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 long-distance 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 1 550 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 all-digital 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.