CN110266398B - Underwater submarine communication method for air-based system - Google Patents

Abstract

The invention discloses a space-based system underwater communication device and a communication method. Using the space-based system underwater communication device, the transmission method includes: firstly splitting a light source to obtain a first light source and a second light source, and then Splitting the second light source into a first horizontally polarized light signal and a first vertically polarized light signal; encoding the downlink signal source; modulating the encoded signal source into the first horizontally polarized light signal, and combining it with the first horizontally polarized light signal. The first vertically polarized optical signal is combined and separated, the separated vertically polarized optical signal is first delayed, and then the horizontally polarized optical signal and the vertically polarized optical signal are coherently detected, decoded and decoded to complete the downlink information. encoding the uplink source; using the encoded source to perform secondary modulation on the second vertically polarized light signal; coherently detect, decode and decode the modulated signal and the first light source, and complete the Transmission of uplink information.

Description

A kind of underwater communication method of space-based system

technical field

The invention belongs to the technical field of communication methods, relates to an underwater-to-submersible communication device of an air-based system, and also relates to a communication method of the above-mentioned underwater-to-submarine communication device of an air-based system.

Background technique

Commonly used submersible communication methods, one is to use ultra-low frequency and very low frequency electromagnetic waves. For ultra-low frequency and very low frequency electromagnetic waves, the size of the antenna is required to be large, and the available frequency band range is limited; the other uses underwater acoustic communication. The attenuation of sound waves in water is proportional to the square of the frequency, which results in a bandwidth-limited underwater acoustic channel. Using the submersible communication method of optical communication, due to the attenuation of the optical signal by the atmospheric channel and the seawater channel, the detection sensitivity of the signal at the receiving end is low, and the uplink communication of the space-based system is rarely mentioned.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a space-based system underwater communication device, which can improve the detection sensitivity of the duplex link receiving end.

The technical solution adopted by the present invention is that an underwater communication device for a space-based system includes a downlink device and an uplink device, the downlink device includes a laser, the laser is connected with a beam splitter, and the beam splitter is a The output end is connected with a first polarization beam splitter, the first output end of the first polarization beam splitter is connected with a modulator, the input end of the modulator is connected with a first encoder, the output end of the modulator is connected with a polarization beam combiner, the first The second output end of the polarization beam splitter is connected with the polarization beam combiner; the output end of the polarization beam combiner is connected with a second polarization beam splitter through an optical antenna, and the first output end of the second polarization beam splitter is connected with a first coherent receiver , the second output end of the second polarization beam splitter is connected with a delay device, and the delay device is connected with the first coherent receiver;

The uplink device includes a reverse modulator, the input end of the reverse modulator is connected with the second output end of the second polarization beam splitter, the input end of the reverse modulator is also connected with a second encoder, and the output end of the reverse modulator is connected with an optical antenna. For the second coherent receiver, the input end of the second coherent receiver is connected to the other output end of the beam splitter.

The feature of the present invention also lies in:

The first coherent receiver includes a coherent detector, a decoder and a decoder connected in sequence.

Another object of the present invention is to provide an underwater-to-submarine communication method for a space-based system, which can realize bidirectional communication of uplink and downlink.

Another technical solution adopted by the present invention is a method for underwater-to-submarine communication of a space-based system, using the above-mentioned underwater-to-submersible communication device of the space-based system, and the transmission method includes:

Step 1. First, split the light source to obtain a first light source and a second light source, and then split the second light source into a first horizontally polarized light signal and a first vertically polarized light signal;

Step 2, encoding the downlink source;

Step 3, modulate the encoded signal source to the first horizontally polarized light signal to obtain the second horizontally polarized light signal;

Step 4, combining the second horizontally polarized light signal and the first vertically polarized light signal;

Step 5, separating the combined optical signals to obtain a third horizontally polarized optical signal and a second vertically polarized optical signal;

Step 6: First delay the second vertically polarized optical signal, then perform coherent detection on the third horizontally polarized optical signal and the delayed second vertically polarized optical signal, and decode and decode the output baseband signal, complete the transmission of downlink information;

Step 7, encode the information source of the uplink;

Step 8, using the encoded signal source to perform secondary modulation on the second vertically polarized light signal to obtain a third vertically polarized light signal;

Step 9: Perform coherent detection, decoding and decoding on the third vertically polarized optical signal and the first light source to complete the transmission of uplink information.

The present invention is also characterized in that,

Step 1, firstly split the light source to obtain a first light source and a second light source E, and then split the second light source E into a first horizontally polarized light signal _Ex and a first vertically polarized light signal _Ey ;

E _x =E _xm cos(ωt-kz+φ _x ) (1);

E _y =E _ym cos(ωt-kz+φ _y ) (2);

where E _xm and E _ym are the amplitudes of the x and y polarization directions of the optical signal, ω is the angular frequency of the light wave, k is the wave vector, φ _x and φ _y are the initial phases of the x and y polarization directions, respectively, t and z are the time Variables and space variables, the synthetic wave optical signal electric field E= _ex E _x +e _y E _y ;

Step 2, using the Turbo code encoding mode to encode the downlink source to obtain the baseband sequence {c _k };

Step 3, using the 16QAM modulation mode to map the baseband sequence { _ck } every 4 code words and then modulate the corresponding amplitude and phase to obtain the second horizontally polarized optical signal light field E _16QAM :

m=(ck _ck+1 ) ₂ , n=( _{ck+2 ck +3} ) ₂ (4);

Step 4, using a polarization beam combiner to combine the second horizontally polarized light signal and the first vertically polarized light signal;

Step 5, separating the combined optical signals to obtain the third horizontally polarized optical signal _Es and the second vertically polarized optical signal E _Lo ;

Step 6, first carry out the delay of time T to the second vertically polarized light signal, then carry out self-homodyne coherent detection to the third horizontally polarized light signal and the second vertically polarized light signal after the delay, and output two-way baseband signals I _i (t) and I _q (t); use the baseband signal processing algorithm to decode the baseband signals I _i (t) and I _q (t) to obtain the recovered baseband sequence {c' _k }; {c' _k } is decoded to complete the transmission of downlink information;

Step 7, carry out Turbo code encoding to the information source of the uplink, obtain the sequence _sk after encoding;

Step 8. Use the cat's eye structure of the inverse modulator to change the defocus amount of the second vertically polarized light signal passing through the lens, and change the second vertically polarized light signal through the corresponding relationship between the sequence _sk and the defocus amount to obtain a third vertically polarized light Signal;

Step 9. Use the homodyne coherent detection method to perform coherent detection on the third vertically polarized optical signal and the first light source, and then decode and decode the output baseband signal from the coherent detection to obtain the second vertically polarized optical signal to complete the uplink. transmission of road information.

Step 6 specifically includes the following steps:

Step 6.1. First use a delay device to delay the second vertically polarized light signal by time T, and then perform self-homodyne coherence detection on the third horizontally polarized light signal and the delayed second vertically polarized light signal, and output the Two baseband signals I _i (t) and I _q (t):

where β is the photoelectric conversion coefficient of the balanced detector, and j is the imaginary dimension;

Step 6.2, filter the baseband signals I _i (t) and I _q (t) in turn, extract the bit timing pulses from the baseband signals I _i (t) and I _q (t) and sample them, and filter them at a specified time. The processed output waveform is sampled and judged to obtain the recovered baseband sequence {c' _k };

Step 6.3. Use the Turbo code iterative decoder to decode the recovered baseband sequence {c' _k }. During the first iterative decoding, input the information symbol probability to the component decoder A in the Turbo code iterative decoder. The log-likelihood ratio Λ(u; I) is initialized to 0, and the decoded output information symbol probability log-likelihood ratio Λ(u; O) is used as the input information symbol probability log-likelihood ratio after interleaving:

After the component decoder B in the turbo code iterative decoder decodes the output, the first iterative decoding ends, and then the information symbol probability logarithm generated by the component decoder B decoding module in the turbo code iterative decoder The likelihood ratio is fed back to the decoding module of the component decoder A after interleaving, as a priori information for the next round of decoding:

In the above formula, the superscript represents corresponding to different component decoding modules, the subscript represents the interleaving process, and I ^-1 represents deinterleaving;

Repeat the above process until a certain number of iterations is reached or a certain iterative condition is met; finally, according to the output information symbol probability log-likelihood ratio Λ(u; O) of the component decoder B in the Turbo code iterative decoder The decision is made, the decoding output is obtained, and the transmission of the downlink information is completed.

Step 8 is specifically:

The reflected echo power p _r of the cat's eye target of the reverse modulator received by the second coherent receiver is:

In the above formula, p _t is the laser emission power, τ _a is the atmospheric transmittance, τ _r is the transmittance of the receiving optical system, ρ _s is the reflection coefficient, D is the aperture of the focusing lens, f is the focal length of the lens, and θ ₀ is the beam divergence angle, d is the defocus amount, r is the distance between the inverse modulator and the second coherent receiver;

The echo power p _r is modulated by controlling the defocus amount d of the cat's eye structure, and the modulated second vertically polarized light signal light field E _4ASK is:

The modulation of the light field E _4ASK is completed by changing the value of d through the arrangement order of (s _k s _k+1 ) ₂ to obtain a third vertically polarized light signal.

The beneficial effects of the present invention are:

In the underwater communication device of the space-based system of the present invention, the downlink device adopts a self-homodyne coherent detector, the uplink adopts a homodyne coherent detector, and the detection sensitivity is improved at the receiving end of the duplex link, so that the space-based The communication in the system is more reliable; the uplink communication adopts the reverse modulator, which uses the reverse modulation return light characteristic to avoid the capture of the tracking system and the receiving light source of the passive end (seawater end), and realize the two-way communication between the submarine and the aircraft.

The underwater communication method of the space-based system of the present invention performs Turbo coding on the transmitted signal source, which can improve the anti-interference capability of the signal; since the same laser light source is used for the uplink and the downlink, the frequencies of the signal light and the local oscillator light are completely Similarly, the homodyne coherent detection method with high sensitivity (homdyne detection has a gain of 23 dB relative to the direct detection) is used, and the uplink communication is realized by adopting the cat's eye reverse modulation method.

Description of drawings

Fig. 1 is the structural representation of a kind of space-based system underwater communication device of the present invention;

2 is a schematic structural diagram of a coherent receiver of a space-based system underwater communication device for submerged communication according to the present invention;

Fig. 3 is the structural representation of the encoder in a kind of space-based system underwater communication method of the present invention;

Fig. 4 is the second horizontally polarized optical signal 16QAM signal constellation diagram in a kind of space-based system underwater communication method of the present invention;

Fig. 5 is the demodulation flow chart of decoder in a kind of space-based system underwater communication method of the present invention;

Fig. 6 is the structural representation of the decoder in a kind of space-based system underwater communication method of the present invention;

7 is a schematic diagram of a cat's eye structure of a reverse modulator in a method for underwater communication of a space-based system of the present invention;

FIG. 8 is a constellation diagram of a third vertically polarized optical signal after 4ASK modulation in an underwater-to-submarine communication method of a space-based system of the present invention.

In the figure, 1. laser, 2. beam splitter, 3. modulator, 4. first encoder, 5. polarization beam combiner, 6. second coherent receiver, 7. second polarization beam splitter, 8 . first coherent receiver, 8-1. coherent detector, 8-2. decoder, 8-3. decoder, 9. inverse modulator, 10. second encoder, 11. delayer, 12 . Beamsplitter.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

An underwater-to-submarine communication device of a space-based system, comprising a downlink (from atmosphere to seawater) device and an uplink (seawater to atmosphere) device, the downlink device includes a laser 1, and the laser 1 is connected with a beam splitter 12 , an output end of the beam splitter 12 is connected with the first polarization beam splitter 2, the first output end of the first polarization beam splitter 2 is connected with the modulator 3, the input end of the modulator 3 is connected with the first encoder 4, and the modulator 3. The output end is connected with the polarization beam combiner 5, the second output end of the first polarization beam splitter 2 is connected with the polarization beam combiner 5, the first output end of the first polarization beam splitter 2 outputs the first horizontally polarized light signal, the first The second output end of a polarization beam splitter 2 outputs a first vertically polarized light signal; the light source emitted by the laser 1 is a single light source blue-green light.

The output end of the polarization beam combiner 5 is connected to a second polarization beam splitter 7 through an optical antenna, the first output end of the second polarization beam splitter 7 is connected to a first coherent receiver 8, and the second output of the second polarization beam splitter 7 A delay device 11 is connected to the end, the first output end of the second polarization beam splitter 7 outputs the third horizontally polarized light signal, the second output end of the second polarization beam splitter 7 outputs the second vertically polarized light signal, and the delay device 11 Connected to the first coherent receiver 8 .

The first polarization beam splitter 2, the modulator 3, the first encoder 4 and the polarization beam combiner 5 are the transmitting ends of the downlink device and are located in the atmosphere. The second polarization beam splitter 7, the delay device 11 and the first A coherent receiver 8 is the receiving end of the downlink device, located in sea water.

The uplink device comprises a reverse modulator 9, the input end of the reverse modulator 9 is connected with the second output end of the second polarization beam splitter 7, the input end of the reverse modulator 9 is also connected with a second encoder 10, and the reverse modulator 9 outputs The second coherent receiver 6 is connected to the end through an optical antenna, and the input end of the second coherent receiver 6 is connected to the other output end of the beam splitter 12 . The inverse modulator 9 and the second encoder 10 are the transmitting end of the uplink, located in sea water, and the second coherent receiver 6 is the receiving end of the uplink, located in the atmosphere.

The first coherent receiver 8 includes a coherent detector 8-1, a decoder 8-2 and a decoder 8-3 connected in sequence.

Preferably, the coherent detector 8-1 is a self-homodyne coherent detector, the decoder 8-2 is a baseband signal decoder, and the decoder 8-3 is a turbo code iterative decoder.

The second coherent receiver 6 has the same structure as the first coherent receiver 8, and its coherent detector is a homodyne coherent detector.

A method for underwater-to-submarine communication of an air-based system, which specifically includes the following steps:

Step 1. First, split the light source to obtain a first light source and a second light source E, and then split the second light source E into a first horizontally polarized light signal and a first vertically polarized light signal;

Specifically, the single light source blue-green light emitted by the laser 1 is firstly split by the beam splitter 12 to obtain the first light source and the second light source E. The first light source is used for coherent detection with the uplink, and then the second light source is used for coherent detection. The E beam splitting _is the first horizontally polarized light signal Ex and the first vertically polarized light signal _Ey ;

E _x =E _xm cos(ωt-kz+φ _x ) (1);

E _y =E _ym cos(ωt-kz+φ _y ) (2);

where E _xm and E _ym are the amplitudes of the x and y polarization directions of the optical signal, ω is the angular frequency of the light wave, k is the wave vector, φ _x and φ _y are the initial phases of the x and y polarization directions, respectively, t and z are the time Variable and space variable, the electric field of the synthetic wave optical signal E= _ex E _x +e _y E _y .

Step 2, using the first encoder 4 to encode the downlink information source to obtain the baseband sequence {c _k };

直接送至复接器，同时{u_k}经过交织器I后的交织序列 {u_n}送入分量编码器B。两个分量编码器输出的校验序列分别为

和

为提高码率和系统频谱效率，可以将两个校验序列经过删除矩阵删余后得到

再与系统输出

一起经过复接构成码字序列{c_k}。Specifically, as shown in Fig. 2, in the encoding process of the turbo code, the input information sequences of the two component codes are the same, and the information sequence {u _k } of length N is sent to the component encoder A for encoding at the same time as system output

It is directly sent to the multiplexer, and the interleaved sequence {u _n } after {u _k } passes through the interleaver I is sent to the component encoder B. The check sequences output by the two component encoders are

and

In order to improve the code rate and system spectral efficiency, the two check sequences can be obtained by puncturing the deletion matrix.

again with the system output

They are multiplexed together to form a codeword sequence {c _k }.

Step 3, using the 16QAM modulation mode to perform every 4 codeword mapping on the baseband sequence { _ck } with the modulator 3 and then modulating with the corresponding amplitude and phase, the second horizontally polarized optical signal light field E _16QAM can be obtained:

m=(ck _ck+1 ) ₂ , n=( _{ck+2 ck +3} ) ₂ (4);

Wherein, (c _k c _k+1 ) ₂ and (c _k+2 c _k+3 ) ₂ represent converting binary values to decimal values;

Specifically, the baseband sequence {c _k } is mapped to every 4 codewords and then modulated with the corresponding amplitudes A, 2A, 3A, 4A and phases 0, π/2, π, 3π/2, and the modulated signal light can be obtained. Field expression E _16QAM , Table 1 shows the coding rules corresponding to 16QAM .

Table 1 16QAM modulation source correspondence table

c_k,c_k+1,c_k+2,c_k+3 S_16QAM 0 0 0 0 E_16QAM＝A·E_xmcos(ωt-kz+φ_x+0) 0 0 0 1 E_16QAM＝A·E_xmcos(ωt-kz+φ_x+π/2) 0 0 1 0 E_16QAM＝A·E_xmcos(ωt-kz+φ_x+π) 0 0 1 1 E_16QAM＝A·E_xmcos(ωt-kz+φ_x+3π/2) 0 1 0 0 E_16QAM＝2A·E_xmcos(ωt-kz+φ_x+0) 0 1 0 1 E_16QAM＝2A·E_xmcos(ωt-kz+φ_x+π/2) 0 1 1 0 E_16QAM＝2A·E_xmcos(ωt-kz+φ_x+π) 0 1 1 1 E_16QAM＝2A·E_xmcos(ωt-kz+φ_x+3π/2) 1 0 0 0 E_16QAM＝3A·E_xmcos(ωt-kz+φ_x+0) 1 0 0 1 E_16QAM＝3A·E_xmcos(ωt-kz+φ_x+π/2) 1 0 1 0 E_16QAM＝3A·E_xmcos(ωt-kz+φ_x+π) 1 0 1 1 E_16QAM＝3A·E_xmcos(ωt-kz+φ_x+3π/2) 1 1 0 0 E_16QAM＝4A·E_xmcos(ωt-kz+φ_x+0) 1 1 0 1 E_16QAM＝4A·E_xmcos(ωt-kz+φ_x+π/2) 1 1 1 0 E_16QAM＝4A·E_xmcos(ωt-kz+φ_x+π) 1 1 1 1 E_16QAM＝4A·E_xmcos(ωt-kz+φ_x+3π/2)

The 16QAM signal constellation diagram of the second horizontally polarized optical signal transmitted after modulation is shown in FIG. 3 .

Step 4. Use the polarization beam combiner 5 to combine the second horizontally polarized light signal and the first vertically polarized light signal.

Step 5, using the second polarization beam splitter 7 to separate the combined optical signals to obtain a third horizontally polarized optical signal (ie, signal light) and a second vertically polarized optical signal (as local oscillator light);

Step 6: Delay the local oscillator light first, then perform coherent detection on the signal light and the delayed local oscillator light, decode the output baseband signal, and complete the transmission of downlink information.

Step 6.1. Use the delayer 11 to first delay the local oscillator light by time T to improve the coherence, and then use the coherent detector 8-1 to perform the third horizontally polarized light signal and the delayed second vertically polarized light signal. Self-homodyne coherent detection, output two baseband signals I _i (t) and I _q (t):

where β is the photoelectric conversion coefficient of the balanced detector, and j is the imaginary dimension;

Step 6.2, utilize the decoder 8-2 to filter the baseband signals I _i (t) and I _q (t), and the synchronous extraction circuit extracts bit timing pulses from the baseband signals I _i (t) and I _q (t) and Sampling is performed, and the filtered output waveform is sampled and judged at a specified time to obtain the baseband sequence {c' _k };

Since the signal light and the local oscillator light have the same carrier frequency, the output analog signal is directly the baseband signal. The self-homodyne coherent detection digital signal processing algorithm is shown in Figure 4. First, the received signal is filtered by the receiving filter to filter out channel noise and other interference. The waveform is sampled and judged to restore the baseband signal. The bit timing pulse used for sampling is extracted from the received signal by the synchronous extraction circuit. The accuracy of the bit timing directly affects the judgment effect. The amplitude judgment is used to restore the actual transmitted baseband sequence { c' _k }; when the bit error is zero, the recovered baseband sequence {c' _k } is the baseband sequence {c _k }.

Step 6.3: Use the decoder 8-3 to decode the recovered baseband sequence {c' _k } to obtain a decoded output, and complete the transmission of the downlink information.

Specifically, as shown in FIG. 5 , in the first iterative decoding, the probability log-likelihood ratio Λ(u; I) of the input information symbol of the component decoder A is initialized to 0, and the decoded output information symbol The probability log-likelihood ratio Λ(u; O) is used as the input information symbol probability log-likelihood ratio after interleaving

After the decoding output of the component decoder B, the first iterative decoding ends, and then the probability log-likelihood ratio of the information symbols generated by the decoding module of the component decoder B is fed back to the decoding module of the component decoder A after interleaving. , as the prior information for the next round of decoding

Repeat the above process until a certain number of iterations is reached or a certain iterative condition is met; finally, a hard decision is made according to the output information symbol probability log-likelihood ratio Λ(u; O) of the component decoder B, and the decoding output can be obtained. .

Step 7, utilize the second encoder 10 to encode the information source of the uplink;

encoding the uplink information source to obtain the encoded sequence _sk ;

Step 8, using the cat's eye structure of the inverse modulator 9 to change the defocus amount of the second vertically polarized light signal passing through the lens, and changing the second vertically polarized light signal light field E _4ASK through the corresponding relationship between the sequence _ck and the defocus amount to obtain The third vertically polarized light signal.

Table 2 4ASK modulation source correspondence table

Specifically, as shown in Figure 6, when the mirror is located on the defocused surface, the incident light beam will return strictly according to the original path, while it will diverge when it is out of focus. The retroreflected light power is modulated by controlling the defocus amount of the "cat's eye" structure. The reflected echo power _pr of the cat's eye target received by the second coherent receiver 6 is

Among them, p _t is the laser emission power, τ _a is the atmospheric transmittance, τ _r is the transmittance of the receiving optical system, ρ _s is the reflection coefficient, D is the aperture of the focusing lens, f is the focal length of the lens, and θ ₀ is the beam divergence angle , d is the defocus amount, and r is the distance from the inverse modulator to the second coherent receiver 6 . Therefore, the expression of the second vertically polarized light signal light field E _4ASK modulated by the cat's eye is as follows:

The modulation of the optical field E _4ASK is completed by changing the value of d through the arrangement order of (s _k s _k+1 ) ₂ . FIG. 7 is a constellation diagram of the third vertically polarized optical signal after 4ASK modulation.

Step 9. Use the second coherent receiver 6 to perform coherent detection on the third vertically polarized optical signal and the first light source by using a homodyne coherent detection method, and then decode and decode the baseband signal output by the coherent detection to obtain a second vertically polarized optical signal. Optical signal to complete the transmission of uplink information. The baseband signal decoding and decoding method is the same as the downlink.

Through the above methods, in the space-based system underwater communication device and communication method of the present invention, the downlink device adopts the self-homodyne coherent detection method, the uplink adopts the homodyne coherent detection method, and the receiving end of the duplex link is improved. The detection sensitivity is improved, which makes the communication in the space-based system more reliable; the uplink communication adopts the reverse modulation method, and uses the reverse modulation return light characteristic to avoid capturing the receiving light source of the tracking system and the passive end (seawater end), realizing the submarine ( Two-way communication between sea water) and aircraft (atmosphere). Taking advantage of the low attenuation characteristics of blue-green light in the atmosphere and seawater, the downlink adopts the self-homodyne coherent detection method, and the uplink adopts the reverse modulation and homodyne coherent detection method, which realizes the full duplex of aircraft and submarines. Communication Systems.

Claims (2)

1. a space-based system underwater communication method for submerging, utilizing the space-based system underwater communication device to transmit, and the space-based system underwater communication device for submerging comprises a downlink device and an uplink device, so The downlink device comprises a laser (1), the laser (1) is connected with a beam splitter (12), and an output end of the beam splitter (12) is connected with a first polarization beam splitter (2), so the The first output end of the first polarization beam splitter (2) is connected with a modulator (3), the input end of the modulator (3) is connected with a first encoder (4), and the output end of the modulator (3) A polarization beam combiner (5) is connected, and the second output end of the first polarization beam splitter (2) is connected with the polarization beam combiner (5); the output end of the polarization beam combiner (5) is connected through an optical antenna There is a second polarization beam splitter (7), the first output end of the second polarization beam splitter (7) is connected with a first coherent receiver (8), and the second polarization beam splitter (7) has a second The output end is connected with a delay device (11), and the delay device (11) is connected with the first coherent receiver (8); the uplink device includes an inverse modulator (9), and the inverse modulator ( 9) The input end is connected with the second output end of the second polarization beam splitter (7), the input end of the inverse modulator (9) is also connected with a second encoder (10), and the inverse modulator (9) outputs The end is connected with a second coherent receiver (6) through an optical antenna, and the input end of the second coherent receiver (6) is connected to another output end of the beam splitter (12), characterized in that the transmission method includes: Step 1: firstly split the light source to obtain a first light source and a second light source E, and then split the second light source E into a first horizontally polarized light signal _Ex and a first vertically polarized light signal _Ey ; E _x =E _xm cos(ωt-kz+φ _x ) (1); E _y =E _ym cos(ωt-kz+φ _y ) (2); where E _xm and E _ym are the amplitudes of the x and y polarization directions of the optical signal, ω is the angular frequency of the light wave, k is the wave vector, φ _x and φ _y are the initial phases of the x and y polarization directions, respectively, t and z are the time Variables and space variables, the synthetic wave optical signal electric field E= _ex E _x +e _y E _y ; Step 2, using the Turbo code encoding mode to encode the downlink source to obtain the baseband sequence {c _k }; Step 3, using the 16QAM modulation mode to perform every 4 codeword mapping on the baseband sequence { _ck } and then modulate the corresponding amplitude and phase to obtain the second horizontally polarized optical signal light field E _16QAM :

m=(ck _ck+1 ) ₂ , n=( _{ck+2 ck +3} ) ₂ (4); Step 4, using a polarization beam combiner to combine the second horizontally polarized light signal and the first vertically polarized light signal; Step 5, separating the combined optical signals to obtain the third horizontally polarized optical signal _Es and the second vertically polarized optical signal E _Lo ;

Step 6: First perform time T delay on the second vertically polarized optical signal, then perform self-homodyne coherent detection on the third horizontally polarized optical signal and the delayed second vertically polarized optical signal, and output two Road baseband signals I _i (t) and I _q (t); baseband signal processing algorithms are used to decode the baseband signals I _i (t) and I _q (t) to obtain the recovered baseband sequence {c' _k } ; Decode the recovered baseband sequence {c' _k } to complete the transmission of downlink information; Step 7, carry out Turbo code encoding to the information source of the uplink, obtain the sequence _sk after encoding; Step 8, using the cat's eye structure of the inverse modulator to change the defocus amount of the second vertically polarized light signal passing through the lens, and changing the second vertically polarized light signal through the corresponding relationship between the sequence _sk and the defocus amount, obtaining a third vertically polarized light signal; The reflected echo power _pr received by the second coherent receiver (6) from the cat's eye target of the inverse modulator is:

In the above formula, p _t is the laser emission power, τ _a is the atmospheric transmittance, τ _r is the transmittance of the receiving optical system, ρ _s is the reflection coefficient, D is the aperture of the focusing lens, f is the focal length of the lens, and θ ₀ is the beam Divergence angle, d is the defocus amount, r is the distance between the inverse modulator (9) and the second coherent receiver (6); The echo power p _r is modulated by controlling the defocus amount d of the cat's eye structure, and the modulated second vertically polarized light signal light field E _4ASK is:

Complete the modulation of the light field E _4ASK by changing the value of d through the arrangement order of (s _k s _k+1 ) ₂ to obtain a third vertically polarized light signal; Step 9. Coherently detect the third vertically polarized optical signal and the first light source by using a homodyne coherent detection method, and then decode and decode the baseband signal output by the coherent detection to obtain a second vertically polarized optical signal, and complete Transmission of uplink information. 2. a kind of space-based system underwater communication method as claimed in claim 1 is characterized in that, step 6 specifically comprises the following steps: Step 6.1. First use the delay device (11) to delay the second vertically polarized optical signal by time T, and then perform auto-zeroing on the third horizontally polarized optical signal and the delayed second vertically polarized optical signal. Differential coherent detection, output two baseband signals I _i (t) and I _q (t):

where β is the photoelectric conversion coefficient of the balanced detector, and j is the imaginary dimension; Step 6.2, filter the baseband signals I _i (t) and I _q (t) in turn, extract the bit timing pulses from the baseband signals I _i (t) and I _q (t) and sample them, and filter them at a specified time. The processed output waveform is sampled and judged to obtain the recovered baseband sequence {c' _k }; Step 6.3. Use the Turbo code iterative decoder to decode the recovered baseband sequence {c' _k }. During the first iterative decoding, the component decoder A in the Turbo code iterative decoder is used for decoding. The probability log-likelihood ratio of the input information symbol Λ(u; I) is initialized to 0, and the decoded output information symbol probability log-likelihood ratio Λ(u; O) is used as the input information symbol probability log-likelihood after interleaving. ratio:

After the component decoder B in the Turbo code iterative decoder is decoded and output, the first iterative decoding ends, and then the information generated by the component decoder B decoding module in the Turbo code iterative decoder The log-likelihood ratio of the symbol probability is fed back to the decoding module of the component decoder A after interleaving, as a priori information for the next round of decoding:

In the above formula, the superscript represents corresponding to different component decoding modules, the subscript represents the interleaving process, and I ^-1 represents de-interleaving; Repeat the above process until a certain number of iterations is reached or a certain iterative condition is met; finally, according to the output information symbol probability log-likelihood ratio Λ(u; O) of the component decoder B in the Turbo code iterative decoder A hard decision is made to obtain the decoded output, and the transmission of the downlink information is completed.

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