r packet is noticeably sim- pler (for example, silence or a sustained tone with little change in modula- tion) and will compress the packet far more. Thus, the packet may involve fewer bits, and the phone will take less time to transmit it. That's where this odd idea of what bandwidth means in CDMA comes in. For in a real sense, bandwidth in CDMA equates to receive power at the cell. CDMA sys- tems constantly adjust power to make sure as little is used as necessary, and compensate for this by using coding gain through the use of forward error correction and other approaches that are much too complicated to go into. The chip rate is constant, and if more actual data is carried by the con- stant chip rate, then less coding gain will occur. Therefore, it's necessary to use more power instead. 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 Chapter 1 TDMA Frames In GSM, a 25-MHz frequency band is divided, using an FDMA scheme, into 124 carrier frequencies spaced one from each other by a 200-kHz frequency band. Normally, a 25-MHz frequency band can provide 125 carrier fre- quencies, but the first carrier frequency is used as a guard band between GSM and other services working on lower frequencies. Each carrier fre- quency is then divided in time using a TDMA scheme. This scheme splits the radio channel, with a width of 200 kHz, into eight bursts, as shown in Figure 1-31. A burst is the unit of time in a TDMA system, and it lasts approximately 0.577 ms. A TDMA frame is formed with eight bursts and lasts, consequently, 4.615 ms. Each of the eight bursts that form a TDMA frame are then assigned to a single user. Time Slot Use One time slot is used for transmission 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 elect