1 / 16

Spread Spectrum Communications for WI-FI LANS

Spread Spectrum Communications for WI-FI LANS. By: Rebecca Sheehan. Definition.

kamala
Download Presentation

Spread Spectrum Communications for WI-FI LANS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Spread Spectrum Communications for WI-FI LANS By: Rebecca Sheehan

  2. Definition “Spread Spectrum is a means of transmission in which the signal occupies a bandwidth in excess of the minimum necessary to send the information; the band spread is accomplished by means of a code which is independent of the data, and a resynchronized reception with the code at the receiver is used for de-spreading and subsequent data recovery.”

  3. Basic Idea of Spread Spectrum • To spread the signal over a wider frequency band than normal so interference from other devices will not have as great an impact • The bandwidth is spread by means of a code which is independent of data. • The receiver synchronizes to the code to recover the data • This is all done while minimizing the impact of interference from other devices

  4. Spread Spectrum • While using spread spectrum means that each transmission utilizes a large amount of the spectrum, this may be compensated for by the interference reduction capability inherent in the use of spread spectrum techniques, so that a considerable number of users may share the same spectral band • Spread Spectrum allows CDMA

  5. History • Spread Spectrum systems have been developed since the mid 1950’s • Originally designed for military use • The first patent was in the 1940’s but the details were held a military secret for years • It wasn’t used for commercial use until after the patent expired • The GPS system is now the largest single spread spectrum system

  6. Benefits of Spreading the Spectrum… • Anti-jamming • Anti-interference • Low probability of intercept • Multiple user random access communications with selective addressing capability • High resolution ranging • Accurate universal timing

  7. Power Spectrum of Data and of Spread Signal

  8. Techniques… • Frequency Hopping • Direct Sequence • Time Hopping

  9. Frequency Hopping • The signal is rapidly switched between different frequencies within the hopping bandwidth pseudo-randomly, and the receiver knows beforehand where to find the signal at any given time • Involves transmitting the signal over a random sequence of frequencies • The random sequence of frequencies is generated by a pseudorandom number generator algorithm • The receiver uses the same algorithm as the sender and it initializes with the same seed – that means it is now able to hop frequencies in sync with the transmitter to correctly receive the frame • Question: Why use frequency hopping? • Answer: Reduces interference (it is unlikely that two signals will use the same frequency)

  10. Generating Pseudo-Random Codes • For each channel the base station generates a unique code that changes for every connection • The base station adds together all the coded transmissions for every subscriber • Subscriber unit generates its own matching code and uses it to extract appropriate signals

  11. Direct Sequence • The digital data is directly coded at a much higher frequency. The code is generated pseudo-randomly, and the receiver knows how to generate the same code, and correlates the received signal with that code to extract the data. • Adds redundancy for greater tolerance of interference • Each bit of data is represented by multiple bits in the transmitted signal • If some of the transmitted bits are damaged by interference, there is usually enough redundancy to recover the original bit • For each bit the sender wants to transmit, what is actually sent is the exclusive-OR (XOR) of that bit and n random bits • The transmitted values (n-bit chipping code), spread the signal across a frequency band that is n times wider than the frame would have otherwise required

  12. Direct Sequence Continued… • Signal Transmission Steps: • A pseudo-random code generated • The information data modulates the pseudo-random code • The resulting signal modulates a carrier • Signal Reception Steps: • Carrier received and amplified • Receiver signal mixed with a local carrier to recover the spread digital signal • Pseudo-random code generated, matching anticipated signal • Receiver acquires received code and phase locks its own code to it • Received signal correlated with generated code, extracting Information data

  13. Example of 4-bit Chipping Sequence

  14. Time Hopping • The carrier is turned on and off by the pseudorandom code sequence • The signal is transmitted in short bursts pseudo-randomly, and the receiver knows beforehand when to expect the burst

  15. Current uses for Spread-Spectrum Communications • Still primarily used for military communications • Also used now for: • Mobile Radio Networks • Timing & Positioning Systems • Some specialized Applications in Satellites, etc.

  16. References • “Theory of Spread Spectrum Communications – A Tutorial”; Picholtz, Shilling, and Milstein; IEEE Transactions on Communication, vol. COM-30, pg 855-884 • Davie, Bruce S. and Peterson, Larry L. Computer Networks: A Systems Approach. 4th ed. New York. Pg 78-79. • http://ss-mag.com/shistory.html • “Spread Spectrum.” http://en.wikipedia.org/wiki//Spread_spectrum.html

More Related