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Swept Jamming against Radio Communications

Written By Fabrizio Vergari
974Swept Jamming against Radio Communications

Barrage jamming can be specifically synthesised as a frequency swept jamming.
It may perform better than noise jamming because its coherency and specific time rule.
The technique generates a relatively narrowband tuned signal which centre frequency is swept in time across the frequency band of interest.
The sweeping is carried out keeping fixed the centre frequency for a short time defined as dwell time (Figure 1).
The full power of the jammer is employed over any dwell bandwidth.
With the proper setting of signal bandwidth and sweep timing and frequency range, this technique can effectively impair narrow band links adapting the jamming to the hopping centre frequency change, while the targeted links performing an EPM capability.
Then, while against frequency fixed signals, a triangle sweep is generated affecting consecutive narrowband spectrum slices (Figure 2) against Frequency Hopping signals the narrowband centre could be actually selected randomly with digital synthesizers generating the jamming waveform.
Current digital synthesisers generate swept signals keeping each centre frequency for dwell times lasting in the order of 100 s, with a bandwidth ranging from some tens of KHz to some MHz.
Against some Frequency Hopping EPM signals operating in the UHF range, like the 220- 400 MHz tactical NATO I region or the L band tactical data link region, the jammer could cover the relevant whole RF band in time ranges in the order of 10 to 100 ms.
The main purpose for sweeping the partial-band noise waveform is to ensure that the jammer enters the frequency spectrum where an FHSS target net is located.

Normally EPM based networks do not use every channel included into their full RF range, but only a portion, called the hop set.
These hop sets need not be that large to be effective.
However, the experience teaches that by sweeping the jamming waveform over the complete operative range, and then the jammer is ensured to jam at the entire set of hop frequencies.
An effective design would take into account sweeping cannot be too fast as an inadequate fraction of the hop could be jammed. The effectiveness of the jamming relates its timing with the target signal’s data rate. In fact, a BER of 10–1 means that it is necessary to jam 1 bit out of 10, or for an EPM system that is sending data at 16 kbps, 1600 bits per second must be jammed to produce this BER. If this system is an FHSS network at 100 hps, each hop with a dwell time of 10 ms will contain some 100 bits with a duty cycle of some 60 %. Therefore, at least 10 hops per second must be jammed. Since these hops can be anywhere in the spectrum from the point of view of the jammer, at least 10 sweeps per second are required. To cover some 60 MHz in the low tactical VHF range, 10 times per second requires a sweep rate of 600 MHz per second. In order to cover this sweep range with single jamming dwell of 10 ms, then 100 dwell per second, the dwell bandwidth will be 6 MHz.

The trade-off with these numbers is that as the instantaneous bandwidth is increased, the power per dwell bandwidth decreases if the total jammer power remains fixed. In addition, the target receiver works with 25KHz IBW in the tactical 30-88 MHz, then just part of the jammer energy is effective against the receiver. At the same time, the increased dwell bandwidth increases the probability to jam the hopping signal while tuned at the unknown centre frequency.

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