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Advances in Intelligent TechnologiesProceedings of the European Conference of Intelligent Technologies

Iasi, Romania, September 25-28, 2000

Secretly of data communication using the

frequency hopping systems

Anastasiu GabrielEngineer University of Electrical engineering Suceava

[email protected]

In the case of using the classic transmission systems, for data secretly is using frequently the data

codification. In the USA is development one efficiently system named Spread Spectrum.The known system is

Directly Sequence Spread Spectrum.

The advantages of using these systems are:

-Interjamming protection

-Interception reduced capacity

One equivalent system but the very performant is Frequency Hopping System.

In this paper I present you this system in the communication D.S.P application.

Keyword: Spread Spectrum, Frequency Hopping Spread Spectrum, D.S.P, and Data Communication

Introduction

I specified in the abstract some advantage of Spread Spectrum system. Using the Frequency Hopping

System is obtaining more advantage. One of these advantages is an interjamming resistance. In concordance

with Shannon theory, the capacity of one single transmission channel is:

C=Wlog(1+S/N) in bits/sec,

where

C is a channel capacity in bits/sec

W is a bandpass signal in Hz

S is the power of signal in WN is power of noise signal in W/Hz

The Frequency Hopping Spread Spectrum system using the M.S.K.modulation, witch represents oneform of F.S.K modulation. The main advantage of this system is a fast changing of frequency in short time by

the speed named hopping speed. The hopping speed is equal by the P.N code speed. The P.N. code generator

generates the P.N code in the pseudoaleatory sequence. The frequency is changing by the one interval the

other interval of transmission. The report S/N is very good in this case in comparation by the other

communication system in the same condition .To consider the one system in Spread Spectrum:

S/N=10log(S/W) (1)

After distribution to have:

S/N=10log(S/W)-10log(x) (2)

Wherexis the band open factors.Noting by BWRF bandpass of RF in Hz and RINFO the transfer rateof data in bits/sec.Is obtain the one factor named GP (processing gain).

GP=BWRF/RINFO (3)GP is a natijamming caracteristic.

Signal and systems component by the Frequency Hopping Spread Spectrum System

The general schematic of Frequency Hopping Spread Spectrum System is show in fig. 1[1]


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Fig.1

Using the specific circuits can be changing rapidly the frequency using the pseudoaleator code. The bit

M represents the initial bit that contain the information. The m-1 bits are the bit that contains the hopping

frequency information. The P.N generator is using by the generate the pseudoaleator code or (m-1) bits. The

frequency synthesizer is command by the pseudoaleator code. The code contains the information by the

frequency used. The Frequency Hopping wave is show in fig.2.

Fig.2

The frequency hopping wave

By the fig.2 results that RF frequency is periodical changing ,by pseudoaleator code in the order know

only the authorizing receiver.The syncroniztion between receiver equipament and transmiting equipment isachieve by the syncronization signal that is generate by the P.N. generator. In absence of this signal one

intruder receive the noise band signal.

The using modulation is F.S.K or M.F.S.K.The F.S.K. is a verry simple form of modulation by the

FM[2].

This modulation is generate by the switching of fc between f1 and f2 who corespund by the digitalsignal show fig. 3.


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Fig.3

The modulation index h is:

h=(f1-f2)Tbh=fTb

where Tb is time.

In the M.F.S.K case the h is 0.5.The F.S.K signal is one signal by the constant cover and constant

power . Spectrum of F.S.K signal is show in fig. 4:

Fig.4

where:

f1, f2=1/This system bandpass is establish by the number of digital sequence.The minimal time necesarly

changing of frequency depend by the sample rate of information.In Frequency Hopping case time is divide in

frame fig5:

Fig. 5


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By the length of the frame , only one frame contain the real information .The frame forming the data

packet that be selected by the P.N code.From to cause of high speed calculation the elementar circuit is non

indicate the development the system.The solution is D.S.P system .This system contain in internal strucure the

subsystem that to allow the develop the PN generator and eventualy other system.The general structures ofD.S.P is show in fig.6.

Fig.6

P.N generator structure

Using the complex structure of D.S.P ,double by the power of calculation in the floating points can be

generate of P.N code.One schematic is show in fig.7.

Fig. 7

The Core procesor to allowing the real time calculation ,FFT analyse and other operation.[4]


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The solution for P.N generator is showing in fig. 8.

Fig.8

The FFT permiting the using this system in the receiver system.In transmiting case using only ALU

,I/O ,Acquisition and memory bloc.

The sequence what be generate is write in the memory ,in the precise order.This order is know only the

receiver system.The A.L.U selected the m sequence by the memory and transmiting in the acquisition

procesorr.This processor command the sequencer bloc and finaly generate the P.N code.

The other block of D.S.P structure is possible to use by the develop the frequency sinthesizer.

Conclusion

The S.S system presented in this paper is rare using .The Direct Sequence System is verry using in the

data transmission and in other present aplication.The next generation of system will be use the FrequencyHopping or Time Hopping system in data transmision.

References

[1].Sisteme militare de transmisiune, Editura Militara Bucuresti.[2].Glass, J.P.F., DS/FHSS Spread Spectrum technique.[3].Glass, J.P.F., Data syncronzation in Spread Spectrum techique.[4].Glass, J.P.F., System architecture of Frequency Hopping System.

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