Optical OFDM (Orthogonal Frequency Division Multiplexing) modulation method combining probabilistic shaping technology and subcarrier packet sending technology

Abstract

The invention discloses an optical OFDM (Orthogonal Frequency Division Multiplexing) modulation method combining a probabilistic shaping technology and a subcarrier packet transmission technology, which belongs to the technical field of optical communication, and comprises the following steps of: optimizing a QAM (Quadrature Amplitude Modulation) constellation diagram based on a probabilistic shaping method to obtain the optimal probability distribution of constellation points, and mapping the optimal probability distribution of the constellation points based on a constant distribution matcher to obtain the optimal probability distribution of the constellation points. An OFDM symbol is obtained; performing frequency domain symbol transformation on the OFDM symbol to generate a transformed frequency domain signal; converting the converted frequency domain signal into an LED group signal based on a filtering block; and processing the LED group signal based on a DCO modulation mode to complete OFDM modulation. According to the invention, different subcarriers are respectively modulated on a plurality of LEDs, so that the peak-to-average power ratio of the OFDM signal modulated on each LED is reduced, and the nonlinear noise interference of the system is further resisted.

Description

Technical Field

[0001] The present invention belongs to the field of optical communication technology, and in particular relates to an optical OFDM modulation method combining a probability shaping technology with a subcarrier grouping transmission technology. Background Art

[0002] Wireless optical communication is one of the key candidate technologies for future 6G communications due to its huge spectrum potential. Optical OFDM (Orthogonal Frequency Division Multiplexing) itself has high spectrum efficiency, strong robustness to multipath effects, strong anti-interference ability and easy implementation, and has been widely studied in the field of wireless optical communications. The current optical OFDM model has the problem of dense distribution of constellation signal points under high-order modulation. The existing optical OFDM constellation modulation method mainly draws on the design of RF signals, and does not take the linear working range of the transmitting device LED into consideration. With the increase of modulation order and the increase of OFDM sampling points, this will introduce large nonlinear noise, which will significantly affect the communication performance.

[0003] In addition, current optical OFDM uses a group of LEDs to simultaneously modulate multiple subcarrier signals to send optical signals. However, OFDM has a high peak-to-average power ratio (PAPR) characteristic, and its performance will be limited by the nonlinear characteristics of the LED itself, resulting in increased nonlinear noise and easy distortion of the transmitted waveform. This is the main challenge facing current optical OFDM systems. Summary of the invention

[0004] In order to solve the above technical problems, the present invention proposes an optical OFDM modulation method combining probability shaping technology with subcarrier grouping transmission technology to solve the above problems existing in the prior art.

[0005] To achieve the above object, the present invention provides an optical OFDM modulation method combining probability shaping technology and subcarrier grouping transmission technology, comprising:

[0006] Optimizing the QAM constellation diagram based on a probability shaping method to obtain an optimal probability distribution of constellation points, mapping the optimal probability distribution of the constellation points based on a constant distribution matcher to obtain OFDM symbols;

[0007] Performing frequency domain symbol transformation on the OFDM symbol to generate a transformed frequency domain signal;

[0008] Converting the transformed frequency domain signal into an LED group signal based on a filter block;

[0009] The LED group signal is processed based on the DCO modulation method to complete OFDM modulation.

[0010] Preferably, the expression of the probability shaping method is:

[0011]

[0012] Among them, x i is the possible amplitude in a single-channel QAM signal, v is the scaling factor, P X () is the probability of taking a single amplitude signal, M is the total number of QAM signals, k is the index of the corresponding signal, Sum all the subterms of e-vx as the denominator.

[0013] Preferably, the process of mapping the optimal probability distribution of the constellation point based on the constant distribution matcher to obtain the OFDM symbol further includes: amplifying the signal amplitude of the optimal probability distribution of the constellation point;

[0014] The expression for amplifying the signal amplitude of the optimal probability distribution of the constellation point is:

[0015]

[0016] Among them, α is the amplification factor, P is the power of the original signal, is the power of the modulated signal, x[n] is the time domain sampling point of the original signal, n is the number of bits, and y[n] is the time domain sampling point of the modulated signal.

[0017] Preferably, the expression for generating the transformed frequency domain signal is:

[0018]

[0019] Among them, X[k] is the kth constellation point symbol in MQAM, and n=0,1,…,N-1 is the time index.

[0020] Preferably, the process of generating the transformed frequency domain signal further comprises: performing Hermitian symmetry processing on the transformed frequency domain signal, and the expression is:

[0021] X Hmt [Nk] = X Hmt* [k];

[0022] Among them, X Hmt [Nk] is the frequency domain sampling point at index Nk after Hermitian transformation, X Hmt* [k] is the frequency domain conjugate sampling point at index k after the Hermitian transform.

[0023] Preferably, the LED group signal includes N / L subcarrier resources;

[0024] The frequency domain signal of the subcarrier resource is expressed as:

[0025]

[0026] in, For H l [k] is H L =[H1,H2…,H l ] is the group filter block vector, l=1,2,…,L is the group number of the LED group.

[0027] Preferably, the process of converting the transformed frequency domain signal into an LED group signal based on the filter block further includes: after grouping by the filter block, converting the frequency domain signal into a time domain signal by performing IFFT, the expression is:

[0028]

[0029] in, is the time domain signal, and IFFT() is the IFFT transform.

[0030] Preferably, the expression for processing the LED group signal based on the DCO modulation method is:

[0031]

[0032] in, Bias is the DCO pre-modulation signal after adding bias. l is the DC bias, is the trimmed DCO modulation signal.

[0033] Compared with the prior art, the present invention has the following advantages and technical effects:

[0034] The purpose of the present invention is to provide a new optical signal transmission technology that combines probability shaping technology with subcarrier grouping transmission technology to reduce the peak-to-average power ratio of optical OFDM. By introducing a probability shaping method to adjust the distribution probability of constellation points in optical OFDM modulation, the transmission probability of constellation points with lower energy is higher than that of constellation points with higher energy, thereby reducing the peak-to-average power ratio of the system and resisting the nonlinear noise interference of the system. In addition, in view of the nonlinear distortion problem of LEDs in optical OFDM, subcarrier grouping transmission technology is adopted to modulate different subcarriers on multiple LEDs respectively, thereby reducing the peak-to-average power ratio of the modulated OFDM signal on each LED, further resisting the nonlinear noise interference of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0036] Figure 1This is a block diagram of the communication system principle after the introduction of probability shaping in an embodiment of the present invention;

[0037] Figure 2 A schematic diagram of the input-output mapping relationship of the CCDM arithmetic encoder according to an embodiment of the present invention;

[0038] Figure 3 This is a principle block diagram of an optical OFDM communication transmission system for subcarrier grouping transmission according to an embodiment of the present invention. DETAILED DESCRIPTION

[0039] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0040] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0041] Embodiment 1

[0042] This embodiment provides an optical OFDM modulation method combining a probability shaping technology and a subcarrier grouping transmission technology, including:

[0043] Optimizing the QAM constellation diagram based on a probability shaping method to obtain an optimal probability distribution of constellation points, mapping the optimal probability distribution of the constellation points based on a constant distribution matcher to obtain OFDM symbols;

[0044] Performing frequency domain symbol transformation on the OFDM symbol to generate a transformed frequency domain signal;

[0045] Converting the transformed frequency domain signal into an LED group signal based on a filter block;

[0046] The LED group signal is processed based on the DCO modulation method to complete OFDM modulation.

[0047] First, the probability shaping method is introduced to optimize the QAM constellation diagram and reduce the peak-to-average power ratio of the system. The block diagram of the communication system after the introduction of probability shaping is as follows: Figure 1 shown. Figure 2 It is a typical schematic diagram of the input-output mapping relationship of the probability shaping CCDM arithmetic encoder.

[0048] Since the IM / DD method is used for data transmission and reception in optical communication, the OFDM signal needs to be processed in a certain way, and then the DCO, ACO or LACO modulation method combined with the positive bias circuit is used to ensure that the transmitter sends a positive real-valued signal. The subcarrier grouping transmission technology focuses on dividing all the original LED array signals into multiple LED light groups. The LEDs in different light groups send signals in different frequency bands, and the LEDs in the same light group send the same signal. The principle block diagram of the communication transmission system of spatial diversity optical OFDM is as follows Figure 3 shown.

[0049] The specific implementation includes the following steps:

[0050] Step 1: According to information theory, the capacity difference between the input signal when it obeys uniform distribution and the Shannon limit is 1.53dB. The probability shaping method is to change the original uniformly distributed QAM signal into an input signal distribution suitable for channel conditions, and the input signal obeying the Maxwell-Boltzmann (MB) distribution can meet this requirement.

[0051] The specific probability distribution of probability shaping adopts MB distribution, and the calculation formula is as follows:

[0052]

[0053] The above formula is a calculation formula for one-dimensional symbols, corresponding to the I or Q channel of the QAM signal. The probability distribution of all QAM signals can be calculated through the probability distribution of a single channel. i is the desirable amplitude in a single-channel QAM signal; v is the scaling factor, which is a key parameter of probability shaping, and v is determined by the SNR of the channel.

[0054] At the same time, in order to ensure that the average power of the signal before and after the probability shaping is equal, it is also necessary to amplify the amplitude of the shaped signal. and X i Represent the probability shaping frequency domain signal and the uniform distribution frequency domain signal respectively. The amplification factor α is first calculated according to the following process:

[0055]

[0056] In order to ensure that the average power of the signal is equal, the amplitude of the time domain signal after probability shaping is amplified at the LED output end:

[0057] y[n]=αx[n] (3)

[0058] Then the codebook mapping is realized through the constant distribution matcher (CCDM), which is a reversible mapping and is also the basis of modulation and demodulation. Figure 1The essence of this process is to map a binary bit stream into a given target probability sequence stream, namely B:{0,1} n →A:{A1,...,A m} m , realizing the mapping relationship from n bits to m symbols.

[0059] To achieve this process, CCDM uses a finite length sequence A m The empirical probability distribution P A To approximate the target probability distribution Specific implementation ideas such as Figure 2 Assume n = 2, m = 2 and P1 = 0.5, P3 = 0.5, then the operation process is as follows:

[0060] Since the input bits are uniformly distributed, the probability interval can be equally divided into [0,0.5) and [0.5,1]. If the first input bit is 0, it is mapped to [0,0.5), otherwise it is mapped to [0.5,1]. Then repeat the above process until all n input bits are divided, such as Figure 2 (a) Four partition intervals are obtained on the left.

[0061] Similarly, the output target sequence symbol interval also needs to be divided to obtain m intervals, but the frequency of occurrence of each symbol in the output sequence must conform to the probability distribution. In this example, in order to meet the above conditions, the output target sequence must consist of two 1s and two 3s. First, the probability of symbols 1 and 3 is 1 / 2. When the first bit is 1, the probabilities of the remaining symbols 1 and 3 are 1 / 3 and 2 / 3 respectively; similarly, when the first bit is 3, the probabilities of the remaining symbols 1 and 3 are 2 / 3 and 1 / 3 respectively. Repeating the above process can obtain the divided intervals, such as Figure 2 (b) In order to ensure a one-to-one mapping relationship between input and output intervals, the mapping criteria are:

[0062] The lower bound of the output interval needs to be included in the input interval.

[0063] If there are multiple output intervals that meet condition 1, the output interval with a lower probability is selected to correspond to the input interval one by one.

[0064] In summary, the arithmetic encoder realizes the one-to-one mapping between the input binary bit sequence and the output symbol sequence through interval division. The probability shaping method can make the constellation point closer to the origin and reduce the peak-to-average power ratio of the system.

[0065] Step 2: First, generate OFDM symbols. Consider the OFDM symbol of N-point MQAM, where N is the number of FFT operation points, and the k-th constellation point symbol in MQAM can be represented by X[k], where k = 0, 1, ..., N-1. The corresponding time domain OFDM signal expression is as follows:

[0066]

[0067] Where n=0, 1,…, N-1 is the time index.

[0068] In actual engineering, N is generally taken as a power of 2. After the frequency domain OFDM symbol is processed as follows, Hermitian symmetry is satisfied, and X[0]=X[N / 2]=0, it can be ensured that the time domain symbols are all real numbers.

[0069] X Hmt [Nk] = X Hmt* [k] (5)

[0070] Step 3: Divide the original single-channel signal into multiple LED light groups to implement subcarrier grouping transmission technology. The LEDs in different light groups send signals in different frequency bands, and the LEDs in the same light group send the same signal.

[0071] After the OFDM signal processing in step 2, the obtained frequency domain signal X[k] is passed through L filter blocks to obtain L LED group signals, each of which contains N / L subcarrier resources. The vector of each filter block is H L =[H1,H2…,H l ], where l = 1, 2, ..., L is the group number. Then the frequency domain symbol of the kth subcarrier of the lth LED group can be obtained by the following formula:

[0072]

[0073] Assume L = 3, then Figure 3 As shown, after filtering block grouping, the frequency domain signal needs to be converted into a time domain signal by IFFT:

[0074]

[0075] Each group of signals has an independent LED branch, and the LED lights in each group transmit the same signal.

[0076] Step 4: In order to make the signal satisfy the positivity in optical communication, the signal processed in step 3 can be modulated by DCO, ACO or LACO to ensure that the output signal is positive.

[0077] Here we take DCO as an example. DCO uses the time domain signal A DC bias is added to the signal, and then the signal that is still less than zero after adding the DC bias is clipped to zero, achieving the positive value of the signal, as shown in the following formula:

[0078]

[0079] From the above steps, we can see that because the power is limited in actual engineering, the DC bias cannot be infinite, and the performance of the DCO will be limited by the peak-to-average power ratio of the time domain signal. The spatial diversity technology can reduce the peak-to-average power ratio of a single LED and effectively resist the clipping noise of the DCO.

[0080] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. An optical OFDM modulation method combining probability shaping technology and subcarrier grouping transmission technology, characterized in that: The following steps are involved: Optimizing the QAM constellation diagram based on a probability shaping method to obtain an optimal probability distribution of constellation points, mapping the optimal probability distribution of the constellation points based on a constant distribution matcher to obtain OFDM symbols; Performing frequency domain symbol transformation on the OFDM symbol to generate a transformed frequency domain signal; Converting the transformed frequency domain signal into an LED group signal based on a filter block; The LED group signal is processed based on the DCO modulation method to complete OFDM modulation.

2. The optical OFDM modulation method combining probability shaping technology and subcarrier grouping transmission technology according to claim 1 is characterized in that: The expression of the probability shaping method is: Among them, x i is the possible amplitude in a single-channel QAM signal, v is the scaling factor, P X () is the probability of taking a single amplitude signal, M is the total number of QAM signals, k is the index of the corresponding signal, Sum all the subterms of e-vx as the denominator.

3. The optical OFDM modulation method combining probability shaping technology and subcarrier grouping transmission technology according to claim 1, characterized in that: The process of mapping the optimal probability distribution of the constellation point based on the constant distribution matcher to obtain the OFDM symbol further includes: amplifying the signal amplitude of the optimal probability distribution of the constellation point; The expression for amplifying the signal amplitude of the optimal probability distribution of the constellation point is: y[n]=αx[n]; Among them, α is the amplification factor, P is the power of the original signal, is the power of the modulated signal, x[n] is the time domain sampling point of the original signal, n is the number of bits, and y[n] is the time domain sampling point of the modulated signal.

4. The optical OFDM modulation method combining probability shaping technology and subcarrier grouping transmission technology according to claim 1, characterized in that: The expression for generating the transformed frequency domain signal is: Among them, X[k] is the kth constellation point symbol in MQAM, and n=0,1,…,N-1 is the time index.

5. The optical OFDM modulation method combining probability shaping technology and subcarrier grouping transmission technology according to claim 1, characterized in that: The process of generating the transformed frequency domain signal further includes: performing Hermitian symmetry processing on the transformed frequency domain signal, and the expression is: Among them, X Hmt [Nk] is the frequency domain sampling point at index Nk after Hermitian transformation. is the frequency domain conjugate sampling point at index k after the Hermitian transform.

6. The optical OFDM modulation method combining probability shaping technology and subcarrier grouping transmission technology according to claim 1, characterized in that: The LED group signal includes N / L subcarrier resources; The frequency domain signal of the subcarrier resource is expressed as: in, For H l [k] is H L =[H1,H2…,H l ] is the group filter block vector, l=1,2,…,L is the group number of the LED group.

7. The optical OFDM modulation method combining probability shaping technology and subcarrier grouping transmission technology according to claim 6 is characterized in that: The process of converting the transformed frequency domain signal into an LED group signal based on the filter block also includes: after grouping by the filter block, converting the frequency domain signal into a time domain signal by IFFT, the expression is: in, is the time domain signal, and IFFT() is the IFFT transform.

8. The optical OFDM modulation method combining probability shaping technology and subcarrier grouping transmission technology according to claim 1, characterized in that: The expression for processing the LED group signal based on the DCO modulation method is: in, Bias is the DCO pre-modulation signal after adding bias. l is the DC bias, is the trimmed DCO modulation signal.

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