ECE 495 — Electromagnetic Warfare and Survivability

By the end of this lesson, you will be able to:

• Explain why a narrowband link is easy to jam and intercept
• Describe DSSS and FHSS as spread-spectrum techniques
• Compute processing gain and relate it to a survivable J/S
• Explain how a signal can hide below the noise floor

A datalink only helps if the message gets through. A plain narrowband signal makes that easy to stop:

• All its energy sits in one small band — easy to find and intercept
• A jammer puts all its power on that band — easy to deny
• Encryption hides the message, not the signal — the link still gets jammed

Take the signal and smear it across a band far wider than the data needs, using a code both ends share.

• The code (the “key”) spreads energy until the signal looks like noise
• No single slice of spectrum carries much power — hard to find, hard to jam
• Only a receiver with the same code can pull the signal back together

Typically 10–60 dB. It is the same quantity L15 wrote as $G_p = B\tau$ for pulse compression — a linear time–bandwidth product, reported here in dB because link budgets are kept in dB.

• Despreading concentrates the signal — its chips line up and add
• The jammer is uncorrelated with the code, so despreading spreads the jammer

So the link survives even when J/S is positive.

One band of spectrum. The wanted signal is spread flat across all of it; the jammer is parked on one slice.

The jammer towers over the signal, and it never has to find anything to do it. What can the receiver do that the jammer cannot?

Multiply by the same code. The signal's chips add coherently and collapse back to narrowband; the jammer, uncorrelated with the code, is smeared across the whole band.

Multiply each data bit by a fast pseudo-noise (PN) chip sequence.

• $N$ chips per bit spreads the bandwidth by $N$ — and sets the processing gain
• The receiver multiplies by the same PN code and integrates per bit
• Signal chips line up and add; the jammer, uncorrelated with the code, gets spread

Instead of spreading every instant, hop the carrier across many channels on a code-driven schedule.

• Transmitter and receiver hop together; an eavesdropper sees only fragments
• A spot jammer cannot follow — the same lose-lose we built in L15
• Spread is over time and frequency, not code — but the payoff is the same J/S relief

With enough processing gain, a signal can live below the noise floor.

• GPS arrives below the noise floor; the code digs it back out
• If the threat cannot find the signal, it cannot intercept or jam it well
• This is low probability of detection — the comms cousin of LPI radar

Type this with me — four codes against two jammers.

N    = [8 64 100 1000];         % chips per bit
JS   = [12 12  12   20];        % jammer above signal, dB
Gp   = 10*log10(N);             % processing gain, dB
marg = Gp - JS;                 % post-despread margin, dB
fprintf('    N   Gp(dB)  J/S(dB)  margin  link\n');
for k = 1:numel(N)
  verdict = 'DOWN';
  if marg(k) > 0, verdict = 'CLOSES'; end
  fprintf('%5d %7.1f %8.0f %7.1f  %s\n', ...
          N(k), Gp(k), JS(k), marg(k), verdict);
end
fprintf('\nDouble the chips -> +%.1f dB of margin.\n', 10*log10(2));

Rows 1 and 2 face the same jammer; one dies, one closes. What changed?

Then: marg is only $G_p -$ J/S. What does the receiver still need?