Spread Spectrum Modulation - Digital Communication: Principles and System Modelling - page 152

The process of frequency hopping is illustrated in Fig. 7.10 . A disadvantage of
Frequency-Hopping over Direct-Sequence is that obtaining a high processing-gain
is hard. The faster the 'hopping-rate' is, the higher the processing gain.

Signal hoping from
one frequency to
other

5

4

3

2

1

0

30

30

20

20

10

10

Time--> Frequency-->

Fig. 7.10 Illustration of the frequency hopping concept

On the other hand, Frequency-Hopping is less affected by the Near-Far effect
than Direct-Sequence. Frequency-Hopping sequences have only a limited number of
'hits' with each other. This means that if a near-interferer is present, only a number
of 'frequency-hops' will be blocked in stead of the whole signal. From the 'hops'
that are not blocked it should be possible to recover the original data-message.
Figure 7.11 shows the general scheme of FHSS modulation. A PN sequence
generator creates a k -bit pattern for every hopping period, T h . The frequency table
uses the pattern to find the frequency to be used for a particular hopping period and
passes it to the frequency synthesizer. The frequency synthesizer creates a carrier
signal of that frequency, and the source signal modulates the carrier signal.
Let's consider a case (for theoretical illustration) where the PN code genera-
tor creates three bit patterns (8 different patterns). The 8 patterns generated by
PN code generator are mapped to eight different frequencies in the frequency table
(Fig. 7.12 ).
The pattern for this station is 101, 111, 001, 000, 010, 011, and 100. Note that
the pattern is pseudorandom it is repeated after eight hoppings. This means that
at hopping period 1, the pattern is 101. The frequency selected is 700 kHz; the
source signal modulates this carrier frequency [ 3 ]. The second k -bit pattern selected