High-spectral-efficiency radio-over-fiber transmission method based on Kramers-Kronig receiving

Abstract

The invention discloses a high-spectral-efficiency radio-over-fiber transmission method based on Kramers-Kronig receiving, and relates to the technical field of microwaves and the technical field of optical communication. Comprising a first laser LD1, a first radio frequency signal source RF1, a second radio frequency signal source RF2, a dual-drive Mach-Zehnder modulator DD-MZM, a single-mode fiber SMF, a first polarization controller PC1, a second polarization controller PC2, a second laser LD2, an optical coupler OC, a photoelectric detector PD, an analog-to-digital converter ADC and a digital signal processing DSP. According to the method, the DD-MZM is used for modulating the intensity and the phase of an optical carrier for two same-frequency radio-frequency signals RF at a sending end, a single PD is used for receiving at a receiving end, a Kramers-Kronig algorithm is used for light current output by the PD to obtain intensity information and phase information of a light electric field so as to reconstruct the light electric field, and after phase jitter of a laser is eliminated, the light electric field is converted into the light electric field. According to the invention, the original coherent receiving structure is simplified, the transmission of the two paths of RF signals with the same frequency can be realized by only one PD, the cost is saved, and the method has better application value in a short-distance communication system and a point-to-point communication system with high requirements on capacity and spectral efficiency.

Description

Technical Field

[0001] The present invention relates to the fields of optical communication technology and microwave technology, and mainly relates to using optical technology to realize the transmission of multiple same-frequency radio frequency signals. Background Art

[0002] With the continuous expansion of communication frequency bands and the continued increase in data rates and user numbers, the communication method of transmitting signals through coaxial cables is no longer able to meet the needs of long-distance, high-capacity transmission due to drawbacks such as high high-frequency loss and heavy weight. RF-over-fiber communication systems achieve communication by modulating RF signals onto optical carriers. These systems not only offer the advantages of wide bandwidth, low loss, and immunity to electromagnetic interference, but also enable multiplexing, enabling low-cost, high-capacity signal transmission.

[0003] Direct detection links, due to their simple structure and low cost, have been widely used in short-distance RF-over-fiber communication systems. However, they can only detect light intensity, not phase, making it difficult to meet growing user demand for transmission capacity. Compared to direct detection links, coherent detection links offer the advantages of high sensitivity, support for multi-dimensional modulation such as intensity and phase, and ease of integration with wavelength division multiplexing systems. However, the receiving end of this link consists of a 90° optical mixer and two balanced detectors, resulting in a complex link structure and high cost, making it difficult to use in short-distance RF-over-fiber communication systems.

[0004] Numerous methods have been developed to improve the spectral efficiency of optical radio frequency communication systems. Coherent receiver-based methods improve spectral efficiency by recovering the optical field to detect the intensity and phase information of light waves, but these methods are complex and costly. Coherent detection methods based on single photodetectors can simplify the receiver structure and improve spectral efficiency by simultaneously transmitting two co-frequency RF signals based on the intensity of the light waves. However, these methods cannot fully recover the optical field and therefore cannot compensate for linear impairments during transmission. Summary of the Invention

[0005] To address the problems in the background technology, the present invention proposes a high-spectral-efficiency optical-based RF transmission method based on Kramers-Kronig reception. This method has the following advantages: First, it can use a single photodetector instead of a coherent receiver at the receiving end, simplifying the link structure while achieving the same functionality, thus meeting the cost and complexity requirements of short-distance optical-based RF transmission systems; second, it can modulate and demodulate two co-frequency RF signals, showing potential in high-spectral-efficiency, high-capacity applications; and third, it can compensate for linear impairments incurred by the signal during transmission in the digital domain.

[0006] The technical solution adopted by the present invention is: the architecture includes 1 a first laser LD1, 2 a first radio frequency signal source RF1, 3 a second radio frequency signal source RF2, 4 a dual-drive Mach-Zehnder modulator DD-MZM, 5 a single-mode optical fiber SMF, 6 a first polarization controller PC1, 7 a second polarization controller PC2, 8 a second laser LD2, 9 an optical coupler OC, 10 a photodetector PD, 11 an analog-to-digital converter ADC, and 12 a digital signal processing DSP. The system is characterized in that a single-wavelength optical carrier generated by LD1 is injected into the DDMZM, and the RF signals generated by RF1 and RF2 are connected to the two RF input ports of the DDMZM. ​​After the intensity and phase of the optical signal output by the DDMZM are modulated and reach the receiving end through the SMF, the receiving end PC1 and PC2 align the polarization of the modulated optical signal and the LD2 output optical signal and couple them into one optical signal through OC. After the coupled optical signal is input into the PD, the photoelectric conversion is completed. The ADC samples the PD output and sends it to the DSP. The DSP restores the optical field based on the Kramers-Kronig algorithm, and performs the two steps of eliminating the laser phase jitter and separation and demodulation in sequence to realize the transmission of the RF1 and RF2 two-way RF signals in the optical radio frequency communication system.

[0007] The DSP algorithm demodulates the two RF signals RF1 and RF2 according to the following steps: first, recover the DC component lost due to the AC coupling output characteristics of the PD; second, obtain the intensity information and phase information of the optical field based on the Kramers-Kronig algorithm; third, remove the DC component of the optical field; fourth, use a bandpass filter to filter out the phase jitter generated by the beat frequency of the two optical signals; fifth, conjugate multiply the original signal and the phase jitter filtered out by the filter to recover the phase modulated signal and eliminate the phase jitter; sixth, obtain the real part and imaginary part of the optical field, perform square addition operations on them, and obtain the intensity modulated signal; seventh, normalize the intensity modulated signal and the phase modulated signal; eighth, add the normalized phase modulated signal and the intensity modulated signal to obtain the RF1 signal, and subtract them to obtain the RF2 signal; and ninth, demodulate the two RF signals RF1 and RF2 respectively.

[0008] The present invention comprises the following steps when working: (1) The optical carrier output by LD1 at the transmitting end of the architecture is injected into the dual-drive Mach-Zehnder modulator DD-MZM; (2) The RF signals output by RF1 and RF2 are connected to the RF input port of the DDMZM. ​​The sum of the RF signals (RF1+RF2) is modulated onto the phase of the optical carrier, and the difference between the RF signals (RF1-RF2) is modulated onto the intensity of the optical carrier. (3) The modulated optical signal output by DDMZM reaches the receiving end through SMF. (4) LD2 at the receiving end of the architecture outputs an optical local oscillator. PC1 and PC2 are first used to align the polarization state of the phase-modulated optical signal and the optical local oscillator. Then, the two are input into the OC. The coupled optical signal output by the OC enters the PD to complete the photoelectric conversion. (5) After the ADC samples the output of the PD, the DSP module further processes the sampled signal in the following steps: the first step is to recover the DC component lost due to the AC coupling output characteristics of the PD; the second step is to obtain the intensity information and phase information of the optical field based on the Kramers-Kronig algorithm; the third step is to isolate the DC; the fourth step is to use a bandpass filter to filter out the phase jitter generated by the beat frequency of the two optical signals; the fifth step is to conjugate and multiply the original signal and the phase jitter filtered out by the filter to restore the phase modulated signal and eliminate the phase jitter; the sixth step is to obtain the real part and the imaginary part of the optical field, perform square addition operations on them, and obtain the intensity modulated signal; and the seventh step is to normalize the intensity modulated signal and the phase modulated signal; the eighth step is to obtain the two RF signals RF1 and RF2; the ninth step is to demodulate the two RF signals RF1 and RF2.

[0009] The present invention proposes a high-spectral-efficiency optical radio frequency transmission method based on Kramers-Kronig reception, which can meet the requirements of short-distance optical radio frequency communication systems for cost and structural complexity, as well as the requirements of optical radio frequency communication systems for high spectral efficiency and large capacity. The transmitting end uses DD-MZM to modulate two RF signals on the phase and intensity of the optical carrier and send them to the receiving end. The receiving end aligns the polarization state of the received modulated optical signal and the local oscillator optical signal, couples the two with OC, and PD performs photoelectric conversion on the coupled output optical signal and outputs it to ADC. After the ADC samples the photocurrent, due to the use of AC-coupled PD, the DSP must first restore the DC component and then eliminate self-frequency interference based on the Kramers-Kronig algorithm. After isolating the restored DC component of the optical field, the two steps of eliminating laser phase jitter and separating and demodulating are performed in sequence. For intensity modulated signals, the signal is recovered by summing the squares of the I / Q signals. For phase modulated signals, the intermediate frequency component is first filtered out and then conjugated and multiplied with the original signal to eliminate frequency deviation and phase noise. The phase angle of the signal is then extracted to obtain the phase modulated signal. Since both the intensity modulated and phase modulated signals contain two microwave vector signals, RF1 and RF2, the intensity modulated and phase modulated signals must be normalized to separate RF1 and RF. The normalized phase modulated and intensity modulated signals are added to obtain RF1, and subtracted to obtain RF2. This enables the transmission of two co-frequency RF signals, RF1 and RF2, in an RF-over-fiber communication system.

[0010] The present invention can simplify the structure of the original coherent detection link under the premise of achieving high spectral efficiency, reduce the complexity and cost of the equipment, and at the same time realize the function of a coherent receiver and compensate for linear damage in signal transmission in the digital domain.

[0011] The present invention realizes the same functions as the coherent detection link, supports multi-dimensional modulation, supports compensation for linear damage in the digital domain, and supports integration with the wavelength division multiplexing system. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 The schematic diagram of the high-spectral-efficiency optical RF transmission architecture based on Kramers-Kronig reception. Figure 2 This is the flow chart of the DSP module. Figure 3 Flowchart for RF1 and RF2 signal separation and phase jitter elimination. Figure 4 Spectrum diagrams obtained by Fourier transform of different signals, where: (a) is the spectrum of the PD output photocurrent, where the small figure is the sideband at 1.94 GHz with the self-beating frequency of the signal aliased. (b) is the output spectrum of the Kramers-Kronig algorithm, where the small figure is the sideband of the self-frequency term of the eliminated signal at 1.94 GHz. (c) is the spectrum diagram of the RF1 radio frequency signal after phase jitter is eliminated. (d) is the spectrum diagram of the RF2 radio frequency signal after phase jitter is eliminated. Figure 5 This is the constellation diagram after demodulation of RF1 and RF2 signals. (a) is the constellation diagram of the demodulated RF1 radio frequency signal. (b) is the constellation diagram of the demodulated RF2 radio frequency signal. Figure 6 The figure is a graph of the error vector magnitude (EVM) of 16-QAM microwave vector signals RF1 and RF2 under different received signal optical powers. Figure 7 The following figure shows the isolation between the two channels when only one RF signal is added. (a) is the isolation of channel 1 when only RF1 signal is loaded. (b) is the isolation of channel 2 when only RF2 signal is loaded. Figure 8 When the center frequency of the two RF signals is 6 GHz and the frequency difference between the two lasers LD1 and LD2 is 7.5 GHz, the constellation diagrams of RF1 and RF2 after 25 km of optical fiber transmission are shown, as well as the constellations of RF1 and RF2 after dispersion compensation. (a) is the constellation diagram of RF1 after 25 km of optical fiber transmission. (b) is the constellation diagram of RF2 after 25 km of optical fiber transmission. (c) is the constellation diagram of RF1 after dispersion compensation. (d) is the constellation diagram of RF2 after dispersion compensation. DETAILED DESCRIPTION

[0013] The following is a detailed description of an embodiment of the present invention in conjunction with the accompanying drawings: This embodiment is implemented on the premise of the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiment:

[0014] Figure 1 The schematic diagram of the high spectrum efficiency optical radio frequency transmission architecture based on Kramers-Kronig reception is shown in the figure. Figure 1 As shown, in this embodiment, the device includes 1 a first laser LD1, 2 a first RF signal source RF1, 3 a second RF signal source RF2, 4 a dual-drive Mach-Zehnder modulator DD-MZM, 5 a single-mode fiber SMF, 6 a first polarization controller PC1, 7 a second polarization controller PC2, 8 a second laser LD2, 9 an optical coupler OC, 10 a photodetector PD, 11 an analog-to-digital converter ADC, and 12 a digital signal processing DSP. The device is characterized in that a single-wavelength optical carrier generated by LD1 is injected into the DDMZM, the RF signals generated by RF1 and RF2 are connected to the two RF input ports of the DDMZM, and the intensity- and phase-modulated optical signal output by the DDMZM reaches the receiving end through the SMF. At the receiving end, PC1 and PC2 align the polarization of the modulated optical signal and the optical signal output by LD2 and couple them into one optical signal via the OC. The coupled optical signal is then input into the PD for photoelectric conversion. The ADC samples the PD output and sends it to the DSP. Because the PD uses AC coupling, the DSP first recovers the DC component and then eliminates self-frequency interference based on the Kramers-Kronig algorithm. After isolating the recovered DC component of the optical electric field, the laser phase jitter elimination and separation demodulation steps are performed sequentially. For the intensity modulated signal, the I / Q signal is squared and summed to recover it. For the phase modulated signal, the intermediate frequency component is first filtered out. The intermediate frequency component is then conjugated and multiplied with the original signal to eliminate the frequency difference and phase noise. At this time, the phase angle of the signal is taken out to obtain the phase modulated signal. Since both the intensity modulated signal and the phase modulated signal contain two microwave vector signals, RF1 and RF2, in order to separate RF1 and RF signals, the obtained intensity modulated signal and phase modulated signal need to be normalized. The RF1 signal is obtained by adding the normalized phase modulated signal and intensity modulated signal, and the RF2 signal is obtained by subtracting them. This realizes the transmission of the two co-frequency RF signals RF1 and RF2 in the optical radio frequency communication system.

[0015] In this example, the specific implementation steps of the method are: Step 1: LD1 generates an optical carrier with an operating wavelength of 1549.61nm and a power of 16dBm, and injects it into the DDMZM with a half-wave voltage of 5V. Step 2: 16-QAM microwave vector signals with a baud rate of 100 MSym / s, a carrier frequency of 1.94 GHz, and a power of 0 dBm, output by both RF1 and RF2, are input into the two RF input ports of the DDMZM to perform phase and intensity modulation on the optical carrier. Step 3: Send the modulated optical signal output by DDMZM to the receiving end through SMF. Step 4: LD2 at the receiving end outputs a true optical signal with an operating wavelength of 1549.58nm and a power of 10dBm. PC1 is used to control the polarization state of the phase-modulated optical signal, and PC2 is used to control the polarization state of the local oscillator optical signal. When the polarizations of the two are aligned, they are input to the OC. The photocurrent generated by the coupled optical signal output by the OC enters the PD and is input to the ADC. Step 5: The DSP module processes the photocurrent sampled by the ADC and recovers the 16-QAM microwave vector signal. Figure 4 It can be seen that the spectrum after different DSP steps is Figure 5 It can be seen that the EVMs of the two recovered 16-QAM microwave vector signals are 7.8% and 8.17% respectively. Step 6: Without changing other parameters, change the power of the modulated optical signal and the power of the local oscillator optical signal at the same time in 1dB steps, and plot the change of EVM with the power ROP of the modulated optical signal received by the PD. Figure 6 It can be seen that when the modulated optical signal power is greater than -9.5dBm, the EVM of the microwave vector signal RF2 meets the requirement of error-free transmission. When the modulated optical signal power is greater than -11.5dBm, the EVM of the microwave vector signal RF1 meets the requirement of error-free transmission. Step 7: Without changing other parameters, turn off the output of RF2 signal to get the isolation of channel 1; then turn off the output of RF1 signal to get the isolation of channel 2. Figure 7 It can be seen that the isolation of channel 1 is 25dB and the isolation of channel 2 is 23dB. Step 8: Without changing other parameters, RF1 and RF2 respectively output 16-QAM microwave vector signals with a baud rate of 100MSym / s, a carrier frequency of 6GHz, and a power of 0dBm. LD2 outputs an operating wavelength of 1549.55nm. Figure 8 It can be seen that the EVMs of RF1 and RF2 before dispersion compensation are 30.74% and 31.92% respectively, while the EVMs of RF1 and RF2 after dispersion compensation are 7.62% and 10.03% respectively.

[0016] In summary, this paper proposes a high-spectral-efficiency RF-over-light transmission method based on Kramers-Kronig reception. In this scheme, the transmitting end requires only a DDMZM to modulate the RF signal in both intensity and phase. At the receiving end, only a PD and an ADC are needed to demodulate the two RF signals, eliminating the need for a complex coherent receiving structure. This method can meet the requirements of high spectral efficiency, large capacity, and low cost in practical applications.

[0017] In summary, the above-described embodiments are merely examples of the present invention and are not intended to limit the scope of protection of the present invention. It should be noted that those skilled in the art will readily appreciate that various equivalent variations and substitutions can be made based on the disclosure of the present invention, including variations in the RF signal center frequency, signal bandwidth, and modulation format. These equivalent variations and substitutions, as well as adjustments to the frequency range, are also within the scope of protection of the present invention.

Claims

1. A high-spectral-efficiency optical radio frequency transmission method based on Kramers-Kronig reception, comprising a first laser LD1, a first radio frequency signal source RF1, a second radio frequency signal source RF2, a dual-drive Mach-Zehnder modulator DD-MZM, a single-mode optical fiber SMF, a first polarization controller PC1, a second polarization controller PC2, a second laser LD2, an optical coupler OC, a photodetector PD, an analog-to-digital converter ADC, and a digital signal processing DSP, characterized in that: A single-wavelength optical carrier generated by LD1 is injected into the DDMZM. ​​The RF signals generated by RF1 and RF2 are connected to the two RF input ports of the DDMZM. ​​The intensity- and phase-modulated optical signal output by the DDMZM passes through the SMF and reaches the receiver. At the receiver, PC1 and PC2 align the polarization of the modulated optical signal with the LD2 output signal and couple them together via OC coupling to form a single optical signal. The coupled optical signal is then input into the PD for photoelectric conversion. The ADC samples the PD output and feeds it into the DSP. Because the PD uses AC coupling, the DSP first recovers the DC component and then uses the Kramers-Kronig algorithm to eliminate self-frequency interference. After isolating the recovered DC component of the optical field, the DSP performs two steps, namely, laser phase jitter removal and separation demodulation. For intensity-modulated signals, the signal is recovered by square-adding the I / Q signals. For phase-modulated signals, the intermediate frequency (IF) component is first filtered out and then conjugated and multiplied with the original signal to eliminate frequency deviation and phase noise. The phase angle of the signal is then extracted to obtain the phase-modulated signal. Because both the intensity-modulated and phase-modulated signals contain two microwave vector signals, RF1 and RF2, the intensity-modulated and phase-modulated signals must be normalized to separate them. The RF1 signal is obtained by adding the normalized phase-modulated and intensity-modulated signals, and the RF2 signal is obtained by subtracting them. This enables the transmission of two co-frequency RF signals, RF1 and RF2, in an RF-over-fiber communication system.

2. The high spectral efficiency radio frequency over light transmission method based on Kramers-Kronig reception according to claim 1, characterized in that: This method only requires one DDMZM at the transmitting end to realize the separate modulation of light wave intensity and phase by two same-frequency RF signals. At the receiving end, only one PD and one ADC are needed to demodulate the two RF signals. Based on the Kramers-Kronig algorithm, this method can restore the light field, eliminate the signal's self-beating frequency interference, and compensate for the linear damage suffered by the signal during transmission, without the need for a complex coherent receiving structure.

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