L16 Pre-flight

L16 Pre-flight#

Block 2: RF Electronic Warfare — Lesson 16: EP for Communications Work this before L16. Pre-flights are never collected — in class, anyone may be cold-called to present any question at the board, and that recitation is the participation grade.

Assumed pre-class reading#

  • L15 wrap-up

  • The Jupyter Book Reading page for L16 — the narrowband-link vulnerability, spread spectrum (DSSS and FHSS), processing gain, and low probability of detection.

Quiz questions (5 items, ~5 minutes)#

Q1. (Multiple choice) Why is a plain narrowband datalink easy to jam?

  • [ ] (a) It radiates too much power, so it is easy to find and overwhelm

  • [ ] (b) Its energy is concentrated in one narrow slice of spectrum

  • [ ] (c) It hops so slowly that a jammer can follow it channel to channel

  • [ ] (d) It is rarely encrypted, so the jammer can read the traffic and time its bursts

Q2. (Multiple choice) Spread spectrum works by:

  • [ ] (a) Raising transmit power until the signal overpowers the jammer

  • [ ] (b) Smearing the signal across a much wider band with a code both ends share

  • [ ] (c) Encrypting the message so the jammer cannot find the carrier

  • [ ] (d) Narrowing the bandwidth so less of the jammer’s power lands in the passband

Q3. (Multiple choice) The processing gain of a spread-spectrum link is:

  • [ ] (a) \(G_p = 10\log_{10}(B_{\text{spread}} / B_{\text{data}})\)

  • [ ] (b) \(10\log_{10}(P_t / P_j)\) — the transmit power over the jammer power

  • [ ] (c) The link’s data rate in bits per second

  • [ ] (d) \(10\log_{10}(f_c / B_{\text{spread}})\) — the carrier frequency over the bandwidth

Q4. (Multiple choice) In DSSS, multiplying the received signal by the matching PN code (despreading) does what?

  • [ ] (a) Spreads the wanted signal even further while concentrating the jammer into a spike

  • [ ] (b) Concentrates the signal back to narrowband while spreading the jammer’s power

  • [ ] (c) Removes the receiver’s thermal noise, so only the jammer is left

  • [ ] (d) Raises the data rate by the spreading factor \(N_{\text{chips}}\)

Q5. (Short answer) A protected link has 30 dB of processing gain and faces a jammer 20 dB above the signal (J/S = +20 dB). Explain whether the link can still close, and name the property that lets GPS be received below the noise floor.

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