What are spread spectrum techniques and their applications?

Spread spectrum spreads signal over bandwidth much larger than minimum required. DSSS (Direct Sequence Spread Spectrum): multiplies data by high-rate pseudo-random code, spreading bandwidth. FHSS (Frequency Hopping Spread Spectrum): rapidly switches carrier among frequencies in pseudo-random sequence. Benefits: Resistance to narrowband interference, LPI/LPD (low probability of intercept/detection), multiple access (CDMA), and multipath resistance. Applications: GPS (DSSS), Bluetooth (FHSS), military communications, and CDMA cellular. Processing gain = BW_spread / BW_signal improves SNR.