Why near-far effect is there in CDMA and not in gsm?

Answer 1

The development of the ability to control the transmission power of the mobiles led to the re-evaluation of a spread spectrum technique invented in World War II, known as CDMA (Code Division Multiple Access). This technique permitted many more users to utilise the same system, but up until then had not been workable in the mobile environment due to a phenomenon known as near-far effect. Initially its use was confined to small aperture satellite services, and also, due to its security virtues, to military applications, where it was used to avoid jamming and detection of sensitive transmissions. The restricted use was due to the inability to eliminate narrowband interference caused by louder signals with greater amplitude drowning out lower power signals. This prevented correct demodulation of the wanted signal. The propagation path loss difference between those located close to the base station and those in fringe areas can be many tens of dB without regulation! (This is not the case in satellite systems where signal levels do not vary greatly.) Through control of the transmission power of the mobiles it became possible to ensure all accessing signals could be received at similar amplitude, thus eliminating near-far effect and enabling the same frequency spectrum to be reused in all the cells. The use of amplifier boosters that are not compatible with CDMA power regulation, therefore, prevents the elimination of near-far effect. This makes it impossible for the system to operate correctly, undermining the foundations of the basic concept of successful CDMA operation.