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Commun Eng
Commun Eng
Communications Engineering
2731-3395
Nature Publishing Group UK London

39266728
279
10.1038/s44172-024-00279-0
Article
Seamlessly merging radar ranging and imaging, wireless communications, and spectrum sensing for 6G empowered by microwave photonics
http://orcid.org/0000-0002-6553-0390
Shi Taixia 1
http://orcid.org/0000-0003-3400-1661
Chen Yang ychen@ce.ecnu.edu.cn

1
http://orcid.org/0000-0002-6877-7057
Yao Jianping jpyao@uottawa.ca

2
1 https://ror.org/02n96ep67 grid.22069.3f 0000 0004 0369 6365 Shanghai Key Laboratory of Multidimensional Information Processing, School of Communication and Electronic Engineering, East China Normal University, Shanghai, 200241 China
2 https://ror.org/03c4mmv16 grid.28046.38 0000 0001 2182 2255 Microwave Photonic Research Laboratory, School of Electrical Engineering and Computer Science, University of Ottawa, Ottawa, K1N 6N5 Canada
12 9 2024
12 9 2024
2024
3 13012 4 2024
2 9 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
To facilitate intelligent interconnection among people, machines, and things, the next generation of communication technology must incorporate various sensing functions besides high-speed wireless communications. Integration of radar, wireless communications, and spectrum sensing is being investigated for 6G with increased spectral efficiency, enhanced system integration, and reduced cost. Microwave photonics, a technique that combines microwave engineering and photonic technology is considered an effective solution for implementing the integration and breaking the bottleneck problems of electronic solutions. Here, we show a photonics-assisted joint radar, wireless communications, and spectrum sensing system that enables precise perception of the surrounding physical and electromagnetic environments while maintaining high-speed communication. Communication signals and frequency-sweep signals are merged optically using a shared system architecture and hardware to achieve signal level sharing, ultimately simultaneously achieving high-accuracy radar ranging and imaging with a measurement error within ± 4 cm and an imaging resolution of 25 × 24.7 mm, high-data-rate wireless communications at 2 Gbaud, and wideband spectrum sensing with a frequency measurement error within ±10 MHz in a 6 GHz bandwidth.

Taixia Shi and colleagues demonstrate a microwave photonics system with integrated capabilities of radar, communication, and spectrum sensing for 6G technologies, simultaneously achieving high-accuracy radar ranging and imaging, high-data-rate wireless communications, and wideband spectrum sensing.

Subject terms

Microwave photonics
Fibre optics and optical communications
https://doi.org/10.13039/501100001809 National Natural Science Foundation of China (National Science Foundation of China) 62371191 61971193 Chen Yang https://doi.org/10.13039/501100003399 Science and Technology Commission of Shanghai Municipality (Shanghai Municipal Science and Technology Commission) 22DZ2229004 Chen Yang issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

In addition to high-speed wireless communications, to realize intelligent interconnection among people, machines, and things, various sensing functions and artificial intelligence will also be incorporated into a 6G network to provide digital infrastructure for new types of applications1,2. Figure 1 shows a scenario in which multi-functions including target sensing, spectrum sensing, and wireless communications are incorporated in a 6G network. Instead of using independent systems for multiple functions, one highly recommended and cost-effective solution is to integrate multidimensional functions into a single system, to unify radar ranging and imaging, wireless communications, and spectrum sensing functions. This integrated system is known as a joint radar, wireless communications, and spectrum sensing (JRCSS) system, which has already been investigated in the electrical domain3,4. However, conventional electronic solutions have encountered challenges in addressing the bottleneck of broadband high-frequency complex waveform generation and have struggled to meet the high data rate and high-resolution application requirements due to the limited tunability and reconfigurability of electronic systems. Microwave photonics5,6, a technique that combines microwave engineering and photonic technology, has received considerable attention due to its numerous advantages, including large bandwidth, high frequency, low loss, wide tunability, and immunity to electromagnetic interference. Over the past few decades, microwave photonics has been extensively explored for the development of advanced radar7, wireless communications8, spectrum sensing9, and multi-function integrated systems10,11.Fig. 1 Application scenario of the integrated radar, wireless communications, and spectrum sensing system.

This figure is created by combining custom drawings with clipart images provided within the Microsoft Visio Standard.

Thanks to the benefits offered by microwave photonics, this technology is widely regarded as a promising solution for 6G systems, and the main research hotspots are the generation of millimeter-wave or terahertz wireless communication signals with high data rates using photonics-assisted signal generation methods12,13, and the realization of low-loss, long-distance wireless signal distribution using wireless-over-fiber techniques14,15. As the advancements in microwave photonics continue to drive innovation, there is a growing recognition of its potential impact on various sectors, fostering developments in radar systems and spectrum sensing techniques. This has the potential to revolutionize industries and facilitate the integration of radar ranging and imaging, spectrum sensing, and communications functions. The first all-optical implementation of microwave photonic radar16 was reported in 2014, and the report of this work has driven the rapid development of research on microwave photonic radar systems. In the follow-up work17–19, people try to integrate the generation of broadband linearly frequency-modulated (LFM) waveforms with the echo de-chirping process in the optical domain to achieve a high range and imaging resolution. Spectrum sensing powered by microwave photonics can achieve frequency measurement20–24 and time-frequency analysis25–30 by mapping the frequency information of the signal under test (SUT) to power20,21, space21, or time22–30. The spectrum sensing via frequency-to-power mapping20 is commonly not suitable for multi-frequency measurement without being combined with other methods, while the spectrum sensing via frequency-to-space mapping is commonly achieved by channelization and commonly needs to be combined with other methods21. Spectrum sensing that relies on frequency-to-time mapping (FTTM) has the ability of multi-frequency measurement and can be effectively implemented by utilizing fiber dispersion24–28 or by sweeping optical signals in combination with optical filtering22,23,29,30.

Building upon the aforementioned investigation of single-function microwave photonic systems, researchers have subsequently extended their studies to explore multi-function systems. These include joint communications and radar (JCR) systems, as well as joint radar and spectrum sensing systems, both benefiting from advanced microwave photonic technology. The radar signal and communication signal can be frequency-division31,32 and time-division33 multiplexed in the JCR system. To share the same waveform in the JCR system, quadrature phase-shift keying (QPSK) signal34 and spectrum-spreading phase-coding signal35 were used, which required a high sampling rate in the radar receivers34,35. Amplitude-shift keying LFM (ASK-LFM) signal36 and QPSK-LFM signal37,38 were used in the photonics-assisted JCR system in conjunction with the de-chirping operation, successfully reducing the complexity of the radar receiver. In the photonics-assisted joint radar and spectrum sensing system, an LFM signal is commonly used for radar detection and spectrum sensing, where the de-chirping operation is used for further radar signal processing and the original LFM signal is used for spectrum sensing via FTTM. The broadband LFM signal can be obtained by photonic frequency doubling39, photonic frequency quadrupling40, or optically injection41.

However, microwave photonic systems that can simultaneously realize the aforementioned three functions, which are highly desired in 6G, have not been studied. To bridge this gap, we introduce a photonics-enabled approach that seamlessly integrates radar ranging and imaging, wireless communications, and spectrum sensing within a unified framework. By sharing hardware and signal-level resources, the integrated system can simultaneously achieve multiple functions while greatly reducing hardware complexity, system volume, cost, and redundancy. An ASK communication signal and an LFM signal are converted to the optical domain, in which the LFM part of the modulated optical signal is used for spectrum sensing, and the LFM part and ASK part of the modulated optical signal are fused after photodetection for radar and communication functions. An experiment is performed to verify the concept. A JRCSS system supporting radar ranging with a measurement error within ± 4 cm, two-dimensional imaging with a resolution of 25 × 24.7 mm, wireless communications with a data rate of 2 Gbaud, and spectrum sensing with a frequency measurement error within ± 10 MHz in a 6 GHz bandwidth, is demonstrated. The proposed approach offers a feasible solution for simultaneously achieving target sensing, spectrum sensing, and wireless communications in 6G with increased spectrum efficiency, enhanced integration, and lower costs.

Results

Principle

The schematic of the proposed JRCSS system is shown in Fig. 2a, while Fig. 2b shows the schematic diagrams of the signals at locations A–H in the system diagram in Fig. 2a. A continuous-wave optical wave generated by a laser diode (LD) is split into two parts. One part from the LD is injected into a dual-parallel Mach–Zehnder modulator (DP-MZM) to which an intermediate frequency (IF) ASK signal and a negatively chirped LFM signal are applied. Note that the ASK signal and the negatively chirped LFM signal have two different phase relationships with one being 90o and 0o and the other being 0o and 90o, as shown in Fig. 2a. Through carrier-suppressed tandem single-sideband (CS-TSSB) modulation at the DP-MZM (Supplementary Note 1), a double-sideband signal with the IF ASK signal and the LFM signal located at the opposite sides of the optical carrier is generated, as shown in Fig. 2b C. A more detailed spectrum evolution is given in Supplementary Fig. 1.Fig. 2 Schematic of the proposed JRCSS system.

a System structure diagram. b Spectra and waveforms at different locations (A–H) in (a). JRCSS joint radar, wireless communications, and spectrum sensing, LD laser diode, DP-MZM dual-parallel Mach–Zehnder modulator, LFM linearly frequency-modulated, ASK amplitude-shift keying, AWG arbitrary waveform generator, PD photodetector, MZM Mach–Zehnder modulator, SUT signal under test, ISO isolator, NM nonlinear medium, CIR circulator, ED envelope detector, OSC oscilloscope, SBS stimulated Brillouin scattering.

For spectrum sensing, one part of the optical signal from the DP-MZM is sent to a Mach–Zehnder modulator (MZM), to which an SUT is applied. By controlling the bias voltage, carrier-suppressed double-sideband modulation with a carrier suppression ratio to make the carrier at a power level close to its sideband is generated. The not fully suppressed optical carrier is used to provide a reference for frequency identification to simplify the operation. At the MZM, the LFM sideband of the CS-TSSB-modulated signal from the DP-MZM is employed as a frequency-sweep carrier. Therefore, after carrier-suppressed double-sideband modulation, a series of frequency-sweep optical sidebands determined by the SUT frequency components will appear together with the partially suppressed optical carrier. The optical signal at the output of the MZM serves as a probe wave and is sent to a nonlinear medium (NM). The other part of the optical signal from the LD functions as a pump wave and is sent to the NM from the other side through an optical circulator (CIR) to trigger the stimulated Brillouin scattering (SBS) effect. The pump wave then interacts with the counter-propagating frequency-sweeping probe wave and generates an SBS gain, which serves as a narrowband optical bandpass filter. If the SBS gain spectrum is within the frequency-sweep ranges of the frequency-sweep optical carrier and SUT sidebands, the frequency-sweep optical carrier and SUT sidebands will be filtered by the SBS gain spectrum at a specific time in a single sweep period, as shown in Fig. 2b D. The specific time associated with the SUT sidebands is determined by the SUT frequency, while that linked to the frequency-sweep optical carrier not fully suppressed is a constant and used as a reference. After SBS interaction, optical pulses are generated at the corresponding time, and FTTM is implemented. Subsequently, the optical signal from the optical CIR is sent to a photodetector (PD1), where the optical pulses are converted into electrical pulses, as shown in Fig. 2b E. The electrical pulses are monitored by an oscilloscope (OSC), enabling the determination of the SUT frequency based on the time of pulse appearance. It is assumed that the carrier frequency of the LD is fc, the center frequency of the SBS gain is fc − fSBS, the frequency-sweep range and period of the frequency-sweep optical signal are fc − fSBS~fc − fSBS + fB and T. Here, fB is the frequency-sweep bandwidth of the frequency-sweep optical signal, and fSBS is the Brillouin frequency shift. Under these circumstances, the spectrum sensing range is 0 ~ fB. After FTTM, if the reference pulse appears at t0 and the signal pulse appears at tSUT, the SUT frequency can be obtained by fSUT = fB × (tSUT − t0)/T. Besides the SUT frequency, the time-frequency information of the SUT can also be obtained under high-speed frequency sweeping conditions by combining the pulses obtained from different sweep periods29. Therefore, spectrum sensing is realized in the system using the optical LFM sideband generated from the DP-MZM.

For radar ranging and imaging, the other part of the optical signal from the DP-MZM is injected into a second PD (PD2) to generate an electrical ASK-LFM signal by beating the LFM sideband and the ASK sideband. One part of the electrical signal from PD2 is sent to a transmitting antenna, which radiates the ASK-LFM signal into the free space. The radar-receiving antenna captures the echo signal reflected by the target, which is then sent to an RF mixer via the RF port. The other part of the electrical signal from PD2 is sent to the mixer via the local oscillator (LO) port. After mixing at the mixer, a de-chirped signal is generated from the IF port of the mixer, which is captured by an OSC and processed to enable target ranging and inverse synthetic aperture radar (ISAR) imaging.

For communications, the communication-receiving antenna receives the ASK-LFM signal radiated from the transmitting antenna. The signal envelope of the ASK signal is then obtained through an envelope detector (ED), and the original communication data can be obtained using a decision circuit. To do so, the received signal from the communication-receiving antenna is split into two by an electrical coupler (EC), and the two output signals from the EC are fed into a mixer via the RF port and LO port through two RF cables with an identical length. After self-mixing and low-pass filtering, the signal envelope carrying the communication data can also be obtained.

Signal generation

An electrical signal with a peak-to-peak amplitude of 1 V is generated by an arbitrary waveform generator (AWG1). The electrical signal consists of an ASK signal and an LFM signal. Figure 3a shows the spectrum of the electrical signal with the ASK signal having a baud rate of 2 Gbaud and Fig. 3b shows the spectrum of the electrical signal with the ASK signal having a baud rate of 0.5 Gbaud. In the two cases, the LFM signal has a negative chirp and a frequency-sweep range of 4.8 ~ 10.8 GHz. The electrical signal is applied to the DP-MZM. Another electrical signal, also consisting of the same ASK signal and LFM signal but with a different phase relationship, is also generated by AWG1 and applied to the DP-MZM but from a different output channel.Fig. 3 Signal generation results.

Electrical spectra of the signal from the AWG when the baud rate of the ASK signal is (a) 2 Gbaud and (b) 0.5 Gbaud. c Optical spectra of the optical signal from the DP-MZM. d Electrical spectra of the electrical signal from PD2. AWG arbitrary waveform generator, ASK amplitude-shift keying, DP-MZM dual-parallel Mach–Zehnder modulator, PD photodetector, LFM linearly frequency-modulated, RBW resolution bandwidth, VBW video bandwidth.

The optical spectra of the LFM optical sideband and the ASK optical sideband at the output of the DP-MZM are shown in Fig. 3c. After CS-TSSB modulation, the ASK signal and the LFM signal are loaded to the opposite sides of the optical carrier due to the specially designed phase relationships. The spectra in the two cases are almost identical due to the limited resolution of 0.015 nm of the optical spectrum analyzer (OSA, ANDO AQ6317B) used to measure the optical spectrum. The LFM optical sideband is used for spectrum sensing while the beating product of the ASK optical sideband and LFM optical sideband is used for radar ranging, imaging, and communication functions.

Figure 3d shows the electrical spectra of the generated ASK-LFM signal from PD2 with a center frequency of 10.8 GHz. The power of this signal is measured to be −18.5 dBm. The spectrum of the ASK-LFM signal is broadened to a certain extent on the basis of the spectrum of the original LFM signal, and the degree of broadening is dependent on the baud rate of the ASK signal. In addition, baseband ASK signals, as well as some single-tone interferences, are also observed. The single-tone interferences are mainly from AWG and harmonics of the ASK signal carrier.

High-speed communications

The two ASK-LFM signals generated above are first used for high-speed communications. Figure 4a shows the 4 μs temporal waveform of the self-mixing signal captured by the OSC with a sampling rate of 10 GSa s−1 when the baud rate of the ASK-LFM signal is 0.5 Gbaud. It should be noted that due to the limited bandwidth of OSC and its intrinsic low-pass filtering properties, the signal envelope can be obtained without additional low-pass filtering in the analog or digital domain. It can be seen that the amplitude of the waveform is noticeably uneven and gradually increases with time. Due to the uneven responses of the mixer and other components in the link over a wide bandwidth, the system exhibits a higher response at lower frequencies and a lower response at higher frequencies (Supplementary Note 2). Because a negatively chirped LFM signal that sweeps from a higher frequency to a lower frequency is used, the ASK-LFM signal also first appears at high frequencies and then at low frequencies within one sweep period of the LFM signal, resulting in a gradual increase in the amplitude of the signal after self-mixing. The amplitude unevenness of the waveform after self-mixing and filtering at different frequency bands is given in Supplementary Fig. 2.Fig. 4 High-speed communication results.

a–f The baud rate is 0.5 Gbaud. a The original waveform captured by the OSC and (b) the compensated waveform. c Electrical spectra corresponding to (a, b). A section of the compensated waveform from (d) 0 to 0.05 μs, and (e) 3.95 to 4 μs. f Eye diagrams of (b). g–k The baud rate is 2 Gbaud. g The original waveform captured by the OSC and (h) the compensated waveform. i Electrical spectra corresponding to (g, h). j A section of the compensated waveform from 0 to 0.05 μs. k Eye diagrams of (h). OSC oscilloscope.

The uneven amplitude of the waveform of the signal after self-mixing will lead to poorer signal-to-noise ratio (SNR) in the self-mixing waveform corresponding to the high-frequency band, making it more prone to bit errors, especially when the received ASK-LFM signal power is low. To mitigate the impact of the uneven mixer frequency response in the signal decision, the temporal waveform envelope is extracted and the original waveform is compensated according to the inverse of the amplitude of the temporal waveform envelope. After compensation, the waveform is flattened, as illustrated in Fig. 4b. Figure 4c displays the electrical spectra of the original waveform and the compensated waveform by fast Fourier transform. Figure 4d, e exhibits a segment of the compensated temporal waveform from 0 to 0.05 μs and from 3.95 to 4 μs, respectively. As can be seen, the original bit information is well represented by the compensated waveform after self-mixing, which indicates that the communication function of the system is well implemented. In addition, it can be clearly observed that the waveform from 3.95 to 4 μs generated by the low-frequency part of the ASK-LFM signal does have a better SNR compared to that from 0 to 0.05 μs generated by the high-frequency part of the ASK-LFM signal. Figure 4f shows the eye diagrams of the compensated waveform, the eyes are widely open, confirming good receiving and detection of the communication signal.

Then, the baud rate is increased to 2 Gbaud. The frequency-sweep range of the LFM signal is the same as the case when the baud rate is 0.5 Gbaud. Therefore, the amplitude unevenness of the waveform in Fig. 4g is very similar to that in Fig. 4a. The compensated waveform is shown in Fig. 4h, with a section from 0 to 0.05 μs shown in Fig. 4j. Again, the original bit information is well recovered. The spectra of the waveform before and after compensation and the eye diagram are shown in Fig. 4i and k. The eye-opening in Fig. 4k is worse than that in Fig. 4f, mainly because of the increased data rate, which is four times that in Fig. 4f. For practical applications, direct envelope detection using an envelope detector or self-mixing using a mixer with a flatter response over the wide bandwidth can mitigate the uneven amplitude of the waveform and better results are expected. In the experiment, 2-Gbaud 2ASK modulation is employed and a communication data rate of 2 Gbps is achieved. To achieve a higher data rate, multi-level ASK modulation can also be employed. In this case, a better SNR is required to achieve the same bit error rate. It should be noted that the phase of the LFM signal cannot be encoded, otherwise the de-chirping operation in this work is not applicable. When the phase of the LFM signal is encoded, the radar receiver commonly needs to sample the echo waveform, which is much more complex than de-chirping and sampling low-frequency de-chirped signals.

Radar ranging and imaging

The same ASK-LFM signal is then used for target radar ranging and imaging. Figure 5a shows the schematic for moving target imaging. To implement ISAR imaging, a cuboid and two cylinders covered with silver paper are placed on a turntable and employed as imaging targets, as shown in Fig. 5b. The dimensions of the cuboid are 10 cm (length) × 8 cm (width) × 18 cm (height). The diameter and height of cylinder 1 are 6.5 and 12 cm, whereas that of cylinder 2 are 8 and 10 cm. The range from the center of the turntable to the antenna pair is ~ 1.47 m. In the ISAR imaging experiment, the turntable is rotated clockwise with a period of 24.56 s. A de-chirped signal is captured by the OSC with an accumulation time of 2.2 seconds when the cuboid is farthest from the antenna, so the theoretical cross-range resolution is 24.7 mm19,42. Since the LFM bandwidth is 6 GHz, the theoretical range resolution is 25 mm19,42. When the turntable is stationary, the electrical spectra of the echo signal after amplification are shown in Fig. 5c, with the corresponding signal power of approximately −8.7 dBm. In Fig. 5c, ripples are observed in the electrical spectra of the echo signal, which is the result of overlapping multiple echo signals from the three targets. Figure 5d, e shows the imaging results of the targets when the baud rates of the ASK-LFM signal are 2 and 0.5 Gbaud, respectively. The three targets can be well distinguished. Because the function of target sensing is mainly determined by the LFM part of the ASK-LFM signal, there is no obvious difference in image quality at the two ASK baud rates. The radar echo used for obtaining the image in Fig. 5d is provided in Supplementary Note 3. The waveform and spectrum in several different imaging frames are given in Supplementary Fig. 3.Fig. 5 Target imaging results.

a Schematic for target imaging. b A cuboid and two cylinders used for target sensing. c Electrical spectra of the echo signal. Imaging result when the ASK baud rate is (d) 2 Gbaud and (e) 0.5 Gbaud. ASK amplitude-shift keying, LFM linearly frequency-modulated, OSC oscilloscope.

Figure 6a shows the schematic for target ranging when only a cylinder is placed on and rotated with the turntable. The range between the turntable and antenna pair is not changed. The rotation period of the turntable is also 24.56 s, during which the range between the cylinder and the antenna is continuously monitored. When the baud rate of the ASK-LFM signal is 0.5 Gbaud, the results at 35 equally spaced sampling points with a 4 μs sampling time in a rotation period are measured 5 times and given in Fig. 6b along with the theoretical curve. The measured target ranges are consistent with the theory. The red circles, blue diamonds, and purple error bars in Fig. 6c show the theoretical target range, the average ranging results of the five measurements in Fig. 6b, and the standard deviations of the range errors corresponding to Fig. 6b. The measurement error is no more than 4 cm. Figure 6d, e shows the measurement results when the baud rate of the ASK-LFM signal is increased to 2 Gbaud, and similar performance can be obtained, which means the communication data rate in the ASK-LFM signal does not have a substantial impact on target ranging.Fig. 6 Moving target ranging results.

a Schematic for moving target ranging. b Five sets of ranging results and (c) average ranging results and the corresponding errors and error bars representing standard deviations when the baud rate of the ASK-LFM signal is 0.5 Gbaud. d Five sets of ranging results and (e) average ranging results and the corresponding errors and error bars representing standard deviations when the baud rate of the ASK-LFM signal is 2 Gbaud. ASK amplitude-shift keying, LFM linearly frequency-modulated.

Figure 7a shows the schematic of ranging for two stationary targets. In this study, the turntable no longer rotates, and two cylinders are used as the targets. The range between cylinder 1 and the antenna pair is 1.34 m, while that between cylinder 2 and the antenna pair is set to 1.405, 1.470, and 1.535 m, respectively. Figure 7b, c shows the ranging results, i.e., the electrical spectra after de-chirping, using the ASK-LFM signal with baud rates of 0.5 and 2 Gbaud, respectively. From the frequency interval of the two strongest frequency components, the range between the two targets can be obtained, which is shown in Fig. 7b, c, with a maximum deviation from the actual value of only 0.7 cm. In addition, the 3 dB bandwidth of the peak after de-chirping is about 0.25 MHz (2.5 cm), and a good range resolution can be observed.Fig. 7 Stationary target ranging results.

a Schematic for two stationary targets ranging. Ranging results when the baud rate of the ASK-LFM signal is (b) 0.5 Gbaud and (c) 2 Gbaud. ASK amplitude-shift keying, LFM linearly frequency-modulated.

Spectrum sensing

The spectrum sensing function is conducted using the same ASK-LFM signal. The LFM sideband after CS-TSSB modulation is the key to spectrum sensing, which has a bandwidth of 6 GHz and a sweep period of 4 μs. Because the Brillouin frequency shift in the experiment is 10.8 GHz and the LFM sideband is 4.8 – 10.8 GHz away from the optical carrier, the spectrum sensing range is from 0 to 6 GHz. Figure 8a, b displays the frequency measurement results for six tones from 1 to 2 GHz, with a frequency step of 0.2 GHz. Figure 8a exhibits the waveforms of the electrical signal obtained from PD1, which is captured by the OSC at a sampling rate of 100 MSa s−1. Six distinct pulses corresponding to the six tones can be generated. The six frequencies are measured 100 times and the averaged values of the measured frequency are 0.9981, 1.1984, 1.3983, 1.5956, 1.8003, and 1.9968 GHz, respectively, which are shown in Fig. 8b. The standard deviations of the 100 measurements, represented by purple error bars in Fig. 8b, are 7.5, 6.0, 7.3, 7.3, 4.8, and 5.8 MHz. It is indicated that the frequency measurement error does not exceed ± 10 MHz.Fig. 8 Frequency and time-frequency measurement results.

Frequency measurement results of six tones, (a) waveforms of the signal from PD1, (b) measurement errors of 100 measurements and error bars representing standard deviations. Measured time-frequency diagrams of different SUTs, (c) triangular-chirp LFM signal, (d) signal with “Sine” time-frequency diagram, (e) quadratic NLFM signal, (f) quaternary NLFM signal, (g) step-frequency signal, (h) signal with “ECNU” time-frequency diagram. PD photodetector, SUT signal under test, LFM linearly frequency-modulated, NLFM non-linear frequency-modulated, ECNU East China Normal University.

Figure 8c–h displays the measured two-dimensional time-frequency diagrams of different SUTs via analog short-time Fourier transform using the proposed system. The diagrams in Fig. 8c–f clearly identify the triangular-chirp LFM signal, signal with a “Sine” time-frequency diagram, quadratic non-linear frequency-modulated (NLFM) signal, and quaternary NLFM signal, with a signal duration of 500 μs. Figure 8g presents the measurement result of a step-frequency signal from 0.1 to 5.9 GHz with a frequency step of 200 MHz and a signal period of 600 μs. Additionally, Fig. 8h presents the measurement result of a signal with an “ECNU” time-frequency diagram, with a signal period of 1200 μs. “ECNU” is the abbreviation of East China Normal University. As can be seen, the time-frequency analysis for all these signals is presented with good clarity. The time-frequency diagrams of more complex signals measured by the proposed system are given in Supplementary Fig. 4 of Supplementary Note 4 to demonstrate the universality of the spectrum sensing function of this system.

In the previous experiment for spectrum sensing, the low-speed pulses from PD1 are sampled at a sampling rate of 100 MSa s−1. To further investigate the spectrum sensing function at different sampling rates, more measurements are carried out. Figure 9a–d displays the measured time-frequency diagrams of an LFM signal with a signal period of 500 μs at a sampling rate of 100, 50, 20, and 10 MSa s−1. The results clearly indicate that the time-frequency diagrams exhibit a satisfactory frequency resolution when the processing sampling rate is 100 and 50 MSa s−1. When the sampling rate is decreased to 20 MSa s−1, there is a corresponding decrease in frequency resolution. When the processing sampling rate is further decreased to 10 MSa s−1, the time-frequency diagram of an LFM signal exhibits a distinct step-frequency signal characteristic, which is caused by the reduced frequency resolution introduced by the low sampling rate.Fig. 9 Time-frequency diagrams at different sampling rates.

Measured time-frequency diagrams of the LFM signal when the sampling rate is (a) 100, (b) 50, (c) 20, and (d) 10 MSa s−1. Measured time-frequency diagrams of the two-tone signal with frequencies of 1.92 and 2 GHz processed at a sampling rate of (e) 100, (f) 50, (g) 20, and (h) 10 MSa s−1. LFM linearly frequency-modulated.

To more clearly observe the influence of the sampling rate on the frequency resolution, an analysis is conducted on a two-tone signal with frequencies of 1.92 and 2 GHz at various processing sampling rates. Figure 9e–h depicts the time-frequency diagrams of the two-tone signal at sampling rates of 100, 50, 20, and 10 MSa s−1, respectively. As can be seen, when the sampling rate is 100 or 50 MSa s−1, we can observe two separate lines in the time-frequency diagrams in Fig. 9e, f. However, as the sampling rate is further reduced to 20 and 10 MSa s−1, the two pulses for the two tones cannot be distinguished. Thus, in the time-frequency diagrams in Fig. 9g, h, the two lines touch each other, which means the frequency resolution of the spectrum sensing is reduced. The pulse waveform from PD1 in spectrum sensing at different sampling rates is given in Supplementary Fig. 5. It can be seen from Fig. 9 that a frequency resolution of 80 MHz can be achieved when a sufficiently high sampling rate is employed. For a given sampling rate, the frequency resolution is determined by the filter bandwidth and the chirp rate of the frequency-sweep optical signal. For a given chirp rate, a filter bandwidth can be found to achieve the best frequency resolution22,30. The greater the chirp rate of the frequency-sweep optical signal, the wider the filter bandwidth is required to achieve the best frequency resolution, but the corresponding best frequency resolution will be poorer. When the chirp rate is very low22, the best frequency resolution will be achieved for the filter with a much narrower bandwidth than the SBS intrinsic bandwidth. In comparison, because the chirp rate is very high30 (several to tens of GHz per microsecond), the best frequency resolution will be achieved for the filter with a bandwidth greater than the SBS intrinsic bandwidth. Thus, the SBS pump wave is swept to generate a broadened SBS gain bandwidth, and the best frequency resolution is achieved at different filter bandwidths for different chirp rates30. The frequency-sweep chirp in this work is 1.5 GHz μs−1, and the frequency resolution obtained under the intrinsic linewidth of SBS is similar to that achieved in the previous study30. Theoretically, this resolution can be further improved by broadening the SBS gain bandwidth. However, considering that 80 MHz is already a good frequency resolution for time-frequency analysis, we do not further enhance it to maintain a reasonable system complexity.

Discussion

System tunability

As shown in Fig. 2, the frequency of the ASK-LFM signal can be adjusted by changing the center frequency of the IF ASK signal or the LFM signal applied to the DP-MZM. In the above experiment, the frequency of the ASK-LFM signal is mainly limited by the working frequency range of the antennas (8 – 18 GHz) and the electrical amplifier (EA, 5.85 – 14.5 GHz) used in the RF link. Thus, in the experiment, the LFM signal is designed to have a frequency-sweep bandwidth from 4.8 to 10.8 GHz, and the IF ASK signal is set with a center frequency at 3 GHz. Consequently, the ASK-LFM signal has a center frequency of 10.8 GHz in Fig. 3d and the subsequent experimental results.

Because the frequency measurement range of the spectrum sensing function is mainly determined by the frequency-sweep range of the LFM optical sideband and the position of the SBS gain spectrum, the frequency change of the LFM signal will affect the measurement range of the spectrum sensing function. Therefore, if only the center frequency of the electrical ASK-LFM signal needs to be adjusted, it is recommended to adjust the center frequency of the ASK-LFM signal. In the experiment, only a binary ASK signal is employed, which is mainly limited by the uneven frequency response of the transmitter link, receiver link, and the mixer used in self-mixing. If the transmitter and receiver link with a flat response, and mixers with a flat response or envelope detector with enough bandwidth are used in addition to greater receiving power, the system is also suitable for multi-level ASK signals.

In addition, the bandwidth of the ASK-LFM signal is jointly determined by the bandwidth of the IF ASK signal and the LFM signal. For different applications, the bandwidth of the ASK signal or LFM signal can be adjusted adaptively to achieve data communication at different data rates or target sensing with different precision. When the system bandwidth is limited, the implementation of the two functions needs to be compromised. When the bandwidth of the IF ASK signal or LFM signal is reduced to 0, the ASK-LFM signal is degraded to the LFM signal or ASK signal, respectively.

In the experiment, the optical carrier from the LD is directly used as the pump wave. Based on the above LFM signal settings, the measurement range of the spectrum sensing function is from 0 to 6 GHz in the experiment. The spectrum sensing bandwidth is limited by the bandwidth of the LFM signal, which can be easily changed by tuning the LFM signal bandwidth. The measurement range of the spectrum sensing function is determined by the relative position of the LFM optical sideband and the pump wavelength. To change the measurement range of the spectrum sensing function, two methods can be used: (1) Tuning the frequency-sweep range of the LFM signal; (2) Adjusting the position of the SBS gain spectrum, i.e., the pump wavelength. In order not to affect the generation of ASK-LFM signals, a more flexible way is the second one.

To adjust the pump wavelength, another DP-MZM (DP-MZM2) as a frequency shift module is added to the pump link immediately after the LD, as shown in Fig. 10a. Figure 10b shows the schematic diagrams of the signals at locations A–D in the system diagram. An RF signal from the microwave signal generator (MSG) is applied to the added DP-MZM, where carrier-suppressed single-sideband (CS-SSB) modulation is implemented to frequency shift the optical carrier from the LD, as shown in Fig. 10b B. When the frequency of the RF signal from the MSG is set to fx, the measurement range of the spectrum sensing function is changed from 0 to 6 GHz to from fx to fx + 6 GHz if the setting of the LFM signal is not changed. Figure 10b C-i and C-ii shows two cases with a measurement range from 0 to fB when the RF frequency is 0 and a measurement range from fx to fB + fx when the RF frequency is fx, respectively. It is worth noting that although the reference signal is used to determine the relationship between time domain pulses and the frequency information more conveniently in the experiment, the reference can be avoided by synchronizing the LFM signal generation with the signal sampling at the OSC.Fig. 10 Schematic of the proposed system.

a System structure diagram that has an additional optical frequency shift module, i.e., DP-MZM2, for tuning the measurement range of the spectrum sensing function. b Spectra and waveforms at different locations (A–D) in (a). LD laser diode, DP-MZM dual-parallel Mach–Zehnder modulator, MSG microwave signal generator, AWG arbitrary waveform generator, LFM linearly frequency-modulated, ASK amplitude-shift keying, MZM Mach–Zehnder modulator, SUT signal under test, ISO isolator, NM nonlinear medium, CIR circulator, PD photodetector, OSC oscilloscope, SBS stimulated Brillouin scattering.

Comparison with other existing methods

A comparison between the proposed JRCSS system and the previously reported photonics-assisted single-function and dual-function systems is provided in Table 1. Compared with other existing systems, the proposed system in this work has the following key features: (1) The proposed system is the only one that can seamlessly integrate spectrum sensing, radar ranging and imaging, and communication functions within a unified framework. (2) At the microwave frequency band, the proposed system has the highest communication data rate compared to other multi-function systems. (3) Compared with other methods with radar function, the radar performance of the proposed method can reach the mainstream level. (4) Compared with other multi-function systems, the proposed method has the time-frequency analysis capability and the minimum frequency measurement error.Table 1 Comparison of different methods

ref.	Integrated approach for multi-functionality	Wireless communications	Radar	Spectrum sensing	
Frequency (GHz)	Data rate (Gbps)	Frequency (GHz)	Ranging resolution (cm) /Error (cm)	ISAR imaging resolution (cm×cm)	Frequency measurement range (GHz) /Error (MHz) /Resolution (MHz)	Time-frequency analysis ability	
12	No	332.55–367.45	122.15	–	–	–	–	–	
17	No	–	–	18–26	2/–	2 × 2	–	–	
25	No	–	–	–	–	–	0–46/–/357	Yes	
30	No	–	–	–	–	–	0–4/ ± 4/35	Yes	
31	Frequency-division multiplexing	4.89–4.91	0.054	2.465–2.485	–/–	–	–	–	
32	>300	56	>300	1.1/–	–	–	–	
33	Time-division multiplexing	335–345	38.1	335–345	1.57/ ± 1.5	–	–	–	
34	Waveform share (QPSK)	23–25	0.3356	23–25	7.5/–	–	–	–	
35	Waveform share (Spectrum-spreading phase-coding signal)	30–40	1	30–40	3.5/–	–	–	–	
36	Waveform share (ASK-LFM)	18–26	0.1	18–26	1.875/–	1.8×2	–	–	
37	Waveform share (QPSK-LFM)	8.5–9.5	0.21052	8.5–9.5	14.99/4	14.99×3.25	–	–	
38	11.5–12.5	0.4	11.5–12.5	–/1.2	–	–	–	
39	Waveform share (LFM)	–	–	12–18	2.6/–	2.6×2.8	28–37/ ± 15/40	No	
40	–	–	18–26	2.06/–	–	28–36/ ± 16/37.6	No	
41	–	–	12–18	1.25/–	1.25×–	0.05–39.95/ ± 50/20	No	
This work	Waveform share (ASK-LFM)	7.8–13.8	2	7.8–13.8	2.5/ ± 4	2.5×2.47	0–6/ ± 10/80	Yes	
QPSK denotes quadrature phase-shift keying, ASK denotes amplitude-shift keying, and LFM denotes linearly frequency-modulated.

Compared to any single-function system, the multi-function system proposed in this work is more complex. However, by implementing waveform sharing through shared system architecture and hardware, the proposed system achieves lower system complexity while fulfilling the same multiple functions, compared to simply combining three different systems with different functions. This is of great significance for the miniaturization and integration of future multi-function systems. The large-bandwidth optoelectronic devices used in the system have relatively high costs at the current stage. As mentioned above, the system architecture is relatively complex due to the need to simultaneously achieve three functions, which makes the system more costly compared to the conventional electronic solutions. However, it is worth noting that the proposed system, based on microwave photonics, offers better system tunability and wider operating bandwidth compared to the conventional electronic solutions. Furthermore, the current system is built using discrete components, which also contributes to the higher system cost. With the continuous development of photonic integrated circuits43,44, the system proposed in this work can be implemented on a chip, which will significantly reduce the system cost to promote large-scale applications.

Conclusion

In summary, we have introduced a photonics-enabled approach that seamlessly integrates radar ranging and imaging, wireless communications, and spectrum sensing within a unified framework. The key to the proposed approach is to generate an optical signal that can be employed to perform radar ranging and imaging, and spectrum sensing in addition to wireless communication. This is achieved here by using photonics to generate an ASK communication sideband and an LFM sideband, where the LFM sideband is used for spectrum sensing and the combination of the LFM and ASK sidebands is used for radar ranging and imaging and wireless communication functions. The proposed approach is evaluated experimentally. A joint radar ranging and imaging, wireless communications, and spectrum sensing system supporting radar ranging with a measurement error within ± 4 cm, two-dimensional imaging with a resolution of 25 × 24.7 mm, wireless communications with a data rate of 2 Gbaud, and spectrum sensing with a frequency measurement error within ± 10 MHz in a 6 GHz bandwidth, is demonstrated. This research holds great significance for 6G technology, enabling precise perception of the surrounding physical and electromagnetic environments while maintaining high-speed data communication, which can be applied to the intelligent transport system, smart factories, and smart public security to meet the needs of various sensing and communication working together and can better promote the integration of future communication systems with the necessary sensing functions in the interconnected world of everything, thereby reducing the overall costs, power consumption, and complexity of the systems.

Methods

The experimental setup is shown in Fig. 2. A light wave, characterized by a wavelength of 1553.096 nm and a power of 16 dBm, is generated by an LD (ID Photonics CoBriteDX1-1-C-H01-FA) and subsequently divided into two equal parts by an optical coupler (OC1). One output of OC1 is sent to a DP-MZM (Fujitsu FTM7961EX) with an insertion loss of 7.5 dB. Two channels of AWG1 (Keysight M8195A, 64 GSa s−1) are used to generate the signals applied to the DP-MZM for CS-TSSB modulation. To implement the CS-TSSB modulation, the negatively chirped LFM signal and the IF ASK signal need to be combined with two different phase relationships, which can commonly be realized by using two 90° hybrid couplers and two 3 dB couplers to couple the negatively chirped LFM signal and the IF ASK signal like the TSSB modulation45. To simplify the experiment, the two signals with different phase relationships are directly generated by AWG1, as shown in Fig. 2. The two signals from AWG1 have a peak-to-peak amplitude of 1 V. The LFM signal has a center frequency of 7.8 GHz, a bandwidth of 6 GHz, and a period of 4 μs. The center frequency and baud rate of the ASK signal are set to 3 GHz and 0.5 or 2 Gbaud. The two signals from AWG1 are applied to the two RF ports of the DP-MZM, respectively, and the bias condition of the DP-MZM for the CS-TSSB modulation is the same as that for the CS-SSB modulation46. Subsequently, the optical signal from the DP-MZM with a power of around –15 dBm is amplified by an erbium-doped fiber amplifier (EDFA, Amonics AEDFA-PA-35-B-FA) and then equally split into two parts using OC2. The gain of the EDFA is set to be 16 dB, so the optical power at the input of OC2 is around 1 dBm.

In one output of OC2 for spectrum sensing, the optical signal is carrier-suppressed double-sideband modulated by the SUT at an MZM (Fujitsu, FTM7938EZ) with an insertion loss of 6 dB. The SUT is generated from AWG2 (Keysight M8190A, 12 GSa s−1) and its peak-to-peak amplitude is 1 V, with a corresponding power of 4 dBm. It is important to note that the measurement can also be conducted even if the power is reduced to −10 dBm29. Furthermore, the use of a low-noise amplifier before applying the SUT to the MZM allows for an even lower SUT power.

The optical signal from the MZM is utilized as the probe wave and sent to an NM, where it interacts with the counter-propagating pump wave from OC1. In this experiment, a section of 25.2 km single-mode fiber with an SBS threshold of around 7 dBm is used as an NM. It should be noted that a much shorter highly nonlinear fiber can also be used as the NM with a reduced weight and volume but an increased SBS threshold. For instance, for a 200 m highly nonlinear fiber, the SBS threshold is typically over 20 dBm. Here, the single-mode fiber is employed because we do not have a highly nonlinear fiber at the time of the experiment. After SBS interaction, the SUT frequency is mapped to the time of occurrence of the optical pulse. Then, the optical pulses from the optical CIR are sent to PD1 (Nortel Networks PP-10 G) to convert them to electrical pulses. The electrical pulses with SUT frequency information from PD1 are captured by the OSC (Rohde & Schwarz RTO2032) with a sampling rate of 100 MSa s−1.

Another part of the optical signal from OC2 is injected into PD2 (u2t MPRV1331A) to generate an electrical ASK-LFM signal for radar and communication functions. The ASK-LFM signal from PD2 is amplified by EA1 (ALM 145-5023-293 5.85 to 14.5 GHz, 23 dB) and then equally split by EC1 (Narda 4456-2, 2 to 18 GHz, −3 dB). One output of EC1 is injected into the LO port of Mixer1 (M/A-COM M14A). Another output of EC1 is emitted by a transmitting antenna (GHA080180-SMF-14, 8–18 GHz). The radar-receiving antenna (GHA080180-SMF-14, 8–18 GHz) captures the reflected signal from the target, which is then amplified by EA2 (CLM 145-7039-293B, 5.85 – 14.50 GHz, 39 dB) and sent to the RF port of Mixer1. The de-chirped signal from the IF port of Mixer1 is captured by the OSC with a sampling rate of 40 MSa s−1 and processed to enable target ranging and ISAR imaging.

The communication-receiving antenna (GHA080180-SMF-14, 8–18 GHz) receives the ASK-LFM signal from the transmitting antenna. The received ASK-LFM signal is amplified by EA3 (CLM 145-7039-293B, 5.85–14.50 GHz, 39 dB). The signal envelope is then obtained by self-mixing via EC2 (Narda 4456-2, 2–18 GHz, −3 dB) and Mixer2 (Miteq m30).

Supplementary information

Peer Review File

Supplementary Information

Supplementary information

The online version contains supplementary material available at 10.1038/s44172-024-00279-0.

Acknowledgements

This work was supported by the National Natural Science Foundation of China under Grant 62371191 and Grant 61971193 and the Science and Technology Commission of Shanghai Municipality under Grant 22DZ2229004. We used Microsoft Visio Standard to create Fig. 1, combining our custom drawings with clipart images from the software.

Author contributions

Y.C. and J.Y. conceived the idea; J.Y., Y.C. and T.S. designed the experiment; T.S. and Y.C. performed the experiment; T.S. analyzed the data; T.S., Y.C. and J.Y. wrote the paper.

Peer review

Peer review information

Communications Engineering thanks Fu-Kang Wang, Eduardo Saia Lima and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Anastasiia Vasylchenkova. A peer review file is available.

Data availability

All data are available from the corresponding author upon reasonable request.

Code availability

All codes are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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References

1. Tong, W. & Zhu, P. 6G: The Next Horizon: From Connected People and Things to Connected Intelligence (Cambridge Univ. Press, Cambridge, 2021).
2. IMT-2030(6G) Promotion Group. 6G Vision and Candidate Technologies. http://www.caict.ac.cn/english/news/202106/t20210608_378637.html (2021).
3. Tang B Stoica P MIMO multifunction RF systems: detection performance and waveform design IEEE Trans. Signal Process. 2022 70 4381 4394 10.1109/TSP.2022.3202315
Tang, B. & Stoica, P. MIMO multifunction RF systems: detection performance and waveform design. IEEE Trans. Signal Process. 70, 4381–4394 (2022).10.1109/TSP.2022.3202315
4. Moo P Difilippo D Multifunction RF systems for naval platforms Sensors 2018 18 2076 10.3390/s18072076 29958465
Moo, P. & Difilippo, D. Multifunction RF systems for naval platforms. Sensors 18, 2076 (2018).29958465 10.3390/s18072076
5. Capmany J Novak D Microwave photonics combines two worlds Nat. Photon. 2007 1 319 330 10.1038/nphoton.2007.89
Capmany, J. & Novak, D. Microwave photonics combines two worlds. Nat. Photon. 1, 319–330 (2007).10.1038/nphoton.2007.89
6. Yao J Microwave photonics J. Lightw. Technol. 2009 27 314 335 10.1109/JLT.2008.2009551
Yao, J. Microwave photonics. J. Lightw. Technol. 27, 314–335 (2009).10.1109/JLT.2008.2009551
7. Pan S Zhang Y Microwave photonic radars J. Lightw. Technol. 2020 38 5450 5484 10.1109/JLT.2020.2993166
Pan, S. & Zhang, Y. Microwave photonic radars. J. Lightw. Technol. 38, 5450–5484 (2020).10.1109/JLT.2020.2993166
8. Kuri T Toda H Olmos J Kitayama K Reconfigurable dense wavelength-division-multiplexing millimeter-waveband radio-over-fiber access system technologies J. Lightw. Technol. 2010 28 2247 2257 10.1109/JLT.2010.2042567
Kuri, T., Toda, H., Olmos, J. & Kitayama, K. Reconfigurable dense wavelength-division-multiplexing millimeter-waveband radio-over-fiber access system technologies. J. Lightw. Technol. 28, 2247–2257 (2010).10.1109/JLT.2010.2042567
9. Zou X Photonics for microwave measurements Laser Photon. Rev. 2016 10 711 734 10.1002/lpor.201600019
Zou, X. et al. Photonics for microwave measurements. Laser Photon. Rev. 10, 711–734 (2016).10.1002/lpor.201600019
10. Rodgers, J. Technologies for RF photonics in wideband multifunction systems. In IEEE Avionics, Fiber-Optics and Photonics Technology Conference 7–8 (San Diego, CA, 2013).
11. Zhu D Pan S Broadband cognitive radio enabled by photonics J. Lightw. Technol. 2020 38 3076 3088 10.1109/JLT.2020.2993021
Zhu, D. & Pan, S. Broadband cognitive radio enabled by photonics. J. Lightw. Technol. 38, 3076–3088 (2020).10.1109/JLT.2020.2993021
12. Zhang L Pang X Jia S Wang S Yu X Beyond 100 Gb/s optoelectronic terahertz communications: Key technologies and directions IEEE Commun. Mag. 2020 58 34 40 10.1109/MCOM.001.2000254
Zhang, L., Pang, X., Jia, S., Wang, S. & Yu, X. Beyond 100 Gb/s optoelectronic terahertz communications: Key technologies and directions. IEEE Commun. Mag. 58, 34–40 (2020).10.1109/MCOM.001.2000254
13. Moon S 6G indoor network enabled by photonics- and electronics-based sub-THz technology J. Lightw. Technol. 2022 40 499 510 10.1109/JLT.2021.3113898
Moon, S. et al. 6G indoor network enabled by photonics- and electronics-based sub-THz technology. J. Lightw. Technol. 40, 499–510 (2022).10.1109/JLT.2021.3113898
14. Li K Yu J Photonics-aided terahertz-wave wireless communication J. Lightw. Technol. 2022 40 4186 4195 10.1109/JLT.2022.3161878
Li, K. & Yu, J. Photonics-aided terahertz-wave wireless communication. J. Lightw. Technol. 40, 4186–4195 (2022).10.1109/JLT.2022.3161878
15. Pereira L Mendes L Filho C Sodre A Amplified radio-over-fiber system linearization using recurrent neural networks J. Opt. Commun. Netw. 2023 15 144 154 10.1364/JOCN.474290
Pereira, L., Mendes, L., Filho, C. & Sodre, A. Amplified radio-over-fiber system linearization using recurrent neural networks. J. Opt. Commun. Netw. 15, 144–154 (2023).10.1364/JOCN.474290
16. Ghelfi P A fully photonics-based coherent radar system Nature 2014 507 341 345 10.1038/nature13078 24646997
Ghelfi, P. et al. A fully photonics-based coherent radar system. Nature 507, 341–345 (2014).24646997 10.1038/nature13078
17. Zhang F Photonics-based broadband radar for high-resolution and real-time inverse synthetic aperture imaging Opt. Exp. 2017 25 16274 16281 10.1364/OE.25.016274
Zhang, F. et al. Photonics-based broadband radar for high-resolution and real-time inverse synthetic aperture imaging. Opt. Exp. 25, 16274–16281 (2017).10.1364/OE.25.016274
18. Li R Demonstration of a microwave photonic synthetic aperture radar based on photonic-assisted signal generation and stretch processing Opt. Exp. 2017 25 14334 14340 10.1364/OE.25.014334
Li, R. et al. Demonstration of a microwave photonic synthetic aperture radar based on photonic-assisted signal generation and stretch processing. Opt. Exp. 25, 14334–14340 (2017).10.1364/OE.25.014334
19. Liang D Jiang L Chen Y Multi-functional microwave photonic radar system for simultaneous distance and velocity measurement and high-resolution microwave imaging J. Lightw. Technol. 2021 39 6470 6478 10.1109/JLT.2021.3101312
Liang, D., Jiang, L. & Chen, Y. Multi-functional microwave photonic radar system for simultaneous distance and velocity measurement and high-resolution microwave imaging. J. Lightw. Technol. 39, 6470–6478 (2021).10.1109/JLT.2021.3101312
20. Chen Y Zhang W Liu J Yao J On-chip two-step microwave frequency measurement with high accuracy and ultra-wide bandwidth using add-drop micro-disk resonators Opt. Lett. 2019 44 2402 2405 10.1364/OL.44.002402 31090691
Chen, Y., Zhang, W., Liu, J. & Yao, J. On-chip two-step microwave frequency measurement with high accuracy and ultra-wide bandwidth using add-drop micro-disk resonators. Opt. Lett. 44, 2402–2405 (2019).31090691 10.1364/OL.44.002402
21. Jiang H Wide-range high-precision multiple microwave frequency measurement using a chip-based photonic Brillouin filter Optica 2016 3 30 34 10.1364/OPTICA.3.000030
Jiang, H. et al. Wide-range high-precision multiple microwave frequency measurement using a chip-based photonic Brillouin filter. Optica 3, 30–34 (2016).10.1364/OPTICA.3.000030
22. Shi T Chen Y Multiple radio frequency measurements with an improved frequency resolution based on stimulated Brillouin scattering with a reduced gain bandwidth Opt. Lett. 2021 46 3460 3463 10.1364/OL.428788 34264238
Shi, T. & Chen, Y. Multiple radio frequency measurements with an improved frequency resolution based on stimulated Brillouin scattering with a reduced gain bandwidth. Opt. Lett. 46, 3460–3463 (2021).34264238 10.1364/OL.428788
23. Singh K Preusler S Misra A Zhou L Schneider T Photonic microwave frequency measurement with high accuracy and sub-MHz resolution J. Lightw. Technol. 2022 40 2748 2753 10.1109/JLT.2022.3147962
Singh, K., Preusler, S., Misra, A., Zhou, L. & Schneider, T. Photonic microwave frequency measurement with high accuracy and sub-MHz resolution. J. Lightw. Technol. 40, 2748–2753 (2022).10.1109/JLT.2022.3147962
24. Chatellus H Cortés L Azaña J Optical real-time fourier transformation with kilohertz resolutions Optica 2016 3 1 8 10.1364/OPTICA.3.000001
Chatellus, H., Cortés, L. & Azaña, J. Optical real-time fourier transformation with kilohertz resolutions. Optica 3, 1–8 (2016).10.1364/OPTICA.3.000001
25. Azaña J Zhu X Rowe C Crockett B Optical time-mapped spectrograms (II): fractional talbot designs J. Lightw. Technol. 2023 41 5284 5295 10.1109/JLT.2023.3260706
Azaña, J., Zhu, X., Rowe, C. & Crockett, B. Optical time-mapped spectrograms (II): fractional talbot designs. J. Lightw. Technol. 41, 5284–5295 (2023).10.1109/JLT.2023.3260706
26. Li M Yao J All-optical short-time fourier transform based on a temporal pulse-shaping system incorporating an array of cascaded linearly chirped fiber Bragg gratings IEEE Photon. Technol. Lett. 2011 23 1439 1441 10.1109/LPT.2011.2162624
Li, M. & Yao, J. All-optical short-time fourier transform based on a temporal pulse-shaping system incorporating an array of cascaded linearly chirped fiber Bragg gratings. IEEE Photon. Technol. Lett. 23, 1439–1441 (2011).10.1109/LPT.2011.2162624
27. Konatham S Real-time gap-free dynamic waveform spectral analysis with nanosecond resolutions through analog signal processing Nat. Commun. 2020 11 3309 10.1038/s41467-020-17119-2 32620871
Konatham, S. et al. Real-time gap-free dynamic waveform spectral analysis with nanosecond resolutions through analog signal processing. Nat. Commun. 11, 3309 (2020).32620871 10.1038/s41467-020-17119-2
28. Xie X Li J Yin F Xu K Dai Y STFT based on bandwidth-scaled microwave photonics J. Lightw. Technol. 2021 39 1680 1687 10.1109/JLT.2020.3042985
Xie, X., Li, J., Yin, F., Xu, K. & Dai, Y. STFT based on bandwidth-scaled microwave photonics. J. Lightw. Technol. 39, 1680–1687 (2021).10.1109/JLT.2020.3042985
29. Zuo P Ma D Chen Y Short-time fourier transform based on stimulated Brillouin scattering J. Lightw. Technol. 2022 40 5052 5061 10.1109/JLT.2022.3174552
Zuo, P., Ma, D. & Chen, Y. Short-time fourier transform based on stimulated Brillouin scattering. J. Lightw. Technol. 40, 5052–5061 (2022).10.1109/JLT.2022.3174552
30. Zuo P Ma D Li X Chen Y Improving the accuracy and resolution of filter-and frequency-to-time mapping-based time and frequency acquisition methods by broadening the filter bandwidth IEEE Trans. Microw. Theory Techn. 2023 71 3668 3677 10.1109/TMTT.2023.3241688
Zuo, P., Ma, D., Li, X. & Chen, Y. Improving the accuracy and resolution of filter-and frequency-to-time mapping-based time and frequency acquisition methods by broadening the filter bandwidth. IEEE Trans. Microw. Theory Techn. 71, 3668–3677 (2023).10.1109/TMTT.2023.3241688
31. Melo, S. et al. Dual-use system combining simultaneous active radar & communication, based on a single photonics-assisted transceiver. In 17th International Radar Symposium (IRS) 1–4 (Krakow, Poland, 2016).
32. Jia S A unified system with integrated generation of high-speed communication and high-resolution sensing signals based on THz photonics J. Lightw. Technol. 2018 36 4549 4556 10.1109/JLT.2018.2863684
Jia, S. et al. A unified system with integrated generation of high-speed communication and high-resolution sensing signals based on THz photonics. J. Lightw. Technol. 36, 4549–4556 (2018).10.1109/JLT.2018.2863684
33. Wang Y Integrated high-resolution radar and long-distance communication based-on photonic in terahertz band J. Lightw. Technol. 2022 40 2731 2738 10.1109/JLT.2022.3143849
Wang, Y. et al. Integrated high-resolution radar and long-distance communication based-on photonic in terahertz band. J. Lightw. Technol. 40, 2731–2738 (2022).10.1109/JLT.2022.3143849
34. Xue Z Li S Xue X Zheng X Zhou B Photonics-assisted joint radar and communication system based on an optoelectronic oscillator Opt. Exp. 2021 29 22442 22454 10.1364/OE.430910
Xue, Z., Li, S., Xue, X., Zheng, X. & Zhou, B. Photonics-assisted joint radar and communication system based on an optoelectronic oscillator. Opt. Exp. 29, 22442–22454 (2021).10.1364/OE.430910
35. Bai W Photonic millimeter-wave joint radar-communication system using spectrum-spreading phase-coding IEEE Trans. Microw. Theory Techn. 2022 70 1552 1561 10.1109/TMTT.2021.3138069
Bai, W. et al. Photonic millimeter-wave joint radar-communication system using spectrum-spreading phase-coding. IEEE Trans. Microw. Theory Techn. 70, 1552–1561 (2022).10.1109/TMTT.2021.3138069
36. Nie, H., Zhang, F., Yang, Y. & Pan, S. Photonics-based integrated communication and radar system. In Proc. 2019 International Topical Meeting on Microwave Photonics (MWP), 1–4 (Ottawa, ON, 2019).
37. Wang S Liang D Chen Y Photonics-assisted joint communication-radar system based on a QPSK-sliced linearly frequency-modulated signal Appl. Optics 2022 61 4752 4760 10.1364/AO.456287
Wang, S., Liang, D. & Chen, Y. Photonics-assisted joint communication-radar system based on a QPSK-sliced linearly frequency-modulated signal. Appl. Optics 61, 4752–4760 (2022).10.1364/AO.456287
38. Liang D Chen Y Photonics-enabled joint communication-radar system with improved detection performance and looser symbol length limitation based on resampling and phase compensation Opt. Laser Techn. 2023 165 109638 10.1016/j.optlastec.2023.109638
Liang, D. & Chen, Y. Photonics-enabled joint communication-radar system with improved detection performance and looser symbol length limitation based on resampling and phase compensation. Opt. Laser Techn. 165, 109638 (2023).10.1016/j.optlastec.2023.109638
39. Shi J Photonics-based dual-functional system for simultaneous high-resolution radar imaging and fast frequency measurement Opt. Lett. 2019 44 1948 1951 10.1364/OL.44.001948 30985782
Shi, J. et al. Photonics-based dual-functional system for simultaneous high-resolution radar imaging and fast frequency measurement. Opt. Lett. 44, 1948–1951 (2019).30985782 10.1364/OL.44.001948
40. Shi J Zhang F Ben D Pan S Simultaneous radar detection and frequency measurement by broadband microwave photonic processing J. Lightw. Technol. 2020 38 2171 2179 10.1109/JLT.2020.2965113
Shi, J., Zhang, F., Ben, D. & Pan, S. Simultaneous radar detection and frequency measurement by broadband microwave photonic processing. J. Lightw. Technol. 38, 2171–2179 (2020).10.1109/JLT.2020.2965113
41. Tang, Z., Zhou, P., Zhu, J., Li, N. & Pan, S. An integrated radar detection and microwave frequency measurement system based on an optically injected semiconductor laser. In 2023 Opt. Fiber Commun. Conf. Exhibition (OFC) 1–3 (San Diego, CA, 2023).
42. Shi T Liang D Han M Chen Y Photonics-based de-chirping and leakage cancellation for frequency-modulated continuous-wave radar system IEEE Trans. Microw. Theory Techn. 2022 70 4252 4262 10.1109/TMTT.2022.3186375
Shi, T., Liang, D., Han, M. & Chen, Y. Photonics-based de-chirping and leakage cancellation for frequency-modulated continuous-wave radar system. IEEE Trans. Microw. Theory Techn. 70, 4252–4262 (2022).10.1109/TMTT.2022.3186375
43. Marpaung D Yao J Capmany J Integrated microwave photonics Nat. Photon. 2019 13 80 90 10.1038/s41566-018-0310-5
Marpaung, D., Yao, J. & Capmany, J. Integrated microwave photonics. Nat. Photon. 13, 80–90 (2019).10.1038/s41566-018-0310-5
44. Wang C Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages Nature 2018 562 101 104 10.1038/s41586-018-0551-y 30250251
Wang, C. et al. Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages. Nature 562, 101–104 (2018).30250251 10.1038/s41586-018-0551-y
45. Lin C Chen J Shih P Jiang W Chi S Ultra-high data-rate 60 GHz radio-over-fiber systems employing optical frequency multiplication and OFDM formats J. Lightw. Technol. 2010 28 2296 2306 10.1109/JLT.2010.2047712
Lin, C., Chen, J., Shih, P., Jiang, W. & Chi, S. Ultra-high data-rate 60 GHz radio-over-fiber systems employing optical frequency multiplication and OFDM formats. J. Lightw. Technol. 28, 2296–2306 (2010).10.1109/JLT.2010.2047712
46. Shi T Han M Chen Y Photonic-based analog and digital RF self-interference cancellation with high spectral efficiency Appl. Opt. 2021 60 10299 10304 10.1364/AO.439709 34807037
Shi, T., Han, M. & Chen, Y. Photonic-based analog and digital RF self-interference cancellation with high spectral efficiency. Appl. Opt. 60, 10299–10304 (2021).34807037 10.1364/AO.439709
