. In spread spectrum transmission technique, the frequency spectrum of a data-signal is spread using a code uncorrelated with that signal. As a result the bandwidth occupancy is much higher than required.