高兼容性亚码元级激光通信/测距一体化系统

Highly compatible sub-symbol level laser communication/rangingintegrated system

随着全社会的数字化转型,空天地一体化信息网络被迅速布局,对卫星间的多调制格式的高速相干光通信和高精度测距的需求愈加迫切。基于相干光通信,提出了一种低成本、低复杂度、高精度的通信/测距一体化系统,通过在相干光通信系统中间隔插入特定的测距数据帧,并利用改进的并行数字时钟恢复(CDR)算法,由其中的鉴相模块提取相位偏差信息,无需消耗额外的硬件资源即可实现高精度测距,由此满足卫星上低功耗、低重量和小尺寸的需求。此外,该系统可兼容不同相干光调制格式,包括OOK、BPSK和QPSK以及不同传输速率的相干光通信链路,体现出了高兼容性。提出的系统在现场可编程门阵列(FPGA)上进行实验验证,在625 Mbps和1 Gbps的BPSK信号下进行通信/测距实验。实验结果表明,码元持续时间为1.6 ns和1 ns,测距精度达到码元时长的2.25%和4.74%,测距精度分别为11 mm和14 mm,达到亚码元量级。通过所提出的通信/测距一体化系统,解决了现有测距系统通用性较差,复杂度较高的问题,提升了系统的灵活性。

Objective Currently,the industry has begun to study the next-generation mobile communication technology(6G),which physically divides the space-air-ground integrated network into three parts of satellite,high-altitude platform,and ground mobile communication network.Low Earth Orbit(LEO)satellite communication network has the advantages of wide coverage,long transmission distance,and large transmission capacity,which makes LEO communication network an indispensable part of the space-air-ground integrated network.Coherent optical communication is suitable for free-space laser communication between LEOs due to its advantages of longdistance transmission and flexible modulation and demodulation.At present,BPSK,QPSK,DP-QPSK and other coherent optical modulation formats are commonly used in laser communication between satellite networks to meet the multi-rate adjustable scenarios.In addition,laser ranging is a particularly important application of laser communication,which provides accurate and valuable information for ground navigation and earth observation.Considering the satellite payload and power consumption,how to achieve high-speed laser communication between satellites and a highly compatible communication/ranging integrated system that can complete accurate ranging is of great significance to the next generation of large-capacity inter-satellite information networks.Methods An integrated system for communication and ranging at the sub-symbol level is proposed(Fig.1).Since the digital clock recovery algorithm can output the frequency and phase offset within the symbol,ranging between the transmitter and the receiver can be achieved by extracting the phase deviation from it.The scheme realizes sub-symbol level ranging without interrupting the normal communication mode(Fig.2).Compared with other ranging schemes,this system does not require additional complex algorithms or hardware.The scheme keeps the ranging data frames aligned with integer seconds by inserting specific ranging data frames and transmits an integer number of frames per second.A bidirectional coherent optical communication device completes a bidirectional one-way ranging process by sending ranging information to each other.Results and Discussions At the transceiver of terminal A and terminal B,a continuous wave laser generates light with a wavelength of 1550 nm and a linewidth of~75 kHz.The light is modulated in a Mach Zehnder Modulator(MZM).It is noteworthy that the proposed ranging scheme is insensitive to both modulation format and rate.Both terminals can send and receive signals with different modulation formats,including OOK,BPSK and QPSK.Moreover,it can flexibly adjust different communication rates.At the receiver of the two terminals,an integrated coherent receiver,an local oscillating laser,an analog-to-digital converter,and a FPGA are integrated on the a small transceiver board(Fig.3).A section of variable optical delay line is added to the link from terminal B to terminal A.The optical delay line varies from 0 to 600 ps with a resolution of 5 ps,which corresponds to an optical transmission distance of 0 m to 0.18 m.The FPGA generates a Pseudo-Random Binary Sequence 23 signal.Multiple ranging experiments were performed with 625 Mbps and 1 Gbps BPSK signal rate.The experiments were conducted with different preset delays using different lengths of optical delay lines.The results show that the symbol duration is 1.6 ns and 1 ns,and the Root Mean Square Error is 36.46 ps and 47.38 ps,which is 2.25%and 4.74%of the symbol duration(Tab.1).Conclusions A low-complexity and high-compatibility integrated communication/ranging system is designed.Considering that clock errors are unavoidable in practical applications because the transmitter and receiver clocks operate independently.The proposed scheme improves the traditional demodulation method for communication terminals by measuring the phase offset.The system calculates the value of the phase offset by means of an alldigital clock recovery algorithm.Remarkably,this high-precision ranging system is able to achieve sub-symbol level accuracy without extra hardware cost.The communication/ranging experiments are performed with 625 Mbps and 1 Gbps BPSK signals.The results show that the ranging accuracy reaches 2.25%and 4.74%of the symbol duration.The communication/ranging system is broadly applicable to most bi-directional laser communication links.It does not rely on a specific communication rate or modulation format,which demonstrates its excellent compatibility.The proposed method solves the problem of poor generalization and high complexity of the existing ranging system.