WO2020024439A1 - 一种自由空间光网络的物理层安全增强方法 - Google Patents

一种自由空间光网络的物理层安全增强方法 Download PDF

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WO2020024439A1
WO2020024439A1 PCT/CN2018/110751 CN2018110751W WO2020024439A1 WO 2020024439 A1 WO2020024439 A1 WO 2020024439A1 CN 2018110751 W CN2018110751 W CN 2018110751W WO 2020024439 A1 WO2020024439 A1 WO 2020024439A1
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signal
physical layer
interference
optical
free
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French (fr)
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吉建华
陈雪梅
王可
徐铭
张志朋
杨淑雯
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深圳大学
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/564—Power control
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H04B10/524—Pulse modulation

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  • the invention belongs to the field of optical communication technology improvement, and particularly relates to a method for enhancing the physical layer security of a free-space optical network.
  • Free-space optical (FSO) communication has the advantages of high bandwidth, no license, fast deployment, and high flexibility. It is also considered as a promising solution to solve the last mile problem in wireless communication.
  • Wireless communication method when it is difficult to achieve communication using optical fiber, FSO can provide very high-speed point-to-point communication.
  • FSO communication can be used for many optical links, such as satellite-to-ground data links, ad hoc relay links that are not limited by fiber optic networks, and so on.
  • FSO is also affected by optical eavesdropping. For example, eavesdroppers deliberately block the light beam to block the line of sight of the intended receiver, or the divergent beam has a large footprint at the receiver, covering both the receiver and the eavesdropper. FSO has security risks.
  • Eavesdroppers can also obtain link information from non-line-of-sight scattered channels, making FSO communication systems suffer from information security problems.
  • atmospheric turbulence and the intensity of the beam will affect the security of the FSO communication system, and the confidentiality and secure transmission of information are the basic requirements for communication between users. Therefore, it is imperative to apply effective technologies to improve the security of FSO communication systems.
  • OCDMA Optical Code Division Multiple Access
  • QNRC Quantum Noise Random Coding
  • the OCDMA system assigns a unique optical domain address code to each user. Unauthorized users cannot obtain the signals of other users transmitted in the system, ensuring that users can only receive their own signals. Dynamically reconfigurable addresses Code, the system can confirm the identity of each user at any time to ensure the trusted transmission of information. 4) Concealment, requiring high confidentiality
  • the transmission of information uses hidden transmission, which increases the technical difficulty of the discovery, thereby increasing its security.
  • the OCDMA system uses its spread spectrum and time spreading characteristics to transform the transmitted signal into noise-like signals, which are hidden in conventional transmission systems and even hidden in background noise.
  • the purpose of the present invention is to provide a physical layer security enhancement method for a free space optical network, which aims to solve the above technical problems.
  • a physical layer security enhancement method of a free space optical network includes the following steps:
  • the legal user uses a demodulator to demodulate the photoelectrically detected signal to obtain a data signal sent by the sender Alice.
  • step S5 further includes the following steps:
  • the receiving end eavesdropping user Eve uses a decoder to perform corresponding non-matching decoding on the received optical signal.
  • step S7 further includes the following steps:
  • the eavesdropping user cannot demodulate the photoelectrically detected signal by using a demodulator and cannot obtain the data signal sent by the sender Alice.
  • a further technical solution of the present invention is: the interference module in step S1 sends an interference signal through power control according to the channel state estimation.
  • a further technical solution of the present invention is that the encoder uses an OCDMA optical encoder to encode a signal using an optical orthogonal code, and the decoder uses an OCDMA optical decoder to decode the signal using an optical orthogonal code.
  • a further technical solution of the present invention is that the modulator uses a MWPPM modulator, and the signal is demodulated on the optical carrier by the transmitter MWPPM modulator; the receiver MWPPM modulator demodulates the signal to obtain the original data signal sent by the transmitter.
  • a further technical solution of the present invention is: the interference signal sent by the interference module reduces the signal-to-noise ratio of the eavesdropping user.
  • the OCDMA optical encoder may adopt one-dimensional encoding or two-dimensional encoding.
  • a further technical solution of the present invention is that the OCDMA encoder uses a fiber delay line or a fiber grating or a liquid crystal or an AWG or a PLC.
  • the beneficial effect of the present invention is that for eavesdropping users, the optical coding encryption of the physical layer must be cracked, thereby improving the security of the FSO transmission system.
  • Add self-interference signals at the transmitting end to reduce the signal-to-noise ratio of eavesdropping users.
  • the use of MWPPM modulation greatly improves the utilization of optical power in FSO / CDMA systems, and also avoids the shortcomings of OOK modulation systems that need to set dynamic decision thresholds.
  • This patent can transform the existing FSO communication system, add interference modules, install optical encoding / decoding security equipment on the existing FSO communication line, improve the anti-interception capability of the communication system, and form a new and reliable FSO security communication system; A new and secure FSO communication network system can also be established.
  • FIG. 1 is a method for enhancing physical layer security of a free-space optical network according to an embodiment of the present invention
  • FIG. 2 is a FSO / CDMA patching system based on self-interfering MWPPM according to an embodiment of the present invention.
  • FIG. 3 is a system block diagram of a scrambling module according to an embodiment of the present invention.
  • FIG. 4 is a first schematic diagram of 2-4-MWPPM pulse position modulation according to an embodiment of the present invention.
  • FIG. 5 is a schematic diagram 2 of a 2-4-MWPPM pulse position modulation according to an embodiment of the present invention.
  • FIG. 6 is a block diagram of an optical fiber delay line encoder and decoder according to an embodiment of the present invention.
  • the physical layer security enhancement method of the free space optical network As shown in FIG. 1, the physical layer security enhancement method of the free space optical network provided by the present invention is detailed as follows:
  • Step S1 Alice, a legitimate user of the sender, sends a data signal, and the interference module sends an interference signal according to the channel state estimation.
  • the interference module sends an interference signal according to the channel state estimation.
  • two legal users Alice and Bob, an eavesdropper, and an interference module are included.
  • Controllable interference signals At the signal transmitting end, after the legitimate user Alice and the interference module send out signals, the interference signals are sent under the channel state estimation by the power control interference module, and there are multiple interference modules.
  • step S2 the data signal and the interference signal are modulated by a modulator according to different data symbols.
  • the transmitted data signal and the interference signal are modulated to the optical carrier using the MWPPM modulator according to the different data symbols in the network. on.
  • Step S3 using different encoders to encode the modulated data signal and the optical signal of the interference signal; passing the demodulated signals through different OCDMA optical encoders, and using optical orthogonal codes (OOC) to encode the information .
  • OOC optical orthogonal codes
  • step S4 the encoded data signal and the interference signal are coupled to the atmospheric turbulent channel for transmission using a coupler; the encoded legal user signal and the interference signal are coupled to the FSO atmospheric channel for transmission using the coupler.
  • step S5 the legal user Bob at the receiving end uses the decoder to perform corresponding matching decoding on the received optical signal.
  • the legal user Bob and the eavesdropping user use the optical encoder to decode the received signal accordingly.
  • step S6 the photodetector APD is used to perform photoelectric detection on the signal; the APD is used as the photodetector at the receiving end.
  • Step S7 The legal user uses a demodulator to demodulate the photoelectrically detected signal to obtain a data signal sent by the sender Alice.
  • the receiver Bob of the legitimate user decodes the matching decoder, and then the MWPPM demodulator recovers the original data information.
  • the eavesdropping user Eve knows the information of the FSO / CDMA system, such as the data rate, encoding type, and codeword structure. However, it does not know the specific codeword used by the user, so Eve can only use a non-matching decoder for decoding. Therefore, for eavesdropping users, the optical coding and encryption at the physical layer must be cracked to correctly recover user data, thereby improving the physical layer security of the FSO transmission system.
  • the proposed self-interfering MWPPM FSO / CDMA patching system includes two legitimate users, Alice and Bob, an eavesdropper, and m interference modules.
  • the interference module sends m power-controllable interference signals.
  • the user data and interference data are first modulated onto the optical carrier by the MWPPM modulator, and then passed through their respective OCDMA optical encoders, using optical orthogonal codes (OOC) Encode the information.
  • OOC optical orthogonal codes
  • a coupler is used to couple the encoded legal user signal and interference signal to the FSO atmospheric channel for transmission.
  • legal user Bob and eavesdropping user use the optical encoder to decode the received signal.
  • Bob of the legal user receiver decodes the matching decoder, and then the MWPPM demodulator restores the original data.
  • Information while eavesdropping on the user, Eve knows the FSO / CDMA system information, such as the data rate, encoding type, and codeword structure, but it does not know the specific codeword used by the user. Therefore, Eve can only use a non-matching decoder to decode . Therefore, for eavesdropping users, the optical coding and encryption at the physical layer must be cracked to correctly recover user data, thereby improving the physical layer security of the FSO transmission system.
  • avalanche photodiodes In high-speed optical communications, avalanche photodiodes (APDs) have a better sensitivity than PIN photodiodes because they amplify weak photocurrents. Therefore, in this patent, APD is used as the photodetector at the receiving end.
  • the FSO / CDMA system bonding channel model proposed in this application adding a self-interference signal, will reduce the reception signal-to-noise ratio of the eavesdropping user Eve, thereby achieving the purpose of improving the security of the FSO / CDMA system at the physical layer.
  • PPM does not need to set a dynamic decision threshold at the receiving end. It only needs to compare the output current of different time slots to determine and recover the signal.
  • PPM has a higher power utilization rate and is resistant to interference. With strong capability, it is currently the most suitable modulation method for FSO communication. However, when the system has a long transmission distance, FSO / CDMA systems using PPM modulation are also affected by pulse stretching. Therefore, in this application, we use multi-wavelength PPM (MWPPM) to further improve the performance of the system.
  • MWPPM multi-wavelength PPM
  • each interference module sends an interference signal to reduce the SNR of the eavesdropping user.
  • the transmitting end estimates the channel state of FSO / CDMA, and then uses the power control module to control the transmission power of the interference signal, thereby achieving the role of improving the physical layer security of the FSO / CDMA system. For example, when the weather is very clear, the attenuation of the FSO atmospheric channel is small. Since the transmit power of the legitimate user Alice remains unchanged, we can use the power control module to increase the power of the interference signal while maintaining the reliability of the FSO / CDMA system. , Thereby increasing the interference of interference signals to eavesdropping users, and improving the security of FSO / CDMA systems.
  • Multi-wavelength PPM is a modulation method in which wavelength shift keying and PPM are combined with each other.
  • MWPPM is characterized by two parameters, the number of wavelengths and the number of time slots.
  • M-L-MWPPM M represents the number of wavelengths
  • L represents the number of time slots.
  • optical pulse modulation is transmitted in a combination of the sum of M wavelengths and L time slots.
  • a combination of 2 wavelengths and 4 time slots is selected to send optical pulses.
  • user data 000, 001, 010, and 011 select the wavelength to be used, while 100, 101, 110, and 111 select the wavelength to be used, and the specific time slot position of the light pulse can be determined according to traditional PPM modulation.
  • Schematic diagram of 2-4-MWPPM pulse position modulation as shown in Figure 4-5.
  • the light pulses used in Figure 4 represent the user data 000, 001, 010, and 011, and the light pulses used in Figure 5 represent the user data.
  • 100, 101, 110, 111 Compared with traditional PPM, M-L-MWPPM can reduce the channel symbol rate.
  • optical encoder is used to encode the optical signal.
  • the optically encoded optical signal is similar to low-power random noise. It is difficult for eavesdroppers to recover data with other address codes. Only specific address codes can recover data.
  • the optical decoder In order to obtain the original optical signal, there are strict requirements for the optical decoder. The original optical signal can be recovered only after the relevant optical decoder is decoded. If the optical decoder does not match the optical encoder, the decoder What is obtained is a noise signal.
  • FIG. 6 is a block diagram of an optical encoder and decoder using a fiber delay line.
  • the optical encoder and optical decoder are composed of several parallel optical fibers and two 1 ⁇ w star couplers. Each of them is determined according to the designed optical orthogonal code.
  • the number of delay line branches equals the number of chips (ie, the weight) of the address code.
  • the role of the optical encoder is to generate a corresponding address code sequence, as shown in Fig. 6 (a).
  • the structure of the optical decoder is similar to that of the encoder, except that the fiber delay is complementary, as shown in Figure 6 (b). Its function is to correlate the input signal with the decoder to achieve matching decoding.
  • the delay of the fiber delay line is
  • a self-interference signal is added at the transmitting end to reduce the signal-to-noise ratio of the eavesdropping user.
  • a power control module can be used to control the power of the interference signal and strengthen the interference of the interference signal to the eavesdropping user. Role, thereby achieving the role of improving the physical layer security of the FSO / CDMA system.
  • MWPPM modulation greatly improves the utilization of optical power in FSO / CDMA systems, and also avoids the shortcomings of OOK modulation systems that need to set dynamic decision thresholds.
  • This patent can transform the existing FSO communication system, add interference modules, install optical encoding / decoding security equipment on the existing FSO communication line, improve the anti-interception capability of the communication system, and form a new and reliable FSO security communication system; A new and secure FSO communication network system can also be established.

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Abstract

本发明涉及一种自由空间光网络的物理层安全增强方法，包括：S1、发送方合法用户Alice发送数据信号，干扰模块根据信道状态估计发送干扰信号；S2、根据不同数据符号对数据信号和干扰信号利用调制器进行信号调制；S3、利用不同的编码器对调制过后的数据信号和干扰信号的光信号进行编码；S4、利用耦合器将编码后的数据信号和干扰信号耦合到大气湍流信道中传输；S5、接收端合法用户Bob利用解码器对接收的光信号进行对应的匹配解码；S6、利用光电探测器APD对信号进行光电检测；S7、合法用户利用解调器对光电检测过的信号进行解调获取发送方Alice发送的数据信号。从而提高了FSO传输系统的安全性。

Description

一种自由空间光网络的物理层安全增强方法 技术领域
本发明属于光通信技术改进领域，尤其涉及一种自由空间光网络的物理层安全增强方法。
背景技术
自由空间光(FSO)通信具有高带宽、不需许可证、快速部署及灵活性高等优点，也被认为是解决无线通信中最后一公里问题的很有前途的一种解决方案,作为一种新型的无线通信方式，当利用光纤很难实现通信的时候，FSO可以提供非常高速的点对点通信。此外，FSO通信可用于许多光链路，比如卫星与地面的数据链路、不受光纤网络限制的ad hoc中继链路等。然而，FSO也会受到光学窃听的影响，例如，窃听者故意阻挡光束以阻拦预期接收端的视线，或者发散光束在接收端的足迹很大，涵盖了接收端和窃听者时,FSO存在安全隐患。窃听者也能从非视线散射信道中获取链路的信息，使得FSO通信系统存在信息安全的问题。另外，大气湍流、光束的强度都会对FSO通信系统的安全性造成影响，而信息保密和安全传输是用户间通信的基本要求。因此，应用有效的技术来提高FSO通信系统的安全性势在必行。
传统的光网络安全性采用网络上层协议的数据加密，并假设物理层已提供畅通且无差错的传输。但是，随着科学技术的发展，所有基于算法的加密手段都已经被证明是可以破解的，已经不能满足信息安全的需求。例如，2009年，日本、法国和德国的研究团队破解了768比特的RSA加密算 法。基于物理的“测不准”和“不可分割”原理的量子密码通信具有绝对安全性，但由于受物理机制的限制，目前的量子通信技术仅适合于低速率的信号传输。物理层加密作为一种适中的安全方案，具有可证明的安全性。
光网络物理层安全的主要技术方案有：混沌光通信、光码分多址(Optical Code Division Multiple Access,OCDMA)和量子噪声随机编码(Quantum Noise Randomized Cipher,QNRC)等。混沌光通信系统的安全性是在混沌信号的复杂性上实现的，目前还没有一种完整系统的理论和手段来对混沌光通信的安全性进行评估分析，且已有实验系统的误码率较高。同时，发送端和接收端的激光器参数必须完全匹配，保证在比较理想的环境下传输。OCDMA通信系统具有多种防护功能，可实现光信息的安全传输。1)抗截获，OCDMA系统基于时频域变换的扩频机理及安全体系，使其具有较强的抗截获的功能。2)抗攻击，面对恶意入侵，OCDMA系统可以采用跳频编码或码字重构等措施，有效避开入侵光信号的影响，保障系统正常运行，从而具有抗攻击能力，确
保信息通信的安全。3)身份认证，OCDMA系统对每个用户赋予一个唯一的光域地址码，非授权用户不能获取到系统中所传输其他用户的信号，确保用户只能接收本身的信号，通过动态可重构地址码，系统可以随时确认每个用户的身份，确保信息的可信传输。4)隐匿性，对机密性要求高
的信息传输，采用隐匿传输，增加被发现的技术难度，从而增加其安全性。OCDMA系统利用其扩频扩时特性，将所传输的信号变为类噪声，隐匿在常规传输系统中，甚至隐匿于背景噪声中。
另外，在FSO通信系统中，存在大气湍流，如果使用OOK调制的话，需要设置动态的判决门限，而PPM不需要在接收端设置判决门限，只需要对不同时隙的输出进行比较，就可以选择需要恢复的信号，此外，PPM对功率的利用率较高，抗干扰能力强。因此，PPM调制是一种目前最适合FSO通信的调制方式。
一方面，随着科学技术的快速发展，使得破解任意一个加密算法理论上都是可行的，例如已经被破解RSA和DES算法等等。另一方面，OCDMA物理层加密能提高现有光通信系统的安全性。因此，为了进一步提高现有FSO通信系统的安全性。
传统的FSO光网络安全性采用网络上层协议的数据加密，并假设物理层已提供畅通且无差错的传输。但是，所有基于算法的加密手段都已经被证明是可以破解的。例如，2009年，日本、法国和德国的研究团队破解了768比特的RSA加密算法。基于物理的“测不准”和“不可分割”原理的量子密码通信具有绝对安全性，但由于受物理机制的限制，目前的量子通信技术仅适合于低速率的信号传输。
发明内容
本发明的目的在于提供一种自由空间光网络的物理层安全增强方法，旨在解决上述的技术问题。
本发明是这样实现的，一种自由空间光网络的物理层安全增强方法，所述自由空间光网络的物理层安全增强方法包括以下步骤：
S1、发送方合法用户Alice发送数据信号，干扰模块根据信道状态估计 发送干扰信号；
S2、根据不同数据符号对数据信号和干扰信号利用调制器进行信号调制；
S3、利用不同的编码器对调制过后的数据信号和干扰信号的光信号进行编码；
S4、利用耦合器将编码后的数据信号和干扰信号耦合到大气湍流信道中传输；
S5、接收端合法用户Bob利用解码器对接收的光信号进行对应的匹配解码；
S6、利用光电探测器APD对信号进行光电检测；
S7、合法用户利用解调器对光电检测过的信号进行解调获取发送方Alice发送的数据信号。
本发明的进一步技术方案是：所述步骤S5中还包括以下步骤：
S51、接收端窃听用户Eve利用解码器对接收的光信号进行对应的非匹配解码。
本发明的进一步技术方案是：所述步骤S7中还包括以下步骤：
S71、窃听用户利用解调器对光电检测过的信号进行解调无法获取发送方Alice发送的数据信号。
本发明的进一步技术方案是：所述步骤S1中所述干扰模块根据信道状态估计通过功率控制发送干扰信号。
本发明的进一步技术方案是：所述编码器采用OCDMA光编码器，利用光正交码对信号进行编码，所述解码器采用OCDMA光解码器，利用光正交码对信号进行解码。
本发明的进一步技术方案是：所述调制器采用MWPPM调制器，在发送方MWPPM调制器将信号解调制光载波上；在接收端MWPPM调制器将信号解调获取发送方发送的原始数据信号。
本发明的进一步技术方案是：所述干扰模块发出的干扰信号降低窃听用户的信噪比。
本发明的进一步技术方案是：所述OCDMA光编码器可采用一维编码或二维编码。
本发明的进一步技术方案是：所述OCDMA编码器采用光纤延时线或光纤光栅或液晶或AWG或PLC。
本发明的有益效果是：对于窃听用户而言，必须破解物理层的光编码加密，从而提高了FSO传输系统的安全性。在发送端加入自干扰信号，降低窃听用户的信噪比，另外，根据FSO/CDMA的信道状态，可以利用功率控制模块，对干扰信号的功率进行控制，加强干扰信号对窃听用户的干扰作用，从而达到提高FSO/CDMA系统物理层安全的作用。使用MWPPM调制方式大大提高了FSO/CDMA系统中光功率的利用率，也避免了OOK调制系统需要设置动态判决门限的不足。本专利可以改造现有的FSO通信系统，在现有的FSO通信线路上，加入干扰模块、安装光编码/解码安全设备，提升通信系统抗截获能力，形成安全可靠的新的FSO安全通信系统； 也可以建立新型、安全的FSO通信网络系统。
附图说明
图1是本发明实施例提供的自由空间光网络的物理层安全增强方法；
图2是本发明实施例提供的基于自干扰MWPPM的FSO/CDMA搭线系统。
图3是本发明实施例提供的扰模块的系统框图。
图4是本发明实施例提供的2-4-MWPPM脉冲位置调制示意图一。
图5是本发明实施例提供的2-4-MWPPM脉冲位置调制示意图二。
图6是本发明实施例提供的光纤延时线编码器和解码器框图。
具体实施方式
如图1所示，本发明提供的自由空间光网络的物理层安全增强方法，其详述如下：
步骤S1，发送方合法用户Alice发送数据信号，干扰模块根据信道状态估计发送干扰信号；在光纤混合网络中，包括两个合法用户Alice和Bob、窃听者Eve以及个干扰模块，干扰模块发送方功率可控的干扰信号。在信号发送端，合法用户Alice和干扰模块发送出信号后，干扰信号是通过功率控制干扰模块在信道状态估计下发出的，其中干扰模块多个。
步骤S2，根据不同数据符号对数据信号和干扰信号利用调制器进行信号调制；在信号发出后，在网络中根据不同的数据符号，对发送的数据信号和干扰信号利用MWPPM调制器调制到光载波上。
步骤S3，利用不同的编码器对调制过后的数据信号和干扰信号的光信 号进行编码；将解调后的信号经过各自不同的OCDMA光编码器，利用光正交码(OOC)对信息进行编码。
步骤S4，利用耦合器将编码后的数据信号和干扰信号耦合到大气湍流信道中传输；利用耦合器将编码后的合法用户信号和干扰信号耦合到FSO大气信道中进行传输。
步骤S5，接收端合法用户Bob利用解码器对接收的光信号进行对应的匹配解码；在接收端，合法用户Bob和窃听用户将接收到的信号利用光编码器进行相应的解码，
步骤S6，利用光电探测器APD对信号进行光电检测；采用APD作为接收端光电探测器。
步骤S7，合法用户利用解调器对光电检测过的信号进行解调获取发送方Alice发送的数据信号。合法用户接收方Bob进行匹配解码器解码，再由MWPPM解调器解调恢复出原始的数据信息，而窃听用户Eve知道FSO/CDMA系统的信息，如数据速率、编码类型及码字结构等，但是它不知道用户使用的具体码字，因此，Eve只能使用非匹配的解码器进行解码。因此，对于窃听用户而言，必须破解物理层的光编码加密，才能正确恢复用户数据，从而提高了FSO传输系统的物理层安全性。
如图2所示本，提出的自干扰MWPPM的FSO/CDMA搭线系统，包括两个合法用户Alice和Bob、窃听者Eve以及m个干扰模块，干扰模块发送m个功率可控的干扰信号。在信号发送端，合法用户Alice和干扰模块发送出信号后，用户数据和干扰数据首先经过MWPPM调制器调制到光 载波上，然后经过各自不同的OCDMA光编码器，利用光正交码(OOC)对信息进行编码。最后，利用耦合器将编码后的合法用户信号和干扰信号耦合到FSO大气信道中进行传输。在接收端，合法用户Bob和窃听用户将接收到的信号利用光编码器进行相应的解码，其中，合法用户接收方Bob进行匹配解码器解码，再由MWPPM解调器解调恢复出原始的数据信息，而窃听用户Eve知道FSO/CDMA系统的信息，如数据速率、编码类型及码字结构等，但是它不知道用户使用的具体码字，因此，Eve只能使用非匹配的解码器进行解码。因此，对于窃听用户而言，必须破解物理层的光编码加密，才能正确恢复用户数据，从而提高了FSO传输系统的物理层安全性。
在高速率的光通信中，由于雪崩光电二极管(APD)具有对微弱的光电流产生放大的作用，比PIN光电二极管具有更好的灵敏度。因此，本专利中，采用APD作为接收端光电探测器。另外，本申请提出的FSO/CDMA系统搭线信道模型，加入自干扰信号，会降低窃听用户Eve的接收信噪比，从而达到在物理层上提高FSO/CDMA系统安全性的目的。同时，相对OOK来说，PPM不需要在接收端设置动态的判决门限，只需要对不同时隙的输出电流进行比较，就可以判决和恢复信号，此外PPM对功率的利用率较高，抗干扰能力强，是目前最适合FSO通信的一种调制方式。然而，当系统传输距离远时，使用PPM调制的FSO/CDMA系统也会受到脉冲展宽的影响。因此，在本申请中，我们使用多波长PPM(MWPPM)来进一步提高系统的性能。
本申请中，在发送端，除了合法用户Alice，还存在m个干扰模块，每个干扰模块都发送干扰信号，用以降低窃听用户的信噪比。如图3所示。首先，发送端估计FSO/CDMA的信道状态，然后利用功率控制模块，对干扰信号的发射功率进行控制，从而达到提高FSO/CDMA系统物理层安全的作用。比如，在天气很晴朗的时候，FSO大气信道的衰减很小，由于合法用户Alice的发送功率保持不变，我们可以在保持FSO/CDMA系统可靠性的同时，利用功率控制模块提高干扰信号的功率，从而加大干扰信号对窃听用户的干扰，提高FSO/CDMA系统的安全性。
多波长PPM(MWPPM)是波长移位键控和PPM相互结合的调制方式，MWPPM有两个参数表征，波长数和时隙数。在M-L-MWPPM中，M表示波长数，L表示时隙数。根据不同的用户数据符号，光脉冲调制在M个波长之和L个时隙中的一个组合中传输。例如，在2-4-MWPPM调制方式中，根据不同的用户数据，选择2个波长与4个时隙的其中一个组合发送光脉冲。例如，用户数据000、001、010、011选择使用波长，而100、101、110、111选择使用波长，而光脉冲具体的时隙位置，可以根据传统的PPM调制确定。2-4-MWPPM脉冲位置调制示意图，如图4-5所示，图4的光脉冲使用波长，分别表示用户数据000、001、010、011，图5的光脉冲使用波长，分别表示用户数据100、101、110、111。与传统PPM相比，M-L-MWPPM可以降低信道符号速率。
利用光编码器对光信号进行编码，光编码后的光信号类似于低功率的随机噪声，窃听者很难用其它地址码恢复数据，只有特定的地址码才能恢 复数据。若要获得原光信号，对光解码器有严格的要求，只有经过唯一的与之匹配的光解码器进行相关解码，才能恢复出原光信号；光解码器若不与光编码器匹配，解码后获得的是噪声信号。
需要说明的是，本专利中提到的OCDMA编码器，可以是一维编码，也可以二维编码，具体实现可以采用光纤延时线、光纤光栅、液晶、AWG、PLC等。图6为采用光纤延时线的光编码器和解码器框图，光编码器和光解码器由几束并行光纤和两个1×w星型耦合器构成，根据所设计的光正交码确定各光纤延迟线的长度，其延迟线分支数目等于地址码的码片的数目(即码重)。光编码器的作用是产生相应的地址码序列，如图6(a)所示，对于(i 1,i 2,…,i j,…,i w)的光正交码，其中，i j表示码字中的第j个“1”在码字中所处的位置(位置编号为0,1,2,…n-1)，用1×w的光分路器、w条并行光纤延迟线和w×1光合路器构成。其中，第j条光纤延迟线的延时为i jτ，τ为码片时宽(即光脉冲的时间宽度)。
在接收端，光解码器的结构与编码器类似，所不同的是光纤时延为互补关系，如图6(b)所示。其功能是把输入信号与解码器作相关运算，实现匹配解码。光解码器中，光纤延时线的延时为
{(n-1-i 1)τ,(n-1-i 2)τ,…,(n-1-i j)τ,…,(n-1-i w)τ}。
对于窃听用户而言，必须破解物理层的光编码加密，从而提高了FSO传输系统的安全性。同时，在发送端加入自干扰信号，降低窃听用户的信 噪比，另外，根据FSO/CDMA的信道状态，可以利用功率控制模块，对干扰信号的功率进行控制，加强干扰信号对窃听用户的干扰作用，从而达到提高FSO/CDMA系统物理层安全的作用。此外，使用MWPPM调制方式大大提高了FSO/CDMA系统中光功率的利用率，也避免了OOK调制系统需要设置动态判决门限的不足。本专利可以改造现有的FSO通信系统，在现有的FSO通信线路上，加入干扰模块、安装光编码/解码安全设备，提升通信系统抗截获能力，形成安全可靠的新的FSO安全通信系统；也可以建立新型、安全的FSO通信网络系统。
以上所述仅为本发明的较佳实施例而已，并不用以限制本发明，凡在本发明的精神和原则之内所作的任何修改、等同替换和改进等，均应包含在本发明的保护范围之内。

Claims (9)

  1. 一种自由空间光网络的物理层安全增强方法，其特征在于，所述自由空间光网络的物理层安全增强方法包括以下步骤：
    S1、发送方合法用户Alice发送数据信号，干扰模块根据信道状态估计发送干扰信号；
    S2、根据不同数据符号对数据信号和干扰信号利用调制器进行信号调制；
    S3、利用不同的编码器对调制过后的数据信号和干扰信号的光信号进行编码；
    S4、利用耦合器将编码后的数据信号和干扰信号耦合到大气湍流信道中传输；
    S5、接收端合法用户Bob利用解码器对接收的光信号进行对应的匹配解码；
    S6、利用光电探测器APD对信号进行光电检测；
    S7、合法用户利用解调器对光电检测过的信号进行解调获取发送方Alice发送的数据信号。
  2. 根据权利要求1所述的自由空间光网络的物理层安全增强方法，其特征在于，所述步骤S5中还包括以下步骤：
    S51、接收端窃听用户Eve利用解码器对接收的光信号进行对应的非匹配解码。
  3. 根据权利要求2所述的自由空间光网络的物理层安全增强方法，其特征在于，所述步骤S7中还包括以下步骤：
    S71、窃听用户利用解调器对光电检测过的信号进行解调无法获取发送方Alice发送的数据信号。
  4. 根据权利要求1-3任一项所述的自由空间光网络的物理层安全增强方法，其特征在于，所述步骤S1中所述干扰模块根据信道状态估计通过功率控制发送干扰信号。
  5. 根据权利要求4所述的自由空间光网络的物理层安全增强方法，其特征在于，所述编码器采用OCDMA光编码器，利用光正交码对信号进行编码，所述解码器采用OCDMA光解码器，利用光正交码对信号进行解码。
  6. 根据权利要求5所述的自由空间光网络的物理层安全增强方法，其特征在于，所述调制器采用MWPPM调制器，在发送方MWPPM调制器将信号解调制光载波上；在接收端MWPPM调制器将信号解调获取发送方发送的原始数据信号。
  7. 根据权利要求6所述的自由空间光网络的物理层安全增强方法，其特征在于，所述干扰模块发出的干扰信号降低窃听用户的信噪比。
  8. 根据权利要求7所述的自由空间光网络的物理层安全增强方法，其特征在于，所述OCDMA光编码器可采用一维编码或二维编码。
  9. 根据权利要求7所述的自由空间光网络的物理层安全增强方法，其特征在于，所述OCDMA编码器采用光纤延时线或光纤光栅或液晶或AWG或PLC。
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