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

11 . 1 Frequency Hopping Spread Spectrum (FHSS) Systems 615
Figure 11.1
Frequency
hopping spread
spectrum system.
RF
Channel
the carrier frequency hopping controlled at the transmitter by the pseudonoise (PN) generator.
To track the hopping carrier frequency, the receiver must utilize the same PN generator in
synchronization with the transmitter PN generator.
We note that most FHSS signals adopt binary or M-ary FSK modulations instead of the
more efficient PAM, PSK, or QAM. The motivation for choosing FSK stems from its ability
to utilize the less complex noncoherent detection. In contrast, coherent detection is generally
needed for PAM, PSK, and QAM modulations. Due to the PN hopping pattern, coherent
detection would require the receiver to maintain phase coherence with the transmitter at every
one of the frequencies used in the hopping pattern. Such requirement would be difficult to satisfy
during frequency hopping. On the other hand, FSK detection can be noncoherent without the
need for carrier phase coherence and can be easily incorporated into FHSS systems.
The frequency upconverter, as discussed in Example 4.2 of Chapter 4, can be a mixer or
a multiplier followed by a bandpass filter. Denote T s as the symbol period. Then the M -ary
FSK modulation signal can be written as
SFSK (t) = A cos (wmt + </>m)
mT s .:5 t .:5 (m + l)T s
(11.la)
in which the M -ary FSK angular frequencies are specified by
1 3 M - 1
W m = W e ±
2 1:!..w, W e ±
2 1:!..w, ... , W e ± - 2
-1:!..w
(11.lb)
The frequency synthesizer output is constant for a period of T c often known as a "chip." If we
denote the frequency synthesizer output as wh in a given chip, then the FHSS signal is
(11.2)
for the particular chip period Tc . The frequency hopping pattern is controlled by the PN
generator and typically looks like Fig. 11.2. At the receiver, an identical PN generator enables
the receiver to detect the FHSS signal within the correct frequency band (i.e., the band the
signal has hopped to). If the original FSK signal only has bandwidth B s Hz, then the FHSS
signal will occupy a bandwidth L times larger
B e
= L · B s
This factor L is known as the spreading factor.
For symbol period T s and chip period T c , the corresponding symbol rate is R s = 1 /T s and
the hopping rate is R e = 1/T c . There are two types offrequency hopping in FHSS. If Tc T s ,
then the FH is known as slow hopping. If T, < T s , it is known as fast FHSS, and there are
multiple hops within each data symbol. In other words, under fast hopping, each data symbol