spread spectrum systems introduction, techniques, applications
TRANSCRIPT
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SPREAD SPECTRUM SYSTEMS
INTRODUCTION, TECHNIQUES, APPLICATIONS
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MEMBERS OF THE GROUP
• 2002-E-202– ALI HUSNAIN SARWAR
• 2002-E-203– NAVEED KHAN SHERWANI
• 2002-E-204– JUNAID SOHAIL
• 2002-E-206– MIAN MUHAMMAD BADAR FAROOQ
• 2002-E-207– RANA MUHAMMAND RIZWAN RIAZ
• 2002-E-209– SYED YASIR ALI GILANI
• 2002-E-210– RIZWAN BIN RAFIQUE CHAUDHRY
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BRIEF HISTORY OF SPREAD SPECTRUMTHE FUTURE LIES WITHIN
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DISCOVERY
• No known scientist is credited for the discovery of SS Systems
• American movie star Hedy Lamarr and her pianist George Antheil.
• They discovered the technique using a player piano to control the frequency hops.
• The Germans also experimented with SS during the second World War.
• The concept was again introduced in 1951 and was coined the term
“Spread Spectrum”
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MILITARY BACKGROUND
• Early research and development efforts tried to provide countermeasures for radar.
• Initially used for military purposes only.
• The U. S. Military alone has used SS signals over satellites for at least 25 years.
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INTRODUCTION SPREAD SPECTRUM SYSTEMS
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DEFINITION
• The basic definition of spread spectrum systems is the spreading of the data plus carrier by a pseudo-noise code.
• Wireless communications frequency of the transmitted signal is deliberately varied.
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EXPLANATION
• Pseudo-noise code independent of the information data, is employed as a modulation wave form to “spread” the signal energy
• The purpose of coding is to transform an information signal so that it looks more like noise.
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• The new "spread" signals the same total power. • Spread Spectrum signals do not interfere with co-
existing signals.• Offer a potential for shared spectrum
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To qualify as a SS signal
• The transmitted signal bandwidth is much greater than the information bandwidth.
• Some function other than information being transmitted is employed to determine the resultant transmitted bandwidth.
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How Does SS Work?WORKING
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WORKING
• Injection of corresponding SS code somewhere in the transmitting chain before the antenna.
• Despreading at a point in the receive chain before data retrieval.
• The key to that the same code must be known at both end in advance
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• Due to special codes signals appear wide band and noise-like.
• Spread spectrum signals are hard to detect on narrow
band equipment.
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• Low power spectral density (watts/hz).• The low PSD helps the SS signals with it non interfering
properties.• The correlation receivers: Interference spreads over large bandwidth
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SSS vs other SystemsCOMPARISON
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PROBLEMS OF COMMON WIRELESS SYSTEMS
• In conventional wireless signal frequency does not change with time .
• Constant frequency communication poses two basic problems.
• In conventional wireless signal frequency does not change with time
• Easy to intercept
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ADVANTAGES
• Low power spectral density
• Privacy due to unknown random codes.
• Good anti-jam performance
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No wastage of bandwidth
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Resistance to interception
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Reduction of multi-path
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PSEUDO NOISE
A pseudo-noise (PN) code sequence acts as a noise-like (but deterministic) carrier used for bandwidth spreading of the signal energy.
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PROPERTIES OF PN CODE
• Random, but appearances can be deceptive.• Long time period.• Deterministic, periodical signal that is known to both the
transmitter and receiver. • Statistical properties• These should have a good autocorrelation• The different codes must be orthogonal (having
correlation index least).
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Important PN sequences
• Walsh-Hadamard codes, M-sequences, Gold-codes and Kasami-codes.
• Walsh sequences fall in the first category of orthagonal sequences.
• M-sequences, Gold-codes and Kasami-codes are non orthagonal codes.
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Processing Gain
• The processing gain is equal to the ratio of the chipping frequency to the data frequency.
• G = transmitted BW/data BW.• There are two major benefits from high processing gain
1. Interference rejection: the ability of the system to reject interference is directly proportional to Gp.
2. System capacity: the capacity of the system is directly proportional to Gp.