on In order to convey the speech on the radio frequency, either in analog or dig- ital form, the transmitted information must be propagated on the radio link. It must be placed on the carrier. A carrier in this respect is a single radio frequency. The process of combining the audio and the radio signals is known as modulation. The resultant waveform is known as a modulated waveform. Modulation is a form of change process where we change the input information into a suitable format for the transmission medium. We also unchanged the information by demodulating the signal. Three normal forms of modulation are used: Amplitude Frequency Phase Amplitude Shift Keying (ASK) In amplitude shift keying (ASK) (Figure 1-24), the radio wave is modulated by shifting on the amplitude. The frequency is left constant, but the ampli- tude is shifted high if the data is a 1 and low if the data is a 0. Normally, we see two amplitude shifts represent a single bit. 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 Input Data Figure 1-24 Normal (Unmodulated Carrier) Modulated Carrier Frequency Shift Keying (FSK) The alternative to ASK is frequency shift keying (FSK). In the case of FSK (Figure 1-25), applying the data onto the radio wave modulates the carrier by changing the frequency. The amplitude is kept constant and the fre- quency is changed. Normally, a single frequency shift represents a bit of data. Phase Shift Keying (PSK) In phase shift keying (PSK), both the amplitude and the frequency are kept constant, SO the changes are represented by a shift in the phase, as shown Input Data Figure 1-25 Normal (Unmodulated Carrier) Modulated Carrier 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