MULTIPLE ACCESS TECHNIQUES

Multiple access is a techniques where by many subscribers or local stations can share the use of a communication channel at the same time of a communication channel at the same time or nearly so, despite the fact that their individual transmissions may originate from widely different locations. Stated in another way a multiple-access technique permits the communication resources of the channel to be shared by a large number of users seeking to communicate with each other.

Multiple Access refers to the remote sharing of a communication channel such as dispersed locations. On the other hand, multiplexing refers to the sharing of a channel such as telephone channel by uses confined to a local site.
In a multiplexed system, user requirements are ordinary fixed. In contrast, in a multiple-access system user requirements can change dynamically with time, in which can provisions are necessary for dynamic channel allocation.

1. FREQUENCY-DIVISION MULTIPLE ACCESS (FDMA)
In this technique, disjoint sub bands of frequency are allocated to the different users on a continuous-time basis. In order to reduce interference between users allocated adjacent channel bands, guard bands are used to act as buffer zones. These guard bands are necessary because of impossibility to achieving ideal filtering for separating the different users.

2. TIME DIVISION MULTIPLE ACCESS (TDMA)
In this technique, each user is allocated the full spectral occupancy of the channel, but only for a short duration of the called a time slot, buffer zones in the form of guard times are inserted between the assigned time slots. This is done to reduce interference between users by allowing for time uncertainty that arise due to system imperfections especially in synchronization schemes.