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

618 SPREAD SPECTRUM COMMUNICATIONS
11 .2 MULTIPLE FHSS USER SYSTEMS
AND PERFORMANCE
Clearly, FHSS systems provide better security against potential enemy jammers or interceptors.
Without full knowledge of the hopping pattern that has been established, adversaries cannot
follow, eavesdrop on, or jam an FHSS user transmission. On the other hand, if an FHSS system
has only one transmitter, then its use of the much larger bandwidth B e would be too wasteful.
To improve the frequency efficiency of FHSS systems. multiple users may be admitted over
the same frequency band B e with little performance loss.
As shown in Fig. 11.4, each of the M users is assigned a unique PN hopping code that
controls its frequency hopping pattern in FHSS. The codes can be chosen so that the users
never or rarely collide in the spectrum with one another. With multiple users accessing the
same L bands, spectral efficiency can be made equal to the original FSK signal without any
loss of FHSS security advantages. Thus, multiple user access becomes possible by assigning
these distinct hopping (spreading) codes to different users, leading to code division multiple
access (CDMA).
Generally, any overlapping of two or more user PN sequences would lead to signal collision
in frequency bands where the PN sequence values happen to be identical during certain chips.
Theoretically, well-designed hopping codes can prevent such user collisions. However, in
practice, the lack of a common synchronization clock observable by all users means that each
user exercises frequency hopping independently. Also, sometimes there are more than L active
users gaining access to the FHSS system. Both cases lead to user collision. For slow and fast
FHSS systems alike, such collision, would lead to significant increases in user detection errors.
Performance of FHSS with Multiple User Access
For any particular FHSS CDMA user, the collision problem would typically be limited to its
partial band. In fact, the effect of such collisions is similar to the situation of partial band
jamming, as analyzed next.
Recall that the performance analysis of FSK systems has been discussed in Chapter 10
(Sec. 10.7) under AWGN channels. It has been shown that the probability of symbol detection
error for noncoherent M-ary FSK signals is
M - l
P ( M-1 e M = 1 - P cM = L )
m
m= l
(I 1.5)
For slow FHSS systems, each data symbol is transmitted using a fixed frequency carrier.
Therefore, the detection error probability of slow FHSS system is identical to Eq. (11.5).
Figure 11.4
CDMA in FHSS
in which each of
the M users is
assigned a
unique PN code.
Code I FHSS
FSK modulator
Code 1 FHSS
FSK receiver
Code 2 FHSS RF Code 2 FHSS
FSK modulator channel FSK receiver
Code MFHSS
FSK modulator
Codc MFHSS
FSK receiver