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Figure 13d is a multi-tone jammer, which poses a better strategy against frequency

hoping systems than the single-tone jamming. This jammer selects the number of tones so that

the optimum degradation occurs when the spread signal hops to a tone frequency. The type of

jammer that is used in this paper is known as the partial-band jammer for frequency hoping

systems, and in time varying systems it is called a pulsed noise jammer. It can be seen in Figure

13b, and is talked about more in depth in the next section.

4.2 Pulsed Noise Jamming Introduction

Instead of just continuously jamming a communication channel, a pulsed noise jammer

(PNJ) can be used to jam the channel at chosen times with a greater power. This proves to be a

more effective way of jamming a spread spectrum system and is often used in electronic counter

measure operations. A PNJ can be defined as a jammer that turns “on” with just sufficient power

to degrade spread spectrum system performance significantly, but does not totally annihilate

system performance when “on”. The PNJ transmits a pulsed band-limited white Gaussian noise

signal whose power spectral density (PSD) just covers the spread spectrum system‟s bandwidth

(W). The duty factor for the jammer (ρ) is the fraction of time during which the jammer is “on”.

When the jammer is “on,” the one-sided received jammer power spectral density can be

e x p r e s s e d b y = / , w h e r e J i s t h e j a m m e r p o w e r , ρ i s j a m m e r d u t y c y c l e , a n d W i s t h e

bandwidth [26]. So, during the time that the jammer is “on”, the jammer voltage is 1/

and when the jammer is “off” the voltage is zero. PNJ in these simulations assumes a jammer

power amplifier is average-power limited rather than peak-power limited to simplify calculations

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