CN115811367B - Balancing method of underwater optical communication system based on combination of hardware and software - Google Patents
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Abstract
The invention discloses an equalizing method of an underwater optical communication system based on combination of hardware and software, which comprises the following steps: s1, acquiring a first amplitude-frequency characteristic of an underwater optical communication system; s2, designing a hardware equalization circuit according to the first amplitude-frequency characteristic and adding the hardware equalization circuit into an underwater optical communication system; s3, under different communication scenes, obtaining a second amplitude-frequency characteristic of the underwater optical communication system added with the hardware equalization circuit, performing inverse Fourier transform on the second amplitude-frequency characteristic, and normalizing the second amplitude-frequency characteristic to obtain a unit impulse response of an underwater optical communication channel; s4, determining the order of a software equalizer and a corresponding tap coefficient according to the unit impulse response and the expected output signal, and adding the software equalizer into an underwater optical communication system in a manner of FPGA and DAC so as to flexibly compensate the equalizing effect of the hardware equalizing circuit according to a communication scene. By combining hardware equalization and software equalization, the communication performance of the underwater optical communication system is remarkably improved.
Description
Technical Field
The invention belongs to the technical field of underwater optical communication, and particularly relates to an equalization method of an underwater optical communication system based on combination of hardware and software.
Background
Approximately 71% of the earth's surface is covered by the ocean, which is rich in mineral resources, biological resources, and power resources. The 21 st century is called a new era of ocean economy, and with the detection, development and utilization of the ocean by humans, the current underwater communication mode cannot meet the increasing communication capacity requirement. Underwater communication can be classified into underwater wired communication and underwater wireless communication, and the underwater wired communication is mainly to connect users by using an optical cable. Submarine optical cable communication is similar to land optical fiber communication, information transmission is carried out by utilizing the optical cable, but submarine environment is different from land, the submarine optical cable is difficult to lay, high in maintenance cost and easy to influence marine organisms. Based on this, the exploration of marine resources is not free from efficient wireless communication technologies.
Currently, the mainstream underwater wireless communication technology includes underwater electromagnetic wave communication, underwater acoustic communication, underwater magnetic induction communication, and underwater wireless optical communication technology. Seawater is a good conductor, and when electromagnetic waves propagate underwater, the energy of the electromagnetic waves is attenuated sharply, and the attenuation of the electromagnetic waves under the water is larger when the frequency is higher. Therefore, the underwater electromagnetic wave communication is mainly divided into very low frequency (3-30 kHz) communication and ultra-low frequency (30-3000 Hz) communication, the communication distance is only a few meters, and the communication speed is extremely low. According to the antenna theory, the size of a transceiver antenna for very low frequency and ultra low frequency communication is large, which also causes the defects of poor maneuverability, concealment and the like of the underwater communication equipment. Underwater acoustic communication is the most mature underwater communication technology at present, and the communication distance is long and can reach tens of kilometers. But the low carrier frequency of the acoustic wave makes the bandwidth of the underwater acoustic communication small, which makes it difficult for the communication rate of the underwater acoustic communication to reach the order of Mbps. In addition, the propagation speed of sound waves in water is only 1500m/s, which is far lower than the propagation speed of light speed, the transmission delay is usually in the order of seconds, and the transmission requirements of high-speed service and low delay which are needed for deep sea data transmission are difficult to meet. Magnetic induction communication requires precise coil alignment and short transmission distances. In contrast, underwater optical communication technology has wider available spectrum resources, higher communication security, stronger background noise suppression capability, and lower power consumption. Therefore, the underwater optical communication technology has great development potential and application value facing to the communication requirements of mass data transmission and high-speed data interaction between underwater communication devices.
In an underwater LED optical communication system, an LED is used as a light source, and is a transmitting end of an optical communication signal and a key device for converting an electric signal into an optical signal. The parameters of the LED itself have a great influence on the whole communication system, for example, the 3dB bandwidth of the LED will not exceed 5MHz, and the amplitude-frequency characteristic of the LED is severely attenuated in the high-frequency part, which can seriously affect the communication performance of the underwater LED optical communication system. Aiming at the problem that the effective modulation bandwidth of an LED is limited, researchers have proposed to adopt a hardware pre-equalization strategy at a transmitting end to compensate a high-frequency part of the LED, and to properly attenuate a low-frequency part of the LED to correct the amplitude-frequency characteristic of the LED, so that the effective modulation bandwidth of the LED is expanded, but the LED bandwidth expansion strategy based on the hardware equalization technology is difficult to easily change equalization parameters to meet different communication scene requirements, so that the communication performance of an underwater optical communication system taking the LED as a light source is greatly influenced.
Disclosure of Invention
Aiming at the defects and improvement demands of the prior art, the invention provides an equalization method of an underwater optical communication system based on combination of hardware and software, and aims to solve the problems that in the prior art, equalization parameters, namely the structure and the electrical element values of a hardware equalization circuit, are difficult to change by using an LED bandwidth expansion strategy of a single hardware equalization technology, and switching of different communication scenes is difficult to flexibly realize, so that the demands of different communication scenes are met.
In order to achieve the above object, the present invention provides an equalization method for an underwater optical communication system based on combination of hardware and software, comprising the steps of:
s1, acquiring a first amplitude-frequency characteristic of an underwater optical communication system;
S2, designing a hardware equalization circuit according to the first amplitude-frequency characteristic, adding the hardware equalization circuit into an underwater optical communication system, and compensating a high-frequency part and suppressing a low-frequency part of an input electric signal;
S3, under different communication scenes, obtaining a second amplitude-frequency characteristic of the underwater optical communication system added with the hardware equalization circuit, performing inverse Fourier transform on the second amplitude-frequency characteristic, and finally normalizing the second amplitude-frequency characteristic to obtain a unit impulse response of an underwater optical communication channel;
s4, determining the order of a software equalizer and a corresponding tap coefficient according to the unit impulse response and the expected output signal, and adding the software equalizer into an underwater optical communication system to compensate the equalizing effect of the hardware equalizing circuit.
Further, the underwater optical communication system comprises a direct current bias device, an electro-optical conversion module LED, a photosensitive detector and a transimpedance amplifier.
Further, the hardware equalization circuit is a bridge T-shaped equalization circuit or an m-order cascaded bridge T-shaped equalization circuit, and m is more than or equal to 2.
Further, the software equalizer is expressed as:
y[n]＝a1-Nx[n+N-1]+...+a-1x[n+1]+a0x[n]+a1x[n-1]+...+aN-1x[n-N+1]
Wherein { x [ n+N-1], …, x [ n+1], x [ N ], x [ N-1], …, x [ N-N+1] } represents the digital baseband signal at the transmission end of { n+N-1, …, n+1, N-1, …, N-N+1} moment, { a 1-N,…,a-1,a0,a1,…,aN-1 } represents tap coefficients of different orders of the software equalizer, and y [ N ] represents the digital baseband signal after equalization at the N moment.
Further, the tap coefficients of the software equalizer are expressed as:
Where { h 2-2N,…,h-2,h-1,h0,h1,h2,…,h2N-2 } represents the unit impulse response of the underwater optical communication channel, { Y 1-N,…Y-1,Y0,Y1,…,Y1-N } represents the desired output signal, { a 1-N,…,a-1,a0,a1,…,aN-1 } represents the tap coefficients of the different orders of the software equalizer.
Further, the tap coefficients of the software equalizer are calculated by zero forcing or minimum mean square error.
Further, the order of the software equalizer and the corresponding tap coefficients are adjusted by the FPGA and the DAC.
In general, through the above technical solutions conceived by the present invention, the following beneficial effects can be obtained:
According to the invention, the hardware equalization circuit is added into the underwater optical communication system, so that the amplitude-frequency characteristic of the underwater optical communication system tends to be flat, and the effective modulation bandwidth of the system is expanded. Furthermore, by adding the software equalizer, the problem that the hardware equalizing circuit is difficult to flexibly adjust equalizing parameters under different communication scenes can be solved, so that the equalizing effect of the hardware equalizing circuit is compensated; the signal waveform of the receiving end can be further modified, and the signal waveform is remodeled so as to facilitate the sampling and the judgment of the receiving end on the user signal. By combining hardware equalization and software equalization, the communication performance of the underwater optical communication system is remarkably improved.
Drawings
Fig. 1 is a flowchart of an equalization method of an underwater optical communication system based on a combination of hardware and software according to an embodiment of the present invention;
fig. 2 is a schematic diagram of amplitude-frequency characteristic test of an underwater optical communication system according to an embodiment of the present invention;
fig. 3 is a schematic diagram of a bridge T-type hardware equalization circuit in accordance with an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. In addition, the technical features of the embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
In the present invention, the terms "first," "second," and the like in the description and in the drawings, if any, are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order.
Referring to fig. 1, in combination with fig. 2 and fig. 3, the present invention provides an equalization method for an underwater optical communication system based on a combination of hardware and software. The method includes operations S1 to S4.
Operation S1, a first amplitude-frequency characteristic of the underwater optical communication system is acquired.
In this embodiment, as shown in the dashed box of fig. 2, a basic underwater optical communication system mainly includes a dc bias device 1, a dc voltage driving light source 2, an electro-optical conversion module LED 3, an underwater channel 4, a photosensitive detector 5, and a transimpedance amplifier 6.
The direct current bias device is used for coupling the transmitted digital baseband signal with the driving current required by the LED and sending the coupled signal into the LED to complete the conversion of the electric signal and the optical signal; the LED is used as a transmitting end antenna for optical communication and sends out signals. The light signal is received by the light sensitive detector at the receiving end through the underwater channel, and the light signal is converted into an electric signal. The electric signal is a weak current signal, and the weak current signal is amplified into a voltage signal which can be used for sampling and judging through a transimpedance amplifier.
Further, as shown in fig. 2, the amplitude-frequency characteristic of the system is tested by using the vector network analyzer 8, the signal sent by the port one 7 of the vector network analyzer has a signal power amplitude of 0dBm, the signal is coupled to the transmitting end of the LED through the direct current bias device to complete the conversion of the electric signal and the optical signal, the optical signal passes through the underwater channel, the photosensitive detector at the receiving end converts the optical signal into the electric signal, then the signal amplification is completed through the transimpedance amplifier, the current signal is converted into the voltage signal, and finally the voltage signal returns to the port two 9 of the vector network analyzer.
Specifically, by testing the signal amplitude of the different frequencies and returning to the vector network analyzer, the amplitude-frequency characteristic (i.e., the first amplitude-frequency characteristic) of the underwater optical communication system based on the LED, such as the 3dB modulation bandwidth of the system, can be obtained.
And S2, designing a hardware equalization circuit according to the first amplitude-frequency characteristic, and adding the hardware equalization circuit into an underwater optical communication system to compensate a high-frequency part of an input electric signal and inhibit a low-frequency part.
Specifically, according to the first amplitude-frequency characteristic of the underwater optical communication system acquired in operation S1, a hardware equalization circuit suitable for the system is designed, including structural design and parameter setting of the hardware equalization circuit. Common hardware equalization circuits include dc bias improvement based equalization circuits, series resonant equalization circuits, passive lead correction equalization circuits, and bridge T-type equalization circuits. The bridge T-shaped equalization circuit is easy to control the final frequency of equalization, and can meet the compensation characteristic of different amplitude-frequency characteristics, and is selected as a hardware equalization circuit of the system, as shown in fig. 3.
For a bridge T-shaped equalization circuit, the following needs to be satisfied in order to satisfy the circuit characteristics:
R2R3＝R1R4＝Z11Z22
Wherein Z 11 is the equivalent resistance of C 1、R1 and L 1, Z 22 is the equivalent resistance of C 2、R4 and L 2, in the circuit design, the equalizing final frequency f 0 of the equalizer can be controlled as:
And in this circuit, Z 11 and Z 22 can be represented as:
Then the forward gain of the bridge T-shaped equalization circuit can be expressed as:
Wherein C 1、L1、R4 is an electronic component of the bridge T-type equalization circuit, R 4 determines a gain of a low frequency portion of the equalization circuit, and R L is a load of the equalization circuit.
In addition, in order to meet the compensation requirements of amplitude-frequency characteristics of different underwater optical communication systems, an m-order cascaded bridge T-shaped equalization circuit can be arranged, and can be expressed as:
Furthermore, before the designed hardware equalization circuit is added into a direct current bias device of the underwater optical communication system, the equalization circuit compensates the high-frequency part of the input electric signal, suppresses the low-frequency part of the input electric signal, realizes the correction of the amplitude-frequency characteristic of the underwater optical communication system using the LED as a light source, and expands the effective modulation bandwidth of the underwater optical communication system. Meanwhile, the hardware equalization circuit can attenuate the amplitude of the signal, before the electric signal is sent to the direct current bias device, the power amplifier and the attenuator are used for regulating and controlling the amplitude of the signal, and the proper optical signal power is obtained after the electro-optic conversion of the LED, so that the response requirement of the photosensitive detector at the receiving end is met, and the optical signal can be successfully collected.
More specifically, the designed hardware equalization circuit is simulated on ADS simulation software, and a verification experiment is carried out by combining a specific underwater optical communication system, so that parameters of the hardware equalization circuit are adjusted to adapt to the communication system.
It will be appreciated that the equalization effect of the hardware equalization circuit may not be optimal when the communication scenario (e.g., different communication rates, different communication distances, different communication environments, etc.) changes. Based on the method, the invention further introduces a software equalizer to waveform-shape the receiving end and compensate the equalizing effect of the hardware equalizing circuit so as to adapt to different communication scenes.
And S3, under different communication scenes, acquiring the second amplitude-frequency characteristic of the underwater optical communication system added with the hardware equalization circuit, performing inverse Fourier transform on the second amplitude-frequency characteristic, and finally normalizing the second amplitude-frequency characteristic to obtain the unit impulse response of the underwater optical communication channel.
Specifically, for different communication scenes, the designed hardware equalization circuit is combined, the second amplitude-frequency characteristic of the underwater optical communication system is tested by using a vector network analyzer, impulse responses of different underwater optical communication channels are obtained through inverse Fourier transform, and a unit impulse response matrix H n＝[h2-2N,…,h-2,h-1,h0,h1,h2,…,h2N-2 is obtained through normalization.
And S4, determining the order of a software equalizer and a corresponding tap coefficient according to the unit impulse response and the expected output signal, and adding the software equalizer into an underwater optical communication system in a manner of FPGA and DAC so as to flexibly compensate the equalizing effect of the hardware equalizing circuit according to a communication scene.
Specifically, after obtaining the unit impulse responses of the underwater optical communication channels in different communication scenarios according to operation S3, determining the order of the software equalizer and the corresponding tap coefficients according to the unit impulse responses and the desired output signals.
The software equalizer may be a high-pass filter, such as a finite impulse response filter, and may be expressed as:
y[n]＝a1-Nx[n+N-1]+...+a-1x[n+1]+a0x[n]+a1x[n-1]+...+aN-1x[n-N+1]
Wherein { x [ n+N-1], …, x [ n+1], x [ N ], x [ N-1], …, x [ N-N+1] } represents the digital baseband signal at the transmission end of { n+N-1, …, n+1, N-1, …, N-N+1} moment, { a 1-N,…,a-1,a0,a1,…,aN-1 } represents tap coefficients of different orders of the software equalizer, and y [ N ] represents the digital baseband signal after equalization at the N moment.
Further, the order of the software equalizer is planned, and the software equalizer is added in the underwater optical communication system to compensate the high-frequency attenuation of the channel, so that other parts of the unit impulse response matrix after equalization except Y 0 can be zero or smaller, namely, the unit impulse response matrix exists:
Where { h 2-2N,…,h-2,h-1,h0,h1,h2,…,h2N-2 } represents the unit impulse response of the underwater optical communication channel, { Y 1-N,…Y-1,Y0,Y1,…,Y1-N } represents the desired output signal, { a 1-N,…,a-1,a0,a1,…,aN-1 } represents the tap coefficients of the different orders of the software equalizer.
Further, tap coefficients of the software equalizer may be determined by a zero forcing algorithm or a minimum mean square error method, etc. The tap coefficient matrix of the software equalizer is expressed as:
Illustratively, the tap coefficients are normalized, e.g., when the filter is two-order, the tap coefficients are set to [1, a 1 ]; when the filter is of third order, the tap coefficient is [ a -1,1,a1 ]; when the filter is of the fifth order, the tap coefficient is [ a -2,a-1,1,a1,a2 ]; and so on.
When determining the order of the software equalizer and the corresponding tap coefficient, firstly presetting an order, and determining the corresponding tap coefficient by the method. And comparing y n with x n to calculate error rate to determine whether the relevant parameters of the software equalizer meet the equalization requirement, if not, adjusting the order of the software equalizer, and determining the corresponding tap coefficient.
It should be noted that, the order of the software equalizer and the corresponding tap coefficient can be adjusted by the FPGA and the DAC, so that the equalization parameters under different communication scenes can be flexibly switched to meet different communication requirements.
In addition, through the software equalizer, upper overshoot and lower overshoot can be realized on the signal of the transmitting end, so that the reshaping of the signal waveform of the receiving end is realized. In an underwater optical communication system, an LED is used as a light source, and the on-off speed of the LED can directly influence the signal waveform of a receiving end, namely the charging time and the discharging time of the LED, so that the judgment of a user signal can be influenced. When a square wave signal is transmitted, it can be understood that the rising time t r and the falling time t f of the signal are. For a linear communication system, i.e. when the LED is operating in the linear regime, the 3dB bandwidth and rise time t r and fall time t f of the LED have the following relationship:
Where f -3dB denotes the 3dB modulation bandwidth size of the light source LED. From the above equation, decreasing t r and t f can increase the 3dB bandwidth of the LED. t r and t f can be reduced by increasing the charge voltage and the discharge voltage. Theoretical calculations and experiments prove that the rise time of photoluminescence of the LED can be reduced by increasing the injection current. This can be explained by carrier scavenging effects and peak effects. The external voltage injection increases the built-in electric field of the LED diode, so that the carriers in the LED diode are rapidly removed from the active region of the LED diode, and t r and t f are reduced to achieve the purpose of waveform reshaping.
The method is combined with a hardware equalization circuit for a software equalizer designed under different communication scenes. On the basis of hardware equalization, the order of the software equalizer and the corresponding tap coefficient are slightly adjusted, so that the equalization of the whole underwater optical communication system can be completed. The hardware equalization circuit is used for correcting amplitude-frequency characteristics of an underwater optical communication system taking an LED as a light source, attenuating low-frequency components of signals and improving high-frequency components of the signals; the software equalizer, namely the feedforward equalizer, is used for correcting the signal waveform of the receiving end of the underwater optical communication system taking the LED as a light source, and enabling the underwater optical communication system to flexibly adapt to different communication scenes so as to meet different communication requirements.
Further, combining the designed hardware equalization circuit and the software equalizer, and carrying out experimental verification of an underwater optical communication system taking the LED as a light source;
specifically, the amplitude-frequency characteristic of the underwater optical communication system is corrected by combining the designed hardware equalization circuit and the software equalizer, and the effective modulation bandwidth of the underwater optical communication system is expanded through the hardware equalization circuit. On the basis of hardware equalization, a designed software equalizer is adopted to further reshape the signal waveform of a receiving end so as to improve the performance of the system. Besides, by means of the balancing mode of combining the hardware balancing circuit and the software equalizer, the underwater optical communication system is beneficial to flexibly coping with different communication scenes. Based on the equalization mode, communication experiments under different communication scenes, including experiments of different communication rates, different communication distances and different communication channel environments, are carried out to verify the feasibility of the proposed equalization mode.
In summary, in the above embodiments of the present invention, a method for correcting the amplitude-frequency characteristic of an underwater optical communication system using an LED as a light source is provided, so that the underwater optical communication system can cope with different communication scenarios and ensure communication performance. According to the method, the hardware equalization circuit is designed according to the actual amplitude-frequency characteristic of the underwater optical communication system, which is measured through experiments, and the hardware equalization circuit comprises the structure of the hardware equalization circuit, the parameters of electronic elements and the like, so that the amplitude-frequency characteristic of the underwater optical communication system tends to be flat, and the effective modulation bandwidth of the system is expanded. And combining with the underwater optical communication system, and testing the effect of the hardware equalization circuit. Further, in the underwater link, communication experiments under different communication scenes are carried out, the error code performance of the system at the moment is tested and analyzed, and according to the experimental result, the hardware equalization circuit is difficult to flexibly switch equalization parameters, so that the software equalizer, namely the finite impulse response filter, is designed, the problem that the hardware equalization circuit is difficult to flexibly adjust the equalization parameters under different communication scenes can be solved, the signal waveform of the receiving end can be further corrected, and the signal waveform is remolded, so that the receiving end can sample and judge user signals conveniently. Thus, the communication performance of the underwater optical communication system can be remarkably improved.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (7)
1. The equalization method of the underwater optical communication system based on the combination of hardware and software is characterized by comprising the following steps of:
s1, acquiring a first amplitude-frequency characteristic of an underwater optical communication system;
S2, designing a hardware equalization circuit according to the first amplitude-frequency characteristic, adding the hardware equalization circuit into an underwater optical communication system, and compensating a high-frequency part and suppressing a low-frequency part of an input electric signal;
S3, under different communication scenes, obtaining a second amplitude-frequency characteristic of the underwater optical communication system added with the hardware equalization circuit, performing inverse Fourier transform on the second amplitude-frequency characteristic, and finally normalizing the second amplitude-frequency characteristic to obtain a unit impulse response of an underwater optical communication channel;
s4, determining the order of a software equalizer and a corresponding tap coefficient according to the unit impulse response and the expected output signal, and adding the software equalizer into an underwater optical communication system to compensate the equalizing effect of the hardware equalizing circuit.
2. The method for equalizing an underwater optical communication system based on a combination of hardware and software as claimed in claim 1, wherein the underwater optical communication system comprises a direct current bias device, an electro-optical conversion module LED, a photosensitive detector, and a transimpedance amplifier.
3. The equalization method of an underwater optical communication system based on the combination of hardware and software according to claim 1, wherein the hardware equalization circuit is a bridge T-shaped equalization circuit or an m-order cascaded bridge T-shaped equalization circuit, and m is larger than or equal to 2.
4. The method for equalizing an underwater optical communication system based on a combination of hardware and software as claimed in claim 1, wherein the software equalizer is expressed as:
y[n]＝a1-Nx[n+N-1]+...+a-1x[n+1]+a0x[n]+a1x[n-1]+...+aN-1x[n-N+1]
Wherein { x [ n+N-1], …, x [ n+1], x [ N ], x [ N-1], …, x [ N-N+1] } represents the digital baseband signal at the transmission end of { n+N-1, …, n+1, N-1, …, N-N+1} moment, { a 1-N,…,a-1,a0,a1,…,aN-1 } represents tap coefficients of different orders of the software equalizer, and y [ N ] represents the digital baseband signal after equalization at the N moment.
5. The method for equalizing an underwater optical communication system based on a combination of hardware and software as claimed in claim 4, wherein the tap coefficients of the software equalizer are expressed as:
Where { h 2-2N,…,h-2,h-1,h0,h1,h2,…,h2N-2 } represents the unit impulse response of the underwater optical communication channel, { Y 1-N,…Y-1,Y0,Y1,…,Y1-N } represents the desired output signal, { a 1-N,…,a-1,a0,a1,…,aN-1 } represents the tap coefficients of the different orders of the software equalizer.
6. The method for equalizing an underwater optical communication system based on a combination of hardware and software as claimed in claim 5, wherein the tap coefficients of the software equalizer are calculated by a zero forcing method or a minimum mean square error method.
7. The method for equalizing an underwater optical communication system based on a combination of hardware and software as claimed in claim 1, wherein the order of the software equalizer and the corresponding tap coefficients are adjusted by FPGA and DAC.
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