An optical OFDM modulation method combining probability shaping technology and subcarrier grouping transmission technology

Abstract

The application discloses an optical OFDM modulation method combining a probability shaping technology and a subcarrier grouping transmission technology, and belongs to the technical field of optical communication, and comprises the following steps: optimizing a QAM constellation diagram based on a probability shaping method, acquiring optimal probability distribution of constellation points, mapping the optimal probability distribution of the constellation points based on a constant distribution matcher, and obtaining OFDM symbols; performing frequency domain symbol transformation on the OFDM symbols, generating transformed frequency domain signals; converting the transformed frequency domain signals into LED group signals based on a filtering block; and processing the LED group signals based on a DCO modulation mode, and completing OFDM modulation. Different subcarriers are modulated on multiple LEDs, so that the peak-to-average power ratio of the OFDM signals modulated on each LED is reduced, and the nonlinear noise interference of the system is further resisted.

Description

Technical Field

[0001] This invention belongs to the field of optical communication technology, and in particular relates to an optical OFDM modulation method that combines probabilistic shaping technology and subcarrier packet transmission technology. Background Technology

[0002] Wireless optical communication, due to its enormous spectral potential, is one of the key candidate technologies for future 6G communication. Optical OFDM (Orthogonal Frequency Division Multiplexing) itself possesses high spectral efficiency, strong robustness to multipath effects, strong anti-interference capabilities, and ease of implementation, leading to its extensive research in the field of wireless optical communication. However, current optical OFDM models suffer from the problem of dense signal point distribution in constellation diagrams under high-order modulation. Existing optical OFDM constellation modulation methods mainly borrow from RF signal design, failing to consider the linear operating range of the transmitting LED. With increasing modulation order and OFDM sampling points, this introduces significant nonlinear noise, substantially impacting communication performance.

[0003] In addition, current optical OFDM transmits optical signals by simultaneously modulating multiple subcarrier signals with a group of LEDs. However, OFDM has a peak-to-average power ratio (PAPR) characteristic, and its performance is limited by the nonlinear characteristics of the LEDs themselves, resulting in increased nonlinear noise and easy distortion of the transmitted waveform. This is the main challenge currently faced by optical OFDM systems. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes an optical OFDM modulation method that combines probabilistic shaping technology with subcarrier packet transmission technology, thereby resolving the issues present in the prior art.

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

[0006] The QAM constellation diagram is optimized based on the probabilistic shaping method to obtain the optimal probability distribution of constellation points. The optimal probability distribution of constellation points is then mapped based on the constant distribution matcher to obtain OFDM symbols.

[0007] The OFDM symbols are subjected to frequency domain symbol transformation to generate the transformed frequency domain signal;

[0008] The transformed frequency domain signal is converted into an LED group signal based on the filter block;

[0009] The LED group signal is processed using DCO modulation to complete OFDM modulation.

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

[0011] ;

[0012] in, Let v be the amplitude that can be taken in a single QAM signal, and v be the scaling factor. The probability of a single amplitude signal taking a value. The total number of QAM signals, This is the index of the corresponding signal. For all The sum of the sub-terms is used as the denominator.

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

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

[0015] ;

[0016] ;

[0017] in, This is the magnification factor. The power of the original signal. The power of the modulated signal, These are the time-domain sampling points of the original signal. For the number of bits, These are the time-domain sampling points of the modulated signal.

[0018] Preferably, the expression for generating the transformed frequency domain signal is:

[0019] ;

[0020] in, Let k be the symbol of the constellation point in MQAM. It is a time index.

[0021] Preferably, the process of generating the transformed frequency domain signal further includes: performing Hermitian symmetry processing on the transformed frequency domain signal, as expressed by:

[0022] ;

[0023] in, This refers to the frequency domain sampling point located at index Nk after the Hermitian transform. This is the frequency domain conjugate sampling point located at index k after the Hermitian transform.

[0024] Preferably, the LED group signal includes N / L subcarrier resources;

[0025] The expression for the frequency domain signal of the subcarrier resource is:

[0026] ;

[0027] in, For the first The first LED group Frequency domain symbols of each subcarrier, For group filter block vectors, This is the group number for the LED group.

[0028] 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, performing an IFFT on the frequency domain signal to convert it into a time domain signal, with the expression:

[0029] ;

[0030] in, For time-domain signals, ( ) represents the IFFT transform.

[0031] Preferably, the expression for processing the LED group signal based on the DCO modulation method is as follows:

[0032] ;

[0033] ;

[0034] in, To add the biased DCO pre-modulated signal, DC bias, This is the cropped DCO modulated signal.

[0035] Compared with the prior art, the present invention has the following advantages and technical effects:

[0036] The purpose of this invention is to provide a novel optical signal transmission technology that combines probabilistic shaping and subcarrier grouping transmission techniques to reduce the peak-to-average power ratio (PAPR) of optical OFDM. By introducing a probabilistic shaping method to adjust the distribution probability of constellation points in optical OFDM modulation, the transmission probability of lower-energy constellation points is increased compared to higher-energy constellation points, thereby reducing the PAPR of the system and thus resisting nonlinear noise interference. Furthermore, to address the nonlinear distortion problem of LEDs in optical OFDM, subcarrier grouping transmission technology is employed, modulating different subcarriers on multiple LEDs respectively, thereby reducing the PAPR of the modulated OFDM signal on each LED and further resisting nonlinear noise interference. Attached Figure Description

[0037] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0038] Figure 1 This is a block diagram illustrating the principle of the communication system after introducing probability shaping according to an embodiment of the present invention.

[0039] Figure 2 This is a schematic diagram of the input-output mapping relationship of a CCDM arithmetic encoder according to an embodiment of the present invention;

[0040] Figure 3 This is a block diagram illustrating the principle of an optical OFDM communication transmission system for subcarrier packet transmission according to an embodiment of the present invention. Detailed Implementation

[0041] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0042] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0043] Example 1

[0044] This embodiment provides an optical OFDM modulation method that combines probabilistic shaping technology and subcarrier packet transmission technology, including:

[0045] The QAM constellation diagram is optimized based on the probabilistic shaping method to obtain the optimal probability distribution of constellation points. The optimal probability distribution of constellation points is then mapped based on the constant distribution matcher to obtain OFDM symbols.

[0046] The OFDM symbols are subjected to frequency domain symbol transformation to generate the transformed frequency domain signal;

[0047] The transformed frequency domain signal is converted into an LED group signal based on the filter block;

[0048] The LED group signal is processed using DCO modulation to complete OFDM modulation.

[0049] First, a probabilistic 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 introducing probabilistic shaping is as follows: Figure 1 As shown. Figure 2 This is a schematic diagram of the input-output mapping relationship of a typical probabilistic shape CCDM arithmetic encoder.

[0050] Since optical communication uses IM / DD for data transmission and reception, OFDM signals require certain processing. Then, modulation methods such as DCO, ACO, or LACO, combined with a positive bias circuit, are used to ensure that the transmitter sends a positive real-value signal. Subcarrier packet transmission technology focuses on dividing the original LED array signal into multiple LED groups. LEDs in different groups transmit signals in different frequency bands, while LEDs within the same group transmit the same signal. The block diagram of a spatial diversity optical OFDM communication transmission system is shown below. Figure 3 As shown.

[0051] The specific implementation includes the following steps:

[0052] Step 1: According to information theory, the capacity difference between the input signal following a uniform distribution and the Shannon limit is 1.53dB. The probabilistic shaping method is to change the originally uniformly distributed QAM signal into an input signal distribution suitable for the channel conditions. An input signal following the Maxwell-Boltzmann (MB) distribution can meet this requirement.

[0053] The specific probability distribution for probability-based integer shaping adopts the MB distribution, and the calculation formula is shown below:

[0054]

[0055] The above formula is the calculation formula for a one-dimensional symbol, corresponding to the I-path or Q-path of a QAM signal. The probability distribution of all QAM signals can be calculated from the probability distribution of a single path. In the formula... is the amplitude that can be taken in a single QAM signal; v is the scaling factor, which is a key parameter for probability shaping, and v is determined by the channel's SNR.

[0056] Meanwhile, in order to ensure that the average power of the signal is the same before and after the application of probability shaping, the amplitude of the shaped signal also needs to be amplified. and Representing a probabilistically shaped frequency domain signal and a uniformly distributed frequency domain signal, respectively, the amplification factor is first determined according to the following process. :

[0057]

[0058] To ensure equal average signal power, the amplitude of the probability-shaped time-domain signal is amplified at the LED output:

[0059]

[0060] Then, the codebook is mapped using a constant distribution matched network (CCDM). This is a reversible mapping, which is the basis of modulation and demodulation. Figure 1 As shown. The essence of this process is to map a stream of binary bits into a given target probability sequence stream, i.e. This implements the mapping relationship from n bits to m symbols.

[0061] To achieve this process, CCDM utilizes finite-length sequences. empirical probability distribution To approximate the target probability distribution The specific implementation idea is as follows: Figure 2 As shown. Assume n = 2, m = 2 and The operation process is as follows:

[0062] 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]. This process is repeated until all n input bits have been divided. Figure 2 (a) Four partitioned intervals were obtained on the left.

[0063] Similarly, the target sequence symbol interval also needs to be divided to obtain m intervals, but the frequency of each symbol in the output sequence must conform to a probability distribution. In this example, to satisfy the above conditions, the target output sequence must consist of two 1s and two 3s. First, the probability of symbols 1 and 3 is 1 / 2. Then, when the first character is 1, the probabilities of the remaining symbols 1 and 3 are 1 / 3 and 2 / 3, respectively. Similarly, when the first character is 3, the probabilities of the remaining symbols 1 and 3 are 2 / 3 and 1 / 3, respectively. Repeating the above process will yield the divided intervals, such as... Figure 2 (b). To ensure a one-to-one mapping between input and output intervals, the mapping criterion is:

[0064] The lower bound of the output interval must be contained within the input interval.

[0065] If multiple output intervals satisfy condition 1, the output interval with the lower probability is selected and matched one by one with the input interval.

[0066] In summary, the arithmetic encoder achieves a one-to-one mapping between the input binary bit sequence and the output symbol sequence through interval partitioning. The probabilistic shaping method can bring the constellation points closer to the origin, thereby reducing the peak-to-average power ratio of the system.

[0067] Step 2: First, generate OFDM symbols. Consider an N-point MQAM OFDM symbol, where N is the number of FFT operation points, and the k-th constellation point symbol in MQAM can be used. To indicate, among which The corresponding time-domain OFDM signal expression is as follows:

[0068]

[0069] in It is a time index.

[0070] In practical engineering, N is generally taken as a power of 2. After processing the frequency domain OFDM symbols as shown in the following formula, Hermitian symmetry is satisfied, and... This ensures that all time-domain symbols are real numbers.

[0071]

[0072] Step 3: Divide the original single signal into multiple LED groups to implement subcarrier group transmission technology. The LEDs in different groups transmit signals in different frequency bands, while the LEDs in the same group transmit the same signal.

[0073] After OFDM signal processing in step two, the resulting frequency domain signal Through L filtering blocks, L LED group signals are obtained, and each LED group signal contains N / L subcarrier resources. The vector of each filtering block is... ,in It is the group number. Then the... The first LED group The frequency domain symbol of each subcarrier can be obtained by the following formula:

[0074]

[0075] Assumption , then Figure 3 As shown, after the signal is grouped into filter blocks, it needs to be converted from a frequency domain signal to a time domain signal using IFFT:

[0076]

[0077] Each signal group has an independent LED branch, and the LEDs in each group transmit the same signal.

[0078] Step 4: To ensure the signal meets the positive value requirement in optical communication, the signal processed in Step 3 can be modulated using DCO, ACO, or LACO to guarantee that the output signal is positive.

[0079] Taking DCO as an example, DCO uses time-domain signals... A DC bias is added, and then all signals that are still less than zero after the DC bias is applied are clipped to zero, thus achieving the positive value of the signal, as shown in the following formula:

[0080]

[0081] As can be seen from the above steps, because the power is limited in actual engineering, the DC bias cannot be infinitely large. The performance of the DCO will be limited by the peak-to-average power ratio of the time domain signal. Spatial diversity technology can reduce the peak-to-average power ratio of a single LED and effectively resist the clipping noise of the DCO.

Claims

1. An optical OFDM modulation method combining probabilistic shaping technology and subcarrier packet transmission technology, characterized in that, Includes the following steps: The QAM constellation diagram is optimized based on the probabilistic shaping method to obtain the optimal probability distribution of constellation points. The optimal probability distribution of constellation points is then mapped based on the constant distribution matcher to obtain OFDM symbols. The OFDM symbols are subjected to frequency domain symbol transformation to generate the transformed frequency domain signal; The transformed frequency domain signal is converted into an LED group signal based on the filter block; The LED group signal is processed based on the DCO modulation method to complete OFDM modulation; The process of mapping the optimal probability distribution of the constellation points to obtain OFDM symbols based on a constant distribution matcher further includes: amplifying the signal amplitude of the optimal probability distribution of the constellation points. The expression for signal amplitude amplification for the optimal probability distribution of the constellation points is: ； ； in, This is the magnification factor. The power of the original signal. The power of the modulated signal, These are the time-domain sampling points of the original signal. For the number of bits, For the time-domain sampling points of the modulated signal, Represents a uniformly distributed frequency domain signal. Represents a probability-shaping frequency domain signal; The expression for the generated transformed frequency domain signal is: ； in, Let k be the symbol of the constellation point in MQAM. It is a time index.

2. The optical OFDM modulation method combining probabilistic shaping technology and subcarrier packet transmission technology according to claim 1, characterized in that, The expression for the probabilistic shaping method is: ； in, Let v be the amplitude that can be taken in a single QAM signal, and v be the scaling factor. The probability of a single amplitude signal taking a value. The total number of QAM signals, This is the index of the corresponding signal. For all The sum of the sub-terms is used as the denominator.

3. The optical OFDM modulation method combining probabilistic shaping technology and subcarrier packet 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, as expressed by: ； in, This refers to the frequency domain sampling point located at index Nk after the Hermitian transform. This is the frequency domain conjugate sampling point located at index k after the Hermitian transform.

4. The optical OFDM modulation method combining probabilistic shaping technology and subcarrier packet transmission technology according to claim 1, characterized in that, The LED group signal includes N / L subcarrier resources; The expression for the frequency domain signal of the subcarrier resource is: ； in, For the first The first LED group Frequency domain symbols of each subcarrier, For group filter block vectors, This is the group number for the LED group.

5. The optical OFDM modulation method combining probabilistic shaping technology and subcarrier packet transmission technology according to claim 4, characterized in that, 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, performing an IFFT to convert the frequency domain signal into a time domain signal, the expression of which is: ； in, For time-domain signals, ( ) represents the IFFT transform.

6. The optical OFDM modulation method combining probabilistic shaping technology and subcarrier packet transmission technology according to claim 1, characterized in that, The expression for processing the LED group signal based on DCO modulation is as follows: ； ； in, To add the biased DCO pre-modulated signal, DC bias, This is the cropped DCO modulated signal.

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