 Use as given at the website. Introduction to GSM Introduction to GSM Input Data Figure 1-26 Normal (Unmodulated Carrier) Modulated Carrier in Figure 1-26. The benefit of phase shifts is that multiple phases can be used to represent more than one modulated bit. Under normal PSK, a shift in the phase represents a single bit; however, multiphase modulation enables multiple bits to be represented. The quadrature phase (QPSK) shifts will allow up to 2 bits per shift, whereas a quadrature and amplitude shift will allow 4 bits per phase shift. Gaussian Minimum Shift Keying (GMSK) GSM modulation works differently, as seen in Figure 1-27. Using Gaussian minimum shift keying (GMSK), the nature of the data moved from the mobile station is digital. For a digital transmission in GSM, the chosen modulation scheme needs to have good error performance in light of the noise and interference in a mobile network environment. GMSK is a com- plex scheme based largely on mathematical functions. The basis of this scheme is an offset quadrature phase shift keying (OQPSK), which offers the advantage of a fairly narrow spectral output. This is combined with a min- imum technique that controls the rate of change of the phase of the carrier and the radiated spectrum will be even lower. This also requires very care- ful planning at the sites to prevent interference and produces only 1 bit per symbol. The combined functions of the baseband filter, the OQPSK and GMSK modulation work to produce a compact transmission spectrum. This is important if adequate adjacent channel interference figures are to be met. The total symbol rate for GSM at 1 bit per symbol in GMSK produces 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 Figure 1-27 GMSK results. 200 KHz 200 KHz 200 KHz 270.833 ksymbols/second. The gross transmission rate of the time slot is 