Spread Spectrum Techniques

Why it matters

Spread Spectrum Techniques are crucial in modern communication systems as they can improve the interference resilience and reliability of data transmission. By spreading the signal over a wide frequency band, these techniques improve resistance to interference, some forms of jamming, making them essential for military and commercial applications.

Key ideas

• Spread Spectrum: A method of transmitting a signal by spreading it over a wide frequency band, much wider than the minimum bandwidth required.
• Types of Spread Spectrum Techniques:
  • Frequency Hopping Spread Spectrum (FHSS): The carrier frequency hops among many frequencies at a fast rate, following a pseudo-random sequence.
  • Direct Sequence Spread Spectrum (DSSS): The original data signal is multiplied by a pseudo-random noise spreading code, spreading the signal over a wider bandwidth.
  • Time Hopping Spread Spectrum (THSS): The signal is transmitted in bursts at pseudo-random time intervals.
• Applications: Examples include DSSS ranging signals and frequency-hopping links. Spreading is not encryption and does not itself guarantee confidentiality.

Formulas

• Processing Gain (G) = Bandwidth of Spread Spectrum Signal (B_s) / Bandwidth of Original Signal (B_o)
  • G: Processing Gain (dimensionless)
  • B_s: Bandwidth of Spread Spectrum Signal (Hz)
  • B_o: Bandwidth of Original Signal (Hz)

Worked example

Given: Original signal bandwidth B_o = 1 kHz, Spread spectrum signal bandwidth B_s = 100 kHz.

1. Calculate Processing Gain

   Formula: G = B_s / B_o

   Substitute values: G = 100,000 Hz / 1,000 Hz

   Calculation: G = 100

   Answer: 100 (dimensionless), or 10log10(100) = 20 dB nominal processing gain. Actual interference suppression depends on synchronization, code properties and the interference model.

Common mistakes

• Confusing FHSS and DSSS: Remember that FHSS involves changing frequencies, while DSSS involves spreading the signal with a code.
• Ignoring the importance of the pseudo-random sequence: Synchronization and code correlation properties affect interference rejection; a spreading sequence alone does not secure the data.
• Miscalculating processing gain by not converting units properly.

For GATE EC

Questions often involve calculating processing gain, understanding the differences between FHSS and DSSS, and their applications. Practice problems on identifying the type of spread spectrum technique used in given scenarios.

Quick check

1. What is the main advantage of using spread spectrum techniques?
2. Name two types of spread spectrum techniques.
3. How is processing gain calculated?

Answers: 1. Resistance to certain interference and jamming conditions; confidentiality requires separate protection. 2. FHSS and DSSS. 3. G = B_s / B_o.