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Spread spectrum is a technique used in telecommunications to spread the bandwidth of a signal over a wider frequency range than necessary for the transmission of the information signal. It involves modulating the data signal with a spreading code, which is a pseudo-random sequence of bits. This spreading code is typically much higher in rate than the original data signal. Spread spectrum offers several advantages such as increased resistance to interference, improved security, and better privacy.

There are two main types of spread spectrum techniques:

1. **Frequency Hopping Spread Spectrum (FHSS)**:
   - In FHSS, the carrier frequency of the transmitted signal hops rapidly between different predetermined frequencies within the available bandwidth. This hopping sequence is synchronized between the transmitter and receiver.
   - The hopping pattern is usually determined by a pseudorandom sequence generated by both the transmitter and receiver, which are kept synchronized.
   - FHSS systems are less susceptible to narrowband interference because the signal is only present on any given frequency for a short period of time.

2. **Direct Sequence Spread Spectrum (DSSS)**:
   - DSSS spreads the signal across a wider bandwidth by multiplying the data signal with a higher-rate spreading code.
   - The spreading code is often a pseudorandom sequence that appears random but is deterministic and known to the transmitter and receiver.
   - The spreading code "chips" the signal, multiplying each bit of the original signal by multiple chips. The more chips per bit, the greater the spreading effect.
   - At the receiver, the original data is recovered by multiplying the received signal with the same spreading code used at the transmitter, which effectively "despreads" the signal.

Key features and benefits of spread spectrum:

1. **Interference Resistance**: Spread spectrum signals are less susceptible to interference from other signals or noise because they spread the signal energy over a wide frequency band. This makes spread spectrum communications more robust in environments with significant interference.

2. **Security**: The use of spreading codes makes spread spectrum signals more difficult to intercept and decode by unauthorized parties. Without knowledge of the spreading code, it is challenging to recover the original data from the spread spectrum signal.

3. **Privacy**: Spread spectrum provides a level of privacy because the spread signal appears as noise to receivers not synchronized with the spreading code. Only receivers synchronized with the correct spreading code can recover the original data.

4. **Low Probability of Intercept (LPI)**: Spread spectrum signals have a low probability of detection by unintended receivers due to their spread nature and low power spectral density.

5. **Multipath Fading Mitigation**: Spread spectrum signals are less affected by multipath fading, where signals take multiple paths due to reflections, refractions, or diffractions in the propagation environment. DSSS can combat this effect by spreading the signal energy across multiple frequencies.

Spread spectrum finds applications in various communication systems, including wireless LANs (such as Wi-Fi), Bluetooth, GPS, CDMA cellular systems, and military communications, among others. It provides a versatile and reliable method for transmitting data in challenging environments.