22.8 Kbps. Access Methods Because radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as pos- sible. The choices are Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) GSM chose a combination of Time and Frequency Division Multiple Access (TDMA/FDMA) as its method. The FDMA part involves the division by frequency of the total 25-MHz bandwidth into 124 carrier frequencies of 200-kHz bandwidth. One or more carrier frequencies are then assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots. One time slot is used for trans- mission by the mobile and one for reception. They are separated in time so that the mobile unit does not receive and transmit at the same time, a fact that simplifies the electronics. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction to GSM Introduction to GSM The FDMA part involves the division by frequency of the total 25-MHz bandwidth into 124 carrier frequencies of 200-kHz bandwidth. One or more carrier frequencies are then assigned to each base station. Using FDMA, a frequency is assigned to a user, as seen in Figure 1-28. Therefore, the larger the number of users in an FDMA system, the larger the number of available frequencies must be. The limited available radio spectrum and the fact that a user will not free its assigned frequency until he or she does not need it anymore explain why the number of users in an FDMA system can be quickly limited. Time Division Multiple Access (TDMA) is digital transmission technology that enables a number of users to access a single radio frequency (RF) chan- nel without interference by allocating unique time slots to each user within each channel. The TDMA d