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  8/2/13 WCDMA (3G) 1/2 GSM Network Basics An Introduction to GSM Networks   WCDMA TECHNOLOGY    By definition, the bandwidth of a WCDMA (Wideband Code Division Multiple Access)system is 5 MHz or more, and this 5 MHz is also the nominal bandwidth of all 3GWCDMA proposals.This bandwidth was chosen because,It is enough to provide data rates of 144 and 384 Kbps (these were 3G targets),and even 2 Mbps in good conditions.   Bandwidth is always scarce, and the smallest possible allocation should beused, especially if the system must use frequency bands already occupied byexisting 2G systems.This bandwidth can resolve more multipaths than narrower bandwidths, thusimproving performance. 3G Network Architecture  In a CDMA system all users occupy the same frequency at the same time, no timescheduling is applied, and their signals are separated from each other by means of orthogonal codes. Each user is assigned a code applied as a secondary modulation,   8/2/13 WCDMA (3G) 2/2 which is used to transform a user’s signals into a spread spectrum-coded version of the user’s data stream. Tree of orthogonal codes  The receiver then uses the same spreading code to transform the spread-spectrumsignal back into the srcinal user’s data stream. These codes are chosen so that theyhave low cross-correlation with other codes. This means that correlating the receivedspread-spectrum signal with the assigned code despreads only the signal that wasspread using the same code. All other signals remain spread over a large bandwidth.That is, only the receiver knowing the right spreading code can extract the srcinalsignal from the received spread-spectrum signal. The figure above shows the tree of orthogonal codes.  Power Control in WCDMA Power control in uplink must make signal powers from different users nearly equal inorder to maximize the total capacity in the cell.In downlink the power control must keep the signal at minimal required level in orderto decrease the interference to users in other cells. Received power at the base station   Home | BSS | NSS | GSM Cell | Transmission Planning | Tower & Antenna | Power System | WCDMA | SS7   © Chathuranga Perera,Electrical Engineer Ceylon Electricity Board  
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