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FIGURE 3.2BPSK space diagram.Instead of the baseband signal vðtÞ assuming 0 to 1 amplitude values, itcould have never-return-to-zero (NRZ) format with 1 amplitude values, eachhaving period T s ; that is, T s ¼ T b from (3.2).3.2.2 Quadrature PSKWhen the index b ¼ 2; M ¼ 4, a 4-phase PSK is derived. It is also referred toas quadrature PSK, or simply QPSK, in which the phase of the carrier in (3.1)is nonzero and can take on one of four values: 45 , 135 , 225 , or 315 . Theangular separation between any two adjacent phasors of a QPSK is 90 (recall2p=M, where M in this case is 4). Figure 3.3 shows the block diagram of aquadrature modulator. Using Fig. 3.3, we can consider the functions requiredof QPSK modulation. The incoming serial bit stream enters the bit splitter(demultiplexer), where it is converted to a parallel, two-channel output: the I-(in-phase) channel and the Q-(in-quadrature) channel. Consequently, the data(bit) rate, r b , in each of the channels is r b =2, since b ¼ 2. The data rate is thesignalling speed in which information can be transferred in bits per second(bit=sec). The data in each channel are fed to the channel’s balanced modulatorwith input carrier’s relative phase of 0 and 90 , respectively. The outputs ofthe product modulators are fed to the bit-combining circuit (linear summer),where they are converted from the I- and Q-data channels to a single binaryoutput data stream. With b ¼ 2, we can code the input bits with correspondingtransmitted phase symbols as shown in Table 3.1, with each symbol having aduration of T s ¼ 2T b . Also, the QPSK constellation (space) diagram is shownin Fig. 3.4.Copyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.