The user having the particular key will receive the desired signal. So each user is separated by separate codes. They have different codes, we have to therefore find good such codes. All users share the entire frequency band for the whole time, that is simultaneously. The advantage is that the code division multiple access systems give softer capacity, graceful degradation, because there is no fixed number of time slots or fixed number of frequency slots.
In case of fixed number of time or slots, the maximum number of frequency user is going to be only that much whatever is the fixed number, but in case of code division multiple access, what happens is that if a system is supporting the five users, the sixth user comes up does not matter. He will still be supported of course at the cost of degradation in the performance of all the users, but then he can come in, and some users when they drop out by thinking that the performance is going down or less important users can drop down, and thus is a graceful degradation allowing you softer capacity limits.
The CDMA has another great advantage, that the multipath fading could be reduced, in fact in mobile communication, because of the urban, suburban flective and reflective environments, you have the problem of multipath fading, and that can be taken care of by the CDMA.
Well you have another problem in CDMA, and that is the near-far problem, some users which a far off, some users may be nearby, and therefore the amount of power which can be received at the receiver could be varying, causing interference, and therefore efficient power control techniques are required to solve the near-far problems. Obviously, code division multiple access, because all users are utilising the entire bandwidth, requires a leager frequency band, which should be contiguous. The complexity is more, and that is the reason it could not be implemented earlier. Only in the second generation system, like IS-95, and third generation systems the CDMA is going to be used primarily.
Optimistically one can find the number of channels using this relationship, which is possible in the CDMA system
