SPREAD SPECTRUM SYSTEMS

INTRODUCTION, TECHNIQUES, APPLICATIONS

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

BRIEF HISTORY OF SPREAD SPECTRUMTHE FUTURE LIES WITHIN

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”

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.

INTRODUCTION SPREAD SPECTRUM SYSTEMS

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.

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.

• The new "spread" signals the same total power. • Spread Spectrum signals do not interfere with co-

existing signals.• Offer a potential for shared spectrum

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.

How Does SS Work?WORKING

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

• Due to special codes signals appear wide band and noise-like.

• Spread spectrum signals are hard to detect on narrow

band equipment.

• 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

SSS vs other SystemsCOMPARISON

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

ADVANTAGES

• Low power spectral density

• Privacy due to unknown random codes.

• Good anti-jam performance

No wastage of bandwidth

Resistance to interception

Reduction of multi-path

PSEUDO NOISE

A pseudo-noise (PN) code sequence acts as a noise-like (but deterministic) carrier used for bandwidth spreading of the signal energy.

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).

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.

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.