Microwave-assisted photon terahertz frequency offset compensation method

Abstract

The invention discloses a microwave-assisted terahertz frequency offset compensation method, and aims to solve the problem of signal distortion caused by frequency offset in terahertz communication. According to the method, a microwave signal is introduced as an auxiliary reference to realize real-time estimation and compensation of terahertz emission signal frequency offset. The specific process comprises the steps of injection of a microwave reference signal, spectrum analysis of a receiving end, offset extraction and frequency correction of a baseband signal. Compared with a traditional frequency offset compensation scheme, the method can effectively improve the precision and stability of system frequency synchronization, reduce the hardware complexity and enhance the anti-interference capability of a communication system, has a good application prospect, and is particularly suitable for a terahertz wireless communication system with high frequency width and high rate.

Description

Technical Field

[0001] The present invention relates to the fields of optical terahertz technology, communication technology, and radar sensing technology, and specifically to a microwave-assisted photonic terahertz frequency deviation compensation method. The method is applicable to high-bandwidth, high-speed terahertz wireless communication systems and aims to solve the signal distortion problem caused by frequency drift in dual-laser heterodyne beat frequency systems. Background Art

[0002] With the rapid development of intelligent connected industries such as smart transportation, industrial interconnection, and drone swarms, 6G communication systems urgently need to achieve ultra-high-speed, ultra-low-latency communication performance to reduce costs and increase efficiency. Terahertz communication, due to its ultra-large bandwidth and ultra-high frequency characteristics, has become an important development direction for 6G ultra-high-speed mobile communications. Due to the electronic bottleneck of electronic devices, all-electronic terahertz communication systems have problems with limited signal frequency and bandwidth; while photonic terahertz communication systems generate, process, and receive terahertz signals through photonic technology, and have superior characteristics such as high signal frequency, large bandwidth, and flexible frequency and bandwidth tunability. One of the core challenges is that in a free-running dual-laser heterodyne beat frequency system, due to the frequency drift and jitter of the two independent lasers, the terahertz signal generated by the beat frequency has significant frequency jitter and spectrum broadening, which limits the application of the system in large-capacity communications or high-precision measurements, and requires compensation for the jitter of the terahertz frequency. There are currently four common methods of frequency offset compensation. The first is the optical frequency comb approach. While this approach can significantly reduce terahertz frequency deviation by leveraging the relative frequency locking between optical frequency comb components, the signal-to-noise ratio (SNR) of its higher-order comb components is significantly degraded. Extracting the higher-order comb signals through optical filtering and then amplifying them with an erbium-doped fiber amplifier (EDFA) significantly increases system complexity and noise accumulation. The second, pilot-assisted approach, inserts a dedicated reference signal, which effectively compensates for terahertz frequency deviation. However, the additional spectrum resources used lead to reduced spectrum utilization, effective power loss, and an increased peak-to-average power ratio (PAPR), which induces nonlinear distortion in the photodetector and power amplifier. The third, envelope detection, while simplifying hardware, suffers from low SNR and receiver sensitivity, as well as difficulty supporting ultra-wideband terahertz communications. Furthermore, signal-to-signal beat frequency crosstalk (SSBI) generated by signal self-mixing degrades signal quality. Although the last purely digital domain compensation algorithm can partially compensate for phase noise and frequency offset, its multidimensional parameter optimization process leads to an exponential increase in computational complexity and requires the configuration of high-precision digital-to-analog conversion / analog-to-digital conversion (ADC / DAC) modules, making it difficult for system integration and energy efficiency indicators to meet the needs of low-cost large-scale networking.

[0003] In order to solve the above problems and achieve efficient frequency offset compensation, the present invention proposes a microwave-assisted photonic terahertz frequency offset compensation method. In a dual-laser heterodyne beat frequency architecture, the optical carrier generated by the laser is divided into two paths through a beam splitter: the first path is loaded with the signal to be transmitted through an electro-optical modulator, and the second path is generated through an electro-optical modulator or a nonlinear optoelectronic device. An optical frequency comb covering a wide-band spectrum is generated. The local oscillator laser (OLO) generated by another laser is also divided into two paths through a beam splitter: the first path is heterodyne beat with the optical modulated signal to form a terahertz modulated signal, and the second path will be used to generate a microwave pilot signal. Through a tunable filter, the comb tooth component closest to the OLO frequency is accurately extracted from the optical frequency comb, and then heterodyne beat is performed with the second OLO to generate a microwave pilot signal. Through the above-mentioned photonic architecture, efficient generation of terahertz signals and frequency locking with microwave pilot signals can be achieved. This invention uses a free-running laser heterodyne beat frequency to generate terahertz signals, which has the characteristics of simple structure and high SNR; it uses high-order comb signals and OLO beat frequency to generate microwave pilot signals, which solves the problems of low SNR and power of high-order signals of optical frequency comb; with the help of existing mature microwave communication technology, the microwave pilot signal and the terahertz communication signal are transmitted separately to solve the frequency deviation problem of the photonic terahertz system, so that there is no need to insert a pilot signal in the communication signal, thereby improving the effective power and spectrum efficiency of the terahertz signal. Summary of the Invention

[0004] The purpose of the present invention is to provide a microwave-assisted photonic terahertz frequency deviation compensation method, which generates a microwave pilot signal through an optical frequency comb, and transmits and processes it separately from the terahertz communication signal and jointly, thereby eliminating homologous frequency jitter and improving the frequency synchronization accuracy and anti-interference capability of the communication system.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A microwave-assisted photon terahertz frequency deviation compensation method comprises the following steps:

[0007] 1.1. The pure optical carrier generated by the first laser is split into two paths through the first optical beam splitter. The first path is loaded with the signal to be transmitted through the electro-optical modulator, and the second path is generated by the optical frequency comb module.

[0008] 1.2. Split the local oscillator light generated by the second laser into two paths via a second optical beam splitter. The first path is heterodyned with the optical carrier modulated in step 1.1 to generate a terahertz communication signal. The second path is heterodyned with the comb component closest to the local oscillator frequency selected from the optical frequency comb via a tunable filter to generate a microwave pilot signal.

[0009] 1.3. Amplifying the terahertz communication signal and the microwave pilot signal by a first terahertz low-noise amplifier and a first intermediate-frequency low-noise amplifier, respectively, and then transmitting them separately by the terahertz transmitting antenna and the microwave transmitting antenna;

[0010] 1.4. Receive a terahertz communication signal and a microwave pilot signal at a receiving end. The received terahertz communication signal and the local oscillator terahertz signal are coherently mixed and down-converted to an intermediate frequency signal in a first terahertz mixer.

[0011] 1.5. After the received microwave pilot signal is calibrated by the second delay compensation module, it is mixed with the intermediate frequency signal obtained in step 1.4 in the first intermediate frequency mixer to offset the same frequency jitter of the two signals and complete frequency offset compensation;

[0012] 1.6. The compensated baseband signal is sequentially filtered by a first electrical low-pass filter, amplified by a first baseband low-noise amplifier, converted by a first analog-to-digital converter, and processed by a communication digital signal processing unit to output digital communication information.

[0013] Preferably, the electro-optical modulator in step 1.1 is used to modulate the digital baseband signal onto a pure optical carrier, and the modulated optical carrier is amplified by an erbium-doped fiber amplifier and then coupled with the local oscillator light to beat the frequency.

[0014] Preferably, the optical frequency comb module in step 1.2 generates a broadband optical frequency comb by a phase modulator under the action of an electrical driving signal, and extracts the comb tooth component closest to the local oscillator optical frequency through an optical bandpass filter.

[0015] Preferably, the frequency of the microwave pilot signal in step 1.2 is the frequency difference between the comb component and the local oscillator light, and the pilot signal frequency is in the Sub-6 GHz frequency band.

[0016] Preferably, in step 1.4, the first terahertz mixer adopts a coherent detection method of a single-row carrier photodiode and a common photodiode combined with local oscillator light to eliminate frequency offset and improve receiving sensitivity.

[0017] Preferably, in step 1.5, the first intermediate frequency mixer offsets the homologous frequency jitter of the terahertz communication signal and the microwave pilot signal through analog mixing, thereby retaining the effective information item of the baseband signal.

[0018] Preferably, in step 1.6, the first electrical low-pass filter is used to filter out the image component and DC noise after mixing, and the communication digital signal processing unit is further used to eliminate residual interference.

[0019] Preferably, the transmitting end and the receiving end are both equipped with a first delay compensation module, a second delay compensation module and a third delay compensation module for matching the transmission path delay of the terahertz communication signal and the microwave pilot signal.

[0020] Preferably, the receiving end includes a communication receiving end and a perception receiving end, which independently complete frequency offset compensation through a first intermediate frequency mixer and a second intermediate frequency mixer respectively, and output communication and perception baseband signals.

[0021] Preferably, the digital signal processing units at the transmitting end and the receiving end are used for pre-distortion, equalization and post-processing of the signal to adapt to the requirements of the high-speed terahertz communication system.

[0022] Beneficial effects:

[0023] The present invention proposes a microwave-assisted photonic terahertz frequency offset compensation method suitable for future mobile networks with high-capacity communications and high-precision sensing. At the transmitting end, a pure optical carrier and an optical optical lock (OLO) are each divided into two channels. One channel of the pure optical carrier is modulated and heterodyned with the other channel of the OLO to generate a terahertz communication signal. Simultaneously, the other channel of the pure optical carrier generates an optical frequency comb, and optical filtering is used to select frequency components close to the OLO frequency. These components are heterodyned with the other channel of the OLO to generate a microwave pilot signal. This method eliminates the need to insert the microwave pilot signal into the terahertz communication signal to establish frequency lock between the microwave pilot signal and the terahertz communication signal, thereby ensuring the spectrum utilization and power efficiency of the terahertz communication signal. At the receiving end, the terahertz communication signal and the microwave pilot signal are received separately. The terahertz local oscillator signal at the receiving end (including the sensing receiving end and the communication receiving end) is coherently mixed with the received terahertz communication signal to achieve down-conversion of the terahertz communication signal. The use of coherent mixing can improve the receiving sensitivity and bandwidth of the terahertz communication signal. The microwave pilot signal is then mixed with the down-converted terahertz communication signal. Mixing methods include analog and digital mixing. Because the two signals have the same frequency offset, this process can compensate for the frequency offset of the terahertz communication signal. By transmitting and processing the pilot signal and the terahertz communication signal separately, the pilot signal frequency can be controlled to sub-6 GHz as needed, maintaining compatibility with currently available microwave communication resources and ensuring the signal-to-noise ratio of the microwave pilot signal.

[0024] The present invention has at least the following advantages:

[0025] (1) Advantages compared with the optical frequency comb terahertz signal generation method:

[0026] By utilizing the coherent beat frequency of the higher-power local oscillator light and the lower-power high-order components of the optical frequency comb, the intensity of the coherent light components is increased, solving the problem of low SNR of the high-order optical frequency comb components.

[0027] (2) Advantages compared to the frequency offset estimation technique using inserted pilots:

[0028] This solution combines local oscillator light with different optical frequency comb components to generate two signals: a microwave pilot signal and a terahertz communication signal. These two signals are transmitted separately, eliminating the need to insert a pilot signal into the modulated terahertz communication signal. This eliminates the need for additional spectrum resources for communication and improves spectrum efficiency. It also reduces the peak-to-average power ratio (PAPR), easing the requirements for the UTC-PD and the linear amplification range of amplifier components.

[0029] (3) Advantages compared with envelope detection technology:

[0030] This solution uses an active receiver to reduce the required received signal power while overcoming the low signal-to-noise ratio, low receiver sensitivity, and narrow operating bandwidth inherent in passive envelope detection. Furthermore, it avoids signal-to-signal beat frequency crosstalk (SSBI) caused by self-mixing, which can degrade signal quality.

[0031] (4) Advantages compared with the frequency offset compensation method of digital signal processing:

[0032] This solution is primarily an analog system that offsets frequency jitter by mixing microwave signals with down-converted terahertz signals. Compared to digital signal processing compensation methods, this approach reduces system complexity and signal processing power consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a schematic diagram of the frequency offset compensation architecture for terahertz communication according to the present invention;

[0034] Figure 2 This is a schematic diagram of the optical spectrum and electrical spectrum of different nodes in the frequency offset compensation architecture of terahertz communication of the present invention. DETAILED DESCRIPTION

[0035] The present invention will be further explained below with reference to the embodiments and accompanying drawings.

[0036] The present invention provides a frequency offset compensation architecture for terahertz communication of 6G, such as Figure 1As shown. The system mainly includes a signal transmitter, a communication receiver and a radar receiver. Among them, the signal transmitter includes: two lasers (LD and LO), four optical couplers (OC1, OC2, OC3, OC4), an electro-optical modulator (Mod), a phase modulator (PM), a digital-to-analog converter (DAC), a transmitter digital signal processing unit (Tx.DSP), an erbium-doped fiber amplifier module (EDFA), an optical bandpass filter (OBPF), a delay compensation module (Delay1), a single-line carrier photodiode (UTC-PD), an ordinary photodiode (PIN-PD), an intermediate frequency low noise amplifier (IF-LNA1), a terahertz low noise amplifier (THz-LNA1), and a terahertz antenna. (THz-Ant.1) and a microwave antenna (IF-Ant.1); the communication receiving end includes: a terahertz antenna (THz-Ant.2) and a microwave antenna (IF-Ant.2), a terahertz low-noise amplifier (THz-LNA2), an intermediate-frequency low-noise amplifier (IF-LNA2), a delay compensation module (Delay2), a baseband low-noise amplifier (BB-LNA1), a terahertz mixer (THz-mixer1), an intermediate-frequency mixer (IF-mixer1), an electrical low-pass filter (ELPF1), an analog-to-digital converter (ADC1) and a communication receiving end digital signal processing unit (Com.DSP). The sensing receiver includes a terahertz antenna (THz-Ant.3), a terahertz low-noise amplifier (THz-LNA3), a delay compensation module (Delay3), a terahertz mixer (THz-mixer2), a mixing module (IF-mixer2), an electrical low-pass filter (ELPF2), an analog-to-digital converter (ADC2), and a sensing receiver digital signal processing unit (Sen.DSP).

[0037] At the signal transmitter, the local oscillator light (OLO) transmitted by LD2 is split into two paths (OLO1, OLO2) via an optical coupler (OC2). Simultaneously, the clean optical carrier (OCA) transmitted by LD1 is split into two paths (OCA1, OCA2) via an optical coupler (OC1). One clean optical carrier (OCA1) is injected into the optical input port of the electro-optical modulator (Mod). Simultaneously, the communication signal, which has undergone digital signal processing (Tx.DSP) and digital-to-analog conversion (DAC) at the transmitter, is injected into the RF input port of the electro-optical modulator (Mod), modulating the clean optical carrier (OCA1) to generate modulated signal light (OMS). Figure 2As shown in (a). Subsequently, the signal light (OMS) amplified by the erbium-doped fiber amplifier (EDFA) and one of the local oscillator lights are coupled through an optical coupler (OC3). They are injected into the single-carrier photodiode (UTC-PD) and a terahertz communication signal is generated through heterodyne beat frequency, as shown in Figure 1. Figure 2 As shown in (b), ΔF1 is in the THz range. After power compensation by the terahertz low-noise amplifier (THz-LNA1), the signal is transmitted through THz-Ant.1. Meanwhile, another optical carrier (OCA2) generates an optical frequency comb through a phase modulator (PM) under the action of an electrical drive signal (LO1). The optical bandpass filter (OBPF) selects frequency components close to the local oscillator frequency (OP, the purpose of this step is to generate a reference signal in the microwave band), as shown in Figure 1. Figure 2 This component is delayed by the delay module (Delay1) and coupled with another local oscillator light through the optical coupler (OC4), and beats the ordinary photodiode (PIN-PD) to generate a microwave signal, as shown in (c). Figure 2 As shown in (d), ΔF2 is in the GHz range. Although the SNR of the high-order optical comb (OP) is low, heterodyning the OLO2 can generate a microwave pilot signal with a higher SNR. After power compensation by the intermediate frequency low-noise amplifier (IF-LNA1), it is transmitted through IF-Ant.1.

[0038] The signals transmitted by THz-Ant.1 and IF-Ant.1 have the same frequency jitter, which can be mathematically derived as follows. The electric field of the pure optical carrier (OCA1, OCA2) after being equally divided by OC1 is: E1=A1exp[j2π(f OCA +Δf1)t+jφ1]; the electric field of the local oscillator light (OLO1, OLO2) after being evenly divided by OC2 is: E2=A2exp[j2π(f OLO +Δf2)t+jφ2]; where Δf1 and Δf2 are the frequency jitters of OCA and OLO respectively, which are independent of each other. After modulation, coupling and beat frequency, the terahertz communication signal ( Figure 1 (1) Signal) is:

[0039] E3=A3s(t)·cos(2πf THz t+2πΔft+Δφ1)where f THz =f OCA -f OLO , Δf=Δf1-Δf2, Δφ1=φ1-φ2, s(t) is the modulation signal output by the digital-to-analog converter. The microwave pilot signal ( Figure 1 (2) signal) is (assuming the delay of the two signals is matched):

[0040] E4=A4cos(2πf P t+2πΔft+Δφ2)

[0041] E3 and E4 have the same frequency jitter Δf = Δf2 - Δf1, where Δφ2 = φ1 - φ2. The pilot signal frequency is the difference between the high-frequency optical frequency comb component (OP) and the local oscillator (OLO) frequency: f P =f OP -f OLO .

[0042] In the communication receiving module, the terahertz local oscillator signal (THz-LO1) is generated by a 12-fold frequency mixer and an electrical drive signal LO2. THz-Ant.2 receives the terahertz communication signal sent by THz-Ant.1, amplifies it through a terahertz low-noise amplifier (THz-LNA2), and then mixes it with the terahertz local oscillator signal (THz-LO1) in a terahertz mixer (THz-mixer1) to generate a down-converted intermediate frequency terahertz communication signal (IF1). The mathematical representation is as follows:

[0043]

[0044] where f IF1 =f THz -f THz-LO1 , Δφ3 is the phase difference between the received terahertz communication signal and the terahertz local oscillator signal

[0045] IF-Ant.2 receives the microwave signal sent by IF-Ant.1, which is first compensated for power by the intermediate frequency low noise amplifier (IF-LNA2). After being compensated by the delay compensation module (Delay2), it is mixed with the down-converted intermediate frequency communication signal (IF1) in the intermediate frequency mixer (IF-mixer1). The mathematical expression is as follows:

[0046] E6=A6s(t)·cos[2πf BB t+2π(Δf-Δf)t+Δφ4]=A6s(t)·cos(2πf BB t+Δφ4)

[0047] where f BB is the frequency difference between the intermediate frequency signal (IF1) after the terahertz communication signal is down-converted and the microwave pilot signal, and Δφ4 is the phase difference between the intermediate frequency signal (IF1) after the terahertz communication signal is down-converted and the microwave pilot signal.

[0048] Since the frequency of the terahertz communication signal and the frequency of the microwave pilot are locked in advance at the transmitting end, after intermediate frequency mixing, the frequency deviations of the two cancel each other out, and a frequency-stable baseband communication signal (BB1) is obtained, such as Figure 2In addition, the low transmission loss of microwave signals can ensure the reception of pilot signals with high SNR, thereby ensuring the accuracy and robustness of frequency offset compensation.

[0049] The signal is then filtered out by an electrical low-pass filter (ELPF1) to remove unwanted components. After amplification by a baseband low-noise amplifier (BB-LNA1), it enters the analog-to-digital converter (ADC1) to generate a frequency-stable digital communication signal. Finally, the digital signal processing module (Com.DSP) at the receiving end recovers the communication information to be transmitted.

[0050] In the sensing receiver module, THz-Ant.3 receives the reflected terahertz sensing signal, which is first amplified by a terahertz low-noise amplifier (THz-LNA3) and then mixed with the terahertz local oscillator signal (THz-LO2) in a terahertz mixer (THz-mixer2) to generate a down-converted intermediate-frequency terahertz sensing signal (IF2). The microwave pilot signal from the signal transmitter is calibrated by a delay compensation module (Delay3) and then mixed with the down-converted intermediate-frequency sensing signal (IF2) in an intermediate-frequency mixer (IF-mixer2) to generate a frequency-stabilized baseband sensing signal (BB2).

[0051] Unwanted components are then filtered out by an electrical low-pass filter (ELPF2). After amplification by a baseband low-noise amplifier (BB-LNA2), the signal enters the analog-to-digital converter (ADC2) to generate a frequency-stable digital sensing signal. Finally, the digital signal processing module (Sensor DSP) at the sensing receiver recovers the desired communication information. The mathematical expression for frequency compensation at the sensing receiver is similar to that at the communication receiver and will not be further explained.

Claims

1. A microwave-assisted photon terahertz frequency offset compensation method, characterized by comprising the following steps: 1.

1. The pure optical carrier generated by the first laser is split into two paths through the first optical beam splitter. The first path is loaded with the signal to be transmitted through the electro-optical modulator, and the second path is generated by the optical frequency comb module. 1.

2. Split the local oscillator light generated by the second laser into two paths via a second optical beam splitter. The first path is heterodyned with the optical carrier modulated in step 1.1 to generate a terahertz communication signal. The second path is heterodyned with the comb component closest to the local oscillator frequency selected from the optical frequency comb via a tunable filter to generate a microwave pilot signal. 1.

3. Amplifying the terahertz communication signal and the microwave pilot signal by a first terahertz low-noise amplifier and a first intermediate-frequency low-noise amplifier, respectively, and then transmitting them separately by the terahertz transmitting antenna and the microwave transmitting antenna; 1.

4. Receive a terahertz communication signal and a microwave pilot signal at a receiving end. The received terahertz communication signal and the local oscillator terahertz signal are coherently mixed and down-converted to an intermediate frequency signal in a first terahertz mixer. 1.

5. After the received microwave pilot signal is calibrated by the second delay compensation module, it is mixed with the intermediate frequency signal obtained in step 1.4 in the first intermediate frequency mixer to offset the same frequency jitter of the two signals and complete frequency offset compensation; 1.

6. The compensated baseband signal is sequentially filtered by a first electrical low-pass filter, amplified by a first baseband low-noise amplifier, converted by a first analog-to-digital converter, and processed by a communication digital signal processing unit to output digital communication information.

2. The method according to claim 1, characterized in that The electro-optical modulator described in step 1.1 is used to modulate the digital baseband signal onto a pure optical carrier, and the modulated optical carrier is amplified by an erbium-doped fiber amplifier and then coupled with the local oscillator light to beat the frequency.

3. The method according to claim 1, characterized in that The optical frequency comb module described in step 1.2 generates a broadband optical frequency comb by a phase modulator under the action of an electrical driving signal, and extracts the comb tooth component closest to the local oscillator optical frequency through an optical bandpass filter.

4. The method according to claim 1, wherein The frequency of the microwave pilot signal in step 1.2 is the frequency difference between the comb component and the local oscillator light, and the pilot signal frequency is in the Sub-6 GHz frequency band.

5. The method according to claim 1, wherein In step 1.4, the first terahertz mixer adopts a coherent detection method combining a single-row carrier photodiode and a common photodiode with local oscillator light to eliminate frequency offset and improve receiving sensitivity.

6. The method according to claim 1, characterized in that In step 1.5, the first intermediate frequency mixer offsets the homologous frequency jitter of the terahertz communication signal and the microwave pilot signal through analog mixing, thereby retaining the effective information item of the baseband signal.

7. The method according to claim 1, characterized in that In step 1.6, the first electrical low-pass filter is used to filter out the image component and DC noise after mixing, and the communication digital signal processing unit is further used to eliminate residual interference.

8. The method according to claim 1, characterized in that The transmitting end and the receiving end are both equipped with a first delay compensation module, a second delay compensation module and a third delay compensation module for matching the transmission path delay of the terahertz communication signal and the microwave pilot signal.

9. The method according to claim 1, characterized in that The receiving end includes a communication receiving end and a perception receiving end, which respectively independently complete frequency offset compensation through a first intermediate frequency mixer and a second intermediate frequency mixer, and output communication and perception baseband signals.

10. The method according to any one of claims 1 to 9, characterized in that The digital signal processing units at the transmitting end and the receiving end are used for pre-distortion, equalization and post-processing of signals to meet the requirements of high-speed terahertz communication systems.

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