IMDD-OFDM optical fiber system and its diversity reception dispersion compensation method

Abstract

The present invention discloses an IMDD-OFDM optical fiber system and a diversity receiving dispersion compensation method thereof. The IMDD-OFDM optical fiber system includes a transmitter and a receiver. The receiver includes: an optical beam splitter, a dispersion element, a first photodetector, a second photodetector and a receiving end digital signal processing module. The OFDM optical signal output by the transmitter is divided into a first OFDM optical signal and a second OFDM optical signal. The first OFDM optical signal is output after being detected by the first photodetector, and the second OFDM optical signal is detected and output by the second photodetector after passing through the dispersion element. Then, the output signal of the frequency selective power fading caused by the compensated dispersion of each subcarrier is obtained through the receiving end digital signal processing module. The present invention can effectively compensate for the problem of frequency selective power fading, so that the signal-to-noise ratio of the output signal is maximized, and the receiver has a simple structure and low system cost.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber communication, and in particular to an IMDD-OFDM optical fiber system and a diversity receiving dispersion compensation method thereof. Background Art

[0002] The fiber optic communication system based on intensity modulation and direct detection (IMDD) has the advantages of low cost, low power consumption and small area, and is an effective solution for short-distance optical communication systems. Compared with the commonly used pulse amplitude modulation (PAM) and carrierless amplitude and phase modulation (CAP), the IMDD orthogonal frequency division multiplexing (OFDM) signal has the advantages of simple frequency domain equalization, flexible modulation, and high tolerance to dispersion. Therefore, the IMDD-OFDM system can be used in passive optical networks (PON), wireless communication over fiber (RoF), and B5G and 6G fronthaul.

[0003] However, the available signal bandwidth or transmission distance of the IMDD-OFDM system is limited by the frequency selective power fading caused by dispersion after square law detection, which affects the system performance. To solve the problem of frequency selective power fading caused by dispersion, existing solutions have proposed single-sideband modulation and dispersion pre-compensation, but the existing compensation solutions rely on complex transmitter configurations, such as additional in-phase orthogonal modulators, digital-to-analog converters, and electrical amplifiers, which increases the cost of the system.

[0004] Therefore, the prior art still needs to be improved and developed. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, an object of the present invention is to provide an IMDD-OFDM optical fiber system and a diversity reception dispersion compensation method thereof, so as to solve the problem of high cost of the existing frequency selectivity function fading solutions caused by dispersion.

[0006] The technical solution of the present invention is as follows:

[0007] In a first aspect, the present invention provides an IMDD-OFDM optical fiber system, comprising a transmitter and a receiver, wherein the receiver comprises: an optical beam splitter, a dispersion element, a first photodetector, a second photodetector and a receiving end digital signal processing module;

[0008] The transmitter is connected to the optical beam splitter and is used to output an OFDM optical signal to the optical beam splitter;

[0009] The optical beam splitter is connected to the first photodetector and the dispersive element respectively, and is used to split the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal and send them to the first photodetector and the dispersive element respectively;

[0010] The first photoelectric detector is connected to the receiving end digital signal processing module, and is used to convert the first OFDM optical signal into a first OFDM electrical signal and output it to the receiving end digital signal processing module;

[0011] The second photodetector is connected to the dispersion element and the receiving end digital signal processing module respectively, and is used to convert the second OFDM optical signal into a second OFDM electrical signal and output it to the receiving end digital signal processing module;

[0012] The receiving end digital signal processing module is used to obtain the received signal of each subcarrier of the first OFDM electrical signal and the received signal of each subcarrier of the second OFDM electrical signal according to the first OFDM electrical signal and the second OFDM electrical signal, and combine the corresponding subcarrier signals of the two OFDM electrical signals through the maximum ratio combining method to obtain the output signal of the frequency selective power fading caused by the compensated dispersion of each subcarrier.

[0013] According to a further configuration of the present invention, the IMDD-OFDM optical fiber system further includes: an oscilloscope; the oscilloscope is respectively connected to the first photodetector, the second photodetector and the receiving end digital signal processing module, and is used to sample the first OFDM electrical signal and the second OFDM electrical signal.

[0014] According to a further configuration of the present invention, the transmitter comprises: a transmitting end digital signal processing module, an arbitrary waveform generator and an electrical amplifier; wherein,

[0015] The transmitting end digital signal processing module is connected to the arbitrary waveform generator, and is used to generate a real number OFDM signal and output it to the arbitrary waveform generator;

[0016] The arbitrary waveform generator is connected to the transmitting end digital signal processing module and the electrical amplifier respectively, and is used to generate an OFDM electrical signal according to the OFDM signal;

[0017] The electrical amplifier is used to amplify the OFDM electrical signal.

[0018] According to a further configuration of the present invention, the transmitter further comprises: a laser, a modulator, a single-mode optical fiber, an erbium-doped optical fiber amplifier and an optical attenuator; wherein,

[0019] The laser is used to generate an optical carrier;

[0020] The modulator is connected to the electrical amplifier and the single-mode optical fiber respectively, and is used to work at an orthogonal point under the drive of the OFDM electrical signal, and the optical carrier outputs an OFDM optical signal to the single-mode optical fiber after passing through the modulator;

[0021] The erbium-doped fiber amplifier is connected to the single-mode optical fiber and the optical attenuator respectively, and is used to amplify the power of the OFDM optical signal;

[0022] The optical attenuator is connected to the erbium-doped fiber amplifier and the optical beam splitter respectively, and is used to adjust the receiving power of the receiving signal of the transmitter.

[0023] According to a further configuration of the present invention, the dispersion element is a standard single-mode optical fiber.

[0024] According to a further configuration of the present invention, both the first photodetector and the second photodetector are single photodiodes.

[0025] In a second aspect, the present invention provides a diversity reception dispersion compensation method for the IMDD-OFDM optical fiber system as described above, comprising:

[0026] Sending the OFDM optical signal output by the transmitter to the optical beam splitter of the receiver to split the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal;

[0027] The first OFDM optical signal is detected by the first photodetector and then outputs a first OFDM electrical signal. The second OFDM optical signal is detected by the second photodetector after passing through the dispersion element and then outputs a second OFDM electrical signal.

[0028] Performing analog-to-digital conversion, synchronization processing, and fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal to obtain received signals of each subcarrier of the first OFDM electrical signal and received signals of each subcarrier of the second OFDM electrical signal respectively;

[0029] The corresponding subcarrier signals of the two OFDM electrical signals are combined by using the maximum ratio combining algorithm to obtain the output signal of each subcarrier with the frequency selective power fading caused by the compensation dispersion.

[0030] In a further configuration of the present invention, the step of sending the OFDM optical signal output by the transmitter to the optical beam splitter of the receiver to split the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal comprises:

[0031] Mapping a pseudo-random binary sequence to a quadrature amplitude modulation symbol;

[0032] Perform parallel-to-serial conversion and subcarrier mapping on quadrature amplitude modulation symbols;

[0033] Use inverse fast Fourier transform to generate real OFDM signal;

[0034] The real OFDM signal is added with a cyclic prefix and is output to an arbitrary waveform generator after parallel-to-serial conversion and amplitude clipping to generate an OFDM electrical signal.

[0035] In a further configuration of the present invention, the step of performing analog-to-digital conversion, synchronization processing and fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal to obtain the received signals of each subcarrier of the first OFDM electrical signal and the received signals of each subcarrier of the second OFDM electrical signal respectively comprises:

[0036] Before performing fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal, serial-to-parallel conversion and cyclic prefix removal processing are performed.

[0037] In a further configuration of the present invention, the step of performing analog-to-digital conversion, synchronization processing and fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal to obtain the received signals of each subcarrier of the first OFDM electrical signal and the received signals of each subcarrier of the second OFDM electrical signal respectively further includes:

[0038] After fast Fourier transform processing is performed on the first OFDM electrical signal and the second OFDM electrical signal, parallel-to-serial conversion, orthogonal amplitude modulation symbol demapping and bit error rate calculation processing are performed.

[0039] The present invention provides an IMDD-OFDM optical fiber system and a diversity receiving dispersion compensation method thereof. The IMDD-OFDM optical fiber system comprises a transmitter and a receiver. The receiver comprises: an optical beam splitter, a dispersion element, a first photodetector, a second photodetector and a receiving end digital signal processing module. The OFDM optical signal output by the transmitter is sent to the optical beam splitter of the receiver to divide the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal. The first OFDM optical signal is then detected by a first photodetector to output a first OFDM electrical signal. The second OFDM optical signal is detected by a second photodetector after passing through a dispersion element and outputs a second OFDM electrical signal. The first OFDM electrical signal and the second OFDM electrical signal are then processed by a digital signal processing module at the receiving end to perform analog-to-digital conversion, synchronization processing and fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal to obtain received signals of each subcarrier of the first OFDM electrical signal and received signals of each subcarrier of the second OFDM electrical signal respectively. The corresponding subcarrier signals of the two OFDM electrical signals are combined by using a maximum ratio combining algorithm to obtain output signals of each subcarrier with frequency selective power fading caused by compensated dispersion. In this way, the present invention combines the corresponding subcarrier signals of two OFDM electrical signals by adopting the maximum ratio combining algorithm through diversity reception, which can effectively compensate for the problem of frequency selective power fading, so that the signal-to-noise ratio of the output signal is maximized, and the receiver has a simple structure and low system cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary personnel in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0041] Figure 1 It is a structural principle diagram of the IMDD-OFDM optical fiber system in the present invention.

[0042] Figure 2 It is a flow chart of the IMDD-OFDM optical fiber system diversity reception dispersion compensation method in the present invention.

[0043] Figure 3 The present invention provides an embodiment of an OFDM signal with a signal bandwidth of 42.7 GHz, which is transmitted through a 10 km standard single-mode optical fiber and uses conventional reception and diversity reception when the received optical power is 8 dBm.

[0044] Figure 4The present invention provides an embodiment of the present invention, which provides a signal-to-noise ratio distribution diagram of two signals and their combined signal using diversity reception when the received optical power is 8 dBm after an OFDM signal with a signal bandwidth of 42.7 GHz is transmitted through a 10 km standard single-mode optical fiber.

[0045] Figure 5 The present invention provides a curve of bit error rate versus received optical power for a 16QAM-OFDM signal with a rate of 170.6 Gbit / s after being transmitted through a 10 km standard single-mode optical fiber using conventional reception and diversity reception.

[0046] Figure 6 A signal constellation diagram of a 16QAM-OFDM signal with a rate of 170.6 Gbit / s provided by an embodiment of the present invention, after being transmitted through a 10 km standard single-mode optical fiber, using conventional reception and diversity reception when the received optical power is 8 dBm.

[0047] Figure 7 The present invention provides an embodiment of an OFDM signal with a signal bandwidth of 50.2 GHz, which is transmitted through a 10 km standard single-mode optical fiber and uses conventional reception and diversity reception when the received optical power is 8 dBm.

[0048] Figure 8 An embodiment of the present invention provides a curve showing the variation of the bit error rate versus transmission rate of an OFDM signal with adaptive bit power loading and a signal bandwidth of 50.2 GHz after being transmitted through a 10 km standard single-mode optical fiber using conventional reception and diversity reception when the received optical power is 8 dBm.

[0049] Fig. 9 An embodiment of the present invention provides a bit power loading configuration diagram of an adaptive bit power loaded OFDM signal with a signal bandwidth of 50.2 GHz, which adopts traditional reception (rate of 180.5 Gbit / s) and diversity reception (rate of 208.1 Gbit / s) when the received optical power is 8 dBm after being transmitted through 10 km of standard single-mode optical fiber.

[0050] The marks in the accompanying drawings are: 1. optical beam splitter; 2. dispersion element; 3. first photodetector; 4. second photodetector; 5. digital signal processing module at the receiving end; 6. oscilloscope; 7. digital signal processing module at the transmitting end; 8. arbitrary waveform generator; 9. electrical amplifier; 10. laser; 11. modulator; 12. single-mode optical fiber; 13. erbium-doped fiber amplifier; 14. optical attenuator. DETAILED DESCRIPTION

[0051] The present invention provides an IMDD-OFDM optical fiber system and a diversity receiving dispersion compensation method thereof. To make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0052] In the embodiments and the scope of the patent application, unless the text specifically defines the article, "a", "an", "the" and "the" may also include plural forms. If there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0053] It should be further understood that the term "comprising" used in the specification of the present invention refers to the presence of the features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof. It should be understood that when we refer to an element as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or there can be intermediate elements. In addition, the "connection" or "coupling" used herein can include wireless connection or wireless coupling. The term "and / or" used herein includes all or any unit and all combinations of one or more associated listed items.

[0054] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless specifically defined as herein.

[0055] In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the fact that ordinary technicians in the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0056] The inventors have found that the available signal bandwidth or transmission distance of the IMDD-OFDM system is limited by the frequency selective power fading caused by the dispersion after square law detection, which affects the system performance. Although O-band transmission can avoid serious dispersion distortion, C-band transmission has great advantages in terms of fiber loss, wavelength division multiplexing devices and the maturity of optical amplifiers. Existing solutions have proposed single-sideband modulation, dispersion pre-compensation and Kramers-Kronig receivers to compensate for C-band dispersion distortion. However, these solutions rely on complex transmitter configurations, such as the need for additional in-phase orthogonal modulators, digital-to-analog converters and electrical amplifiers, which greatly increases the cost of the system. In addition to the above solutions, adaptive bit power loading algorithms based on digital signal processing (DSP) have been widely used in IMDD-OFDM systems, which counteract the frequency selective power fading effect by using only non-power fading subcarriers, but power fading still exists and cannot increase the available signal bandwidth. In addition, diversity technology based on Spectrally Efficient Frequency Division Multiplexing (SEFDM) can also alleviate the problem of frequency selective power fading caused by dispersion. However, SEFDM demodulation requires a complex inter-symbol interference elimination algorithm instead of a simple single-tap frequency domain equalization, and the signal-to-noise ratio of high-performance subcarriers will deteriorate after using SEFDM-based diversity technology, thus limiting its application in low-cost, high-performance IMDD optical fiber transmission systems.

[0057] In view of the above technical problems, the present invention provides an IMDD-OFDM optical fiber system and a diversity reception dispersion compensation method thereof. The IMDD-OFDM optical fiber system includes a transmitter and a receiver. The receiver includes: an optical beam splitter, a dispersion element, a first photodetector, a second photodetector and a receiving end digital signal processing module. The present invention sends an OFDM optical signal output by a transmitter into an optical beam splitter of a receiver to divide the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal. Then, the first OFDM optical signal is detected by a first photodetector and then outputs a first OFDM electrical signal. The second OFDM optical signal is detected by a second photodetector after passing through a dispersion element and then outputs a second OFDM electrical signal. Then, a digital signal processing module at the receiving end performs analog-to-digital conversion, synchronization processing, and fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal to obtain received signals of each subcarrier of the first OFDM electrical signal and received signals of each subcarrier of the second OFDM electrical signal respectively. A maximum ratio combining algorithm is used to combine the corresponding subcarrier signals of the two OFDM electrical signals to obtain output signals of frequency selective power fading of each subcarrier that has been compensated for dispersion. In this way, the present invention combines the corresponding subcarrier signals of two OFDM electrical signals by adopting the maximum ratio combining algorithm through diversity reception, which can effectively compensate for the problem of frequency selective power fading, so that the signal-to-noise ratio of the output signal is maximized, and the receiver has a simple structure and low system cost.

[0058] See also Figure 1 The present invention provides a preferred embodiment of an IMDD-OFDM optical fiber system.

[0059] like Figure 1As shown, an IMDD-OFDM optical fiber system provided by the present invention includes a transmitter and a receiver, wherein the receiver includes: an optical beam splitter 1, a dispersion element 2, a first photodetector 3, a second photodetector 4 and a receiving end digital signal processing module 5; the transmitter is connected to the optical beam splitter 1, and is used to output an OFDM optical signal to the optical beam splitter 1; the optical beam splitter 1 is respectively connected to the first photodetector 3 and the dispersion element 2, and is used to divide the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal and send them to the first photodetector 3 and the dispersion element 2 respectively; the first photodetector 3 is connected to the receiving end digital signal processing module 5, and is used to convert the first OFDM optical signal into a first OFDM optical signal. OFDM electrical signal and output it to the receiving end digital signal processing module 5; the second photodetector 4 is respectively connected to the dispersion element 2 and the receiving end digital signal processing module 5, and is used to convert the second OFDM optical signal into a second OFDM electrical signal and output it to the receiving end digital signal processing module 5; the receiving end digital signal processing module 5 is used to obtain the receiving signals of each subcarrier of the first OFDM electrical signal and the receiving signals of each subcarrier of the second OFDM electrical signal according to the first OFDM electrical signal and the second OFDM electrical signal, and combine the corresponding subcarrier signals of the two OFDM electrical signals through the maximum ratio combining method to obtain the output signal of the frequency selective power fading caused by the compensated dispersion of each subcarrier.

[0060] Specifically, the transmitter acts as a transmitting end, and the generated OFDM optical signal is divided into two paths after passing through the optical beam splitter 1, namely, a first OFDM optical signal and a second OFDM optical signal. The first OFDM optical signal is directly detected by the first photodetector 3, and the first OFDM electrical signal is output to the receiving end digital signal processing module 5 for analog-to-digital conversion, synchronous processing, serial-to-parallel conversion, removal of cyclic prefix, and fast Fourier transformation to obtain the receiving signal of all data subcarriers of the first OFDM electrical signal. The second OFDM optical signal is directly detected by the second photodetector 4 after passing through the dispersion element 2, and the second OFDM electrical signal is output to the receiving end digital signal processing module 5 for analog-to-digital conversion, synchronous processing, serial-to-parallel conversion, removal of cyclic prefix, and fast Fourier transformation to obtain the receiving signal of all data subcarriers of the second OFDM electrical signal.

[0061] Subsequently, the maximum ratio combining algorithm is used to combine the received signal Y1(k) of the kth (k=1,2,…,K, where K is the number of data subcarriers) subcarrier of the first OFDM electrical signal with the received signal Y2(k) of the kth subcarrier of the second OFDM electrical signal, and the output signal S(k) of the kth subcarrier with frequency selective power fading caused by dispersion compensation is obtained, which is expressed as:

[0062] ;

[0063] Among them, Hn (k) is the frequency response of the kth subcarrier of the system composed of the transmitter, optical fiber and photodetector n (n=1,2) in the receiver, which can be obtained by training estimation before signal transmission. Finally, the output signal obtained by combining the received signal Y1 (k) of the kth subcarrier of the first OFDM electrical signal and the received signal Y2 (k) of the kth subcarrier of the second OFDM electrical signal is converted into parallel and serial, orthogonal amplitude modulation symbol demapping is performed, and bit error rate calculation processing is performed.

[0064] In this way, the present invention combines the corresponding subcarrier signals of two OFDM electrical signals by adopting the maximum ratio combining algorithm through diversity reception, which can effectively compensate for the problem of frequency selective power fading, so that the signal-to-noise ratio of the output signal is maximized, and the receiver has a simple structure and low system cost.

[0065] In some embodiments, the first photodetector 3 and the second photodetector 4 may each be a single photodiode.

[0066] Please continue reading Figure 1 In a further implementation of an embodiment, the IMDD-OFDM optical fiber system also includes: an oscilloscope 6; the oscilloscope 6 is respectively connected to the first photodetector 3, the second photodetector 4 and the receiving end digital signal processing module 5, and is used to sample the first OFDM electrical signal and the second OFDM electrical signal.

[0067] Specifically, the oscilloscope 6 uses a real-time oscilloscope with a sampling rate of 256GS / s. The first OFDM electrical signal output by the first photodetector 3 and the second OFDM electrical signal output by the second photodetector 4 are received by the oscilloscope 6 and processed offline, so that the subsequent receiving end digital signal processing module 5 can perform resampling, synchronization processing, serial-to-parallel conversion, cyclic prefix removal, fast Fourier transform, maximum ratio merging, parallel-to-serial conversion, orthogonal amplitude modulation demodulation and bit error rate calculation. For each transmitted OFDM data frame, except for the first symbol used for synchronization, each OFDM data frame consists of 900 OFDM symbols, of which the first 50 symbols are used for channel estimation and the remaining symbols are valid data symbols.

[0068] Please continue reading Figure 1 In a further implementation of an embodiment, the transmitter includes: a transmitting end digital signal processing module 7, an arbitrary waveform generator 8 and an electrical amplifier 9; wherein the transmitting end digital signal processing module 7 is connected to the arbitrary waveform generator 8, for generating a real number OFDM signal and outputting it to the arbitrary waveform generator 8; the arbitrary waveform generator 8 is respectively connected to the transmitting end digital signal processing module 7 and the electrical amplifier 9, for generating an OFDM electrical signal according to the OFDM signal; the electrical amplifier 9 is used to amplify the OFDM electrical signal.

[0069] Furthermore, the transmitter also includes: a laser 10, a modulator 11, a single-mode optical fiber 12, an erbium-doped fiber amplifier 13 and an optical attenuator 14; wherein the laser 10 is used to generate an optical carrier; the modulator 11 is respectively connected to the electrical amplifier 9 and the single-mode optical fiber 12, and is used to operate at an orthogonal point under the drive of the OFDM electrical signal, and the optical carrier outputs an OFDM optical signal to the single-mode optical fiber 12 after passing through the modulator 11; the erbium-doped fiber amplifier 13 is respectively connected to the single-mode optical fiber 12 and the optical attenuator 14, and is used to amplify the power of the OFDM optical signal; the optical attenuator 14 is respectively connected to the erbium-doped fiber amplifier 13 and the optical beam splitter 1, and is used to adjust the receiving power of the receiving signal of the transmitter.

[0070] Specifically, the sampling rate of the arbitrary waveform generator 8 can be 120 GSa / s. In the transmitting end digital signal processing module 7, the pseudo-random binary sequence is mapped to a hexadecimal quadrature amplitude modulation (QAM) symbol or mapped to a QAM symbol with a different number of bits using an adaptive bit and power loading algorithm. The QAM symbol is then subjected to serial-to-parallel conversion and subcarrier mapping, wherein the hexadecimal QAM symbol (adaptive bit and power loaded QAM symbol) is loaded to the 2nd to 183rd (2nd to 215th) subcarriers. A 512-point inverse fast Fourier transform with complex conjugation is then used to generate a real OFDM signal, and after adding a 16-point prefix, parallel-to-serial conversion and clipping, the signal generated offline is input into the arbitrary waveform generator 8 to generate an OFDM electrical signal, and the OFDM electrical signal is amplified after passing through the electrical amplifier 9.

[0071] The optical carrier is generated by an external cavity laser with a wavelength of 1550.12nm, the modulator 11 is a 32GHz Mach-Zehnder modulator 11, and the length of the single-mode optical fiber 12 can be set to 10km. The modulator 11 works at the orthogonal point under the drive of the OFDM electrical signal output by the electrical amplifier 9. After passing through the modulator 11, the optical carrier outputs the OFDM optical signal to the single-mode optical fiber 12, and the signal power is amplified to 9dBm through the erbium-doped fiber amplifier 13. Thereafter, the optical attenuator 14 can be used to adjust the received optical power of the received signal, and then the OFDM optical signal is directly detected by the receiver.

[0072] In some embodiments, the dispersion element 2 is a standard single-mode optical fiber with the same length as the single-mode optical fiber 12, for example, 10 km, or a dispersion element with a dispersion value similar to that of the single-mode optical fiber 12. It should be noted that if a standard single-mode optical fiber with the same length as the transmission optical fiber is used as a dispersion element, an additional 2 to 3 dB insertion loss will be introduced into the lower branch of the receiver (the second photodetector 4). Among them, a transimpedance amplifier can be used to amplify the output electrical signal of the second photodetector 4 to avoid the use of an optical amplifier. In addition, the insertion loss of the lower branch of the receiver can also be reduced by using a low-loss dispersion element.

[0073] See also Figure 2 In some embodiments, the present invention provides a diversity reception dispersion compensation method for the IMDD-OFDM optical fiber system as described above, comprising the steps of:

[0074] S100, sending the OFDM optical signal output by the transmitter to the optical splitter of the receiver to split the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal; the details are as described in an embodiment of an IMDD-OFDM optical fiber system and will not be repeated here.

[0075] S200, the first OFDM optical signal is detected by the first photodetector and then output as the first OFDM electrical signal, the second OFDM optical signal is detected by the second photodetector after passing through the dispersion element and then outputs the second OFDM electrical signal; the details are as described in an embodiment of an IMDD-OFDM optical fiber system and will not be repeated here.

[0076] S300, performing analog-to-digital conversion, synchronization processing, and fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal to obtain received signals of each subcarrier of the first OFDM electrical signal and received signals of each subcarrier of the second OFDM electrical signal respectively; the specific details are as described in an embodiment of an IMDD-OFDM optical fiber system and will not be repeated here.

[0077] S400, using a maximum ratio combining algorithm to combine the corresponding subcarrier signals of the two OFDM electrical signals to obtain an output signal of each subcarrier that has compensated for the frequency selective power fading caused by dispersion. The details are as described in an embodiment of an IMDD-OFDM optical fiber system, which will not be described in detail here.

[0078] In some embodiments, step S100 includes the steps of:

[0079] S110, mapping a pseudo-random binary sequence to an orthogonal amplitude modulation symbol;

[0080] S120, performing parallel-to-serial conversion and subcarrier mapping on the orthogonal amplitude modulation symbols;

[0081] S130, generating a real OFDM signal by using inverse fast Fourier transform;

[0082] S140, adding a cyclic prefix to the real OFDM signal and outputting it to an arbitrary waveform generator after parallel-to-serial conversion and amplitude clipping to generate an OFDM electrical signal.

[0083] In some embodiments, step S300 includes the steps of:

[0084] S310, performing serial-to-parallel conversion and removing cyclic prefix processing before performing fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal;

[0085] S320: After performing fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal, perform parallel-to-serial conversion, orthogonal amplitude modulation symbol demapping and bit error rate calculation processing.

[0086] In order to verify the technical effect of the present invention, Figure 3As shown in FIG. 1 , the present invention provides a signal-to-noise ratio distribution diagram of an OFDM optical signal with a signal bandwidth of 42.7 GHz after being transmitted through a 10 km standard single-mode optical fiber and using conventional reception and diversity reception when the received optical power is 8 dBm. Figure 4 The signal-to-noise ratio distribution diagram of the two-way signal and its combined signal using traditional reception is given. It can be seen that the signal-to-noise ratio of the combined signal of the diversity receiver using the maximum ratio combining algorithm is better than the signal-to-noise ratio of the signals of each branch of the diversity receiver. In addition, compared with the traditional receiver, the diversity reception method proposed in the present invention can improve the signal-to-noise ratio of the power fading subcarrier by 17.6 dB, while the signal-to-noise ratio gain of the non-power fading subcarrier is relatively small. The diversity reception method of the present invention can effectively compensate for the frequency selective power fading caused by dispersion within the 42.7 GHz signal bandwidth.

[0087] like Figure 5 As shown, Figure 5 The bit error rate curves of the 16QAM-OFDM signal with a rate of 170.6 Gbit / s and received optical power after transmission through 10 km of standard single-mode optical fiber using traditional reception and diversity reception are given. For traditional reception, due to the frequency selective power fading caused by severe dispersion, the bit error rate is still higher than 0.03 when the received optical power increases to 8 dBm. Compared with traditional reception, the diversity reception method effectively compensates for the frequency selective power fading (see Figure 3 and Figure 4 ), which reduces the bit error rate of 16QAM-OFDM signal by more than an order of magnitude, below the 7% hard decision forward error correction coding threshold of 3.8×10-3. In order to show the effectiveness of using diversity reception method to compensate for frequency selective power fading, Figure 6 The results show that the 16QAM-OFDM signal with a rate of 170.6 Gbit / s is transmitted through 10 km of standard single-mode optical fiber and the received optical power is 8 dBm using traditional reception ( Figure 6 a) and diversity reception ( Figure 6 b) Signal constellation diagram. Due to the effective compensation of frequency selective power fading, the constellation diagram using diversity reception is clearer than that of traditional reception.

[0088] In order to maximize the system capacity, the present invention uses an adaptive bit and power loading algorithm to compare the capacity difference between the traditional reception and diversity reception methods. Since the adaptive bit and power loading algorithm can adaptively allocate bits and power according to the frequency response of the subcarrier, the bandwidth of the adaptive bit and power loading OFDM signal is selected to be 50.2 GHz, which is higher than the 42.7 GHz of the 16QAM-OFDM signal. Figure 7The test SNR distributions of conventional reception and diversity reception within the 50.2 GHz signal bandwidth are given. It can be seen that diversity reception can effectively compensate for frequency selective power fading. However, due to the limitation of the bandwidth of the modulator used, the SNR drops sharply in the high frequency range above 42 GHz. Based on the measured SNR distributions, adaptive bit and power loading QAM mapping is performed on the OFDM systems of conventional reception and diversity reception, respectively. Figure 8 The variation curves of bit error rate with transmission rate of OFDM signal with adaptive bit power loading and signal bandwidth of 50.2 GHz after transmission over 10 km standard single-mode optical fiber with traditional reception and diversity reception at the received optical power of 8 dBm are given. Fig. 9 ( Fig. 9 a is traditional reception, Fig. 9 b is diversity reception) then it gives Figure 8 The bit power loading configuration diagram of traditional reception (rate of 180.5 Gbit / s) and diversity reception (rate of 208.1 Gbit / s) in FIG. It can be seen that: 1) The adaptive bit and power loading algorithm can avoid using subcarriers with strong power fading, thereby improving the transmission performance, but it cannot increase the available signal bandwidth. 2) Due to the compensation for frequency selective power fading, the diversity reception system has very few zero bit loaded subcarriers and can utilize a bandwidth of up to about 50 GHz, greatly expanding the available signal bandwidth. 3) Under the 7% hard decision forward error correction coding threshold with a bit error rate lower than 3.8×10-3, the diversity reception method of the present invention can achieve adaptive bit and power loading OFDM signal transmission at a rate of 208.1 Gbit / s after 10 km of standard single-mode optical fiber transmission, which is 15.3% higher than the rate of 180.5 Gbit / s of traditional reception.

[0089] In summary, the present invention provides an IMDD-OFDM optical fiber system and a diversity reception dispersion compensation method thereof. The IMDD-OFDM optical fiber system includes a transmitter and a receiver. The receiver includes: an optical beam splitter, a dispersion element, a first photodetector, a second photodetector and a receiving end digital signal processing module. The OFDM optical signal output by the transmitter is sent to the optical beam splitter of the receiver to divide the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal. The first OFDM optical signal is then detected by a first photodetector to output a first OFDM electrical signal. The second OFDM optical signal is detected by a second photodetector after passing through a dispersion element and outputs a second OFDM electrical signal. The first OFDM electrical signal and the second OFDM electrical signal are then processed by a digital signal processing module at the receiving end to perform analog-to-digital conversion, synchronization processing and fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal to obtain received signals of each subcarrier of the first OFDM electrical signal and received signals of each subcarrier of the second OFDM electrical signal respectively. The corresponding subcarrier signals of the two OFDM electrical signals are combined by using a maximum ratio combining algorithm to obtain output signals of each subcarrier with frequency selective power fading caused by compensated dispersion. In this way, the present invention combines the corresponding subcarrier signals of two OFDM electrical signals by adopting the maximum ratio combining algorithm through diversity reception, which can effectively compensate for the problem of frequency selective power fading, so that the signal-to-noise ratio of the output signal is maximized, and the receiver has a simple structure and low system cost.

[0090] It should be understood that the application of the present invention is not limited to the above examples. For ordinary technicians in this field, improvements or changes can be made based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. An IMDD-OFDM optical fiber system, comprising a transmitter and a receiver, It is characterized in that The receiver comprises: an optical beam splitter, a dispersion element, a first photodetector, a second photodetector and a receiving end digital signal processing module; The transmitter is connected to the optical beam splitter and is used to output an OFDM optical signal to the optical beam splitter; The optical beam splitter is connected to the first photodetector and the dispersive element respectively, and is used to split the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal and send them to the first photodetector and the dispersive element respectively; The first photoelectric detector is connected to the receiving end digital signal processing module, and is used to convert the first OFDM optical signal into a first OFDM electrical signal and output it to the receiving end digital signal processing module; The second photodetector is connected to the dispersion element and the receiving end digital signal processing module respectively, and is used to convert the second OFDM optical signal into a second OFDM electrical signal and output it to the receiving end digital signal processing module; The receiving end digital signal processing module is used to obtain the received signal of each subcarrier of the first OFDM electrical signal and the received signal of each subcarrier of the second OFDM electrical signal according to the first OFDM electrical signal and the second OFDM electrical signal, and combine the corresponding subcarrier signals of the two OFDM electrical signals through the maximum ratio combining method to obtain the output signal of the frequency selective power fading caused by the compensated dispersion of each subcarrier.

2. The IMDD-OFDM optical fiber system according to claim 1, It is characterized in that Also includes: Oscilloscope; The oscilloscope is connected to the first photodetector, the second photodetector and the receiving end digital signal processing module respectively, and is used for sampling the first OFDM electrical signal and the second OFDM electrical signal.

3. The IMDD-OFDM optical fiber system according to claim 1, It is characterized in that The transmitter includes: a transmitting end digital signal processing module, an arbitrary waveform generator and an electrical amplifier; wherein, The transmitting end digital signal processing module is connected to the arbitrary waveform generator, and is used to generate a real number OFDM signal and output it to the arbitrary waveform generator; The arbitrary waveform generator is connected to the transmitting end digital signal processing module and the electrical amplifier respectively, and is used to generate an OFDM electrical signal according to the OFDM signal; The electrical amplifier is used to amplify the OFDM electrical signal.

4. The IMDD-OFDM optical fiber system according to claim 3, It is characterized in that The transmitter also includes: a laser, a modulator, a single-mode optical fiber, an erbium-doped optical fiber amplifier and an optical attenuator; wherein, The laser is used to generate an optical carrier; The modulator is connected to the electrical amplifier and the single-mode optical fiber respectively, and is used to work at an orthogonal point under the drive of the OFDM electrical signal, and the optical carrier outputs an OFDM optical signal to the single-mode optical fiber after passing through the modulator; The erbium-doped fiber amplifier is connected to the single-mode optical fiber and the optical attenuator respectively, and is used to amplify the power of the OFDM optical signal; The optical attenuator is connected to the erbium-doped fiber amplifier and the optical beam splitter respectively, and is used to adjust the receiving power of the receiving signal of the transmitter.

5. The IMDD-OFDM optical fiber system according to claim 1, It is characterized in that The dispersive element is a standard single-mode optical fiber.

6. The IMDD-OFDM optical fiber system according to claim 1, It is characterized in that The first photodetector and the second photodetector are both single photodiodes.

7. A diversity reception dispersion compensation method for an IMDD-OFDM optical fiber system according to any one of claims 1 to 6, It is characterized in that include: Sending the OFDM optical signal output by the transmitter to the optical beam splitter of the receiver to split the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal; The first OFDM optical signal is detected by the first photodetector and then outputs a first OFDM electrical signal. The second OFDM optical signal is detected by the second photodetector after passing through the dispersion element and then outputs a second OFDM electrical signal. Performing analog-to-digital conversion, synchronization processing, and fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal to obtain received signals of each subcarrier of the first OFDM electrical signal and received signals of each subcarrier of the second OFDM electrical signal respectively; The corresponding subcarrier signals of the two OFDM electrical signals are combined by using the maximum ratio combining algorithm to obtain the output signal of each subcarrier with the frequency selective power fading caused by the compensation dispersion.

8. The diversity reception dispersion compensation method for the IMDD-OFDM optical fiber system according to claim 7, It is characterized in that The step of sending the OFDM optical signal output by the transmitter to the optical beam splitter of the receiver to split the OFDM optical signal into a first OFDM optical signal and a second OFDM optical signal comprises: Mapping a pseudo-random binary sequence to a quadrature amplitude modulation symbol; Perform parallel-to-serial conversion and subcarrier mapping on quadrature amplitude modulation symbols; Use inverse fast Fourier transform to generate real OFDM signal; The real OFDM signal is added with a cyclic prefix and is output to an arbitrary waveform generator after parallel-to-serial conversion and amplitude clipping to generate an OFDM electrical signal.

9. The diversity reception dispersion compensation method for the IMDD-OFDM optical fiber system according to claim 8, It is characterized in that The step of performing analog-to-digital conversion, synchronization processing, and fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal to obtain received signals of each subcarrier of the first OFDM electrical signal and received signals of each subcarrier of the second OFDM electrical signal respectively comprises: Before performing fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal, serial-to-parallel conversion and cyclic prefix removal processing are performed.

10. The diversity reception dispersion compensation method for the IMDD-OFDM optical fiber system according to claim 8, It is characterized in that The step of performing analog-to-digital conversion, synchronization processing, and fast Fourier transform processing on the first OFDM electrical signal and the second OFDM electrical signal to obtain received signals of each subcarrier of the first OFDM electrical signal and received signals of each subcarrier of the second OFDM electrical signal respectively further includes: After fast Fourier transform processing is performed on the first OFDM electrical signal and the second OFDM electrical signal, parallel-to-serial conversion, orthogonal amplitude modulation symbol demapping and bit error rate calculation processing are performed.

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