Question E8D01:
Why are received spread spectrum signals resistant to interference?  

Choices:  
A: Signals not using the spread spectrum algorithm are suppressed in the receiver  
B: The high power used by a spread spectrum transmitter keeps its signal from being easily overpowered  
C: Built-in error correction codes minimize interference  
D: If the receiver detects interference, it will signal the transmitter to change frequencies    

Correct Answer is  A: Signals not using the spread spectrum algorithm are suppressed in the receiver  

To calculate why spread spectrum resists interference, remember “processing gain.”

1) Why the correct answer is correct (A)
- In DS-SS and FHSS, the receiver uses a matching code (or hop pattern) to de-spread the desired signal.
- With the correct code, the desired signal is “collapsed” back to a narrow bandwidth, increasing its spectral density.
- Any other signal that does not match the code remains spread-out and appears as low-level noise after the de-spreading and filtering. In other words, non-matching signals are suppressed in the receiver.

2) Why the other choices are incorrect
- B: Spread spectrum does not rely on high power. In fact, it typically uses low power spectral density; resistance to interference comes from processing gain, not brute force power.
- C: Error correction may help overall link reliability, but it is not the primary reason for interference resistance. The key mechanism is code-based de-spreading (processing gain).
- D: Standard spread spectrum does not require the receiver to signal the transmitter to change frequencies upon interference. FHSS uses a prearranged hop pattern; resistance comes from the hop pattern or code correlation, not adaptive feedback.

3) Useful formulas and rules of thumb
- Processing gain (for DS-SS):
  $$G_p = \frac{W}{R} \approx \frac{R_c}{R_b}$$
  where:
  - $W$ ≈ spread bandwidth
  - $R_c$ = chip rate
  - $R_b$ = data (bit) rate
- In decibels:
  $$G_p\text{(dB)} = 10 \log_{10}\!\left(\frac{R_c}{R_b}\right)$$
- Effect on interference and SNR (rule of thumb):
  - After de-spreading, an uncorrelated interferer is reduced by about $G_p$ (in dB) at the detector output.
  - The desired signal’s post-despread SNR improves by roughly $G_p$ (in dB).
- FHSS rule of thumb:
  - Narrowband interference only affects the fraction of time the hop lands on the interfered channel(s); the rest of the hops are clean.