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Abstract
The joint radar and communication (JRC) system providing both large-capacity transmission and high-resolution ranging will play a pivotal role in the next-generation wireless networks (e.g., 6G and beyond) and defense applications. Here, we propose and experimentally demonstrate a novel photonics-assisted millimeter-wave (mm-wave) JRC system with a multi-Gbit/s data rate for communication function and centimeter-level range resolution for radar function. The key is the design of the intermediate-frequency (IF) JRC signal through the angle modulation of the linear frequency modulation (LFM) radar carrier using orthogonal frequency division multiplexing (OFDM) communication signal, inspired by the idea of constant-envelope OFDM (CE-OFDM). This IF angle-modulated waveform facilitates the broadband photonic frequency (phase)-multiplying scheme to generate mm-wave JRC signal with multiplied instantaneous bandwidth and phase modulation index for high-resolution LFM radar and noise-robust CE-OFDM communication. It is with fixed low power-to-average power ratio to render robustness against the nonlinear distortions. In proof-of-concept experiments, a 60-GHz JRC signal with an instantaneous bandwidth over 10-GHz is synthesized through a CE-LFM-OFDM signal encoded with a 2-GBaud 16-QAM OFDM signal. Consequently, a 1.5-cm range resolution of two-dimension imaging and an 8-Gbit/s data rate are achieved for both radar and communication functions, respectively. Furthermore, the proposed JRC system is able to achieve higher radar range resolution and better anti-noise communication, when using higher-order photonic frequency multiplying.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Six-generation and beyond (6G/B6G) wireless networks have been forecasted as key enablers for a number of emerging applications, including but not limited to smart cities and industries, ubiquitous wireless connections, and metaverse [1–5]. The integrated communication and sensing featuring high-quality wireless connectivity and robust high-accuracy sensing capability is expected to play an important role in future 6G/B6G networks. In particular, the joint radar and communication (JRC) systems [3–5] have attracted substantial attention for integrating data transmission and environment sensing, thanks to the advantages of spectrum and hardware sharing, low cost, and high-power efficiency. These foreseeable advantages form the intrinsic impetus for the recent rapid development of the JRC system in terms of waveform design [6], and system architecture [5], in order to simultaneously offer high-speed data transmission and high-resolution radar sensing abilities.
On the other hand, unlocking the bandwidth capabilities of millimeter-wave (mm-wave) and Terahertz-wave (THz-wave) [7–8], is one of the pillars of 5G beyond and future 6G wireless networks to accommodate high-capacity wireless transmission as well as high-accuracy sensing capability. The 28-GHz and 73-GHz frequency bands with up to 2 GHz available bandwidth have been envisioned by the 5G New Radio (NR) to provide a multi-Gbit/s wireless data rate [9]. Commercial mm-wave radars operating at 76-81 GHz show a high detection resolution for harvesting the 5-GHz available bandwidth [10]. Moreover, the phase-modulated continuous wave (PMCW)-based [11] and orthogonal frequency division modulation (OFDM)-based mm-wave JRC systems [12] centered at the 76-81 GHz frequency band are proposed and investigated through simulations. Nevertheless, experimental demonstrations of the electrical mm-wave JRC system are very recent with few published works. Concerns for practically implementing high-frequency broadband mm-wave JRC systems using traditional electronic approaches remain unresolved, which are exemplified by the narrow bandwidth, high cost, and complexity.
Microwave/mm-wave/THz-wave photonics technologies [13–21] with distinct advantages of high operating frequency, large instantaneous bandwidth, and strong immunity to electromagnetic interferences have been intensively explored to offer an alternative to the electronic method, in order to achieve high-resolution radar imaging [20] and large-capacity wireless communication [21]. Also, the photonic JRC schemes [22–30] have recently attracted much interest. Table 1 summarizes state-of-the-art photonic solutions for high-performance JRC systems. In [22], a low-cost dual-band radar and communication system is facilitated by a single photonics-assisted transceiver with an inefficient utilization of spectrum resource. Thus, in [23,24], the JRC signals encoding an amplitude shift keying (ASK) modulation communication signal onto the linear frequency modulation (LFM) radar waveform are synthesized through cascaded and paralleled optical intensity modulators. Although it can achieve high range resolution (< 1.8 cm) for radar detection, the power-domain mutual interference between LFM radar pulse and ASK communication signal highly restricts the peak-to-sidelobe ratio (PSLR) for the radar range profile (e.g., < 9.5 dB) and capacity for data communication (typically, lower than 100 Mbit/s).
Table 1. State-of-art photonic solutions for JRC systems
Subsequently, two photonics-assisted JRC systems based on the spectrum-spreading phase-coding [25] and the optoelectronic oscillator (OEO) [26] have been proposed to generate orthogonal phase-coded signals with higher PLSR (> 12 dB) and data rate (> 1 Gbit/s). However, compared to the LFM regime, the phase-coded one [27] is unfavorable for radar imaging systems demanding low complexity. In [27], a quadrature phase-shift keying (QPSK)-sliced LFM JRC waveform is generated based on a dual-polarization dual-parallel Mach-Zehnder modulator. It demonstrates high-resolution radar imaging (14.99 cm×3.25 cm), while the communication data rate is limited (∼105 Mbit/s). Accordingly, the high-spectral-efficiency OFDM technique is introduced into the photonics-assisted JRC system to improve the communication transmission capacity. For instance, an OFDM-based JRC system using the OEO technique renders a data rate as high as 6.4 Gbit/s and a radar range resolution up to 7.5 cm [28]. However, the OFDM signal suffers from a major limitation of the high power-to-average power ratio (PAPR). This issue causes a vulnerability to nonlinear distortions in the nonlinear high power amplifiers (PAs) for long-distance radar detection.
Furthermore, in [29], a photonics-assisted W-band mm-wave JRC system operating at the time-division multiplexing (TDM) mode is proposed to independently transmit LFM and QPSK signals. The achieved radar range resolution and communication data rate are 0.94 cm and 10 Gbit/s, respectively. Subsequently, the TDM-based JRC system is scaled to the THz-band [30], obtaining a net data rate of 38.1 Gbit/s and a range resolution of 1.58 cm for communication and radar detection separately. Although the TDM efforts in [29,30] impose concerns on the spectral efficiency and processing delay, they still throw inspiring light on realizing mm-wave/THz-wave JRC systems with distinguished communication capacity and radar detection resolution by reaping the unique advantages of photonics technologies.
Therefore, we propose and experimentally demonstrate a novel mm-wave photonics-assisted JRC system with multi-Gbit/s data rate (for communication) and centimeter-level range resolution (for radar). It is based on the photonic frequency-multiplying scheme to generate mm-wave constant envelope LFM-OFDM JRC signal in a co-frequency and co-time full-duplex (CCFD) mode. The constant envelope intermediate frequency (IF) JRC signal for photonic frequency-multiplying is formed from the angle modulation of the LFM radar carrier with the OFDM communication signal. Thanks to its low PAPR characteristic, such a JRC signal is insensitive to nonlinear distortions, which is significant for achieving the largest transmit power budget using nonlinear high-power amplifiers. More importantly, its phase modulation feature facilitates the photonic frequency (phase)-multiplying scheme to obtain multiplied instantaneous bandwidth and phase modulation index for high-resolution radar and high-robust communication. In experiments, based on the photonic frequency-doubling, the 60-GHz V-band JRC signal with a 10-GHz instantaneous bandwidth and 2-GBaud 16-QAM OFDM signal is generated from the IF 16-QAM LFM-OFDM JRC signal having a 5-GHz instantaneous bandwidth. This experiment achieves a communication data rate of 8 Gbit/s and two-dimension radar imaging with a range resolution below 1.5 cm. Furthermore, the scalability of our proposal towards better anti-noise communication performance and higher-resolution radar detection through higher-order photonic frequency-multiplying, such as quadrupling, is validated through simulations.
2. Principle of operation
2.1 Design of the CE-LFM-OFDM JRC signal
Inspired from [31–37], the OFDM signal of high PAPR can be transformed into a constant envelope one through angle modulation, to achieve the lowest (0 dB) PAPR characteristic. The baseband CE-OFDM signal can be written as
where A and $\phi $ are the signal amplitude and phase offset, separately. $m(t)$ denotes the real-valued OFDM signal, and h is the phase modulation index. The low-PAPR feature of CE-OFDM is highly important for radar applications using nonlinear PAs of high-power gain to support long-distance target detections [31]. Thus, we here design a constant envelope JRC signal by modulating the phase of the LFM continuous-wave (LFM-CW) signal using the real-valued OFDM signal, which can be formulated as where ${\omega _0}$ is the initiative angle frequency of the CE-LFM-OFDM signal. $k = {B / {{T_c}}}$ denotes the slope of the LFM-CW carrier, which is related to its instantaneous bandwidth B and pulse width ${T_c}$. In addition, the employment of CE-OFDM is paid by a decrease of at least 50% in the spectral efficiency compared with the traditional OFDM scheme [31,32]. Notwithstanding, this additional bandwidth expense can be largely compensated by the dramatically-improved power efficiency for its low PAPR, when nonlinear PAs of high-power gain are used for long-range radar detections. In addition, this concern can be further alleviated in the high-frequency mm-wave applications featuring a vast amount of bandwidth available.2.2 Mm-wave CE-LFM-OFDM JRC system using photonic frequency-multiplying
Figure 1 shows the schematic diagram of the proposed photonic frequency-multiplying mm-wave CE-LFM-OFDM JRC system. In the photonic mm-wave JRC signal generator, the optical carrier from a tunable laser source (TLS) is injected into the first Mach-Zehnder modulator (MZM1). MZM1 is driven by a single-tone radio frequency (RF) signal from the microwave generator and biased at the minimum transmission point (MITP) to attain a carrier-suppressed double sideband (CS-DSB) optical signal. Under the small-signal modulation assumption, the output electrical field of MZM1 can be expressed as
Fig. 1. Schematic diagram of proposed photonic frequency-multiplying CE-LFM-OFDM JRC system, of which the operation principle is illustrated by the evolutions of electrical and optical spectra at different locations. BER: bit error rate; ELPF: electrical lowpass filter; JRC: joint radar and communication; LFM-CW: linear frequency-modulated continuous-wave; LNA: low noise amplifier; LO: local oscillator; MZM: Mach-Zehnder modulator; MG: microwave generator; OFDM: orthogonal frequency division multiplexing; OSC: oscilloscope; OBPF: optical bandpass filter; OC: optical coupler; PA: power amplifier; PC: personal computer; PD: photodetector; QAM: quadrature amplitude modulation; TLS: tunable laser source; WS: waveshaper.
Then, the CS-DSB optical signal is sent to MZM2. The IF CE-LFM-OFDM JRC signal illustrated by Eq. (2) is synthesized by an arbitrary waveform generator (AWG) and applied to MZM2 to modulate the CS-DSB optical signal. Under the large-signal modulation, the optical output of MZM2 would have different-order harmonic sidebands around two input optical tones (${\omega _c} \pm {\omega _{RF}}$):
In the communication receiver, the mm-wave CE-LFM-OFDM JRC signal is received by the antenna followed by a low noise amplifier (LNA) to compensate for the propagation loss. Heterodyne coherent reception is then implemented through an mm-wave mixer along with the local oscillator (LO) signal to down-convert the mm-wave JRC signal to the IF one:
Noted that, the phase modulation index of the received JRC signal is twice that of the transmitted one from AWG due to the photonic frequency-doubling. This is beneficial for improving the signal-to-noise (SNR) of the communication function as the anti-noise performance of the analog angular modulation is highly related to the phase modulation index. Specifically, an N-fold increase in the phase modulation index leads to an SNR enhancement of $10{\log _{10}}{N^2}$ dB [35]. According to [34], the communication SNR of the JRC signal can be quantitatively expressed as follows:
where $\alpha $ denotes the photonic frequency-multiplying factor, N is the noise spectral density, ${B_{OFDM}}$ denotes the bandwidth of the OFDM baseband signal. From Eq. (9), the SNR increases with the square of the product of the frequency-multiplying factor and phase modulation index. Moreover, the SNR is inversely proportional to ${B_{OFDM}}$. Therefore, for a given SNR requirement, a large communication bandwidth (capacity) demands a large frequency-multiplying factor and phase modulation index.In the radar receiver, the echo signal reflected by the target is received by a mm-wave antenna and amplified by an LNA. The output of LNA is applied to MZM3 to modulate the reference optical signal, as shown in Fig. 1. An optical bandpass filter (OBPF) is used to select the optical components around the $+ 1st$ order sideband. Its output is
Generally, by properly designing the parameters of the transmitted JRC signal according to the detection range, ${\omega _{dechirped}}$ should be with a small value (e.g., tens of MHz) [38], imposing little impact on the initial frequency setting of the transmitted OFDM signal.
According to Eq. (11), the de-chirped radar signal is an angle modulation signal. Thus, its SNR is accordingly related to the phase modulation index as that for the communication signal. However, the phase carrying the OFDM information here is detrimental to the SNR of the de-chirped radar signal, which can be quantitatively described as
Therefore, based on Eqs. (9) and (15), an SNR trade-off exists between the radar and communication functions for our proposal, which is regulated by the phase modulation index and the bandwidth. Accordingly, for implementing the proposed JRC system, the phase modulation index and the bandwidth of the communication signal should be carefully chosen to balance the performances between the radar and communication functions.
3. Experiments and results
3.1 Generation of the mm-wave CE-LFM-OFDM JRC signal
Experiments are carried out based on the setup shown in Fig. 1. A 1564.2-nm optical carrier from the TLS is with a 12-dB power level. Then, it is injected into MZM1 having a bandwidth of 40 GHz and being biased at the MITP. A 21-GHz single-tone RF signal generated by the microwave generator drives the RF port of MZM1 to modulate the optical carrier in the CS-DSB modulation mode. As can be seen in Fig. 2 (blue line), the obtained CS-DSB optical signal can achieve a carrier suppression ratio of 29 dB.
The off-line generated IF CE-LFM-OFDM JRC signal centered at 9 GHz is loaded into the AWG for digital-to-analog conversion and applied to the RF port of MZM2 through an electrical amplifier. The measured optical spectrum at the output of MZM2 is shown in Fig. 2 (red line), indicating a carrier suppression ratio over 18-dB. After being boosted by the erbium-doped fiber amplifier (EDFA), a WS working at the bandstop filtering mode is leveraged to eliminate the unwanted sidebands and two of them spaced by around 47 GHz remains, indicated by the blue line in Fig. 3. Finally, the output of WS is sent into a 70-GHz PD for generating the 60-GHz mm-wave CE-LFM-OFDM JRC signal through the heterodyne beating between the two selected optical sidebands.
The key parameters for the transmitted IF CE-LFM-OFDM JRC signal are summarized in Table 2. Figure 4 shows the obtained temporal waveforms of traditional OFDM and the proposed CE-LFM-OFDM signals captured at the output of AWG. The OFDM signal exhibits a high PAPR [see Fig. 4(a)], while the CE-LFM-OFDM signal has a desirable constant envelope property for combating nonlinear distortions [see Fig. 4(b)].
Fig. 4. Temporal waveforms of (a) traditional OFDM and (b) the proposed CE-LFM-OFDM signals captured at the output of AWG.
Table 2. Parameters for generating IF CE-LFM-OFDM JRC signal
When the 9-GHz CE-LFM-OFDM JRC signal from the AWG is applied to the photonic mm-wave signal generator, the 60-GHz CE-LFM-OFDM JRC signal is generated. Its electrical spectrum is measured by a spectrum analyzer with an ultra-wideband frequency measurement range from 9 kHz to 90 GHz. The attained result is shown in Fig. 5(a). Figure 5(b) presents its corresponding time-frequency characteristic by calculating the spectrogram of the temporal waveform of its 8.4-GHz down-converted copy, demonstrating a 10-GHz instantaneous bandwidth from 55 GHz to 65 GHz. Noted that, this generated mm-wave JRC signal is able to achieve a dual-functional system in a CCFD mode, where the temporal, spectral, and signaling resources are utilized in a shared manner between radar and communication. Afterwards, this mm-wave JRC signal is emitted into the free space through the mm-wave PA and horn antenna operating at 50-75 GHz. For radar function demonstrations, its reflections from targets are gathered by a radar receiver antenna. While for communication functions showcase, its replicas after free-space propagation are captured by a communication receiver antenna. In addition, limited by available optoelectronic devices in the laboratory, the experimental demonstrations of radar and communication functions are separately performed.
3.2 Demonstrations of wireless communication function
For wireless communication function demonstration, a 1.2-m 60-GHz wireless transmission link is established by a pair of mm-wave antennas, a transmit PA and a receive LNA, as shown in Fig. 6(a). The wireless 60-GHz JRC signal captured by the communication receiver antenna is amplified by a LNA to obtain a power gain of 38 dB. Then, it is down-converted to an 8.4-GHz IF one using a 51.6-GHz LO signal from the electrical frequency-quadrupling of the 12.9-GHz single-tone microwave signal. The down-converted IF JRC signal is finally digitized by a real-time OSC running at a sample rate of 40 GSa/s. Further off-line DSPs are then performed to evaluate the performance of the communication system, which include coherent down-conversion using the LFM carrier, phase demodulation and OFDM demodulation.
When the bandwidth of the baseband OFDM is 3 GHz, Fig. 7(a) gives the spectrum of the down-converted IF JRC signal centered at 8.5-GHz and with a 10-GHz instantaneous bandwidth. After the coherent down-conversion using the LFM carrier and phase demodulation, the 4-QAM communication OFDM baseband signal of 3-GHz bandwidth is successfully obtained, of which the spectrum is shown in Fig. 7(b). Accordingly, a clear and separated constellation diagram for the demodulated signal can be observed in Fig. 7(c), demonstrating a 16.8% error-vector-magnitude (EVM) satisfying the 17.5% threshold specified by the 3rd Generation Partnership Project (3GPP) for 4-QAM modulation. Then, the OFDM baseband signal is varied to the 16-QAM one with a 2-GHz bandwidth. Figures 7(d) and (e) present the corresponding spectra before and after coherent down-conversion and phase modulation, respectively. In this case, the OFDM communication signal is also recovered. From the obtained constellation diagram of demodulated signal shown in Fig. 7(f), a 12.1% EVM is achieved, satisfying the specified 12.5% requirement in 3GPP for 16-QAM modulation.
Fig. 7. 4-QAM OFDM case: measured spectra (a) before and (b) after phase information recovery; (c) the corresponding constellation diagram of demodulated signal. 16-QAM OFDM case: measured spectra (d) before and (e) after phase information recovery; (f) corresponding constellation diagram of the demodulated signal.
Then, we evaluate the communication BERs of the system versus different received optical power for the transmitted CE-LFM-OFDM signal with a phase modulation index ranging from 0.1 to 0.6. As can be seen in Fig. 8, when the phase modulation index increases from 0.1 to 0.35, the received optical power margin to keep a BER below the 7% pre-forward error correction (pre-FEC) threshold of $3.8 \times {10^{ - 3}}$ is improved by about 5 dB. Within the range of 0.1-0.5, increasing the phase modulation index is beneficial for the BER reduction or anti-noise performance enhancement. However, when phase modulation index exceeds the threshold of 0.5, nonlinear distortion rises during phase demodulation such that the BER increases in the CE-LFM-OFDM JRC system. It is revealed that the value of phase modulation index should be carefully specified to achieve optimal communication performance.
Fig. 8. Measured BERs under different received optical power for the phase modulation index ranging from 0.1 to 0.6.
It is worth noting that, the phase modulation index of the received CE-LFM-OFDM signal is twice that of the transmitted signal on account of the photonic frequency (phase) doubling. As the bandwidth of CE-OFDM is governed by the phase modulation index [31]: ${B_{CE - OFDM}}\textrm{ = }\max (2\pi h,1) \cdot {B_{OFDM}}$, the doubled phase modulation index in our system has an attractive trait that relieves the bandwidth requirement of the transmitter when large phase modulation index causing spectral spreading is demanded to combat noisy environments.
Also, an additional experiment is carried out to validate the effectiveness of the photonic frequency-doubling scheme for improving SNR by doubling the phase modulation index. In this case, MZM2 is biased at the normal quadrature point for achieving the fundamental-frequency CE-LFM-OFDM signal at the receiver. Thus, when the phase modulation index of the transmitted 4-QAM CE-LFM-OFDM IF signal is set as 0.5, Fig. 9 presents the measured EVM as a function of received optical power for the frequency-doubling and fundamental frequency cases. From Eq. (9), the frequency-doubling method allows the optical sensitivity to be ameliorated by over 6.5 dB, in comparison to the fundamental-frequency scheme. Such an advantage is brought by the doubled phase modulation index for SNR enhancement.
Fig. 9. Measured EVMs as a function of the received optical power for the cases of frequency-doubling and fundamental frequency schemes.
3.3 Demonstrations of radar function
For radar function demonstrations, the transmitted mm-wave JRC signal is radiated into free space by the PA and radar receiver antenna to detect the targets ∼2 m away from them, as shown in Fig. 6(b). Then, the echo signals reflected by the targets are gathered by a receive antenna adjacent to the transmitted one, and applied to MZM3 having a wideband bandwidth of 65 GHz. An EDFA is inserted between MZM3 and OBPF for optical amplification. An OBPF is used to select the optical components around the +1st order sideband. The measured optical spectrum of output of OBPF is shown in Fig. 3 (red line).
Eventually, a low-speed PD is utilized to implement radar de-chirping process through the coherent heterodyne beating between the optical carriers carrying reference and reflected (delayed) LFM signals. After low-pass filtering, the de-chirped radar signal from the low-speed PD is digitized by an OSC with a sampling rate of 250 MSa/s. Two metal reflectors are used to emulate the targets of interest for radar range demonstrations, as shown in Fig. 10(a). The sizes of two metal reflectors are all 2 cm ${\times} $ 3 cm, and the distance between them is around 1.5 cm. As the range results shown in Fig. 10(b), the measured de-chirped frequency tones for two reflectors are 28.39 MHz and 28.60 MHz, respectively, corresponding to the derived distances of 2.129 m and 2.145 m. Therefore, the detected distance between two targets is 1.58 cm, which is consistent with the theoretical 1.5-cm range resolution for the JRC signal having a 10-GHz instantaneous bandwidth.
Fig. 10. (a) Photograph of two metal targets for radar detection; (b) obtained radar ranging results of two reflectors separated at 1.5 cm.
Then, experiments concerning the turntable inverse synthetic aperture radar (ISAR) imaging are implemented to further highlight the superiority of the proposed JRC system. As displayed in Fig. 11(a), an electric fan with three silver-paper-packed blades is utilized to emulate the moving targets. The size of each blade is about 20 cm ${\times} $ 5 cm and the speed of rotation of the fan is $2\pi $ rad/s. The LFM radar detection signal is with a duration of 10 ms consisting of 2000 pulses. Figures 11(b)-(d) show two-dimensional imaging results at different radar imaging frames. It can be seen from these figures that the profile of the electric fan can be clearly distinguished. Noted that, several pieces of wave-absorbing material are used in our experiments to eliminate the strong background reflections, see Fig. 6(b). The incomplete wave-absorption at the joint between the upper and down pieces leads to a transverse spot line in the middle of ISAR imaging results shown in Figs. 11(b)-(d).
Fig. 11. (a) Photograph of the electric and its three blades packed with silver paper; (b)-(d): obtained ISAR imaging results at different imaging frames.
4. Discussions
4.1 Performance trade-off between radar and communication functions
According to Eqs. (9) and (15), the SNRs for the communication and radar functions are proportional and in inverse proportion to the phase modulation index and the bandwidth of the original OFDM. Thus, performance trade-off can be uncovered for the communication and radar communication in our system.
First, Fig. 12 shows the measured SNR of the de-chirped radar signal and the EVM of the demodulated OFDM signal, with a fixed 3-GHz bandwidth of OFDM signal and variable phase modulation indexes of transmitted CE-LFM-OFDM signal ranging from 0.1 to 0.7. As shown in Fig. 12 (blue line), the radar’s SNR decreases with the increase of phase modulation index, which is consistent with the theoretical result revealed by Eq. (15). The communication performance indicated by EVM can be gradually improved by increasing the phase modulation index within the range of 0-0.5, but then degraded cross the threshold of 0.5, being consistent with the results shown in Fig. 8. Therefore, the suggested value of phase modulation index for our system should be 0.5, which is able to achieve an excellent communication performance at a comparatively low cost of radar performance degradation. Moreover, the constellations of the demodulated 4-QAM OFDM signal and radar ISAR imaging results for different the phase modulation indexes are shown in Fig. 13, showing that the phase modulation index significantly affects the communication BER and radar imaging performances.
Fig. 12. Measured SNRs of the de-chirped radar signal and EVMs of the demodulated OFDM signal under different phase modulation indexes of CE-LFM-OFDM signal.
Fig. 13. Constellations of the demodulated 4-QAM OFDM signal for a phase modulation index of (a) PMI = 0.2 or (b) PMI = 0.6. ISAR images for a phase modulation index of (c) PMI = 0.2 or (d) PMI = 0.6.
Then, by keeping the phase modulation index unchanged as 0.5, Fig. 14 shows the SNR of the de-chirped radar signal and the EVM of the demodulated communication signal as a function of the bandwidth of transmit OFDM baseband signal. Revealed by Fig. 14, the SNR for two functions degrades with the increase of bandwidth, which is in accordance with Eqs. (9) and (15). To meet the 17.5% EVM requirement for the 4-QAM OFDM signal, the bandwidth of the transmitted OFDM baseband signal should be below 3 GHz. As demonstrated in Fig. 15, a broad bandwidth (e.g., 4 GHz) for the transmitted OFDM baseband signal brings degradations not only to the communication reliability but also to the radar imaging quality.
Fig. 14. SNRs of de-chirped radar signal and EVMs of demodulated communication signal for different bandwidths of the transmitted OFDM baseband signal.
Fig. 15. Constellations of the demodulated 4-QAM OFDM signal with a bandwidth of (a) B = 1 GHz and (b) B = 4 GHz. radar ISAR images for the 4-QAM OFDM signal with a bandwidth of (c) B = 1 GHz and (d) B = 4 GHz.
4.2 Simulations for higher-order photonic frequency multiplying
To further verify the scalability of our proposal to higher radar range resolution and better anti-noise communication performance, the mm-wave JRC system based on a high-order photonic frequency-multiplying such as the frequency-quadrupling is investigated based on a commercially available optical system simulation platform, VPItransmissionMaker. The basic structure of the investigated system is similar to Fig. 1, except that MZM2 is replaced by a dual-parallel MZM (DPMZM). The biasing voltages of the DPMZM are judiciously adjusted to achieve the carrier-suppressed optical signal with only even-order sidebands [38]. Consequently, a mm-wave CE-LFM-OFDM signal with quadrupled frequency/instantaneous bandwidth/phase modulation index is generated, which can be written as
This is significant for achieving a high-frequency JRC signal with a wide instantaneous bandwidth (for higher radar range resolution) and large phase modulation index (for better communication anti-noise performance) with low-speed electrical components.
In simulations, the IF 64-QAM CE-LFM-OFDM JRC signal with 5-GHz radar instantaneous bandwidth and OFDM communication bandwidth is applied to the photonic frequency-quadrupling system for mm-wave JRC signal generation. When its phase modulation index is set as 0.5, the electrical spectrum and spectrogram of the obtained mm-wave JRC signal around 60 GHz are shown in Figs. 16(a) and (b), demonstrating a 20-GHz instantaneous bandwidth from 50 GHz to 70 GHz. Using this generated JRC signal, communication and radar functionalities are subsequently validated through simulations. For communication function, the benefits of higher-order photonics-assisted frequency-multiplying particularly in terms of anti-noise performance are highlighted by a comparison to the frequency-doubling case discussed above. Accordingly, Fig. 17(a) gives the demodulated EVMs as a function of the received optical power for frequency-quadrupling and -doubling cases. Obviously, the frequency-quadrupling scheme outperforms by a 4-dB enhancement in the optical receiving sensitivity for meeting the 8% limit for 64-QAM modulation in 3GPP.
Fig. 16. Obtained (a) spectrum and (b) spectrogram of the generated 60-GHz JRC signal for the frequency-quadrupling system.
Fig. 17. (a) Measured EVMs as a function of the received optical power for the frequency-quadrupling and -doubling systems; (b) measured radar range profile for two simulated objects.
For simulating radar function, the mm-wave JRC signal with an instantaneous bandwidth of 20 GHz is delayed by 20 ns and 20.05 ns to emulate the reflections from two objects at 3 m and 3.0075 m, respectively. After the photonic de-chirped process, the radar de-chirped frequency tones for two objects are measured as 80 MHz and 80.21 MHz shown in Fig. 17(b), indicating ranging the distances of 3 m and 3.0079 m, respectively. Consequently, the sensed distance between two objects is about 7.9-mm, which is consistent with the theoretical 7.5-mm range resolution for the JRC signal of 20-GHz instantaneous bandwidth.
4.3 Comparison between our proposed scheme and previous JRC systems
Moreover, the detailed comparison of key indicators between our proposal and previously reported (electronics and photonics) JRC systems are shown in Table 3. Here, only the cases that the radar and communication functions are integrated through CCFD mode are listed for comparison. The FDM [22] and TDM [29,30] enabled ones are not considered here. From Table 3, one may find that our proposal has obvious advantages in both radar range resolution and communication data rate.
Table 3. Comparison between our proposal and different reported JRC systems with a CCFD mode
5. Conclusion
We have proposed and experimentally validated a high spectrum efficiency mm-wave JRC system with multi-Gbit/s data rate and centimeter-level resolution detection based on the photonic frequency-multiplying CE-LFM-OFDM technique. In our proposal, the constant envelope JRC signal with a fixed low PAPR trait is designed by the angle modulation of the LFM carrier using OFDM communication signal. With the assistance of photonic frequency-multiplying system, a mm-wave CE-LFM-OFDM JRC signal with multiplying frequency/phase/ instantaneous bandwidth is generated to facilitate the high-resolution LFM-based radar and noise-robust CE-OFDM communication. Proof-of-concept experiments based on the photonic frequency-doubling are performed and a 60-GHz V-band JRC signal with a 10-GHz instantaneous bandwidth is generated. Validated by experimental results, the proposed system can achieve a 1.5-cm range resolution of two-dimension imaging and an 8-Gbit/s data rate. This proposed method is expected to offer a viable photonics-assisted alternative for achieving spectrum and hardware efficient mm-wave JRC system with extreme high radar detection resolution and communication capacity and thus accommodate the visions of future 6G and beyond wireless networks.
Funding
National Key Research and Development Program of China (2019YFB2203204); National Natural Science Foundation of China (61922069, U21A20507, 6200140); Sichuan Science and Technology Program (2022JDTD0013); Fundamental Research Funds for the Central Universities (2682021CX045).
Acknowledgments
The authors would like to thank the anonymous reviewers for their valuable comments that help improve this paper.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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