SPREAD SPECTRUM COMMUNICATION SCHEMES

MATLAB IMPLEMENTATION

AIM AND NEED

AIM

Weaknesses of present schemes

• Conservation of bandwidth

• Good spectral density

• Concentrated spectrum easy to detect and intercept

• Low redundancy makes signal susceptible to jamming

INITIAL USAGE

The spread spectrum technologies were initially developed for the military and intelligence communities to overcome interception and jamming.

WORKING

Digital (telecommunication) signal is transmitted on a bandwidth considerably larger than the frequency content of the original information.

For FIXED TRANSMISSION POWER, a broader spectrum implies:

i. Lower power signalii. High spectral redundancy

Low signal power makes the communication signals difficult to be intercepted and detected

High spectral density makes signals resistant to partial band-jamming

BASIC CHARACTERISTICS

Carrier is an unpredictable/ “pseudo-random” wide-band signal

Bandwidth of carrier is much wider than the bandwidth of the data modulation

Reception is accomplished by cross-correlation of the received wide-band signal with a synchronously generated replica of the wide-band carrier

Pseudorandom- Seemingly random but reproducible

Except wideband FM, conventional comm systems

multiplicity factor = 1

SS systems multiplicity factor in

thousands

Lower power level per format

SS SYSTEM

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HOPPING SPREAD SPECTRUM

DIRECT SEQUENCE SPREAD SYSTEM

FREQUENCY HOPPING SPREAD SPECTRUM

SYSTEMS(FHSS)

FHSS SYSTEMS, PROPERTIES, PERFORMANCE INDICATORS,

APPLICATIONS AND SIMULINK

INTRODUCTION

FHSS uses a narrow band signal (less than 1 MHz), which modulates a data signal causing it to ‘hop’ in random and the hopping happens in a pseudorandom ‘predictable’ sequence in a regular time from frequency to frequency which is synchronized at both transmitting and receiving ends.

DEVIANCE FROM THE REGULARThe main difference now is that the carrier frequency varies over intervals in accordance to a predetermined pseudo-random pattern. The evasive signal so generated takes a broader spectrum and becomes difficult to jam and intercept.

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PREFERENCE OF M-ary FSK MODULATION SCHEME

Though PAM, PSK and QAM schemes are more efficient, M-ary modulation is used due to its less complex non-coherent detection abilities

PN hopping pattern demands the reciever to maintain phase coherence with every one of the frequencies used in the hopping pattern.

FSK is allowed as it allows non-coherent detection

APPLICATIONS

GSM Cellular systems

WLAN

WiFi NetworkWPAN

Bluetooth

MATLAB CODE AND GRAPHS