 and control. This channel is tougher than AMPS for two reasons. First, its broader carrier band- width of 200 kHz makes it more resistant to Rayleigh fading. Second, it is sending only 13 kbps in 25 kHz of bandwidth after error correction, a fairly light data load. In com- parison, a quadraphase phase shift keying (QPSK) modulated channel sends 2 bits per second per hertz of bandwidth, so GSM's speech channel of half a bit per hertz leaves a lot of room to manage interference. The result is a channel that can withstand as S/I ratio of 12 dB with the same audio quality as AMPS gets with an S/I ratio of 17 dB. The 25-MHz of GSM spectrum allows 12.5 MHz forward and 12.5 MHz reverse for total of 62 carriers of 200 kHz each, 480 channels. Each cell sector in GSM system re- serves its own paging and access channels, so the voice channel count is about 450 channels. 27.3 Signal-to-Interference Performance While Rayleigh fading is changing the signal and interference instant to instant, we can think of the S/I ratio as a signal value over some area. 3 As long as this area has a favorable S/I ratio, we expect a wireless telephone call to sound good. The radio path between an antenna face at a base station and a wireless user ter- minal depends on many factors. The distance between the base station and the user terminal is the most obvious determinant of radio path gain, and we expect it to fol- low the fourth-power-of-distance rule from Sec. 1.7. Other factors affecting the radio path are terrain, buildings, and vegetation. The result of all these factors is a radio At frequencies of 900 MHz and higher, wavelengths of 33 cm in) or less, an area can be very small, not even a meter across, and still have global statistical properties. 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 Capacity and Quality Principles Received