 expect in the pilot signal, so it can adjust its own inter- nal parameters using the pilot as a reference. In systems that use pilots, the signal is demodulated at the receiving end by a com- parison of the two waveforms, the pilot signal and the modulated signal, at a single time t. In noncoherent modulation, a single signal is used, and the demodulator at the receiving end notes the changes in the signal over time. In noncoherent modulation, low amplitude might be a 0 and high amplitude might be a 1. Or the absence of a phase change might represent a 0 and the presence of a phase change might represent a 1. One of the simplest digital modulation schemes is a coherent scheme called binary phase-shift keying (BPSK). The rule is simple enough: We broadcast a single frequency, and at predetermined time intervals, we broadcast the wave in the same phase as the pilot for a 0 bit and in opposite phase for a 1 bit, as shown in Fig. 1.9. Each of these predetermined time periods is called a symbol. 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. Radio Engineering Concepts Key Radio Concepts V (t) Pilot Signal Figure 1.9 Binary phase-shift keying (BPSK). The next level of digital sophistication is called quadraphase phase-shift keying (QPSK). The rule is not quite SO simple: We broadcast a single frequency the same as BPSK, we compare to the pilot phase at predetermined time intervals the same as BPSK, but QPSK allows four different phase relations instead of two. The four phase shifts are 0, 90, 180, or 270 degrees for 0, 1, 2, or 3, as shown in Fig. 1.10. Using QPSK, we send 2 bits per symbol instead of just one. (Our four options, 0, 1, 2, and 3 in decimal, are represented as 2 bits in binary notation: 00, 01, 10, and 11.) QPSK makes twice as much use of the radio bandwidth as BPSK, and we can send twice as much informa- tion. We get a free lunch