CN101958871A - Adaptive free space optical communication (FSO)-orthogonal frequency division multiplexing (OFDM) transmission system and transmission method - Google Patents

Abstract

The invention discloses an adaptive free space optical communication (FSO)-orthogonal frequency division multiplexing (OFDM) transmission system. The transmission system comprises an adaptive orthogonal frequency division multiplexing (AOFDM) transmission part, an optical up-conversion module, an optical down-conversion module and an AOFDM receiving part which are sequentially connected. An adaptive FSO-OFDM transmission method of the invention has the advantages of expanding a carrier deviation estimation range and simultaneously solving problems on the estimation of carrier deviations under the condition of inconsistency of each carrier deviation caused by the frequency-selective fading characteristic of an atmosphere channel by adopting a high-order cumulant-based carrier blind-synchronization algorithm, providing an effective way for improving a transmission rate and reducing the bit error rate of the system for free space optical communication by realizing the modeling of the optimal adaptive FSO-OFDM transmission system with maximum transmission capacity, and providing novel methods and theoretical bases for further improving the transmission efficiency of the adaptive system and improving system performance by adopting high-order cumulant-based blind channel estimation and blind signal detection technology.

Description

A kind of self adaptation FSO-OFDM transmission system and transmission method

Technical field

The invention belongs to communication technical field, be specifically related to a kind of self adaptation FSO-OFDM transmission system, the invention still further relates to this transmission system is platform, utilizes the Higher Order Cumulants information of received signal to realize that accurately blind Channel Estimation, Blind Source detect and the transmission method of the blind estimation of sub-carrier frequencies.

Background technology

Free space optical communication (FSO) is to realize point-to-point as information carrier, in the space or put a kind of technology of multiple spot message transmission with light beam.This technology has become a big research focus of current information technology owing to have that cost is low, networking flexibility, need not advantage such as frequency permission.Though free space optical communication is by people's extensive concern, also there are some problem demanding prompt solutions in it: 1. transmission rate is limited.2. atmospheric channel disturbs.In order to give full play to the potentiality of FSO, support the higher rate of information throughput and improve characteristic of BER in System, necessary employing spectrum efficiency height, the novel transmission technology that Chinese People's Anti-Japanese Military and Political College's gas effect capability is strong.OFDM (OFDM) is a kind of multicarrier transmission mode in essence, it be digital stream with a high speed serial transmission be converted into a plurality of on subchannel the rate data streams of parallel transmission, therefore possess availability of frequency spectrum height, antijamming capability is strong and a series of characteristics such as bandwidth favorable expandability, obtaining application widely aspect the short-distance wireless access, it is the competitive key technology of next generation mobile communication system, apply it to the bottleneck that to break through restriction optical communication development among the FSO, improve the rate of information throughput and the reliability of optical communication.

The research of OFDM technology in the wireless optical field just just begins, the researcher is simple in order to realize, all adopt fixing modulation system for all subcarriers, and utilize the insertion pilot frequency information to carry out channel estimating and estimating carrier frequencies, will comprise a lot of supplementarys in the system like this, cause the loss of system power and information rate, thereby reduced the efficiency of transmission of system.Simultaneously, in free-space optical communication system, because related is at random atmospheric channel, its channel capacity is a stochastic variable with time variation, to utilize channel capacity to greatest extent, the transmission rate that must make information also is an amount with the channel volume change, even the transmitting terminal modulation system has adaptive characteristic.The purpose of OFDM modulation technique is that the broad-band channel that will have frequency selectivity is divided into several smooth narrowband subchannels, one of advantage that it is maximum is exactly can be according to the actual transmissions situation of each subchannel allocation of transmit power and information bit flexibly, thereby utilizes channel capacity to greatest extent.Therefore self adaptation OFDM modulation technique (AOFDM) is combined with FSO, rationally set up the FSO-AOFDM system model, can give full play to the advantage of technology separately, utilize channel capacity to greatest extent according to characteristics separately.

Summary of the invention

The purpose of this invention is to provide a kind of self adaptation FSO-OFDM transmission system, solved existing transmission system and all adopted fixing modulation system, the problem that the efficiency of transmission of system is low for all subcarriers.

Another object of the present invention provides the transmission method of above-mentioned transmission system.

The technical solution adopted in the present invention is, a kind of self adaptation FSO-OFDM transmission system, comprise the AOFDM radiating portion, light up-conversion module, light down conversion module and the AOFDM receiving unit that connect successively, the AOFDM radiating portion comprises Channel Modeling module, Adaptive Modulation module, serial to parallel conversion module a, IFFT conversion module, loading protection interval module, parallel serial conversion module a and the D/A converter module that connects successively, and the AOFDM radiating portion is connected with light up-conversion module by D/A converter module; The AOFDM receiving unit comprises successively the carrier frequency synchronization module, analog-to-digital conversion module, channel estimation module, the serial to parallel conversion module b that connect, goes to protect interval module, FFT conversion module, parallel serial conversion module b, signal detection module and adaptive demodulation module; the AOFDM receiving unit is connected with the light down conversion module by the carrier frequency synchronization module; light up-conversion module and light down conversion module are carried out transfer of data by plasmas channel, and the Adaptive Modulation module also is connected with channel estimation module.

Another technical scheme of the present invention is, a kind of self adaptation FSO-OFDM transmission method is specifically implemented according to following steps:

Step 1: build the FSO-OFDM system model with monte carlo method,, set up channel model, the information of channel model is passed to the Adaptive Modulation module according to the transmission characteristic of FSO-OFDM system model;

Step 2: the Adaptive Modulation module is carried out Adaptive Modulation according to the modulation system of selecting to the information source information of input according to the Information Selection modulation system of the channel model that obtains, and obtains modulated signal;

Step 3: the modulated signal that step 2 is obtained passes to serial to parallel conversion module a and carries out conversion, obtains the modulated signal parallel data stream;

Step 4: the modulated signal parallel data stream that step 3 is obtained passes to the IFFT conversion module to carry out contrary Fourier and changes, and obtains ofdm signal;

Step 5: the ofdm signal that step 4 is obtained passes to loading protection interval module, loads protection at interval;

Step 6: the loading protection ofdm signal at interval that step 5 is obtained passes to parallel serial conversion module a and carries out conversion, obtains the ofdm signal serial data stream;

Step 7: the ofdm signal serial data stream that step 6 is obtained passes to D/A converter module and carries out digital-to-analogue conversion, obtains analog information;

Step 8: the analog information that step 7 is obtained passes to light up-conversion module and carries out the light up-conversion, obtains modulated optical signal, by transmission antennas transmit in plasmas channel;

Step 9: the modulated optical signal that light down conversion module receiving step 8 is transmitted in the plasmas channel, carry out the light down-conversion and recover the signal of telecommunication;

Step 10: give the carrier frequency synchronization module with the electrical signal transfer that step 9 obtains, carry out carrier wave frequency deviation and correct the OFDM symbol after obtaining correcting;

Step 11: the OFDM symbol after the correction that step 10 is obtained passes to analog-to-digital conversion module, carries out analog-to-digital conversion, the digital signal of the OFDM symbol after obtaining correcting;

Step 12: the digital signal transfers of the OFDM symbol after the correction that step 11 is obtained is given channel estimation module, digital signal according to the OFDM symbol after correcting, channel condition information is carried out blind estimation, obtain channel condition information, the digital signal transfers of the OFDM symbol after will correcting is on the one hand then given serial to parallel conversion module b, on the other hand channel condition information is fed back to simultaneously the Adaptive Modulation module, the Adaptive Modulation module is selected next modulation system of each subcarrier constantly according to channel condition information;

Step 13: the digital signal of the OFDM symbol after the correction that serial to parallel conversion module b will obtain is carried out serial to parallel conversion, and the parallel data stream of the OFDM symbol after obtaining correcting passes to and protects interval module;

Step 14: the parallel data diffluence protection of the OFDM symbol after going to protect interval module to the correction that obtains passes to the FFT conversion module at interval;

The parallel data stream of the OFDM symbol after the correction after step 15:FFT conversion module is protected at interval to the removal that obtains carries out the FFT conversion, obtains parallel modulation signal, passes to parallel serial conversion module b;

Step 16: the parallel modulation signal that parallel serial conversion module b will obtain carries out parallel serial conversion, obtains serial modulation signal, passes to signal detection module;

Step 17: signal detection module is judged the modulation system that the Adaptive Modulation module is selected according to the serial modulation signal that receives, and passes to the adaptive demodulation module;

Step 18: the modulation system that the adaptive demodulation module is selected according to the Adaptive Modulation module that obtains, carry out adaptive decoding, recover the information source information of input.

Characteristics of the present invention also are,

Selecting modulation mode in step 2 and the step 12 wherein, implement according to following rule: under specific bit error rate demand condition, if the threshold value of these five kinds of modulation system correspondences of BPSK, QPSK, 8PSK, 16QAM and 64QAM is respectively: R1, R2, R3, R4 and R5, make SNR represent the instantaneous signal-to-noise ratio of channel, then the selective rule of modulation system is: during R1≤SNR＜R2, select the BPSK modulation system; R2≤SNR＜R3 selects the QFSK modulation system; R3≤SNR＜R4 selects 8PSK modulation mode; R4≤SNR＜R5 selects the 16QAM modulation system; R5≤SNR＜R6 selects the 64QAM modulation system.

Wherein the analog information that in the step 8 step 7 is obtained passes to light up-conversion module and carries out the light up-conversion, obtain modulated optical signal, implement according to following steps: at first, the analog information signal of telecommunication that step 7 is obtained is decomposed into real part, the imaginary part two path signal removes to modulate MZM respectively, obtains two ways of optical signals; Then, again with two ways of optical signals respectively by the orthogonal polarizer of the direction of shaking thoroughly, obtain two bunch polarised lights, through behind the bundling device, the elliptically polarized light that obtains having the certain elliptical angle of polarization.

The modulated optical signal that light down conversion module receiving step 8 is transmitted in the plasmas channel in the step 9 wherein, carry out the light down-conversion and recover the signal of telecommunication, implement according to following steps: at first, to be divided into two paths of signals through beam splitter by the light signal that optical antenna collects, one the tunnel enters horizontal analyzer, and one the tunnel enters vertical analyzer; Then, measure the light intensity P of horizontal polarization and vertical polarization respectively by photo-detector _∥With P _⊥At last, the angle of polarization of the elliptically polarized light that receives by calculating is recovered the information of information source.

Wherein the electrical signal transfer that in the step 10 step 9 is obtained is given the carrier frequency synchronization module, and carry out carrier wave frequency deviation and correct, the OFDM symbol after obtaining correcting, implement according to following steps:

Suppose ofdm signal of x (t) expression, receiving terminal receives and be expressed as by the atmospheric channel interference signals:

$y\left(t\right)=x\left(t\right)+n\left(t\right)=\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{A}_i{\left(t\right)e}^{j({2\mathrm{\πf}}_{i}t+{\mathrm{\θ}}_i+{\mathrm{\φ}}_i)}+n\left(t\right),$

A wherein _i(t), f _i, θ _iWith

Expression respectively: the amplitude of i way carrier wave, frequency, baseband phase, initial phase deviation, n (t) represents additive white Gaussian noise; The quadravalence circulative accumulation amount of the OFDM that the definition receiving terminal receives is:

$C_{40}^a\left(\mathrm{0,0,0}\right)={<x^4\left(t\right)e^{-\mathrm{<figure-callout\; id="8"\; label="j"\; filenames="HSA00000274793700011.png"\; state="\{\{state\}\}">j</figure-callout>}8\mathrm{\&pi;at}}>}_t-3{<x^2\left(t\right)e^{-j4\mathrm{\&pi;at}}>}_t^2,$

Wherein＜ _tExpress time is average;

After obtaining the Higher-Order Cyclic cumulant of the ofdm signal that receiving terminal receives, detect normalization Higher-Order Cyclic cumulative amount again

Value, if be not equal to zero, be sub-carrier frequencies then, if equal zero then be not sub-carrier frequencies.

The wherein blind estimation in the step 12, implement according to following steps:

The ofdm signal that the definition receiving terminal receives is independent identically distributed, then can obtain the estimated value of channel parameter according to the character of convolution algorithm and Higher Order Cumulants:

$h\left(\mathrm{\&tau;}\right)=\frac{C_4({\mathrm{\&tau;}}_1,\mathrm{\&tau;},L-1)}{C_3({\mathrm{\&tau;}}_1,L-1)}\&CenterDot;\frac{C_3^2(L-\mathrm{1,0})}{C_3(L-1,L-1)}\&CenterDot;\frac{C_4(L-1,L-\mathrm{1,0})}{C_4^2(L-\mathrm{1,0,0})},$

τ wherein, τ ₁∈ [0, L-1] is for each fixing τ, τ ₁Value when to L-1, changing from 0, obtain L corresponding h (τ), utilize least-squares estimation again, promptly obtain the estimated value of channel parameter.

Wherein signal detection module is judged the modulation system that the Adaptive Modulation module is selected according to the serial modulation signal that receives in the step 17, implements according to following steps:

The detected characteristics vector of definition signal cumulant is:

S＝[|C ₄₀/C ₄₂|，|C ₄₁/C ₄₂|]，

Wherein, { C _4n, n=0,1,2} is the 4 rank cumulants of signal x (t), is defined as: C ₄₁=M ₄₁-3M ₂₁M ₂₀,

Wherein, M _IjEach rank square of representation signal, according to following formula, the detected characteristics vector value of average power normalized signal and the detected characteristics vector value of received signal on the theory of computation are passed through to calculate the Euclidean distance of two vectors at last, thereby are carried out Modulation Identification respectively.

The invention has the beneficial effects as follows,

(1) the optimization self adaptation FSO-OFDM transmission system modeling of realization maximum transfer capacity is that free space optical communication improves transmission rate, and improving system's error performance provides a kind of valid approach;

(2) adopt based on the blind synchronized algorithm of the carrier wave of Higher Order Cumulants, can enlarge the carrier deviation estimation range, can solve the estimation problem of selecting carrier deviation under the inconsistent situation of caused each carrier deviation of fading characteristic by the atmospheric channel frequency simultaneously;

(3) adopt blind Channel Estimation and Blind Source detection technique, for further improving the Adaptable System efficiency of transmission, improving systematic function new method and theoretical foundation are provided based on Higher Order Cumulants.

Description of drawings

Fig. 1 is the structural representation of self adaptation FSO-OFDM transmission system of the present invention;

Fig. 2 is the flow chart of the transmission method of self adaptation FSO-OFDM transmission system of the present invention.

Among the figure; 1. Channel Modeling module; 2. Adaptive Modulation module, 3. serial to parallel conversion module a, 4.IFFT conversion module; 5. load the protection interval module; 6. parallel serial conversion module a, 7. D/A converter module, 8. smooth up-conversion module; 9. light down conversion module; 10. carrier frequency synchronization module, 11. analog-to-digital conversion modules, 12. channel estimation modules; 13. serial to parallel conversion module b; 14. go to protect interval module, 15.FFT conversion module, 16. parallel serial conversion module b; 17. signal detection module, 18. adaptive demodulation modules.

Embodiment

The present invention is described in detail below in conjunction with the drawings and specific embodiments.

The structure of self adaptation FSO-OFDM transmission system of the present invention, as shown in Figure 1, comprise the AOFDM radiating portion, light up-conversion module 8, light down conversion module 9 and the AOFDM receiving unit that connect successively, the AOFDM radiating portion comprises Channel Modeling module 1, Adaptive Modulation module 2, serial to parallel conversion module a3, IFFT conversion module 4, loading protection interval module 5, parallel serial conversion module a6, the D/A converter module 7 that connects successively, and the AOFDM radiating portion is connected with light up-conversion module 8 by D/A converter module 7; The AOFDM receiving unit comprises successively the carrier frequency synchronization module 10, analog-to-digital conversion module 11, channel estimation module 12, the serial to parallel conversion module b13 that connect, goes to protect interval module 14, FFT conversion module 15, parallel serial conversion module b16, signal detection module 17 and adaptive demodulation module 18, and the AOFDM receiving unit is connected with light down conversion module 9 by carrier frequency synchronization module 10.Light up-conversion module 8 and light down conversion module 9 are carried out transfer of data by plasmas channel.Adaptive Modulation module 2 also is connected with channel estimation module 12.

The transmission method of self adaptation FSO-OFDM transmission system of the present invention, as shown in Figure 2, specifically implement according to following steps:

Step 1: Channel Modeling module 1 is according to the characteristics of FSO and OFDM, rationally build the FSO-OFDM system model with monte carlo method, according to the transmission characteristic of FSO-OFDM system model, set up channel model, the information of channel model is passed to Adaptive Modulation module 2;

Step 2: the information of the channel model that Adaptive Modulation module 2 bases obtain is determined the choice mechanism of different modulating mode, according to choice mechanism the information source information of input is carried out Adaptive Modulation, obtains modulated signal; Specifically implement according to following steps:

According to the channel model of having set up, the channel capacity of research FSO-OFDM communication system, and the errored bit performance of calculating different modulating mode, the signal-noise ratio threshold of selection different modulating mode.Adopt the method for coming selecting modulation mode according to channel signal to noise ratio size in the present invention, the criterion of this method is in the channel model that step 1 has been built, satisfies certain target bit, recently determines the change-over gate limit value of modulation system according to noise.

The selectable modulation system of FSO-OFDM has: BPSK, QPSK, 8PSK, 16QAM and 64QAM, and under specific bit error rate demand condition, every kind of all corresponding signal-noise ratio threshold value of modulation system.If the threshold value of these five kinds of modulation system correspondences is respectively: R1, R2, R3, R4 and R5, make SNR represent the instantaneous signal-to-noise ratio of channel, then the selective rule of modulation system is: during R1≤SNR＜R2, select the BPSK modulation system; R2≤SNR＜R3 selects the QPSK modulation system; R3≤SNR＜R4 selects 8PSK modulation mode; R4≤SNR＜R5 selects the 16QAM modulation system; R5≤SNR＜R6 selects the 64QAM modulation system;

Step 3: the modulated signal that step 2 is obtained passes to serial to parallel conversion module a3 and carries out conversion, obtains the modulated signal parallel data stream;

Step 4: the modulated signal parallel data stream that step 3 is obtained passes to IFFT conversion module 4 to carry out contrary Fourier and changes, and obtains ofdm signal;

Step 5: in order to improve system's antijamming capability, the ofdm signal that step 4 is obtained passes to loading protection interval module 5, loads protection at interval;

Step 6: the loading protection ofdm signal at interval that step 5 is obtained passes to parallel serial conversion module a6 and carries out conversion, obtains the ofdm signal serial data stream;

Step 7: the ofdm signal serial data stream that step 6 is obtained passes to D/A converter module 7 and carries out digital-to-analogue conversion, obtains analog information;

Step 8: the analog information that step 7 is obtained passes to light up-conversion module 8 and carries out the light up-conversion, obtains modulated optical signal, and in plasmas channel, embodiment is by transmission antennas transmit:

Because the signal that step 7 obtains is the signal of plural form, when carrying out light intensity modulation, receiving terminal can only be described out the amplitude of light vector, phase information can't be described out, the electric light conversion that therefore need utilize light polarization to modulate to realize signal, here (Mach-Zehndermodulation MZM) comes the real part of difference performing step 7 gained signals and the electric light conversion of imaginary part to need to use two Mach-Zehnder modulators.

At first, the step 7 gained signal of telecommunication is decomposed into real part imaginary part two path signal removes to modulate MZM respectively, obtain two ways of optical signals; Then, again with it respectively by the orthogonal polarizer of the direction of shaking thoroughly, obtain two bunch polarised lights, will be elliptically polarized light through the light beam behind the bundling device with certain elliptical angle of polarization.

Step 9: light down conversion module 9 receives to go up and goes on foot the modulated optical signal that is transmitted in the plasmas channel, carries out the light down-conversion and recovers the signal of telecommunication, and embodiment is:

At first, will be divided into two paths of signals through beam splitter by the light signal that optical antenna collects, the one tunnel enters horizontal analyzer, and one the tunnel enters vertical analyzer; Then, measure the light intensity P of horizontal polarization and vertical polarization respectively by photo-detector _∥With P _⊥At last, the angle of polarization of the elliptically polarized light that receives by calculating is recovered the information of information source.

Step 10: give carrier frequency synchronization module 10 with the electrical signal transfer that step 9 obtains, carry out carrier wave frequency deviation and correct, the OFDM symbol after obtaining correcting, specific implementation method is:

Because the high availability of frequency spectrum and the transmission reliability of FSO-OFDM system be prerequisite with quadrature between subcarrier all,, therefore sub-carrier frequencies is accurately estimated it is unusual important problem even less deviation also can make demodulation performance sharply descend.In order to improve system transmissions efficient and to select the unfixed phenomenon of the caused subcarrier spacing of fading characteristic in order to overcome the atmospheric channel frequency, the present invention adopts the frequency based on each subcarrier of method direct estimation of high-order cumulative amount.

Suppose ofdm signal of x (t) expression, receiving terminal receives and be expressed as by the atmospheric channel interference signals:

$y\left(t\right)=x\left(t\right)+n\left(t\right)=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{A}_i{\left(t\right)e}^{j({2\mathrm{\&pi;f}}_{i}t+{\mathrm{\&theta;}}_i+{\mathrm{\&phi;}}_i)}+n\left(t\right)---\left(1\right)$

A wherein _t(t), f _i, θ _iWith

Expression respectively: the amplitude of i way carrier wave, frequency, baseband phase, initial phase deviation, n (t) represents additive white Gaussian noise.

Because white Gaussian noise is zero greater than the circulative accumulation amount on 2 rank, the quadravalence circulative accumulation amount that therefore defines the OFDM that receiving terminal receives is:

$C_{40}^a\left(\mathrm{0,0,0}\right)={<x^4\left(t\right)e^{-\mathrm{<figure-callout\; id="8"\; label="j"\; filenames="HSA00000274793700011.png"\; state="\{\{state\}\}">j</figure-callout>}8\mathrm{\&pi;at}}>}_t-3{<x^2\left(t\right)e^{-j4\mathrm{\&pi;at}}>}_t^2---\left(2\right)$

Wherein＜ _tExpress time is average.

After obtaining the Higher-Order Cyclic cumulant of the ofdm signal that receiving terminal receives, detect normalization Higher-Order Cyclic cumulative amount again

Value, if be not equal to zero, be sub-carrier frequencies then, if equal zero then be not sub-carrier frequencies.

Step 11: the OFDM symbol after the correction that step 10 is obtained passes to analog-to-digital conversion module 11, carries out analog-to-digital conversion, the digital signal of the OFDM symbol after obtaining correcting;

Step 12: the digital signal transfers of the OFDM symbol after the correction that step 11 is obtained is given channel estimation module 12, digital signal according to the OFDM symbol after correcting, channel condition information is carried out blind estimation, obtain channel condition information, the digital signal transfers of the OFDM symbol after will correcting is on the one hand then given serial to parallel conversion module b13, on the other hand channel condition information is fed back to simultaneously Adaptive Modulation module 2, Adaptive Modulation module 1 is according to channel condition information, according to the modulation system benchmark in the step 2, the modulation system of next each subcarrier of the moment of choose reasonable.

Blind estimation specific implementation method is:

The ofdm signal that the definition receiving terminal receives is independent identically distributed, then can obtain the estimated value of channel parameter according to the character of convolution algorithm and Higher Order Cumulants:

$h\left(\mathrm{\&tau;}\right)=\frac{C_4({\mathrm{\&tau;}}_1,\mathrm{\&tau;},L-1)}{C_3({\mathrm{\&tau;}}_1,L-1)}\&CenterDot;\frac{C_3^2(L-\mathrm{1,0})}{C_3(L-1,L-1)}\&CenterDot;\frac{C_4(L-1,L-\mathrm{1,0})}{C_4^2(L-\mathrm{1,0,0})}---\left(3\right)$

τ wherein, τ ₁∈ [0, L-1] is for each fixing τ, τ ₁Value when to L-1, changing from 0, obtain L corresponding h (τ), utilize least-squares estimation again, can obtain the estimated value of channel parameter.

Step 13: the digital signal of the OFDM symbol after the correction that serial to parallel conversion module b13 will obtain is carried out serial to parallel conversion, and the parallel data stream of the OFDM symbol after obtaining correcting passes to and protects interval module 14;

Step 14: the parallel data diffluence protection of removing to protect the OFDM symbol after 14 pairs of correction that obtain of interval module passes to FFT conversion module 15 at interval;

The parallel data stream of the OFDM symbol after the correction after the removal protection at interval that 15 pairs of step 15:FFT conversion modules obtain carries out the FFT conversion, obtains parallel modulation signal, passes to parallel serial conversion module b16;

Step 16: the parallel modulation signal that parallel serial conversion module b16 will obtain carries out parallel serial conversion, obtains serial modulation signal, passes to signal detection module 17;

Step 17: signal detection module 17 judges that according to the serial modulation signal that receives which kind of modulation system is Adaptive Modulation module 2 select for use, passes to adaptive demodulation module 18.Specific implementation method is:

The detected characteristics vector of definition signal cumulant:

S＝[|C ₄₀/C ₄₂|，|C ₄₁/C ₄₂|] (4)

Wherein, { C _4n, n=0,1,2} is the 4 rank cumulants of signal x (t), is defined as:

C ₄₁=M ₄₁-3M ₂₁M ₂₀, Wherein, M _IjEach rank square of representation signal.

According to (4) formula, the detected characteristics vector value of average power normalized signal and the detected characteristics vector value of received signal on the theory of computation are passed through to calculate the Euclidean distance of two vectors at last, thereby are carried out Modulation Identification respectively.

Step 18: the modulation system that adaptive demodulation module 18 is selected for use according to the Adaptive Modulation module 2 that obtains, carry out adaptive decoding, recover original source information.

Self adaptation FSO-OFDM transmission system of the present invention and transmission method, according to optical communication and OFDM modulation technique characteristics separately, on the basis of original FSO-OFDM system, design a kind of self adaptation FSO-OFDM system transmissions system model, and be that platform utilizes received signal Higher Order Cumulants information to realize that accurately blind Channel Estimation, Blind Source detect and the blind estimation of sub-carrier frequencies with this model.Finally hand over the frequency multiplexing technique development for light positive and lay the first stone, also for free space optical communication improves transmission rate, improving systematic function provides a kind of valid approach.

Claims (8)

1. self adaptation FSO-OFDM transmission system, it is characterized in that, comprise the AOFDM radiating portion, light up-conversion module (8), light down conversion module (9) and the AOFDM receiving unit that connect successively, described AOFDM radiating portion comprises Channel Modeling module (1), Adaptive Modulation module (2), serial to parallel conversion module a (3), IFFT conversion module (4), loading protection interval module (5), parallel serial conversion module a (6) and the D/A converter module (7) that connects successively, and the AOFDM radiating portion is connected with light up-conversion module (8) by D/A converter module (7); Described AOFDM receiving unit comprises the carrier frequency synchronization module (10) that connects successively; analog-to-digital conversion module (11); channel estimation module (12); serial to parallel conversion module b (13); go to protect interval module (14); FFT conversion module (15); parallel serial conversion module b (16); signal detection module (17) and adaptive demodulation module (18); the AOFDM receiving unit is connected with light down conversion module (9) by carrier frequency synchronization module (10); described smooth up-conversion module (8) and light down conversion module (9) are carried out transfer of data by plasmas channel, and described Adaptive Modulation module (2) also is connected with channel estimation module (12).

2. self adaptation FSO-OFDM transmission method, it is characterized in that, adopt a kind of self adaptation FSO-OFDM transmission system, its structure is: comprise the AOFDM radiating portion that connects successively, light up-conversion module (8), light down conversion module (9) and AOFDM receiving unit, described AOFDM radiating portion comprises the Channel Modeling module (1) that connects successively, Adaptive Modulation module (2), serial to parallel conversion module a (3), IFFT conversion module (4), load protection interval module (5), parallel serial conversion module a (6) and D/A converter module (7), the AOFDM radiating portion is connected with light up-conversion module (8) by D/A converter module (7); Described AOFDM receiving unit comprises the carrier frequency synchronization module (10) that connects successively; analog-to-digital conversion module (11); channel estimation module (12); serial to parallel conversion module b (13); go to protect interval module (14); FFT conversion module (15); parallel serial conversion module b (16); signal detection module (17) and adaptive demodulation module (18); the AOFDM receiving unit is connected with light down conversion module (9) by carrier frequency synchronization module (10); described smooth up-conversion module (8) and light down conversion module (9) are carried out transfer of data by plasmas channel; described Adaptive Modulation module (2) also is connected with channel estimation module (12)

Specifically implement according to following steps:

Step 1: build the FSO-OFDM system model with monte carlo method,, set up channel model, the information of channel model is passed to Adaptive Modulation module (2) according to the transmission characteristic of FSO-OFDM system model;

Step 2: Adaptive Modulation module (2) is carried out Adaptive Modulation according to the modulation system of selecting to the information source information of input according to the Information Selection modulation system of the channel model that obtains, and obtains modulated signal;

Step 3: the modulated signal that step 2 is obtained passes to serial to parallel conversion module a (3) and carries out conversion, obtains the modulated signal parallel data stream;

Step 4: the modulated signal parallel data stream that step 3 is obtained passes to IFFT conversion module (4) to carry out contrary Fourier and changes, and obtains ofdm signal;

Step 5: the ofdm signal that step 4 is obtained passes to loading protection interval module (5), loads protection at interval;

Step 6: the loading protection ofdm signal at interval that step 5 is obtained passes to parallel serial conversion module a (6) and carries out conversion, obtains the ofdm signal serial data stream;

Step 7: the ofdm signal serial data stream that step 6 is obtained passes to D/A converter module (7) and carries out digital-to-analogue conversion, obtains analog information;

Step 8: the analog information that step 7 is obtained passes to light up-conversion module (8) and carries out the light up-conversion, obtains modulated optical signal, by transmission antennas transmit in plasmas channel;

Step 9: the modulated optical signal that light down conversion module (9) receiving step 8 is transmitted in the plasmas channel, carry out the light down-conversion and recover the signal of telecommunication;

Step 10: give carrier frequency synchronization module (10) with the electrical signal transfer that step 9 obtains, carry out carrier wave frequency deviation and correct the OFDM symbol after obtaining correcting;

Step 11: the OFDM symbol after the correction that step 10 is obtained passes to analog-to-digital conversion module (11), carries out analog-to-digital conversion, the digital signal of the OFDM symbol after obtaining correcting;

Step 12: the digital signal transfers of the OFDM symbol after the correction that step 11 is obtained is given channel estimation module (12), digital signal according to the OFDM symbol after correcting, channel condition information is carried out blind estimation, obtain channel condition information, the digital signal transfers of the OFDM symbol after will correcting is on the one hand then given serial to parallel conversion module b (13), on the other hand channel condition information is fed back to simultaneously Adaptive Modulation module (2), Adaptive Modulation module (2) is selected next modulation system of each subcarrier constantly according to channel condition information;

Step 13: the digital signal of the OFDM symbol after the correction that serial to parallel conversion module b (13) will obtain is carried out serial to parallel conversion, and the parallel data stream of the OFDM symbol after obtaining correcting passes to and protects interval module (14);

Step 14: the parallel data diffluence protection of the OFDM symbol after going to protect interval module (14) to the correction that obtains passes to FFT conversion module (15) at interval;

The parallel data stream of the OFDM symbol after the correction after step 15:FFT conversion module (15) is protected at interval to the removal that obtains carries out the FFT conversion, obtains parallel modulation signal, passes to parallel serial conversion module b (16);

Step 16: the parallel modulation signal that parallel serial conversion module b (16) will obtain carries out parallel serial conversion, obtains serial modulation signal, passes to signal detection module (17);

Step 17: signal detection module (17) is judged the modulation system that Adaptive Modulation module (2) is selected according to the serial modulation signal that receives, and passes to adaptive demodulation module (18);

Step 18: the modulation system that adaptive demodulation module (18) is selected according to the Adaptive Modulation module (2) that obtains, carry out adaptive decoding, recover the information source information of input.

3. self adaptation FSO-OFDM transmission method according to claim 2, it is characterized in that, selecting modulation mode in described step 2 and the step 12, implement according to following rule: under specific bit error rate demand condition, if the threshold value of these five kinds of modulation system correspondences of BPSK, QPSK, 8PSK, 16QAM and 64QAM is respectively: R1, R2, R3, R4 and R5, make SNR represent the instantaneous signal-to-noise ratio of channel, then the selective rule of modulation system is: during R1≤SNR＜R2, select the BPSK modulation system; R2≤SNR＜R3 selects the QPSK modulation system; R3≤SNR＜R4 selects 8PSK modulation mode; R4≤SNR＜R5 selects the 16QAM modulation system; R5≤SNR＜R6 selects the 64QAM modulation system.

4. self adaptation FSO-OFDM transmission method according to claim 2, it is characterized in that, the analog information that in the described step 8 step 7 is obtained passes to light up-conversion module (8) and carries out the light up-conversion, obtain modulated optical signal, implement according to following steps: at first, the analog information signal of telecommunication that step 7 is obtained is decomposed into real part, the imaginary part two path signal removes to modulate MZM respectively, obtains two ways of optical signals; Then, again with two ways of optical signals respectively by the orthogonal polarizer of the direction of shaking thoroughly, obtain two bunch polarised lights, through behind the bundling device, the elliptically polarized light that obtains having the certain elliptical angle of polarization.

5. self adaptation FSO-OFDM transmission method according to claim 2, it is characterized in that, the modulated optical signal that light down conversion module (9) receiving step 8 is transmitted in the plasmas channel in the described step 9, carry out the light down-conversion and recover the signal of telecommunication, implement according to following steps: at first, to be divided into two paths of signals through beam splitter by the light signal that optical antenna collects, the one tunnel enters horizontal analyzer, and one the tunnel enters vertical analyzer; Then, measure the light intensity P of horizontal polarization and vertical polarization respectively by photo-detector _∥With P _⊥At last, the angle of polarization of the elliptically polarized light that receives by calculating is recovered the information of information source.

6. self adaptation FSO-OFDM transmission method according to claim 2, it is characterized in that the electrical signal transfer that in the described step 10 step 9 is obtained is given carrier frequency synchronization module (10), carry out carrier wave frequency deviation and correct, OFDM symbol after obtaining correcting, implement according to following steps:

Suppose ofdm signal of x (t) expression, receiving terminal receives and be expressed as by the atmospheric channel interference signals:

$y\left(t\right)=x\left(t\right)+n\left(t\right)=\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{A}_i{\left(t\right)e}^{j({2\mathrm{\&pi;f}}_{i}t+{\mathrm{\&theta;}}_i+{\mathrm{\&phi;}}_i)}+n\left(t\right),$

A wherein _i(t), f _i, θ _iWith

Expression respectively: the amplitude of i way carrier wave, frequency, baseband phase, initial phase deviation, n (t) represents additive white Gaussian noise; The quadravalence circulative accumulation amount of the OFDM that the definition receiving terminal receives is:

$C_{40}^a\left(\mathrm{0,0,0}\right)={<x^4\left(t\right)e^{-j8\mathrm{\&pi;at}}>}_t-3{<x^2\left(t\right)e^{-j4\mathrm{\&pi;at}}>}_t^2,$

Wherein＜ _tExpress time is average;

After obtaining the Higher-Order Cyclic cumulant of the ofdm signal that receiving terminal receives, detect normalization Higher-Order Cyclic cumulative amount again

Value, if be not equal to zero, be sub-carrier frequencies then, if equal zero then be not sub-carrier frequencies.

7. self adaptation FSO-OFDM transmission method according to claim 2 is characterized in that, the blind estimation in the described step 12 is implemented according to following steps:

The ofdm signal that the definition receiving terminal receives is independent identically distributed, then can obtain the estimated value of channel parameter according to the character of convolution algorithm and Higher Order Cumulants:

$h\left(\mathrm{\&tau;}\right)=\frac{C_4({\mathrm{\&tau;}}_1,\mathrm{\&tau;},L-1)}{C_3({\mathrm{\&tau;}}_1,L-1)}\&CenterDot;\frac{C_3^2(L-\mathrm{1,0})}{C_3(L-1,L-1)}\&CenterDot;\frac{C_4(L-1,L-\mathrm{1,0})}{C_4^2(L-\mathrm{1,0,0})},$

τ wherein, τ ₁∈ [0, L-1] is for each fixing τ, τ ₁Value when to L-1, changing from 0, obtain L corresponding h (τ), utilize least-squares estimation again, promptly obtain the estimated value of channel parameter.

8. self adaptation FSO-OFDM transmission method according to claim 2, it is characterized in that, signal detection module in the described step 17 (17) is judged the modulation system that Adaptive Modulation module (2) is selected according to the serial modulation signal that receives, and implements according to following steps:

The detected characteristics vector of definition signal cumulant is:

S＝[|C ₄₀/C ₄₂|，| ₄₁/C ₄₂|]，

Wherein, { C _4n, n=0,1,2} is the 4 rank cumulants of signal x (t), is defined as:

C ₄₁=M ₄₁-3M ₂₁M ₂₀,

Wherein, M _IjEach rank square of representation signal, according to following formula, the detected characteristics vector value of average power normalized signal and the detected characteristics vector value of received signal on the theory of computation are passed through to calculate the Euclidean distance of two vectors at last, thereby are carried out Modulation Identification respectively.

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