Inter-satellite Laser Communication Link Research Articles

Research on angular displacement detection technology of inter-satellite laser communication based on coherent system

With the rapid development of spatial information networks technologies, inter-satellite laser communication links have played an important role in the satellite Internet. High-precision relative angular displacement detection is the foundation for achieving real-time tracking of laser communication links, and its accuracy directly impacts the tracking performance and communication quality of the laser communication link. Traditional laser communication systems utilize intensity-based position detection methods to calculate angular displacement, requiring setting separate Charge Coupled Device (CCD) or Four-quadrant Detector (QD) as well as independent optical tracking branches. This setup makes it difficult to integrate with high-speed coherent communication systems. Additionally, the method employing intensity detection to determine angular displacement based on spot position has lower accuracy in theory. A angular displacement detection of coherent-based inter-satellite laser communication is proposed in this paper. Firstly, real-time calculation of phase differences at different positions of the beam is achieved through heterodyne coherent array detection and dual high-precision phase-locked loops. Then, a model correlating wave front phase difference with relative angular displacement is established, phase noise during beam transmission and detection processes is analyzed, and algorithm for detecting angular displacement in coherent systems is studied. Finally, a laser communication link is constructed under laboratory condition, and experimental validation work is carried out. When the relative angle deviation is 1°, the phase difference changes by 0.089049795 rad, and the angular displacement detection error of 0.66697μrad(σ). When the relative angle error is 0.603873μrad(σ), the detection sensitivity is −43.93dBm. The experimental results show that in the coherent system, the relative angular displacement detection with high precision can be realized by using the wavefront phase difference of beam, which can support the laser communication tracking system to achieve higher precision beam tracking.

Research on Inter-Satellite Laser Communication Link Transmission and Routing Switching Technology of Satellite Internet (Invited)

Research on Inter-Satellite Laser Communication Link Transmission and Routing Switching Technology of Satellite Internet (Invited)

On-orbit demonstration of inter-satellite free-space optical stable communication enabled by integrated optical amplification of HPA and LNA.

Satellite free-space optical (FSO) communication is very promising in improving the bandwidth and capacity of space information networks in the future. However, the inter-satellite transmission distance of over 1000km leads to unstable optical beam pointing, acquisition, and tracking and then generates optical power jitter by a large margin before detection-demodulation. Therefore, it is difficult to realize high-stability and long-time FSO communication between satellites due to the generated bit error rate (BER) by jitter. In this paper, we report an autonomously self-designed and high-integration laser communication payload (LCP) and on-orbit-demonstrated inter-satellite 145min, zero-BER FSO stable communication with a line rate of 2.8Gbps. Moreover, based on the inter-satellite laser communication link, a video phone was clearly implemented for more than 10min, and authentic user data transmitted 459,149 packets, achieving results of zero-packet loss. Summarily, this on-orbit experiment demonstrated an excellent performance of the LCP owing to the distinctive design of integrating a high-power amplifier and low-noise amplifier optical amplification function. Our space mission was successfully completed, and the on-orbit demonstration results may offer a significant reference for the field of satellite laser communication and space information networks.

Beacon correction method for inter-satellite laser communication

Positioning errors are caused by wavefront distortion owing to the thermal deformation of optical systems. To address this issue, we propose a restoration algorithm to correct for the beacon centroid, which is important for inter-satellite laser communication links. Based on the far-field distribution model of the distortion facula, the objective function is established to solve the wavefront distortion for reducing the pointing error of the optical communication system. The feasibility of this approach is verified by theoretical analysis and experimental verification, including in-orbit experimental verification. The proposed approach is likely to be useful for designing inter-satellite laser communication systems.

Research on simulation methods for Doppler frequency shift of a coherent inter-satellite laser link in a ground test system

Spatial Doppler frequency shift occurs during ground integration testing of an inter-satellite laser communication link and is difficult to simulate it consistently; therefore, to address this problem, a Doppler frequency shift simulation method for an optical communication carrier in space based on a dual-parallel Mach–Zehnder modulator (DP-MZM) is proposed. By adjusting the RF signal added to the MZM and the bias voltage of the DP-MZM, the output carrier suppresses single-sideband modulation to achieve a controllable optical carrier frequency shift. In this study, theoretical analysis and experimental research on this simulation method was performed, and a Doppler frequency shift simulator for ground testing of two laser communication terminals was developed, which can realize controllable optical carrier frequency shift adjustment in the range of at least ±18 GHz and a sideband rejection ratio that is greater than 40 dB. The research results have been applied to an inter-satellite laser link ground test, obtaining good simulation and test results.

Time synchronization based on intersatellite lasercommunication links

Time synchronization is essential for navigation, positioning, and timing services. It is widely used in space applications. Laser communication technology is widely used and has the advantages of large communication capacity, strong confidentiality, and anti-interference. In this paper, a time synchronization technique based on laser communication links is proposed. Estimate ranging was realized using the data frame head, and precise ranging was realized by receiving the clock phase. Through theoretical analysis, the theoretical limit of time synchronization accuracy based on laser communication links was obtained, and simulation was performed. Millimeter-level measurement accuracy and picosecond-level time synchronization accuracy can be obtained at a communication rate of 1 Gbps under two-way and one-way ranging conditions. A time synchronization test based on laser communication links was conducted in a laboratory using a coherent binary phase shift keying communication system; test results are consistent with theoretical expectations.

Received power attenuation due to the wave-front aberrations induced by the receiving optical antenna in an inter-satellite laser communication link

Due to the design defect, adjustment error, thermal deformation and gravity deformation in orbit, the bad optical antenna of the communication receiving terminal in an inter-satellite laser communication link will induce aberrations to the received wave-front. This will cause attenuation of the received power and deterioration of the bit error rate (BER) performance. The calculation model of the compensated and normalized received power is established when the received light spot is pointed at the center of the focal plane and the limited receiving radius is considered. The numerical results show that, for receivers with a single first-order Zernike mode wave-front aberration, the primary spherical aberration has the most influence on the received power attenuation, and for receivers with the same type aberrations, higher-order Zernike aberrations have more influence on that. The universality of the result for different antennas is verified by introducing a characteristic radius and a relative receiving radius. Some suggestions are proposed to lower the requirements of the wave aberrations control, thus reducing the difficulty of optical design, structural design, thermal design, assembly and adjustment. The outcome of this study would contribute to the design of the inter-satellite laser communication systems.

High-dynamic wavelength tracking and millimeter-level ranging inter-satellite laser communication link with feedback-homodyne detection.

The Integrated Laser Communication/Ranging System, which uses a coded signal as the ranging information carrier, is of great importance to the next large-capacity inter-satellite information network. In this paper, a system design with a high-sensitivity feedback-homodyne detection scheme and an asynchronous ranging algorithm is demonstrated with real-time field-programmable gate array-implementation (FPGA). The parallel fast Fourier transformation (FFT) estimation is applied to improve the speed and the range of the wavelength drift tracking, which can handle a dynamic wavelength drift up to 2.4pm/s (300MHz/s). Meanwhile, for clock sources with subtle dynamic frequency offset and sufficient stability, the proposed fractional symbol ranging method is proven to achieve millimeter-level measurement accuracy. The designed system is shown to perform well in terms of both laser linewidth tolerance and noise resistance.

Pointing and tracking errors due to low-frequency deformation in inter-satellite laser communication

ABSTRACTA Gaussian random-phase screen (GRPS) model is proposed to represent the low-frequency wave-front deformation in inter-satellite laser-communication links. The model is used to investigate mutual-alignment errors comprising pointing and tracking errors caused by the low-frequency deformation. Theoretical analysis and numerical simulation show that the GRPS root-mean-square (RMS) value, pointing error, and tracking error increase with the amplitude factor and the scale length in the model. The expectation of RMS increases in the form of a linear function with and increases in the form of a cubic function with . The expectations of the pointing and tracking errors increase with increases in the expected values for and . For a specific pointing error or tracking error, the 99.5% confidence interval of the RMS of the low-frequency wave-front deformation is provided. From this confidence interval, the lower limit of the processing precision of optical antennas corresponding to certain mutual-alignment errors can be predicted. These conclusions are expected to be instrumental in designing inter-satellite laser-communication systems.

10.23 Mcps laser pseudo-code ranging system with 0.33 mm (1σ) pseudo-range measurement precision.

The inter-satellite laser link is the backbone of the next inter-satellite information network, and ranging and communication are the main functions of the inter-satellite laser link. This study focuses on the inter-satellite laser ranging based on the pseudo-code correlation technology. In this paper, several typical laser-ranging methods have been compared and we determined that the laser pseudo-code ranging architecture is more suitable for the inter-satellite laser communication link. The pseudo-code ranging system is easy to combine with a digital communication system, and we used it to calculate integer ambiguity by modulating the time information. The main challenge of the ranging system is range precision, which is the main focus of this paper. First, the framework of the pseudo-code ranging system is introduced; the ranging architecture of dual one-way ranging is used to eliminate the clock error between the two transceivers, and then the uncertainty of the phase detector is analyzed. In the analysis, the carrier to noise ratio and the ranging code rate are constrained by the laser communication link margin and the electronic hardware limitation. Therefore, the relationship between the sampling depth and the phase detector uncertainty is verified. A series of optical fiber channel laser pseudo-code ranging experiments demonstrated the effects of sampling depth on the ranging precision. By adjusting the depth of storage, such as the depth of 1.6 Mb, we obtained a pseudo-range measurement precision of 0.33 mm (1σ), which is equivalent to 0.0001 times code subdivision of 10.23 Mcps pseudo-code. This paper has achieved high precision in a pseudo-range measurements, which is the foundation of the inter-satellite laser link.

Impact of Pointing Errors on the Error Performance of Intersatellite Laser Communications

The effect of pointing errors on the average bit error probability (ABEP) of an intersatellite laser communication link is studied. A closed-form expression in terms of the Marcum $Q$ -function is given for calculating the instantaneous channel gain. This expression provides great potential in further performance analysis and system optimization. In addition, tight, closed-form upper and lower bounds on the ABEP are derived, by using bounds on the Gaussian $Q$ -function. These bounds are in simpler form than existing results and are much more efficient in numerical evaluations. Further simplifications of these bounds are performed and invertible ABEP expressions are given. Via numerical comparisons, these invertible ABEP expressions are shown to be accurate approximations within a wide range of transmit power of interest and thus provide efficient error prediction methods. The diversity gain is easily obtained using the invertible ABEP expressions, which is related to the ratio of the equivalent beam radius to the pointing error displacement standard jitter at the receiver. Moreover, we show explicitly how the ABEP depends on different system parameters.

Design and performance testing of an avalanche photodiode receiver with multiplication gain control algorithm for intersatellite laser communication

An avalanche photodiode (APD) receiver for intersatellite laser communication links is proposed and its performance is experimentally demonstrated. In the proposed system, a series of analog circuits are used not only to adjust the temperature and control the bias voltage but also to monitor the current and recover the clock from the communication data. In addition, the temperature compensation and multiplication gain control algorithm are embedded in the microcontroller to improve the performance of the receiver. As shown in the experiment, with the change of communication rate from 10 to 2000 Mbps, the detection sensitivity of the APD receiver varies from −47 to −34 dBm. Moreover, due to the existence of the multiplication gain control algorithm, the dynamic range of the APD receiver is effectively improved, while the dynamic range at 10, 100, and 1000 Mbps is 38.7, 37.7, and 32.8 dB, respectively. As a result, the experimental results agree well with the theoretical predictions, and the receiver will improve the flexibility of the intersatellite links without increasing the cost.

Optical Coherent Receiver Front End for Inter-Satellite Laser Communication Link

This paper presents the new concept of optical coherent receiver front end which can obtain optical phase error signals in PSK (Phase Shift Keying) communication data as well as spatial tracking error without any extra tracking sensor, leading to downsizing of inter-satellite laser communication terminals. This new receiver front end consists an optical 90 deg hybrid mixer with free space optical components, segmental photo detectors and RF electronics, to realize not only balanced detection of both in-phase and quadrature components but also heterodyning detection of tip/ tilt error signals.

Received power analysis due to antenna deformation based on wavelet in inter-satellite laser communication links

Arbitrary deformation of optical antenna caused by processing technique and complex outer-space environment in inter-satellite laser communications can degrade the performance of the system. It is crucial to study the influence of deformation on the high precision system. Wavelet expression is proposed in the paper for diversified distortions. The influence on received power is investigated with the consideration of location, scale and depth of the deformation. Theoretical and numerical analysis show that the distortion attenuation factor increases with scale factor; with the location of the deformation shift from the center to the edge of the antenna, the distortion attenuation factor will increase till the whole deformation is away from the shelter of the secondary mirror, and then decrease; with the linear increase of depth, when the pattern of deformation can be well approximated by a constant value, the distortion attenuation factor will fluctuate periodically, or there will be no obvious variation regularity between them. It is hoped the study can help the optical devices selection and system compensation.

Effect of Doppler shift on differential phase-shift keying receivers using interferometric demodulation and balanced detection in intersatellite laser communication links

Effect of Doppler shift on differential phase-shift keying receivers using interferometric demodulation and balanced detection in intersatellite laser communication links

Pointing and tracking errors due to localized deformation in inter-satellite laser communication links

Instead of Zernike polynomials, ellipse Gaussian model is proposed to represent localized wave-front deformation in researching pointing and tracking errors in inter-satellite laser communication links, which can simplify the calculation. It is shown that both pointing and tracking errors depend on the center deepness h, the radiuses a and b, and the distance d of the Gaussian distortion and change regularly as they increase. The maximum peak values of pointing and tracking errors always appear around h=0.2lambda. The influence of localized deformation is up to 0.7microrad for pointing error, and 0.5microrad for tracking error. To reduce the impact of localized deformation on pointing and tracking errors, the machining precision of optical devices, which should be more greater than 0.2?, is proposed. The principle of choosing the optical devices with localized deformation is presented, and the method that adjusts the pointing direction to compensate pointing and tracking errors is given. We hope the results can be used in the design of inter-satellite lasercom systems.