B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

624 SPREAD SPECTRUM COMMUNICATIONS
Figure 11.7
Figure 1 from the
Aug. 11, 1942.
H. K. MARKEY Kr AL
2,292,387
Lamarr-Antheil
patent (From
U.S. Patent and
SECRET COMMUNICATION SYSTEM
Fi led June 10, 1941
2 Sheets-Sheet 1
Trademark
Office.) L. R.
35
23
22
Amplifier
24e
38
37
11 .4 DIRECT SEQUENCE SPREAD SPECTRUM
FHSS systems exhibit some important advantages, including low-complexity transceivers and
resistance to jamming. However, the difficulty of carrier synchronization under frequency
hopping means that only noncoherent demodulations for FSK and DPSK are actually practical.
As shown in the analysis from Sec. 10.11, FSK and DPSK tend to have poorer BER performance
(power efficiency) and poorer bandwidth efficiency compared with QAM systems, which
require coherent detection. Furthermore, its susceptibility to collision makes FHSS a less
effective technology for CDMA. As modern communication systems have demonstrated, direct
sequence spread spectrum (DSSS) systems are much more efficient in bandwidth and power
utilization. 7 Today, DSSS has become the dominant CDMA technology in advanced wireless
communication systems. It is not an exaggeration to state that DSSS and CDMA are almost
synonymous.
Optimum Detection of DSSS PSK
Direct sequence spread spectrum is a technology that is more suitable for integration with
bandwidth-efficient linear modulations such as QAM/PSK. Although there are several different
ways to view DSSS, its key operation of spectrum spreading is achieved by a PN sequence,
also known as the PN code or PN chip. The PN sequence is mostly binary, consisting of 1 s
and Os, which are represented by polar signaling of + I and -1. To minimize interference
and to facilitate chip synchronization, the PN sequence has some nice autocorrelation and
cross-correlation properties.
Direct sequence spread spectrum (DSSS) expands the traditional narrowband signal by
utilizing a spreading signal c(t). As shown in Fig. 11.8, the original data signal is linearly