Recently i have learnt that unlike GSM that is used in mobile handset for communication, cdma uses broad spectrum frequency ie. the full available spectrum. Basically it is a form of spread spectrum. Do a user uses a unique code to transmit the information and receiver uses the code to pick up thesignal ? Won't it will be difficult for one to pick up the complete signal if send over several discrete frequencies
You imagine that broad spectrum frequency is the frequency of the signal, but that is not so. The frequency refers to how frequent one can find the data of one communication in the data signal stream.
I quote from the paper Basic note on Spread Spectrum CDMA signals:
In Code Division Multiple Access (CDMA) systems all users transmit in the same bandwidth simultaneously. Communication systems following this concept are "spread spectrum systems". In this 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 then required. The codes used for spreading have low cross-correlation values and are unique to every user. This is the reason that a receiver which has knowledge about the code of the intended transmitter, is capable of selecting the desired signal.
For more detailed info, see Wikipedia Code-division multiple access.