rying in its own indepen- dent Rayleigh fading pattern, as shown in Fig. 27.2. This adds more variation to the -83dBm Signal -100dBm Interferer Figure 27.2 A 17-dB S/I ratio with Rayleigh fading. 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. Conventional Reuse Principles Conventional Reuse Principles instant-to-instant S/I ratio, but it still sounds good enough in listening tests to be an acceptable call standard for AMPS. There is also interference from adjacent channels in AMPS. The 30-kHz frequency spacing is large enough that the adjacent-channel interference effect is 25 dB less than the same interference would be for cochannel. Thus an adjacent-channel interferer 8 dB stronger than the signal would have the same destructive effect as a cochannel in- terferer 17 dB below the signal. While cochannel interference causes dropouts from mo- mentary loss of SAT, adjacent-channel interference adds noise to the actual voice, some- times a whistling sound. While it is hard to make an exactly equivalent comparison, 25 dB seems to be an accepted value. This means that we can use adjacent channels on ad- jacent cells without too much worry, but the signal range of same-cell calls is too great to allow adjacent-channel use on the same cell, even on different sectors of the same cell. The 25-MHz of AMPS spectrum allows 12.5 MHz forward and 12.5 MHz reverse for a total of 416 channels of 30 kHz each. An AMPS system reserves 21 channels for pag- ing and access, leaving 395 two-way voice channels. 27.2 The GSM Channel (TDMA) As we described in Sec. 7.3, the Global System for Mobility (GSM) channel is one of eight time slots in a 200-kHz carrier. Without being too cavalier, we can think of the GSM channel as 25 kHz of digital radio carrying 13 kbps of regular pulse excitation-long-term prediction (RPE-LTP) speech coding along with digital supervi- sion