Spread Spectrum1

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Spread Spectrum

Introduction


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Parameter Disain Sistem

energi

bandwitdh

Tahan terhadap interferensi

Beroperasi pada spektral energi yg rendah memungkinkan multiple access tanpa

external control

Menanggulangi eaves-dropper


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2 Kriteria Utama SS1. Bandwidth yg digunakan untuk

mentransmisikan sinyal akan jauh lebihbesar dari sinyal itu sendiri

2. Bandwidth yg ditransmisikan harusindependent terhadap sinyal informasi,

tetapi harus mengikuti suatu fungsisehingga dapat dimengerti oleh penerima.

Bandwidth expansion in spread spectrum systemsis achieved by using a function that is independentof the message, thus it is more susceptible to

white noise as opposed to other communicationtechniques, such as FM and PCM


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Spread Spectrum

Militer Menyebar bandwidth sinyal agar tidak mudah

dideteksi Jamming & Anti jamming In the end, it can be

stated that the best anti-jamming is simply good

engineering design and the spreading of the operatingfrequencies

Low Probability Intercept

Sipil Menyebar bandwidth sinyal untuk deteksi sinyal

khusus dan agar memungkinkan dilakukan

peningkatan kapasitas kanal (multiple access) GPS

CDMA


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SEJARAH (1)

Radar Techniques mid-1920's: birth of RADAR

1940: chirp pulse radar, patented by Prof. E. Huttman(Germany)

1945: chirp pulse radar, R. H. Dickie (USA)

mid-1940's: formulation of the matched filter concept early 1950's: P.M. Woodward, resolution, accuracy, and

ambiguity properties of pulse waveforms finally wereplaced on a sound theoretical basis

1938: among Gunella's approximately 100 patents isone containing all the technical characteristics of an SR-

SS radar.


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SEJARAH (2)

Communication Theory 1930: Norbert Wiener dari MIT menemukan

"Generalized Harmonic Analysis". Wiener penemupemodelan probabilitas aliran informasi pada sistemkomunikasi dan kontrol.

Mulai thn 1947: Shanon mengembangkancommunication theories. Shannon's and Wiener's theoriesstimulate the Institute of Radio Engineers (IRE) to form theProfessional Group on Information Theory, which begun publishing in

1953.

Nathan Marchand dan Louis deRosa, ketua dariIRE, memegang peranan penting dlm

pengembangan SS systems.


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SEJARAH (3)

1942: A 26-year-old engineer, Nathan Marchand,working for ITT's Federal Telephone and Radio Corporation in New

York, patented first band-pass correlator.

1947: Lee, Wiesner and Cheatham of M.I.T. developed

first high performance electronic correlators. They alsoreported applications of correlation techniques to detectionproblems.

1924: Alfred Goldsmith, one of the founders of IRE, fileda patent being spread spectrum in nature. His patentdescribed a system using a form of FM-SS transmission.


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SEJARAH (4) 1935: German engineers, Paul Kotowski and KurtDannehl applied for a patent on a device used formasking voice signals by combining them with anequally broad-band noise signal produced by arotating generator. Receiver had another rotatinggenerator which was synchronized and used inorder to recover the noisy voice signal. This

invention was a starting point in the developmentof DS-SS communication systems.

World War II: Henri Businger, inventor ofmoving-target indicator for radars, together withLouis deRosa applied for a on facsimilecommunication system with anti-jamming

capabilities. It was a time wobbling system whichwas an early relative of modern TH-SS systems.


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SEJARAH (5) World War II: Many missile guidance

systems were developed, i.e. radarcontrolled glide bombs. Many ofthese systems were protected from

jamming by SS-like systems. WHYN (Wobbulated HYperbolic

Navigation) was developed bySylvania during mid 1940's. Thisnavigation system was used toguide missiles and was one of thefirst TR-FM-SS systems.


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SEJARAH (6) : Shannon dan SS

"It is hard to picture the world before Shanon as it seemedto those who lived in it. In the face of publications nowknown and what we now read into them, it is difficult torecover innocence, ignorance, and lack of understanding. Itis easy to read into earlier work a generality that came only

later." J.R. Pierce The developments such as Shannon's theories and other

research mentioned earlier contributed to the emergence ofSpread Spectrum as it is known today. Although spreadspectrum systems were being developed for military, andunder secrecy, there was still some exchange of

information between different researchers. Many spreadspectrum systems emerged during that time. Some of themare WHYN, noise wheels, and NOMAC.


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SEJARAH (7) Another example of SS systems is a full-fledged SR-SS

single-sideband communication system based on

Rogoff's noise wheels. Patent for this invention wasfiled in March, 1953.

One of the ITT engineers: Mortimer Rogoff, an avidphotographic hobbyist, conceived of usingphotographic techniques to store a noise-like signaland for building an ideal cross correlator. Rogoff

prepared sheets of film whose transmitivity variedlinearly in both directions, thus creating a mask. Atevery point (X,Y) on the mask, transmissioncharacteristics were proportional to the product XY.The two signals would then be correlated by usingthem as the X and Y inputs to the oscilloscope, reading

the light emitted from the masked oscilloscope facewith a photo-multiplier, and low-pass filtering theresultant output.


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SEJARAH (8)

Extensive studies of spread spectrum techniques weredone at MIT and MIT's Lincoln Laboratories. Much ofthe research was performed by graduate students,guided by Group Leaders Fano and Davenport. Thetechniques under study were named NOise ModulationAnd Correlation or NOMAC. The first of NOMAC's

systems was used for comparison of the performancesof transmitted- and stored-reference systemsoperating in the presence of broad-band Gaussiannoise. Research done at M.I.T led to development ofseveral TR-SS systems [1].


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SEJARAH (9)

Many spread spectrum technologies have beendeclassified since World War II and are nowavailable for commercial use. Commercialapplications of spread spectrum have attractedconsiderable attention because of its possible use

for CDMA. Spread spectrum also reducesdetrimental effects of multipath. Today, bothvoice and data oriented systems employ spreadspectrum systems. Modern applications of spreadspectrum range from low speed fire safetydevices to high speed wireless local areanetworks [7].


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SS Performance1. Low probability of intercept (LPI) can be achieved with

high processing gain and unpredictable carrier signalswhen power is spread thinly and uniformly in thefrequency domain, making detection against noise by thesurveillance receiver difficult. A low probability of positionfix (LPPF) attribute goes one step further in including bothintercept and direction finding (DFing) in its evaluation.Low probability of signal exploitation (LPSE) may includeadditional effects, e.g., source identification, in addition to

intercept and DFing.2. Anti-jam (AJ) capability can be secured with an

unpredictable carrier signal. The jammer cannot use signalobservations to improve its performance in this case, andmust rely on jamming techniques which are independentof the signal to be jammed.

3. High time resolution is attained by the correlation

detection of wide-band signals. Differences in the time ofarrival (TOA) of the wide-band signal, on the order of thereciprocal of the signal bandwidth, are detectable. Thisproperty can be used to suppress multipath and, by thesame token, to render repeater jammers ineffective.


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SS Performance (cont)

4. Transmitter-receiver pairs using independentrandom carriers can operate in the same bandwidthwith minimal co-channel interference. Thesesystems are called spread-spectrum code-divisionmultiple-access (CDMA) systems.

5. Cryptographic capabilities result when the datamodulation cannot be distinguished from the carriermodulation, and the carrier modulation is effectivelyrandom to an unwanted observer. In this case theSS carrier modulation takes on the role of a key in acipher system. A system using indistinguishable

data and SS carrier modulation is a form of privacysystem.


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Teknik SS

1. Direct sequence (pseudo-noise)

DS/CDMA

2. Frequency hopping.

WIFI/bluetooth

3. Time hopping. WIFI

4. Chirp.

RADAR

5. Hybrid methods.


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Direct Sequence SS

Pseudonoise (PN) Spread Spectrum

It can be assumed that the information signal inDS SS transmission is spread at baseband, andthe spread signal is then modulated in a second

stage. By using this approach the modulation isseparate from the spreading and "stretching"operation and the baseband spreading and"stretching" can be discussed separately. At thereceiver, the signal is first demodulated and then

"stretched" back to recover original information.


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DS-SS System


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DSSS


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Transmitter DS-SS (1)


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Transmitter DS-SS (2)


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Konsep Spreading dg DS


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PN Generator (1)


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PN-Code Generation (2)


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Spreading Results


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Ilustrasi Spektrum


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Photo Spektrum DS-SS

A Spectrum Analyzer Photo of a Direct Sequence (DS) Spread Spectrum signal.


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Konsep AutoKorelasi (1)

Ideally, the spreading code should be designedso that the chip amplitudes are statisticallyindependent of one another. The entire period ofPN sequence consists of N time chips. In case ofmaximal linear PN generator, the value on N is

(2^n)-1, where n is the number of stages in thecode generator. Another important reason forusing PN generator to modulate a signal are theproperties of the resulting signal's autocorrelationfunction. It has a maximal value of one repeatingitself every period, and a constant value of -(1/N)

in between the peaks :


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Konsep AutoKorelasi (2) The autocorrelation function will be very small in the regions

between the peaks if N is very large. By using shift register codesillustrated earlier PN sequence can be made very large. Forexample, if the clock frequency is 10MHz, which means that chiptime is 10^-7 seconds, and the shift register with 41 stages isused, the resulting is: N = 2^41-1=2.199x10^12, N*Tc=N*10^-7=2.199x10^5 seconds or 2.545 days .

The mathematical properties of the PN sequences play animportant role in the DS systems. A PN sequence consists of aseries of plus or minus ones. It must possess certain specifiedautocorrelation properties. Aperiodic sequences that meet thecriteria of pseudo-randomness are called Barker sequences andare known to exist only for short sequences oflength=1,2,3,4,5,7,11 and 13. For this reason such sequences are

typically too short for appropriate spreading of the signals (fordetailed discussion of aperiodic signals refer to [1]). In generalonly periodic sequences are of interest to the designer of DS SSsystems.


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Fungsi autokorelasi

Spread Spectrum1

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