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

616 SPREAD SPECTRUM COMMUNICATIONS
Fi g ure 11.2
Ty pical Bluetooth
frequency
hopping pattern.
111---+--+--+---+--+--+----l---""--
10
9 i----t- - --t- - t--.--- - ----i - ---t- - --t--
] 8 1--t----t,,,_.,1--t--+-----1---+--+--
8 7
i 5 P-.""'l---+-+---l--+-+--'""I---+--­
6 1----l-----,llll-llq---l--+-..,,,.._l--l---+---
& 4 1--t--t---t--t-----..-.,----1---+----
£ 3 1---+--+-+---l--l"-"l--1--1----1""-
2 t---. ..... .,,;---........i--+-+--1---+--+--
Time slots (chips)
Fi g ure 11.3
Effects of
(a) narrowband
jamming,
(b) FHSS under
broadband
jamming, and
(c) partial band
jamming.
Signal
spectrum
Jammer
spectrum
FH signal
spectrum
Bs
(a)
(b)
Bs
(c)
Bs
is spread across multiple frequency bands because of the fast hopping, and must be detected
by detection over these frequency bands.
One major advantage of FHSS lies in its ability to combat jamming. Suppose a jamming
source has a finite level of jamming power P 1 . Against a narrowband signal with bandwidth
B s , the jamming source can transmit within B s at all time, creating an interference
PSD level of P1 /B s . Hence, the signal-to-interference ratio (SIR) for the narrowband (NB)
transmission is
(11.3a)
On the other hand, against FHSS signal with total bandwidth of B e , the jamming source must
divide its limited power and will generate a much lower level of interference PSD with average
value P1 /B e . As a result, at any given time, the signal bandwidth is still B s and the SIR is
(11.3b)
Therefore, with a spreading factor of L, an FH signal is L times more resistant to a jamming
signal with finite power than a narrowband transmission. Figure 11.3a and b illustrate the
different effects of the finite jamming power on narrowband and FHSS signals.
On the other hand, the jammer may decide to concentrate all its power P 1 in a narrow signal
bandwidth against FHSS. This will achieve partial band jamming. If the frequency hopping
is slow, such that T c = T s , then on average one out of every L user symbols will encounter