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SS (spread spectrum)

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Spread Spectrum (SS) refers to a technique used in telecommunications and wireless communication systems to spread the signal over a wide frequency band. The purpose of spread spectrum is to improve the reliability, security, and efficiency of data transmission in various applications, including wireless networks, satellite communication, cellular systems, and military communication.

The fundamental principle of spread spectrum is to spread the transmitted signal over a larger bandwidth than the minimum required for its information content. This spreading is achieved by modulating the signal with a pseudorandom noise (PN) sequence, also known as a spreading code or spreading sequence. The spreading code is a binary sequence that appears random but has specific mathematical properties. The same code is used at the receiver to de-spread the signal and recover the original information.

There are several types of spread spectrum techniques, including Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). Both techniques use different approaches to achieve signal spreading.

  1. Frequency Hopping Spread Spectrum (FHSS): FHSS involves rapidly changing the frequency of the carrier signal according to a predetermined hopping sequence. The transmitter and receiver synchronize their hopping sequences to ensure proper communication. The frequency hopping pattern may change several times per second. By hopping over multiple frequency channels, FHSS provides resistance to interference, noise, and intentional jamming. It also allows multiple users to share the same frequency band without significant mutual interference.
  2. Direct Sequence Spread Spectrum (DSSS): DSSS spreads the signal by multiplying the original data signal with a high-rate pseudorandom noise code. The noise code consists of a sequence of chips, where each chip represents multiple bits of data. The chip rate is significantly higher than the original data rate. By spreading the signal over a wide bandwidth, DSSS provides resistance to narrowband interference, fading, and multipath effects. It also allows multiple users to coexist in the same frequency band by using different spreading codes.

DSSS further distinguishes between two subcategories: Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK). BPSK uses a single phase shift to represent one bit of information, while QPSK uses four different phase shifts to represent two bits of information per symbol. QPSK offers higher spectral efficiency compared to BPSK.

Benefits of Spread Spectrum:

  1. Resistance to Interference: Spread spectrum techniques provide robustness against various types of interference, such as narrowband interference, multipath fading, and intentional jamming. The wide bandwidth and spreading codes help mitigate the effects of interference, ensuring reliable communication.
  2. Security: The random-like nature of the spreading codes used in spread spectrum makes it difficult for unauthorized users to intercept or decode the transmitted signal. This property provides a level of security in wireless communication systems.
  3. Multiple Access Capability: Spread spectrum allows multiple users to share the same frequency band without significant interference. Each user can use a different spreading code, enabling multiple access and increasing the capacity of the communication system.
  4. Low Probability of Intercept (LPI): Spread spectrum signals have a low probability of being detected by unintended receivers due to their low power spectral density. This feature is advantageous in military and covert communication applications.

Applications of Spread Spectrum:

  1. Wireless Networks: Spread spectrum is widely used in wireless local area networks (WLANs), such as Wi-Fi, to provide reliable and secure wireless communication. The use of spread spectrum helps to mitigate interference from other wireless devices and improves the overall performance of the network.
  2. Cellular Systems: Spread spectrum is an essential component of modern cellular systems, including 2G, 3G, and 4G. It enables multiple users to share the available frequency spectrum efficiently, increases resistance to interference, and improves the overall quality of service.
  3. Satellite Communication: Spread spectrum is utilized in satellite communication systems to enhance signal quality, reduce interference, and provide secure and reliable communication links.
  4. Military Communication: Spread spectrum plays a crucial role in military communication systems, providing resistance to interception, jamming, and improving overall system security.

In summary, spread spectrum is a valuable technique in telecommunications and wireless communication systems. It spreads the signal over a wider bandwidth using pseudorandom noise codes, providing resistance to interference, enhancing security, and enabling efficient multiple access. Its applications range from wireless networks and cellular systems to satellite communication and military applications.