Multiple access schemes are used whenever there is a need for a number of independent users to share a resource that may be scarce or expensive. For example, in a communication environment, users may need to communicate with one another over a shared medium. The term “multiple access” means that many devices can connect to and share the same medium. One feature of such a shared medium is that when any user transmits their information, many of the users that are connected to the medium can receive the transmission. The role of a multiple access scheme is to coordinate the transmissions from the users to avoid a “collision” in the medium and define steps that can be taken to resolve collisions when they occur. A collision occurs when two or more users transmit their information in the medium in such a manner that at least one bit from one user overlaps with another bit from another user. In this case, the transmissions are unusable, and the information needs to be retransmitted.
A multiple access scheme must be able to handle several users without mutual interference. Also, it should be able to maximize the spectrum efficiency. In this chapter, we consider some of the popular multiple access schemes. These schemes can be divided into three categories: