CN114978311A

CN114978311A - Linear companding method for inhibiting visible light communication LED nonlinear amplitude limiting noise

Info

Publication number: CN114978311A
Application number: CN202210287440.4A
Authority: CN
Inventors: 李鹍鹏; 贾科军
Original assignee: Lanzhou University of Technology
Current assignee: Lanzhou University of Technology
Priority date: 2022-03-22
Filing date: 2022-03-22
Publication date: 2022-08-30

Abstract

The invention discloses a linear companding method for inhibiting nonlinear amplitude limiting noise of a visible light communication LED, which mainly comprises the following steps: (1) mapping the frequency domain data into a Hermite symmetric structure; (2) pre-scale transformation; (3) performing adaptive linear compression transformation after IFFT; judging whether the amplitude of the signal meets the requirement or not according to the amplitude limiting threshold, compressing the signal which does not meet the requirement, and not processing the signal which meets the requirement; (4) dividing the signal into two groups of compressed and uncompressed symbols, and framing and transmitting the signals in sequence after amplitude limiting; (5) splitting a frame after photoelectric conversion, deleting a CP, keeping a first group of symbols unchanged, performing expansion transformation on a second group of compressed symbols, and adding according to bits to obtain an O-OFDM symbol; (6) and extracting information after FFT, and demodulating to obtain original information. The method solves the problems that the traditional symbol decomposition serial transmission technology decomposes an O-OFDM symbol into a plurality of symbols with smaller amplitude to inhibit LED nonlinear distortion, the system frequency band utilization rate and the information transmission rate are low, and the system optical power loss is reduced.

Description

Linear companding method for inhibiting visible light communication LED nonlinear amplitude limiting noise

Technical Field

The invention relates to a method for improving LED nonlinear distortion in a visible light communication OFDM system, in particular to a method for suppressing amplitude limiting noise caused by LED nonlinear distortion in an OFDM-based visible light communication system.

Background

Visible Light Communication (VLC) is a communication method in which light signals are directly transmitted in a free space using visible light (380 nm to 780 nm) emitted from a Light Emitting Diode (LED) as an information carrier. VLC spectrum resource is abundant, can realize illumination and two kinds of functions of high-speed communication, and visible light communication has advantages such as equipment cost is low, the security is high simultaneously, do not receive radio frequency spectrum control and no electromagnetic radiation, has become one of the most effective way of solving the scarce problem of radio frequency spectrum resource.

The visible light communication system is a light intensity modulation/direct detection (IM/DD) system, and commonly used modulation methods include on-off keying (OOK), Pulse Position Modulation (PPM), and the like, but the transmission rate of these modulation methods is low, and meanwhile, an optical signal emitted by an LED may cause multipath effect through multipath propagation, thereby causing inter-symbol interference (ISI) of the signal. Optical orthogonal frequency division multiplexing (O-OFDM) is a multi-carrier modulation technique with advantages of inter-symbol interference resistance, high spectral efficiency, etc. Therefore, the optical OFDM technology has become one of the most potential modulation techniques for high-speed VLC systems.

Although OFDM modulation can effectively increase the transmission rate of VLC systems, O-OFDM systems also face a large peak-to-average power ratio (PAPR) problem, as do wireless communication systems. In VLC systems, nonlinear distortion is one of the factors affecting the system performance, where LED is the main source of nonlinear distortion, and LED nonlinear distortion mainly comes from two aspects, on one hand, since LED has limited dynamic range, and normally emits light when the input signal is greater than the on-voltage (TOV), but at the same time, needs to be lower than the allowed cut-off voltage, so that when O-OFDM signal with high PAPR passes through LED, serious cut-off distortion is generated; on the other hand, the electrical-to-optical conversion characteristic in the LED working region is also nonlinear, so that severe nonlinear conversion distortion is generated, resulting in severe degradation of VLC system performance.

Various methods have been proposed for suppressing nonlinear clipping distortion outside the LED operating region. The method mainly comprises the steps of reducing the peak-to-average ratio of an O-OFDM system, a power back-off method, improving a novel O-OFDM modulation system with a lower peak-to-average ratio and an iterative signal amplitude limiting (ISC) method.

There are many methods for reducing PAPR, for example, considering the special structure of the time domain of asymmetric amplitude-limited Optical orthogonal frequency division multiplexing (ACO-OFDM) signals, proposed Recoverable amplitude limiting methods With better PAPR and Bit Error Rate (BER) performance (Wei X, man w, Hua Z, et al. ACO-OFDM-specific Recoverable Upper Clipping communication [ J ]. IEEE Photonics Journal,2014,6(5):1-17), probability-based methods such as partial transmission sequence method (PTS) (huenh Vo trunk D, Hakjeon B, Chang-Soo P, PAPR reduction communication With low probability communication in coherent Optical system [ C ]/201218, emission method (P), noise component mapping (H629), no J, et al.A Modified SLM Scheme With Low Complexity for PAPR Reduction of OFDM Systems [ J ]. IEEE Transactions on Broadcasting,2007, 53(4): 804-. The above methods all have a good effect of suppressing the PAPR, but because the algorithms of the methods are relatively complex, the methods have the disadvantages of large calculation amount, high implementation difficulty and the like.

The traditional O-OFDM system is improved, and a novel modulation system with lower peak-to-average ratio characteristic is obtained. For example, an interleaved discrete Fourier transform based extended layered/enhanced asymmetric amplitude limited optical orthogonal frequency division multiplexing (IDFTS-L/E-ACO-OFDM) system (Bai R, Wang Z, Jiang R, et al. interleaved DFT-spread layered/enhanced ACO-OFDM for intensity-modulated direct-detection systems [ J ]. Journal of light wave Technology, 2018,36(20): 4713-; a Hybrid Optical orthogonal frequency division multiplexing (PHO-OFDM) scheme Based on pulse amplitude modulation (Zhang T, Zou Y, Sun J, et al design of PAM-DMT-Based Hybrid Optical OFDM for visual Light Communications [ J ]. IEEE Wireless Communications Letters,2019,8(1):265 and 268); an improved asymmetric shear optical orthogonal frequency division multiplexing (M-ACO-OFDM) system and an improved generalized Light emitting diode refractive index modulation orthogonal frequency division multiplexing (M-GLIM-OFDM) system (Assaidah, Chow C W. Performance of M-ACO-OFDM, DCO-OFDM and M-GLIM OFDM in visual Light Communication Systems [ C ]// 201925 th Asia-Pacific Conference Communication (APCC),2019: 297-300); an interleaved discrete Fourier transform based extended layered/enhanced asymmetric clipped optical orthogonal frequency division multiplexing (IDFTS-L/E-ACO-OFDM) system (Bai R, Wang Z, Jiang R, et al, interleaved DFT-spread layered/enhanced ACO-OFDM for intensity-modulated direct-detection systems [ J ]. Journal of Lightwave Technology, 2018,36(20): 4713-. The new modulation system can reduce the peak-to-average ratio, but can increase the complexity of the system.

The companding transform algorithm has the advantages of low algorithm complexity, simple design, no limitation of signal modulation mode and subcarrier quantity, obvious PAPR reduction effect and the like, and is widely applied. For example, a nonlinear exponential companding transform method (Bandara K, Niroopan P, Chung Y H.PAPR reduced OFDM visual light communication using explicit nonlinear compression combining [ C ]//2013 IEEE International Conference on Microwaves, Communications, extensions and Electronic Systems (COMCAS2013),2013: 1-5). While non-linear companding generally degrades BER performance, linear companding can strike a performance tradeoff between reducing PAPR and BER.

The document (Mesleh R, Elgal H, Haas H. LED non-linearity transmission techniques in Optical wireless OFDM communication systems [ J ]. Optical communication and Networking, IEEE/OSA Journal of 2012,4(11):865-875) proposes an iterative signal amplitude limiting (ISC) technique, which is based on the principle of iteratively limiting an O-OFDM symbol with a larger amplitude into a plurality of symbols with a smaller amplitude, so as to ensure that the limited symbols are within the range of an LED working area, then the symbols are transmitted simultaneously through a plurality of LEDs, and a receiving end PD superposes a plurality of Optical signals to restore the original signals. Although the ISC can better suppress the nonlinear distortion of the LEDs, the ISC technique requires a plurality of LEDs to transmit signals simultaneously, which requires a small distance between the LEDs, and increases the requirement of the system for synchronization, thereby increasing the complexity of the system. And when the transmission distance is large, the BER performance of the system is degraded due to the difference of the channel gains of the multiple optical signals. The document (Jiakejun, Yangbo, Haohao, Haeli, Symbol Decomposition and Serial Transmission (SDST) technology is proposed in the symbol decomposition technology of visible light communication optical orthogonal frequency division multiplexing system for suppressing LED nonlinear distortion research [ J ] Chinese laser, 2020,47(04):234-, the method reduces the complexity of the system and the requirement on synchronization, avoids the BER deterioration problem caused by channel gain difference, however, the number of decompositions in the SDST technique is fixed, when the input signal power is small, there are all-zero symbols that do not carry useful information, at which time the clipping noise is almost zero, the noise superposed on the symbols causes the BER performance of the system to be poor, and the SDST technology inevitably has the defects of low system frequency band utilization rate and low information transmission rate; the design and research of a visible light communication self-adaptive O-OFDM symbol decomposition serial transmission system [ J ] communication science report, 2020,41(09):179-189 ] in the literature (Jiakejun, Yangbao, Cao Minghua, Li Suo, Haeli. A self-Adaptive Symbol Decomposition Serial Transmission (ASDST) technology is provided aiming at the defects of the SDST technology, self-adaptive decomposition is carried out according to the actual size of the amplitude of the O-OFDM symbol, the problem that the BER performance is poor when a plurality of all-zero symbols without carrying information are transmitted along with the increase of the decomposition times when the signal power is small in the SDST technology is solved, the average optical power loss is reduced when a small signal is generated, and the frequency band utilization rate and the information transmission rate are improved. Although the ASDST technique solves some disadvantages in the SDST technique, it is only effective for the case of small input signal power, and when the input power is large, the performance parameters of the ASDST technique, such as average symbol decomposition times, are the same as the SDST technique, so that there are inevitably a series of problems, such as low system band utilization rate and large optical power loss, which will seriously affect the practicability of the method.

By combining the advantages and disadvantages of the above technologies, the present invention adopts a Linear Companding technology to design a method for suppressing LED nonlinear distortion, and proposes an Adaptive Linear Companding Transformation (ALCT) technology to solve some disadvantages of the conventional symbol decomposition serial transmission technology.

Disclosure of Invention

The invention aims to provide a linear companding method for inhibiting visible light communication LED nonlinear amplitude limiting noise, which aims to solve the problems that the frequency band utilization rate and the information transmission rate of a system are reduced, the optical power consumption is high and the like along with the increase of symbol decomposition times when the variance of an input symbol is large by an O-OFDM symbol decomposition serial transmission technology and an adaptive O-OFDM symbol decomposition serial transmission technology; the method can obviously improve the frequency band utilization rate and the information transmission rate of the system, can reduce the optical power loss in large signal, and has higher practicability compared with the two methods.

In order to solve the technical problem, the invention adopts the technical scheme that:

a linear companding method for inhibiting visible light communication LED nonlinear amplitude limiting noise comprises the following steps:

a sending end:

step 1: carrying out quadrature amplitude modulation on input binary bit data;

step 2: mapping the frequency domain data to enable the output data to have a Hermitian symmetric structure;

and 3, step 3: pre-scaling the mapped signal, and presetting the variance of an O-OFDM symbol;

and 4, step 4: obtaining real bipolar O-OFDM symbols after inverse Fourier transform, and then performing parallel-serial conversion;

and 5: performing adaptive linear compression transformation on the O-OFDM symbols;

the O-OFDM symbol of the invention refers to a real signal which is subjected to parallel-serial conversion after passing through an inverse Fourier transform module. Firstly, according to the effective dynamic range of an LED, judging whether the actual amplitude of an O-OFDM symbol is in an allowed dynamic range in advance, if the amplitude of a signal is in the dynamic range, carrying out compression transformation on the signal without amplitude limiting distortion, otherwise, carrying out compression transformation on the amplitude of the signal which does not meet the requirement, thereby reducing the dynamic range of the signal and reducing amplitude limiting noise;

step 6: decomposing the signal into two groups of compressed symbols and uncompressed symbols, adding a Cyclic Prefix (CP) to each symbol, and then serially framing for sequential transmission;

and 7: and D, adding direct current offset to the analog signal subjected to the digital-to-analog conversion, and finally transmitting the analog signal through an LED.

Receiving end:

step 1: the photoelectric detector receives the optical signal and performs photoelectric conversion;

step 2: converting the analog electric signal into a digital signal through analog-to-digital conversion;

and step 3: splitting a frame, then deleting a cyclic prefix, performing linear expansion transformation on a compressed symbol, and then combining two groups of signals;

a receiving end receives a frame symbol, splits the frame according to a frame synchronization signal to obtain two groups of decomposition symbols in the frame, then deletes the cyclic prefix of each group of symbols, simultaneously carries out corresponding linear expansion transformation on the compressed symbols, then delays the two groups of symbols to time alignment, and recovers the original O-OFDM symbol after combination;

and 4, step 4: inputting the signals after the serial-parallel conversion into a Fourier transform module;

and 5: and the frequency domain equalization module equalizes and compensates channel attenuation, then extracts effective information of the frequency domain signal according to the signal mapping structure of the sending end to obtain a component carrying information, and finally demodulates and restores the original information.

Compared with the prior art, the invention has the beneficial effects that:

(1) the error rate performance of small signals is improved;

the traditional iterative amplitude limiting technology and the O-OFDM symbol decomposition serial transmission technology have fixed symbol decomposition times, and are decomposed into fixed symbols no matter the amplitude of an input symbol, so when the variance of the O-OFDM symbol is small, a plurality of all-zero symbols can be superimposed with noise after being transmitted through an optical channel, and the combined signal can cause the error rate performance of a system to be poor.

The self-adaptive linear companding conversion technology determines whether a signal needs to be compressed according to the actual amplitude of an input O-OFDM symbol and a system amplitude limiting threshold, and performs self-adaptive companding conversion on the signal, so that the signal in an LED working area range with a small amplitude value is not subjected to companding conversion and is directly passed through, thereby avoiding the problem of system error rate performance deterioration caused by small signal amplification of a receiving end, and remarkably improving the error rate performance of the small signal.

(2) Average optical power loss is reduced;

the traditional iterative signal amplitude limiting technology needs a plurality of LEDs to simultaneously transmit a plurality of O-OFDM symbols, and an O-OFDM symbol decomposition serial transmission system needs to serially transmit a plurality of decomposition symbols, so that when the input signal power is the same, the optical power loss of the input signal power and the optical power loss of the O-OFDM symbol decomposition serial transmission system are the same, and the required average optical power is increased due to the increase of the number of the LEDs or the number of the symbol decomposition times, so that the energy utilization rate of the system is reduced; the adaptive O-OFDM symbol decomposition serial transmission technique can solve the average optical power loss in small signals, but when transmitting signals with large input power, the required average optical power is the same as that of the ISC and SDST techniques.

The invention introduces a pre-decision module, performs self-adaptive linear companding conversion on the signal according to the actual amplitude of the signal, and only needs to send one O-OFDM symbol when the signal power is small, so the optical power is the same as the ASDST technology when the signal power is small, and when the signal power is large, information transmission can be completed only by two decomposition symbols at most, so the optical power loss can be effectively reduced, and the energy utilization rate of a system is improved.

(3) The utilization rate of a system frequency band and the information transmission rate are improved;

the SDST technique needs to decompose an O-OFDM symbol into a plurality of symbols and delay one symbol period for serial transmission, so that when the number of times of decomposing the symbol is large, the frequency band utilization rate and the information transmission rate of the system are seriously reduced, and thus the practicability is poor; ASDST solves the disadvantages of low bandwidth utilization and information rate of SDST techniques in small signals, but when the signal input variance is large, the number of decompositions is the same as that of SDST techniques, so the improvement on system performance is very limited.

The self-adaptive linear companding technology only sends one uncompressed O-OFDM symbol when the input variance is small, so that the information transmission rate is the same as that of the ISC technology; when the variance of the input symbols is increased to a certain degree, the input symbols are decomposed into two groups of symbols for transmission, and at the moment, information transmission can be completed only by two symbols, so that the frequency band utilization rate and the information transmission rate are remarkably improved compared with the SDST and ASDST technologies.

Description of the drawings:

FIG. 1 is a schematic diagram of a transmitting end of the present invention;

FIG. 2 is a frame structure diagram of the present invention;

FIG. 3 is a schematic diagram of a receiving end of the present invention;

FIG. 4 is a schematic diagram of O-OFDM symbol compression and grouping of the ACO-OFDM system of the present invention;

FIG. 5 is a schematic diagram of O-OFDM symbol compression and grouping of the DCO-OFDM system of the present invention;

FIG. 6 is a graph comparing bit error rates of the techniques ISC, SDST, ASDST and ALCT of the 4QAM modulated ACO-OFDM system;

FIG. 7 is a graph of bit error rate comparison of ISC, SDST, ASDST and ALCT techniques for DCO-OFDM system under 4QAM modulation;

FIG. 8 is a diagram of bit error rate analysis of 4QAM modulated ACO-OFDM system under Gaussian channel according to the present invention;

FIG. 9 is a diagram of bit error rate analysis of a 4QAM modulated DCO-OFDM system under Gaussian channel according to the present invention;

FIG. 10 is a graph of the average optical power analysis of the ACO-OFDM system under 4QAM modulation according to the present invention;

FIG. 11 is a diagram of an average optical power analysis of a DCO-OFDM system under 4QAM modulation according to the present invention;

FIG. 12 is a graph comparing information transfer rates for the ACO-OFDM, DCO-OFDM systems SDST, ASDST, and ALCT techniques of the present invention;

Detailed Description

Referring to fig. 1, a linear companding method for suppressing nonlinear clipping noise of a visible light communication LED, the present invention only uses two most commonly used optical OFDM modulation modes: the specific implementation is illustrated by taking asymmetric limiting optical OFDM (ACO-OFDM) and direct current offset optical OFDM (DCO-OFDM) as examples, and other O-OFDM modulation modes can be implemented according to the principle.

The specific implementation mode of the sending end is as follows:

(1) the sending end firstly carries out serial-to-parallel (S/P) conversion on input binary bit data, then distributes the binary bit data to N subcarriers with lower transmission rate, and obtains a QAM modulation symbol on the kth subcarrier as X through M-order Quadrature Amplitude Modulation (QAM) _k ＝a _k +jb _k 。

(2) The visible light communication system adopts an intensity modulation direct detection (IM/DD) technology, directly utilizes a baseband OFDM signal for modulation, necessarily requires the modulation signal to be a real signal, and maps (Mapping) frequency domain data to ensure that output data has a Hermitian symmetric structure, namely

For the ACO-OFDM system, data transmission is performed by using only odd subcarriers, and if even subcarriers are set to 0, the mapped signal is:

wherein, (.) ^* Denotes the conjugation operation, (. cndot.) T denotes the transpose of the matrix, and N is the IFFT transform length.

For the DCO-OFDM system, since the serial number of the subcarrier corresponding to the DC data is 0, in order to avoid DC offset and ensure that the time domain signal is a real number, the 0 th subcarrier and the N/2 th subcarrier are mapped to be 0, namely X ₀ ＝X _N/2 When the signal is 0, the mapped signal is:

(3) in order to study the relation between the variance of the input O-OFDM symbol and the linear companding parameters and the VLC system performance, an Inverse Fast Fourier Transform (IFFT) module is requiredThe variance (power) of the output time domain O-OFDM symbol is preset, so that the signal X before being input to the IFFT module is preset _mapping Performing a pre-scaling transformation, the transformed signal being represented as: x _scaled (n)＝αX _mapping (n) where α is a pre-scaling factor and the total average power input and output by the IFFT are equal, i.e.

Where E (-) represents the mathematical expectation, the resulting pre-scale transformation factor is:

in the formula: sigma ₀ Representing the IFFT output signal x _IFFT Standard deviation of (2).

(4) The pre-scaled signal is subjected to an Inverse Fast Fourier Transform (IFFT) algorithm, and the output time domain signal is expressed as:

x _IFFT ＝F ^H X _scaled ＝αF ^H X _mapping ，

where (·) H denotes a matrix conjugate transpose operation, and F is an N × N normalized discrete fourier transform matrix, expressed as:

wherein the content of the first and second substances,

according to the Central Limit Theorem (CLT), when the number of subcarriers is large (N ≧ 64), x _IFFT Approximate obedience mean is 0 and variance is

A gaussian distribution of (a).

For the ACO-OFDM system, due to the characteristics of the frequency domain mapping structure, a real signal with an anti-symmetric characteristic is obtained after N-point IFFT transformation, that is:

x _IFFT (n)＝-x _IFFT (n+N/2)，n＝0,1,…,N/2-1，

the negative part of the signal is 0-point clipped resulting in a unipolar signal without any loss of useful information.

In a DCO-OFDM system, a time-domain signal x _IFFT Of the nth time domain value x _IFFT (n) may be expressed as:

as can be seen from the above formula, the DCO-OFDM signal is a bipolar real signal, so that the signal has a large dynamic range and is more sensitive to LED nonlinear distortion.

(5) Outputs a time domain signal x after parallel-to-serial (P/S) conversion _IFFT And then an adaptive linear compression transform is performed. The method of the invention is different from the traditional linear companding method, the companding function demarcation point is set according to the effective dynamic range of the modulation signal, when the signal is in the dynamic range, the companding conversion is not carried out, and when the signal exceeds the dynamic range, the linear companding conversion is carried out on the part of the signal.

To generate a positive real signal for driving the LED and to meet the illumination and communication requirements, a dc offset B is usually added to the driving signal _DC So that the dynamic range of the modulated signal is defined by B _DC And the effective working area of the LED is determined together.

Let the dynamic range of the LED be V _min ～V _max In the case of the ACO-OFDM system, since the negative value part of the IFFT output time domain signal is subjected to 0-point clipping, the output signal is not negative, and if B is the case _DC ＜V _min Signal x _IFFT Requiring lower clipping with a clipping threshold of epsilon _min ＝V _min -B _DC . When B is present _DC ≥V _min When the amplitude is not limited, the threshold value is 0, so the lower amplitude limiting threshold can be expressed as epsilon _min ＝max(V _min -B _DC ,0). The lower clipping threshold is generally less than the upper clipping threshold, so the upper clipping threshold is lower than the upper clipping thresholdThe clipping threshold is expressed as epsilon _max ＝V _max -B _DC . For the DCO-OFDM signal, because the DCO-OFDM signal is a bipolar real number signal, no matter what value the direct current bias takes, the upper limit limiting threshold is epsilon _max ＝V _max -B _DC The lower limiting threshold is epsilon _min ＝V _min -B _DC 。

The self-adaptive linear companding technology mainly outputs a time domain signal x to IFFT _IFFT Performing adaptive linear compression transformation, wherein the amplitude size demarcation point of the signal is mainly composed of epsilon _min 、ε _max The distinction is made, and the specific companding function is as follows:

wherein, the multiplication factor of the compression number is more than 0 and less than or equal to 1; x is the number of _IFFT (n) represents x _IFFT N-1, 0,1, …, N-1.

Firstly, the time domain sampling amplitude of each O-OFDM symbol is pre-judged, if the amplitude is in the dynamic range [ epsilon ] _min ,ε _max ]At this time, since the signal is not clipped, the signal is passed without being subjected to compression conversion. For an excess interval [ epsilon ] _min ,ε _max ]In order to reduce clipping distortion, the sampling value of the time domain signal with larger amplitude is multiplied by a coefficient a smaller than 1 to carry out compression transformation.

(6) The signal is decomposed into two groups of uncompressed and compressed symbols, a Cyclic Prefix (CP) is added to each symbol after amplitude limiting, and then serial framing is carried out for sequential transmission.

The signal amplitude is subjected to self-adaptive linear compression conversion by the sending end according to the signal amplitude, so that the signal amplitude has a fuzzy problem, namely whether the signal amplitude is subjected to compression conversion cannot be determined, the receiving end cannot correctly decompress the signal, and the signal is divided into uncompressed symbols in order to accurately restore the signal at the receiving end

And compressed symbols

Two groups, which are respectively represented as:

wherein the content of the first and second substances,

and

respectively represent

And

n-0, 1, N-1.

Thus, the original compressed symbol

Then compressing the symbols

Delaying by one O-OFDM symbol period T, will

Prior to the generation of

And time division transmission is carried out, so that the problem of poor system performance caused by transmitting the sideband information of whether each sampling point is compressed or not can be effectively avoided. Meanwhile, when the signal variance is small, there may be

In the case of all zeros, in order to eliminate the influence of all zeros on the system performance, these all zeros are deleted and not transmitted.

The output symbol after clipping the two sets of symbols is

Wherein, the first and the second end of the pipe are connected with each other,

the slicing procedure can be expressed as follows:

wherein the content of the first and second substances,

and

respectively represent

And

n-0, 1, N-1.

The amplitude of the signal is reduced after adaptive linear compression transformation, but if the variance of the input signal is reduced

If the preset value is too large, the signal after the compression processing may still exceed the dynamic range of the LED, so it needs to be doneThe surface amplitude limiting operation is performed, but it should be noted that since the signal is adaptively companded, it is equivalent to linearly expanding the modulation range of the LED when the original O-OFDM symbol is in

When in range, there is no clipping noise, when out of range

In range, clipping noise will be generated. The specific clipping process can be expressed as:

wherein x is _IFFT (n) represents a signal x _IFFT N-0, 1, …, N-1.

According to the Bussgang theorem, after a Gaussian random process passes through a nonlinear amplitude limiting transformation process, the output can be modeled as that an original signal passes through a linear attenuation system and is superposed with an additive amplitude limiting noise, and the amplitude limiting output signal is expressed as:

in the formula, n _clip And K is a clipping attenuation coefficient and is expressed as:

in the formula (I), the compound is shown in the specification,

and

representing a normalized lower clipping threshold and an upper clipping threshold;

a probability density function representing an IFFT output signal;

denotes the probability that a standard normal random variable exceeds x, i.e., Q (x) P [ N (0,1) > x]。

In order to resist ISI caused by multipath propagation and optical dispersion effect of O-OFDM symbol, two symbols after compression and amplitude limiting are subjected to

Cyclic Prefixes (CPs) are added respectively and framing is performed, and the frame structure is shown in fig. 2.

(7) A group of frame signals are subjected to digital-to-analog (D/A) conversion, discrete time domain signals are converted into continuous signals s (t), and direct current offset B is added to the continuous signals s (t) _DC Generating a signal x for driving the LED _LED (t), namely:

x _LED (t)＝s(t)+B _DC 。

the generated driving signal directly modulates the light intensity of the LED, and the light power is the mean value E [ x ] of the electric signal _LED (t)]It is determined that if there is only one O-OFDM symbol, the average optical power is:

if there are two O-OFDM symbols, the average optical power is:

in the formula (I), the compound is shown in the specification,

representing the probability density function of the IFFT output signal.

Referring to fig. 3, a linear companding method for suppressing nonlinear clipping noise of a visible light communication LED includes:

(1) after transmission through the wireless optical channel, the receiving end adopts Photoelectric Detector (PD) to detect the light intensity signal, converts the light intensity signal into an electrical signal, and the expression is as follows:

y(t)＝γx _LED (t)+n _AWGN (t)，

wherein γ represents the photoelectric conversion efficiency of PD; n is _AWGN (t) represents the sum of background light shot Noise and thermal Noise of the device in the system, which can be modeled as Additive White Gaussian Noise (AWGN) independent of the signal.

(2) The received signal y (T) is subjected to analog-to-digital (A/D) conversion to obtain a digital signal, and then uniform sampling is carried out with the period of T/N, namely N discrete sampling values are obtained by sampling one O-OFDM symbol in the period of T.

(3) The following description is made only for the case where two O-OFDM symbols exist. Sequentially receiving two decomposed symbols in a frame, then decomposing the frame and deleting the cyclic prefix of each decomposed symbol, then performing linear expansion transformation on a second compressed signal, and delaying a first signal until the first signal is aligned with the second signal in time; finally, adding the corresponding bits to obtain a combined O-OFDM symbol;

the two decomposed symbols received by the receiving end can be expressed as:

wherein n is ₁ And n ₂ Respectively, representing additive white gaussian noise superimposed on the first and second O-OFDM decomposed symbols.

The method comprises the following steps of carrying out self-adaptive linear expansion transformation on signals in a received frame, wherein the specific process comprises the following steps: receive the first symbol y ₁ The next received compressed symbol y is not subjected to expansion transformation ₂ Multiplying by 1/a for expansion transformation, and then multiplying y ₁ Delayed by one symbol period T and y ₂ Time-aligned and combined, the combined signal is represented as:

in the formula, n ₁ And n ₂ Which represents white gaussian noise on each O-OFDM symbol in the received signal y, respectively, assuming the same power spectral density.

(4) The decompressed and combined O-OFDM symbols are serial-to-parallel (S/P) converted and then input to an N-point Fast Fourier Transform (FFT) module, outputting a frequency domain signal represented as:

in the formula, N _clip Clipping noise n representing a non-gaussian distribution _clip The FFT of (2) is subject to gaussian distribution after FFT by the central limit theorem. Because the sending end carries out self-adaptive linear compression conversion on the signal, the dynamic range of the LED is expanded to epsilon _min /a,ε _max /a]So when x is _IFFT The amplitude is located at [ epsilon ] _min /a,ε _max /a]When in interval, there is no clipping noise, N _clip When x is equal to 0 _IFFT Amplitude exceeding [ epsilon ] _min /a,ε _max /a]Clipping noise will be generated at intervals.

Representing the DC offset signal B _DC The fourier transform of (d). N is a radical of _AWGN Representing the fourier transform of white gaussian noise superimposed on an O-OFDM symbol.

When the number of subcarriers is large, according to CLT, the amplitude limiting noise which is not distributed in Gaussian distribution is converted into noise distributed in Gaussian distribution after FFT, the average value is 0, the power of the signal before and after conversion is kept unchanged, and the amplitude limiting noise n of the ACO-OFDM and DCO-OFDM systems is kept unchanged _clip The variances of (a) are:

in the formula (I), the compound is shown in the specification,

the bandwidth utilization rate of the O-OFDM system is shown, only the odd subcarriers of the ACO-OFDM system carry useful information, so the ACO-OFDM system has the advantages of high bandwidth utilization rate, low bandwidth consumption and low cost

The 0 th subcarrier and the N/2 th subcarrier in the DCO-OFDM system do not carry useful information, so that

(5) The FFT output signal enters a frequency domain equalization module, effective information of the frequency domain signal is extracted according to a mapping structure of the O-OFDM symbol of the sending end, and the component carrying the information is obtained as follows:

in the formula (I), the compound is shown in the specification,

is Gaussian white noise, the mean value is 0, and the variance is 1; sigma _clip Representing clipping noise mean square error, σ _AWGN1 And σ _AWGN2 Mean square error of AWGN without and after the extended transform, respectively. Due to Fourier transformation of DC bias signal

Only the subcarrier signal with sequence number 0 is affected, so the dc term does not affect the extracted subcarriers and is therefore deleted. For the ACO-OFDM system, extracting information on the first half N/4 odd subcarriers of Y according to a frequency domain mapping structure, wherein k is 1,3,5, …, N/2-1; for DCO-OFDM system, extracting subcarrier signals with serial numbers of 1-N/2-1 of YAnd k is 1,2,3, …, N/2-1.

Inputting the extracted information into an M-QAM demodulator, and recovering original binary data by adopting maximum likelihood detection, wherein the Bit Error Rate (BER) expression of the M-QAM is as follows:

in the formula, gamma _SNR Represents the ratio of the bit power and the noise power of the input of the QAM demodulator for the bit signal to noise ratio, expressed as:

in the formula (I), the compound is shown in the specification,

representing clipping noise variance; a is ₁ 、a ₂ Respectively representing the average number of the uncompressed symbols and the average number of the compressed symbols;

representing a white Gaussian noise variance, where B represents the modulation bandwidth of the O-OFDM system and N ₀ Representing a Gaussian white noise single-side power spectral density; p _s,elec Represents the average electrical symbol power of the information-carrying subcarriers, expressed as:

in the formula: p _b,elec In order to average the bit electric power,

representing the O-OFDM system bandwidth utilization.

Simulation experiment

By combining MATLAB simulation experiments with theoretical analysis, the rationality, feasibility and benefit of the method for inhibiting the LED nonlinear amplitude limiting distortion of the visible light communication OFDM system are verified.

Simulation experiment parameters: adopting normalized 4QAM modulation, the number of subcarriers is N-256, and the variance of input signal

Ranging from-20 dBm to 50 dBm. The modulation bandwidth B of the O-OFDM system is 50MHz and the gaussian white noise variance is set to-20 dBm. The emitting device adopts OSRAMLUWW5SM white light LED (model: LUWW5SM-KXKY-6P7Q-Z, series: Golden)

The manufacturer: OSRAM Opto Semiconductors Inc.), the working region range of which is V _min ＝0.1V，V _max Assuming that the non-linear characteristics of the LED operating region have been compensated for using predistortion techniques, 1V. Taking DC offset B _DC The photoelectric conversion factor γ is 1A/W, 0.4V. Therefore, the lower and upper limiting values of the ACO-OFDM system are epsilon _min 0V and ε _max The amplitude limiting value of the lower side and the upper side of the DCO-OFDM system is epsilon at 0.6V _min 0.3V and ε _max ＝0.6V。

Establishing a Monte Carlo (Monte Carlo) error rate simulation model, and firstly carrying out pre-scale transformation on a signal when carrying out simulation analysis on the error rate performance of a system, wherein the variance of an xIFFT (modulation and control signal IFFT) is

Thus, the output O-OFDM symbol of the IFFT transformation module obeys the mean value of 0 and the variance of 0

(ii) a gaussian distribution of; calculating the error rate for 10000 times circularly, and then calculating the average error rate to obtain the final product

The bit error rate is a simulated value. When calculating the theoretical error rate, firstly, according to the given sum of the limiting thresholds

And calculating a theoretical amplitude limiting noise variance, substituting the theoretical amplitude limiting noise variance into a formula to calculate a theoretical bit signal to noise ratio, and finally substituting the theoretical amplitude limiting noise variance into a QAM demodulation bit error rate formula to calculate and obtain a theoretical error rate.

Simulation result

FIGS. 4 and 5 show 4QAM modulation with 32 subcarrier numbers,

When the linear companding coefficient a is 0.5, the amplitude variation graphs of the ACO-OFDM and DCO-OFDM original signals and the signals after adaptive linear compression transformation and grouping. The abscissa represents the number of time domain discrete samples and the ordinate represents the signal amplitude voltage in volts (V).

The amplitude limiting threshold of the ACO-OFDM system is respectively epsilon _min 0 and ε _max 0.6. Fig. 4(a) shows the original ACO-OFDM signal output by IFFT, and it can be seen that the amplitude fluctuation range of the original signal is large, the maximum amplitude reaches about 2.1V, and it can be seen that the ACO-OFDM time domain signal has an anti-symmetric property, so that the original information can be recovered without distortion after all signals smaller than 0 are limited. Fig. 4(b) and (c) show two O-OFDM symbols after 0-value amplitude limiting and compression grouping, and it can be seen that the maximum amplitude of a group of symbols without linear compression transformation is within the range of the amplitude limiting threshold of 0-0.6V, while the maximum peak value of a group of symbols with linear compression transformation is also significantly reduced to about 1.1V, and the amplitude of the signal is reduced by half.

The amplitude limiting threshold of the DCO-OFDM system is respectively epsilon _min 0.3 and ε _max 0.6. FIG. 5(a) shows the original DCO-OFDM symbol x output after IFFT transformation and parallel-to-serial conversion _ALC A bipolar real signal with an amplitude variation range of approximately-2.1 to 1.8V and a large dynamic range, and symbols without linear compression transformation are shown in FIGS. 5(b) and (c), respectively

And the symbol after linear compression transformation

The amplitude of the uncompressed signal can be found to be within an amplitude limiting threshold of-0.3-0.6V, the amplitude of the compressed signal is limited to-1.1-0.9V, and the dynamic range of the signal is effectively reduced. Unlike the ACO-OFDM signal, the DCO-OFDM signal may be negative in magnitude and satisfy

The abscissa in fig. 6 and 7 represents the O-OFDM symbol variance in decibel-milliwatts (dBm) and the ordinate represents the bit error rate. The solid line with the symbol "□" represents the simulated value of the error rate of the original O-OFDM system, the solid line with the symbol "O" represents the simulated value of the error rate of the iterative clipping (ISC) technique when the number of LEDs is 2, and the solid line with the symbol "Δ" represents the simulated value of the error rate of the ISC technique when the number of LEDs is 4. With a symbol

The solid line of (a) represents the simulated value of the bit error rate for the symbol-resolved serial transport (SDST) technique at a resolution number of 2, and the solid line with the symbol "represents the simulated value of the bit error rate for the SDST technique at a resolution number of 4. With a symbol

The solid line represents the bit error rate simulation value of the Adaptive Symbol Decomposition Serial Transmission (ASDST) technology when the maximum symbol decomposition time is 2, and the solid line is signed

The solid line of (b) represents the simulated value of the bit error rate of the ASDST technique when the maximum symbol decomposition number is 4. With a symbol

The solid line of (a) represents the bit error rate simulation value when the linear compression coefficient of the Adaptive Linear Companding Transform (ALCT) technology is 1/4, and the solid line with the sign "fourstar" represents the bit error rate simulation value when the linear compression coefficient of the ALCT technology is 1/4.

As can be seen from FIGS. 6 and 7, when

When the signal is combined at the receiving end, the noise is amplified to cause system performance deterioration, and the performance is worse when the frequency of symbol decomposition is larger; the ASDST technology and the ALCT technology can not affect the system performance when the signal is small, because the ASDST technology eliminates an all-zero signal, for the ALCT technology, whether the signal needs to be compressed or not is judged at a sending end according to the actual amplitude of the signal, and when the signal meets the requirement, the signal is not processed, so the error rate is superposed with the original system. When in use

When the amplitude limiting noise becomes the main noise component of the system, it can be seen that several methods can effectively reduce the amplitude limiting distortion in the system and improve the error rate performance, and the ALCT technology can obtain the same performance as the conventional symbol decomposition technology, for example, when the symbol decomposition times of the ISC, SDST, ASDST systems are 2, the same error rate performance can be achieved by setting the linear compression coefficient in the ALCT technology to 1/2, and similarly, when the symbol decomposition times of the ISC, SDST, ASDST systems are 4, the same system performance as that of the ALCT technology when the linear compression coefficient is set to 1/4 is achieved, but different from that, the ALCT technology can achieve the transmission of information by only two symbols at most, so that the optical power can be saved and the information transmission rate can be improved.

The abscissa in fig. 8 and 9 represents the O-OFDM symbol variance in decibel-milliwatts (dBm) and the ordinate represents the bit error rate. The solid line with the symbol "■" represents the theoretical value of the error rate of the original O-OFDM system, and the dotted line with the symbol "□" represents the simulated value of the error rate of the original O-OFDM system. The solid line with the symbol "Δ" represents the theoretical value of the error rate when the linear compression coefficient is 0.5, and the broken line with the symbol "Δ" represents the simulated value of the error rate when the linear compression coefficient is 0.5. A solid line with symbol ″, represents an error rate theoretical value when the compression coefficient is 0.4, and a broken line with symbol ″, represents an error rate simulated value when the linear compression coefficient is 0.4. The solid line with the symbol ". diamond" represents the bit error rate theoretical value when the compression coefficient is 0.3, and the broken line with the symbol ". diamond" represents the bit error rate simulation value when the linear compression coefficient is 0.3.

As can be seen from fig. 8 and 9, the bit error rate simulation result is consistent with the theoretical calculation result, and the correctness of the system design scheme, the specific implementation process and the theoretical analysis of the present invention is verified. When the temperature is higher than the set temperature

When the signal amplitude is small, the signal amplitude can meet the requirements, linear compression transformation is not needed, the error rate performance does not change along with the change of compression parameters, and therefore error rate curves under all conditions are almost overlapped. On the other hand, when

When the signal amplitude is increased to a certain degree, the probability that the signal amplitude exceeds the working area range of the LED is obviously increased, at the moment, serious nonlinear amplitude limiting noise is introduced into the system, so that the error rate performance is continuously deteriorated, the amplitude limiting noise is obviously reduced after partial linear companding conversion is carried out on the signal, and the error rate of the system is obviously reduced. When the compression coefficient is larger, the compression degree is smaller, the amplitude limiting noise is larger, the error rate performance is poorer, and when the compression coefficient is gradually reduced, the error rate performance is gradually better. For example, a 4QAM modulation, for example,

when the BER of the ACO-OFDM original system is about 3 multiplied by 10 ^-2 When a is 0.5, BER is reduced to about 2.6 × 10 ^-3 Compared with the original system, the BER is reduced by about one order of magnitude; when a is 0.4, BER is reduced to about 2.1 × 10 ^-4 The BER is reduced by about two orders of magnitude compared to the original system.

In fig. 10 and 11, the abscissa represents the O-OFDM symbol variance in decibel-milliwatts (dBm), and the ordinate represents the average optical power in watts (W). The solid line with the symbol "□" represents the simulated value of the average optical power of the SDST system at a symbol resolution of 4. The solid line with the symbol "Δ" represents the simulated value of the average optical power of the ASDST system when the maximum symbol decomposition degree is 4. The solid line with the sign ″) represents the theoretical value of the average optical power of the ALCT system when the linear compression coefficient is 1/4, and the broken line with the sign ″) represents the simulated value of the average optical power of the ALCT system when the linear compression coefficient is 1/4.

In order to compare the average optical power of each system under the same condition, the parameters of each system are set to values having the same bit error rate performance for comparison, where the number of sign decompositions is 4 and the linear compression coefficient is 1/4. As can be seen from fig. 10 and fig. 11, the simulation result of the average optical power of the ALCT transmission system matches the theoretical calculation value. The average optical power of the SDST system is the largest and therefore the energy utilization is the lowest, while the average optical power of the ASDST system saves more optical power than the SDST system with a fixed number of symbol decompositions in the case of a smaller symbol variance, but when the system variance increases to a certain extent, the optical power of both systems will be the same and therefore the ASDST technique cannot improve the problem of the SDST system that the optical power loss is large in the case of a larger signal input variance. Compared with the above two technologies, the ALCT technology requires the minimum average optical power for transmitting information.

In fig. 12, the abscissa indicates the O-OFDM symbol variance in decibel-milliwatts (dBm), and the ordinate indicates the information transmission rate in megabits per second (Mbit · s) ^-1 ). Marking off

Represents the information rate of the ACO-OFDM system when the SDST technique fixes the symbol decomposition number of 4, the solid line with the symbol "■" represents the information rate of the ACO-OFDM system when the maximum symbol decomposition number of 4 of the ASDST technique, and the dotted line with the symbol "□" represents the information rate of the ACO-OFDM system when the linear compression coefficient of the ALCT technique is 1/4. Dashed line

Representing the DCO-OFDM system with the fixed symbol decomposition times of 4 in the SDST technologyThe information rate, the solid line with symbol ″, represents the information rate of the DCO-OFDM system when the maximum symbol decomposition number of the ASDST technique is 4, and the dotted line with symbol ″, represents the information rate of the DCO-OFDM system when the linear compression coefficient of the ALCT technique is 1/4.

Also in order to compare the information transmission rates of the respective systems under the same condition, the parameters of the respective systems are controlled to have the same error rate performance for comparison. As can be seen from FIG. 12, the symbol decomposition times of the SDST technique is constant, and the information transmission rate is not dependent

And (4) changing. ASDST solves the problem of low transmission rate of SDST technology when signal variance is small, but then

The information transmission rate of the ASDST technique is continuously decreased, and finally, the transmission rate of the ASDST technique is the same as that of the SDST technique. Compared with the two technologies, the ALCT technology has the maximum information transmission rate, because the maximum number of O-OFDM symbols required by the ALCT system for transmitting the same amount of information is 2, the ALCT technology can effectively solve the defect of low information transmission rate of SDST and ASDST technologies under the condition of ensuring the same performance of restraining amplitude limiting noise, and meanwhile, the frequency band utilization rate of the system is improved.

The above is the specific implementation scheme of the present invention and the verification of computer simulation experiment. It should be noted that the above examples and simulations of the design of the system of the present invention are presented only to illustrate and verify the correctness, feasibility and benefits of the method of the present invention, and are not intended to limit the method of the present invention, as will be clearly understood by those skilled in the art. While the invention has been effectively illustrated and described by way of examples, there are numerous variations of the invention that may be made without departing from the spirit of the invention. Various corresponding changes or modifications can be made by those skilled in the art according to the method of the present invention without departing from the spirit and substance of the method of the present invention, and these corresponding changes or modifications are within the scope of the method of the present invention as claimed.

Claims

1. A linear companding method for inhibiting visible light communication LED nonlinear amplitude limiting noise comprises the following steps:

a sending end:

step 1: carrying out quadrature amplitude modulation on input binary bit data;

step 2: mapping the frequency domain data to ensure that the output data has a Hermite symmetrical structure;

and step 3: pre-scaling the mapped signal, and presetting an optical OFDM (O-OFDM) symbol variance;

and 4, step 4: obtaining a real bipolar O-OFDM symbol after inverse Fourier transform;

and 5: performing adaptive linear compression transformation on the O-OFDM symbols after the parallel-serial conversion;

step 6: decomposing the signal into two groups of symbols, namely compressed symbols and uncompressed symbols, adding a Cyclic Prefix (CP) to each symbol after amplitude limiting, and then serially framing for sequential transmission;

Receiving end:

step 1: the photoelectric detector receives the optical signal to perform photoelectric conversion, and the optical signal is converted into an electric signal;

step 2: converting the electric signal into a digital signal through analog-to-digital conversion;

and step 3: splitting the two groups of symbols into frames, deleting the cyclic prefix, performing linear expansion transformation on the compressed symbols, and then combining the two groups of symbols;

and 4, step 4: inputting the serial-parallel converted signals to a Fourier transform module;

and 5: and the frequency domain equalizing module equalizes and compensates the channel attenuation, extracts effective information of the frequency domain signal according to the signal mapping structure of the sending end to obtain a component carrying information, and finally demodulates and restores the original information.

2. The linear companding method for suppressing nonlinear clipping noise of the visible light communication LED according to claim 1, wherein the method comprises the following steps: a sending end step 5, wherein the object processed by the adaptive linear companding transform technology in the invention is a time domain O-OFDM vector signal which is output after inverse Fourier transform and parallel-serial conversion; the self-adaptive linear companding conversion process is mainly to pre-judge the signal according to the actual amplitude of the O-OFDM symbol and the amplitude limiting threshold, so as to find out the signal exceeding the amplitude limiting threshold and to perform compression conversion to reduce the signal amplitude, and the signal lower than the amplitude limiting threshold is not processed;

assuming a clipping threshold, i.e. the dynamic range of the modulated signal, is ∈ _min ,ε _max ]O-OFDM symbol period is T, and symbol x is output by parallel-to-serial conversion _IFFT (ii) a The amplitude division point of the signal mainly consists of epsilon _min 、ε _max The distinction is made, and the specific companding function is:

wherein, the multiplication factor of the compression number is more than 0 and less than or equal to 1; x is the number of _IFFT (n) represents x _IFFT N is 0,1, …, N-1;

the self-adaptive linear compression transformation carries out self-adaptive linear compression transformation on signals according to the actual amplitude of the O-OFDM symbols and the amplitude limiting threshold value, firstly, the time domain sampling amplitude of each O-OFDM symbol is pre-judged, and if the time domain sampling amplitude is in the dynamic range [ epsilon ] _min ,ε _max ]In this case, the signal is not limited, so that the signal is not compressed and converted, and is directly passed through; for an excess interval [ epsilon ] _min ,ε _max ]When the time domain signal with larger amplitude passes through the LED, serious top-cut distortion is generated, in order to reduce amplitude-limiting distortion, the amplitude which does not meet the requirement is found out, and then the amplitude is multiplied by a coefficient a which is less than 1 to carry out linear compression transformation.

3. The linear companding method for suppressing nonlinear clipping noise of the visible light communication LED according to claim 1, wherein the method comprises the following steps: in the step 6 of the sending end, the signal after the adaptive linear compression conversion has amplitude ambiguity, so that the receiving end has no problemThe method correctly decompresses the signal, and divides the signal into uncompressed symbols in order to accurately recover the signal at the receiving end

And compressed symbols

Two groups, which are respectively represented as:

wherein the content of the first and second substances,

and

respectively represent

And

n-0, 1, N-1;

thus, the original compressed symbol

Then compressing the symbols

Delaying by one O-OFDM symbol period T, will

Prior to the generation of

Under the condition of all-zero, in order to eliminate the influence of all-zero signals on the system performance, the all-zero signals which do not carry information are deleted and are not transmitted;

the clipped symbol is represented as

Wherein the content of the first and second substances,

the slicing procedure can be expressed as follows:

wherein the content of the first and second substances,

and

respectively represent

And

n-0, 1, N-1;

the amplitude of the compressed signal is reduced, but if the variance of the input signal is reduced

If the preset is too large, the compressed signal still exceeds the dynamic range of the LED, so the above clipping operation is required, but it should be noted that since the signal is adaptively and linearly companded, the modulation range of the LED is expanded, and when the original signal is in the range of the LED, the signal is compressed and amplified

When in range, there is no clipping noise, when out of range

wherein x is _IFFT (n) represents a signal x _IFFT N-0, 1, …, N-1;

in the formula: n is _clip And K is a clipping attenuation coefficient and is expressed as:

in the formula (I), the compound is shown in the specification,

and

a probability density function representing the IFFT output signal;

In order to resist intersymbol interference caused by multipath propagation and optical dispersion effect of O-OFDM symbols, the two symbols after amplitude limiting are subjected to amplitude limiting

Cyclic Prefixes (CPs) are added, respectively, and framing is performed.

4. The linear companding method for suppressing nonlinear clipping noise of the visible light communication LED according to claim 1, wherein the method comprises the following steps: receiving end step 3, receiving two decomposition symbols in the frame in sequence, then dividing the frame and deleting the cyclic prefix of each decomposition symbol, then performing linear expansion transformation on the second compressed symbol, and then delaying the first symbol to be aligned with the second symbol in time; finally, adding the corresponding bits to obtain a combined O-OFDM symbol;

the two decomposed symbols received by the receiving end can be expressed as:

wherein n is ₁ And n ₂ Respectively, additive white Gaussian noise superimposed on the first and second O-OFDM decomposed symbols, all of which have the same probability density, B _DC Is a direct current bias, gamma represents a photoelectric conversion factor;

the specific decompression process is as follows: receive the first symbol y ₁ The next received compressed symbol y is not subjected to expansion transformation ₂ Multiplying by 1/a for linear expansion transformation, and then multiplying y ₁ Delayed by one symbol period T and y ₂ Time alignment and combination are performed, and the combined symbol is represented as: