Multiple access schemes are used to allow many simultaneous users to use the same fixed radio bandwidth spectrum. Multiple access is a technique whereby many subscribers or local stations can share the use of a communication channel at the same time or nearly so, despite the fact that their individual transmissions may originate from widely different locations. In any radio system, the bandwidth allocated to it is always limited. Multiple access schemes are used to allow many mobile users to share simultaneously a finite amount of radio spectrum. For high quality communications, this must be done without severe degradation in the performance of the system. Sharing of the spectrum is needed in order to increase the user capacity of any wireless network. FDMA, TDMA and CDMA are the three major methods of sharing the available bandwidth to multiple users in wireless system. There are many extensions, and hybrid techniques for these methods, such as OFDM, and hybrid TDMA and FDMA systems. However, an understanding of the three major methods is required for understanding of any extensions to these methods.
