CN106936448A - A kind of Turbo code suitable for laser communication buoy encodes FDAPPM methods - Google Patents

Abstract

Fixed length double width degree pulse position modulation (Fixed dual amplitude pulse position modulation, FDAPPM) method is encoded the invention discloses a kind of Turbo code suitable for laser communication buoy.Wireless light communication technology has message capacity big；Strong anti-interference performance, good confidentiality；Protocol transparent, without special application frequency range；Support price is cheap, the advantages of networking is convenient, flexible.Therefore, there is good application scenario in the communication system of sonobuoy and aerial platform.Fixed length double width degree pulse position modulating method (FDAPPM) is a kind of improved procedure of conventional pulse position modulation (PPM), its have both from sign synchronization and modulation symbol length fix the advantages of.The technology such as Turbo code coding FDAPPM method combinations FDAPPM modulation and Turbo codings, effectively reduces the error performance of FDAPPM systems.Therefore, Turbo code coding FDAPPM methods realize between buoy and aerial platform it is real-time, at a high speed, there is in safe and secret communication application value higher.

Description

A kind of Turbo code suitable for laser communication buoy encodes FDAPPM methods

Technical field

The invention belongs to field of signal processing, it is related to the communication means of all kinds of sonar buoy and comprehensive place.

Background technology

Buoy can realize the functions such as communication, investigation.But buoy is easily arrived by neighbouring ship and aircraft observation simultaneously, or buoy The radio wave sent during work is detected positioning.Therefore find Novel Communication mode, realize in real time, at a high speed, it is safe and secret, double To communication capacity, be the important prerequisite for giving full play to naval warfare efficiency.The buoy communicated using pulse laser has following Several features：

1. small volume, energy consumption are small, and crypticity is good：Only need to carry the transmitting and reception of pulse laser communication system on buoy Antenna；Optical emitting and reception antenna small volume, even without sea is stretched out, crypticity is good, and optical antenna is basic for antenna Belong to passive device, energy consumption is smaller；2. high speed data transfer, level security be strong, electromagnetism interference.Pulsed light belongs to light wave, Light frequency is more high than wave frequency, has natural advantage at the aspect such as high speed data transfer and electromagnetism interference, And light wave has stronger directionality, the probability for being detected positioning is smaller, and level security is strong；3. can turn into blue green light to dive Efficient implementation.It is well known that blue green optical communication mode is a submarine subsurface communication technology for great technical advantage, pass through Development for many years, blue green light subsurface communication does not obtain the application of naval of various countries but yet.Except Partial key device it is not mature enough it Outward, main cause also includes：Bluish-green light emission system difficulty is equipped on submarine larger, i.e. two-way communication has larger challenge；By In two-way communication is difficult to, cause the aerial platforms such as satellite, aircraft big to the positioning difficulty of submarine, i.e., communication link is difficult to set up. Way communication can provide one effectively to tethered buoy submarine based on pulse laser for blue green optical communication technology under water Platform is realized in two-way communication, solves the difficult point that blue green optical communication link is difficult to set up.

Choosing suitable modulator approach can effectively improve the performance of radio telecommunicaltion system.Wireless light communication sets mostly Intensity modulated/direct detection (IM/DD) system is calculated as, because optic communication is difficult the reality that interference and system can be fairly simple It is existing.The modulation system being applied in IM/DD optical communication systems at present has a lot, wherein fairly simple modulation system is binary system On-off keying (OOK).In OOK systems, each bit is carried out by opening or closing light source pulse in each bit interval Send.This is the most basic form of modulated optical signal, only need to make light source scintillation i.e. codified.In Manchester's code, sequence In each bit be made up of 2 switching pulses.Generally, light source carries out intensity modulated by coded pulse waveform, while direct detection Receiver is decoded to signal after intensity modulated.In order to further improve transmission channel antijamming capability, air letter is applied to The optical communication system in road much employs pulse position modulation (PPM).PPM is a kind of orthogonal modulation mode, is modulated compared to OOK Mode, its mean power is reduced, but the cost paid for this simultaneously is the increase in the demand to bandwidth, and receiving terminal is needed Sign synchronization and slot synchronization are wanted, receiver complexity is high；The methods such as pulse interval modulation (DPIM) have preferably relative to OOK Power efficiency and error performance, have preferable bandwidth efficiency relative to PPM, and each modulation symbol is started or tied with pulse Beam, that is, possess from sign synchronization feature, can greatly simplify receiver design complexities, but because the length of symbol is not fixed, holds Easily cause modulator cache overflow or add empty slot so that demodulator can not be demodulated correctly.FDAPPM be fusion PPM and A kind of new modulator approach that DPIM is proposed, similar to traditional DPIM, there is FDAPPM length to fix and carry sign synchronization Etc. characteristic.Similar to traditional PPM, there are FDAPPM symbol lengths to fix, therefore do not wait for or be input into modem Multiple zero and make the probability reduction that demodulation malfunctions.

Turbulence effect causes the error performance of radio optical communication system greatly to decline, and can be effectively improved using channel coding The error performance of system.Typical error correcting code includes RS yard, convolutional code, Turbo code etc., and RS yard suitable for detecting and correction decoder The unexpected error that device is produced, but correct the limited in one's ability of random error；Convolutional code is applied to corrects random error, but decodes Unexpected error is easily caused in journey；Turbo code is provided simultaneously with correcting random and two kinds of abilities of mistake of burst, code efficiency ratio Traditional RS+ convolutional codes will get well.Therefore, the performance of research Turbo code encoding novel modulator approach is to radio optical communication system Extensive use has great importance.

The content of the invention

The technical problem to be solved

In order to avoid the deficiencies in the prior art part, the present invention proposes a kind of Turbo code suitable for laser communication buoy With coding techniques be combined modulation technique by coding FDAPPM methods, the method, increase pulse width, raising bandwidth efficiency, increasing Big transmission capacity and possess feature from sign synchronization, and substantially reduce the bit error rate of system.

Technical scheme

A kind of Turbo code suitable for laser communication buoy encodes FDAPPM methods, it is characterised in that step is as follows：

Step 1：Hydrophone receives the source signal a that signal is converted into binary form through analog, according to light Turbo code Coding criterion, carries out source signal a channel coding and forms signal sequence b；

Step 2：According to FDAPPM modulator approach symbolic construction criterions, the signal sequence b after coding is further modulated：

Signal sequence b is resolved into some sections of symbols first, every section of symbol includes M bit, the decimal system of each symbol It is v to be worth；Fixed length double width degree pulse position modulation FDAPPM uses two kinds of pulses, and its amplitude is respectively A and δ A, 0<δ ＜ 1；By one Individual binary M-bit data group is mapped as 2^M-1Dipulse signal on+3 time periods of time slot composition：If v ＜ 2^M-1, then In FDAPPM mappings, the original position of each symbol is " 1 " pulse, is that another amplitude is " δ " in the v+3 time slot position Pulse；If v >=2^M-1, the original position of each symbol is the pulse of " δ ", in v-2^M-1+ 3 time slot positions are the pulse of " 1 "；

Electric signal expression formula is after the modulation of FDAPPM is obtained according to above-mentioned mapping relations：

In formula, A=(2^M-1+ 3) P/ (1+ δ), P are average emitted power, T_b=M/ (2^M-1+3)/R_bRepresent time slot width, R_b It is transmission rate；Pulse amplitude c_i∈{0,1,δ}；P (t) is rectangular pulse functions, and k is the call number for modulating postamble sequence；

Step 3：Electric signal x (t) after the modulation of FDAPPM is converted into optical signals optical transmitting set to launch, is being received End converts optical signal into the electric signal with noise by optical receiver

In formula, R=e η/h ν are the responsiveness of photodetector, and e, η, ν and h are respectively quantum charge, detector quantum effect Rate, the frequency and Blanc constant that receive signal；G is the gain of avalanche photodide APD；I (t) decays for atmospheric turbulance Under, the random process of turbulent perturbation is received in light pulse, and its distribution function is Γ () is Gamma functions, K_α-β() is alpha-beta level Equations of The Second Kind modified Bessel function, and α and β represents large scale and small respectively Effective number of yardstick scattering unit；N (t) is the equivalent summation of thermal noise and Johnson noise in receiving circuit, can be approximately zero Average, variance are σ²White Gaussian noise；

Step 4：It is the sequence of M for number of modulation levels, when MLSD is demodulated, 2 is calculated first^MThe sequence likelihood value of the possible symbol of group GRAc_k, k=1,2 ..., 2^M；When symbol sebolic addressing is expressed asThen decision procedure is WhereinRepresent 2^MThe modulation symbol of level FDAPPM, r corresponds to the information sequence before FDAPPM modulation；

Step 5：Source signal a is demodulated and recovered using Iterated MAP decoding method_t：

Described Iterated MAP decoding method is comprised the following steps that：

Step 5a：The prior probability of " 1 " in the initialization input information of decoder 1Equal in the input information of decoder 2 The prior probability of " 1 "

Step 5b：Calculate the log-likelihood ratio output of MAP decoders 1：

In formula, a_tThe value of t transmission signal a is represented, p () is probability function, and l', l represents t to t+1 moment State,It is by a to all_t=1 or a_t=0 a for causing_tState transfer carry out；Order

Represent forward recursion,Represent backward recursion,Represent input bit a_tThe reference value of posterior probability during=i；r_t,jRepresent that t receives signal；N-1 is block Verification number in coding, j represents the searching number of check number in block coding；

Step 5c：The external information of the output signal of decoder 1 is calculated, makees the prior information of next decoding procedure：

Step 5d：Calculate the log-likelihood ratio output of MAP decoders 2：

In formula,Represent r_t,jBy the reception signal after interleaver；

Step 5e：The external information of the output signal of MAP decoders 2 is calculated, makees the prior information of next decoding procedure：

Step 5f：By Λ_2e(a_t) used as the prior information of next iteration decoder 1, repeat step 5b-5e is changed Generation；For nth iteration n=1,2 ..., I, wherein I represents iteration total degree；By after I iteration, by gained Value input bit a is can obtain compared with 0_tEstimate a_t：

Beneficial effect

A kind of Turbo code suitable for laser communication buoy proposed by the present invention encodes FDAPPM methods, and modulator is used Fixed length double width value pulse position modulating method, relative to OOK, improves power efficiency, so as to reduce its bit error rate；Relative to PPM increased the mean breadth of pulse, and letter rate is passed so as to improve bandwidth efficiency and unit, and from sign synchronization.Turbo code is compiled Code technology can effectively reduce the bit error rate of FDAPPM systems, it is ensured that radio optical communication system remains to keep normal under turbulent-flow conditions Work.Receiver uses Maximum likelihood sequence detection algorithm, using the relevance between each modulation symbol, further improves detection Performance.MAP decoders are finally used, by iterative decoding, it is ensured that the error performance high of communication system.

Brief description of the drawings

Fig. 1 is that Turbo code encodes FDAPPM system models；

Fig. 2 is FDAPPM modulator approach coding schematic diagrams；

Fig. 3 compares figure for the symbolic construction of OOK, PPM and FDAPPM modulator approach；

Fig. 4 is Turbo code decoder schematic diagram；

Fig. 5 compares figure for the normalization bandwidth demand of OOK, PPM and FDAPPM；

Fig. 6 compares for the transmission rate of OOK, PPM and FDAPPM；

Fig. 7 for OOK, PPM and FDAPPM normalization average emitted power ratio compared with；

Fig. 8 compares for the FDAPPM bit error rates of different modulating series, turbulent-flow conditions

Fig. 9 be different Turbulence Channels in the FDAPPM bit error rates compare (M=4)；

Figure 10 is in Turbulence Channels, Turbo code encodes the error performance (M=4) of lower FDAPPM；

Figure 11 is in Turbulence Channels, Turbo code encodes the error performance of different modulating series FDAPPM.

Specific embodiment

In conjunction with embodiment, accompanying drawing, the invention will be further described：

The present invention proposes a kind of Turbo code suitable for laser communication buoy and encodes FDAPPM methods.The method will be adjusted Technology processed is combined with coding techniques, increases pulse width, improves bandwidth efficiency, increase transmission capacity, possesses same from symbol The feature of step, and substantially reduce the bit error rate of system.Therefore, Turbo code encodes FDAPPM methods in wireless light communication application There is certain advantage.

Turbo code coding FDAPPM method system models are as shown in Figure 1.As can be seen that input message sequence a is sent first Enter Turbo encoder to be encoded, x is encoded sequence, it is mapped as FDAPPM sign formats and is converted into optical signal hair It is shot out.In receiving terminal, receive signal and optical signal is changed into by electric signal by a photodiode, it is assumed that noise can be approximate For bilateral power spectral density is N₀/ 2 white Gaussian noise, is then soft demodulated to signal and iterative decoding obtains a_k。

Therefore, the technical solution adopted for the present invention to solve the technical problems is mainly included the following steps that：

A () hydrophone receives signal and is converted into binary form through analog, criterion is encoded according to light Turbo code, by information source Signal carries out channel coding；

B () further modulates the signal sequence after coding according to FDAPPM modulator approach symbolic construction criterions；

C (), because the change of air pressure, wind speed causes turbulence effect, is divided according to marine atmosphere surface using Gamma-Gamma Cloth theory sets up optical signal in marine atmosphere surface transmission attenuation model；

D () receiving terminal optimizes detection using maximum likelihood algorithm, soft demodulation simultaneously recovers coded signal.；

E () uses MAP iterative decodings, finally demodulate and recover source signal.

Each step of the invention is further described in detail below：

The step (a), is implemented as follows:

One common Turbo encoder is as shown in Fig. 2 main by encoder, interleaver and residual matrix and multiple connection Device is constituted.In Turbo cataloged procedures, identical information sequence is encoded twice, but the order of information sequence is different twice. Length is encoded for the information sequence a of N sends into first component coder first.The Part I of encoder have two it is defeated Go out, first output b₀It is identical with input message sequence, because the encoder is systematic code, another output b₁It is check code.a Second component coder is sent into by the interleaved sequence a after interleaver, its check code b is only transmitted in the part₂.Information sequence b₀ And the verification sequence of two component coders collectively constitutes the coded sequence of Turbo code, therefore the total bitrate of Turbo code is 1/ 3, but encoder bit rate can be improved using residual matrix, wherein interleaver is the key for realizing Turbo code approximate random coding, its Effect is to be reset the bit position in input message sequence, to reduce the correlation of component coder output verification sequence And improve code weight.

The step (b), is implemented as follows：

Fixed length double width degree pulse position modulation (FDAPPM) uses two kinds of pulses, and its amplitude is respectively A and δ A (0<δ ＜ 1). One binary M (M represents the bit number that each symbol is included) position data group is mapped as (2^M-1+ 3) individual time slot is constituted Dipulse signal on time period.Decimal numbers of the v represented by symbol is made, if v ＜ 2^M-1, then in FDAPPM mappings, each symbol Number original position be " 1 " pulse, the v+3 time slot position be another amplitude be " δ " pulse；If v >=2^M-1, each The original position of symbol is the pulse of " δ ", in (v-2^M-1+³) individual time slot position for " 1 " pulse, Fig. 3 be OOK, PPM and The symbolic construction of the modulation systems such as FDAPPM.

The character expression of FDAPPM is：

In formula, A=(2^M-1+ 3) P/ (1+ δ), P are average emitted power, T_b=M/ (2^M-1+3)/R_bRepresent time slot width, R_b It is transmission rate；Pulse amplitude c_i∈{0,1,δ}；P (t) is rectangular pulse functions, and k is the call number for modulating postamble sequence；

The step (c), is implemented as follows：

Turbulence effect is because the change of pressure, height and wind speed in marine atmosphere top layer causes the random of atmospheric temperature Change, so that the transmitting beam ripple of laser does random fluctuation, causes beam jitter, intensity fluctuation (flicker) and picture point shake etc. Effect.Under Turbulence Channels, signal intensity obeys the light that Gamma-Gamma distributions can more comprehensively meet under all atmospheric conditions Strong flicker, therefore receive optical signal obedience GG distributions：

Γ () is Gamma functions, K_α-β() is alpha-beta level Equations of The Second Kind modified Bessel function, and α and β is represented greatly respectively Yardstick and effective number of small yardstick scattering unit.Scintillation index σ²Relation with α and β is：σ²The α β of+1/ β of=1/ α+1/.

The step (d), is implemented as follows：

To concentrate performance of the analysis combination pulse position modulating method under turbulence effect, it is considered to which light intensity is when each is transmitted For constant and in the absence of intersymbol interference in gap, and think that path attenuation coefficient is 1.Modulated for a combining pulse "ON" time slot, correspondence time slot reception electric current be：Wherein R=e η/h ν are photodetector Responsiveness, e, η, ν and h be respectively quantum charge, detective quantum efficiency, receive the frequency and Blanc constant of signal, and G is Avalanche photodide (Avalanche Photo Diode, APD) gain, I (t) is that light pulse is by rapids under atmospheric turbulance is decayed The random process of flow disturbance, n (t) is the equivalent summation of thermal noise and Johnson noise in receiving circuit, can be approximately zero-mean, Variance is σ²White Gaussian noise.Combining pulse does not have light pulse when being modulated at "Off" time slot, receive electric signal and be equal to Average is zero, and variance is σ²=N₀/ 2 white Gaussian noise y (t)=n (t), defining electric signal to noise ratio is：Then Average electricity signal to noise ratio is represented by：Wherein W is the pulse spacing.

Because each symbol lengths of FDAPPM are consistent, the soft demodulation of MLSD can make full use of the relevance between each modulation symbol, Therefore performance is demodulated firmly better than pointwise.It is the sequence of M for number of modulation levels, when MLSD is demodulated, 2 is calculated first^MThe sequence of the possible symbol of group Row likelihood value, symbol sebolic addressing is represented byThen decision procedure is WhereinRepresent 2^MThe modulation symbol of level FDAPPM, r corresponds to the information sequence before FDAPPM modulation.The condition of FDAPPM is paired Error probability (Pairwise Error Probability, PEP) is represented by working asDuring transmission, it is detected asProbability.

When turbulent flow condition is considered, condition PEP is represented by：Therefore, unconditional P_pCan be obtained by Hermite Gauss quadrature：

To 2^MLevel FDAPPM modulation, during central high s/n ratio, the joint upper bound of SER is represented by：

Therefore, the joint upper bound of the bit error rate (Bit Error Rate, BER) of FDAPPM FSO systems is represented by：

The step (e), is implemented as follows：

Assuming that the receiving sequence r' of decoder 1 is represented by：

R'=..., (r_t,0,r_t,1),(r_t+1,0,r_t+1,1),…} (6)

The receiving sequence of decoder 2 by deinterleaving after information sequenceWith the output sequence r of encoder₂Composition：

Log-likelihood ratio based on Turbo code is：

By the Λ (a of gained_t) value input bit a is can obtain compared with 0_tHard estimate a_t：

Wherein subscript t represents the time, it is assumed that r' and r " it is uncorrelated, then

P_r(r', r " | a)=P_r(r'|a)P_r(r”|a) (10)

Log-likelihood ratio is represented by：

The typical Turbo code iterative decoder structure based on MAP algorithms is as shown in Figure 4：It is worth noting that, in algorithm The input of decoder is coded bit number b_tLikelihood estimator LLR (b_t), the value is non-fully independent with interchannel noise, that is, give A fixed r_t, its corresponding LLR (b_t) non-critical observe Gaussian Profile.But in high s/n ratio, σ²/2·LLR(b_t) obey Variance is σ²Gaussian Profile, therefore binary system Turbo decoders are still applicable.First MAP decoding in Fig. 4 is considered first Device, encoding rate is 1/n, and the log-likelihood ratio output of MAP decoders 1 is represented by：

A in formula_tThe value of t transmission signal a is represented, p () is probability function, and l', l represents t to t+1 moment State,It is by a to all_t=1 (or a_t=a for 0) causing_tState transfer carry out.Order

Represent forward recursion,Represent backward recursion,Represent input bit a_tThe reference value of posterior probability during=i, here we be normalized to WithThe prior probability of " 1 " and " 0 " signal respectively in the input information of decoder 1.For the ease of area Not, the prior probability of " 1 " and " 0 " signal is used respectively in the input information of decoder 2WithRepresent.We assume that decoding In device 1WithInitial value be：

Therefore formula (12) it is rewritable into：

In formula n-1 be block coding in verification number, in systematic code becauseWith coding grid and state l It is uncorrelated, i.e.,Therefore Λ₁(a_t) can be further simplified as：

Wherein

Λ_1e(a_t) it is the external information of the output signal of decoder 1, this is only tediously long with encoder information-related, with input Information r_t,0It is unrelated, therefore this can be used as the prior information of next decoding procedure.Secondly second MAP decoding in Fig. 4 is considered Device, the input information of MAP decoders 2 includes the signal sequence after interweavingReceive signalSoft output with decoder 1 is believed Breath is by the signal Λ after intertexture₁(a_t) relevant.Therefore, in order to reduce relevance, we must be Λ₁(a_t) in relevant r_t,0's Item is removed, it is clear that Λ_1e(a_t) in and do not include r_t,0, therefore can be by Λ_1e(a_t) general as the priori of decoding second stage Rate, i.e., by the external information Λ of the output of decoder 1 after intertexture_1eThe prior information of process can be decoded as next step：

Obtained from formula (17) and probability and formula：

Prior probability when we can calculate second decoding is：

In the second step of decoding, the log-likelihood ratio of MAP decoders output is designated as Λ₂(a_t).It is similar with formula (15) (15), Λ₂(a_t) be represented by：

The prior probability that formula (17) is represented substitutes into formula (21) and can obtain：

Wherein Λ_2e(a_t) for decoder 2 output information external information, this can be used as the prior probability of decoder 1 An estimate feed back to decoder 1 and carry out the iterative decoding of next round, such as formula (17).Therefore the logarithm that decoder 1 is exported Likelihood ratio is represented by：

Summarize Iterated MAP decoding method：

1) initialization order

2) for nth iteration n=1,2 ..., I, wherein I represents iteration total degree,

- calculated according to formula (12)With

- calculateValue

- calculateValue

3) by after I iteration, a_tEstimate a_tCan basisCarry out hard decision gained.

Instance parameter sets as follows：

Consider that light intensity is constant and in the absence of intersymbol interference in each combining pulse modulation symbol transmission time slot, and Think that path attenuation coefficient is 1.

This example is implemented as follows：

A () source rate is 1Gbps, detective quantum efficiency η=0.8, optical wavelength v=1550nm, APD average gain G =150.

(b) information sequence a_k={ 0,1 } represents binary information bit " 0 " and " 1 ", b_kIt is encoded sequence, c_kTo adjust Sequence after system, with 1/T_bSpeed by emission filter, and optical signal launch converted electrical signals to by transmitter go out；

C () optical signal x (t) is transmitted by atmospheric turbulence channels, channel model uses GG distributed models；

D () receives signal and optical signal is changed into electric signal by a photodiode, receiving terminal uses direct detection And assume that transmitter and receiver front end bandwidth is enough.It is N that signal is received plus bilateral power spectral density₀/ 2 Gauss white noise Sound, then by receiving filter.During demodulation, using Maximum Likelihood Detection principle, recovery obtains the bit stream after M coding

E () Fig. 5 represents that PPM and FDAPPM, to the normalized bandwidth demands of OOK, as a result show bandwidth of the FDAPPM than OOK Demand is high, but lower than PPM, and as M=4, the bandwidth demand of FDAPPM is 2.75 and 0.68 times of OOK and PPM.

Fig. 6 is given under time slot width the same terms, compares PPM and FDAPPM to the normalized transmission rates of OOK, as a result Show that transmission rates of the FDAPPM than OOK is low, but it is higher than PPM, and as M=4, the transmission rate of FDAPPM is about OOK and PPM 0.36 and 1.45 times；When M increases, although the transmission rate of FDAPPM is still to higher than PPM, both reach unanimity.

Fig. 7 is given under peak power the same terms, compares PPM and FDAPPM to the normalized mean powers of OOK.Knot Fruit shows the mean power of FDAPPM higher than PPM but well below OOK, and as M=4, the mean power of FDAPPM and PPM is 0.27 and 0.125 times of OOK.When M increases, the transimission power of PPM and FDAPPM is reduced, and both reach unanimity.

Fig. 8 represented under the conditions of average signal-to-noise ratio identical, BERs of the FDAPPM in different modulating series.As can be seen that The BER of FDAPPM reduces with the increase of SNR.Therefore in middle rank and during strong turbulence, it usually needs with reference to channel coding and point Collection technology improves the performance of system.

Fig. 9 represents the BER of FDAPPM under different turbulent-flow conditions.As can be seen that the average BER of FDAPPM withIncrease And increase, for example work as BER=10^-6When, FDAPPM existsWithTurbulence Channels needed for SNR respectively may be about 22dB, 32.2 and 51.5dB, it is seen that under intermediate turbulent-flow conditions, and the error performance of FDAPPM is than weak turbulent flow bar It is far short of what is expected during part.CompareBER during with other conditions can be obtained, and under the conditions of strong turbulence, the error performance of FDAPPM enters One step is deteriorated.

Figure 10 and Figure 11 are represented in Turbulence ChannelsTurbo code encodes FDAPPM systems and uncoded system Error performance compare, Turbo code coding and is not compiled FDAPPM systems when wherein Figure 10 represents that iterations is respectively 1,2,3,5 The error performance of code system；Figure 10 represents that under conditions of iteration 5 times number of modulation levels is respectively 4,8,16 Turbo code coding The error performance of FDAPPM systems and uncoded system.As can be seen that in Turbulence Channels, FDAPPM is encoded using Turbo code The error performance of system when the error performance of system is substantially better than uncoded, and iterations is more, number of modulation levels is higher, error code Rate is lower.As shown in Figure 10, when the bit error rate is 10^-5When, decoding respectively iteration 1,2,3,5 times when error performance for not compiling The error performance of FDAPPM systems can improve signal to noise ratio about 6.1dB~6.5dB respectively under the conditions of code.As shown in figure 11, error code is worked as Rate is 10^-5When, 16 grades of FDAPPM are encoded in Turbo code, and the average signal-to-noise ratio after 5 iterative decodings needed for system is for uncoded FDAPPM systems reduction about 7.2dB.In addition it can also be seen that 2 iteration and the above than 1 iteration there is larger performance to change It is apt to, but the performance improvement trend for continuing to increase caused by iterations slows down.

Claims (1)

1. a kind of Turbo code suitable for laser communication buoy encodes FDAPPM methods, it is characterised in that step is as follows：

Step 1：Hydrophone receives the source signal a that signal is converted into binary form through analog, is encoded according to light Turbo code Criterion, carries out source signal a channel coding and forms signal sequence b；

Step 2：According to FDAPPM modulator approach symbolic construction criterions, the signal sequence b after coding is further modulated：

Signal sequence b is resolved into some sections of symbols first, every section of symbol includes M bit, and the decimal value of each symbol is v；Fixed length double width degree pulse position modulation FDAPPM uses two kinds of pulses, and its amplitude is respectively A and δ A, 0<δ ＜ 1；By one two The M-bit data group of system is mapped as 2^M-1Dipulse signal on+3 time periods of time slot composition：If v ＜ 2^M-1, then FDAPPM In mapping, the original position of each symbol is " 1 " pulse, is that another amplitude is the pulse of " δ " in the v+3 time slot position； If v >=2^M-1, the original position of each symbol is the pulse of " δ ", in v-2^M-1+ 3 time slot positions are the pulse of " 1 "；

Electric signal expression formula is after the modulation of FDAPPM is obtained according to above-mentioned mapping relations：

$x\left(t\right)=\underset{k=1}{\overset{\mathrm{\&infin;}}{\&Sigma;}}{\mathrm{Ac}}_{i}p(t-{\mathrm{kT}}_b)$

In formula, A=(2^M-1+ 3) P/ (1+ δ), P are average emitted power, T_b=M/ (2^M-1+3)/R_bRepresent time slot width, R_bTo pass Defeated speed；Pulse amplitude c_i∈{0,1,δ}；P (t) is rectangular pulse functions, and k is the call number for modulating postamble sequence；

Step 3：Electric signal x (t) after the modulation of FDAPPM is converted into optical signals optical transmitting set to launch, receiving terminal by Optical receiver converts optical signal into the electric signal with noise

In formula, R=e η/h ν for photodetector responsiveness, e, η, ν and h be respectively quantum charge, detective quantum efficiency, Receive the frequency and Blanc constant of signal；G is the gain of avalanche photodide APD；I (t) is light under atmospheric turbulance is decayed The random process of turbulent perturbation is received in pulse, and its distribution function isΓ () is Gamma functions, K_α-β() is alpha-beta level Equations of The Second Kind modified Bessel function, and α and β represents large scale and small chi respectively Effective number of degree scattering unit；N (t) is the equivalent summation of thermal noise and Johnson noise in receiving circuit, can be approximately zero equal Value, variance are σ²White Gaussian noise；

Step 4：It is the sequence of M for number of modulation levels, when MLSD is demodulated, 2 is calculated first^MThe sequence likelihood value GRAc of the possible symbol of group_k, k= 1,2,…,2^M；When symbol sebolic addressing is expressed asThen decision procedure is WhereinRepresent 2^MThe modulation symbol of level FDAPPM, r corresponds to the information sequence before FDAPPM modulation；

Step 5：Source signal a is demodulated and recovered using Iterated MAP decoding method_t：

Described Iterated MAP decoding method is comprised the following steps that：

Step 5a：The prior probability of " 1 " in the initialization input information of decoder 1In the input information of decoder 2 " 1 " Prior probability

Step 5b：Calculate the log-likelihood ratio output of MAP decoders 1：

${\mathrm{\&Lambda;}}_1\left(a_t\right)=log\frac{\underset{a_t=1}{\underset{l^{\&prime;},l}{\mathrm{\&Sigma;}}}{\mathrm{\&alpha;}}_{t-1}\left(l^{\&prime;}\right)p_t^1\left(1\right)\exp (-\frac{{\mathrm{\&Sigma;}}_{j=0}^{n-1}{(r_{t,j}-x_{t,j}^1(l\left)\right)}^2}{2{\mathrm{\&sigma;}}^2})\&CenterDot;{\mathrm{\&beta;}}_t\left(l\right)}{\underset{a_t=0}{\underset{l^{\&prime;},l}{\mathrm{\&Sigma;}}}{\mathrm{\&alpha;}}_{t-1}\left(l^{\&prime;}\right)p_t^1\left(0\right)\exp (-\frac{{\mathrm{\&Sigma;}}_{j=0}^{n-1}{(r_{t,j}-x_{t,j}^0(l\left)\right)}^2}{2{\mathrm{\&sigma;}}^2})\&CenterDot;{\mathrm{\&beta;}}_t\left(l\right)}$

In formula, a_tThe value of t transmission signal a is represented, p () is probability function, and l', l represents t to the shape at t+1 moment State,It is by a to all_t=1 or a_t=0 a for causing_tState transfer carry out；Order Represent forward recursion,Represent backward recursion,Represent input bit a_t= The reference value of posterior probability during i；r_t,jRepresent that t receives signal；N-1 is the verification number in block coding, and j represents that block is encoded The searching number of middle check number；

Step 5c：The external information of the output signal of decoder 1 is calculated, makees the prior information of next decoding procedure：

${\mathrm{\&Lambda;}}_{1e}\left(a_t\right)={\mathrm{\&Lambda;}}_1\left(a_t\right)-log\frac{p_t^1\left(1\right)}{p_t^1\left(0\right)}-\frac{2r_{t,0}}{{\mathrm{\&sigma;}}^2}$

Step 5d：Calculate the log-likelihood ratio output of MAP decoders 2：

${\mathrm{\&Lambda;}}_2\left(a_t\right)={\mathrm{\&Lambda;}}_{1e}\left(a_t\right)+log\frac{\underset{a_t=1}{\underset{l^{\&prime;},l}{\mathrm{\&Sigma;}}}{\mathrm{\&alpha;}}_{t-1}\left(l^{\&prime;}\right)\exp (-\frac{{({\tilde{r}}_{t,0}-x_{t,0}^1)}^2+{\mathrm{\&Sigma;}}_{j=1}^{n-1}{({\tilde{r}}_{t,j}-x_{t,j}^1(l\left)\right)}^2}{2{\mathrm{\&sigma;}}^2})\&CenterDot;{\mathrm{\&beta;}}_t\left(l\right)}{\underset{a_t=1}{\underset{l^{\&prime;},l}{\mathrm{\&Sigma;}}}{\mathrm{\&alpha;}}_{t-1}\left(l^{\&prime;}\right)\exp (-\frac{{({\tilde{r}}_{t,0}-x_{t,0}^0)}^2{\mathrm{\&Sigma;}}_{j=1}^{n-1}{({\tilde{r}}_{t,j}-x_{t,j}^0(l\left)\right)}^2}{2{\mathrm{\&sigma;}}^2})\&CenterDot;{\mathrm{\&beta;}}_t\left(l\right)}$

In formula,Represent r_t,jBy the reception signal after interleaver；

Step 5e：The external information of the output signal of MAP decoders 2 is calculated, makees the prior information of next decoding procedure：

${\mathrm{\&Lambda;}}_{2e}\left(a_t\right)={\mathrm{\&Lambda;}}_2\left(a_t\right)-{\mathrm{\&Lambda;}}_{1e}\left(a_t\right)-\frac{2{\tilde{r}}_{t,0}}{{\mathrm{\&sigma;}}^2}$

Step 5f：By Λ_2e(a_t) used as the prior information of next iteration decoder 1, repeat step 5b-5e is iterated；It is right In nth iteration n=1,2 ..., I, wherein I represents iteration total degree；By after I iteration, by gainedValue Input bit a is can obtain compared with 0_tEstimate a_t：

$a_t=\left\{\begin{array}{cc}1& \mathrm{\&Lambda;}\left(a_t\right)\&GreaterEqual;0\\ 0& else\end{array}\right..$