What is SS (spread spectrum)

Spread Spectrum (SS) Explained in Detail

Spread spectrum (SS) is a communication technique used in various applications to transmit information over a wider bandwidth than the actual data itself. This wider bandwidth offers several advantages, including:

• Increased Resistance to Interference: The spread signal is less susceptible to interference from other signals occupying the same frequency band.
• Improved Security: By spreading the information across a wider band, it becomes more difficult for unauthorized receivers to intercept and understand the data.
• Reduced Power Spectral Density: Spreading the signal lowers its power density in any particular frequency range, making it less likely to interfere with other users of the spectrum.

Core Principle of Spreading:

• In traditional narrowband communication, the information signal occupies a relatively small portion of the frequency spectrum.
• Spread spectrum, however, utilizes a "pseudo-random" code to spread the information signal over a much wider bandwidth. This pseudo-random code is a complex, noise-like signal that has specific statistical properties.

Types of Spread Spectrum:

There are three main categories of spread spectrum techniques, each with its own characteristics:

1. Direct Sequence Spread Spectrum (DSSS):
   • In DSSS, the information signal is multiplied by the pseudo-random code in the time domain. This results in a spread signal with a wider bandwidth and a lower power spectral density.
   • The receiver uses a replica of the same pseudo-random code to de-spread the signal and recover the original information.
2. Frequency Hopping Spread Spectrum (FHSS):
   • In FHSS, the carrier frequency of the information signal is rapidly switched between a set of available frequencies according to a pseudo-random code sequence.
   • The receiver needs to be synchronized with the hopping pattern to know which frequency to listen to at any given time.
3. Time Hopping Spread Spectrum (THSS):
   • In THSS, the information signal is transmitted in short bursts at specific time slots within a frame. These time slots are determined by a pseudo-random code sequence.
   • The receiver needs to be synchronized with the hopping pattern to know which time slots to listen in on to receive the complete information.

Applications of Spread Spectrum:

Spread spectrum finds applications in various domains due to its unique properties:

• Cellular Networks: CDMA (Code Division Multiple Access) used in cellular networks like 3G and some 4G variants employs DSSS for increased capacity and interference resistance.
• Wireless Local Area Networks (WLANs): Wi-Fi (IEEE 802.11) utilizes DSSS in some variations for improved co-existence with other devices.
• Bluetooth: Bluetooth technology also employs a form of FHSS for short-range wireless communication.
• GPS: The Global Positioning System leverages a form of CDMA to transmit navigation signals with high resistance to jamming.
• Military Communication: Spread spectrum plays a vital role in military communication for its security and anti-jamming capabilities.

Benefits and Trade-offs:

• Spread spectrum offers significant advantages in terms of interference resistance, security, and efficient spectrum utilization.
• However, the wider bandwidth usage can also lead to increased complexity in receiver design and potential efficiency trade-offs compared to traditional narrowband communication.

Additional Notes:

• The specific choice of spread spectrum technique and its parameters depends on the specific application requirements and the regulatory environment.
• Modern spread spectrum systems often combine elements of different techniques, like using DSSS with frequency hopping for added security and flexibility.