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5.1 PRINCIPLES OF MULTIPLE ACCESSAs an illustration, consider a satellite transponder of B bandwidth with nchannels as shown in Fig. 5.1. When the transponder is in an operational stateat any time t i , each channel amplifies every carrier f i of the earth terminals ES i(where i ¼ 1; 2; ...; n). If no safeguards are in place, it is likely that severalcarriers will occupy a particular channel simultaneously and will mutuallyinterfere.To avoid such mutual interference, it is important that the receivers ofother earth terminals be able to discriminate among the received carriers.Discrimination can be achieved by any of three techniques: filtering, temporalgating, or signature assignment [1].1. Filtering. Take for instance the carrier energies to be in the frequencydomain. If we subdivide the available channel bandwidth into a number, say n,as shown in Fig. 5.1, and assign a subchannel to each user upon that user’srequest, then the receiver can discriminate among carriers by filtering. Thismultiple-access method is the principle of multiple access by frequencydivision (FDMA). It is commonly used in wireline channels to accommodatemultiple users for voice and data transmission.2. Temporal gating. If, for instance, we subdivide the frame duration T sinto, say, n nonoverlapping subintervals, each of duration T s =n, and assigneach user to a particular time slot within each frame, then several carrierenergies can be received sequentially by the receiver. The receiver can thusFIGURE 5.1Principle of multiple access.Copyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.