An adaptive post-compensation method and system for a dual-layer optical OFDM receiver

Abstract

The application is suitable for the field of optical wireless communication technology, and relates to a kind of adaptive post-compensation method and system of double-layer optical OFDM receiving end, comprising: the optical-electric conversion and analog-digital conversion of double-layer optical orthogonal frequency division multiplexing optical signal are carried out, and frequency domain transformation is executed;Extract the characteristic quantity related to clipping noise and nonlinear distortion, generate distortion state parameter;Determine the recovery order of two layers of signal, preferentially judge one layer, reconstruct the signal component of the layer and strip from the received signal, obtain the remaining layer of signal to be compensated;The clipping noise estimator is constructed to the signal to be compensated and is deducted;Nonlinear post-compensation is carried out to the signal after deduction, and compensation parameters are updated according to the mapping relationship between input signal and reference signal;The compensated signal is judged, and if two layers are recovered, it is output, otherwise, the parameters are adjusted according to the residual error and recovered again.The application improves the recovery ability and engineering applicability of the system under the condition of actual nonlinear device and high PAPR.

Description

Technical Field

[0001] This invention belongs to the field of optical wireless communication technology, and particularly relates to an adaptive post-compensation method and system for a dual-layer optical OFDM receiver. Background Technology

[0002] Visible Light Communication (VLC) utilizes light-emitting diodes (LEDs) for lighting and communication multiplexing, offering advantages such as abundant spectrum resources, low electromagnetic interference, and low deployment costs, making it an important technology in optical wireless communication. To improve transmission rates and spectral efficiency, Orthogonal Frequency Division Multiplexing (OFDM) has been widely adopted in VLC systems. Since VLC often employs intensity modulation and direct detection (IM / DD) methods, the transmitted signal must meet real-valued and non-negative constraints. Therefore, considerable research has focused on the modulation structure, biasing methods, layered transmission, and reception recovery methods of optical OFDM. Recent published research has demonstrated that dual-layer optical OFDM, through flexible arrangement of two data subcarrier layers combined with adaptive DC bias, achieves a better overall performance in terms of spectral efficiency, computational complexity, peak-to-average power ratio, and bit error rate.

[0003] The existing technology for receiver recovery in dual-layer optical OFDM has the following drawbacks:

[0004] There is a lack of a unified receiving framework for dual-layer optical OFDM. While existing dual-layer optical OFDM receiving schemes incorporate noise suppression, they primarily focus on demodulation of the interlayer structure, failing to adequately address the coupling relationship between clipping noise and LED nonlinear distortion. Insufficient targeted processing of nonlinear residuals at the receiver leads to errors in the layered recovery process easily propagating step-by-step, affecting the accuracy of subsequent decisions.

[0005] The disconnect between clipping noise suppression and receiver recovery. Existing research on clipping noise mainly focuses on waveform shaping (such as PAPR suppression) and theoretical analysis at the transmitter, while the estimation, reconstruction, and residual suppression of clipping noise at the receiver are usually not core design goals. However, in practical systems, even if PAPR suppression is implemented at the transmitter, residual clipping noise still exists, and the lack of a dedicated receiver suppression mechanism limits the system's recovery stability under different clipping conditions.

[0006] General post-compensation schemes lack adaptability to layered structures. Existing high-performance post-compensation schemes (such as kernel methods and dictionary learning) are mostly based on single-layer VLC receiver models, and have limited consideration for the order of interlayer recovery in two-layer optical OFDM, the distortion propagation paths of signal components in different layers, and the impact of clipping residues on subsequent decisions. In addition, although some studies have reduced complexity through sparsification, these schemes are still mainly aimed at general VLC scenarios and have not been specifically optimized for the characteristics of two-layer structures (such as the availability of decision results for priority recovery layers), resulting in suboptimal allocation of computational resources.

[0007] In summary, existing technologies still lack a receiver processing scheme for dual-layer optical OFDM visible light communication that can coordinate interlayer recovery, clipping noise suppression, and low-complexity nonlinear post-compensation in the same receiving process, thereby improving the recovery capability and engineering applicability of such systems under practical nonlinear devices and high PAPR conditions. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the purpose of this invention is to provide an adaptive post-compensation method for a dual-layer optical OFDM receiver, thereby solving the problem that existing technologies cannot coordinate interlayer recovery, clipping noise suppression, and low-complexity nonlinear post-compensation within the same receiving process, resulting in poor recovery capability and low engineering applicability of dual-layer optical OFDM under nonlinear devices and high PAPR conditions. In addition, this invention also provides an adaptive post-compensation system for a dual-layer optical OFDM receiver.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides an adaptive post-compensation method for a dual-layer optical OFDM receiver, comprising the following steps:

[0011] S10. After receiving the dual-layer optical orthogonal frequency division multiplexing optical signal, the receiving end performs photoelectric conversion and analog-to-digital conversion, and performs synchronization, normalization and frequency domain transformation on the received signal to obtain the dual-layer frequency domain signal to be recovered.

[0012] S20. Extract the feature quantities related to clipping noise and nonlinear distortion based on the current receiving block, and generate distortion state parameters;

[0013] S30. After obtaining the distortion state parameters, determine the recovery order of the two layers of signals, prioritize the decision on one layer, reconstruct the signal component of that layer using the decision result and strip it from the received signal to obtain the signal to be compensated for the remaining layer.

[0014] S40. Construct a clipped noise estimate for the signal to be compensated and subtract it;

[0015] S50. Perform nonlinear post-compensation on the signal after deducting clipping noise. The nonlinear post-compensation adaptively updates the compensation parameters according to the mapping relationship between the input signal and the reference signal.

[0016] S60. Make a decision on the compensated signal. If the two layers have been fully recovered, output the result. Otherwise, adjust the parameters according to the residual error and recover the signal again.

[0017] Furthermore, in S20, the characteristic quantities include one or more of the following: received sample amplitude distribution, peak occurrence frequency, pilot deviation, error vector amplitude, and initial constellation dispersion degree.

[0018] Furthermore, in S30, the recovery order includes: first recovering the main layer signal, then recovering the auxiliary layer signal; or adaptively selecting the layer to be recovered based on pilot quality, interlayer energy ratio, or initial error magnitude.

[0019] Furthermore, after the priority layer completes the preliminary decision, it reconstructs the time-domain or frequency-domain components corresponding to that layer based on the decision symbol, and then separates them from the original received signal to obtain the remaining layer's signal to be compensated.

[0020] Furthermore, in S40, the construction of the clipping noise estimate includes generation based on pilot error, reconstruction error of the decided layer, sample information of received waveform exceeding a preset threshold, or frequency domain abnormal energy distribution.

[0021] Furthermore, in S50, the nonlinear post-compensation adopts a polynomial model, a kernel method model, a sparse dictionary model, or a lightweight neural network model.

[0022] Furthermore, selective compensation is performed based on the distortion state parameters. When the clipping noise is determined to be weak and the constellation spread is small, low-complexity compensation is performed; when the clipping noise is determined to be strong or the nonlinear distortion is significant, the enhanced compensation mode is switched.

[0023] Furthermore, it also includes step S70: updating the noise suppression and compensation parameters for the next cycle based on the current recovery error.

[0024] Secondly, the present invention also provides an adaptive post-compensation system for a dual-layer optical OFDM receiver, comprising:

[0025] An optical receiver module is used to receive optical signals and perform photoelectric conversion;

[0026] The preprocessing module is used to sample, synchronize, process the DC component, and transform the frequency domain of the converted signal to obtain the two-layer frequency domain signal to be recovered.

[0027] The interlayer recovery control module is used to extract feature quantities related to clipping noise and nonlinear distortion from the dual-layer frequency domain signal and generate distortion state parameters. Based on the distortion state parameters, the recovery order of the two-layer signal is determined, a decision is made on the priority layer, and the signal components of that layer are reconstructed and stripped using the decision result to obtain the signal to be compensated for the remaining layers.

[0028] A clipping noise estimation module is used to construct and subtract clipping noise estimates from the signal to be compensated.

[0029] The nonlinear post-compensation module is used to perform nonlinear post-compensation on the signal after deducting clipping noise, and adaptively update the compensation parameters according to the mapping relationship between the input and the reference signal.

[0030] The decision output module is used to make decisions on the compensated signal and output data.

[0031] Furthermore, it also includes a feedback update module for updating the parameters of the clipping noise estimation module and / or the nonlinear post-compensation module based on the current recovery error.

[0032] Compared with the prior art, the adaptive post-compensation method and system for a dual-layer optical OFDM receiver provided by this invention have at least the following advantages:

[0033] Existing dual-layer optical OFDM technologies suffer from poor recovery capabilities and low engineering applicability under nonlinear device and high PAPR conditions. This invention unifies the inter-layer recovery process of dual-layer optical OFDM with the receiver's nonlinear compensation process. Instead of treating noise suppression, inter-layer demodulation, and post-compensation as independent processing steps, it coordinates clipping noise suppression, inter-layer data recovery, and nonlinear distortion correction within the same receiver link. This helps reduce the impact of error propagation during layered reception on subsequent decisions, making the receiver recovery process more suitable for dual-layer optical OFDM scenarios. The dual-layer optical OFDM scheme introduces noise suppression at the receiver and combines it with a single FFT to extract data subcarriers from different layers, balancing spectral efficiency, complexity, PAPR, and BER. This invention further integrates this receiver recovery process with nonlinear post-compensation. This invention incorporates clipping noise suppression as a dedicated processing element at the receiver, allowing the system to move beyond relying solely on transmitter bias control or PAPR suppression to indirectly improve receiver performance. Instead, it enables targeted mitigation of residual clipping effects before the receiver decision. The addition of clipping noise estimation and suppression at the receiver further enhances the system's recovery stability under various clipping conditions. Furthermore, by introducing adaptive nonlinear post-compensation at the receiver, this invention addresses the inherent nonlinearity of LEDs and the resulting waveform distortion, helping to mitigate the impact of nonlinear distortion on constellation decision-making and bit error rate performance. Combining the interlayer recovery characteristics of dual-layer optical OFDM, this invention makes post-compensation more than just a general processing method for single-layer VLC signals; it is more aligned with layered optical signals. The present invention relates to an OFDM receiver structure. While ensuring compensation capability, it also controls the complexity of implementation through sparse dictionaries, low-complexity iteration, or hierarchical selective compensation, making it more suitable for engineering deployment. EX-KRLS post-compensation, while maintaining the same bit error rate performance, can significantly compress the dictionary size through ALD and coherence sparsification. The dictionary size reduction for the two sparsification strategies reaches 52.6% and approximately 77%, respectively. In summary, compared to schemes that only optimize bias, control PAPR, or arrange inter-layer subcarriers at the transmitter, this invention focuses on improving the receiver recovery stage. It adds a compensation path for clipping noise and device nonlinearity without significantly changing the existing transmitter framework. This not only inherits the existing advantages of the two-layer optical OFDM structure in terms of spectral efficiency and complexity but also enhances the system's practical applicability under nonlinear device conditions. Attached Figure Description

[0034] To more clearly illustrate the solution of the present invention, a brief introduction will be given to the drawings used in the description of the embodiments below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0035] Figure 1 A flowchart illustrating an adaptive post-compensation method for a dual-layer optical OFDM receiver provided in an embodiment of the present invention;

[0036] Figure 2 This is an architecture diagram of an adaptive post-compensation system for a dual-layer optical OFDM receiver, provided in an embodiment of the present invention. Detailed Implementation

[0037] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0039] This invention provides an adaptive post-compensation method for a dual-layer optical OFDM receiver, applied to the receiver processing of an IM / DD visible light communication system. It focuses on addressing the problem that dual-layer optical OFDM signals are simultaneously affected by interlayer superposition, clipping noise, and LED nonlinear distortion during receiver recovery. The adaptive post-compensation method for the dual-layer optical OFDM receiver includes the following steps:

[0040] S10. After receiving the dual-layer optical orthogonal frequency division multiplexing (OFDM) optical signal, the receiving end performs photoelectric conversion and analog-to-digital conversion, and performs synchronization, normalization, and frequency domain transformation on the received signal to obtain the dual-layer frequency domain signal to be recovered. S20. Based on the current receiving block, feature quantities related to clipping noise and nonlinear distortion are extracted to generate distortion state parameters. S30. After obtaining the distortion state parameters, the recovery order of the two layers of signals is determined. One layer is prioritized for decision-making. The decision result is used to reconstruct the signal components of that layer and remove them from the received signal to obtain the remaining layer's signal to be compensated. S40. A clipping noise estimate is constructed for the signal to be compensated and subtracted. S50. Nonlinear post-compensation is performed on the signal after deducting clipping noise. The nonlinear post-compensation adaptively updates the compensation parameters based on the mapping relationship between the input signal and the reference signal. S60. A decision is made on the compensated signal. If both layers are fully recovered, the signal is output; otherwise, the parameters are adjusted based on the residual error, and the signal is recovered again.

[0041] This invention enables the coordinated completion of interlayer recovery, clipping noise suppression, and low-complexity nonlinear post-compensation within the same receiving process, thereby improving the recovery capability and engineering applicability of such systems under practical nonlinear devices and high PAPR conditions.

[0042] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0043] This invention provides an adaptive post-compensation method for a dual-layer optical OFDM receiver, applied to the receiver processing of IM / DD visible light communication systems. It focuses on the problem of dual-layer optical OFDM signals being simultaneously affected by interlayer superposition, clipping noise, and LED nonlinear distortion during receiver recovery. The basic concept is to organize interlayer recovery, clipping noise suppression, and nonlinear post-compensation into a continuous joint recovery process at the receiver, instead of processing them separately. After the received signal enters the receiver, photoelectric conversion, synchronization, and basic demodulation are first completed to obtain the dual-layer optical OFDM baseband signal to be recovered. Then, the clipping noise intensity, nonlinear distortion degree, and interlayer interference level in the current received signal are estimated, and the compensation mode for the current reception cycle is determined accordingly. Based on this, the layer signals less affected or with higher decision-making power are recovered first. Then, the decision results of the recovered layers are used to construct a noise reconstruction signal or distortion reference signal to jointly correct the remaining layer signals. Finally, an adaptive post-compensation module is introduced to further suppress residual nonlinearity and output the corrected symbol decision result. This technical approach aligns with the overall direction of publicly available research on noise suppression at dual-layer optical OFDM receivers, clipping noise analysis, and VLC post-distorters. For example... Figure 1 As shown, in this embodiment, the adaptive post-compensation method of the dual-layer optical OFDM receiver includes the following steps:

[0044] S10. After receiving the dual-layer optical orthogonal frequency division multiplexing optical signal, the receiving end performs photoelectric conversion and analog-to-digital conversion, and performs synchronization, normalization and frequency domain transformation on the received signal to obtain the dual-layer frequency domain signal to be recovered. Its output is used as the input for subsequent inter-layer recovery and noise suppression.

[0045] S20. Extract the feature quantities related to clipping noise and nonlinear distortion based on the current receiving block, and generate distortion state parameters.

[0046] Specifically, in this embodiment, after obtaining the frequency domain signal, a state analysis is performed on the current receiving block to extract features related to clipping noise and nonlinear distortion. These features may include the received sample amplitude distribution, peak frequency, pilot bias, error vector amplitude, initial constellation dispersion, or other information that reflects the degree of clipping and the strength of nonlinear distortion. Using these features, distortion state parameters for the current receiving block are generated. Existing research has shown a clear correlation between clipping level, PAPR control method, and BER performance; therefore, these distortion state parameters can serve as the basis for subsequent noise suppression and post-compensation mode selection.

[0047] S30. After obtaining the distortion state parameters, determine the recovery order of the two layers of signals, prioritize the decision on one layer, reconstruct the signal component of that layer using the decision result and strip it from the received signal to obtain the signal to be compensated for the remaining layers.

[0048] Specifically, in this embodiment, after obtaining the distortion state parameters, the inter-layer recovery control module determines the recovery order of the current two-layer signals: first recovering the main layer signal, then the auxiliary layer signal; or adaptively selecting the layer to be recovered based on pilot quality, inter-layer energy ratio, or initial error magnitude. After the priority layer completes its preliminary decision, the corresponding time-domain or frequency-domain components can be reconstructed based on the decision symbol and stripped from the original received signal to obtain the remaining layer's signal to be compensated. This reduces the impact of inter-layer coupling on the recovery of the second layer.

[0049] S40. Construct a clipping noise estimate for the signal to be compensated and subtract it.

[0050] Specifically, in this embodiment, for the residual signal after stripping, a clipping noise estimation module constructs a clipping noise estimate. This estimate can be based on pilot error, reconstruction error of the decided layer, sample information of the received waveform exceeding a preset threshold, or abnormal energy distribution in the frequency domain. The estimated clipping noise component is then used to subtract or suppress the residual signal. Existing public research has shown that clipping noise directly affects the BER performance of VLC-OFDM, therefore, explicitly estimating and suppressing this noise at the receiver has a practical basis.

[0051] S50. Perform nonlinear post-compensation on the signal after deducting clipping noise. The nonlinear post-compensation adaptively updates the compensation parameters according to the mapping relationship between the input signal and the reference signal.

[0052] Specifically, in this embodiment, after clipping noise suppression is completed, nonlinear post-compensation is performed on the received signal. The post-compensation can employ a polynomial model, kernel method model, sparse dictionary model, or lightweight neural network model. Preferably, the post-compensation module adaptively updates the compensation parameters based on the mapping relationship between the current input signal and the reference signal, enabling the compensator to adjust according to changes in LED nonlinearity and the receiving environment. Existing VLC research has shown that receiver post-distorters such as KLMS-DFE, KRLS, and EX-KRLS can all improve the recovery performance caused by LED nonlinearity, with EX-KRLS further reducing computational complexity through sparsification strategies.

[0053] S60. Make a decision on the compensated signal. If the two layers have been fully recovered, output the result. Otherwise, adjust the parameters according to the residual error and recover the signal again.

[0054] Specifically, in this embodiment, after compensation is completed, a final decision is made on the current layer signal, and the decision result is used to construct the reference signal required for the next layer recovery. If both layers are recovered, the final data result is output. If the residual error of the current block is still higher than the preset threshold, the clipping noise estimation parameters, post-compensation parameters, or inter-layer recovery order are readjusted according to the error magnitude, and recovery is performed again. This forms an iterative reception process of "preliminary recovery - noise estimation - nonlinear compensation - decision correction".

[0055] Furthermore, in this embodiment, the post-compensation module does not uniformly perform high-complexity compensation on all received symbols, but instead performs selective compensation based on the distortion state parameters of the current block. When it is determined that the current block has weak clipping noise and small constellation spread, only low-complexity compensation is performed; when it is determined that the current block has strong clipping noise or significant nonlinear distortion, it switches to enhanced compensation mode. This maintains good compensation capability while also helping to control the computational overhead at the receiver. Public research has demonstrated that sparse dictionaries and low-complexity kernel recursion methods can significantly reduce model size while maintaining compensation performance, thus this implementation has a feasible technical basis.

[0056] The adaptive post-compensation method for a dual-layer optical OFDM receiver provided in this invention achieves its purpose because the distortion in the received dual-layer optical OFDM signal does not only originate from additive noise, but also includes waveform distortion caused by LED nonlinearity, nonlinear noise caused by clipping, and interlayer recovery error propagation. If only conventional FFT demodulation and universal equalization are used, the receiver does not fully utilize these distortion components. By incorporating interlayer recovery, clipping noise estimation, and adaptive post-compensation into the same link, structural information can be used to reduce interlayer interference, distortion estimation can be used to reduce clipping residue, and post-compensation can be used to correct nonlinear mapping deviations, thereby improving the recovery capability and decision stability of the dual-layer optical OFDM signal under nonlinear device conditions.

[0057] This invention also provides an adaptive post-compensation system for a dual-layer optical OFDM receiver, combined with... Figure 1 and Figure 2 In this embodiment, the adaptive post-compensation system of the dual-layer optical OFDM receiver includes:

[0058] An optical receiver module is used to receive optical signals and perform photoelectric conversion;

[0059] The preprocessing module is used to sample, synchronize, process the DC component, and transform the frequency domain of the converted signal to obtain the two-layer frequency domain signal to be recovered.

[0060] The interlayer recovery control module is used to extract feature quantities related to clipping noise and nonlinear distortion from the dual-layer frequency domain signal and generate distortion state parameters. Based on the distortion state parameters, the recovery order of the two-layer signal is determined, a decision is made on the priority layer, and the signal components of that layer are reconstructed and stripped using the decision result to obtain the signal to be compensated for the remaining layers.

[0061] A clipping noise estimation module is used to construct and subtract clipping noise estimates from the signal to be compensated.

[0062] The nonlinear post-compensation module is used to perform nonlinear post-compensation on the signal after deducting clipping noise, and adaptively update the compensation parameters according to the mapping relationship between the input and the reference signal.

[0063] The decision output module is used to make decisions on the compensated signal and output data.

[0064] The feedback update module is used to update the parameters of the clipping noise estimation module and / or the nonlinear post-compensation module based on the current recovery error.

[0065] Example 1

[0066] Basic recovery implementation method of dual-layer optical OFDM receiver

[0067] This embodiment focuses on a two-layer optical OFDM system in IM / DD visible light communication. The transmitting end employs a two-layer subcarrier structure, while the receiving end utilizes photoelectric detection, synchronization, frequency domain transformation, and layered recovery processing. The receiving process in this embodiment includes: performing photoelectric conversion and analog-to-digital conversion on the received optical signal to complete synchronization and frequency domain transformation; performing noise suppression on the current receiving block; determining the priority recovery layer based on the decisionability of the two signal layers; after preliminary decision-making on the priority layer, reconstructing the signal components corresponding to that layer and stripping them from the received signal to reduce inter-layer interference during the recovery of the other layer; and then performing recovery and final decision-making on the remaining layers. The structural basis of this embodiment is consistent with the two-layer optical OFDM scheme published in IEEE Communications Letters in 2024. This paper clearly presents a two-layer structure based on subcarrier activation factors, a receiving end that first performs noise suppression, and then extracts QAM and PAM subcarriers through a single FFT, and reports higher spectral efficiency, lower complexity, lower PAPR, and a competitive BER.

[0068] In this embodiment, the subcarrier activation factor can be set to three example values: 0.3, 0.5, and 0.7, corresponding to low, medium, and high inter-layer activation intensities, respectively. The receiver performs the same "priority layer recovery—signal reconstruction—remaining layer recovery" process under all three conditions. When the activation factor is low, the residual interference after priority layer recovery is small, making it suitable for lower-complexity subsequent compensation; when the activation factor is high, inter-layer coupling is enhanced, making it more suitable for the receiver to introduce enhanced noise suppression or nonlinear post-compensation after signal stripping. The purpose of this embodiment is to illustrate that the present invention does not rely on a fixed layer structure but can maintain a unified reception recovery process even when the two-layer structure changes.

[0069] Example 2

[0070] Implementation methods combining clipping noise estimation and suppression

[0071] This embodiment, based on Embodiment 1, introduces a clipping noise estimation and suppression module. After completing the initial FFT, the receiver generates a clipping noise estimate for the current receiving block using pilot offset, constellation spread, number of amplitude distribution anomalies, or high-energy frequency domain components. Subsequently, the residual signal recovered from the priority layer is corrected based on this estimate, and then a decision is made for the remaining layers. In this embodiment, the clipping intensity can be set using three example operating points, such as corresponding to tight, intermediate, and loose clipping conditions; alternatively, the clipping level or the equivalent PAPR operating interval can be directly used as input. The purpose is to illustrate that the receiver can switch the suppression intensity for different clipping conditions, rather than always using a fixed processing method.

[0072] This implementation has a clearly disclosed technical basis. A 2024 Photonics study analyzed clipping noise in DCO-OFDM visible light communication and examined the BER performance of pilot-assisted PAPR reduction at different clipping levels. The paper reported that DCO-OFDM using pilot-assisted PAPR reduction outperformed conventional DCO-OFDM without PAPR reduction at all three analyzed clipping levels. Based on this published result, this embodiment further extends the principle that "different clipping conditions cause BER differences" to the receiver, using it as a basis for noise estimation and suppression intensity selection.

[0073] Example 3

[0074] Implementation method combining adaptive nonlinear post-compensation

[0075] This embodiment adds a nonlinear post-compensation module to Embodiment 2. After completing clipping noise suppression, the receiver inputs the current signal to be decided into the post-compensator. The post-compensator corrects the amplitude compression and waveform distortion caused by LED nonlinearity based on the mapping relationship between the input signal and the reference signal. The post-compensator can employ a polynomial model, kernel method model, sparse dictionary model, or lightweight neural network model. For ease of explanation, the compensation modes can be divided into three levels: low complexity compensation, medium complexity compensation, and enhanced compensation, corresponding to light, medium, and heavy distortion reception states, respectively. The receiver adaptively switches between the three compensation modes based on the error vector amplitude of the current block, the constellation dispersion, or the pilot residual.

[0076] This embodiment connects with existing research on VLC receiver post-distorters. The 2019 KLMS-DFE work proposed an adaptive post-distorter that jointly mitigates LED nonlinearity and ISI in the reproducing kernel Hilbert space; its abstract and conclusions show that, with a BER of (10^-4), the KLMS-DFE-NC relative contrast algorithm with network-size control can achieve an SNR gain of approximately 3–4 dB. The 2023 KRLS-GN-ENC work demonstrates that the KRLS post-distorter with adaptive kernel width has better nonlinearity compensation capability and faster convergence speed compared to classical polynomial filtering. The 2024 EX-KRLS work further shows that, by combining ALD and coherence sparsity, the dictionary size can be reduced by 52.6% and approximately 77%, respectively, while maintaining the same bit error rate performance. This embodiment does not limit itself to using any one of these algorithms, but rather uses this approach as the feasible basis for the receiver post-compensation module.

[0077] Example 4

[0078] Implementation methods of hierarchical collaborative recovery and selective compensation

[0079] This embodiment illustrates the synergistic relationship between "inter-layer recovery" and "selective compensation" in this invention. The receiver first performs a fast state determination for each received block. When the clipping noise of the current block is detected to be weak and the priority layer decision is reliable, low-complexity compensation is applied only to the remaining layers. When significant constellation dispersion, large pilot deviation, or strong amplitude compression characteristics are detected in the current block, an enhanced compensation step is inserted between the two recovery layers, or noise reconstruction is performed again before the second layer decision. Therefore, the compensation is not applied with the same intensity to all received blocks, but rather adaptively configured based on the current block state and the inter-layer recovery results.

[0080] The significance of this embodiment lies in illustrating that the present invention is not simply a superposition of a "noise suppression module + post-compensation module," but rather enables the two to work collaboratively around the hierarchical recovery sequence. Publicly available dual-layer optical OFDM schemes have demonstrated the feasibility of receiver-side hierarchical recovery and single-shot FFT extraction; low-complexity EX-KRLS studies have shown that post-compensation complexity can be controlled through sparsity. Combining the two allows for the selection of a more suitable recovery path under different reception conditions, making it easier to achieve a balance between implementation complexity and recovery performance.

[0081] Performance Description: From the perspective of receiver structure performance, the above embodiment organizes noise suppression, interlayer recovery, and post-compensation at the receiver into a unified process for dual-layer optical OFDM. This allows the receiver to first weaken interlayer interference using the layer structure, then reduce residual nonlinear noise using clipping noise estimation, and finally correct LED nonlinear mapping deviations using post-compensation. Compared to processing methods that rely solely on general FFT decisions or conventional equalization, this structure is more suitable for the recovery characteristics of layered optical OFDM signals. Publicly available results for dual-layer optical OFDM have demonstrated that this approach offers better overall performance in terms of spectral efficiency, complexity, PAPR, and BER.

[0082] From the perspective of clipping noise control, Example 2 illustrates that the receiver can dynamically adjust the noise suppression intensity according to different clipping conditions, thereby extending the clipping risk perception from the transmitter to the receiver decision stage. Publicly available research has verified that pilot-assisted PAPR reduction can improve BER at multiple clipping levels, meaning that the strength of clipping noise does indeed change the system's bit error rate performance. Therefore, there is a clear technical basis for estimating and suppressing clipping residue at the receiver.

[0083] From the perspective of nonlinear compensation effects, Examples 3 and 4 illustrate that introducing a receiver post-distorter in a layered optical OFDM scenario not only inherits the improvement effects of existing VLC post-compensation methods on LED nonlinearity and ISI, but also controls the complexity through selective compensation and sparsification strategies. Existing published results provide evidence for BER improvement, convergence speed improvement, and significant dictionary size reduction at the same bit error rate. These results collectively support the conclusion that "receiver post-compensation can not only improve recovery performance but also adapt to engineering implementation through structural optimization."

[0084] In summary, the above embodiments demonstrate that by incorporating dual-layer recovery, clipping noise suppression, and adaptive nonlinear post-compensation into the same receiving link, the present invention can improve the receiving recovery capability of the system under nonlinear devices and high PAPR conditions without significantly changing the existing dual-layer optical OFDM transmitting structure, and enhance the engineering applicability of this type of VLC system in practical applications.

[0085] The adaptive post-compensation method and system for dual-layer optical OFDM receivers described in the above embodiments, compared with existing technologies, suffer from poor recovery capability and low engineering applicability under nonlinear device and high PAPR conditions in dual-layer optical OFDM. This invention unifies the inter-layer recovery process of dual-layer optical OFDM with the receiver's nonlinear compensation process, eliminating noise suppression, inter-layer demodulation, and post-compensation as independent processing steps. Instead, clipping noise suppression, inter-layer data recovery, and nonlinear distortion correction are coordinated within the same receiver link. This helps reduce the impact of error propagation during layered reception on subsequent decisions, making the receiver recovery process more suitable for dual-layer optical OFDM scenarios. The dual-layer optical OFDM scheme introduces noise suppression at the receiver and combines it with a single FFT to extract data subcarriers from different layers, balancing spectral efficiency, complexity, PAPR, and BER. This invention further combines this receiver recovery process with nonlinear post-compensation. This invention incorporates clipping noise suppression as a dedicated processing element at the receiver, allowing the system to move beyond relying solely on transmitter bias control or PAPR suppression to indirectly improve receiver performance. Instead, it enables targeted mitigation of residual clipping effects before the receiver decision. The addition of clipping noise estimation and suppression at the receiver further enhances the system's recovery stability under various clipping conditions. Furthermore, by introducing adaptive nonlinear post-compensation at the receiver, this invention addresses the inherent nonlinearity of LEDs and the resulting waveform distortion, helping to mitigate the impact of nonlinear distortion on constellation decision-making and bit error rate performance. Combining the interlayer recovery characteristics of dual-layer optical OFDM, this invention makes post-compensation more than just a general processing method for single-layer VLC signals; it is more aligned with layered optical signals. The present invention relates to an OFDM receiver structure. While ensuring compensation capability, it also controls the complexity of implementation through sparse dictionaries, low-complexity iteration, or hierarchical selective compensation, making it more suitable for engineering deployment. EX-KRLS post-compensation, while maintaining the same bit error rate performance, can significantly compress the dictionary size through ALD and coherence sparsification. The dictionary size reduction for the two sparsification strategies reaches 52.6% and approximately 77%, respectively. In summary, compared to schemes that only optimize bias, control PAPR, or arrange inter-layer subcarriers at the transmitter, this invention focuses on improving the receiver recovery stage. It adds a compensation path for clipping noise and device nonlinearity without significantly changing the existing transmitter framework. This not only inherits the existing advantages of the two-layer optical OFDM structure in terms of spectral efficiency and complexity but also enhances the system's practical applicability under nonlinear device conditions.

[0086] Obviously, the embodiments described above are merely preferred embodiments of the present invention, and not all embodiments. The accompanying drawings illustrate preferred embodiments of the present invention, but do not limit the scope of the patent. The present invention can be implemented in many different forms; rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing specific embodiments, or make equivalent substitutions for some of the technical features. Any equivalent structures made using the content of this specification and drawings, directly or indirectly applied to other related technical fields, are similarly within the scope of patent protection of this invention.

Claims

1. An adaptive post-compensation method for a dual-layer optical OFDM receiver, characterized in that, Includes the following steps: S10. After receiving the dual-layer optical orthogonal frequency division multiplexing optical signal, the receiving end performs photoelectric conversion and analog-to-digital conversion, and performs synchronization, normalization and frequency domain transformation on the received signal to obtain the dual-layer frequency domain signal to be recovered. S20. Extract the feature quantities related to clipping noise and nonlinear distortion based on the current receiving block, and generate distortion state parameters; S30. After obtaining the distortion state parameters, determine the recovery order of the two layers of signals, prioritize the decision on one layer, reconstruct the signal component of that layer using the decision result and strip it from the received signal to obtain the signal to be compensated for the remaining layer. S40. Construct a clipped noise estimate for the signal to be compensated and subtract it; S50. Perform nonlinear post-compensation on the signal after deducting clipping noise. The nonlinear post-compensation adaptively updates the compensation parameters according to the mapping relationship between the input signal and the reference signal. S60. Make a decision on the compensated signal. If the two layers have been fully recovered, output the result. Otherwise, adjust the parameters according to the residual error and recover the signal again.

2. The adaptive post-compensation method for a dual-layer optical OFDM receiver according to claim 1, characterized in that, In S20, the characteristic quantities include one or more of the following: received sample amplitude distribution, peak occurrence frequency, pilot deviation, error vector amplitude, and initial constellation dispersion degree.

3. The adaptive post-compensation method for a dual-layer optical OFDM receiver according to claim 1, characterized in that, In S30, the recovery order includes: first recovering the main layer signal, then recovering the auxiliary layer signal; or adaptively selecting the layer to be recovered based on the pilot quality, interlayer energy ratio, or initial error magnitude.

4. The adaptive post-compensation method for a dual-layer optical OFDM receiver according to claim 3, characterized in that, After the priority layer completes the initial decision, the time or frequency domain component corresponding to that layer is reconstructed according to the decision symbol, and then stripped from the original received signal to obtain the signal to be compensated for the remaining layers.

5. The adaptive post-compensation method for a dual-layer optical OFDM receiver according to claim 1, characterized in that, In S40, the construction of the clipping noise estimate includes generation based on pilot error, reconstruction error of the decided layer, sample information of received waveform exceeding a preset threshold, or frequency domain abnormal energy distribution.

6. The adaptive post-compensation method for a dual-layer optical OFDM receiver according to claim 1, characterized in that, In S50, the nonlinear post-compensation adopts a polynomial model, a kernel method model, a sparse dictionary model, or a lightweight neural network model.

7. The adaptive post-compensation method for a dual-layer optical OFDM receiver according to claim 6, characterized in that, Selective compensation is performed based on the distortion state parameters. When the clipping noise is determined to be weak and the constellation spread is small, low-complexity compensation is performed. When the clipping noise is determined to be strong or the nonlinear distortion is obvious, the enhanced compensation mode is switched.

8. The adaptive post-compensation method for a dual-layer optical OFDM receiver according to claim 1, characterized in that, It also includes step S70: updating the noise suppression and compensation parameters for the next cycle based on the current recovery error.

9. A system employing the method as described in any one of claims 1 to 8, characterized in that, include: An optical receiver module is used to receive optical signals and perform photoelectric conversion; The preprocessing module is used to sample, synchronize, process the DC component, and transform the frequency domain of the converted signal to obtain the two-layer frequency domain signal to be recovered. The interlayer recovery control module is used to extract feature quantities related to clipping noise and nonlinear distortion from the dual-layer frequency domain signal and generate distortion state parameters. Based on the distortion state parameters, the recovery order of the two-layer signal is determined, a decision is made on the priority layer, and the signal components of that layer are reconstructed and stripped using the decision result to obtain the signal to be compensated for the remaining layers. A clipping noise estimation module is used to construct and subtract clipping noise estimates from the signal to be compensated. The nonlinear post-compensation module is used to perform nonlinear post-compensation on the signal after deducting clipping noise, and adaptively update the compensation parameters according to the mapping relationship between the input and the reference signal. The decision output module is used to make decisions on the compensated signal and output data.

10. The system according to claim 9, characterized in that, It also includes a feedback update module for updating the parameters of the clipping noise estimation module and / or the nonlinear post-compensation module based on the current recovery error.

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