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Spread Spectrum Communications for WI-FI LANS. By: Rebecca Sheehan. Definition.
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Spread Spectrum Communications for WI-FI LANS By: Rebecca Sheehan
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.”
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
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
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
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
Techniques… • Frequency Hopping • Direct Sequence • Time Hopping
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)
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
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
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
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
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.
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