Systems and methods for wavelength diversity based low probability to intercept (LPI) free space optical communications

Abstract

A system and method are provided for low probability to intercept (LPI) free-space optical (FSO) communications. A transmitter transmits a broadband light (e.g., but not limited to a full width half maximum of 10 nm or more) that is in a solar-radiation spectrum, and the broadband light is modulated using a modulation scheme (e.g., binary phase shift keying). The modulated light encodes a data packet that includes a header and a codeword generated using an error correction code. After propagating through an FSO channel, the receiver collects the transmitted modulated light and used one or more balanced detectors heterodyne detectors at different center wavelengths to convert the received light to respective RF signals, which are then processed to recover the information data encoded in the codeword.

Description

SYSTEMS AND METHODS FOR WAVELENGTH DIVERSITY BASED LOW PROBABILITY TO INTERCEPT (LPI) FREE SPACE OPTICAL COMMUNICATIONSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a PCT application that claims priority to U.S. Provisional Application No. 63 / 588,626 filed on October 6, 2023, which is expressly incorporated by reference herein in its entirety.FIELD

[0002] The present disclosure generally relates to communications; and in particular to systems and methods for wavelength diversity based low probability to intercept (LPI) free space optical communications in strong turbulence regime.BACKGROUND

[0003] In low probability of intercept (LPI) communications we are concerned with both users’ privacy and secure communications. The field of free-space optical (FSO) communication is at the forefront of the latest trends in communication systems, in particular for defense applications. However, FSO communication is full of challenges with major obstacles originating from the atmospheric turbulence effects.

[0004] It is with these observations in mind, among others, that various aspects of the present disclosure were conceived and developed.SUMMARY

[0005] The issues raised above are addressed by the proposed system.

[0006] In some aspects, the techniques described herein relate to a system for low probability to intercept (LPI) free space optical (FSO) communications, the system including: a transmitter that includes: a broadband optical source that generates light with a spectrum in a solarradiation spectrum and has a full width half maximum of at least 10 nm, an encoder that encodes information data using an error correction technique to generate a codeword and combines the codeword with a header to provide a data packet, a modulator that modulates the light according to a predefined modulation scheme to generate a modulated optical signal, andoutput optics that couple the modulated optical signal from the transmitter to a free space channel.

[0007] In some aspects, the techniques described herein relate to a receiver, including: input optics that couple a transmitted modulated optical signal from a free space channel to a receiver to provide a received optical signal; one or more balanced detectors that mix the received optical signal with coherent optical signals from one or more local oscillators to provide mixed signals that are detected by balanced photoreceivers to generate one or more analog RF signals; and an analog-to-digital converter that converts the one or more analog RF signals to one or more digital signals; and one or more processors that process the one or more digital signals to reconstruct information data, wherein the modulated optical signal has been modulated to carry a data packet that includes a header and a codeword, the codeword representing information data that has been encoded according to an error correction technique.

[0008] In some aspects, the techniques described herein relate to a method for low probability to intercept (LPI) free-space optical (FSO) communications, the method including: generating, at a broadband optical source, light having a spectrum within a solar-radiation spectrum and has full width half maximum of at least 10 nm; encoding, by an encoder, information data using an error correction technique to generate a codeword and adding a header before the codeword to provide a data packet; modulating, by a modulator, the light according to a predefined modulation scheme to generate a modulated optical signal; and transmitting, from a transmitter, the modulated optical signal into a free space channel that includes solar radiation.

[0009] In some aspects, the techniques described herein relate to an optical communication system including: a broadband light source; a phase modulator; an erbium-doped fiber amplifier (EDFA); a beam expander; a periscope; a compressing telescope; a fiber coupler; a receive EDFA; a 50:50 splitter; a first balanced heterodyne detector including a first local oscillator laser operating at a first wavelength and connected to the 50:50 splitter; a second balanced heterodyne detector including a second local oscillator laser at a second wavelength and connected to the 50:50 splitter, wherein the second wavelength differs from the first wavelength. The communication system further includes one or more processors; and a computer-readable storage medium storing instructions which, when executed by the one or more processors, cause the one or more processors to perform operations including: generating, by the light source, the phase modulator and the erbium-doped fiber amplifier (EDFA), a phase-modulated signal; transmitting, via the

[0010] beam expander and the periscope, the phase-modulated signal to yield a transmitted signal, wherein the transmitted signal is hidden in solar radiation; receiving, via the compressing telescope, a broadband signal including the transmitted signal over a turbulent free-space optical channel; splitting, via the 50:50 splitter, the broadband signal into a first broadband signal and a second broadband signal; detecting, via the first balanced heterodyne detector, a first received beam by interacting the first local oscillator laser with the first broadband signal to generate a first radio frequency output; detecting, via the second balanced heterodyne detector, a second received beam by interacting the second local oscillator laser with the second broadband signal to generate a second radio frequency output; and simultaneously capturing, via analog-to-digital converters (ADCs), the first radio frequency output and the second radio frequency output to generate a received electrical signal.

[0011] In some aspects, the techniques described herein relate to a method of operating a communication system, the method including: generating, by a broadband light source, a phase modulator and an erbium-doped fiber amplifier (EDFA), a phase-modulated signal; transmitting, via a beam expander and a periscope, the broadband phase-modulated signal to yield a transmitted signal, wherein the transmitted signal is hidden in solar radiation; receiving, via a compressing telescope, a broadband signal including the transmitted signal over a turbulent channel; splitting, via a 50:50 splitter, the broadband signal into a first broadband signal and a second broadband signal; detecting, via a first balanced heterodyne detector, a first received beam by interacting a first local oscillator laser with the first broadband signal to generate a first radio frequency output; detecting, via a second balanced heterodyne detector, a second received beam by interacting a second local oscillator laser with the second broadband signal to generate a second radio frequency output; and simultaneously capturing, via an analog-to-digital converters (ADCs), the first radio frequency output and the second radio frequency output to generate a received signal.

[0012] In some aspects, the techniques described herein relate to a communication system including: an optical transmitter including a light source, a phase modulator and a erbium- doped fiber amplifier (EDFA); an optical receiver including a compressing telescope; one or more processors; and a computer-readable storage medium storing instructions which, when executed by the one or more processors, cause the one or more processors to perform operations including: generating, via the optical transmitter, a phase-modulated signal; transmitting, via a beam expander and a periscope, the broadband phase-modulated signal toyield a transmitted signal, wherein the transmitted signal is hidden in solar radiation; receiving, via the optical receiver, a broadband signal including the transmitted signal received over a turbulent channel; splitting, via a 50:50 splitter, the broadband signal into a first broadband signal and a second broadband signal; detecting, via a first balanced heterodyne detector, a first received beam by interacting a first local oscillator laser with the first broadband signal to generate a first radio frequency output; detecting, via a second balanced heterodyne detector, a second received beam by interacting a second local oscillator laser with the second broadband signal to generate a second radio frequency output; and simultaneously capturing, via analog- to-digital converters (ADCs), the first radio frequency output and the second radio frequency output to generate a received signal.

[0013] In some aspects, an optical communication system includes: a broadband light source; a phase modulator; an erbium-doped fiber amplifier (EDFA); a beam expander; a periscope; a compressing telescope; a fiber coupler; a receive EDFA; a 1 :M beam splitter or a wavelength division (WDM) demultiplexer; M balanced heterodyne detectors each comprising a respective local oscillator laser, wherein M equals two or more, wherein a respective wavelength for each respective local oscillator is different from each other wavelength of other local oscillators in the M balanced heterodyne detectors; one or more processors; and a computer-readable storage medium storing instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: generating, by the light source, the phase modulator and the erbium-doped fiber amplifier (EDFA), a phase-modulated signal; transmitting, via the beam expander and the periscope, the phase-modulated signal to yield a transmitted signal, wherein the transmitted signal is hidden in solar radiation; receiving, via the compressing telescope, a broadband signal comprising the transmitted signal over a turbulent free-space optical channel; splitting, via the 1:M beam splitter, the broadband signal into M broadband signals; detecting, via each of the M balanced heterodyne detectors, a respective broadband signal of the M broadband signals to generate M radio frequency outputs; and simultaneously capturing, via analog-to-digital converters (ADCs), the M radio frequency outputs to generate a received electrical signal.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0014] In order to describe the manner in which the above-recited and other advantages and features of the disclosure can be obtained, a more particular description of the principles brieflydescribed above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only exemplary embodiments of the disclosure and are not therefore to be considered to be limiting of its scope, the principles herein are described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0015] FIG. 1 illustrates a first block diagram of a first system for free-space optical communications using a broadband source, in accordance with some embodiments.

[0016] FIG. 2 illustrates a second block diagram of a second system for free-space optical communications using a broadband source, in accordance with some embodiments.

[0017] FIG. 3 illustrates a flow diagram of a method for free-space optical communications using a broadband source, in accordance with some embodiments.

[0018] FIG. 4 illustrates a third block diagram of a third system for free-space optical communications using a broadband source, in accordance with some embodiments.

[0019] FIG. 5 A illustrates a plot of bit error rates of channel realizations for a first case study, in accordance with some embodiments.

[0020] FIG. 5B illustrates a histogram plot of received power for the first case study, in which a Rayleigh-like distribution indicates a strong turbulence regime, in accordance with some embodiments.

[0021] FIG. 6A illustrates a plot of bit error rates of channel realizations for a second case study comparing performance with and without adaptive optics (AO), in accordance with some embodiments.

[0022] FIG. 6B illustrates a histogram plot of received power for results without AO in the second case study, in which a Rayleigh-like distribution indicates a strong turbulence regime, in accordance with some embodiments.

[0023] FIG. 6C illustrates a histogram plot of received power for results with AO in the second case study, in which a Gaussian-like distribution indicates a medium turbulence regime, in accordance with some embodiments.

[0024] FIG. 7 illustrates a block diagram of a computing device, in accordance with some embodiments.DESCRIPTION OF EXAMPLE EMBODIMENTS

[0025] Various embodiments of the disclosure are discussed in detail below. While specific implementations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the disclosure.

[0026] Additional features and advantages of the disclosure will be set forth in the description which follows, and in part will be obvious from the description, or can be learned by practice of the herein disclosed principles. The features and advantages of the disclosure can be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the disclosure will become more fully apparent from the following description and appended claims, or can be learned by the practice of the principles set forth herein.

[0027] The disclosed technology addresses the need in the art for free-space optical (FSO) communication for low probability to intercept (LPI) applications to provide a cost-effective, simple alternatives.

[0028] According to certain non-limiting examples, the systems and methods disclosed herein provide improvements in bit error rates (BERs) using wavelength diversity for free-space optical (FSO) communication for low probability to intercept (LPI) applications

[0029] According to certain non-limiting examples, the systems and methods disclosed herein use low-density parity check (LDPC) coded data packets (e.g., codewords) to realize low probability of intercept (LPI) communication for free-space optical communication in the presence of strong atmospheric turbulence.

[0030] In LPI communications, the concern is with both users’ privacy and secure communications. The field of free-space optical (FSO) communication is at the forefront of the latest trends in communication systems, in particular for defense applications. However, FSO communication is full of challenges with major obstacles originating from the atmospheric turbulence effects.

[0031] According to certain non-limiting examples, the systems and methods disclosed herein can provide an alternative to using adaptive optics (AO) techniques to mitigate the losses incurred from atmospheric turbulence. A drawback of AO is that devices used in AO, such as deformable mirrors and wavefront sensors are essentially designed and developed forastronomical observations. They are designed to work optimally only in a low turbulence regime and their performance is limited under strong turbulence effects. The AO systems are highly complex to set up in an FSO case study and generally quite expensive and given the cost of these devices one quickly arrives at a point of diminishing returns.

[0032] This disclosure proposes a wavelength-based diversity method wherein the system encodes the information on a broadband source such as an EDFA source and detects the received signal with two (or more) different center wavelengths on different balanced detectors simultaneously. Afterwards, the system properly combines the log-likelihood ratios and produce the results for diversity.

[0033] This concept is relevant in LPI communications and represents a low-cost solution of operating the system in the strong turbulence regime and beyond. Given that EDFA source output follows Gaussian distribution, the same as for the solar radiation, a low-power phase modulated signal over a large bandwidth (39.2 nm) is extremely difficult to intercept.

[0034] FIG. 1 illustrates a non-limiting example of free space optical (FSO) communication for low probability to intercept (LPI) applications. LPI communications can be used to protect users’ privacy and to provide secure communications. The field of free-space optical (FSO) communication can be used in communication systems and in particular for defense applications. FSO communication is presented with challenges due the atmospheric turbulence effects. The systems and methods disclosed herein can use wavelength diversity to provide LPI FSO communications in strong turbulence regime without the need for adaptive optics (AO). Wavelength diversity can provide a less expensive less complicated alternative to AO with many of the same benefits (e.g., reduced bit error rate (BER)).

[0035] FSO communication system 100 includes an optical transmitter (e.g., TX 102) that encodes information data 112 on optical beam 104, which is transmitted through FSO channel 106 to an optical receiver (e g., RX 108) where information data 112 is recovered as recovered signal 126.

[0036] TX 102 includes broadband source 110, code generator 114, modulator 116, and coupling and beam expanding optics 118.

[0037] According to certain non-limiting examples, broadband source 110 is a broadband optical source that generates light with a spectrum in a solar-radiation spectrum and has a full width half maximum that spans many times the modulation bandwidth of modulator 116 toprovide wavelength diversity. For example, according to certain non-limiting examples, the full width half maximum of the light from broadband source 110 can span at least 10 nm. Because the light from broadband source 110 has a spectrum in the solar-radiation spectrum that spans a wide range of wavelengths and FSO channel 106 includes solar radiation that at least partial obscures / masks the light from broadband source 110, the signal on the light from broadband source 110 can be difficult to intercept. For example, broadband source 110 can generate light that provides wavelength diversity across many center wavelengths that are spaced by more than a modulation bandwidth of modulator 116. For example, the spectrum of the light of interest in LPI FSO communication is between a wavelength of 1500 nm and 2200 nm.

[0038] According to certain non-limiting examples, code generator 114 is an encoder that encodes information data using an error correction technique to generate a codeword and combines the codeword with a header to provide a data packet. The data packet is then sent to modulator 116 where the data packet is encoded onto the light from broadband source 110. For example, code generator 114 can encode information data 112 using a low-density parity-check code, a Reed-Solomon code, a turbo code, a Bose-Chaudhuri-Hocquenghem (BCH) code, a convolutional code, a polar code, or a product code, to mention few. These examples of error correction codes are non-limiting and other error correction codes can be used without departing from the scope of the disclosure.

[0039] According to certain non-limiting examples, modulator 116 is a modulator that modulates the light according to a predefined modulation scheme to generate a modulated optical signal. For example, modulator 116 can be a phase modulator that modulates the light using phase shift keying at the modulation bandwidth. Further, the phase shift keying can be binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), or some other scheme for phase shift keying or quadrature amplitude modulation (QAM). The modulation data rate can correspond to a bit rate in a range from 10 Gbps to 100 Gbps.

[0040] According to certain non-limiting examples, coupling and expanding optics 118 can be a beam expanding telescope.

[0041] According to certain non-limiting examples, FSO channel 106 can be a turbulent channel, which is also affected by scattering, diffraction, and absorption effects.

[0042] RX 108 includes compressing and coupling optics 120, detector 122, and forward error correction decoder 124.

[0043] According to certain non-limiting examples, compressing and coupling optics 120 can be a compressing telescope. Further, coupling optics 120 can include reflective and / or refractive optics to collect and focus light into RX 108.

[0044] According to certain non-limiting examples, detector 122 can include one or more balanced detectors that mix the received optical signal with coherent optical signals from one or more local oscillators (e.g., lasers) to provide mixed signal that are detected by balanced photoreceivers to generate one or more analog RF signals. A beam splitter, such as a fiberbased optical splitter or wavelength division multiplexing (WDM) demultiplexer can demultiplex the received signal into different wavelength bands that are each processed by mixing them with coherent light from a laser operating at a center wavelength corresponding to the given wavelength band. The mixed signal can be generated by combining the wavelength band from the received light with the coherent laser light on a 50:50 beam splitter and detecting the mixed light signals using balanced photoreceivers. The RF signals generated by detector 122 can be converted by an analog-to-digital converter (ADC) to one or more digital signals that are sent to the digital signal processing module followed by forward error correction decoder 124.

[0045] For example, the local oscillators in detector 122 can be lasers operating at respective center wavelengths that are spaced by more than a modulation signal bandwidth, thereby providing wavelength diversity for detecting the data packet. The heterodyne detectors can generate respective RF signals corresponding to the center wavelengths, and each of the RF signals is converted to a corresponding digital signal.According to certain non-limiting examples, forward error correction decoder 124 includes one or more processors that process the one or more digital signals to reconstruct information data 112 (i.e., to generate recovered signal 126, which is the same as information data 112 after all errors have been corrected). For example, the one or more processors can process the combination of digital signals by determining log-likelihood ratios (LLRs) for each of the respective center wavelengths and combining the LLRs for each of the respective center wavelengths to generate aggregate log-likelihood ratios. Further, a sum-product algorithm is used to decode the LDPC codeword using the aggregate log-likelihood ratios, when the codeword is an LDPC codeword. Otherwise, when the codeword has been encoded using another forward error correction code than LDCP, a decoding algorithm corresponding to the other forward error correction code is used to decode the codeword.

[0046] The LLRs can be determined according to the techniques described in I. B. Djordjevic, Advanced Optical and Wireless Communications Systems, 2ndEdi. (Springer Nature Switzerland AG, Switzerland, 2022) (hereinafter ''Advanced Optical and Wireless Communications System""), which is incorporated herein by reference in its entirety. For example, the LLRs can be determined as described on pp. 562, 590-591,598-599, and 604 of Advanced Optical and Wireless Communications Systems. Further, when the codeword is encoded using a low-density parity-check (LDPC) code, a sum-product algorithm is used to decode the codeword as described on pp. 565-570 of Advanced Optical and Wireless Communications Systems. The modulation scheme used by modulator 116 can be a modulation scheme described in chapter 5 of Advanced Optical and Wireless Communications Systems. Detector 122 can be coherent detectors as described in sections 3.1.9 and 6.8 of Advanced Optical and Wireless Communications Systems. The encoding can be performed using a technique described in chapter 9 of Advanced Optical and Wireless Communications Systems. The forward error correction code that is not LDPC code should apply corresponding decoding algorithm described in Chapter 9 of the book cited above.

[0047] According to certain non-limiting examples, code generator 114 can encode information data 112 using an error correction code that is a low-density parity-check code, a Reed-Solomon code, a turbo code, a Bose-Chaudhuri-Hocquenghem (BCH) code, a convolutional code, a polar code, or a product code. For example, low-density parity-check (LDPC) codes can use sparse parity-check matrices, allowing efficient decoding while providing near-optimal performance. Turbo codes can be used in communication systems and combine two or more convolutional codes with iterative decoding to achieve near-Shannon-limit performance. Bose-Chaudhuri- Hocquenghem (BCH) codes are a class of cyclic error-correcting codes that can correct multiple random errors. Convolutional codes can encode data in a way that spreads it over several output bits, allowing for error correction using Viterbi or sequential decoding algorithms. Polar codes can be effective for large block sizes and achieve capacity for symmetric channels. Product codes can be formed by combining two or more simpler codes, allowing them to correct multiple errors in a systematic way. Each of these codes has its own strengths and is suited to different applications, depending on the error characteristics of the channel and the required performance.

[0048] FIG. 2 illustrates another non-limiting example of free space optical (FSO) communication for low probability to intercept (LPI) applications. In FIG. 2, FSOcommunication system 100 includes a transmitter (i.e., TX 102), channel 224, and a receiver (i.e., RX 108).

[0049] TX 102 includes amplified spontaneous emission source 204 with a seed from a broadband LED source, amplifier 202a, amplifier 202b, processor 206a, arbitrary waveform generator 208, phase modulator 210, and fiber-to-free space optics 220.

[0050] RX 108 includes free space to fiber optics 222, amplifier 202c, 1560 nm laser 212a, 1565 nm laser 212b, beam splitter 214, balanced heterodyne detector 216a, balanced heterodyne detector 216b, analog-to-digital converter 218, and processor 206b. The processor 206B can be a computer, processor (e g., a central processing unit), a field programable gate array (FPGA), or application specific integrated circuit (ASIC) based.

[0051] FSO LPI communication is realized by TX 102 encoding information data on beam 226 as a modulated signal, which propagates through channel 224 and is received by RX 108 where the transmitted modulated signal is detected and used to reconstruct the information data.

[0052] FIG. 3 illustrates an example method 300 for FSO LPI communication. Although the example method 300 depicts a particular sequence of operations, the sequence may be altered without departing from the scope of the present disclosure. For example, some of the operations depicted may be performed in parallel or in a different sequence that does not materially affect the function of the method 300. In other examples, different components of an example device or system that implements the method 300 may perform functions at substantially the same time or in a specific sequence.

[0053] According to some examples, the method includes generating an optical signal at process 304. For example, the TX 102 illustrated in FIG. 1 may generate an optical signal. FSO communication system 100 include a transmitter (e.g., TX 102) that generates a broadband optical signal and modulates the optical signal according to a specified pattern and modulation scheme (e.g., quadrature phase shift keying (QPSK) or some other modulation scheme). This can all be done using fiber-optic components, photonic waveguides (.e.g., a photonic system on a chip, photonic integrated circuit-PIC), Mac-Zehnder modulator, phase modulator, or electro- optical I / Q modulator.

[0054] According to certain non-limiting examples, TX 102 can perform signal generation using an ASE noise source (e.g., an erbium-doped fiber amplifier (EDFA)), which uses a broadbandLED output signal as a seed, that feds a Mach-Zehnder (MZ) modulator, followed by another EDFA to compensate for insertion loss of the MZ modulator. The RF signal to the MZ modulator is generated by an arbitrary waveform generator (AWG) at an information rate of 10 Gbps. Alternatively, the AWG can be replaced by the analog-to-digital converters (ADCs), pulse pattern generators, FGPA, or ASIC device. The signal sent to the AWG has a payload that is a low-density parity check (LDPC) encoded binary phase shift keying (BPSK) signal with a forward error correction (FEC) frame rate of 0.625, codeword length of 3992 bits, with 2497 information bits and 1495 parity-bits, which is provided as a non-limiting, illustrative example. A 616-bit header is added to the data packet before the payload, which is used by processor 206b to find the start of the FEC frame.

[0055] According to some examples, the method includes generating a broadband optical signal at block 306. For example, the amplified spontaneous emission source 204 illustrated in FIG. 2 may generate a broadband optical signal. To provide low probability of intercept (LPI) communications, a broadband source is used to generate the optical signal. The broadband source generates light in a bandwidth range within the solar spectrum, such that the signal is difficult to distinguish from solar radiation, making interception of the signal challenging. The combination of the broadband signal in the solar spectrum together with the modulation and signal processing techniques described below are beneficial for LPI communications and represent a low-cost solution of operating the system in the strong turbulence regime without requiring adaptive optics (OA) techniques.

[0056] According to certain non-limiting examples, a broadband signal is generated using an ASE noise source (EDFA), with seed input form an LED broadband source. The EDFA source output follows a Gaussian distribution, which is the same as for the solar radiation. consequently, the optical signal can be extremely difficult to intercept, when the broadband optical signal is a low-power phase modulated signal over a large bandwidth (e.g., 39.2 nm), in particular when the broadband signal level is lower than that of the solar radiation.

[0057] According to some examples, the method includes modulating the optical signal at block 308. For example, the phase modulator 210 illustrated in FIG. 2 may modulate the optical signal. According to certain non-limiting examples, the optical signal can be modulated using a Mach-Zehnder (MZ) modulator. For example, the signal from an ASE noise source (EDFA) can feed into the MZ modulator, which is followed by an EDFA to compensate for insertion loss ofthe MZ modulator. The RF signal to the MZ modulator can be generated by an arbitrary waveform generator (AWG) at an information rate of 10 Gbps or 20 Gbps.

[0058] According to some examples, the method includes generating a data packet that includes a header and a payload (e.g., codeword), which is generated using an error-correcting code at block 310. For example, the processor 206a illustrated in FIG. 2 may generate a data packet that includes a header and a payload (e.g., codeword), which is generated using an errorcorrecting code. According to certain non-limiting examples, an MZ modulator modulates the phase of the broadband optical signal. The RF signal to the MZ modulator can be generated by an AWG. The signal sent to the AWG has a payload that is a low-density parity check (LDPC) encoded binary phase shift keying (BPSK) signal with a forward error correction (FEC) frame rate of 0.625, codeword length of 3992 bits, with 2497 information bits and 1495 parity-bits. A 616-bit header is added to the data packet before the payload, which is used by processor 206b in RX 108 to find the start of the FEC frame.

[0059] According to some examples, the method includes amplifying the modulated optical signal at block 312. For example, the amplifier 202b illustrated in FIG. 2 may amplify the modulated optical signal. According to certain non-limiting examples, an EDFA is placed after the MZ modulator. For example, this amplifier can compensate for the insertion loss of the MZ modulator. As indicated by the dashed line this amplification step is optional, and this step can be omitted, e.g., when the optical signal has a large enough amplitude without additional amplification. According to certain non-limiting examples, the amplifier 202b amplifies the optical signal to and intensity to the desired level before launching it to the free space using a beam expander and a periscope.

[0060] According to some examples, the method includes launching the modulated optical signal to a free space channel at block 314. For example, the fiber to free space optics 220 illustrated in FIG. 2 may launch the modulated optical signal to a free space channel. The optical signal transitions to free space propagation from an optical fiber, a photonic waveguide, free space optics, or other optical format that is used to generate, condition, modulate, and otherwise prepare the optical signal.

[0061] According to certain non-limiting examples, the optical signal is launched to the free space using a beam expander or a combination of a beam expander and a periscope.

[0062] According to some examples, the method includes propagating the signal through the channel at block 316. For example, the signal may propagate through channel 224 illustrated in FIG. 2. The beam 226 propagates through a free space channel (e.g., channel 224), which can be in a strong turbulence regime.

[0063] According to certain non-limiting examples, FSO channel 106 or channel 224 can be a turbulent channel. Free space optical (FSO) communication is at the forefront of communication systems, especially for defense applications. However, FSO communication faces challenges due to atmospheric turbulence effects.

[0064] The systems and methods disclosed herein provide a low-cost solution for operating an FSO communication system in the strong turbulence regime and beyond, which can be challenging for other proposed solutions, such as those relying only on adaptive optics (AO) techniques to combat the effects of turbulence. For example, a challenge of AO is that devices used in AO, (e.g., deformable mirrors and wavefront sensors) are primarily designed and developed for astronomical observations. They are designed to work optimally only in a low turbulence regime and their performance is limited under strong turbulence effects. Further, AO systems tend to be highly complex to set up in FSO applications and generally quite expensive. As a result of the cost of these devices, the point of diminishing returns is quickly approached when using AO.

[0065] According to some examples, the method includes detecting the optical signal at process 318. For example, the RX 108 illustrated in FIG. 1 may detect the optical signal. After propagating through the FSO channel (e.g., channel 224), the light can be collected by a condensing telescope, such as a reflective telescope or a refractive telescope. The collected light can then be coupled into an optical fiber, a photonic waveguide, or another optical system for detecting the received light and converting it into a digitized electrical signal that can be processed by a personal computer (e.g., processor 206b).

[0066] According to certain non-limiting examples, beam 226 is captured using a compression telescope and passed to an optical bench where it is coupled into an optical fiber. The received signals are then amplified by pre-amp EDFA and split into two branches using a 50:50 beam splitter. Each of these branches feeds another balanced detector. For wavelength diversity, one branch is mixed with 1560 nm laser 212a and detected using balanced heterodyne detector 216a. The other branch is mixed with 1565 nm laser 212b and detected using balanced heterodyne detector 216b. The resulting RF signals are then converted from analog to digitalsignals using analog-to-digital converter 218. According to certain non-limiting examples, the analog-to-digital conversion process is performed by capturing the RF outputs from the balanced heterodyne detectors by feeding the RF signals to a real-time scope and then using screen capture on the real-time scope to simultaneously capture both of the RF signals.

[0067] According to some examples, the method includes collecting and compressing the optical signal from free space channel at block 320. For example, the free space to fiber optics 222 illustrated in FIG. 2 may collect and compress the optical signal from free-space channel. According to certain non-limiting examples, beam 226 is captured using a compression telescope and passed to an optical bench where it is coupled into an optical fiber.

[0068] According to some examples, the method includes amplifying the optical signal at block 322. For example, the amplifier 202c illustrated in FIG. 2 may amplify the optical signal. According to certain non-limiting examples, the received signals are then amplified by preamplifier EDFA.

[0069] According to some examples, the method includes detecting the optical signal to generate a digitized electrical signal at block 324. For example, the beam splitter 214 illustrated in FIG. 2 may detect the optical signal to generate a digitized electrical signal. According to some examples, the method includes splitting the received optical signal into two branches using a 50:50 beam splitter, each of these branches feeds another balanced detector. For wavelength diversity, one branch is mixed with 1560 nm laser 212a and detected using balanced heterodyne detector 216a. The other branch is mixed with 1565 nm laser 212b and detected using balanced heterodyne detector 216b. The resulting RF signals are then converted from analog to digital signals using an analog-to-digital converter 218. According to certain non-limiting examples, the analog-to-digital conversion process is performed by capturing the RF outputs from the balanced heterodyne detectors by feeding the RF signals to a real-time scope, and then using screen capture on the real-time scope to simultaneously capture both of the RF signals. Alternatively, the analog-to-digital converters can be used instead of a real-time scope.

[0070] According to some examples, the method includes decoding the detected signal at process 326. For example, the processor 206b illustrated in FIG. 2 may decode the detected signal. According to certain non-limiting examples, the transmitted data encoded on the optical signal is recovered by various digital signal processing methods performed by processor 206b. For example, to determine the start of the codeword (e.g., the FEC frame) a cross-correlationmethod can be applied to find the header. For example, the header is known by both the transmitter and the receiver. Thus, the receiver can calculate the cross-correlation between the known header and the recovered signal, and the time corresponding to a maximum of the crosscorrelation can be identified as the start time.

[0071] After determining the start time for the data packet, the log-likelihood hood ratios (LLRs) can be computed for both 1560 nm and 1565 nm channels. These LLRs can be used to recover the bits, and an error correction process based on the LDPC can be used to recover the original data (e.g., the 2497 information bits) from the recovered bits (e.g., the codeword of 3992 bits). Further, the LLRs can also be used to determine the bit error rates (BERs).

[0072] For wavelength diversity results the LLRs of both channels can be summed, after proper weighting. According to certain non-limiting examples, the LLRs are summed using a weighted ratio. For example, it was observed in one case that a ratio of 0.25: 1 ratio (for 1560 nm and 1565 nm, respectively) provided good results.

[0073] According to some examples, the method includes determining the start time of the payload (e.g., codeword) at block 328. For example, the processor 206b illustrated in FIG. 2 may determine the start time of the payload (e.g., codeword). According to certain non-limiting examples, the start of the codeword (e.g., the FEC frame) can be determined using a crosscorrelation method. For example, the header is known by both the transmitter and the receiver. Thus, the receiver can calculate the cross-correlation between the known header and the recovered signal, and the time corresponding to a maximum of the cross-correlation can be identified as the start time.

[0074] According to some examples, the method includes detecting the values of bits in the payload (e.g., codeword) at block 330. For example, the processor 206b illustrated in FIG. 2 may detect the values of bits in the payload (e.g., codeword). According to certain non-limiting examples, log-likelihood hood ratios (LLRs) can be computed for both 1560 nm and 1565 nm channels. These LLRs can be used to recover the codeword of 3992 bits.

[0075] According to some examples, the method includes using error correction to recover the information data from the bit values at block 332. For example, the processor 206b illustrated in FIG. 2 may use error correction to recover the information data from the bit values. According to certain non-limiting examples, the LLRs can be used to recover the bits, and an error correction process based on the LDPC code can be used to recover the original data (e.g.,the 2497 information bits) from the recovered bits (e.g., the codeword of 3992 bits). Further, the LLRs can also be used to determine the bit-error rates (BERs). The method 300 comprises an initial data 302 and a recovered data 334.

[0076] FIG. 4 illustrates an example implementation of the systems and methods disclosed herein. Further, this example implementation was used in a case study demonstrating the efficacy of the systems and methods disclosed herein.

[0077] The case study was performed over an FSO link from the ECE Quantum communication Lab, located in ECE Rm 549, to a second (Meinel) building rooftop, which are separated by distance “d,” i.e., approximately 750 m. Corner cube 414 is a retroreflector that is used to double the distance to a total path length of about 1.5 km. The information-carrying beam is pointed from the ECE Lab towards the second building where a retroreflector (corner cube 414), on the rooftop is placed, which reflects the beam to the room, thus giving the link an effective length of approximately 1.5 km.

[0078] A low-power phase modulated signal is generated on a light beam with a large bandwidth (i.e., about 39.2 nm). The wideband light is generated using an amplified spontaneous emission noise source (e g., ASE 402) that is amplified using an erbium-doped fiber amplifier (i.e., EDFA 404a). Because the light output from this ASE-EDFA source follows Gaussian distribution, which is the same as for the solar radiation making the radat signal difficult to intercept. Alternatively, in place of ASE 402, any other thermal light source can be used instead such as light emitting diode (LED).

[0079] Next, the light is phase modulated according to a binary phase shift key (BPSK) scheme using a Mach-Zehnder (MZ) modulator (i.e., PM 408), followed by EDFA 404b to compensate for insertion loss of the MZ modulator. The RF signal to the MZ modulator is generated using an arbitrary waveform generator (i.e., AWG 406) at an information rate of 10 Gbps, resulting is an LDPC encoded BPSK signal with a payload of the data packet being a codeword length of 3992 bits, with 2497 information bits and 1495 parity-bits that has a forward error correction (FEC) frame rate of 0.625. A header having 616 bits is added to the beginning of the data packet, and this header is used during offline digital signal processing (DSP) to find the start of the FEC payload. The output is then further amplified by EDFA 404b to lOOmW before launching the modulated optical signal to the free space using beam expander 410 and periscope 412.

[0080] After reflecting from corner cube 414 and returning to periscope 412, the beam is captured using compression telescope 416 and passed through various mirrors 418 and lenses 422 before being coupled into the fiber via fiber coupler 420. The received signals are then amplified by EDFA 404c and split by 50:50 beam splitter 432 and fed to two separate balanced detectors (i.e. balanced detector 426a and balanced detector 426b). For wavelength diversity, local oscillator lasers were respectively, tuned to 1560 nm and 1565 nm (i.e., laser 424a and laser 424b), which are used as reference signals for the balanced detectors.

[0081] The RF outputs of balanced detector 426a and balanced detector 426b are then simultaneously screen captured from real-time scope 428, which functions as an analog-to- digital converter (ADC). Offline digital signal processing (DSP) was performed by computer 430 to recover the modulated signal and compute bit-error rates (BERs). The start of the codeword (i.e., FEC frame) was determined using a cross-correlation method to find the header. Computer 430 computed the log-likelihood hood ratios (LLRs) for both 1560 nm and 1565 nm channels, which are used to determine the BERs. For wavelength diversity results, the system first sums the LLRs of both channels in 0.25: 1 ratio (for 1560 nm and 1565 nm, respectively). Other ratios might also be optimal depending on various factors. Thus, the concept is not limited to any specific ratio. The PC can be replaced by FPGA or ASIC hardware.

[0082] The results from the case study are summarized in FIG. 5 A and FIG. 5B. FIG. 5 A shows the uncoded bit-error rates (BERs) for different time instances (turbulence realizations). More particularly, FIG. 5 A shows 1560 nm BER 502, 1565 nm BER 504, and wavelength diversity BER 506.

[0083] In this case study, the 1565 nm channel exhibited better tolerance to atmospheric turbulence effects compared to the 1560 nm channel. The results from either channel alone are improved when the wavelength diversity is applied. The BER for all three cases are sufficient that the LDPC decoder was able to correct all errors in each of the three cases under study.

[0084] FIG. 5B shows a histogram of received power, collected in front of the compressing telescope, which is similar to a Rayleigh distribution, indicating that the turbulence of the FSO channel falls within a strong atmospheric turbulence regime.

[0085] FIG.6A, FIG. 6B, and FIG. 6C illustrate results from a second case study. This case study compares the performance of case-study system 400 with and without adaptive optics(AO). When AO is used in case-study system 400, the AO system is placed between compression telescope 416 and fiber coupler 420.

[0086] Similar to the above description of FIG. 4, the test bed used for the second case study includes capturing the returned signal via periscope 412 and compression telescope 416., after which the optical beam passes through an AO system before being coupled into fiber coupler 420. Next, the beam is bifurcated into a 1550 nm wavelength channel and a 1570 nm wavelength channel. In the second case study, this bifurcation is performed using a compact coarse WDM (CCWDM) de-multiplexer, rather than 50:50 beam splitter 432. The two channels are respectively sent to two separate balanced homodyne detectors and the other two inputs to balanced detectors are local lasers signals with wavelengths 1550 nm and 1570 nm, respectively. As described above with reference to FIG. 4, the RF outputs of these balanced detectors are then sampled by a real-time scope (RTS) for offline DSP. The processing of the RF signals is performed as described above for FIG. 4.

[0087] In the second case study, the AO system uses a 140-actuator deformable mirror (DM) together with a Shack-Hartmann wavefront sensor (WFS) operating in a servo-loop configuration. BER results are collected for cases when the AO subsystem is active and inactive.

[0088] FIG. 6A compares BERs (vertical axis) for C+L-bands-based wavelength-diversity and AO schemes for different FSO channel realizations (horizontal axis).

[0089] FIG. 6A summarizes uncoded BER results from the second case study using case-study system 400 for different turbulence realizations (time instances). More particularly, FIG. 6A plots BERs for cases when using a single balanced detector with the LO wavelengths are 1550 nm and 1570 nm wavelength, and FIG. 6A plots BERs using wavelength diversity (e.g., using multiple balanced detectors with different LO wavelengths). The plotted BERs were collected during the strong turbulence regime, as indicated by FIG. 6B.

[0090] In FIG. 6A, it can be observed that the L-band wavelength channel outperforms the C- band wavelength channel, while the C+L-bands-based diversity scheme outperforms both. The C+L-bands-based wavelength diversity scheme operated in the strong turbulence regime outperforms the AO scheme operated in the medium turbulence regime, as indicated in FIG. 6C. The LDPC decoder was able to correct all errors in all cases under study.

[0091] FIG. 6B shows a histogram plot of received power during the acquisition represented in FIG. 6A. As seen in FIG. 6B, the Rayleigh distribution indicates strong turbulence. That is, FIG, 6B shows a Rayleigh distribution histogram of the received power for the second case study, indicating the strong turbulence regime.

[0092] FIG. 6C shows a Gaussian-like distribution histogram received power in the 1550 nm channel during adaptive optics data collection, which is indicative of the medium turbulence regime.

[0093] The second case study demonstrates that the C+L-bands-based wavelength diversity scheme operated in strong turbulence regime can outperform the adaptive optics scheme operated in medium turbulence regime in terms of the BERs. Both the complexity and cost of the C+L-bands-based wavelength diversity can be significantly lower than the adaptive optics scheme. According to certain non-limiting examples, the wavelength diversity scheme can use a compact coarse wavelength division multiplexing (CCWDM) de-multiplexer and one additional balanced detector. The advantages of the C+L-bands-based wavelength diversity scheme include being more affordable and easier to implement realtive to AO, which can be bulky, introduce additional complexity (e.g., requiring precise alignment and the added complexity of the servo loop control), and additional expense (e.g., a deformable mirror and wavefront sensor are more expensive than an additional balanced detector).

[0094] FIG. 7 shows an example of computing system 700. The computing system 700 can be code generator 114, forward error correction decoder 124, processor 206a, processor 206b, and / or computer 430. The computing system 700 can perform the functions of method 300. The computing system 700 can be part of a distributed computing network in which several computers perform respective steps in method 300 and / or the functions of FSO communication system 100. The computing system 700 can be connected to the other parts of the distributed computing network via connection 702 or communication interface 728. Connection 702 can be a physical connection via a bus, or a direct connection into processor 704, such as in a chipset architecture. Connection 702 can also be a virtual connection, networked connection, or logical connection.

[0095] In some embodiments, computing system 700 is a distributed system in which the functions described in this disclosure can be distributed within a datacenter, multiple data centers, a peer network, etc. In some embodiments, one or more of the described system components represents many such components each performing some or all of the function forwhich the component is described. In some embodiments, the components can be physical or virtual devices.

[0096] Example computing system 700 includes at least one processing unit (CPU or processor) 704 and connection 702 that couples various system components including system memory 708, such as read-only memory (e.g., ROM 710) and random access memory (e.g., RAM 712) to processor 704. Computing system 700 can include a cache of high-speed memory 706 connected directly with, in close proximity to, or integrated as part of processor 704. Processor 704 may essentially be a completely self-contained computing system, containing multiple cores or processors, a bus, memory controller, cache, etc. A multi-core processor may be symmetric or asymmetric.

[0097] Processor 704 can include any general -purpose processor and a hardware service or software service, such as services 720, 722, and 724 stored in storage device 714, configured to control processor 704 as well as a special-purpose processor where software instructions are incorporated into the actual processor design.

[0098] To enable user interaction, computing system 700 includes an input device 730, which can represent any number of input mechanisms, such as a microphone for speech, a touch- sensitive screen for gesture or graphical input, keyboard, mouse, motion input, speech, etc. Computing system 700 can also include output device 726, which can be one or more of a number of output mechanisms known to those of skill in the art. In some instances, multimodal systems can enable a user to provide multiple types of input / output to communicate with computing system 700. Computing system 700 can include a communication interface 728, which can generally govern and manage the user input and system output. There is no restriction on operating on any particular hardware arrangement, and therefore the basic features here may easily be substituted for improved hardware or firmware arrangements as they are developed.

[0099] Storage device 714 can be a non-volatile memory device and can be a hard disk or other types of computer-readable media that can store data that are accessible by a computer, such as magnetic cassettes, flash memory cards, solid state memory devices, digital versatile disks, cartridges, random access memories (RAMs), read-only memory (ROM), and / or some combination of these devices.

[0100] The storage device 714 can include software services, servers, services, etc., that when the code that defines such software is executed by processor 704, it causes the system to perform a function. In some embodiments, a hardware service that performs a particular function can include the software component stored in a computer-readable medium in connection with the necessary hardware components, such as processor 704, connection 702, output device 726, etc., to carry out the function.

[0101] For clarity of explanation, in some instances, the present technology may be presented as including individual functional blocks including functional blocks comprising devices, device components, steps or routines in a method embodied in software, or combinations of hardware and software.

[0102] Any of the steps, operations, functions, or processes described herein may be performed or implemented by a combination of hardware and software services or services, alone or in combination with other devices. In some embodiments, a service can be software that resides in memory of a code generator 114, forward error correction decoder 124, processor 206a, processor 206b, and / or computer 430 and performs one or more functions of method 300 when a processor executes the software associated with the service. In some embodiments, a service is a program or a collection of programs that carry out a specific function. In some embodiments, a service can be considered a server. The memory can be a non-transitory computer-readable medium.

[0103] In some embodiments, the computer-readable storage devices, mediums, and memories can include a cable or wireless signal containing a bit stream and the like. However, when mentioned, non-transitory computer-readable storage media expressly exclude media such as energy, carrier signals, electromagnetic waves, and signals per se.

[0104] Methods according to the above-described examples can be implemented using computer-executable instructions that are stored or otherwise available from computer-readable media. Such instructions can comprise, e.g., instructions and data that cause or otherwise configure a general-purpose computer, special-purpose computer, or special-purpose processing device to perform a certain function or group of functions. Portions of computer resources used can be accessible over a network. The executable computer instructions may be, e.g., binaries, intermediate format instructions such as assembly language, firmware, or sourcecode. Examples of computer-readable media that may be used to store instructions, information used, and / or information created during methods according to described examples include magnetic or optical disks, solid-state memory devices, flash memory, USB devices provided with non-volatile memory, networked storage devices, and so on.

[0105] Devices implementing methods according to these disclosures can comprise hardware, firmware and / or software, and can take any of a variety of form factors. Typical examples of such form factors include servers, laptops, smartphones, small form factor personal computers, personal digital assistants, and so on. The functionality described herein also can be embodied in peripherals or add-in cards. Such functionality can also be implemented on a circuit board among different chips or different processes executing in a single device, by way of further example.

[0106] The instructions, media for conveying such instructions, computing resources for executing them, and other structures for supporting such computing resources are means for providing the functions described in these disclosures.

[0107] For clarity of explanation, in some instances the present technology may be presented as including individual functional blocks including functional blocks comprising devices, device components, steps or routines in a method embodied in software, or combinations of hardware and software.

[0108] Any of the steps, operations, functions, or processes described herein may be performed or implemented by a combination of hardware and software services or services, alone or in combination with other devices. In some embodiments, a service can be software that resides in memory of a client device and / or one or more servers of a content management system and perform one or more functions when a processor executes the software associated with the service. In some embodiments, a service is a program, or a collection of programs that carry out a specific function. In some embodiments, a service can be considered a server. The memory can be a non-transitory computer-readable medium.

[0109] In some embodiments, the computer-readable storage devices, mediums, and memories can include a cable or wireless signal containing a bit stream and the like. However, when mentioned, non-transitory computer-readable storage media expressly exclude media such as energy, carrier signals, electromagnetic waves, and signals per se.

[0110] Methods according to the above-described examples can be implemented using computer-executable instructions that are stored or otherwise available from computer-readable media. Such instructions can comprise, e.g., instructions and data that cause or otherwise configure a general-purpose computer, special-purpose computer, or special-purpose processing device to perform a certain function or group of functions. Portions of computer resources used can be accessible over a network. The computer-executable instructions may be, For example, binaries, intermediate format instructions such as assembly language, firmware, or source code. Examples of computer-readable media that may be used to store instructions, information used, and / or information created during methods according to described examples include magnetic or optical disks, solid state memory devices, flash memory, USB devices provided with non-volatile memory, networked storage devices, and so on.

[0111] Devices implementing methods according to these disclosures can comprise hardware, firmware and / or software, and can take any of a variety of form factors. Typical examples of such form factors include servers, laptops, smart phones, small form factor personal computers, personal digital assistants, and so on. Functionality described herein also can be embodied in peripherals or add-in cards. Such functionality can also be implemented on a circuit board among different chips or different processes executing in a single device, by way of further example.

[0112] The instructions, media for conveying such instructions, computing resources for executing them, and other structures for supporting such computing resources are means for providing the functions described in these disclosures.

[0113] Although a variety of examples and other information was used to explain aspects within the scope of the appended claims, no limitation of the claims should be implied based on particular features or arrangements in such examples, as one of ordinary skill would be able to use these examples to derive a wide variety of implementations. Further and although some subject matter may have been described in language specific to examples of structural features and / or method steps, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to these described features or acts. For example, such functionality can be distributed differently or performed in components other than those identified herein. Rather, the described features and steps are disclosed as examples of components of systems and methods within the scope of the appended claims.

[0114] Claim clauses include:

[0115] Clause 1. A system for low probability to intercept (LPI) free-space optical (FSO) communications, the system comprising: an optical transmitter that includes: a broadband incoherent optical source that generates light with a spectrum in a solar-radiation spectrum and has a full width half maximum of at least 10 nm, an encoder that encodes information data using an error correction technique to generate a codeword and combines the codeword with a header to provide a data packet, a modulator that modulates the light according to a predefined modulation scheme to generate a modulated optical signal, and output optics that couple the modulated optical signal from the transmitter to a free-space optical channel.

[0116] Clause 2. The system of clause 1, wherein broadband optical source that generates light that provides wavelength diversity at a plurality of center wavelengths that are spaced by more than a modulation bandwidth, and the encoder encodes information data using a low- density parity-check code, a Reed-Solomon code, a turbo code, a Bose-Chaudhuri- Hocquenghem (BCH) code, a convolutional code, a polar code, or a product code.

[0117] Clause 3. The system of clause 2, wherein the modulator is a phase modulator that modulates the light using phase shift keying at the modulation bandwidth.

[0118] Clause 4. The system of clause 3, wherein: the phase shift keying is binary phase shift keying or quadrature phase shift keying, and the modulation data rate corresponds to a bit rate in a range from 10 Gbps to 100 Gbps, and the spectrum of the light is between a wavelength of 1500 nm and 2200 nm.

[0119] Clause 5. The system of clause 1 or clause 2, further comprising: an optical receiver that includes: input optics that couple the transmitted modulated optical signal from the free space channel to the receiver to provide a received optical signal, one or more balanced detectors that mix the received optical signal with coherent optical signals from one or more local oscillators to provide mixed signal that are detected by balanced photoreceivers to generate one or more analog RF signals, and an analog-to-digital converters that convert the one or more analog RF signals to one or more digital signals; and one or more processors that process the one or more digital signals to reconstruct the information data.

[0120] Clause 6. The system of clause 5, wherein the one or more processors detect a start time of the data packet using the header.

[0121] Clause 7. The system of clause 5 or clause 6, wherein: the one or more balanced detectors include a plurality of heterodyne detectors, the one or more local oscillators includelasers operating at respective center wavelengths that are spaced by more than a modulation bandwidth, thereby providing wavelength diversity for detecting the data packet, the plurality of heterodyne detectors generating respective RF signals corresponding to the respective center wavelengths, each of the respective RF signals being converted to a corresponding digital signal of the one or more digital signals, and the one or more processors process a combination of digital signals of the one or more digital signals to reconstruct the information data.

[0122] Clause 8. The system of clause 7, wherein the one or more processors process the combination of digital signals by determining log-likelihood ratios for each of the respective center wavelengths and combining the log-likelihood ratios for each of the respective center wavelengths to generate aggregate log-likelihood ratios.

[0123] Clause 9. The system of clause 8, wherein a sum-product algorithm is used to decode the LDPC codeword using the aggregate / combined log-likelihood ratios.

[0124] Clause 10. An optical receiver, comprising: input optics that couple a transmitted modulated optical signal from a free space channel to a receiver to provide a received optical signal; one or more balanced detectors that mix the received optical signal with coherent optical signals from one or more local oscillators to provide mixed signals that are detected by balanced photoreceivers to generate one or more analog RF signals; and an analog to digital converter that converts the one or more analog RF signals to one or more digital signals; and one or more processors that process the one or more digital signals to reconstruct information data, wherein the modulated optical signal has been modulated to carry a data packet that includes a header and a codeword, the codeword representing information data that has been encoded according to an error correction technique.

[0125] Clause 11. The receiver of clause 10, wherein the one or more processors detect a start time of the data packet using the header.

[0126] Clause 12. The receiver of clause 10 or clause 11, wherein: the one or more balanced detectors include a plurality of heterodyne detectors, the one or more local oscillators include lasers operating at respective center wavelengths that are spaced by more than a modulation bandwidth, thereby providing wavelength diversity for detecting the data packet, the plurality of heterodyne detectors generating respective RF signals corresponding to the respective center wavelengths, each of the respective RF signals being converted to a corresponding digitalsignal of the one or more digital signals, and the one or more processors process a combination of digital signals of the one or more digital signals to reconstruct the information data.

[0127] Clause 13. The receiver of clause 12, wherein the one or more processors process the combination of digital signals by determining log-likelihood ratios for each of the respective center wavelengths and combining the log-likelihood ratios for each of the respective center wavelengths to generate aggregate / combined log-likelihood ratios.

[0128] Clause 14. The receiver of clause 13, wherein a sum-product algorithm is used to used to decode the LDPC codeword using the aggregate / combined log-likelihood ratios.

[0129] Clause 15. A method for low probability to intercept (LPI) free space optical (FSO) communications, the method comprising: generating, at a broadband incoherent optical source, light having a spectrum within a solar-radiation spectrum and has full width half maximum of at least 10 nm; encoding, by an encoder, information data using an error correction technique to generate a codeword and adding a header before the codeword to provide a data packet; modulating, by a modulator, the light according to a predefined modulation scheme to generate a modulated optical signal; and transmitting, from a transmitter, the modulated optical signal into a free space channel that includes solar radiation.

[0130] Clause 16. The method of clause 15, wherein: the broadband incoherent optical source generates the light at a plurality of center wavelengths that are spaced by more than a modulation bandwidth, thereby providing wavelength diversity, the encoder of the transmitter encodes the information data using a low-density parity-check code, a Reed-Solomon code, a turbo code, a Bose-Chaudhuri-Hocquenghem (BCH) code, a convolutional code, a polar code, or a product code, and the modulator is a phase modulator that modulates the light using phase shift keying at the modulation bandwidth.

[0131] Clause 17. The method of clause 16, wherein: the phase shift keying uses binary phase shift keying or quadrature phase shift keying, and the modulation bandwidth corresponds to a bit rate in a range from 10 Gbps to 100 Gbps, and the spectrum of the light is between a wavelength of 1500 nm and 2200 nm.

[0132] Clause 18. The method of any of clause 15 through clause 17, further comprising: receiving, at a receiver, the transmitted modulated optical signal from the free space channel to provide a received optical signal; mixing the received optical signal with coherent optical signals from one or more local oscillators to provide mixed signal that are detected by balancedphotoreceivers to generate one or more analog RF signals; converting the one or more analog RF signals to one or more digital signals; and processing, by one or more processors, the one or more digital signals to reconstruct the information data.

[0133] Clause 19. The method of clause 18, wherein mixing the received optical signal with coherent optical signals further includes: generating the one or more analog RF signals using a plurality of heterodyne detectors, wherein the one or more local oscillators include lasers operating at respective center wavelengths that are spaced by more than a modulation, thereby providing wavelength diversity for detecting the data packet, the plurality of heterodyne detectors generate respective RF signals corresponding to the respective center wavelengths, converting the one or more analog RF signals to the one or more digital signals includes converting the respective RF signals being converted to a plurality of digital signals, and processing the one or more digital signals includes processing a combination of the plurality of digital signals to reconstruct the information data.

[0134] Clause 20. The method of clause 19, wherein processing the combination of the plurality of digital signals to reconstruct the information data includes: determining loglikelihood ratios for each of the respective center wavelengths and combining the loglikelihood ratios for each of the respective center wavelengths to generate aggregate loglikelihood ratios, and using a sum-product algorithm to decode the LDPC codeword based on the aggregate / combined log-likelihood ratios.

[0135] Clause 21. An optical communication system comprising: a broadband incoherent light source; a phase modulator; an erbium-doped fiber amplifier (EDFA); a beam expander; a periscope; a compressing telescope; a fiber coupler; a receive EDFA; a 50:50 splitter; a first balanced heterodyne detector comprising a first local oscillator laser operating at a first wavelength and connected to the 50:50 splitter; a second balanced heterodyne detector comprising a second local oscillator laser at a second wavelength and connected to the 50:50 splitter, wherein the second wavelength differs from the first wavelength; one or more processors; and a computer-readable storage medium storing instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: generating, by the light source, the phase modulator and the erbium-doped fiber amplifier (EDFA), a phase-modulated signal; transmitting, via the beam expander and the periscope, the phase-modulated signal to yield a transmitted signal, wherein the transmitted signal is hidden in solar radiation; receiving, via the compressing telescope, a broadband signalcomprising the transmitted signal over a turbulent free-space optical channel; splitting, via the 50:50 splitter, the broadband signal into a first broadband signal and a second broadband signal; detecting, via the first balanced heterodyne detector, a first received beam by interacting the first local oscillator laser with the first broadband signal to generate a first radio frequency output; detecting, via the second balanced heterodyne detector, a second received beam by interacting the second local oscillator laser with the second broadband signal to generate a second radio frequency output; and simultaneously capturing, via analog to digital converters (ADCs), the first radio frequency output and the second radio frequency output to generate a received electrical signal.

[0136] Clause 22. The communication system of clause 21, wherein the one or more processors comprises one of an application-specific integrated circuit, a field programmable gate array, or a computer processor.

[0137] Clause 23. The communication system of clause 21 or clause 22, wherein the light source comprises one or more of an amplified spontaneous emission (ASE) or an erbium-doped fiber amplifier (EDFA).

[0138] Clause 2 4. The communication system of any of clause 21 through clause 23, wherein the phase modulator comprises a Mach-Zehnder (MZ) modulator.

[0139] Clause 25. The communication system of clause 21 through clause 24, wherein the first detection wavelength comprises 1560 nm laser and the second wavelength comprises 1565 nm laser.

[0140] Clause 26. The communication system of clause 21 through clause 25, wherein the first wavelength and the second wavelength differ by between 3 nm and 40 nm.

[0141] Clause 27. The communication system of clause 21 through clause 26, wherein the computer- readable storage medium stores further instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: combining a first log-likelihood ratio associated with the first radio frequency output with a second log-likelihood ratio associated with the second radio frequency output to generate the resulting log-likelihood ratio used for diversity.

[0142] Clause 28. The communication system of clause 21 through clause 27, wherein the computer-readable storage medium stores further instructions which, when executed by the oneor more processors, cause the one or more processors to perform operations comprising: processing the received signal to compute a bit error rate.

[0143] Clause 29. The communication system of clause 21 through clause 28, wherein the computer-readable storage medium stores further instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: determining a start position of a forward error correction header having a codeword in the received signal by applying a cross-correlation method to find the header.

[0144] Clause 30. The communication system of clause 21, wherein the computer-readable storage medium stores further instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: computing a first log-likelihood ratio associated with the first radio frequency output; computing a second log-likelihood ratio associated with the second radio frequency output; summing the first loglikelihood ratio with the second log-likelihood ratio.

[0145] [Clause 31. The communication system of clause 30, wherein summing the first loglikelihood ratio with the second log-likelihood ratio is performed in an optimum fashion ratio related to the first wavelength and the second wavelength, respectively.

[0146] Clause 32. The communication system of clause 31, wherein the optimum fashion ratio comprises 0.25: 1 for the first wavelength and the second wavelength, respectively.

[0147] Clause 33. A method of operating a communication system, the method comprising: generating, by a broadband light source, a phase modulator and an erbium-doped fiber amplifier (EDFA), a phase-modulated signal; transmitting, via a beam expander and a periscope, the broadband phase-modulated signal to yield a transmitted signal, wherein the transmitted signal is hidden in solar radiation; receiving, via a compressing telescope, a broadband signal comprising the transmitted signal over a turbulent channel; splitting, via a 50:50 splitter, the broadband signal into a first broadband signal and a second broadband signal; detecting, via a first balanced heterodyne detector, a first received beam by interacting a first local oscillator laser with the first broadband signal to generate a first radio frequency output; detecting, via a second balanced heterodyne detector, a second received beam by interacting a second local oscillator laser with the second broadband signal to generate a second radio frequency output; and simultaneously capturing, via analog-to-digital converters (ADCs), the first radio frequency output and the second radio frequency output to generate a received signal.

[0148] Clause 34. The method of clause 33, wherein the implements one or more of an application-specific integrated circuit, a field programmable gate array, or a computer processor.

[0149] Clause 35. The method of clause 33 of clause 34, wherein the broadband light source comprises one or more of an amplified spontaneous emission (ASE), an erbium-doped fiber amplifier (EDFA), or light emitting diode (LED).

[0150] Clause 36. The method of clause 33, wherein the phase modulator comprises a Mach- Zehnder (MZ) modulator.

[0151] Clause 37. The method of clause 33, wherein the phase modulator is replaced by electro- optical I / Q modulator.

[0152] Clause 38. The method of clause 33 through clause 37, wherein the first detection wavelength comprises 1560 nm laser and the second wavelength comprises 1565 nm laser.

[0153] Clause 39. The method of clause 33 through clause 38, wherein the first wavelength and the second wavelength differ by between 3 nm and 40 nm.

[0154] Clause 40. The method of clause 33 through clause 39, wherein the method further comprises: combining a first log-likelihood ratio associated with the first radio frequency output with a second log-likelihood ratio associated with the second radio frequency output to generate the resulting log-likelihood ratio used for diversity.

[0155] Clause 41. The method of clause 33 through clause 40, wherein the method further comprises: processing the received signal to compute a bit error rate.

[0156] Clause 42. The method of clause 33 through clause 41, wherein the method further comprises: determining a start position of a forward error correction header having a codeword in the received signal by applying a cross-correlation method to find the header.

[0157] Clause 43. The method of clause 33 through clause 42, wherein the method further comprises: computing a first log-likelihood ratio associated with the first radio frequency output; summing the first log-likelihood ratio with the second log-likelihood ratio. Clause 43. The communication system of clause 22, wherein summing the first log-likelihood ratio with the second log-likelihood ratio is performed in an optimum fashion ratio related to the first wavelength and the second wavelength, respectively.

[0158] Clause 44. The communication system of clause 23, wherein the optimum fashion ratio comprises 0.25: 1 for the first wavelength and the second wavelength, respectively.

[0159] Clause 45. A communication system comprising: an optical transmitter comprising a light source, a phase modulator and a erbium-doped fiber amplifier (EDFA); an optical receiver comprising a compressing telescope; one or more processors; and a computer-readable storage medium storing instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: generating, via the optical transmitter, a phase-modulated signal; transmitting, via a beam expander and a periscope, the broadband phase-modulated signal to yield a transmitted signal, wherein the transmitted signal is hidden in solar radiation; receiving, via the optical receiver, a broadband signal comprising the transmitted signal received over a turbulent channel; splitting, via a 50:50 splitter, the broadband signal into a first broadband signal and a second broadband signal; detecting, via a first balanced heterodyne detector, a first received beam by interacting a first local oscillator laser with the first broadband signal to generate a first radio frequency output; detecting, via a second balanced heterodyne detector, a second received beam by interacting a second local oscillator laser with the second broadband signal to generate a second radio frequency output; and simultaneously capturing, via analog-to-digital converters (ADCs), the first radio frequency output and the second radio frequency output to generate a received signal.

[0160] Clause 46. An optical communication system comprising: a broadband incoherent light source; a phase modulator; an erbium-doped fiber amplifier (EDFA); a beam expander; a periscope; a compressing telescope; a fiber coupler; a receive EDFA; a 1 :M beam splitter; M balanced heterodyne detectors each comprising a respective local oscillator laser, wherein M equals two or more, wherein a respective wavelength for each respective local oscillator is different from each other more processors; and a computer-readable storage medium storing instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: generating, by the light source, the phase modulator and the erbium-doped fiber amplifier (EDFA), a phase-modulated signal; transmitting, via the beam expander and the periscope, the phase-modulated signal to yield a transmitted signal, wherein the transmitted signal is hidden in solar radiation; receiving, via the compressing telescope, a broadband signal comprising the transmitted signal over a turbulent free-space optical channel; splitting, via the 1 :M beam splitter, the broadband signal into M broadband signals; detecting, via each of the M balanced heterodyne detectors, a respectivebroadband signal of the M broadband signals to generate M radio frequency outputs; and simultaneously capturing, via analog to digital converters (ADCs), the M radio frequency outputs to generate a received electrical signal.

[0161] Clause 47. The communication system of clause 46, wherein the phase modulator is replaced by the electro-optical I / Q modulator to support quadrature amplitude modulation (QAM) formats.

Claims

CLAIMSWhat is claimed is:

1. A system for low probability to intercept (LPI) free-space optical (FSO) communications, the system comprising: a transmitter that includes: a broadband incoherent optical source that generates light with a spectrum in a solar-radiation spectrum and has a full width half maximum of at least 10 nm, an encoder that encodes information data using an error correction technique to generate a codeword and combines the codeword with a header to provide a data packet, a modulator that modulates the light according to a predefined modulation scheme to generate a modulated optical signal, and output optics that couple the modulated optical signal from the transmitter to a free space channel.

2. The system of claim 1, wherein broadband incoherent optical source that generates light that provides wavelength diversity at a plurality of center wavelengths that are spaced by more than a modulation bandwidth, and the encoder encodes information data using a low-density parity-check code, a Reed- Solomon code, a turbo code, a Bose-Chaudhuri-Hocquenghem (BCH) code, a convolutional code, a polar code, or a product code.

3. The system of claim 2, wherein the modulator is a phase modulator that modulates the light using phase shift keying at the modulation bandwidth.

4. The system of claim 3, wherein: the phase shift keying is binary phase shift keying or quadrature phase shift keying, and the modulation bandwidth corresponds to a bit rate in a range from 10 Gbps to 100 Gbps, and the spectrum of the light is between a wavelength of 1500 nm and 2200 nm.

5. The system of claim 1, further comprising: a receiver that includes:input optics that couple the modulated optical signal from the free space channel to the receiver to provide a received optical signal, one or more balanced detectors that mix the received optical signal with coherent optical signals from one or more local oscillators to provide mixed signal that are detected by balanced photoreceivers to generate one or more analog RF signals, and analog-to-digital converters that convert the one or more analog RF signals to one or more digital signals; and one or more processors that process the one or more digital signals to reconstruct the information data.

6. The system of claim 5, wherein the one or more processors detect a start time of the data packet using the header.

7. The system of claim 5, wherein: the one or more balanced detectors include a plurality of heterodyne detectors, the one or more local oscillators include lasers operating at respective center wavelengths that are spaced by more than a modulation bandwidth, thereby providing wavelength diversity for detecting the data packet, the plurality of heterodyne detectors generating respective RF signals corresponding to the respective center wavelengths, each of the respective RF signals being converted to a corresponding digital signal of the one or more digital signals, and the one or more processors process a combination of digital signals of the one or more digital signals to reconstruct the information data.

8. The system of claim 7, wherein the one or more processors process the combination of digital signals by determining log-likelihood ratios for each of the respective center wavelengths and combining the log-likelihood ratios for each of the respective center wavelengths to generate combined log-likelihood ratios.

9. The system of claim 8, wherein: a sum-product algorithm is used to decode the codeword using the combined loglikelihood ratios, when the codeword is an LDPC codeword, andwhen the codeword has been encoded using another forward error correction code than LDCP, a decoding algorithm corresponding to the another forward error correction code is used to decode the codeword.

10. A receiver, comprising: input optics that couple a transmitted modulated optical signal from a free space channel to a receiver to provide a received optical signal; one or more balanced detectors that mix the received optical signal with coherent optical signals from one or more local oscillators to provide mixed signals that are detected by balanced photoreceivers to generate one or more analog RF signals; and one or more analog-to-digital converters that convert the one or more analog RF signals to one or more digital signals; and one or more processors that process the one or more digital signals to reconstruct information data, wherein the modulated optical signal has been modulated to carry a data packet that includes a header and a codeword, the codeword representing information data that has been encoded according to an error correction technique.

11. The receiver of claim 10, wherein the one or more processors detect a start time of the data packet using the header.

12. The receiver of claim 10, wherein: the one or more balanced detectors include a plurality of heterodyne detectors, the one or more local oscillators include lasers operating at respective center wavelengths that are spaced by more than a modulation bandwidth, thereby providing wavelength diversity for detecting the data packet, the plurality of heterodyne detectors generating respective RF signals corresponding to the respective center wavelengths, each of the respective RF signals being converted to a corresponding digital signal of the one or more digital signals, and the one or more processors process a combination of digital signals of the one or more digital signals to reconstruct the information data.

13. The receiver of claim 12, wherein the one or more processors process the combination of digital signals by determining log-likelihood ratios for each of the respective centerwavelengths and combining the log-likelihood ratios for each of the respective center wavelengths to generate combined log-likelihood ratios.

14. The receiver of claim 13, wherein: a sum-product algorithm is used to decode the codeword using the combined loglikelihood ratios, when the codeword is an LDPC codeword, and when the codeword has been encoded using another forward error correction code than LDCP, a decoding algorithm corresponding to the another forward error correction code is used to decode the codeword.

15. A method for low probability to intercept (LPI) free-space optical (FSO) communications, the method comprising: generating, at a broadband incoherent optical source, light having a spectrum within a solar-radiation spectrum and has full width half maximum of at least 10 nm; encoding, by an encoder, information data using an error correction technique to generate a codeword and adding a header before the codeword to provide a data packet; modulating, by a modulator, the light according to a predefined modulation scheme to generate a modulated optical signal; and transmitting, from a transmitter, the modulated optical signal into a free space channel that includes solar radiation.

16. The method of claim 15, wherein: the broadband incoherent optical source generates the light at a plurality of center wavelengths that are spaced by more than a modulation bandwidth, thereby providing wavelength diversity, the encoder of the transmitter encodes the information data using a low-density paritycheck code, a Reed-Solomon code, a turbo code, a Bose-Chaudhuri-Hocquenghem (BCH) code, a convolutional code, a polar code, or a product code, and the modulator is a phase modulator or an I / Q modulator, the modulator modulates the light using phase shift keying modulation or quadrature amplitude modulation, and the modulator operates at the modulation bandwidth.

17. The method of claim 16, wherein: the phase shift keying modulation uses binary phase shift keying or quadrature phase shift keying, and the modulation bandwidth corresponds to a bit rate in a range from 10 Gbps to 100 Gbps, and the spectrum of the light is between a wavelength of 1500 nm and 2200 nm.

18. The method of claim 15, further comprising: receiving, at a receiver, the transmitted modulated optical signal from the free space channel to provide a received optical signal; mixing the received optical signal with coherent optical signals from one or more local oscillators to provide mixed signal that are detected by balanced photoreceivers to generate one or more analog RF signals; converting the one or more analog RF signals to one or more digital signals; and processing, by one or more processors, the one or more digital signals to reconstruct the information data.

19. The method of claim 18, wherein mixing the received optical signal with coherent optical signals further includes: generating the one or more analog RF signals using a plurality of heterodyne detectors, wherein the one or more local oscillators include lasers operating at respective center wavelengths that are spaced by more than a modulation bandwidth, thereby providing wavelength diversity for detecting the data packet, the plurality of heterodyne detectors generate respective RF signals corresponding to the respective center wavelengths, converting the one or more analog RF signals to the one or more digital signals includes converting the respective RF signals being converted to a plurality of digital signals, and processing the one or more digital signals includes processing a combination of the plurality of digital signals to reconstruct the information data.

20. The method of claim 19, wherein processing the combination of the plurality of digital signals to reconstruct the information data includes: determining log-likelihood ratios for each of the respective center wavelengths and combining the log-likelihood ratios for each of the respective center wavelengths to generate combined log-likelihood ratios, and using a sum-product algorithm to decode the codeword based on the combined loglikelihood ratios, when the codeword is an LDPC codeword.

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