CN112636830A

CN112636830A - Time synchronization method, system and storage medium based on DCO-OFDM visible light communication system

Info

Publication number: CN112636830A
Application number: CN202011404293.1A
Authority: CN
Inventors: 蒋宇飞; 杨建国; 朱旭
Original assignee: Shenzhen Graduate School Harbin Institute of Technology
Current assignee: Shenzhen Graduate School Harbin Institute of Technology
Priority date: 2020-12-03
Filing date: 2020-12-03
Publication date: 2021-04-09
Anticipated expiration: 2040-12-03
Also published as: CN112636830B

Abstract

The application relates to a time synchronization method, a time synchronization system and a storage medium based on a DCO-OFDM visible light communication system, wherein a blind virtual subcarrier time synchronization algorithm uses the property of a first subcarrier in the DCO-OFDM system, does not need to use a training sequence, and estimates timing offset by using the maximum value of a ratio, so that the time synchronization performance is prevented from being reduced by error noise caused by obtaining channel information by approximate processing in a direct current offset time synchronization algorithm, and the inconvenience of a threshold value is avoided. Similarly, the virtual subcarrier time synchronization algorithm is a blind algorithm, so that the extra overhead caused by the transmission of the training sequence is avoided, and the influence of the limited bandwidth of the LED is effectively reduced.

Description

Time synchronization method, system and storage medium based on DCO-OFDM visible light communication system

Technical Field

The present application relates to the field of communications technologies, and in particular, to a time synchronization method and system based on a DCO-OFDM visible light communication system, and a storage medium.

Background

Nowadays, in the era of information technology, people's life is closely related to mobile devices, and people can not leave the use of mobile devices such as mobile phones, computers, smart bracelets and the like. Meanwhile, the proliferation of wireless mobile devices has made fourth generation mobile communications unable to meet the demands of people for high speed, large system capacity, etc. Therefore, the development of the fifth Generation (5th Generation,5G) mobile communication technology has become a research hotspot in the field of mobile communication at home and abroad. However, 5G mobile communication technology faces the scarcity of available radio frequency spectrum resources below 10 GHz. Visible Light Communication (VLC) is a potential technology for overcoming the scarcity of radio frequency spectrum resources by using a Visible Light spectrum in a wavelength range of 380nm to 780nm corresponding to a frequency range of several hundred terahertz. Light Emitting Diodes (LEDs) are lighting devices, and wireless data connection function can be realized through Light intensity conversion. An LED is used as a transmitter of a visible light communication system, and a Photodiode (PD) is used as a receiver. However, the limited bandwidth characteristics of LEDs can cause intersymbol interference, thereby limiting data transmission rates and affecting system error rate performance. Direct Current Biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) is introduced into a visible light communication system as an effective scheme that can resist the problem of intersymbol interference.

Synchronization is a very important issue for DCO-OFDM systems, and small synchronization errors all cause system performance degradation. The visible light communication system transmits data through the intensity change of the LED at a transmitting end and directly receives the data by using the photodiode at a receiving end, so that the DCO-OFDM system has no frequency synchronization problem of carrier frequency offset. Therefore, the time synchronization problem is mainly considered in the DCO-OFDM system for visible light communication. The time synchronization requires that the receiving end obtains the start position of the DCO-OFDM data frame, so that the receiving end can correctly remove the Cyclic Prefix (CP) and perform accurate Fast Fourier Transform (FFT). If the time synchronization is wrong, intersymbol Interference and intercarrier Interference (ICI) can be caused, which leads to the performance reduction of the Bit Error Rate (BER) of the system.

Many methods have been proposed for time synchronization, and these methods can be divided into two categories: the time synchronization algorithm is mainly divided into two categories, namely a training sequence-based time synchronization algorithm and a blind time synchronization algorithm. The main idea of the time synchronization algorithm based on the training sequence is to perform time synchronization after performing correlation operation by using a special structure of the training sequence. In 1997, Schmidl and Cox proposed an SC algorithm that uses two pieces of repeated data as training sequences, calculates an autocorrelation function at the receiving end and finds the maximum value thereof, thereby roughly estimating the start position of a data frame. However, due to the existence of the cyclic prefix in the OFDM system, a maximum platform exists in the autocorrelation function calculated at the receiving end, which may cause a time synchronization error. In 2003, Park et al further proposed an improved Park algorithm. The algorithm redefines the structure of the training sequence, and also divides the training sequence into four parts, but takes two sections of data which are in mirror symmetry as the first two parts, and the second two parts are in a conjugate relation with the first two parts, so that the Park algorithm solves the platform problem of the SC algorithm. However, when the length of the OFDM data block is less than 4 times of the length of the cyclic prefix, the heights of the secondary peak and the main peak in the autocorrelation function of the Park algorithm are the same, which may cause misjudgment of the start position of the data frame when performing peak detection. The time synchronization algorithm performs autocorrelation operation by using a special structure of a training sequence, and finally peak detection is performed to obtain an estimated value of the initial position of the data frame. However, they are not suitable for visible light communication DCO-OFDM systems. Not only does the bandwidth limitation of the LED destroy the correlation between the training sequences of the received signals, but also the correlation operation is changed from complex signal operation to real signal operation because the visible light communication system requires the transmitted signals to be real and non-negative, thereby reducing the correlation between the training sequences.

In 2018, Jiang et al proposed a Minimum Received Signal Power (MRSP) algorithm in a DCO-OFDM system for visible light communication. Time synchronization is performed by minimizing the difference between the received signal and the reconstructed signal based on the training sequence. The algorithm has robustness on the LED limited bandwidth, but the algorithm has overlarge complexity because the algorithm needs to perform channel estimation to obtain a reconstructed signal.

There are only a small fraction of blind time synchronization algorithms. In 1997, Beek et al proposed a cyclic prefix algorithm that did not require a training sequence. The algorithm utilizes the characteristic of the cyclic prefix, namely the cyclic prefix and the last data in the current OFDM data block are repeated, self-correlation operation is carried out on a receiving end by utilizing the redundant information, and then the estimated value of the initial position of the data frame is obtained by peak value detection. However, the cyclic prefix algorithm cannot be applied to the visible light communication system, and the LED-limited bandwidth destroys the correlation of the cyclic prefix, thereby affecting the time synchronization performance. There is only one blind time synchronization algorithm for visible light communication systems. The algorithm realizes synchronization by utilizing the characteristic that asymmetric amplitude limiting optical ACO-OFDM signals are symmetrical in a time domain, does not need any additional overhead of a training sequence, and is low in complexity. However, the algorithm does not consider the LED-limited bandwidth, and the symmetry of the time-domain signal at the receiving end is easily destroyed by the intersymbol interference due to the LED-limited bandwidth. Meanwhile, the algorithm cannot be used in the visible light communication DCO-OFDM system because symmetry exists only in the ACO-OFDM signal and not in the DCO-OFDM signal. To date, there is no blind time synchronization algorithm for DCO-OFDM Fidelity (LiFi) systems.

Disclosure of Invention

The present application is directed to a time synchronization method, system and storage medium based on a DCO-OFDM visible light communication system, so as to solve at least one of the above technical problems.

The application provides a time synchronization method based on a direct current bias light orthogonal frequency division multiplexing DCO-OFDM visible light communication system, which comprises the following steps:

obtaining receiving frequency domain signals which correspond to N/2 th subcarriers of a plurality of DCO-OFDM blocks and are obtained through cyclic prefix removal and Fourier transform processing, wherein the receiving frequency domain signals are defined by virtual subcarrier timing offset, and a first value range of the virtual subcarrier timing offset is determined by cyclic prefix length and the total number N of subcarriers;

determining a virtual subcarrier timing offset estimation value when the virtual subcarrier timing metric function value meets a first preset condition within the first value range based on a virtual subcarrier timing metric function defined by the received frequency domain signal;

and determining the receiving frequency domain signal with the data frame starting position meeting the requirement according to the virtual subcarrier timing offset estimation value.

Further, determining, based on a virtual subcarrier timing metric function defined by the received frequency domain signal, a virtual subcarrier timing offset estimation value when the virtual subcarrier timing metric function value satisfies a first preset condition within the first value range, specifically including:

subtracting the adjacent timing measurement functions of the virtual subcarriers to obtain a difference absolute value function, and calculating T for the difference absolute value function_vPoint variance, T_vDetermined by the cyclic prefix length and the total number of subcarriers, at T_vAnd determining the virtual sub-carrier timing offset estimation value when the variance value meets a second preset condition in a second value range determined by the cyclic prefix length and the total number of the sub-carriers.

Further, T is calculated for the difference function_vThe point variance specifically includes:

calculating T for said difference function_vPoint variance to obtain a first intermediate variance;

recalculating T for the first intermediate variance_vAnd point variance is obtained to obtain the variance value.

The present application also provides a receiving device in a DCO-OFDM visible light communication system, which is characterized by comprising: a processor and a memory, the memory storing a computer program for invocation by the processor, the computer program, when invoked, being executable to perform the method as described above.

The present application also provides a DCO-OFDM visible light communication system, including: a receiving device as described above, and a transmitting device.

Further, the transmission apparatus includes: the transmitting device comprises an M-QAM module, a Hermite symmetry module, an IFFT module, a direct current bias module, a negative value elimination module and a cyclic prefix adding module, wherein the M-QAM module, the Hermite symmetry module, the IFFT module and the direct current bias module are used for carrying out multi-system quadrature amplitude modulation (M-QAM) modulation, Hermite symmetry and Inverse Fast Fourier Transform (IFFT) on binary input data, adding direct current bias, eliminating negative values and adding cyclic prefix processing to obtain a transmitting signal, and: a lighting module for transmitting the transmission signal.

Further, the reception apparatus includes: a receiver module for performing photoelectric conversion on an optical signal to obtain a received time domain signal, a traversal de-cyclic prefix module for performing de-cyclic prefix on the received time domain signal to obtain a received secondary signal when different direct current offset timing offset values are taken, an FFT module for performing Fast Fourier Transform (FFT) processing on the received secondary signal to obtain the received frequency domain signal for processing by a time synchronization module, a frequency domain equalization module for sequentially performing frequency domain equalization and M-QAM demodulation processing on the received frequency domain signal determined by the time synchronization module to obtain recovery data corresponding to the binary input data, and an M-QAM demodulation module,

the time synchronization module is used for obtaining receiving frequency domain signals which correspond to the N/2 th subcarriers of a plurality of DCO-OFDM blocks and are obtained through cyclic prefix removal and Fourier transform processing, wherein the receiving frequency domain signals are defined by virtual subcarrier timing offset, and a first value range of the virtual subcarrier timing offset is determined according to cyclic prefix length and the total number N of subcarriers; determining a virtual subcarrier timing offset estimation value when the virtual subcarrier timing metric function value meets a first preset condition within the first value range based on a virtual subcarrier timing metric function defined by the received frequency domain signal; and determining the receiving frequency domain signal with the data frame starting position meeting the requirement according to the virtual subcarrier timing offset estimation value.

The present application further provides a computer storage medium, wherein the computer storage medium stores a computer program, and the computer program is called by a processor to execute the method as described above.

The beneficial effect of this application lies in:

by providing the time synchronization method, the time synchronization system and the storage medium based on the DCO-OFDM visible light communication system, the blind virtual subcarrier time synchronization algorithm uses the property of the first subcarrier in the DCO-OFDM system, does not need to use a training sequence, and estimates the timing offset by using the maximum value of the ratio, thereby not only avoiding the reduction of time synchronization performance caused by error noise due to the fact that channel information is obtained by approximate processing in the DC offset time synchronization algorithm, but also avoiding the inconvenience of a threshold value. Similarly, the virtual subcarrier time synchronization algorithm is a blind algorithm, so that the extra overhead caused by the transmission of the training sequence is avoided, and the influence of the limited bandwidth of the LED is effectively reduced.

Drawings

Fig. 1 is a schematic structural diagram of a visible light communication system according to a first embodiment of the present application.

Fig. 2 is a schematic diagram of a dc offset timing metric function under a noise-free condition according to a first embodiment of the present application.

Fig. 3 is a schematic diagram illustrating the influence of the number of subcarriers and the number of DCO-OFDM blocks required for synchronization on theorem 1 according to an embodiment of the present application.

Fig. 4 is a schematic diagram of a virtual subcarrier timing metric function in the case of no noise according to the first embodiment of the present application.

Fig. 5 is a comparison graph of time synchronization performance of the blind algorithm and other time synchronization algorithms according to the first embodiment of the present application.

Fig. 6 is a graph comparing the error rate performance of the blind algorithm and other time synchronization algorithms according to the first embodiment of the present application.

Detailed Description

The principle of the invention according to the present application will be described in detail with reference to some embodiments, which are used for explaining the invention and do not represent that the scope of protection of the present application only includes the embodiments, and other embodiments not listed below and belonging to the inventive concept are still within the scope of protection of the present application.

The first embodiment is as follows:

the embodiment of the application provides a time synchronization method based on a DCO-OFDM visible light communication system. The method will be described in detail below in several sections.

Aiming at the influence of timing offset on a received frequency domain signal in a visible light communication DCO-OFDM system, two blind time synchronization algorithms without a training sequence are provided. The main contributions herein are as follows:

firstly, as far as we know, the first work of carrying out the blind time synchronization algorithm for the visible light communication DCO-OFDM system realizes high spectral efficiency, and a large number of pilot frequencies are required to be used as training sequences, which is different from the previous work. A blind time synchronization algorithm based on direct current offset and a blind time synchronization algorithm based on virtual subcarriers are provided for a visible light communication DCO-OFDM system. The blind direct current offset algorithm is realized by the inherent direct current offset of a DCO-OFDM system, the blind virtual subcarrier algorithm is realized by minimizing the power of a virtual subcarrier, and then an accurate timing offset estimation value is obtained by a threshold value.

Second, time synchronization is performed in the frequency domain. Most of the conventional time synchronization algorithms based on training sequences generally use correlation between the training sequences in the time domain to perform peak detection and then obtain a timing offset estimation value, but because the bandwidth limitation of LEDs causes intersymbol interference, the correlation between the training sequences is reduced at a receiving end, and the time synchronization performance is reduced. The time synchronization is carried out on the frequency domain, the channel convolution on the time domain is changed into the product on the frequency domain, the channel problem is convenient to process, the defect of time synchronization in the time domain is avoided, and the influence caused by the limitation of the bandwidth of the LED is convenient to solve to a certain extent.

System model

The system block diagram of the blind time synchronization algorithm research based on visible light communication DCO-OFDM is shown in FIG. 1:

since the visible light channel has no higher randomness than a general wireless communication channel, it can be estimated according to its unique characteristics, and the estimation method also depends on its characteristics, so the light channel is explained first. Meanwhile, due to the problem that the bandwidth of the LED is limited in visible light communication, modeling analysis is also carried out on the visible light communication. Then both the optical channel and the LED-limited bandwidth are considered for the total channel.

A common visible light communication scenario is short-range indoor transmission. In contrast to Line-of-Sight (LoS) transmission, fading due to scattering is very small and therefore negligible. The optical channel is generally considered to be a LoS channel, whose impulse response can be expressed as [9 ]:

wherein m is-ln (2)/ln [ cos (phi) ]_1/2)]Denotes the Lambertian transmission order, phi_1/2Is the half power angle of the LED, A is the receiving area of the photodetector, d is the distance between the emitting end and the receiving end, phi and

respectively the light radiation angle of the LED and the light incidence angle of the photodetector. The LED-limited modulation bandwidth due to the low-pass characteristic of the light emission front-end can be expressed as 10]：

Wherein f is_bIs the 3dB cut-off frequency of the LED.

The overall channel impulse response can be modeled as:

wherein the content of the first and second substances,

representing a linear convolution.

A description needs to be defined for several important parts and symbols therein to facilitate the understanding of the blind time synchronization algorithm later. First is X_i(k) Is represented in the ith (i ═ 0, 1.., N_s-1) a multi Quadrature Amplitude Modulation (M-QAM) signal on a k (k 0, 1.,. N-1) th subcarrier of the DCO-OFDM block. Since visible light transmits a real-valued non-negative time-domain signal, the signal is used for a time-domain signal after Inverse Fast Fourier Transform (IFFT)Number x_i(n) is a real, complex signal X_i(k) The conjugate symmetry needs to be satisfied:

where superscript denotes complex conjugation. To guarantee the time domain signal x_i(n) is non-negative, we apply DC bias and remove negative values, as follows:

wherein V_DCIndicating a dc bias. We give x first_i(n) applying a DC bias, and when a negative value is found, directly rejecting the signal to be 0. Then add a cyclic prefix with length L_cp。

After passing through the optical channel, the received time domain signal obtained at the receiving end is recorded as:

wherein w_i(n) is additive white gaussian noise. We define θ (θ ═ -L)_cp…, N-1) is a timing offset occurring at the time synchronization, and when θ is 0, perfect time synchronization is indicated, and otherwise, time synchronization error is indicated. The received frequency domain signal is affected by the timing offset as follows:

(ii) Effect of timing offset on System

From the last of the previous section we know θ (θ ═ -L)_cp…, N-1) is a timing offset, where L_cpIs the length of the cyclic prefix employed, and N is the number of subcarriers. It is necessary to utilize the nature of the effect of the timing offset on the received frequency domain signal to obtain the correspondingBlind time synchronization algorithm, therefore, we need to analyze the effect of timing offset on the received frequency domain signal. The effect of timing offset on the received frequency domain signal is divided into four cases:

when θ is 0, perfect time synchronization is indicated, and the received frequency domain signal is not interfered by any other DCO-OFDM block. Namely, it is

When theta is in [ - (L)_cp-L+1),-1]When the received frequency domain signal is shifted in phase, i.e. Y_i,θ(k)＝Y_i,0(k)e^j2πkθ/NAnd L is the number of channel paths.

When theta is equal to [ -L [ - ]_cp,-(L_cp-L+2)]And the detected DCO-OFDM block is influenced by the tailing of the last DCO-OFDM block, and intersymbol interference is introduced.

When theta epsilon (0, N-1), the detected DCO-OFDM block is influenced by the latter block, and intersymbol Interference and intercarrier Interference (ICI) are introduced.

(III) the blind DC offset time synchronization algorithm

The blind time synchronization algorithm mainly uses the property of the signal, and according to the above analysis, when theta is equal to 0,

from equation (5), assuming that the applied DC bias is sufficient, the culling effect can be ignored and thus can be obtained

The expression of (a) is as follows:

it is found that there is an influence on the DC offset in the 0 th subcarrier, and X is known from the formula (4)_i(0) Therefore, when θ is 0, we can obtain the received frequency domain signal on the 0 th subcarrierComprises the following steps:

when theta is in [ - (L)_cp-L+1),-1]From the previous analysis, we can obtain the received frequency domain signal on the 0 th subcarrier as: y is_i,θ(k)＝Y_i,0(k)e^j2πk0/N＝Y_i,0(k)。

When θ > 0, the received frequency domain signal on the 0 th subcarrier may be affected by intersymbol interference and intercarrier interference.

Thus, when we use the 0 th sub-carrier and

when measured as a change by autocorrelation, only if θ ∈ [ - (L)_cp-L+1),0]The difference is only affected by noise. In other cases, the difference is not only affected by noise but also by intersymbol interference and intercarrier interference, so that the result of autocorrelation is large. Therefore, a sudden jump occurs at θ ═ 0, and this characteristic is shown in fig. 2, according to which it is possible to perform time synchronization. Therefore, we can use this property to derive a timing metric function based on the dc offset timing metric function on the 0 th subcarrier as:

where Np denotes the 0 th subcarrier using the first Np DCO-OFDM blocks,

is defined as follows:

when N and Np are sufficiently large (N)64, Np 128), one can obtain:

n and Np values to the average received frequency domain signal on the 0 th subcarrier

And

the effect of mean square error performance between is shown in figure 3. Theorem 1: as the values of N and Np increase,

and

the mean square error performance between becomes good. When using N-64 and Np-128 consistent with the simulation parameters,

and

mean square error value of less than 5 x 10^-3Therefore, it can be considered that

As shown in fig. 2, at the correct frame start point M_DCB,0When a jump is generated, we can detect the jump point and can accurately synchronize time. Therefore, a method using a threshold value is conceived to detect the trip point. The blind dc offset time synchronization algorithm obtains an estimate of the timing offset in two steps. First, a rough timing offset estimate is obtained by taking the subscript of the dc offset timing metric function minimum:

then, pass through the threshold λ_DCBTo obtain an accurate timing offset estimate:

wherein arg first (-) denotes obtaining the subscript value satisfying the condition for the first time, C_DCB,τIs a decision timing metric function obtained by differencing dc offset timing metric functions adjacently:

the direct current offset time synchronization algorithm utilizes the influence of timing offset on a receiving frequency domain signal of DCO-OFDM to carry out time synchronization, does not need to utilize a training sequence, but uses the property of direct current offset on the 0 th subcarrier in a DCO-OFDM system, so the direct current offset time synchronization algorithm is a blind algorithm, avoids extra overhead caused by the transmission of the training sequence, and effectively reduces the influence of LED limited bandwidth.

(IV) the blind virtual subcarrier time synchronization algorithm

According to the formula (9), the blind dc offset time synchronization algorithm utilizes the dc offset property on the 0 th subcarrier, but because the channel information is required to be known in the blind dc offset time synchronization algorithm, we utilize the approximation processing of the formula (11), which avoids the requirement of knowing the channel information, but also causes the reduction of the time synchronization performance. Therefore, we receive information of the nth/2 sub-carrier of the frequency domain signal from the DCO-OFDM for time synchronization.

From the equations (8) and (4), it is possible to obtain

Therefore, when θ is 0, we can obtain the received frequency domain signal on the nth/2 th sub-carrier as:

Y_i,0(N/2)＝W_i(N/2) (15)

we can use this property to derive a timing metric function based on the virtual sub-carrier timing metric function on the nth/2 th sub-carrier as:

similarly, a sudden jump occurs at θ ═ 0, and this characteristic is shown in fig. 3, and can be used for time synchronization. The former blind dc offset time synchronization algorithm uses a threshold to detect its trip point, but the magnitude of the threshold is known in advance at the receiving end. Then, as can be seen from fig. 4, M is in the noise-free case _NS,00, then we can find M_NS,θ+1/M_NS,θTo determine the start point of the jump. However, when noise is present, M is used directly_NS,θ+1/M_NS,θCan cause misjudgment and belongs to (0, N-1) at the time of theta]Larger ratios are also possible within the range. So the ratio is maximized after the timing metric function is processed. We first found by differencing the neighbors:

C_NS,θ＝|M_NS,θ-M_NS,θ-1|,θ∈[-L_cp+1,N-1] (17)

then calculating Tv ═ L_cp-L +2 point variance, resulting in:

wherein the content of the first and second substances,

after one time of Tv point variance, we can eliminate the difference between theta and epsilon (0, N-1)]Within range suddenly have M_NS,θErrors caused by very small valuesAnd (5) carrying out the steps. But at theta ∈ (0, N-1)]In the range of M_NS,θIt is also possible for a nearly flat portion to appear, thus obtaining V_NS,pOr will be close to 0, then V is direct_NS,p+1/V_NS,pErroneous determination may be caused. Therefore we get the Tv point variance again

Wherein the content of the first and second substances,

finally, the estimated value of the timing offset obtained by the blind virtual subcarrier time synchronization algorithm is as follows:

the blind virtual subcarrier time synchronization algorithm uses the property of the N/2 th subcarrier in a DCO-OFDM system, does not need to use a training sequence, and estimates the timing offset by using the maximum value of a ratio, thereby not only avoiding the reduction of time synchronization performance by error noise caused by obtaining channel information by approximate processing in a direct current offset time synchronization algorithm, but also avoiding the inconvenience of a threshold. Similarly, the virtual subcarrier time synchronization algorithm is a blind algorithm, so that the extra overhead caused by the transmission of the training sequence is avoided, and the influence of the limited bandwidth of the LED is effectively reduced.

(V) complexity analysis

In order to prove the high efficiency of the provided blind direct current offset time synchronization algorithm and the blind virtual subcarrier time synchronization algorithm, the complexity of the algorithm is analyzed according to the times of multiplication in the algorithm. The proposed dc offset algorithm and the virtual subcarrier algorithm are shown in table 1 in comparison with the complexity of other time synchronization algorithms.

TABLE 1 complexity comparison of time synchronization algorithms

P is the length of the training sequence in the minimum received signal power algorithm. The process of channel estimation in minimum received signal power algorithm results in 2P³+4NP²The high complexity of +3NP + N, the DC offset algorithm and the virtual subcarrier algorithm proposed herein have only Np + NpNlog respectively₂N and Np + NpNlog₂Complexity of N +2 Tv. In the DC bias algorithm, NpNlog₂The complexity of N comes from the FFT of the received time domain signals of Np DCO-OFDM blocks, and the computation of the dc offset timing metric function brings Np complexity. And in the virtual sub-carrier algorithm, NpNlog₂The complexity of N comes from the process of performing FFT on received time domain signals of Np DCO-OFDM blocks, the complexity of Np is brought by the calculation of a virtual subcarrier timing measurement function, and finally, when an estimated value of timing offset is obtained, the Tv point variance is solved twice, so that the time complexity of 2Tv is brought. When the length P of the training sequence is close to the number N of the subcarriers, compared with the minimum received signal power algorithm, the complexity reduction multiple of the proposed direct current offset algorithm and the virtual subcarrier algorithm is approximately 6N²/(Nplog₂N). Similar performance as the minimum received signal power algorithm can be obtained for the proposed dc offset algorithm and the dummy subcarrier algorithm when N is 64, Np is 128 and P is 53, where the complexity is reduced by a factor of 20.

Although SC, Park, and cyclic prefix blind algorithms based on training sequences have lower complexity, in DCO-OFDM visible light communication systems considering LED limited bandwidth, performance of these algorithms is greatly affected by intersymbol interference caused by LED limited bandwidth, and performance is very poor, which will be verified in simulation results of the next section.

(VI) Effect and Experimental data

In order to better show the performance of the dc offset and virtual sub-carrier blind time synchronization algorithm proposed herein, the proposed method was simulated, and the system parameter settings are shown in table 2.

TABLE 2 main parameters table of system

One, time synchronization performance

Fig. 5 shows a comparison graph of time synchronization performance of the dc offset blind algorithm and the dummy subcarrier blind algorithm proposed herein with other time synchronization algorithm methods. The false synchronization probability reflects the synchronization performance of the algorithm, i.e. a lower false synchronization probability indicates a better synchronization performance. It can be seen from the figure that, in terms of time synchronization performance, the proposed dc offset blind algorithm and the dummy subcarrier blind algorithm are significantly better than SC, Park and cyclic prefix algorithms, because the SC and Park algorithms that perform time synchronization by using the correlation of the training sequence have their training sequence correlation destroyed by the intersymbol interference caused by the LED-limited bandwidth, while the cyclic prefix algorithm performs synchronization by using the cyclic prefix of the DCO-OFDM system and the correlation of the subcarrier data behind the DCO-OFDM block, although the cyclic prefix is a blind algorithm, and the LED-limited bandwidth also seriously affects the correlation of the cyclic prefix. The time synchronization performance of the direct current bias blind algorithm and the virtual subcarrier blind algorithm is close to that of the minimum received signal power algorithm, but the complexity of the minimum received signal power algorithm is high, and a training sequence is needed for time synchronization, so that the bandwidth efficiency is reduced. The time synchronization performance of the blind time synchronization algorithm proposed herein approaches the minimum received signal power algorithm while reducing complexity and not requiring additional spectral overhead. Comparing the dc offset blind algorithm and the virtual subcarrier blind algorithm proposed herein, it can be found that the time synchronization performance of the dc offset blind algorithm is slightly better than that of the virtual subcarrier blind algorithm under the condition of low signal-to-noise ratio, because the dc offset blind algorithm uses a threshold method, even under the influence of large noise, the optimal threshold is selected, so the performance is good, and the virtual subcarrier blind algorithm is more likely to have the wrong synchronization under the condition of large noise. However, under the condition of large signal-to-noise ratio, the time synchronization performance of the virtual subcarrier blind algorithm is superior to that of the direct current bias blind algorithm and is the same as that of the minimum received signal power algorithm, because in the direct current bias blind algorithm, approximate processing is utilized to replace directly required channel information when a timing measurement function of the direct current bias blind algorithm is calculated, error noise is generated, and the time synchronization performance is influenced.

Second, bit error rate performance

The bit error rate performance of the dc offset and dummy subcarrier blind algorithm proposed herein versus other time synchronization algorithms is shown in fig. 6. In the DCO-OFDM system for visible light communication, synchronization performance affects synchronization effect at a receiving end, and if time synchronization is wrong, a recovered signal is inconsistent with a signal at a transmitting end, so that the time synchronization performance directly affects the quality of error rate performance. The dotted line in the figure represents the lower bound of the ideal system error rate performance obtained by perfect time synchronization, perfect channel information and zero-forcing equalization, based on the ideal system error rate, i.e. the system error rate performance under the current simulation parameters. It can be seen from the figure that the error rate performance of the SC, Park and cyclic prefix algorithms is very poor, because the synchronization performance of the SC, Park and cyclic prefix algorithms is very poor. It can be known from the figure that the system error rate performance of the direct current bias blind algorithm and the virtual subcarrier blind algorithm provided herein is close to the ideal system error rate performance, and particularly after the signal-to-noise ratio is 10dB, the error rate performance of the blind time synchronization algorithm provided herein is the same as the theoretical error rate performance. Meanwhile, the error rate performance of the blind algorithm proposed herein is significantly better than SC, Park and cyclic prefix algorithms.

Example two:

the embodiment mainly provides a receiving device in a DCO-OFDM visible light communication system, including: a processor and a memory, the memory storing a computer program that can be called by the processor, the computer program, when called, performing the method according to the first embodiment.

The present embodiment further provides a DCO-OFDM visible light communication system, including: a receiving device as described above, and a transmitting device.

The embodiment also provides a computer storage medium, which is characterized in that the computer storage medium stores a computer program, and the computer program is called by a processor to execute the method.

The implementations shown in the above systems, devices, media are implemented by hardware components, devices, units, modules, apparatuses, and other components of the operations described herein with respect to the first embodiment or the second embodiment. Examples of hardware components include controllers, sensors, generators, drivers, memories, comparators, arithmetic logic units, adders, subtractors, multipliers, dividers, integrators, processors, and any other electronic component known to one of ordinary skill in the art that is configured to perform the operations described herein. In one example, the hardware components are implemented by one or more processors or computers. A processor or computer is implemented by one or more processing elements such as an array of logic gates, a controller and arithmetic logic unit, a digital signal processor, a microcomputer, a programmable logic controller, a field programmable gate array, a programmable logic array, a microprocessor, or any other device or combination of devices known to those of ordinary skill in the art that is capable of responding to and executing instructions in a defined manner to achieve a desired result.

In one example, a processor or computer includes or is connected to one or more memories that store instructions or software for execution by the processor or computer. Instructions or software (such as an operating system, OS, and one or more software applications running on the OS) are executed by a processor or computer-implemented hardware component to perform the operations described herein with respect to embodiment one or embodiment two. The hardware components also access, manipulate, process, create, and store data in response to execution of instructions or software. For simplicity, the singular terms "processor" or "computer" may be used in the description of the examples described herein, but in other examples, multiple processors or computers are used, or a processor or computer includes multiple processing elements or multiple types of processing elements, or both. In one example, the hardware component includes a plurality of processors, and in another example, the hardware component includes a processor and a controller. Examples of hardware components having any one or more of a variety of different processing configurations include single processors, independent processors, parallel processors, single instruction single data SISD multiprocessing, single instruction multiple data SIMD multiprocessing, multiple instruction single data MISD multiprocessing, and multiple instruction multiple data MIMD multiprocessing.

The method shown in embodiment one or embodiment two to perform the operations described in this application is performed by computing hardware (e.g., by one or more processors or computers) implemented as executing instructions or software to perform the operations described in this application to be performed by the method described in this application as described above. For example, a single operation or two or more operations may be performed by a single processor, or two or more processors, or a processor and a controller. One or more operations may be performed by one or more processors, or a processor and a controller, and one or more other operations may be performed by one or more other processors, or another processor and another controller. One or more processors, or a processor and a controller, may perform a single operation or two or more operations. Instructions or software for controlling a processor or computer-implemented hardware component and performing a method as described above may be written as a computer program, code segments, instructions, or any combination thereof, to individually or collectively instruct or configure the processor or computer to operate as a machine or special purpose computer for performing the operations performed by the hardware component and the method as described above. In one example, the instructions or software include machine code that is executed directly by a processor or computer, such as machine code produced by a compiler. In another example, the instructions or software include higher level code that is executed by a processor or computer using an interpreter. A person of ordinary skill in the art can easily write instructions or software based on the block diagrams and flowcharts shown in the figures and the corresponding description in the specification, which disclose algorithms for performing operations performed by hardware components and methods as described above.

The instructions or software and any associated data, data files, and data structures used to control the processor or computer-implemented hardware components and perform the methods described above may be recorded, stored, or fixed in or on one or more non-transitory computer-readable storage media. Examples of non-transitory computer-readable storage media include: read-only memory ROM, random-access memory RAM, flash memory, CD-ROM, CD-R, CD + R, CD-RW, CD + RW, DVD-ROM, DVD-R, DVD + R, DVD-RW, DVD + RW, DVD-RAM, BD-ROM, BD-R, BD-RLTH, BD-RE, magnetic tape, floppy disk, magneto-optical data storage device, hard disk, solid state disk, and any device known to those of ordinary skill in the art that is capable of storing instructions or software and any associated data, data files and data structures in a non-transitory manner and providing the instructions or software and any associated data, data files and data structures to a processor or computer so that the processor or computer can execute the instructions. In one example, the instructions or software and any associated data, data files, and data structures are distributed across networked computer systems such that the instructions and software and any associated data, data files, and data structures are stored, accessed, and executed by a processor or computer in a distributed fashion.

While the present disclosure includes specific examples, it will be apparent to those of ordinary skill in the art, after having had a full understanding of the disclosure of the subject application, that: various changes in form and detail may be made to these examples without departing from the spirit and scope of the claims and their equivalents. The examples described herein are to be considered in a descriptive sense only and not for purposes of limitation. The description of features or aspects in each example will be considered applicable to similar features or aspects in other examples. Suitable results may be achieved if the described techniques are performed in a different order and/or if components in the described systems, architectures, devices, or circuits are combined in a different manner and/or replaced or supplemented by other components or their equivalents. Therefore, the scope of the present disclosure is defined not by the detailed description but by the claims and their equivalents, and all changes within the scope of the claims and their equivalents are to be construed as being included in the present disclosure.

Claims

1. A time synchronization method based on a direct current bias light orthogonal frequency division multiplexing (DCO-OFDM) visible light communication system is characterized by comprising the following steps:

2. The method for time synchronization according to claim 1, wherein determining the virtual subcarrier timing offset estimation value when the virtual subcarrier timing metric function value satisfies a first predetermined condition within the first range of values based on the virtual subcarrier timing metric function defined by the received frequency domain signal comprises:

3. The time synchronization method of claim 2, wherein T is calculated for the difference function_vThe point variance specifically includes:

4. A receiving apparatus in a DCO-OFDM visible light communication system, comprising: a processor and a memory, the memory storing a computer program that can be invoked by the processor, the computer program, when invoked, performing the method of any of claims 1-3.

5. A DCO-OFDM visible light communication system, comprising: the receiving device of claim 4, and a transmitting device.

6. The visible light communication system according to claim 5, wherein the transmitting device comprises: the transmitting device comprises an M-QAM module, a Hermite symmetry module, an IFFT module, a direct current bias module, a negative value elimination module and a cyclic prefix adding module, wherein the M-QAM module, the Hermite symmetry module, the IFFT module and the direct current bias module are used for carrying out multi-system quadrature amplitude modulation (M-QAM) modulation, Hermite symmetry and Inverse Fast Fourier Transform (IFFT) on binary input data, adding direct current bias, eliminating negative values and adding cyclic prefix processing to obtain a transmitting signal, and: a lighting module for transmitting the transmission signal.

7. The visible light communication system according to claim 5, wherein the receiving device comprises: a receiver module for performing photoelectric conversion on an optical signal to obtain a received time domain signal, a traversal de-cyclic prefix module for performing de-cyclic prefix on the received time domain signal to obtain a received secondary signal when different direct current offset timing offset values are taken, an FFT module for performing Fast Fourier Transform (FFT) processing on the received secondary signal to obtain the received frequency domain signal for processing by a time synchronization module, a frequency domain equalization module for sequentially performing frequency domain equalization and M-QAM demodulation processing on the received frequency domain signal determined by the time synchronization module to obtain recovery data corresponding to the binary input data, and an M-QAM demodulation module,

8. A computer storage medium, characterized in that the computer storage medium stores a computer program that is invoked by a processor to perform the method according to any one of claims 1-3.