4 ms·

In this case, I'm sure you're right. But it might be worth noting that you can use spread-spectrum techniques (http://en.wikipedia.org/wiki/Spread_spectrum htt

by mturmon 14y ago

In this case, I'm sure you're right. But it might be worth noting that you can use spread-spectrum techniques (http://en.wikipedia.org/wiki/Spread_spectrum http://en.wikipedia.org/wiki/Spread_spectrum) to overcome jamming. Spread spectrum has some similarities to an encryption scheme, because the method used to smear the signal around uses a pseudorandom sequence which is known to the encoder and the decoder. CDMA (http://en.wikipedia.org/wiki/Code_division_multiple_access http://en.wikipedia.org/wiki/Code_division_multiple_access) cell phones use this idea. As you note, this type of encryption takes place at the RF level.

JosephRedfern 14y ago
Surely the "jammers" could just broadcast their jamming signal over a large range of frequencies?
- InclinedPlane 14y ago
  Of course, but that raises the amount of resources necessary for jamming all that much higher, perhaps taking it out of the realm of feasibility.
  - mturmon 14y ago
    Right, and you now have variables under your control to drive their power requirements arbitrarily high -- namely, the better your timing, the higher your effective S/N ratio (because your spread-spectrum signal can be, in effect, very-very narrowband, but hopping all over). You increase your timing accuracy by 10x, and the jammer's power requirements increase by 10x (crudely speaking -- not a 1:1 trade, but directly linked).
    - dhimes 14y ago
      That's a different kind of spread-spectrum. That's a frequency-hopper, where the carrier changes frequency (the first SS- an idea attributed to Hedy Lemarr in the lore). We're talking about direct sequence, which increases the bandwidth of the signal but keeps the same carrier. To jam that (theoretically) you need large power through the entire band.
      - mturmon 14y ago
        I was talking about both -- for the purposes of explanation, frequency-hopping is more intuitive. (I was trying to be responsive to @Redfern above, who mentions frequency-domain jamming.) Both frequency-hopping and direct-sequence spread energy over a larger bandwidth. Direct-sequence does not explicitly frequency-hop, but the time-domain switching it uses will spread the energy out in Fourier space.

by mturmon 14y ago

JosephRedfern 14y ago

Surely the "jammers" could just broadcast their jamming signal over a large range of frequencies?

InclinedPlane 14y ago
Of course, but that raises the amount of resources necessary for jamming all that much higher, perhaps taking it out of the realm of feasibility.
- mturmon 14y ago
  Right, and you now have variables under your control to drive their power requirements arbitrarily high -- namely, the better your timing, the higher your effective S/N ratio (because your spread-spectrum signal can be, in effect, very-very narrowband, but hopping all over). You increase your timing accuracy by 10x, and the jammer's power requirements increase by 10x (crudely speaking -- not a 1:1 trade, but directly linked).
  - dhimes 14y ago
    That's a different kind of spread-spectrum. That's a frequency-hopper, where the carrier changes frequency (the first SS- an idea attributed to Hedy Lemarr in the lore). We're talking about direct sequence, which increases the bandwidth of the signal but keeps the same carrier. To jam that (theoretically) you need large power through the entire band.
    - mturmon 14y ago
      I was talking about both -- for the purposes of explanation, frequency-hopping is more intuitive. (I was trying to be responsive to @Redfern above, who mentions frequency-domain jamming.) Both frequency-hopping and direct-sequence spread energy over a larger bandwidth. Direct-sequence does not explicitly frequency-hop, but the time-domain switching it uses will spread the energy out in Fourier space.