bsite. Introduction to GSM Chapter 1 Radio Assignment Each BTS is assigned a group of channels with which to operate. Any fre- quency can be assigned to the BTS, as they are frequency agile. This enables the system to reallocate frequencies as needed to handle load bal- ancing. Normally, the BTS can support upwards of 31 channels (frequen- cies); however, in actual operation, the operators usually assign from 1 to 16 channels per BTS. This is a business and practicality issue. The Absolute Radio Frequency Channel Number (ARFCN) is used in the channel assign- ment at each of the frequencies. Frequency Pairing The pairing is shown as the way of handling the 45-MHz separations. Remember that channel 0 was not used. It was reserved as a guard band from the lower frequencies to prevent interference. Extended GSM Radio Frequencies After the ETSI assigned the initial block of frequencies, a later innovation was to assign an additional block of 10 MHz on the bottom of the original block. The reasoning was that future demands would require this capacity. This meant that the frequencies from 880 to 890 MHz for the uplink and 915 to 925 MHz were added. This created an additional 50 carriers. The car- riers were numbered 974 to 1,023 so that the channel assignments would not be confused with the initial GSM standard. Once the added channels were implemented, the additional channels were still paired at 45-MHz separation. Channel 974 was not used; it became the guard band for the lower frequencies below 880 MHz and 925 MHz. The initial channel 0 in the primary GSM band is now used because of this shift, as shown in Figure 1-23. 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 Figure 1-23 Extended GSM. 974575 123124 974575 123124 915 925 MHz MHz Channel 974 not used. Acts as guardband. Channel 0 is now used. Modulati