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Abstract: Spread spectrum signaling technique is used for security purpose in wireless network because it becomesdifficult to access the wireless network by unwanted users. Weare developing the algorithm to retrieve the PN sequence &pattern of spectrum in spread spectrum system. This algorithmwill increase the wireless network security & solve the problemof accessing the wireless network in secured way. Because,when only sender end knows the PN sequence & pattern ofspread spectrum communication system the network securitywill be enhanced.Spread spectrum is a means of transmission in which datasequence occupies a much more bandwidth than minimumrequired bandwidth necessary to send it. The spectrumspreading at transmitter & de-spreading at receiver isobtained by PN sequence which is independent of datasequence. There are two types of the spread spectrumtechniques direct sequence spread spectrum & frequencyhopped spread spectrum. Direct sequence spread spectrum istechnique in which data sequence directly modulates thepseudo noise sequence known to only transmitter & receiver.Frequency hopping means to transmit data in differentfrequency slots.

Keywords: Direct sequence spread spectrum (DSSS), frequencyhopping spread spectrum (FHSS), hopping pattern recognition(HPR), pseudo-noise sequence (PN sequence), local areanetwork (LAN), initial frequency detection (IFD), remaining

frequency detection (RFD), compute hopping pattern (CHP).

I. INTRODUCTION

Spread spectrum technique has low probability ofinterception and anti jamming capability. Therefore spreadspectrum technique has been utilized widely in defensecommunications. The spread spectrum communication has

been used in many Ad hoc networks, for example, cell phone wireless LAN (local area network), GPS (global positioning system). Ad hoc networks are dynamiccollections of self organizing mobile nodes with links that

are changing in an unpredictable way. Because the Ad hocnetwork does not have any fixed infrastructure such asstations or routers, they are highly applicable for emergencydeployments disasters search and rescue missions &military operations.

For secure communication in a hostile environment,requirement of the system such that transmitted data cannoteasily detected or easily recognized by unwanted listeners.This requirement is fulfilled by the kind of signalingtechniques altogether known as spread spectrum methods or

techniques. The merit of spread spectrum communicationsystem is its potential to eliminate interference. Theinterference may be the unconscious interference by anyuser, who is concurrently trying to transmit through thechannel. The interference may be conscious by anunfavorable transmitter trying to block the transmission.

In combating to the interference (conscious interference),it is important for communicators that jammer who is tryingto disrupt the communication does not have priorknowledge of signal characteristics except for the overallchannel bandwidth & the modulation type. The spectrumspreading is accomplished before transmission by usingcode that is independent of the data sequence . The signal istransmitted over low average power. The data may behidden in background noise by spreading its bandwidth withthe coding. Due to its low power level, the transmittedsignal is said to be covert. The data is demodulated by theintended receiver from same code which is not known toothers. The spread spectrum systems are of two types directsequence spread spectrum & frequency hopped spreadspectrum.

In direct sequence spread spectrum (DSSS) the binary data

is directly multiplied with the PN sequence at transmitterside. The direct sequence spread spectrum is like whitenoise. The amplitude & thus power in direct sequencespread spectrum signal is same as in original signal [13].Due to the increased bandwidth of direct sequence spreadspectrum signal the power spectral density must be lower.After dispreading the received signal with the same PNsequence data is obtained. The properties of the directsequence spread spectrum are efficient modulation, broadmodulation bandwidth, continuous transmission, quicksynchronization, low power spectral density minimizesinterference.

Frequency hopping spread spectrum (FHSS) istechnique in which carrier frequencies changes randomly ina predetermined way known only to the transmitting andreceiving devices [5]. Frequency hopping means to transmitdata in different frequency slots. The total B.W of theoutput signal is equal to sum of all these frequency slots orhops. The properties of the frequency hopping spreadspectrum are simple modulation, narrowband, discontinuoustransmission, and more network overhead.

In this paper we are taking the advantages of both spreadspectrum techniques. If the interferer is within the spreading

band, then the direct sequence spread spectrum (DSSS)

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system can tolerate and completely reject it while thefrequency hopping spread spectrum (FHSS) system can becompletely jammed on that channel. For a large out-of-bandinterferer, the opposite is true. The DSSS process issensitive to such interferers, where the FHSS system is not.

We are developing a novel method to detect frequencyhopping pattern & retrieve the PN code. This algorithm willincrease the wireless network security & solve the problemof accessing the wireless network in secured way. As, onlysender end knows the pattern of frequency hopping insteadof both. Therefore in this manner the network security will

be enhanced. First consider that PN code used is same inDS-SS & FH-SS. The data signal is spread by using PNcode. Then, this signal is modulated using M-ary FSK,which is again modulated with spread spectrum carrierfrequencies or hops randomly to generate the widebandsignal. The hopping pattern is pseudo-random across a set ofknown frequencies. After the current pattern is exhausted anew iteration occurs to allow the rest of the message to betransmitted. Finding the sequence refers to finding the

starting frequency and the following frequencies in thecorrect order. Then retrieve the PN code for decoding theDS-SS (direct sequence spread spectrum) signal. Thendecode the DS-SS signal by using that retrieved PN code.

II. SYSTEM DESCRIPTION

In this paper we are explaining the dual encryptiontransmission of spread spectrum. And then decrypt thatencrypted data without any prior knowledge of encryptingPN sequence. First data sequence is directly encrypted byPN sequence in direct sequence spread spectrum (DSSS).Then this encrypted data is modulated & its modulation isagain encrypted by PN sequence in frequency hoppingspread spectrum (FHSS). As data is encrypted two times thedata is more secure from the unwanted user. To describe indetails the operations of spread spectrum detection & PNsequence retrieval, the simplified block diagram of atransmitter & receiver for spread spectrum multiple accesssystem is illustrated in Fig. 1 & Fig. 2.

Fig.1. Transmitter block diagram

Fig.2. Receiver block diagram

Here, in this block diagram of transmitter first we the datasequence b(n) is directly multiplied with the PN sequencec(t) in product modulator or modulator. The output of themodulator is wide spectrum signal m(N). The spectrum ofthis signal is quite high compared to that of narrow banddata signal b(n).

Then, this signal m(N) is applied to the M-ary FSK

modulator. The modulator output is particular frequencydepending upon the input symbol. The output of themodulator is applied to mixer. The other input of the mixeris the particular frequency from frequency synthesizer. Theoutput of the frequency synthesizer at particular instant isfrequency slot or hop. The output of the mixer isDS/FH/M-ary FSK signal & is transmitted over thewideband channel.

The frequency hops given to mixer are generated by thefrequency synthesizer. The input of the frequencysynthesizer is controlled by the pseudo-noise (PN) sequencegenerator. Thet successive bits of PN sequence generatorcontrol the frequency hops generated by synthesizer. Sincethe bits of the PN sequence generator change randomly, thefrequency hops generated also change randomly. Since t

bits PN sequence controls frequency hops, there will bedistinct 2 t frequency hops generated. The total bandwidthof the output signal is very high.

The received DS/FH/ MFSK signal is applied to hopping pattern recognition (HPR) block. This block consists of thefunctional stages initial frequency detection (IFD),remaining frequency detection (RFD), compute hopping

pattern (CHP). The goal of the detection device is to detectthe HP (hopping pattern) without any prior knowledge aboutthe HP used by the sender at the receiver. The HP ofwireless network will be known using this block withoutany prior knowledge, about the HP used by the sender at the

receiver.Hopping pattern (HP) may consist of two to m randomfrequencies. Each frequency may exist only once. AssumeH.P is F with frequencies F 0, F 1, F 2,.., F x. To detect that afrequency being transmitted clear channel assessment isrequired. It requires 50 s to indicate busy. To recognizeHopping Pattern formula is expressed as:

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Here d x is the dwell time of frequency F x. Only onefrequency will be detected by detection device at a time.This detection occurs by calling the CCA procedure. Todetect the specific frequency transmitted currently by clearchannel assessment transmission should be of complete test

period. When on transmission stops on the detectedfrequency before test period then, the correct frequency maynot be available. This failure when the frequency hop is lessthan 50 s can overcome in stage six. The output of thethird stage is then given to the decision device, then to thefourth stage M-ary FSK detector. The detector detects the

particular symbol transmitted. In the FH/M-ary FSK theindividual frequency of smallest duration is called chip.

Now, at the output of the M-ary FSK detector we get the N bit symbol in parallel. From these N bits t bit LSBs arethen transferred to the PN sequence register r(t). These arethe PN sequence for that N bit symbol. Then the N bitsymbol is converted to the serial bit stream by parallel toserial converter. Then this N serial bits are applied to themixer with thet bit LSBs. The output of the mixer is the

data signal b(n). Thist bit LSBs are then used to generatethe frequency hops by frequency synthesizer. This is fed tothe mixer with the input signal & compared with the outputof HPR algorithm in decision block. The output of the bothHPR algorithm & the mixer should be same. If thefrequency hop is lesser than the 50 s than HPR algorithmfails. Then the output of the mixer is passed to the M-aryFSK detector.

III. SPREAD SPECTRUM PATTERN & PN SEQUENCERETRIEVAL

Earlier analysis on non-coherent frequency-hoppedmultiple access (FHMA) systems assume that: 1) a single-stage detector is employed by the receiver and 2) randommemory less FH patterns (users addresses) are used.3) They also assume that a chip (hop)-synchronous mode ismaintained, i.e., all users switch their carrier frequenciessimultaneously. Here the Spread spectrum pattern & PNsequence retrieval algorithm to detect hopping pattern &retrieve PN sequence in wireless ad hoc network will bedeveloped. The algorithm consists of six stages forrecognizing the hopping pattern & retrieving PN sequence.Each stage is linked with other stage, such that the firststage output is taken as the input to the next stage. But theoutput of last stage is compared with the output of thirdstage. This method will perform in six stages: Stage 1Initial Frequency Detection (IFD), locatesthe first frequency in the network. Stage 2 Remaining Frequencies Detection (RFD),detects the remaining frequencies. Stage 3 Compute Hopping Pattern (CHP),calculates the entire hopping pattern. Stage 4 Demodulate the M-ary FSK signal. Stage 5 Retrieve the PN sequence & decode data. Stage 6 Demodulate the input & compare.

The objective of the first stage is to detect one particularfrequency that is transmitting, regardless of its location inthe HP. The second stage is to detect the rest of thefrequencies and use a time-based marker to associate themwith the first frequency. The third stage takes thefrequencies and the time markers from the second stage andcalculates the order of the frequencies and the individualdwell times. Fourth stage demodulates the M- ary FSKsignal & generates the N bit data at its output. Fifth stageretrieves the PN sequence & gives the original data b(n).Sixth stage again demodulates the input data stream &compares with output of third stage to avoid the error if thehop is less than 50 s .

A. Initial frequency detection algorithm (IFD)

The initial frequency detection stage (IFD) is used toidentify the first frequency in F , denoted as F o. This methodis used to scan through all the frequencies, and if nofrequency is found scan all the frequencies again. When F o

is found then it is passed to the second stage. This stage isdescribed as follows:

Let the frequency 1 is set to c. F or 50 s scan the F c. After channel recognition record F o= F c. Increment c and move to stage 2.Come to stage 1 if

c = 79.

B. Remaining frequencies detection (RFD)

When the IFD is complete & passes F o to the RemainingFrequencies Detection (RFD). The RFD records whenfrequencies stops transmission with respect to the end of the

Fo signal. This stage is described as follows: Start timer from 0 & test continuously to find F o. Increment the timer & begin new scan Fscan1. Record the timer related with the end of transmission

in the respective tx position, when frequency found isnot equal to F o.

Test continuously to find it, whenever F o is found.Record that timer duration as the timer 1 and reset it.

If F o is found or timer surpassed the period exit thestage. Increment the timer & begin new scan F scan2.

Record the timer related with the end of transmissionin the respective tx position, when frequency found isnot equal to F o.

C. Compute hopping pattern (CHP)

The output of the RFD fed to the CHP i.e the array of tx.Then calculate the hopping patterns hierarchy and the dwelltimes. This stage works as follows: Classify the tx array of input. For F o compute dwell time. Compute the remaining dwell times.

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We classify the array of tx depending on the timers in CHP.

D. Demodulate the M-ary FSK signal

In this stage the output of the third stage is fed to thenon-coherent M-ary FSK detector [12]. The output ofthis detector consists of the N bit symbol or data.

E. Retrieve the PN sequence & decode the data

In this stage the PN sequence is retrieved from N bitsymbol by transferring bit LSBs to the register r(t). Thisstage is described as follows:

The t bit LSBs of N bit symbol are transferred toregister r(t) of t bit.

This t bit LSBs are PN code for that symbol. Then the N bit parallel data are converted into the

serial data. Hence bit LSBs are used to decode that N bit data.

F. Demodulate the input & compare

In this stage the incoming FH/ MFSK is againdemodulated by the frequency hops generated by theretrieved PN sequence & compare it with the output ofFHPR block. This stage is described as follows:

The t bit LSBs in r(t) are used to generate thefrequency hops as in transmitter.

These hops are used to demodulate the FH/ MFSKsignal.

The demodulated signal is compared with the output ofthird stage.

If output of third stage is same as sixth stage thenretrieved PN code is correct otherwise not.

If the hop is less than the 50 s then for that instancestarting three stage fails & the output of sixth stage will

be considered.

IV. CONCLUSION

The proposed spread spectrum pattern & PN sequenceretrieval algorithm will provide more security to thewireless AD HOC network. The algorithm has six stages:initial frequency detection (IFD), remaining frequenciesdetection (RFD), compute hopping pattern (CHP),demodulate the M-ary FSK signal, retrieve the PN sequence& decode data, demodulate the input & compare. Our futurework in this field will include multiple pattern detection,

adaptive detection, and multiple interference reduction.

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