Lesson 16 Flashcards#
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1. Why is a narrowband link easy to find and intercept?
All its energy sits in one small slice of spectrum, so a wideband receiver sees a tall spike at a known frequency — an obvious feature to lock onto and intercept.
2. Why is a narrowband link easy to jam?
Its energy is concentrated in one band, so a jammer can aim all of its power at that band and overwhelm the receiver — nothing dilutes the attack.
3. Why doesn't encryption defeat a jammer?
Encryption hides the message content, not the signal. The spike is still there to be jammed; a jammer denies the channel regardless of what the bits mean. You can encrypt everything you send and still be silenced.
4. What is the core idea of spread spectrum?
Smear the signal across a band far wider than the data needs, using a code both ends share. The energy spreads until the signal looks like noise — low and flat, hard to find and hard to jam — and only a receiver with the same code can pull it back together.
5. What is processing gain, and what is the formula?
The ratio of spread bandwidth to data bandwidth, in dB: \(G_p = 10\log_{10}(B_{\text{spread}}/B_{\text{data}}) = 10\log_{10} N_{\text{chips}}\), where \(N_{\text{chips}}\) is chips per bit. Typically 10–60 dB.
6. A code runs 1000 chips per bit. What is the processing gain?
\(G_p = 10\log_{10}(1000) = 30\) dB. (Each factor of ten in chip rate is another 10 dB; a 64-chip code gives \(\approx 18\) dB.)
7. What does despreading do to the signal and to the jammer?
Multiplying by the same code concentrates the wanted signal (its chips line up and add, collapsing it back to narrowband) and spreads the jammer (uncorrelated with the code, it gets smeared across the band). The picture before and after despreading is simply reversed.
8. How does processing gain let a link survive a positive J/S?
After despreading, the effective ratio is roughly \((\text{J/S})_{\text{in}} - G_p\). So even when the jammer is louder than the signal at the antenna, the link closes if \(G_p\) exceeds the deficit with margin. With \(G_p = 30\) dB against a 20 dB jammer, ~10 dB of margin remains.
9. How does processing gain relate to bit-error rate?
BER is set by \(E_b/N_0\) after despreading, and despreading raises the effective \(E_b/N_0\) by \(G_p\). Drive the post-despread margin positive and the BER collapses — bits lost in the jammer come through clean.
10. How does DSSS spread the signal?
It multiplies each data bit by a fast pseudo-noise (PN) chip sequence. \(N\) chips per bit spreads the bandwidth by \(N\) and sets \(G_p\). The receiver multiplies by the same PN code and integrates per bit, so the signal re-collapses and the jammer smears.
11. How does FHSS spread the signal, and how does it relate to L15?
It hops the carrier across many channels on a code-driven schedule; transmitter and receiver hop together. A spot jammer can't follow and must spread its power across the whole hop set (partial-band/barrage). It is L15's radar frequency agility pushed onto our links — same lose-lose, same J/S relief, spread in frequency instead of code.
12. What is LPD, and what is the canonical example?
Low probability of detection — with enough processing gain a signal rides below the noise floor and only correlation digs it out. GPS is the example: it arrives below thermal noise. If the threat can't find the signal, it can't intercept or jam it well. LPD is the comms cousin of LPI radar — "you cannot jam what you cannot find."