A Bidirectional Multi-Format/Rate-Adjustable Integrated Laser Communication System for Satellite Communication

Satellite optical communication networks have become a promising core network structure, surpassing 5G and 6G, and are expected to realize seamless connections between terrestrial and non-terrestrial networks. Constructing an optical satellite network that can achieve various link schemes and enable bidirectional communication with multiple optical satellites at a low cost (including low power consumption, low weight, and low space occupation for each satellite) poses a technical challenge. In this paper, we propose and establish a multi-format/rate-tunable integrated laser communications hardware platform that can switch between different links by controlling the modulation format and rate. The platform supports OOK, QPSK, and BPSK modulation with rates ranging from 0.625 to 10 Gbps. Experimental demonstration of bidirectional communication is conducted over a free-space optical link. At a distance of 510 meters, sensitivity tests are conducted for signals with various modulation formats and rates. The receiving sensitivity at the 3.8e-3 FEC threshold is −46 dBm for 10 Gbps PDM-QPSK signal, and is −58 dBm for 625 Mbps BPSK signal. Results show excellent signal sensitivity across different formats and rates. Additionally, the platform is capable of performing ranging while communicating with a minimal ranging error of 2.9% of the width of a code word.


I. INTRODUCTION
I N RECENT years, satellite communications have become more sophisticated and widespread globally.With the advancement of space technology, the digitization of communication satellites, the deployment of small satellites-based satellite Xiaoliang Li is with the School of Electronics and Information Engineering, Beihang University, Beijing 100191, China, and also with the Beijing Research Institute of Telemetry, Beijing 100094, China (e-mail: lix79@163.com).
Yizhou Wang, Xiaoxiao Dai, and Qi Yang are with the Wuhan National Lab for Optoelectronics (WNLO) and National Engineering Laboratory for Next Generation Internet Access System, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China (e-mail: yangqi@hust.edu.cn).
Digital Object Identifier 10.1109/JPHOT.2024.3366805constellations, and the volume of data exchanged with satellites are all on the rise.However, due to limited Radio Frequency (RF) spectrum resources, the spectrum will eventually be exhausted, making spectrum allocation a serious issue.This will ultimately lead to a shift towards higher RF frequencies or even directly to optical bands in space [1].Space laser communication has a wider potential bandwidth in the optical spectrum and offers several advantages, including a small aperture and light weight of the transmitting antenna, high data transmission rate, good data confidentiality, and strong anti-jamming ability [2].Various countries have implemented mega-constellation programs, utilizing optical communication networks comprised of thousands of optical satellites.Huawei plans to build a 10000-satellite Low Earth Orbit (LEO) constellation called Massive VLEO for beyond 5G [3].Telesat has developed a global network consisting of 198 advanced LEO satellites that are seamlessly integrated with on-ground data networks.SpaceX intends to launch a multitude of low-orbit satellites with the goal of forming a constellation of stars.OneWeb is constructing a constellation of 648 LEO satellites.Kaskilo will establish a low Earth orbit constellation consisting of 288 satellites [4].Satellite optical communication networks have emerged as a promising core network architecture, surpassing 5G and 6G, and offering seamless connectivity between terrestrial and non-terrestrial networks.Many satellite systems employing laser communications are already in operation.A PPM signal with a downlink rate of 622 Mbps and an uplink rate of 20 Mbps was demonstrated on a communication link of over 400000 km as part of a lunar laser communication experiment [5].A 1.8 Gbps DPSK/IMDD intersatellite laser communication link has been used by the Japanese Data Relay System to transmit data between LEO satellites and data relay satellites [6].A 1.8 Gbps satellite in BPSK modulation format was launched into a geosynchronous orbit of 36000 km as the European Space Agency's next-generation data relay system [7].And a 10 Gbps satellite utilizing QPSK modulation format was successfully launched, enabling signal communication between the satellite and an optical ground station [8].The sensitivity of several real-time free-space optical (FSO) coherent communication transceivers using different modulation formats and rates are investigated, as shown in Table I [9], [10], [11], [12], [13].In the future, achieving high-capacity satellite laser communication will be possible due to the advantages of coherent optical communication.A single-channel polarization division multiplexed -quadrature phase shift keying (PDM-QPSK) transmission between satellites at 160 Gbps was simulated for 40000 km [14].A 100 to 600 Gbps 16-QAM WDM system with a transmission of more than 5000 km was simulated and verified [15].Given the variety of satellites already in orbit, newly launched satellites must be able to communicate with the previous ones.This requires the capability to communicate with different types of satellites and terminals, as satellite network communications involve multiple links.The communication distance, transmission loss, rate, and modulation format may vary due to different links and communication terminals.The specific selection of modulation format signal and rate to achieve optimal performance depends on the characteristics of the link.If the system is compatible with various modulation formats and can switch between modulation formats and rates based on demand, the satellite network can support multiple optical satellite networks and enable bi-directional communication with different communication terminals across various links.The concept of adaptive optical satellite network proposed by [16] is applicable to the above situation, but it only focuses on the design of link parameters rather than the development of communication modules.A relay system is proposed to address the issue of link damage between the satellite and the ground [17], [18].However, they all mainly focus on theoretical derivation and simulation of link performance of RF/FSO relay systems, which may be not applicable to satellite networks.
Future development of satellite communication terminals will inevitably require high compatibility of various modulation formats and data rates to support bi-directional communication among satellite networks of different types and generations.This can greatly enhance the flexibility and reduce the cost of space laser communications.Relevant organizations have also done some work on the design of satellite communication terminals [19], [20].However, all of these devices can only support certain modulation formats with low data rate, which may not meet the required demand.
In this paper, we propose and establish a multi-format/ratetunable integrated laser communications (MR-ILC) hardware platform.The advantages of the proposed technique are mainly in two folds: firstly, the proposed MR-ILC utilizes an automatic bias control algorithm to adjust the bias voltage to facilitate signal transmission in various modulation formats.Secondly, it uses only coherent detection to receive signals for all the modulation formats, including intensity modulations.Different signals can be transmitted and received on one same hardware platform.Besides, this system also includes the ranging measurement function while communicating.The ranges are calculated using the frequency difference information from the clock recovery algorithm in the communication, without additional hardware resource consuming.

A. Architecture of MR-ILC System
The MR-ILC platform is capable of transmitting and receiving signals with various modulation formats and rates.The modulation and demodulation algorithms for different signals are optimized and integrated into a single set of algorithms.This approach minimizes computational complexity and aligns well with the limited resources available in satellite communication.
Concept of the proposed MR-ILC system is depicted in Fig. 1.This system is capable of choosing modulation formats and signal rates for communications based on the characteristics of the link (such as transmission distance and loss) and the signals from the respective communication terminals.For example, BPSK signals can be used when the distance is long and the loss is high.PDM-QPSK signals, on the other hand, are suitable when a high information rate is required.Optical satellites equipped with the MR-ILC platform can establish bi-directional communication for different types of satellites.It can also serve as a relay satellite to connect different types of satellites.Additionally, it can serve as a third communication terminal to establish a third-party communication link with two other terminals, enabling communication to continue in case of a link interruption or damage.It also allows for two-way communication with various types of satellites that are already in operation.The function of ranging is also integrated into the system without extra hardware, which is accomplished along with communication.MR-ILC greatly improves the flexibility of satellite optical communication networks, and a single satellite that can fulfill the functions of different types of satellites also greatly reduces the cost of the communication network.
Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

B. Principle of the MR-ILC System
The optical signals of different modulation formats are generated on the same hardware platform by controlling the dualparallel Mach-Zehnder modulators (MZM), structure of which is shown in Fig. 2(a).
For a Dual Polarization IQ Modulator, the output can be approximated as . (1) where E in and E out are the input electric field and output light of the modulator, respectively.ϕ xi , ϕ xq , ϕ yi , and ϕ yq are the phases of four MZMs.From (1), it is evident that when the variation in phase shift ϕ conforms to a particular law, the sign of cos(ϕ/2) can oscillate between positive and negative.This phenomenon causes the phase of the output optical field to toggle between 0 and π.As a result, phase modulation can be achieved.The input voltage difference at which the phase shift ϕ changes by π is defined as V π of MZM.The phase shift ϕ exhibits a linear relationship with the input voltage of the electrical signal.The phase shift of the input electrical signal can be expressed as: Where V bias and V s are bias voltage and RF data.When the bias voltage reaches V bias = −V π /2 ± kV π (where k is an integer), the MZM enters the operating state of intensity modulation.When the bias voltage is V bias = −V π ± 2kV π , the MZM enters the operating state of amplitude and phase modulation.Fig. 2(c) shows the modulation curves of a MZM, where the red curve represents the input electric fields and the black curve represents the output optical intensity.Since the optical modulator comprises multiple MZM structures, this paper employs an automatic bias control algorithm to regulate several input bias voltages.The four arms are observed and managed separately.The undesired arm is adjusted to the NULL position and no radio frequency signal is applied to render it as invalid.This results in the least possible leakage of light signals, thereby reducing the impact of the unoperated MZM on the modulation signal.For example, to apply BPSK modulation, the XQ, YI, and YQ arms need to be set to NULL and no RF signals should be loaded to them to make them invalid, which actually means only the XI arm is working.The modulator in this paper has multiple operating points, and different operating points can be used to realize different functions.Fig. 2(d) shows the RF signal and voltage values loaded on each arm corresponding to the different signals.Table II shows the modulation formats supported on the platform and their corresponding rates.
Signals with different modulation formats and rates are transmitted from the transmitter, received by the receiver module of the communication platform after passing through the channel, and then sent to the FPGA for signal recovery.The FPGA automatically selects the corresponding algorithms based on the type of received signals, which are primarily divided into two categories: Coherent and IMDD.For coherent signals, the demodulation algorithm is only slightly different in the CMA.The overall algorithm flow remains the same.For direct detection of signals, the clock recovery algorithm does not need to be changed, after the clock recovery, the FFE can be used for judgment.So, the overall algorithmic complexity of the optimized integration is not high.
Communication modules for satellites must prioritize low power, low weight, and small footprint due to limited resources and space constraints.Therefore, the hardware platform of the overall real-time adaptive communication module is a small-sized (18 * 16 cm), low-power (∼48 W), and integrated transceiver development board, as shown in Fig. 3

III. EXPERIMENT SETUP
The experimental setup and physical display for the proposed MR-ILC system are shown in Fig. 4. The red and blue links in the diagram, respectively represent the transmission link of optical and electrical signals.The MR-ILC system consists of two identical FPGA-based terminals and a FSO transmission link.The communication between terminal A and B is bidirectional coherent optical communication.The FSO link is used for representing the inter-satellite transmission medium Ranging is achieved by varying the length of the optical delay line in the experiment.The optical delay line ranges from 0 to 600 ps with a resolution of 5 ps, thus the corresponding transmission distance is from 0 m to 0.18 m.
At the transmitter side, a laser with a wavelength of 1550 nm and a linewidth of ∼75 KHz, and an optical power setting of 13 dBm is used as the light source [21].The resulting laser is then injected into a DP-IQM with an output optical power of −2 dBm.In addition, an in-house developed board integrates the DP-IQM and an FPGA at the transmitter side to generate signals with different modulation formats and rates.The digital electrical signals are pseudo-random binary sequence-23 (PRBS-23) signals generated internally by the FPGA.Its internal IP core calculates the bit error rate (BER) at the receiver side.These signals are then transmitted to the DP-IQM inputs through digital ports instead of digital-to-analog converters (DACs).The modulated optical signal is then fed into the FSO transmitter collimator, as shown in Fig. 4(b).
The 510-meter-long FSO link is located between the two academic buildings shown in Fig. 4(c) and consists of two terminals.The experiment was conducted during the winter season under cloudy weather conditions.The temperature ranged from 3 to 15 degrees Celsius.The wind speed and condition were measured on the Beaufort wind force scale between 1 to 3. The experiment was conducted in the evening.These terminals enable full-duplex communication, allowing for the implementation of FSO applications.The academic buildings are located within Huazhong University of Science and Technology.Considering the distance and the requirement for an unobstructed transmission link, Terminal A is situated on the roof of Academic Building A, while Terminal B is positioned at a window on the 6th floor of Academic Building B. In order to achieve the highest possible accuracy in capturing and aligning the two terminals, we utilize a charge-coupled device (CCD) to acquire the optical image of the beacon.This allows us to calculate the pixel coordinates and deflection angle.Then, a proportional-integral-derivative (PID) controller is used to adjust the deflection angle of the optical gimbal and correct the pointing of the optical antenna.Finally, the stage and fine steering mirror (FSM) are adjusted for precise aiming and optimal coupling efficiency in this experiment.
At the receiver side, the integrated coherent receiver (ICR), local oscillator laser (LO), analog-to-digital converter (ADC), and FPGA are also integrated on the same transceiver-integrated board, as shown in Fig. 4(a).The beam is captured by an Rx collimator, transmitted through a 510-meter FSO link, and then coupled into an optical fiber.The fiber-coupled optical signal is attenuated by a variable optical attenuator (VOA) and amplified by an erbium-doped fiber amplifier (EDFA).The amplified signal is then received by the ICR.The VOA is used to simulate loss for extremely sensitive testing.The EDFA is used to amplify the optical signal and regulate the power within the appropriate receiving range of the ICR.The detected signals are sequentially sampled by four 6-bit 5 GSa/s ADCs with an analog bandwidth of 6 GHz.The sampled signals are sent to the FPGA for real-time digital signal processing (DSP) and recovery.The algorithmic changes are automatic and vary depending on the type of received signal.In the case of BPSK, QPSK, and PDM-QPSK signals, the algorithm incorporates ADC synchronization, clock recovery, constant mode algorithm (CMA), frequency offset recovery, phase recovery, and symbol determination.Gardner algorithm is utilized for clock recovery.The number of taps for the CMA equalizer is 5.The frequency and phase recovery are based on Viterbi-Viterbi (V-V) algorithm [22].The BPSK and QPSK recovery algorithms have minor differences from the CMA, specifically in how errors are updated.In the process of distance measurement between terminals A and B, the required information for mutual transmission is conveyed through the communication link.This information is subsequently demodulated by DSP for further computation.

IV. RESULTS
Variable-rate coherent transmission and ranging measurement performance are tested on the real-time platform.The receiver at different rates (0.625 Gbps 1.25 Gbps 2.5 Gbps 5 Gbps 10 Gbps) employs the same set of FPGA receiver, and their results of receiver sensitivity are illustrated in Fig. 5(a).It can be seen that adjacent curves exhibit an approximate 3 dB difference on the horizontal axis at the same BER.The results align with the theoretical expectations, where the theoretical prediction indicates a 3 dB decrease in receiver sensitivity when the communication rate is doubled.For instance, the sensitivity of PDM-QPSK at 10 Gbps is −43.1 dBm, while the corresponding sensitivities for the other rates decrease by ∼3 dB in sequence.[23].
Since there is a two-way communication between terminal A and terminal B and the length of the two links is the same, in order to get the specific accuracy of ranging it is necessary to add a section of error length to the link from B to A for one-way communication.Therefore, a section of adjustable high precision optical delay line is added as a source of error.Multiple sets of tests on ranging measurement were conducted at a communication rate of 625 Mbps, with varying lengths (100 ps 200 ps 300 ps 400 ps 550 ps) of optical delay lines by different preset delay.Each set underwent a substantial number of tests, and the final results were obtained by averaging their values.As shown in Table IV, assuming no error in the values of the optical delay lines, the calculated ranging error can be obtained by subtracting the measured value from its designated (preset)  V shows the sensitivity limits for this test with bit error rate of 3.8e-3, which is based on a 7% overhead hard-decision forward error correction (HD-FEC) [23].They are used to compare the sensitivities of the MR-ILC proposed in this paper at different rates.The comparison results show that, despite integrating the signals of multiple modulation formats in the same transceiver, the receive sensitivity performance of each of them is satisfactory.

V. CONCLUSION
In this paper, we propose and establish an adaptive optical satellite communication platform that supports data rates ranging from 0.625 to 10 Gbps using OOK, QPSK, and BPSK modulation.We also conduct two-way communication experiments over a ∼510 m free-space optical link.Investigations of the sensitivity of signals with different modulation formats at various rates demonstrate comparable sensitivity performance.The receiving sensitivity at the 3.8e-3 FEC threshold is −46 dBm for 10 Gbps PDM-QPSK signal, and is −58 dBm for 625 Mbps BPSK signal.Results show excellent signal sensitivity across different formats and rates.The ranging error is only 2.9% of the codeword width.The experiments demonstrate the significance of our MR-ILC in advancing the next-generation high-capacity inter-satellite information network.This technology greatly improves the flexibility of satellite optical networks and significantly reduces costs.

Manuscript received 17
December 2023; revised 2 February 2024; accepted 13 February 2024.Date of publication 16 February 2024; date of current version 7 March 2024.This work was supported in part by the National Key Research and Development Plan Project under Grant 2022YFB2803205, in part by the National Natural Science Foundation of China under Grant 62275091 and Grant 62205115, and in part by the Innovation Project of Optics Valley Laboratory under Grant OVL2021BG002.(Corresponding authors: Feng Fan; Rongke Liu.)

Fig. 2 .
Fig. 2. (a) Schematic diagram of an I/Q modulator.(b) Algorithm processing flow of the FPGA.(c) Modulation curve of a MZM (d) control methods for the format-adjustable/rate-tunable communication.
. Considering the hardware resources required for real-time algorithms, the FPGA chip used in this paper is the 5SGS model from the Stratix V family of chips manufactured by Altera.The transmitter is mainly composed of a signal laser, a DP-IQM, and an IO port for data transmission.It does not use a DAC, high-speed bit stream is sent to the DP-IQM through the IO port for modulation, which differs from conventional satellite communication systems.This configuration allows for the modulation and transmission of different signals.The automatic bias control algorithm is used to adjust the bias point of the signals, enabling them to be modulated into various optical signals for transmission based on specific requirements.The receiving end of the system consists of a local laser source, an integrated coherent receiver (ICR), and an analog-to-digital converter (ADC).The local laser source and the ICR are used to coherently receive the optical signals.The received signals are sampled by the ADC, and the data is then processed by the FPGA using real-time algorithms.Modulation and demodulation of multiple signals on a highly integrated, small-sized hardware platform greatly reduces costs, which meets the trend of future large-scale satellite launches and satisfies communication needs.

Fig. 5 .
Fig. 5. BER versus ROP performance of received signals at different rates for BPSK and QPSK.

TABLE I PERFORMANCE
OF DIFFERENT PUBLISHED REAL-TIME FSO COHERENT TRANSCEIVERS

TABLE II TYPE
OF MR-ILC SIGNAL

TABLE III RECEIVED
SENSITIVITY LIMIT WITH OOK As indicated in Table III, the received sensitivity of the OOK signal at a rate of 10Gbps is −42.89 dBm, when a 20.5% overhead Forward Error Correction (FEC) with a BER threshold of 2e-2 is applied . At a 625 Mbps BPSK signal rate, with a symbol duration of 1.6 ns, the calculated Root Mean Square Error (RMSE) based on measured data is 47 ps, reaching 2.9% of the symbol duration.It shows superior functionality of the proposed bidirectional MR-ILC system.TableIlists several real-time FSO coherent communication transceivers with different modulation formats and rates, and Table value