CN105207715A - Chaotic sequence based CDMA (Code Division Multiple Access) access method - Google Patents

Abstract

The invention discloses a chaotic sequence based CDMA access method. The method comprises the following steps that S1) carrier modulation is carried out on information bits sent by each subscriber, and the spectrum is spread via a chaotic spreading code generated by a chaos generator; S2) a transmission filter filters signals after spectrum spreading, and filtered signals are overlapped; and S3) the overlapped signals are emitted to a VLC channel via the LED end for transmission, the transmitted signals are separated, a coupling filter filters the separated signals, and the filtered signals are de-spread and demodulated to obtain original information bits of the subscribers. In the S3) de-spreading is carried out by using the chaotic spreading code generated by the chaos generator in the S1). The method uses the chaotic sequence to replace a traditional optical orthogonal code to realize spectrum spreading, and the method is applied to a VLC system to enhance the security of subscriber access in the VLC process and enable the VLC system to realize safe multi-subscriber access.

Description

A kind of CDMA access method based on chaos sequence

Technical field

The present invention relates to visible light communication field, more specifically, relate to a kind of CDMA access method based on chaos sequence with high security.

Background technology

Compared with traditional radio-frequency (RF) communication system, visible light communication (VisibleLightCommunication, VLC) has high-speed transfer, low consumption, high security and exempts from the advantages such as mandate.VLC system adopts light-emitting diode (LightEmittingDiodes, LED) as light source, transmitting terminal adopts intensity modulation, receiving terminal adopts light intensity detector (Photodiode, PD) direct-detection (IntensityModulationwithDirectDetection, IM/DD) receives information.

Terminal uses all in visible ray WLAN (wireless local area network) shares identical main light source, therefore different subscriber signals must take on a different character, different user signal segmentation could be opened when such adapter receives, namely adopt multiple access technique identification different user.Multiple access technique can support that multi-user communicates simultaneously efficiently, and unduly increases system complexity, thus reduces system Construction and maintenance cost.

Multiple access technique generally includes time division multiple access, frequency division multiple access and code division multiple access mode.When identifying user with the transmission time slot of signal transmission, be called time division multiple access way; When distinguishing user with the carrier frequency difference of signal transmission and setting up multiple access access, be called FDMA; When distinguishing user with the pattern difference of signal transmission and setting up multiple access access, be called code division multiple access mode.Wherein, in code division multiple access mode, user is because of can the simultaneously frequency range of shared system and time interval resource, and compared with other two kinds of modes, power system capacity is larger.

Based in multi-user's visible light communication system of code division multiple access, although the line-of-sight propagation characteristic of visible ray is visible light communication provide better fail safe compared with Radio-Frequency Wireless Communication, but, multi-user under same light beam covers is because of the broadcast characteristic of light beam, and the information propagated is easy to be intercepted and captured by malicious user.Although the existing optical CDMA system based on unipolar code achieves the identification of multi-user, because the address identifier code adopted is easy to be known by malicious user, the secure accessing of multi-user therefore cannot be ensured.

Therefore, propose a kind of multi-user access technology providing high security user to access to be necessary.

Summary of the invention

The present invention, for overcoming at least one defect (deficiency) described in above-mentioned prior art, provides a kind of CDMA access method based on chaos sequence with high security.

For solving the problems of the technologies described above, technical scheme of the present invention is as follows:

Based on a CDMA access method for chaos sequence, comprise the following steps:

S1. the information bit of each user's transmission is through carrier modulation, then carries out spread spectrum with the chaos spread spectrum code produced by chaos generator respectively;

S2. adopt transmission filter to filter the signal after spread spectrum, the signal after filtering is superposed;

S3. the signal after superposition is transmitted into VLC channel by LED end to transmit, is separated, then adopts matched filter to carry out filtering to the signal after transmission, last despread-and-demodulation recovers the original information bits obtaining each user;

In above-mentioned steps S3, despreading is employing is that the chaos spread spectrum code produced by chaos generator in step S1 carries out despreading.

In above-mentioned steps S1, the information bit that user sends after carrier modulation is carried out in Quadrature Phase Shift Keying modulation mapping, then carries out spread spectrum by the chaos spread spectrum code that multiplier and chaos generator produce.Key of the present invention is when carrying out despreading, the chaos spread spectrum code produced by corresponding chaos generator must be adopted to carry out despreading, so just can accurately realize the decoding of information.

This is because chaos phenomenon is a kind of similar random process in non-linear deterministic system, this process bounded but very responsive to initial value.Chaos sequence has extremely strong autocorrelation and extremely weak cross correlation, simultaneously to initial value extreme sensitivity, can be used for the fail safe of raising system.And different mapping modes can produce the demand that a large amount of chaos sequences meets multi-user.

Chaos sequence correlation properties are utilized to have high security to the access user distinguished in VLC system.Noise like due to chaos sequence and the exquisite sensitivity to initial value, therefore only have and legal accept user and could recover initialization information, and eavesdrop user and can not intercept and capture transmission data.

Due to the mode of the IM/DD that optical channel adopts, the signal propagated in optical channel must be non-negative signal, so the bipolar signal of mapping will be changed into the unipolar signal of non-negative before signal transmission enters VLC optical channel; Therefore also need to adopt the method for direct current biasing to make bipolar signal all become non-negative signal to signal after superposition in described step S2.

Further, by the signal power P of VLC channel _rcan be expressed as:

P _r＝[H _los(0)+H _nlos(0)]P _t(1)

Wherein H _losand H (0) _nlos(0) transfer function of light signal direct projection and reflection is represented, P _trepresent average transmission optical signal power, calculated by following formula:

$P_t={\lim }_{T\→\mathrm{\∞}}\frac{1}{2T}{\∫}_{-T}^{T}x\left(t\right)dt---\left(2\right)$

In formula (2): the temporal light intensity value that waveform x (t) that transmits in system is LED light source, know from formula (2), LED instantaneous transmission luminous power can not be negative value, and therefore LED holds waveform input signal to should be non-negative.

Compared with prior art, the beneficial effect of technical solution of the present invention is: The present invention gives a kind of CDMA access method based on chaos sequence being applied to VLC system, the method chaos sequence replaces traditional light orthogonal code to realize the function of spread spectrum, apply it in VLC system, thus enhance the fail safe of user's access in visible light communication process, make VLC system achieve the secure accessing of multi-user.

Accompanying drawing explanation

Fig. 1 is the CDMA-VLC system block diagram that the present invention is based on chaos sequence.

Fig. 2 is visible light communication system reflections propagate link model schematic diagram.

Fig. 3 is the SER performance curve comparison diagram using different spreading code.

Fig. 4 is the performance curve comparison diagram of the different user quantity using chaos sequence.

Fig. 5 is legitimate receipt user and the reception error rates of data comparison diagram eavesdropping user.

Embodiment

Accompanying drawing, only for exemplary illustration, can not be interpreted as the restriction to this patent; In order to better the present embodiment is described, some parts of accompanying drawing have omission, zoom in or out, and do not represent the size of actual product;

To those skilled in the art, in accompanying drawing, some known features and explanation thereof may be omitted is understandable.Below in conjunction with drawings and Examples, technical scheme of the present invention is described further.

The present invention proposes the CDMA access method based on chaos sequence, namely adopts the multiple users in chaos sequence identification visible light communication system, effectively can strengthen the multiple access fail safe of visible light communication system.

The present invention proposes and chaos sequence is used for CDMA-VLC system to realize Multi-user recognition that is efficient and high safety.It is the structure chart of the CDMA-VLC system based on chaos sequence that the inventive method realizes shown in Fig. 1.First the information bit that each user sends maps (QuadraturePhaseShiftKeying, QPSK) by Quadrature Phase Shift Keying modulation, is then multiplied by the chaos spread spectrum code produced by chaos generator and carries out spread spectrum.By transmission filter, optical filter, VLC channel, light concentrator, matched filter, last despread-and-demodulation recovers the original information bits obtaining each user, makes user that chaos sequences different separately can be relied on to carry out Division identification.Due to the mode of the IM/DD that optical channel adopts, the signal propagated in optical channel must be non-negative signal, so the bipolar signal of mapping will be changed into the unipolar signal of non-negative before signal transmission enters optical channel, what adopt here is that the method for adding direct current biasing makes bipolar signal all become non-negative signal.

The performance evaluation of this programme in VLC system adopts general visible light channel model, namely comprise can sighting distance (LineofSight, LOS) and non-can sighting distance (NonLineofSight, NLOS) two type.By the signal power P after optical channel _rcan be expressed as:

P _r＝[H _los(0)+H _nlos(0)]P _t(1)

Wherein H _losand H (0) _nlos(0) transfer function of light signal direct projection and reflection is represented, P _trepresent average transmission optical signal power, calculated by following formula:

$P_t={\lim }_{T\→\mathrm{\∞}}\frac{1}{2T}{\∫}_{-T}^{T}x\left(t\right)dt---\left(2\right)$

In formula: the temporal light intensity value that waveform x (t) that transmits in system is LED light source, as can be seen from formula (2), LED instantaneous transmission luminous power can not be negative value, and therefore LED light source waveform input signal should be non-negative.

The noise of VLC system under bias light impact can regard the superposition of the independently noise variance of some Gaussian distributed as.Get its limiting value and utilize central-limit theorem, interchannel noise can be considered as the Gaussian noise irrelevant with signal.When not considering bias light, the noise of receiver preamplifier is also the Gaussian noise irrelevant with signal.Therefore, the interchannel noise of visible light communication system is regarded as additive white Gaussian noise n (t) irrelevant with signal here.Now Received signal strength y (t) can be expressed as:

$y\left(t\right)=Rx\left(t\right)\⊗h\left(t\right)+n\left(t\right)---\left(3\right)$

R represents photoelectric conversion efficiency; X (t) expression transmits; N (t) represents the additive white Gaussian noise irrelevant with signal, can ignore; H (t) represents that the available pulse that affects of indoor wireless optical communication multipath dispersion responds;

Available pulse response h (t) that affects of indoor wireless optical communication multipath dispersion is described as follows: direct projection link and reflection link,

$h\left(t\right)=\left\{\begin{array}{cc}\frac{2t_0}{t^3{\sin }^2\left(FOV\right)}& t_0\≤t\≤\frac{t_0}{cos\left(FOV\right)}\\ 0& elsewhere\end{array}\right.---\left(4\right)$

In formula, t ₀represent minimal time delay, FOV (FieldofView) represents visual angle, visual field.

For direct projection link, the energy of transmitting can be concentrated into narrow beam and obtain higher through-put power efficiency.Transmitter uses Lambertian radiation model, then channel impulse response is:

$h_{los}\left(t\right)=\frac{A_r(m+1)}{2{\mathrm{\πd}}^2}{\cos }^m\left(\mathrm{\φ}\right)T_s\left(\mathrm{\ψ}\right)g\left(\mathrm{\ψ}\right)cos\mathrm{\ψ}\mathrm{\σ}(t-\frac{d}{c})---\left(5\right)$

Wherein A _rbe the capture area of receiver, ψ is the angle of incident light, and φ is radiative angle.For a typical VLC transmission system, in order to reduce the noise entering receiver, need the wavelength using the non-object region of optical filter filtering, wherein T _s(ψ) gain of optical filter is represented; Make the problem that can not infinitely increase with the effective area that light concentrator effectively can solve receiver in practical application, wherein g (ψ) is the gain of light concentrator, and d is the distance of transmitter and receiver.M is the radiation index of Lambert's model, with the half-power angle Φ of LED light source _1/2it is relevant,

$m=\frac{-ln\left(2\right)}{ln\left({\mathrm{cos\Φ}}_{1/2}\right)}---\left(6\right)$

For reflection link, if consider the independent light source S inside a room and receiver R, then impulse response h _nloscan be expressed as,

$h_{nlos}={\mathrm{\Σ}}_{k=0}^{\mathrm{\∞}}{h}_{nlos}^{\left(k\right)}(t,S,R)---\left(7\right)$

Wherein represent the impulse response after k light reflection, reflecting surface as shown in Figure 2, is divided into by reflection model part, the area of each part is Δ A, and the impulse response after primary event just can be expressed as,

Wherein Φ _sjthe light source angle of departure, Ψ _sjthe angle of light source and pip, Ψ _rjit is the angle of light source and receiver.D _sjthe distance of LED light source and pip, d _rjbe the distance of pip and receiver, reflection coefficient generally gets ρ=0.8.

For simplifying the analysis, reflection impulse response once is only considered, because the impulse response after secondary reflection is almost negligible.

$h\left(t\right)=h_{los}\left(t\right)+h_{nlos}^{\left(1\right)}(t,S,R)---\left(9\right)$

Based on the visible light channel model shown in the visible ray multiple access system shown in Fig. 1 and Fig. 2, the performance of BER of the visible light communication system based on chaos sequence, security performance are analyzed, respectively as shown in Fig. 3, Fig. 4 and Fig. 5.

Fig. 3 gives the symbol error rate (SymbolErrorRate of the access of the multiple access based on the chaos sequence visible light communication system adopting the inventive method, SER) with the performance curve of SNR change, and with unipolarity light orthogonal code (OpticalOrthogonalCodes, OOC) serve as the performance of spreading code, and traditional bipolarity orthogonal sequence M sequence (MaximumLengthSequence) contrasts.As shown in Figure 3, the performance that the chaos sequence (LogisticSequence) that the present invention proposes serves as spreading code in visible light communication is obviously better than the algorithm of OOC.When BER equals 10 ^-4time chaos sequence snr gain ratio " S.-J.Bae; Y.-J.Hong; andK.-S.Lee; " Performanceofvlc-cdmacommunicationsystemusingled; " TheJournalofTheKoreaInstituteofIntelligentTransportSyste ms, vol.8, no.2, pp.83 – 90,2009. " algorithm that proposes will exceed about 3dB.

Fig. 4 carries the performance curve that the symbol error rate that gives different user changes with SNR.Spreading code uses the chaos sequence (LogisticSequence) of 128, as we can see from the figure, when number of users increases, the error rate can increase, this is because along with the increase of number of users, interference between user strengthens, thus causes the performance of BER of system to decline.

Finally, Fig. 5 gives the comparison curves that CDMA-VLC system uses the security performance of different spreading code, as shown in Figure 5, visible light communication system uses the spreading code identification user of 64 M sequence and 64,128 chaos sequences (Logisticsequence) respectively, as can be seen from the figure:

(1) legitimate receipt user can realize proper communication, and malice is eavesdropped user and then can not be obtained user profile.

Due to the producing method of legitimate receipt user known spreading code before Received signal strength, so can recover to have sent information, realize the normal reception of information, and due to the characteristic of chaos sequence pair and initial value extreme sensitivity, even if the information that eavesdropping user has stolen partial spreading code can not the initial value of Exact recovery chaos sequence.So in chaos sequence initial value difference 10 ^-8situation under, eavesdropping user reception error rates of data all close to 0.5.

(2) visible light communication system based on M sequence cannot ensure the fail safe that user accesses.

As we can see from the figure, because M sequence is easy to regeneration, malicious user adopts the M sequence of regeneration, can recover to obtain the transmission information intercepted and captured.On the contrary, based on the present invention program, chaos sequence initial value difference 10 ^-8time, malicious user also cannot recover information.Therefore, chaos sequence is used significantly can to strengthen the security performance of visible light communication.

Obviously, the above embodiment of the present invention is only for example of the present invention is clearly described, and is not the restriction to embodiments of the present invention.For those of ordinary skill in the field, can also make other changes in different forms on the basis of the above description.Here exhaustive without the need to also giving all execution modes.All any amendments done within the spirit and principles in the present invention, equivalent to replace and improvement etc., within the protection range that all should be included in the claims in the present invention.

Claims (3)

1. based on a CDMA access method for chaos sequence, it is characterized in that, comprise the following steps:

S1. the information bit of each user's transmission is through carrier modulation, then carries out spread spectrum with the chaos spread spectrum code produced by chaos generator respectively;

S2. adopt transmission filter to filter the signal after spread spectrum, the signal after filtering is superposed;

S3. the signal after superposition is transmitted into VLC channel by LED end to transmit, is separated, then adopts matched filter to carry out filtering to the signal after transmission, last despread-and-demodulation recovers the original information bits obtaining each user;

In above-mentioned steps S3, despreading is employing is that the chaos spread spectrum code produced by chaos generator in step S1 carries out despreading.

2. the CDMA access method based on chaos sequence according to claim 1, is characterized in that, also needs to adopt the method for direct current biasing to make bipolar signal all become non-negative signal in described step S2 to signal after superposition.

3. the CDMA access method based on chaos sequence according to claim 1, is characterized in that, by the signal power P of VLC channel _rcan be expressed as:

P _r＝[H _los(0)+H _nlos(0)]P _t(1)

Wherein H _losand H (0) _nlos(0) transfer function of light signal direct projection and reflection is represented, P _trepresent average transmission optical signal power, calculated by following formula:

$P_t={\lim }_{T\→\mathrm{\∞}}\frac{1}{2T}{\∫}_{-T}^{T}x\left(t\right)dt---\left(2\right)$

In formula (2): the temporal light intensity value that waveform x (t) that transmits in system is LED light source, know from formula (2), LED instantaneous transmission luminous power can not be negative value, and therefore LED holds waveform input signal to should be non-negative.