Spread Spectrum

SPREAD SPECTRUM MODULATION

PRESENTED BY SUSHANTA SARKAR 4TH YEAR ETC REG NO.120507004 ID NO.120705304

Need for spread spectrum Spread spectrum modulation was originally developed for military to be used in the battle ground and in the hostile territories where the enemy always tries to intrude into the communication system of the friendly forces to steal information and to jam the systems.

The main objectives of spread spectrum modulation were :• To avoid being detected. • To prevent eavesdropping. • To prevent the jamming of signals.

Characteristics of Spread Spectrum Modulation • They are difficult to intercept by an unauthorized person. • They are easily hidden. • They are resistant to jamming. • They have multiple access capability.

WHAT IS SPREAD SPECTRUM

• A type of modulation in which the modulated signal bandwidth is much greater than the message signal bandwidth. • The spreading of the message signal spectrum is done by a spreading code called Pseudo Noise Code (PN Code) which is independent of the message signal.

Types of Spread Spectrum • Direct Sequence Spread Spectrum (DS/SS) • Frequency Hopping Spread Spectrum (FH/SS) • Time Hopping Spread Spectrum (TH/SS)

Direct Sequence Spread Spectrum Modulation • In DS/SS a binary polar signal representing PN code is used to multiply the message signal which is also represented by a binary polar signal. Data m(t) Sequence

×

S(t)

C(t) PN Code generator

Modulator

Direct Sequence Spread Spectrum

Direct Sequence Spread Spectrum ∑ Y(t)

X(t)

X(t) = transmitter output J(t) = jamming signal Y(t) = X(t) + J(t)

J(t)

X

Y(t)

Low-pass filter

M(t)

C(t) Local PN sequence generator

U(t)

Demodulator

Advantages of DS/SS Modulation • The DS/SS allows greater capacity by allowing multiple access communication. • Several users can occupy the same frequency spectrum simultaneously, and frequency bands can be reused without separation distance of users. • Resistant to multipath fading in ground based mobile radio communication.

Limitations of DS/SS • It suffers from Near-Far problem. • Device limitation imposes restriction in the length of the PN code which in turn imposes limitation on the frequency band over which the spreading takes place. This signal can be jammed by some wide band jammers.

Frequency Hopping Spread Spectrum Modulation •

The type of spread spectrum in which carrier hops randomly from one frequency to another is called Frequency Hop (FH) spread spectrum.

FH/SS are of two types:• Slow Frequency Hopping (SFH) :- In slow frequency hopping the symbol rate of the input signal is an integer multiple of the frequency hopping rate. That is several symbol are transmitted on each frequency hop. • Fast Frequency Hopping (FFH) :- In fast frequency hopping the frequency hopping rate is an integer multiple of the input symbol rate . That is the carrier frequency will change or hop several times during the transmission of the one symbol.

Frequency Hop Spread Spectrum

Binary data

FSK modulator

×

Band pass filter

FH/SS signal

Frequency synthesizer

. . . . .. PN code generator

Modulator

Frequency Hop Spread Spectrum Received Signal

×

Band pass filter

FSK Demodulator

Estimation of binary data

Frequency Synthesizer

.. ... Local PN sequence generator

Demodulator

Frequency Hop Spread Spectrum Frequency selection in FH/SS

Frequency Hop Spread Spectrum FH/SS Cycles

Frequency Hop Spread Spectrum Bandwidth Sharing

Frequency Hop Spread Spectrum Advantages of FH/SS

• FH/SS is less susceptible to NearFar problem then DS/SS. • It can achieve much higher processing gains than DS/SS.

Limitation of FH/SS

• Does not have the same degree of jamming resistance as DS/SS. • SFH has limited multipath immunity.

Application of Spread Spectrum Modulation • • • •

In cellular telephony using CDMA. Global Positioning System. Secured military communication. In satellite television.

References:• www.wikipedia.org • Modern Digital and Analog Communication Systems, B.P Lathi • Communication Systems, Simon Haykins • Data Communication and Networking, Forouzan • www.freepatentsonline.com

Thank You