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In direct sequence spread spectrum, which is the basis of CDMA systems, the baseband signal is multiplied with a pseudo noise (PN) sequence which has a high chip rate, thus the resulting modulated signal will have high rate and thus larger bandwidth. But the power of the original baseband signal and the power of spreaded signal resulting from the multiplication process is the same. Then, why this spreading process provides jamming resistance property?

Viewed in time domain, they have the same power. Does the jammer depend on the power value to detect the signal or not? I read in a reference that the spreaded signal appears as noise for the unintended receiver. But this only when viewed in frequency domain. Does the jammer depends on frequency domain measurements for detecting the signal, not on time domain power measurements?

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  • $\begingroup$ "but only when viewed in frequency domain" <-- that's wrong. $\endgroup$ – Marcus Müller Feb 28 at 12:00
  • $\begingroup$ To expand on what @MarcusMüller just said: if you demodulate a non-coherent jamming signal, then in the time domain it looks like noise in that the output of your reception process appears to be random. (That's a poor explanation, but there are things that just don't make easy sense in the time domain that are obvious in the frequency domain -- yet no information is lost going from one to the other, so if some explanation is true in one, it must be true in the other). $\endgroup$ – TimWescott Feb 28 at 14:19
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In order to work well, a jamming signal must be well-correlated with the signal it is trying to jam. In the case of sine-wave modulated signals, it's easy to detect the signal one wishes to jam, to infer that it's narrowband (and, hence, riding on a real or suppressed sine-wave carrier), to infer its bandwidth, and from there to choose a suitable jamming signal.

To jam a CDMA spread spectrum signal you'd either need to overwhelm it with an uncorrelated signal (random, single tone, pseudo-random noise) or you'd have to find it, find its PN sequence, synchronize to its PN sequence, and then in most cases adjust your synchronization to match the time offset between your reception of the signal and the receiver's, then you'd have something to send -- but if I were trying to receive the signal, I could defeat you by having multiple receivers, located more than a wavelength of the chipping rate away from each other, each trying to receive the signal. One receiver may get the jamming signal and the intended signal on top of each other, but unless your jamming signal is co-located with the intended transmission, other receivers would be able to pick out the good from the bad.

But keep in mind that civilian CDMA is intended to minimize unintentional jamming, and does pretty well. It's not intended to defeat malicious jamming.

I suspect that there are smart military types out there that know a lot more than I do about jamming CDMA -- I'm just making this up as I go from first principles. I also suspect that military systems will use something more complicated than civilian CDMA, for just that reason.

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    $\begingroup$ Also to add, to jam the signal you just need to occupy the spectrum with a white noise signal with a power greater than the processing gain of the signal less the margin the signal needs to properly demodulate. (As received at the receiver). Single tone jammers would also work well for jamming under the same criteria but they are much easier to defeat with a sophisticated receiver that can excise the tone prior to despreading (which is simple to implement). $\endgroup$ – Dan Boschen Feb 28 at 19:48
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    $\begingroup$ I've incorporated that into my answer -- good point. $\endgroup$ – TimWescott Mar 1 at 3:24
  • $\begingroup$ Do you mean that the detection process of a CDMA signal is difficult? I couldn't understand the answer well, and I would be grateful if you could tell me if the following paragraph is correct or not: detecting a CDMA signal is difficult since doesn't only depend on measuring the signal frequency and its bandwidth, but also on detecting its code, which cannot be detected easily. The detection process doesn't depend on measuring the signal power. $\endgroup$ – Noha Mar 9 at 12:22
  • $\begingroup$ @TimWescott @ Dan Boschen $\endgroup$ – Noha Mar 14 at 11:12
  • $\begingroup$ @Noha sorry seeing your comment now- the system doesn't allow you to tag two names. I suggest adding that as another question (detection ability) since it's no longer specific to this question. But in short, in general that is not true- you cannot say that CDMA signals are difficult to detect if you don't know the code. If it is a linear code, and there is strong enough SNR, they are quite easy to detect, as well as spoof without other prevention mechanisms $\endgroup$ – Dan Boschen Apr 29 at 2:35
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I think that the jamming properties of CDMA may be better intuitively understood by looking at its predecessor: frequency hopping spread spectrum.

If you transmitted signal jumps between 100 different frequency slots one at a time, a jammer is faced with: 1) blasting out enough power to disrupt all frequency bins, all of the time (100x) 2) figuring out the hop sequence and concentrating jamming power in the bin that contains signal at any one time

Code division make use of «all time, all frequency», but use a spreading sequence as a key.

-k

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