空间激光通信相干探测

空间相干光通信被认为是突破现有高速空间通信瓶颈的重要手段, 但其应用受到大气湍流的极大限制. 为此, 本文首先基于Huygens-Fresnel原理和低频补偿功率谱反演法, 研究了高斯光束经大气湍流传输后振幅和相位的随机分布特性; 然后, 利用相干混频效率及通信误码率模型, 获得大气湍流对空间相干光通信系统性能的影响规律; 最后, 搭建激光外差探测实验系统, 定量研究了大气湍流对空间相干光通信相干探测性能的影响. 结果表明: 弱湍流条件下, 空间相干光通信性能几乎不受大气湍流的影响; 中等强度湍流影响下, 相干混频效率会随着湍流强度的增大而迅速下降, 但通过提高单比特光子数可以有效抑制湍流对通信性能的负面影响; 强湍流会显著破坏光束相干性, 使得相干混频效率趋近于零, 即使提高单比特光子数也无法有效改善通信性能. 大气湍流是空间相干光通信发展的重要限制因素, 该研究可为空间相干光通信系统性能评估提供有益参考.

为了探究卫星激光通信中对于各种外部干扰的安全防护技术，介绍卫星激光通信链路中存在的大气湍流、太空背景噪声、弱光干扰和强激光干扰等4种干扰源及其作用原理，并给出前两种干扰在理论分析中常用的数值模型。其中弱光干扰和强激光干扰又被称为激光有源干扰，是一种人为干扰手段。从通信体制维度出发，分别分析这4种干扰源对于相干和非相干通信体制系统的影响，给出不同体制下应对这4种干扰的当前主流的防护技术，从通用性、成本以及易用性等角度，比较各种防护手段的技术特点并说明它们在卫星激光通信中的主要应用场景。

激光通信技术突破了传统微波通信的带宽限制, 成为实现高速率、大容量星地通信的重要手段, 特别适用于海量空间科学数据的传输. 星地激光通信技术在空间科学中具有广泛的应用前景, 是实现空间科学数据高效、快速传输的关键技术之一. 本文系统梳理了国内外星地激光通信的系统组成及实验成果, 详细介绍了实现稳定可靠通信的关键技术, 例如捕获跟踪瞄准、星上激光器技术等. 针对大气湍流对激光信道的影响, 分析了自适应光学等有效的抑制方法, 并汇总了基于新型结构光场的激光通信技术发展现状. 结合空间科学对数据传输的需求, 对星地激光通信的研究现状进行了总结, 并展望了未来的发展方向, 强调了其在空间科学和深空探测中的重要应用潜力.

为了缓解大气湍流对星地激光通信系统性能的影响，基于 M 分布大气信道模型，在星地激光通信上行链路和下行链路中提出了一种多载波相干正交频分复用（OFDM）调制系统，推导了星地激光通信上行链路和下行链路相干 OFDM 调制系统误码率的闭合表达式。在弱大气湍流和强大气湍流下，分别研究了天顶角、接收孔径、信噪比、最佳束散角以及最佳发射半径与系统误码率的关系，并与二进制相干差分相移键控（DPSK）调制进行比较。理论和仿真对比结果表明，星地激光通信系统中采用相干OFDM调制系统的误码性能优于相干DPSK调制系统。

随着商业航天的快速发展，众多行业对天基信息服务提出了更高要求，传统单颗卫星的中继方式已难以满足日益增长的业务需求。与此同时，地面通信对速率和容量的需求不断提升，催生了星间激光通信技术。该技术有效弥补了现有通信方式的局限，实现了更高速率、更大容量的空间通信，为通信技术开辟了新的发展方向。文章介绍了星间激光通信技术的基本传输原理及特点；梳理了国际上星间激光通信技术的研究进展；从激光光源技术、捕获跟踪与瞄准技术、复用技术和光学收发技术四个方面分析了星间激光通信的关键技术；最后，对该技术的应用前景以及面临的挑战进行了归纳，并提出了未来星间激光通信技术在设备总体架构、路由交换、自抗扰控制和终端设备小型化方面的发展建议，以期为星间激光通信技术的完善发展提供新的思路。

… Additionally, the method employing intensity detection to determine angular displacement … A angular displacement detection of coherent-based inter-satellite laser communication is …

Satellite-to-ground coherent laser communication is constrained by atmospheric turbulence. Although array detection can mitigate atmospheric turbulence, outdoor experiments are expensive and difficult to conduct. This study presents a satellite-to-ground optical communication downlink model using equivalent Rytov index-interval phase screens with a non-Kolmogorov power spectrum to simulate atmospheric turbulence between satellites and ground stations. The performance of single and array detector systems was evaluated based on mixing efficiency (ME) and bit error rate (BER). The results demonstrated that array detectors enhanced both ME and BER performance, with the coherent ME increasing approximately linearly with the number of unilateral array elements. The methodologies presented in this study serve as valuable tools for forecasting communication-link budgets and establishing optical system design requirements in satellite-to-ground atmospheric coherent laser communications.

… ground station have been performed to the TDP1 Alphasat GEO satellite and the EDRS … homodyne detection using an OPLL. For higher data rates > 20 Gbps, coherent detection with …

… Satellite laser communication can be applied to space ground communication, low orbit and high orbit satellite … based on coherent optical detection and space division multiplexing over …

Amidst the next industrial revolution, advanced spaceborne optical communication technologies that offer terabit per second throughput enable seamless exploration, communication, and efficient information capacity allocation. The current paper aims to provide profound insight into the major developments of laser communication activities in deep space. To achieve this objective, a comprehensive review and comparison of the most prominent ESA‐supported (European Space Agency) initiatives, including the Lunar Optical Communication Link (LOCL) and the Deep Space Optical Communications (DSOC) demonstrations, among other activities, are provided. While ESA has gained sophisticated heritage by means of manufacturing and testing a number of cutting‐edge optical communication technologies within LOCL activity, it also intends to demonstrate an augmented ground infrastructure for establishing an end‐to‐end High Photon Efficiency (HPE) optical communication link between Earth and DSOC payload of NASA's (National Aeronautics and Space Administration) Psyche Spacecraft. To this end, critical and leading system designs including specific issues that are required for the realization of next‐generation systems, along with examples of high‐level architectures, are provided in the current work. Considering the enhanced technical expertise, the paper further addresses the technological prospects and envisaged deep‐space optical data‐return channels for future missions, encompassing the giant planets and beyond at distances larger than 4.2 Astronomical Units (AU), as part of the forthcoming planning cycle, Voyage 2050, of ESA's Space Science Programme. All those prominent goals are addressed and evaluated in terms of fundamental limitations that apply to the information capacity of the HPE optical communication system, which is then compared with a radio frequency (RF) Ka‐band link. The demonstrated capabilities to extend the range over 100 AU of optical communication links, while supporting capacity characterized by a high signal‐to‐noise regime, have the potential to revolutionize planetary exploration.

Coherent detection can provide enhanced receiver sensitivity and spectral efficiency in free-space optical (FSO) communications. However, turbulence can cause modal power coupling effects on a Gaussian data beam and significantly degrade the mixing efficiency between the data beam and a Gaussian local oscillator (LO) in the coherent detector. Optical phase conjugation (OPC) in a photorefractive crystal can"automatically"mitigate turbulence by: (a) recording a back-propagated turbulence-distorted probe beam, and (b) creating a phase-conjugate beam that has the inverse phase distortion of the medium as the transmitted data beam. However, previously reported crystal-based OPC approaches for FSO links have demonstrated either: (i) a relatively fast response time of 35 ms but at a relatively low data rate (e.g.,<1 Mbit/s), or (ii) a relatively high data rate of 2-Gbit/s but at a slow response time (e.g.,>60 s). Here, we report an OPC approach for the automatic mitigation of dynamic turbulence that enables both a high data rate (8 Gbit/s) data beam and a rapid (<5 ms) response time. For a similar data rate, this represents a 10,000-fold faster response time than previous reports, thereby enabling mitigation for dynamic effects. In our approach, the transmitted pre-distorted phase-conjugate data beam is generated by four-wave mixing in a GaAs crystal of three input beams: a turbulence-distorted probe beam, a Gaussian reference beam regenerated from the probe beam, and a Gaussian data beam carrying a high-speed data channel. We experimentally demonstrate our approach in an 8-Gbit/s quadrature-phase-shift-keying coherent FSO link through emulated dynamic turbulence. Our results show ~10-dB improvement in the mixing efficiency of the LO with the data beam under dynamic turbulence with a bandwidth of up to ~260 Hz (Greenwood frequency).

… In the first phase, the LCRD flight modem will perform a coherent clock loopback (ie, the … provide a coherent clock loopback, enabling the first optimetric measurements through an …

The deployment of non-terrestrial networks (NTNs) is envisioned to achieve global coverage for 6G and beyond. In addition to space nodes, aerial NTN nodes such as high-altitude platform stations (HAPSs) and rotary-wing unmanned aerial vehicles (UAVs) could be deployed, based on the intended coverage and operational altitude requirements. NTN nodes have the potential to support both wireless access and backhauling. While the onboard base station provides wireless access for the end users, the backhauling link connects the airborne/space-borne base station to the core network. With its high data transmission capability comparable to fiber optics and its ability to operate in the interference-free optical spectrum, free space optical (FSO) communication is ideally suited to backhauling requirements in NTNs. In this paper, we present a comprehensive tutorial on airborne FSO backhauling. We first delve into the fundamentals of FSO signal transmission and discuss aspects such as geometrical loss, atmospheric attenuation, turbulence-induced fading, and pointing errors, all of which are critical for determining received signal levels and related link budget calculations. Then, we discuss the requirements of airborne backhaul system architectures, based on use cases. While single-layer backhaul systems are sufficient for providing coverage in rural areas, multi-layer designs are typically required to establish connectivity in urban areas, where line of sight (LoS) links are harder to maintain. We review physical layer design principles for FSO-based airborne links, discussing both intensity modulation/direct detection (IM/DD) and coherent modulation/coherent demodulation (CM/CD). Another critical design criteria for airborne backhauling is self-sustainability, which is further discussed in our paper. We conclude the paper by discussing current challenges and future research directions. In this context, we discuss reconfigurable intelligent surfaces (RIS) and spatial division multiplexing (SDM), for improved performance and an extended transmission range. We emphasize the importance of advanced handover techniques and scalability issues for practical implementation. We also highlight the growing role of artificial intelligence/machine learning (AI/ML) and their potential applications in the design and optimization of future FSO-based NTNs.

… is less significant for laser communication systems employing intensity modulation/direct detection, it can degrade the communication system performance for coherent detection. This …

We experimentally demonstrate a self-coherent free-space optical communications (FSOC) system based on the pilot-assisted scheme. By co-propagating the data-carrying beam with a continuous-wave pilot, the scheme reduces turbulence-induced losses in spatial coherence between the beams, which can adversely affect the performance of local oscillator (LO)-based coherent systems. The system utilizes a single free-space photodetector and self-coherent detection based on the Kramers-Kronig receiver to minimize receiver hardware complexity. Using an 800 m free-space link, we transmit a 10 Gbit/s 16-QAM single polarization signal over real atmospheric turbulence. Additionally, we present a method for simultaneous characterization of the atmospheric channel by recording image sequences of the received laser spot. Results demonstrate the robustness of the system in moderate turbulence, and serve as the first field demonstration of this approach over a real free-space link.

Free-space optical (FSO) communications has potential advantages of higher data capacity and lower probability of interception, when compared to radio-frequency communications. However, atmospheric turbulence generally limits performance of FSO links because it induces modal coupling from the fundamental Gaussian mode to many higher-order Laguerre-Gaussian (LG) spatial modes. We review pilot-assisted self-coherent (PASC) approach that can enable turbulence-resilient FSO communications. In PASC, a frequency-offset continuous-wave pilot beam is co-transmitted with the data beam and eventually the two beams are optoelectronically mixed at receiver's photodetector (PD). During square-law mixing in PD, a turbulence conjugate distortion is automatically generated and applied to the distorted data beam. Thus, all the spatial modes can be efficiently mixed between pilot and data beams. As a result, the recovered data quality is not severely affected by turbulence-induced modal coupling effects. We also review the extended applications of PASC approaches, including: (a) increasing spectral efficiency by Kramers-Kronig detection; (b) enhancing system bandwidth by PASC with a PD array; (c) improving PD bandwidth utilization by PASC with self-homodyne detection. Finally, we discuss the enhanced misalignment tolerance by PASC in FSO links.

We investigate the performance of free-space optical communication systems in the presence of atmospheric turbulence to assess the advantages that a coherent communication system can bring with respect to a conventional intensity modulation and direct detection (IM/DD) system. The perspective is an information-theoretic one, hence we evaluate the mutual information and the corresponding outage probability of both channels, with various traditional symbol constellations, as a pragmatic approximation to the capacity, or to the outage capacity, of those channels. In addition, we analyze non-uniform symbol constellations to evaluate the possible shaping gain that can be achieved under different channel conditions. We propose a method to quantify the gain that the coherent solution can achieve, in terms of signal-to-noise ratio (SNR), so that it can be compared, on a techno-economical basis, against the higher cost that it implies.

High data rate optical deep space communication links for future space missions need large capture area receivers that can efficiently couple light into a single-mode fiber. Coherent detection is attractive as it offers both high spectral efficiency and sensitivity. Here, we numerically investigate two such large area receivers in the context of weak signal reception; the multi-aperture array and the multi-mode fiber-coupled receiver, together with optical coherent combining. We find that the number of speckles captured by the aperture should match the number of modes supported by the receiver-fiber for high efficiency and sensitivity. Using an optically preamplified dither-optical phase locked loop for tip-tilt, phase, and amplitude compensation, we predict that efficient reception of signals can be maintained down to -80 dBm of received power per mode for realistic atmospheric channels.

… In particular, we detail how turbulence-induced wavefront aberrations degrade coherent detection efficiency and present wave-optics simulations that quantify the resulting losses. …

Compared with single-pass propagation in traditional free-space optical (FSO) systems, the round trip double-pass propagation through atmospheric turbulence in modulating retroreflector (MRR) FSO systems would experience more turbulence-induced fading that impairs system performance. As the transceiver transmits and receives the same laser beam in the MRR FSO system, the coherent detection (CD) could be inherently formed at the transceiver to improve the system performance by virtue of enhanced receiver sensitivity. However, since the CD is susceptible to optical phase distortion caused by atmospheric turbulence, turbulence-resilient methods based on self-interferometric phase compensation and self-coherent mode-mixing enhancement are considered and first integrated into the coherent MRR FSO system in this work. For the direct detection (DD) and the CD with the two turbulence-resilient methods in the MRR FSO system, we describe the theoretical definition of received signal current, wave-optics simulation for channel modelling and Monte-Carlo simulation for bit error rate (BER) evaluation, but we have not yet conducted the corresponding experiments due to the equipment limitations. The wave-optics simulation results show that the channel fading of the DD and the CD could be better fitted by Lognormal and Rician distributions, respectively. The Monte-Carlo simulation results show that the CD could generally outperform the DD and the BER performance of the CD with self-coherent mode-mixing enhancement is the highest, which achieves a signal-to-noise ratio gain of about 6dB compared with the DD.

… Free-space optical (FSO) communications has attracted wild … With the rapid growth of data volume, coherent modulation … technology [7] have been adopted in coherent detection. …

In free-space optical (FSO) communication systems, on–off keying (OOK) is a widely used modulation format. Coherent and non-coherent OOK receivers with sensitivities of −54.60 dBm and −51.25 dBm, respectively, were built with a communication rate of 1 Gbit/s and a bit error rate of 10−3. In an FSO communication system, the parameters must be designed to ensure a sufficient link margin. In contrast to optical fiber systems, FSO systems have ambient light (AL) noise such as sunlight. The efficiency of sunlight coupling in the single-mode fiber (SMF) of the receivers was calculated in this study. For a signal light with AL, the change in the main components of noise and the sensitivity deterioration were theoretically analyzed and experimentally verified in conditions of coherent reception and non-coherent reception with a preamplifier. For coherent reception, the theoretical sensitivity deterioration results are consistent with the experimental results which indicate that coherent reception exhibits better anti-AL noise performance than non-coherent reception when the power spectral density of the AL is the same. Coherent and non-coherent receivers coupled with SMF can work in direct sunlight. When the receiver lens diameter is greater than 4.88 × 10−4 m, the anti-AL noise performance of the receiver can be improved by increasing the receiver lens diameter.

… freespace optical (FSO) communication by inducing modal power coupling and wavefront distortion. This paper investigates a pilot-assisted self-coherent detection … -coherent detection …

There are various performance advantages when using temporal phase-based data encoding and coherent detection with a local oscillator (LO) in free-space optical (FSO) links. However, atmospheric turbulence can cause power coupling from the Gaussian mode of the data beam to higher-order modes, resulting in significantly degraded mixing efficiency between the data beam and a Gaussian LO. Photorefractive crystal-based self-pumped phase conjugation has been previously demonstrated to "automatically" mitigate turbulence with limited-rate free-space-coupled data modulation (e.g., <1 Mbit/s). Here, we demonstrate automatic turbulence mitigation in a 2-Gbit/s quadrature-phase-shift-keying (QPSK) coherent FSO link using degenerate four-wave-mixing (DFWM)-based phase conjugation and fiber-coupled data modulation. Specifically, we counter-propagate a Gaussian probe from the receiver (Rx) to the transmitter (Tx) through turbulence. At the Tx, we generate a Gaussian beam carrying QPSK data by a fiber-coupled phase modulator. Subsequently, we create a phase conjugate data beam through a photorefractive crystal-based DFWM involving the Gaussian data beam, the turbulence-distorted probe, and a spatially filtered Gaussian copy of the probe beam. Finally, the phase conjugate beam is transmitted back to the Rx for turbulence mitigation. Compared to a coherent FSO link without mitigation, our approach shows up to ∼14-dB higher LO-data mixing efficiency and achieves error vector magnitude (EVM) performance of <16% under various turbulence realizations.

Adaptive optics (AO) technology can correct wavefront distortion in coherent free space optical communication (FSOC), with wavefront sensors playing a vital role in this process. However, traditional wavefront sensors are large and expensive. Therefore, we propose using the inexpensive and easy-to-deploy flat optics angle-based wavefront sensor (FOA-WFS) to measure the wavefront aberration. It aims to meet the needs of various FSOC applications. We first establish the relationship between the energy ratio and the Zernike coefficient through theoretical studies and analyze the feasibility of applying the FOA-WFS to the FSOC. We then generate experimental datasets based on the relevant principles. Through numerical simulation, we verify that it can reconstruct wavefront aberration accurately and improve system performance. Finally, we analyze the mixing efficiency and bit error rate based on the collected aberration data by the experimental platform. The results indicate that the AO system based on the FOA-WFS can efficiently improve the performance of the FSOC. This study provides a novel wavefront aberration detection method for designing the AO systems in the FSOC.

The self-coherent scheme offers significant advantages for free-space optical (FSO) communications, particularly due to its higher oscillator-signal mixing efficiency compared to conventional coherent schemes. This paper presents an analysis and simulation results of an FSO system utilizing the dual-polarization (DP) self-coherent scheme, where independent quadrature amplitude modulation (QAM) data is transmitted for each polarization. Atmospheric turbulence, however, introduces random polarization rotation and polarization-dependent phase shifts, which cause the effective channel matrix to become singular at certain angles. This leads to an increased bit error rate (BER) in conventional systems that use two photodetectors (PDs). To address this issue, we propose a novel detection scheme using three PDs. This study evaluates the performance of long-haul FSO transceivers, comparing the DP self-coherent scheme with the conventional coherent scheme under a variety of conditions, including satellite-to-ground, ground-to-satellite, and satellite-to-satellite links. The analysis spans different levels of atmospheric turbulence, ranging from weak to moderate to strong, providing a comprehensive assessment of transmission efficacy.

We demonstrate mid-IR FSO coherent detection at $\sim 3.4 \mu \text{ m}$ using a native detector for (i) a single 1-Gbaud data channel with various modulation formats, including OOK, QPSK, 16 QAM, and 64 QAM, and (ii) five multi-carrier 1-Gbaud 16-QAM channels. We show >10-dB improvement in receiver sensitivity compared to direct detection using the same detector.

… -based coherent laser receiver as a promising candidate for forthcoming inter-satellite networks… data rate, QPSK and BPSK coherent transmissions using an FPGA-based transmitter and …

Coherent free-space optical communication offers significant advantages in terms of communication capacity, making it particularly suitable for high-speed inter-satellite transmission within satellite communication networks. Nonetheless, the presence of Doppler frequency offset (FO) and phase noise (PN) associated with lasers adversely affects the bit error rate (BER) performance of these communication systems. Conventional methods for FO and phase estimation are usually hindered by high computational demands and phase cycle slips, especially in environments characterized by elevated channel noise. To address these challenges, a noise-tolerant method is proposed to facilitate accurate carrier phase recovery (CPR) with reduced complexity. This method merges a second-order feedback loop and a feedforward stage to achieve accurate estimation. The simulation results indicate that the proposed method surpasses traditional methods in terms of noise tolerance and resource efficiency. Particularly, the BER of the proposed method can be decreased to 6.7×10−3 at a signal-to-noise ratio (SNR) of 4.5 dB, in contrast to a BER of 0.25 for the traditional method. Additionally, the resource consumption of the proposed method can be decreased by 64% under equivalent conditions. Furthermore, the experimental results reveal that the phase estimation error and BER for the proposed method are 2.1×10−4 and 7.5×10−4, respectively, when the received power is −41 dBm. These values are significantly lower than those achieved with traditional methods, which obtain errors of 1.85×10−3 and a BER of 0.48, respectively.

The problem of carrier frequency drift in intersatellite coherent laser communication, mainly caused by the Doppler frequency shift and the frequency shift of the laser itself, can cause the circumferential rotation of the constellation and an increase in the bit error rate (BER). Based on the principles of laser internal modulation and in-phase and quadrature (IQ) orthogonal modulation, a carrier arbitrary frequency difference locking method was designed to overcome the problem of carrier frequency drift. When the light signal modulation format is binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK), this method can lock the carrier frequency difference at any point within a certain range (BPSK: ± 240 MHz & QPSK: ± 120 MHz). When the Doppler frequency shift was not applied to the light signal, the standard deviations of frequency locking error of the BPSK signal and the QPSK signal were 26.1 kHz and 29.2 kHz, respectively. When a frequency shift with a speed of ±5.2 MHz/s was applied to the light signal, and the modulation format of the signal is QPSK, the standard deviation of frequency locking error was 149.4 kHz. By using this method, the tracking of the local oscillator carrier frequency to the light signal carrier frequency can be achieved. Therefore, more bandwidth in subsequent demodulation algorithms can be used to overcome the random phase noise of the lasers, ensuring a low BER for intersatellite coherent laser communication.

Coherent optical satellite links enable high‐throughput communication and high accuracy ranging to and between satellites. Due to the ever‐increasing demand for throughput, wavelength division multiplexing of polarization multiplexed optical signals is being considered as a solution to provide high‐speed optical satellite links. Fiber‐optic systems solve the implementation scalability problem of these systems by shifting design complexity to integrated circuits, thereby massively reducing the system footprint. As a result of the major advances in complementary metal‐oxide‐semiconductor (CMOS) technology, the implementation scalability of such systems in terrestrial fiber systems has been solved by shifting the system complexity to digital hardware, enabling intradyne reception and complex signal recovery algorithms. While the use of fiber‐optic transceivers provides a fast path to high‐speed coherent optical satellite links (OSLs), it requires additional mitigation techniques to combat the effects of both the OSL channel and the space environment. To support future satellite networks with Tbit/s optical links, it will be critical to further minimize the size, weight, and power (SWaP), cost and reliability of the transceivers. Thus, the development of custom intradyne optical transceivers for OSLs is emerging as an attractive option as the demand for throughput in satellite networks continues to grow. This would not only enable the use of a more optimized signal processing chain but also enable the use of radiation mitigation techniques optimized for the signal processing architecture and the use of soft‐decision forward error correction (FEC) optimized for OSLs. The signal processing of coherent optical satellite receivers can be divided into three key subsystems: timing recovery, carrier synchronization, and equalization. This paper reviews state‐of‐the‐art digital signal processing for optical communication to identify suitable algorithms for timing recovery, carrier frequency and phase compensation, equalization, and polarization demultiplexing with emphasis on high‐throughput optical satellite links. Finally, the performance of different digital signal processing algorithms is assessed by numerical simulations considering different optical satellite link scenarios.

Following the transition of terrestrial networks to optical fibers in the 1980s, optical laser links are increasingly being explored as an alternative to Radio Frequency (RF) communications in space, especially to address congestion in the microwave bands. For downlinks, free-space propagation remains a major challenge, mainly because of atmospheric turbulence. Coherent detection is becoming the preferred approach due to its superior overall performance. In Low Earth Orbit (LEO)-to-ground downlinks, an additional issue for coherent demodulation arises from the Doppler frequency drift caused by the orbital motion of the satellite.In this study, a Z-transform modeling of a coherent receiver based on a hybrid Optical-Digital Phase-Locked Loop (ODPLL) for Doppler compensation and frequency synchronization is proposed. In the envisioned architecture, the optical signals are processed in the analog domain by optoelectrical components, with no speed limitations other than propagation delays; in turn, the control for frequency tracking is performed in the digital domain, thus easing the design. The system latency is taken into account, in view of implementing the algorithm on a Field Programmable Gate Array (FPGA) and to later integrate the photonic elements to build the hardware receiver. The simulated behavioral model achieves successful Quadrature Phase-Shift Keying (QPSK) demodulation even when submitted to strong Doppler rates, and under realistic received power levels.

This paper examines the technical challenges and potential solutions for implementing optical communications in low-Earth-orbit (LEO) constellations, with an emphasis on achieving high photon efficiency, enabling adaptable high data rates, and minimizing power consumption.

The performance of a coherent, high-data-rate DP-QPSK optical link between an optical ground station and a geostationary satellite is described. The signal processing blocks were optimized and tested within a representative simulation environment, accounting for various impairments, implementation losses and atmospheric power fluctuations. The optimized transceiver exhibits an absence of desynchronization threshold even at low Optical Signal-to-Noise Ratio (OSNR), and achieves performance levels at approximately 0.3 dB on average of the theoretical limits.

… oscillator lasers to sub-cards, allowing N optical signals with different apertures to be coherently … The coherent receiver is composed of a modular design, which facilitates subsequent …

… and compensating Doppler effects in inter-satellite coherent laser communication systems. … implemented in digital coherent receivers. For example, a digital coherent transceiver based …

Free-space optical (FSO) communication technologies constitute a solution to cope with the bandwidth demand of future satellite-ground networks. They may overcome the RF bottleneck and attain data rates in the order of Tbit/s with only a handful of ground stations. Here, we demonstrate single-carrier Tbit/s line-rate transmission over a free-space channel of 53.42 km between the Jungfraujoch mountain top (3700 m) in the Swiss Alps and the Zimmerwald Observatory (895 m) near the city of Bern, achieving net-rates of up to 0.94 Tbit/s. With this scenario a satellite-ground feeder link is mimicked under turbulent conditions. Despite adverse conditions high throughput was achieved by employing a full adaptive optics system to correct the distorted wavefront of the channel and by using polarization-multiplexed high-order complex modulation formats. It was found that adaptive optics does not distort the reception of coherent modulation formats. Also, we introduce constellation modulation – a new four-dimensional BPSK (4D-BPSK) modulation format as a technique to transmit high data rates under lowest SNR. This way we show 53 km FSO transmission of 13.3 Gbit/s and 210 Gbit/s with as little as 4.3 and 7.8 photons per bit, respectively, at a bit-error ratio of 1 ∙ 10^−3. The experiments show that advanced coherent modulation coding in combination with full adaptive optical filtering are proper means to make next-generation Tbit/s satellite communications practical. This paper demonstrates a 53 km free-space-optical communication link mimicking a satellite-downlink. It achieves 1Tbit/s transmission by addressing turbulence and low-SNR issues through adaptive optics and a novel four-dimensional modulation format.

… -satellite coherent laser communication experimental system is bulit. The use of fast steering mirror (FSM) serves to simulate inter-satellite … that when the transmitter and receiver are in a …

… The previous results are valid under the assumption that there is no loss of synchronization in the coherent receiver. We have further exploited the time series from both Alpha-up …

… receiver aperture is about 0.4 to 0.6 times the coherence length. For larger pointing errors, the optimal receiver aperture is approximately 1 time the coherence … optimal receiver aperture …

Coherent Binary Phase Shift Key (BPSK) has been proposed for Low-Earth-Orbit LEO) -satellite-to-ground communications. This paper will present new models for the turbulent channel BPSK signal-to-noise ratio and average Bit Error Rate (BER). These models include equations for the linear and square-law receiver signal-to-noise ratios, turbulent channel heterodyne efficiency, turbulent carrier-to-noise and signal-to-noise ratios, signal-local oscillator beam position mismatch, and tracked beam-wander intensity variance. This paper will also show some example calculations assuming These models will be evaluated using the Round-Earth Model and the Hufnagel-Andrews-Phillips (HAP) refractive index structure parameter vertical profile model. Results show that the beam-wander intensity variance will cause high asymptotic average BERs for all receiver zenith angles and high carrier-to-noise ratios under moderate-to-strong turbulence levels. The conclusion is that additional means like forward error correction coding are needed to reduce the average BER to more desirable levels.

We introduce the reservoir computing (RC) concept into the laser processing unit for the scenario of the inter-satellite communication to mitigate the impacts from the multi-distortion sources, enabling significantly improving the quality of the coherent signals through the intelligent approach in outer space. We carry out the experimental investigations to quantify the impacts from the Sun radiation, the Doppler frequency shift and the vibration on the transmitted quadrature-phase-shift-keying (QPSK) signals, and perform the signal compensation through only one RC module at the receiver end. A maximum 5.59dB signal-to-noise ratio (SNR) improvement has been achieved in the experiment when the three distortion-sources applied simultaneously, confirming the compensation capability for the laser inter-satellite communication.

… While we show the coherent heterodyne detection can improve the receiver sensitivity by >… detection, this result might be changed for a larger baud rate. Since our heterodynedetection …

… Compared to the self-heterodyne detection, our approach has twice the utilization efficiency of the detector’s bandwidth. Compared to conventional LO-based homodyne or intradyne …

… In the KK-based few-mode heterodyne detection system, we use a 50:50 six-mode fiber-type mixer to mix an equal number of modes of signal light with the LO light, and the mixed …

Satellite quantum communications have received a surge of interest in recent years, encouraged by the success of the QUESS experiments and as a reaction to advances in quantum computing. This work focuses on low Earth orbit nanosatellite missions, which offer several benefits (e.g. lower costs, greater launch availability, easier to replenish) over satellites at different orbit heights (medium Earth orbit, geosynchronous Earth orbit) and satellite size classes (e.g. small-sats, etc). Recent work on low Earth orbit satellite Quantum Key Distribution (QKD) has focused on transmitters and quantum sources, with less attention paid to the receiver side of the link. This review addresses that imbalance by focusing on the receiver and detection systems of satellite quantum communications. This work has found that two-way beacon acquisition and tracking systems are both a common element of state-of-the-art missions, and likely to remain a critical subsystem to ensure high-quality satellite-to-ground station links; however, improvements to detection and tracking algorithms could be made. For discrete variable QKD, receivers and detection systems in nanosatellites at low Earth orbit are likely to remain using silicon single photon avalanche diodes, while receivers and detection systems in optical ground stations may opt for superconducting nanowire single photon detectors, as cryogenic cooling is feasible. For continuous variable QKD, the receivers and detection systems, using coherent detection systems, share many hardware elements with classical optical satellite communications. Combined with coherent detection systems’ inherent resilience to operating during daytime conditions, this makes continuous variable QKD an attractive prospect for future missions, if its fundamental challenges can be overcome.

For terrestrial fiber-optical data communication, the digital-coherent intradyne detection method is commonly used for data recovery at the receiver. This method has several advantages compared to optical phase locked homodyne systems. For example, the handling of phase noise is shifted into the digital domain and the fabrication of the complete system and the setup for operation are easier. Furthermore, frequency acquisition can be performed much faster, which is a major advantage in the presence of fades as in GEO space to ground links. In this paper, the demonstration of the technology in optical freespace communication is shown with the TDPI testbed.

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… detection method, optical heterodyne detection has excellent advantages over direct detection… method for the performance analysis of heterodyne detection systems, including wavefront …

… The coherent optical frontend is a free-space optical 90-degree hybrid. By inputting the … confirmed for direct detection and heterodyne detection. Heterodyne angle detection is superior …

… in the field of space optical communication. In November 2011, … heterodyne detection approach. Two beams with different frequencies are usually required in the heterodyne detection. …