CN115361070A - Space-time hybrid diversity FSO communication system based on optical coding - Google Patents
Space-time hybrid diversity FSO communication system based on optical coding Download PDFInfo
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Abstract
The invention is applicable to the field of communication technology improvement, and provides a space-time hybrid diversity FSO communication system based on optical coding. The transmitting end comprises a transmitter, a time/code division light collection coding module and an optical code space diversity light emitting module, wherein the output end of the transmitter is connected with the input end of the time/code division light collection coding module, and the output end of the time/code division light collection coding module is connected with the input end of the optical code space diversity light emitting module; the receiving end comprises a receiver, a time/code division light collecting and decoding module and a light code space diversity light receiving module, wherein the output end of the light code space diversity light receiving module is connected with the input end of the time/code division light collecting and decoding module, and the output end of the time/code division light collecting and decoding module is connected with the input end of the receiver. The method not only effectively reduces the influence caused by the atmospheric turbulence and increases the reliability of legal users, but also improves the physical layer security of the FSO transmission system.
Description
Technical Field
The invention belongs to the field of communication technology improvement, and particularly relates to a space-time hybrid diversity FSO communication system based on optical coding.
Background
Based on the fact that the optical cable is intercepted, the security problem of optical information transmission in the optical fiber communication system is still to be solved urgently. The secure transmission of optical information requires that a communication system has good security, and should have the functions of survivability resistance, interception resistance, attack resistance, identity authentication and hiding.
Conventional optical network security employs data encryption of network upper layer protocols and assumes that the physical layer has provided smooth and error-free transmission. However, all algorithmic based encryption approaches have proven to be hackable. For example, in 2009, research teams in japan, france, and germany cracked a 768-bit RSA encryption algorithm. Quantum cryptography based on the principles of physical "misdetection" and "indivisible" has absolute security, but due to the limitations of physical mechanisms, current quantum communication technologies are only suitable for low-rate signal transmission.
The main technical scheme of the optical network physical layer security comprises the following steps: chaotic optical communication, optical code division multiple access
(Optical Code Division Multiple Access, OCDMA) and Quantum Noise random encoded Cipher (QNRC) ], etc. The OCDMA communication system has multiple protection functions and can realize the safe transmission of optical information. 1) Anti-interception, "prism door" events have exposed 200 cables to interception, which severely threatens the security of information transmission. The OCDMA system is based on a spread spectrum mechanism of time-frequency domain transformation and a security system, so that the OCDMA system has a strong anti-interception function. 2) And in the face of malicious intrusion, the OCDMA system can adopt measures such as frequency hopping coding or code word reconstruction and the like, effectively avoids the influence of an intrusion optical signal, and ensures the normal operation of the system, so that the OCDMA system has the anti-attack capability and ensures the safety of information communication. 3) The OCDMA system gives a unique optical domain address code to each user, an unauthorized user cannot acquire signals of other users transmitted in the system, and the system can only receive the signals of the user, and can confirm the identity of each user at any time through the dynamically reconfigurable address code, so that the credible transmission of information is ensured. 4) The confidentiality is realized, the information transmission with high confidentiality requirement is adopted, and the technical difficulty of discovery is increased, so that the security is improved. The OCDMA system utilizes the characteristic of spread spectrum and spread time to change the transmitted signal into noise-like signal which is hidden in the conventional transmission system and even in the background noise.
The OCDMA technology has certain security and confidentiality, but the basic security guarantees are not enough when aiming at certain more targeted and powerful eavesdropping strategies such as energy detection, brute force search, code word interception and differential detection eavesdropping. For example, in an OOK-modulated OCDMA system, an eavesdropper can successfully recover the data information in the channel by only using the energy of each bit in the encoded signal as a decision criterion without knowing the address codeword information. Therefore, the OCDMA physical layer encryption can improve the security of the existing FSO communication system, but cannot ensure absolute security, and there is a possibility of code word cracking, so that it is necessary to combine with other technologies to improve the performance of the system.
FSO communications systems suffer from reliability and security issues. Common methods to solve the reliability problem are spatial diversity and time diversity techniques. However, space diversity requires a sufficient distance between transmitting/receiving end lenses, and in the case of a limited field, the reliability of the system cannot be improved by increasing the diversity number, and meanwhile, for a space diversity system with multiple inputs and multiple outputs, the physical layer security cannot be improved by space diversity. Time diversity requires sufficient time delay between branches and minimum correlation, and when the diversity number increases, multiple access interference increases, which decreases the signal-to-noise ratio of a legitimate user, thereby decreasing the reliability of the legitimate user.
Disclosure of Invention
The invention aims to provide a space-time hybrid diversity (FSO) communication system based on optical coding, and aims to solve the technical problem.
The invention is realized in this way, a space-time hybrid diversity FSO communication system based on optical coding, the space-time hybrid diversity FSO communication system based on optical coding includes a sending end, an atmospheric channel and a receiving end, the sending end modulates user data and sends the modulated user data to the receiving end through the atmospheric channel to restore the original signal to be displayed to the user, the sending end includes a transmitter, a time/code division optical coding module and an optical code space diversity optical emission module, the output end of the transmitter is connected with the input end of the time/code division optical coding module, the output end of the time/code division optical coding module is connected with the input end of the optical code space diversity optical emission module; the output end of the optical code space diversity optical transmission module transmits the coupling signal to an atmosphere channel through a plurality of collimation transmission lenses; the receiving end comprises a receiver, a time/code division optical collecting and decoding module and an optical code space diversity optical receiving module, wherein the output end of the optical code space diversity optical receiving module is connected with the input end of the time/code division optical collecting and decoding module, and the output end of the time/code division optical collecting and decoding module is connected with the input end of the receiver.
The further technical scheme of the invention is as follows: at a sending end, data signals respectively enter different optical encoders in the time/code division light collection encoding module to be encoded, time/code diversity encoding is realized by respectively giving different delays to each path of encoding signals, each path of encoding signals are combined and sent to an optical code space diversity light emitting module by using a coupler, and the optical code space-time mixed diversity sending is realized by sending through a plurality of paths of FSO links.
The further technical scheme of the invention is as follows: the method comprises the steps that uncoded optical pulse signals are divided into w paths at a sending end through an optical splitter, different optical encoders are adopted for coding the w paths, different time delays are given to the w paths according to the correlation between code words and the correlation time of an atmospheric channel, and finally the w paths of coded signals are combined through a coupler and sent to an optical code space diversity transmission module.
The further technical scheme of the invention is as follows: in a time/code division light collection decoding module at a receiving end, a received light coding signal is divided into w paths through an optical splitter, each path adopts a corresponding matching decoder, complementary delay is given at the same time, the complementary delay is used for aligning the decoding signals of the w paths, after the light decoding signals are coupled, photoelectric conversion is realized by an optical receiver, and a user data signal is recovered through judgment, so that a space-time mixed diversity mechanism based on light coding is realized.
The further technical scheme of the invention is as follows: the time/code division optical coding module performs physical layer time diversity optical coding on user data in a time diversity mode, different optical encoders are used in different branches, relative delay is set according to code word construction and atmospheric turbulence coherence time, and multi-path coded data are combined.
The further technical scheme of the invention is as follows: the optical code space diversity light emitting module emits combined signals of different codes of each branch into the atmosphere through a plurality of collimation emitting lenses, and the number of the lens pairs is flexibly selected according to the environment condition.
The further technical scheme of the invention is as follows: the optical code space diversity optical receiving module receives optical signals through a plurality of collimating receiving lenses and combines the optical signals through a coupler to complete optical code space diversity reception.
The further technical scheme of the invention is as follows: the time/code division light collection decoding module performs light decoding in a time diversity mode, performs matching decoding on the light signals of different branches by using different matching light decoders, and completely aligns the light decoding signals of different branches through different complementary delays to complete time/code division light collection decoding.
The further technical scheme of the invention is as follows: the relative delay must be larger than the coherence time of the atmospheric channel and ensure that the multiple access interference between different branches is minimal.
The further technical scheme of the invention is as follows: the relative time delay between the time diversity optical decoders is complementary to the relative time delay of the transmitting end.
The beneficial effects of the invention are: as the time/code division optical coding module is adopted at the transmitting end, for the eavesdropping user, the received signal is the result after a plurality of different coded signals are coupled, and the safety of the FSO communication system can be improved by comparing with a single coded signal. Meanwhile, for a legal user, the time/code division light collection coding/decoding module and the optical code space diversity light emitting/receiving module are adopted for transmission at the same time, namely, the time/code division and space diversity are adopted at the same time, so that the influence caused by turbulence is effectively reduced, and the reliability of the legal user is improved. Therefore, by adopting the hybrid diversity FSO communication system based on the time/code division and the space diversity of the optical coding, the reliability and the safety of the system can be simultaneously improved.
Drawings
Fig. 1 is a diagram of a single-user system architecture of an FSO communication system based on optical coding and space-time hybrid diversity according to an embodiment of the present invention.
Fig. 2 is a general scheme architecture diagram of the system provided by the embodiment of the invention.
Fig. 3 is a schematic block diagram of a transmit-side time/code division optical coding module of an FSO communications system according to an embodiment of the present invention.
Fig. 4 is a schematic block diagram of a time/code division optical decoding module at a receiving end of the FSO communication system according to an embodiment of the present invention.
Fig. 5 is a structural diagram of a 2-diversity time/code division optical coding module according to an embodiment of the present invention.
Fig. 6 is a cross-correlation diagram of two codewords provided by an embodiment of the invention.
Detailed Description
As shown in fig. 1, the space-time hybrid diversity FSO communication system based on optical coding provided by the present invention includes a transmitting end, an air channel and a receiving end, the transmitting end modulates user data and transmits the modulated user data to the receiving end via the air channel to restore the modulated user data into original signals, and displays the original signals to a user, the transmitting end includes a transmitter, a time/code division optical coding module and an optical code space diversity optical transmission module, an output end of the transmitter is connected to an input end of the time/code division optical coding module, and an output end of the time/code division optical coding module is connected to an input end of the optical code space diversity optical transmission module; the output end of the optical code space diversity optical transmission module transmits the coupling signal to an atmospheric channel through a plurality of collimation transmission lenses; the receiving end comprises a receiver, a time/code division light collection decoding module and a light code space diversity light receiving module, wherein the output end of the light code space diversity light receiving module is connected with the input end of the time/code division light collection decoding module, and the output end of the time/code division light collection decoding module is connected with the input end of the receiver.
At a sending end, after data signals respectively enter different optical encoders at a time/code division optical encoding module for encoding, different delays are respectively given to each path of encoded signals, so that time/code diversity encoding is realized; then, the coupler is used for combining and sending each path of coded signals to the optical code space diversity light emitting module, and the coded signals are sent through multiple paths of FSO links, so that optical code space-time hybrid diversity sending is realized. At the receiving end, the received signal of the optical code space diversity is a mixed signal of a plurality of coded signals, and the time/code division optical decoding module and the receiver recover corresponding data information from the signal.
The time/code division light collection coding/decoding module is used, at a sending end, an optical pulse signal is divided into w paths through an optical splitter, each path is coded by adopting a different optical coder, meanwhile, each path gives different time delays according to the correlation between code words and the correlation time of an atmospheric channel, and finally, the w paths of coded signals are combined through a coupler and sent to an optical code space diversity sending module; to achieve time diversity, the relative time delay between time-diversity optical encoders must satisfy two conditions: (1) greater than the coherence time (ms order) of the FSO channel; (2) The cross correlation value between the code words is ensured to be minimum, so that the multiple access interference is reduced, and the signal-to-noise ratio and the reliability are improved. Then, the optical code space diversity optical transmission module transmits through multiple FSO links to realize optical code space diversity. Then, optical decoding is performed in a time/code diversity manner, so as to restore the coded optical signal transmitted by the user to the original optical signal. And finally, sending the decoded optical signal to an optical receiver, and recovering the decoded optical signal to an original signal after photoelectric detection, amplification, filtering and sampling judgment.
In a time/code division light collection decoding module at a receiving end, a received light coding signal is divided into w paths through an optical splitter, each path adopts a corresponding matching decoder, and complementary delay is given at the same time, wherein the complementary delay is used for aligning the decoding signals of the w paths. The relative time delay between the time diversity optical decoders is complementary to the relative time delay of the transmitting end. Then, after the signals after optical decoding are coupled, the optical receiver realizes photoelectric conversion, and recovers user data signals through judgment, thereby realizing a space-time hybrid diversity mechanism based on optical coding.
At a receiving end, a received signal is a mixed coding signal of a plurality of spatial links, in a time/code diversity optical decoder module, the mixed coding signal is input to the time/code diversity optical decoder through an optical splitter for matching decoding, complementary delay is given, and then after the decoded signal is coupled, an optical receiver recovers corresponding data information.
The time/code division light collecting encoder comprises a 1 xw optical splitter, w different optical encoders, (w-1) different optical delay lines and a w x 1 optical coupler which are connected in sequence. The optical splitter can divide an original optical signal into w parts of the same optical signal, each path of optical signal is subjected to different optical coding, and different optical encoders adopt different code words. Then, each optical code signal is delayed differently, and the relative delay must satisfy two conditions: (1) greater than the coherence time of the FSO channel; (2) The cross-correlation value between the code words is guaranteed to be minimum, so that the multiple access interference is reduced. And finally, coupling the w optical coding signals through a coupler to form the time/code division optical collecting encoder module. Similarly, at the time/code diversity optical decoder, w paths of the same coded optical signals are decoded by corresponding matched decoders through the splitters, and the decoded signals are aligned through parallel complementary delay lines and finally combined to the output of the coupler.
The optical transmitter is used for converting an original electric signal of user data into an optical signal;
the time/code division optical collecting and coding module is used for carrying out optical coding of a physical layer on user data in a time diversity mode to realize time diversity coding. Different optical encoders are adopted for different branches, and meanwhile, proper time delay needs to be set according to the code word construction and the atmospheric turbulence coherence time. The relative delay must be larger than the coherence time of the atmospheric channel and ensure that the multiple access interference between different branches is minimal.
And the optical code space diversity light emitting module emits coupling signals of different codes of each branch into the atmosphere through a plurality of collimation emitting lenses. According to the weather condition and the field size, the number of the lens pairs can be flexibly selected. Meanwhile, under the conditions of limited places and bad weather, the reliability of the system can be ensured by increasing the time diversity number instead of increasing the space diversity number.
The optical code space diversity optical receiving module receives optical code signals by adopting a plurality of collimating receiving lenses and then combines the optical code signals by a coupler, thereby realizing optical code space diversity reception. Then, the optical signals are divided into different branch optical signals by an optical splitter.
The time/code collection optical decoding module performs optical decoding in a time diversity mode, and optical signals of different branches adopt different optical decoders and are respectively subjected to matching decoding. Then, complementary delay is carried out, so that optical decoding signals of different branches are completely aligned, and the coded optical signals transmitted by the user are reduced into original optical signals, thereby realizing time/code diversity decoding.
The relative delay must be larger than the coherence time of the atmospheric channel and ensure that the multiple access interference between different branches is minimal.
The relative time delay between the time diversity optical decoders is complementary to the relative time delay of the transmitting end.
The optical receiver is used for recovering the decoded optical signal into an original signal after photoelectric detection, amplification, filtering and sampling judgment.
In the present invention, at the transmitting end, a legal user 1 first converts data into an optical signal through a transmitter, and then performs time/code division optical coding. Taking two-diversity as an example, an optical signal is divided into two identical parts by a coupler, and different optical codes are respectively carried out, and one of the two paths of delays is given at the same time, and the relative delay must satisfy two conditions: (1) greater than the coherence time of the FSO channel; (2) The cross-correlation value between the code words is guaranteed to be minimum, so that the multiple access interference is reduced. And then coupling the encoded data to realize time/code division optical encoding. Then, the two parts divided into the same are emitted into the atmosphere through two collimating emission lenses. When there is sufficient distance or delay between the collimating lens pair that is greater than the coherence time of the atmospheric turbulence, the effect of the atmospheric turbulence on the signal is uncorrelated, and therefore the reliability of the system can be increased. At a receiving end, the two corresponding collimating receiving lenses receive optical signals and then combine the optical signals through the coupler to realize space diversity FSO-CDMA. Then, time/code division optical decoding is performed. The optical splitter divides the optical signal into two identical parts, respectively carries out matching decoding, and simultaneously gives complementary time delay to the corresponding branch. Finally, the decoded signal is coupled and transmitted to the legal user 2. Meanwhile, assuming that an eavesdropping user exists among legal users, the eavesdropping user eavesdrops a certain proportion of signals at a position close to a receiving end, but the eavesdropping user cannot know specific code words of the legal users, so that only non-matching decoding can be adopted. Because the time/code division optical coding is adopted at the sending end, the information obtained by the eavesdropping user is the result after two different coding signals are coupled, the difficulty of the eavesdropping user in cracking is increased, and the safety of a physical layer of the system is further improved. Meanwhile, for legal users, the reliability of the system is further improved due to the simultaneous adoption of time/code division and space diversity technologies. Therefore, the scheme of the space-time hybrid diversity FSO system based on optical coding can simultaneously improve the reliability and the safety of the existing FSO communication system.
As shown in fig. 2, at the transmitting end, after n user data are modulated respectively, m OCDMA codes of different groups and (m-1) different relative delays are performed to implement time/code division optical coding. Then, space diversity is realized through L collimation transmitting lenses respectively, and the signals are sent to an atmospheric link to realize optical code space diversity FSO-CDMA. At the receiving end, the user firstly carries out corresponding optical code space diversity reception through L collimating receiving lenses, and then realizes space-time hybrid diversity reception through corresponding time/code diversity optical decomposition and complementary time delay. And finally, coupling the aligned signals to an optical receiver for judgment, and restoring the signals into user data.
After user data passes through the optical modulator, the user data is converted into optical signals and then subjected to time/code division optical coding, and under the condition that a legal user address code is unknown, an eavesdropping user cannot restore the user data by using non-matching decoding. Meanwhile, as time/code diversity is used at the transmitting end, and different optical coding signals are coupled, the interference to eavesdropping users is increased, so that the information is more difficult to crack. The effect is more pronounced when multiple users are present. The optical signals after coupling of each user are transmitted to the atmosphere in a diversity mode through the collimating transmitting lens group, in order to guarantee the quality of received signals, enough distance exists between the collimating lens pairs, and the number of the collimating lens pairs can be flexibly adjusted according to weather conditions and field sizes.
In a hybrid diversity system, each user employs m different optical codes and m-1 delays (i.e., m time diversity) while transmitting using L collimating lenses (i.e., L space diversity). The time diversity number and the space diversity number can be flexibly adjusted by requirements, for example, when the requirement of the system on safety is higher, the time diversity number can be increased. Although the increase of the time diversity number can generate multiple access interference to influence the reliability of a legal user, compared with single time diversity, the space diversity increased by the hybrid diversity system improves the reliability of the system; and when the system has higher requirement on reliability, the amount of space diversity can be increased. Compared with single space diversity, proper time diversity can not only improve the safety of the system, but also improve the reliability. When the system resource is limited, compared with single time diversity or space diversity, the hybrid diversity system is more flexible and can meet various requirements, and when the diversity number is the same, the hybrid diversity can provide better performance.
The time delay of the time/code collection optical coding is described as follows: the coherence time interval of the atmospheric turbulence is 0.1ms to 10ms. Therefore, to ensure that the two signals are completely uncorrelated, the relative delay of each signal should be greater than the coherence time, at which time the atmospheric turbulence experienced by the two signals is completely uncorrelated. The cross-correlation interference between the encoded signals depends on the relative delays of the two optical address codes. In the following, the optical orthogonal code of (40, 3, 1) is used to explain, the code words of the two branches of the time diversity are (1, 5, 13) (1, 8, 21), and the cross-correlation diagram of the two code words is shown in fig. 6. It can be seen that delays of different chirp widths result in different cross-correlations, such as delay 3 with a cross-correlation of 1 and delay 4 with a cross-correlation of 0. Therefore, when the number of time/code diversity increases, multiple access interference inevitably occurs due to the increase of the number of encoders, thereby reducing the reliability of the system, and therefore, the time/code diversity system is not as good as the higher the number of diversity. In order to further improve the reliability of the FSO-CDMA communication system, it is necessary to adopt the space-time hybrid diversity method proposed by this patent.
As shown in fig. 3, at the transmitting end, the optical signal is divided into two paths by the optical splitter, and two paths of different optical codes are provided, while two paths of different delays are provided. Finally, the coupled time/code diversity signal is output through the coupler. As shown in fig. 4, at the receiving end, the received time/code diversity coded signal is divided into two parts by an optical splitter to perform respective matching decoding, and meanwhile, one of the two parts is given a complementary delay, and finally, the coupler outputs the coupled optical signal. The relative delay must satisfy two conditions: (1) greater than the coherence time of the FSO channel; (2) The cross-correlation value between the code words is guaranteed to be minimum, so that the multiple access interference is reduced.
FIG. 5 is a diagram of a two-diversity time/code division optical encoding module. After an optical signal modulated by a transmitting end is divided into two paths by a coupler, one path of signal is divided into three paths by the coupler, and the three paths of signal are coupled to form a path of optical coding signal after different time delays; the other path is firstly passed through tau 1 =10ms and τ 2 A delay of 0.025ns after which a different optical coding is performed. And finally, merging the two paths of coded signals to finish time/code division optical coding. It should be noted that the optical codes of different users must use different address codes to reduce the effect of multiple access interference.
In the invention, as the time/code division optical coding is adopted at the transmitting end, for the wiretap user, the received signal is the result after two different coding signals are coupled, thereby improving the safety of the FSO communication system. Meanwhile, for the legal user, the method of space diversity and time/code division light coding is adopted for transmission, so that the reliability of the legal user is improved. Therefore, the reliability and the safety of the system can be simultaneously improved by the hybrid diversity FSO communication system based on the space-time diversity of the optical coding. The invention is particularly suitable for areas which have requirements on both communication reliability and safety but are inconvenient for erecting optical fibers, such as interconnection among urban mansions, cross-valley communication, post-disaster communication establishment, communication connection among islands and the like, and has urgent market demand and wide application prospect.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.
Claims (10)
1. A space-time hybrid diversity FSO communication system based on optical coding comprises a sending end, an atmospheric channel and a receiving end, wherein the sending end modulates user data and sends the modulated user data to the receiving end through the atmospheric channel to restore the modulated user data into original signals to be displayed to a user; the output end of the optical code space diversity optical transmission module transmits the coupling signal to an atmosphere channel through a plurality of collimation transmission lenses; the receiving end comprises a receiver, a time/code division light collection decoding module and a light code space diversity light receiving module, wherein the output end of the light code space diversity light receiving module is connected with the input end of the time/code division light collection decoding module, and the output end of the time/code division light collection decoding module is connected with the input end of the receiver.
2. The space-time hybrid diversity FSO communication system based on optical coding as claimed in claim 1, wherein at the transmitting end, the transmitter converts the original electrical signal of the user data into optical signals, the optical signals enter different optical encoders in the time/code division optical coding modules for coding, different delays are given to the coded signals to implement time/code diversity coding, the couplers are used to combine the coded signals to transmit to the optical code space diversity optical transmission module, and the signals are transmitted through multiple FSO links to implement the space-time hybrid diversity transmission of the optical codes.
3. The space-time hybrid diversity FSO communication system based on the optical coding as claimed in claim 2, wherein the uncoded optical pulse signals are divided into w paths at the transmitting end through an optical splitter, each path is coded by using a different optical coder, and meanwhile, each path gives different time delays according to the correlation between the code words and the correlation time of the atmospheric channel, and finally, the w paths of coded signals are coupled and transmitted to the optical code space diversity transmission module through a coupler.
4. The space-time hybrid diversity FSO communication system based on the optical coding as claimed in any of claims 1 to 3, wherein in the time/code division optical collecting and decoding module at the receiving end, the received optical coding signal is divided into w paths by an optical splitter, each path adopts a corresponding matching decoder, and complementary delay is given at the same time, the complementary delay is used for aligning the decoded signals of the w paths, after the optical decoded signals are coupled, the optical receiver realizes the photoelectric conversion, and the user data signal is recovered through judgment, thereby realizing the space-time hybrid diversity mechanism based on the optical coding.
5. The space-time hybrid diversity FSO communication system based on optical coding as claimed in claim 4, wherein the time/code division optical coding module performs physical layer time diversity optical coding on the user data in a time diversity manner, uses different optical encoders in different branches, sets relative delay according to the codeword construction and the atmospheric turbulence coherence time, and couples multiple channels of coded data.
6. The space-time hybrid diversity FSO communication system based on optical coding as claimed in claim 5, wherein the optical code space diversity optical transmitter module transmits the coupled signal after different coding of each branch to the atmosphere via a plurality of collimating transmitting lenses, and the number of lens pairs is flexibly selected according to the environment.
7. The space-time hybrid diversity FSO communication system based on optical coding as claimed in claim 6, wherein the optical code space-division optical receiving module receives optical signals through a plurality of collimating receiving lenses and combines the optical signals by a coupler to complete optical code space-division reception.
8. The FSO communication system according to claim 7, wherein the time/code-division optical receiver decoder module performs optical decoding in a time-diversity manner, performs matched decoding on the optical signals of different branches by using different matched optical decoders, and performs time/code-division optical decoding by performing different complementary delays to completely align the optical decoded signals of different branches.
9. An optical code based space-time hybrid diversity FSO communication system according to claim 8, wherein the relative delay must be larger than the coherence time of the atmospheric channel and to ensure minimum multiple access interference between different branches.
10. The FSO communication system according to claim 9, wherein the relative delay between the time diversity optical decoders is complementary to the relative delay at the transmitting end.
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