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Quoted from here:

Certain military communication systems are based on the jam-resistant burst transmission of binary telegrams which consist of orthogonal spread-spectrum waveforms (BOK). To regain the message content at the receiver the a priori not known arrival times of the telegrams have to be detected by means of a synchronization circuit. In this paper a novel power-level adaptive synchronization circuit for producing the telegram alarms is proposed. This circuit shows CFAR-performance in the case of noise jammers but does not show the degradation typical for other synchronization systems (e.g. systems employing a hard bandpass limiter at the receiver front end) if other types of jamming signals are received. The performance of the synchronization circuit is theoretically analyzed for noise jam mers and CW-sine jammers. Some considerations concerning the influence of pulsed jammers are also included. Measurements finally confirm the theoretical results.

Isn't anything transmitted through the ether potentially jammable? I mean, what stops someone from transmitting orthogonal spread-spectrum waveforms with a higher powered radio frequency constantly to overwhelm any other lower frequency radio transmission? Shouldn't any type of data transmittion, including very short bursts be incapacitated at that point?

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Orthogonal Spread Spectrum Waveforms are more commonly known as Direct Sequence Spread Spectrum when working in the digital domain. Directly jamming such signals is very hard. You need to use a PN sequence very close (short hamming distance) to the one used to spread the original signal. Raw power is insufficient to jam the signal.

If your PN sequence is long enough, you can spread the signal wide enough that the energy density falls below the noise floor, which makes detection of a transmission very difficult unless you know the PN sequence.

By definition a PN has the property that the auto-correlation is 1 and the correlation with other signals is << 1.

However, if the system uses a poor PRNG (i.e. not a CSPRNG and/or having a small internal state) then it may be possible to recover the PN sequence (or one sufficiently close) to jam the signal.

Another type of attack on DSSS is a time shifting attack. If the attacker can detect and isolate the signal they can retransmit with a small delay. The overlapping signals both use the same PN sequence, and the output will be the sum of the two signals. If this is done repeatedly you can jam the signal. A good analogy of this attack involves trying to talk in the presents of lots of echos. It becomes two difficult to distinguish one symbol (word) from the clutter.

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Yes. This is why it was not described as "jam-proof". However, it just so happens that transmitting powerful RF waves across the entire RF spectrum is a) very, very power intensive (like "you can't do this outside of a fixed installation near a dedicated power plant"-type power intensive), b) ruins your own systems as well, and c) makes you an excellent target for anti-radiation missiles. The whole point of jam-resistant stuff is to make it so that the enemy, who has finite amounts of power, doesn't know when and where to transmit to jam the signal. It's simply not practical to transmit high power across a broad spectrum for long periods of time.

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