阅读说明：本技术 一种基于时域预处理的混合光ofdm方法 (Mixed light OFDM method based on time domain preprocessing ) 是由 李宝龙 陆波 丁文杰 薛晓妹 于 2021-06-23 设计创作，主要内容包括：本发明公开了一种基于时域预处理的混合光OFDM方法,属于无线光通信领域。本发明以时域预处理的方式在发射端消除削波失真,且发送端仅需一个IFFT模块,接收端则为标准的OFDM接收器,显著降低了系统的复杂度和时延。仿真结果表明本发明所提出的方案具有更好的抑制非线性失真的能力,在非线性情况下能进行良好的数据传输,具有较优的误码率性能。(The invention discloses a mixed light OFDM method based on time domain preprocessing, and belongs to the field of wireless optical communication. The invention eliminates clipping distortion at the transmitting end in a time domain preprocessing mode, and the transmitting end only needs one IFFT module, and the receiving end is a standard OFDM receiver, thereby obviously reducing the complexity and time delay of the system. Simulation results show that the scheme provided by the invention has better capability of inhibiting nonlinear distortion, can perform good data transmission under the nonlinear condition and has better error rate performance.)

1. A mixed light OFDM method based on time domain preprocessing is characterized in that,

(1) the transmitting end comprises the following steps:

step 1: the two paths of sending bits are respectively subjected to Quadrature Amplitude Modulation (QAM) mapping and Pulse Amplitude Modulation (PAM) mapping and then are subjected to hybrid transmission, the number of subcarriers of an OFDM system is represented as N, and within one OFDM symbol time, a transmitted QAM symbol is represented as X_iWhere i is 0, 1, …, N/4-1, and the transmitted PAM symbol is denoted as Y_mWherein m is 0, 1, …, N/4-2, serial/parallel connectedAlternatively, after hermitian symmetric transformation, the frequency domain signal of the hybrid optical OFDM can be expressed as:

wherein the imaginary compensation coefficient

Step 2: to W_kPerforming inverse Fourier transform operation, and performing parallel-to-serial conversion to generate time domain signal w_n，n＝0，1，...，N-1；

And step 3: computing a time-domain preprocessed signal s_n：

And 4, step 4: will w_nAnd s_nAnd (4) superposing, performing digital-to-analog conversion, generating an analog signal, and transmitting the analog signal through a light emitting diode. The final transmission signal t_nThe calculation formula of (2) is as follows:

t_n＝w_n+s_n，n＝0，1，…，N-1；

(2) the receiving end adopts a standard OFDM receiver.

2. The hybrid light OFDM method of claim 1, wherein in step 2, the time domain signal w_n＝x_n+y_n，x_nRepresented as an unclipped ACO-OFDM signal, y_nRepresenting an unclipped PAM-DMT signal.

3. The mixed-light OFDM method according to claim 1 or 2, wherein the time-domain preprocessed signal is calculated in step 3 by:

to ensure that the transmitted signal is not negative, and the receiving end x_nAnd y_nDo not interfere with each other, transmit signal t_nCan be expressed as:

whereinRepresenting operation of the negative part of clipping, a_nMeaning if no time domain pre-processing is performed, x_nPair of clipping operations y_nThe noise interference generated; consider x_nHalf-wave symmetry and sort through x_nPositive and negative, t can be further simplified_nAnd then combine with y_nTo obtain the final transmission signal t_nThe calculation formula of (2) is as follows:

finally according to s_n＝t_n-w_nDetermining a time-domain preprocessed signal s_n。

Technical Field

The invention relates to a time domain preprocessing-based mixed light OFDM (Orthogonal Frequency Division Multiplexing) method, belonging to the technical field of optical wireless communication.

Background

With the increasing scarcity of Radio Frequency (RF) spectrum resources, researchers have gradually turned their focus to the area of Optical Wireless Communications (OWC) with higher spectrum. By modulating the bit stream at light intensity, OWCs are capable of achieving both high-speed transmission and individualized illumination, providing an attractive complementary technique for RF communication. OWC has stimulated a great deal of research interest in both academia and industry due to its inherent advantages, such as abundant license-free spectrum resources, strong electromagnetic interference resistance and a secure communication environment.

In order to realize ideal data rate transmission, Orthogonal Frequency Division Multiplexing (OFDM) is widely used in OWC due to its advantages of high spectrum efficiency, strong inter-symbol interference resistance, and the like. Unlike conventional RF communications, OWC is based on an intensity modulation/direct detection (IM/DD) system, requiring the transmitted signal to be real and positive, which has led to the field of optical OFDM (O-OFDM) research.

Aiming at a system that optical wireless communication is based on intensity modulation/direct detection and requires that a transmitted signal is real and positive, a plurality of O-OFDM schemes have been proposed at present, but all have certain performance defects. Among them, HACO-OFDM has a better tradeoff between spectral efficiency, computational complexity, and transmission performance. However, as the clipping distortion of the ACO-OFDM branch pollutes the subcarriers carrying the PAM symbol, the HACO-OFDM receiver must regenerate and eliminate the clipping distortion to recover the PAM symbol, and the detection process increases complexity and detection error.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a mixed light OFDM method based on time domain preprocessing, so as to reduce the complexity and the time delay of the system.

The technical scheme of the invention is as follows: the invention relates to a mixed light OFDM method based on time domain preprocessing,

(1) the transmitting end comprises the following steps:

step 1: the two paths of sending bits are respectively subjected to Quadrature Amplitude Modulation (QAM) mapping and Pulse Amplitude Modulation (PAM) mapping and then are subjected to hybrid transmission, the number of subcarriers of an OFDM system is represented as N, and within one OFDM symbol time, a transmitted QAM symbol is represented as X_iWhere i is 0, 1, …, N/4_1, and the transmitted PAM-DMT symbol is denoted as Y_mWhere m is 0, 1, …, N/4-2, and after serial/parallel conversion and hermitian symmetric conversion, the frequency domain signal of the mixed light OFDM can be expressed as:

wherein the imaginary compensation coefficientk＝0，1，…，N-1；

Step 2: to W_kPerforming inverse Fourier transform operation, and performing parallel-to-serial conversion to generate time domain signal w_n，n＝0，1，...，N-1；

And step 3: computing a time-domain preprocessed signal s_n：

And 4, step 4: will w_nAnd s_nAnd (4) superposing, performing digital-to-analog conversion, generating an analog signal, and transmitting the analog signal through a light emitting diode. The final transmission signal t_nThe calculation formula of (2) is as follows:

t_n＝w_n+s_n，n＝0，1，…，N-1；

(2) the receiving end adopts a standard OFDM receiver.

Further, in step 2, the time domain signal w_n＝x_n+y_n，x_nRepresented as an unclipped ACO-OFDM signal, y_nRepresenting an unclipped PAM-DMT signal.

Further, in step 3, the time domain preprocessed signal is calculated as follows:

to ensure that the transmitted signal is not negative, and the receiving end x_nAnd y_nDo not interfere with each other, transmit signal t_nCan be expressed as:

whereinRepresenting operation of the negative part of clipping, a_nMeaning if no time domain pre-processing is performed, x_nPair of clipping operations y_nThe noise interference generated; consider x_nHalf-wave symmetry and sort through x_nPositive and negative, t can be further simplified_nAnd then combine with y_nTo obtain the final transmission signal t_nThe calculation formula of (2) is as follows:

finally according to s_n＝t_n-w_nDetermining a time-domain preprocessed signal s_n。

Advantageous effects

The method of the invention does not need clipping processing at the sending end, ensures that the sending signal is nonnegative at the sending end in a time domain preprocessing mode, and also eliminates clipping distortion generated in HACO-OFDM. Compared with the HACO-OFDM, the transmitting end only needs one IFFT module, the complexity of the transmitting end is reduced, the receiving end does not need to regenerate and eliminate the clipping loss, and the OFDM receiver is a standard OFDM receiver and also reduces the complexity and the time delay of the receiving end;

drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

Fig. 1 is a block diagram of a transmitting end and a receiving end of a mixed light OFDM method based on time domain preprocessing according to an embodiment;

FIG. 2 is a graph comparing PAPR performance of a mixed light OFDM method (TPHO-OFDM) based on time domain preprocessing with HACO-OFDM according to an embodiment;

FIG. 3 is a graph of the BER performance of an embodiment of a time-domain pre-processing based hybrid optical OFDM method (TPHO-OFDM) versus HACO-OFDM in a linear case;

FIG. 4 is a graph of the BER performance of the time-domain pre-processing based mixed light OFDM method (TPHO-OFDM) compared with the HACO-OFDM under the nonlinear condition.

Detailed Description

The technical solution of the present invention is further described in detail below with reference to the drawings and examples of the specification, and the embodiments of the present invention include, but are not limited to, the following examples.

Examples

Considering that PAM-DMT is polluted by clipping distortion of ACO-OFDM branch in hybrid asymmetric reduction orthogonal frequency division multiplexing (HACO-OFDM), a receiving end must regenerate and eliminate the clipping distortion to recover PAM symbols, and the iterative detection process increases the complexity and processing delay of the receiving end. In order to solve the problem, the present invention provides a mixed light OFDM system based on time domain preprocessing, which includes a transmitting end and a receiving end, as shown in fig. 1, which is a block diagram of the transmitting end and the receiving end in this embodiment. The method eliminates clipping distortion at a transmitting end in a time domain preprocessing mode, only one inverse Fourier transform (IFFT) module is needed at the transmitting end, and a receiving end is a standard OFDM receiver, so that the complexity and the time delay of the system are obviously reduced.

Specifically, the invention relates to a mixed light OFDM method based on time domain preprocessing, which comprises the following steps:

step 1: the two paths of sending bits are respectively subjected to Quadrature Amplitude Modulation (QAM) mapping and Pulse Amplitude Modulation (PAM) mapping and then are subjected to hybrid transmission, the number of subcarriers of an OFDM system is represented as N, and within one OFDM symbol time, a transmitted QAM symbol is represented as X_iWhere i is 0, 1, …, N/4-1, and the transmitted PAM symbol is denoted as Y_mWhere m is 0, 1, …, N/4-2, and after hermitian symmetric transformation by serial/parallel conversion, the frequency domain signal of the mixed light OFDM can be expressed as:

wherein the imaginary compensation coefficientk＝0，1，…，N-1。

Step 2: to W_kPerforming inverse Fourier transform operation, and performing/parallel-serial conversion to generate time domain signal w_n. Wherein w_n＝x_n+y_n，x_nRepresented as an unclipped ACO-OFDM signal, y_nRepresenting an unclipped PAM-DMT signal, N-0, 1.

And step 3: computing a time-domain preprocessed signal s_n。

And 4, step 4: will w_nAnd s_nAnd (4) superposing, performing digital-to-analog conversion, generating an analog signal, and transmitting the analog signal through a light emitting diode. The final transmission signal t_nCan be expressed as

The specific derivation is as follows: to ensure that the transmitted signal is not negative, and the receiving end x_nAnd y_nDo not interfere with each other, transmit signal t_nCan be expressed as

WhereinRepresenting operation of the negative part of clipping, a_nMeaning if no time domain pre-processing is performed, x_nPair of clipping operations y_nThe noise generated interferes.

Clipping signalSatisfy the requirement of

WhereinIt is indicated that the clipping operation is performed,representing clipping distortion noise on even subcarriers, where a_nRepresenting imaginary noise interference, b_nReal part noise interference, and satisfies

Clipping distortion can be obtainedInterference a generated in the imaginary part of even-indexed sub-carriers_nComprises the following steps:

and further t_nCan be written as

Consider x_nHalf-wave symmetry ofThe relation between the clipped time domain signal and the unclipped time domain signal is

t_nCan be further written as:

extraction oft_nCan be further written as:

when x is_n＞x_N/2+nWhen is in accordance with x_nHalf-wave symmetry of (2), x can be obtained_nIf greater than 0, then

When x is_n≤x_N/2+nWhen is in accordance with x_nHalf-wave symmetry of (2), x can be obtained_nWhen the ratio is less than or equal to 0, then

Consider y_nOf (a) is antisymmetry, i.e. y_n＝y_N/2+n＝-y_N/2-n＝-y_N-nThe final transmitted signal t_nCan be reconstructed into

And 5: the receiving end of the invention is a standard OFDM receiver.

The PAPR performance comparison graph of the mixed light OFDM method (TPHO-OFDM) based on time domain preprocessing and the mixed asymmetric reduction orthogonal frequency division multiplexing HACO-OFDM of the invention is shown in figure 2, and the horizontal coordinate power-division average ratio PAPR of the graph₀For the transformation range of the set peak-to-average power ratio, the ordinate is a complementary cumulative distribution function curve Pr [ PAPR >₀]Indicating that the system has large PAPR distributionProbability of PAPR 0. As can be seen, a given PAPR₀Pr [ PAPR > PAPR of TPHO-OFDM₀]Smaller, this indicates that the proposed method has better ability to suppress nonlinear distortion.

The BER performance comparison graph of the hybrid optical OFDM method (TPHO-OFDM) based on time domain preprocessing and the hybrid asymmetric reduction orthogonal frequency division multiplexing HACO-OFDM under the linear condition of the invention is shown in fig. 3, and it can be seen that the BER performance of the TPHO-OFDM method is almost equal to that of the HACO-OFDM under the linear condition compared with the HACO-OFDM, but the complexity of the invention is far lower than that of the HACO-OFDM, and the processing delay of the invention is lower.

The BER performance comparison graph of the mixed light OFDM method (TPHO-OFDM) based on time domain preprocessing and the mixed asymmetric reduction orthogonal frequency division multiplexing HACO-OFDM under the nonlinear condition is shown in FIG. 4, and the LED can not always keep linear characteristics due to the change of the temperature and the environment. It can be seen that the BER performance of the TPHO-OFDM method under the nonlinear condition is far better than that of the HACO-OFDM method, namely, the system can perform good data transmission under the nonlinear condition, and the method has certain significance.

The above examples are only preferred embodiments of the present invention, it should be noted that: it will be apparent to those skilled in the art that various modifications and equivalents can be made without departing from the spirit of the invention, and it is intended that all such modifications and equivalents fall within the scope of the invention as defined in the claims.