TABLE OF CONTENTS The spread spectrum can provide the redundancy of the message bits while in transmission. The two direct sequence techniques can be illustrated in Fig. 8.30. The first one is shown in Fig. 8.30(a). The data x( t) transmitted at a rate R b are modulated, first by carrier f 0 and then by a spreading code G( t) to form a DS signal s t( t) with a chip rate R c, which takes a DS bandwidth B ss. Then the DS signal s t( t - T), after a propagation time delay T, is received and goes through a correlator that consists of two functions: multiplier and averager. The correlator uses the same prestored spreading code G( t) to despread the DS signal. Then the despread signal x( t - T) is covered. The second DS technique is to spread first, then modulate, as shown in Fig. 8.30(b). These two DS techniques provide the same DS signal s t( t). The following analysis uses the technique from Fig. 8.30(a).
Let x( t) be a data stream modulated by a binary phase shift keying (BPSK) so that
| (8.145) |  |
where x( t) = 1 and its data rate = R b . At the...
