CN110971549B - MIMO-ADO-OFDM visible light communication system and receiving method - Google Patents

MIMO-ADO-OFDM visible light communication system and receiving method Download PDF

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CN110971549B
CN110971549B CN201911250243.XA CN201911250243A CN110971549B CN 110971549 B CN110971549 B CN 110971549B CN 201911250243 A CN201911250243 A CN 201911250243A CN 110971549 B CN110971549 B CN 110971549B
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CN110971549A (en
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肖海林
翟雷
周迪
刘海龙
潘永才
吴彬
毛淑霞
刘小兰
张文倩
覃琦超
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Hubei University
Guilin University of Electronic Technology
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Guilin University of Electronic Technology
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
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    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
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Abstract

The invention discloses a visible light communication system of MIMO-ADO-OFDM, which comprises a transmitting end and a receiving end and is characterized in that the transmitting end is provided with K LED units with the same structure, the receiving end is provided with K receiving units with the same structure, wherein K is integral multiple of 2, and the transmitting end and the receiving end of an MIMO array are in a transmitting and receiving state with the theoretically optimal angle. Such a communication system is low cost and fast and simple to deploy. The invention also discloses a receiving method of the visible light communication system of the MIMO-ADO-OFDM, which can reduce the system error rate and improve the system signal-to-noise ratio and the intersymbol interference (ISI) capability.

Description

MIMO-ADO-OFDM visible light communication system and receiving method
Technical Field
The invention relates to the technical field of visible light communication, in particular to a visible light communication system and a visible light receiving method of MIMO-ADO-OFDM.
Background
Visible light wireless communication is a new wireless optical communication technology developed after a white light LED lamp is widely applied, and the technology refers to that a signal is modulated and then a high-speed modulated optical carrier signal is sent out through an LED by using the characteristics that an LED device is easy to modulate and can be rapidly switched between on and off, air is used as a transmission medium, a photoelectric conversion device such as a Photodetector (PD for short) is used for receiving the optical carrier signal, and the optical carrier signal is demodulated and information is obtained. Compared with the traditional incandescent lamp, the white light emitting diode LED can meet the requirements of high efficiency, low current and voltage, and has the advantages of long service life, high response speed, high stability, environmental friendliness and the like. The visible light communication system integrates illumination and communication, has the characteristics of strong safety and confidentiality, strong anti-interference performance, large communication capacity, no need of frequency license, quick and simple deployment and the like, can realize the communication function under the self characteristic of maintaining lower cost, and has good application prospect in military, aerospace and civil aspects.
Orthogonal Frequency Division Multiplexing (OFDM), as a relatively mature technology in radio, has the advantages of inter-symbol interference (ISI) resistance, high spectrum resource utilization, strong frequency selective fading resistance, and the like. However, in the conventional OFDM system, the transmitted signal is usually bipolar complex value, and is not suitable for intensity modulation/direct detection (IM/DD) in VLC. At present, the basic schemes proposed in VLC are direct current offset optical OFDM (DCO-OFDM), asymmetric clipping optical OFDM (ACO-OFDM), pulse amplitude discrete multitone modulation (PAM-DMT), Flip-OFDM, and the like. These approaches have respective advantages and disadvantages: DCO-OFDM ensures that signals are not negative by adding a direct current bias mode, and has a simple structure but higher power overhead; the ACO-OFDM can directly clip the signal zero value without adding direct current offset, and the power of the transmitted signal is obviously reduced. In order to realize the compromise between the transmitting power and the frequency spectrum utilization rate, an ADO-OFDM superposition type transmission scheme is provided, and due to the fact that the modulation bandwidths of LEDs and the like are limited, a visible light SISO system is difficult to realize high-speed data transmission. In recent years, a Multiple Input Multiple Output (MIMO) technology is applied to a visible light communication system, and a system light source adopts an LED array to meet the requirement of illumination intensity, so as to improve the signal-to-noise ratio of the system, thereby realizing high-rate data transmission. However, the traditional MIMO visible light communication system has a simple receiving algorithm, poor interference resistance, low signal-to-noise ratio error rate, and low practicability, and cannot meet the high-performance transmission of multi-array visible light communication.
Disclosure of Invention
The invention aims to provide a visible light communication system and a visible light receiving method of MIMO-ADO-OFDM aiming at the defects of the prior art. Such a communication system is low cost and fast and simple to deploy. The method can reduce the system error rate, and improve the system signal-to-noise ratio and intersymbol interference (ISI) capability.
The technical scheme for realizing the purpose of the invention is as follows:
a visible light communication system of MIMO-ADO-OFDM comprises a transmitting end and a receiving end, wherein the transmitting end is provided with K LED units with the same structure, the receiving end is provided with K receiving units with the same structure, K is an integral multiple of 2, the transmitting end and the receiving end of an MIMO array are both in a transmitting and receiving state with the theoretically optimal angle, and one LED unit of the transmitting end comprises sequentially connected LED units
A serial-parallel conversion unit: the serial-parallel conversion unit converts the input serial structure data into parallel structure data;
a constellation mapping unit: the constellation mapping unit carries out QAM mapping on the signal, conjugate data are added to the complex signal subjected to QAM constellation mapping, so that the signal forms a Hermitian conjugate symmetric structure, and the output of IFFT is ensured to be a real number;
an ACO-OFDM signal generation unit: the ACO-OFDM signal generating unit divides an information vector X into odd subcarriers and even subcarriers, wherein XMagic cardRepresenting QAM constellation points on odd subcarriers, with even subcarrier modulation zeroed, XMagic card＝[0,X1,0,X3,...,0,XN-1]Wherein X isNMeans for mapping and modulating to Nth subcarrierAfter IFFT operation, zero value clipping is performed on the frequency domain data symbol, and the obtained bipolar real-value OFDM signal is changed into a unipolar non-negative signal by clipping at the zero value, that is:
Figure BDA0002308809780000021
and setting a threshold value A for the obtained non-negative real number signal x', wherein the judgment criterion is as follows:
Figure BDA0002308809780000022
performing frequency domain filtering after threshold judgment to obtain a baseband signal
Figure BDA0002308809780000023
Wherein n isACOIs the clipping noise of the ACO-OFDM signal, the signal obtained after FFT operation exists in the even carrier wave, xMagic cardIs XMagic cardObtaining a time domain signal after IFFT operation;
DCO-OFDM signal generation unit: x in DCO-OFDM signal generating unitDollRepresenting QAM constellation points on even carriers, zero setting of odd subcarrier modulation data, XDoll＝[X0,0,X2,...,XN-2,0]The signal is converted into a bipolar real-valued OFDM signal through N-point inverse fast fourier transform, i.e. N-point IFFT, and DC is added to make the signal of the negative part greater than zero, so that the obtained bipolar real-valued OFDM signal becomes a unipolar non-negative signal, i.e.:
x′＝x+a,a＞|x| (3)，
judging the obtained non-negative real number signal X', and performing frequency domain filtering after threshold judgment to obtain baseband signal XDCO；
A cyclic prefix adding unit: a cyclic prefix adding unit adds XACO、XDCOAdding the signals into a Cyclic Prefix (CP for short) after adding;
a parallel-serial conversion unit: the parallel-serial conversion unit converts the parallel data into serial data,form a new ACO-OFDM modulation signal X ═ XACO+XDCOFinally, an optical signal is sent out through the LED;
each receiving unit of the receiving end comprises 2 branches, and the first branch is provided with a plurality of sequentially connected branches
First photodetector PD: the optical signal is used for receiving the optical signal sent by the light source;
a first A/D conversion and serial-parallel conversion unit: the first A/D conversion and serial-parallel conversion unit carries out analog-to-digital conversion and serial-parallel conversion on an optical signal received by the photoelectric detector PD to enable the signal to be a parallel digital signal;
first STO estimation and de-CP unit: the first STO estimation and CP removal unit is used for measuring and calculating a timing deviation (STO value) by adopting the similarity between a Cyclic Prefix (CP) of a signal and a corresponding sequence at the tail of a symbol;
a first bidirectional maximum likelihood detection unit: the first bidirectional maximum likelihood detection unit establishes a candidate emission vector set capable of bidirectional detection to reduce the search range of a maximum likelihood vector (ML vector), so that the maximum likelihood vector can be obtained in limited searches during signal detection;
a DCO-OFDM signal demodulation and reconstruction modulation unit: the DCO-OFDM signal demodulation and reconstruction modulation unit has the function of obtaining the DCO-OFDM signal X on the even carrierDCO；
The second branch being provided with connections in series
The second photodetector PD: the optical signal is used for receiving the optical signal sent by the light source;
a second A/D conversion and serial-parallel conversion unit: the second A/D conversion and serial-parallel conversion unit performs analog-to-digital conversion and serial-parallel conversion on the optical signal received by the photoelectric detector PD to enable the signal to be a parallel digital signal;
second STO estimation and de-CP unit: the second STO estimation and CP removal unit is used for measuring and calculating a timing deviation (STO value) by adopting the similarity between a Cyclic Prefix (CP) of a signal and a corresponding sequence at the tail of a symbol;
a second bidirectional maximum likelihood detection unit: the second bidirectional maximum likelihood detection unit establishes a candidate emission vector set capable of bidirectional detection to reduce the search range of a maximum likelihood vector (ML vector), so that the maximum likelihood vector can be obtained in limited searches during signal detection;
an ACO-OFDM signal demodulation and reconstruction modulation unit: the DCO-OFDM signal demodulation and reconstruction modulation unit is connected with the ACO-OFDM signal demodulation and reconstruction modulation unit to output to obtain a final signal XACO。
The receiving method of the visible light communication system using the MIMO-ADO-OFDM comprises the following steps:
1) the receiving end divides the optical signal into an upper branch and a lower branch to respectively receive the optical signal, the optical signal is converted into an electric signal by adopting a first photoelectric detector PD and a second photoelectric detector PD respectively, and then the visible light analog signal is converted into a digital signal by a first A/D conversion and serial-parallel conversion unit and a second A/D conversion and serial-parallel conversion unit;
2) the first STO estimation and CP removal unit and the second STO estimation and CP removal unit respectively perform STO estimation on Symbol timing Offset (STO for short) existing in the ADO-OFDM electric signal to obtain a synchronous signal and remove a Cyclic Prefix (CP);
3) the first bidirectional maximum likelihood detection unit and the second bidirectional maximum likelihood detection unit respectively carry out Maximum Likelihood (ML) detection, detect and separate complex signals in a multiple-input multiple-output (MIMO) system;
4) the signal output by the first bidirectional maximum likelihood detection unit is divided into two paths, the main path signal is demodulated and reconstructed and modulated by ACO-OFDM, the bypass is the odd subcarrier of the signal separated after the maximum likelihood detection in the step 3), the odd subcarrier is subtracted from the signal demodulated and reconstructed by the main path, and then N-point FFT operation and judgment are carried out to obtain a DCO-OFDM modulated signal X without carrier noise interferenceDCO；
5) Modulating the DCO-OFDM modulation signal X in the step 4)DCOPerforming DCO-OFDM reconstruction modulation, subtracting the signal output by the second bidirectional maximum likelihood detection unit in the second branch in the step 3), performing N-point FFT, taking odd carrier, and finally expanding by two times to obtain the final signal XACO。
The symbol timing offset STO described in step 2) is estimated as:
calculating timing deviation (STO) value by using similarity of signal Cyclic Prefix (CP) and tail corresponding sequence of symbol, calculating square difference of data with first CP length, adding a subsequent square difference and subtracting the square difference of the front CP of the sequence each time to obtain the square difference of the corresponding part of the second CP, thus obtaining excellent performance by using self-character of the symbol1And W2Two sliding windows with the same length as CP and a distance of one symbol length are used to measure the similarity of data in the two windows in a sliding mode, and when the content in the sliding window one is CP, the sliding window one W is used1And a sliding window II W2The received signal in time domain can be represented as y n when the timing error STO is delta]＝x[x+δ]The corresponding frequency domain signal is:
Figure BDA0002308809780000041
by calculating the estimated STO value, the signal can be cut and modified to obtain a signal suitable for the next operation,
the STO value is calculated as in equation (4):
Figure BDA0002308809780000042
wherein: n is a radical ofgDenotes the length of the CP, ylRepresenting the l-th received signal in the time domain, after finding the STO, subtracting the first and the last incomplete OFDM symbols, and estimating the rest part, wherein after the STO is estimated, the cyclic prefix CP of the signal is removed;
the bidirectional maximum likelihood detection ML in the step 3) is as follows:
firstly, a candidate transmitting vector set capable of being detected in two directions is established, the search range of the ML vector is reduced through the vector set, and therefore the maximum likelihood vector is obtained in the limited search, and the steps are as follows:
1) to findCandidate send vector set S capable of bidirectional detectionMLIn a 2 × 2 MIMO system, since
y＝Hx+n＝h1x1+h2x2+n (5)，
Wherein h is1,h2Two column vectors of H matrix, when x1When known, the following can be obtained:
Figure BDA0002308809780000051
the set of candidate transmit vectors is therefore formed as:
Figure BDA0002308809780000052
all the same reason is x2When known, can obtain
Figure BDA0002308809780000053
So that the set of candidate transmit vectors is formed as
Figure BDA0002308809780000054
That is, all constellation points in the constellation diagram are x respectively1From x1The corresponding most probable signal x can be determined2If from the x2Can reversely push out x1', and x1′＝x1Then, the vector x can be determined as [ x ]1,x2]TIs two-way detectable and is otherwise discarded, and therefore a set of candidate transmit vectors can be constructed:
Figure BDA0002308809780000055
wherein x1、x2The corresponding most likely transmitted signals are found for the constellation points in the constellation diagram and,
Figure BDA0002308809780000056
omega' is x2The set of components is composed of a plurality of groups,
Figure BDA0002308809780000057
2) in a candidate send vector set SMLIn which a maximum likelihood search is performed according to equation (10):
Figure BDA0002308809780000058
Figure BDA0002308809780000059
in the set SMLThe maximum likelihood vector can be obtained by limited searching, and the preparation work of signal demodulation is completed.
The reconstruction process of the ACO-OFDM in the step 4) comprises the following steps:
the upper branch takes the odd carrier signals as:
Figure BDA00023088097800000510
X′ACO(odd) + NACO(odd), the signal after the lower branch performs the ACO-OFDM demodulation on the signal is:
Figure BDA00023088097800000511
X′ACO(odd) + NACO(even) + XDCO(even), after the ACO-OFDM signal is reconstructed, the odd carrier signal taken by the upper branch is subtracted, and then the result is expressed by formula (11):
Figure BDA0002308809780000061
then N-point FFT operation and judgment are carried out to obtain X for eliminating carrier noiseDCOAnd signals, wherein the signals X and N are frequency domain signals, and the signal X' is a time domain signal.
Reconstructing DCO-OFDM in the step 5) comprises the following steps:
x to be outputDCOCarrying out N-point IFFT operation again, adding direct current offset and cyclic prefix to obtain a reconstructed time domain signal X'DCOAnd (even), subtracting the received signal of the lower branch circuit to obtain a signal as shown in formula (12):
Figure BDA0002308809780000062
then N-point FFT is carried out, the odd number part is taken to be enlarged by 2 times, and the final signal X is obtainedACO。
According to the technical scheme, the received ACO-OFDM and DCO-OFDM signals are demodulated, reconstructed, modulated and subtracted, recovery of odd and even subcarrier signals is achieved, interference of carrier noise on even carrier signals is reduced to the minimum, and receiving performance of the ADO-OFDM system is effectively improved.
Such a communication system is low cost. The method can reduce the system error rate, and improve the system signal-to-noise ratio and intersymbol interference (ISI) capability.
Drawings
Fig. 1 is a schematic diagram of a conventional indoor visible light communication system model;
FIG. 2 is a schematic diagram of the IM/DD technology of the conventional visible light communication;
FIG. 3 is a diagram of a conventional NxN visible MIMO system;
FIG. 4 is a schematic diagram illustrating the structure and principle of a transmitting end in the ADO-OFDM visible light communication system in the embodiment;
FIG. 5 is a diagram illustrating a comparison of signal states before and after amplitude limiting at a zero value at a transmitting end in an ADO-OFDM visible light communication system in an embodiment;
fig. 6 is a schematic diagram illustrating comparison of states of signals before/after DC bias is added to a transmitting end in an ADO-OFDM visible light communication system in the embodiment; (ii) a
Fig. 7 is a schematic diagram illustrating a structure and a principle of a receiving end in an ADO-OFDM visible light communication system according to an embodiment;
FIG. 8 is a flowchart illustrating an STO estimation method for MIMO-ADO-OFDM receiving end dual sliding windows in an embodiment.
Detailed Description
The invention is described in further detail below with reference to the following figures and specific examples, but the invention is not limited thereto.
Example (b):
referring to fig. 1, 2 and 3, the visible light communication technology is a new wireless communication technology developed after the application of white light LEDs, and is currently applied in indoor scenes, and the system integrates illumination and communication, and has the advantages of large communication capacity, low cost, rapid and simple deployment and the like, but the traditional nxn visible light MIMO communication system is simple in receiving algorithm, poor in interference resistance, low in signal-to-noise ratio and low in bit error rate.
A visible light communication system of MIMO-ADO-OFDM comprises a transmitting end and a receiving end, wherein the transmitting end is provided with K LED units with the same structure, the receiving end is provided with K receiving units with the same structure, K is an integral multiple of 2, the transmitting end and the receiving end of an MIMO array are both in a transmitting and receiving state with a theoretically optimal angle, as shown in figure 4, one LED unit of the transmitting end comprises sequentially connected LED units
A serial-parallel conversion unit: the serial-parallel conversion unit converts the input serial structure data into parallel structure data;
a constellation mapping unit: the constellation mapping unit carries out QAM mapping on the signal, conjugate data are added to the complex signal subjected to QAM constellation mapping, so that the signal forms a Hermitian conjugate symmetric structure, and the output of IFFT is ensured to be a real number;
an ACO-OFDM signal generation unit: the ACO-OFDM signal generating unit divides an information vector X into odd subcarriers and even subcarriers, wherein XMagic cardRepresenting QAM constellation points on odd subcarriers, with even subcarrier modulation zeroed, XMagic card＝[0,X1,0,X3,...,0,XN-1]Wherein X isNRepresenting the frequency domain data symbol modulated onto the nth subcarrier after mapping, performing zero value amplitude limiting after IFFT operation, and changing the obtained bipolar real-valued OFDM signal into a unipolar non-negative signal by amplitude limiting at the zero value, as shown in fig. 5, which is a schematic diagram comparing signal states before/after amplitude limiting at the zero value of the transmitting end in an ADO-OFDM visible light communication system, that is:
Figure BDA0002308809780000071
and setting a threshold value A for the obtained non-negative real number signal x', wherein the judgment criterion is as follows:
Figure BDA0002308809780000072
performing frequency domain filtering after threshold judgment to obtain a baseband signal
Figure BDA0002308809780000073
Wherein n isACOIs the clipping noise of the ACO-OFDM signal, the signal obtained after FFT operation exists in the even carrier wave, xMagic cardIs XMagic cardObtaining a time domain signal after IFFT operation;
DCO-OFDM signal generation unit: x in DCO-OFDM signal generating unitDollRepresenting QAM constellation points on even carriers, zero setting of odd subcarrier modulation data, XDoll＝[X0,0,X2,...,XN-2,0]The signal is converted into a bipolar real-valued OFDM signal through N-point inverse fast fourier transform, namely IFFT, and DC is added to make the signal of the negative part greater than zero, so that the obtained bipolar real-valued OFDM signal becomes a unipolar non-negative signal, that is:
x′＝x+a,a＞|x| (3)，
judging the obtained non-negative real number signal X', performing frequency domain filtering after threshold judgment to obtain a baseband signal XDCOFig. 6 is a schematic diagram showing a comparison between states of signals before/after adding DC direct current bias to a transmitting end in an ADO-OFDM visible light communication system;
a cyclic prefix adding unit: a cyclic prefix adding unit adds XACO、XDCOAdding the signals into a cyclic prefix CP after adding;
a parallel-serial conversion unit: the parallel-serial conversion unit converts the parallel data into serial data to form a new ACO-OFDM modulation signal X ═ XACO+XDCOFinally, an optical signal is emitted through the LED；
The transmitting terminal modulates a randomly generated information sequence by M-order QAM to generate a complex signal, performs serial-parallel conversion and QAM mapping on input serial data, adds conjugate data to the complex signal subjected to QAM constellation mapping to form a Hermitian symmetric structure, divides the signal into odd subcarriers and even subcarriers, selects the odd subcarriers to transmit modulation data by the ACO-OFDM signal, sets the even numbers to zero to obtain XMagic card＝[0,X1,0,X3,...,0,XN-1]The DCO-OFDM signal selects even number sub-carrier to transmit modulation data, and the odd number is set to zero to obtain XDoll＝[X0,0,X2,...,XN-2,0]The odd carrier signal is taken and converted into a bipolar real-valued OFDM signal through N-point Inverse Fast Fourier Transform (IFFT).
As shown in fig. 7, each receiving unit of the receiving end includes 2 branches, and the first branch is provided with sequentially connected branches
First photodetector PD: the optical signal is used for receiving the optical signal sent by the light source;
a first A/D conversion and serial-parallel conversion unit: the first A/D conversion and serial-parallel conversion unit carries out analog-to-digital conversion and serial-parallel conversion on an optical signal received by the photoelectric detector PD to enable the signal to be a parallel digital signal;
first STO estimation and de-CP unit: the first STO estimation and CP removal unit is used for measuring and calculating a timing deviation (STO value) by adopting the similarity between a Cyclic Prefix (CP) of a signal and a corresponding sequence at the tail of a symbol;
a first bidirectional maximum likelihood detection unit: the first bidirectional maximum likelihood detection unit establishes a candidate emission vector set capable of bidirectional detection to reduce the search range of a maximum likelihood vector (ML vector), so that the maximum likelihood vector can be obtained in limited searches during signal detection;
a DCO-OFDM signal demodulation and reconstruction modulation unit: the DCO-OFDM signal demodulation and reconstruction modulation unit has the function of obtaining the DCO-OFDM signal X on the even carrierDCO；
The second branch being provided with connections in series
The second photodetector PD: the optical signal is used for receiving the optical signal sent by the light source;
a second A/D conversion and serial-parallel conversion unit: the second A/D conversion and serial-parallel conversion unit performs analog-to-digital conversion and serial-parallel conversion on the optical signal received by the photoelectric detector PD to enable the signal to be a parallel digital signal;
second STO estimation and de-CP unit: the second STO estimation and CP removal unit is used for measuring and calculating a timing deviation (STO value) by adopting the similarity between a Cyclic Prefix (CP) of a signal and a corresponding sequence at the tail of a symbol;
a second bidirectional maximum likelihood detection unit: the second bidirectional maximum likelihood detection unit establishes a candidate emission vector set capable of bidirectional detection to reduce the search range of a maximum likelihood vector (ML vector), so that the maximum likelihood vector can be obtained in limited searches during signal detection;
an ACO-OFDM signal demodulation and reconstruction modulation unit: the DCO-OFDM signal demodulation and reconstruction modulation unit is connected with the ACO-OFDM signal demodulation and reconstruction modulation unit to output to obtain a final signal XACO。
The receiving method of the visible light communication system using the MIMO-ADO-OFDM comprises the following steps:
1) the receiving unit divides the optical signal into an upper branch and a lower branch which respectively receive the optical signal, the first photoelectric detector PD and the second photoelectric detector PD are respectively adopted to convert the optical signal into an electric signal, and then the first A/D conversion and serial-parallel conversion unit and the second A/D conversion and serial-parallel conversion unit convert the visible light analog signal into a digital signal;
2) the first STO estimation and CP removal unit and the second STO estimation and CP removal unit respectively perform STO estimation on symbol timing deviation STO existing in the ADO-OFDM electric signal to obtain a synchronous signal and remove a cyclic prefix CP;
3) the first bidirectional maximum likelihood detection unit and the second bidirectional maximum likelihood detection unit respectively carry out maximum likelihood ML detection, detect and separate complex signals in a multi-input multi-output (MIMO) system;
4) the signal output by the first bidirectional maximum likelihood detection unit is divided into two paths, the main path signal is demodulated and reconstructed and modulated by ACO-OFDM, and the odd number subcarrier of the signal separated after the maximum likelihood detection in the step 3) are taken as the bypassSubtracting the demodulated and reconstructed signal of the sub-carrier and the main path, and then carrying out N-point FFT operation and judgment to obtain a DCO-OFDM modulation signal X without carrier noise interferenceDCO；
5) Modulating the DCO-OFDM modulation signal X in the step 4)DCOPerforming DCO-OFDM reconstruction modulation, subtracting the signal output by the second bidirectional maximum likelihood detection unit in the second branch in the step 3), performing N-point FFT, taking an odd carrier, and finally expanding by two times to obtain a final signal XACO。
As shown in fig. 8, the symbol timing offset STO described in step 2) is estimated as:
calculating timing deviation (STO) value by using similarity of signal Cyclic Prefix (CP) and tail corresponding sequence of symbol, calculating square difference of data with first CP length, adding a subsequent square difference and subtracting the square difference of the front CP of the sequence each time to obtain the square difference of the corresponding part of the second CP, thus obtaining excellent performance by using self-character of the symbol1And W2Two sliding windows with the same length as CP and a distance of one symbol length are used to measure the similarity of data in the two windows in a sliding mode, and when the content in the sliding window one is CP, the sliding window one W is used1And a sliding window II W2The received signal in time domain can be represented as y n when the timing error STO is delta]＝x[x+δ]The corresponding frequency domain signal is:
Figure BDA0002308809780000101
by calculating the estimated STO value, the signal can be cut and modified to obtain a signal suitable for the next operation,
the STO value is calculated as in equation (4):
Figure BDA0002308809780000102
wherein：NgDenotes the length of the CP, ylRepresenting the l-th received signal in the time domain, after finding the STO, subtracting the first and the last incomplete OFDM symbols, and estimating the rest part, wherein after the STO is estimated, the cyclic prefix CP of the signal is removed;
the bidirectional maximum likelihood detection ML in the step 3) is as follows:
firstly, a candidate transmitting vector set capable of being detected in two directions is established, the search range of the ML vector is reduced through the vector set, and therefore the maximum likelihood vector is obtained in the limited search, and the steps are as follows:
1) solving a set S of candidate transmission vectors that can be detected bidirectionallyMLIn a 2 × 2 MIMO system, since
y＝Hx+n＝h1x1+h2x2+n (5)，
Wherein h is1,h2Two column vectors of H matrix, when x1When known, the following can be obtained:
Figure BDA0002308809780000103
the set of candidate transmit vectors is therefore formed as:
Figure BDA0002308809780000104
all the same reason is x2When known, can obtain
Figure BDA0002308809780000105
So that the set of candidate transmit vectors is formed as
Figure BDA0002308809780000106
That is, all constellation points in the constellation diagram are x respectively1From x1The corresponding most probable signal x can be determined2If, ifFrom the x2Can reversely push out x1', and x1′＝x1Then, the vector x can be determined as [ x ]1,x2]TIs two-way detectable and is otherwise discarded, and therefore a set of candidate transmit vectors can be constructed:
Figure BDA0002308809780000111
wherein x1、x2The corresponding most likely transmitted signals are found for the constellation points in the constellation diagram and,
Figure BDA0002308809780000112
omega' is x2The set of components is composed of a plurality of groups,
Figure BDA0002308809780000113
2) in a candidate send vector set SMLIn which a maximum likelihood search is performed according to equation (10):
Figure BDA0002308809780000114
Figure BDA0002308809780000115
in the set SMLThe maximum likelihood vector can be obtained by limited searching, and the preparation work of signal demodulation is completed.
The reconstruction process of the ACO-OFDM in the step 4) comprises the following steps:
the upper branch takes the odd carrier signals as:
Figure BDA0002308809780000116
X′ACO(odd) + NACO(odd), the signal after the lower branch performs the ACO-OFDM demodulation on the signal is:
Figure BDA0002308809780000117
X′ACO(odd) + NACO(even) + XDCO(even), ACO-OFDM signal reconstructionThen, subtracting the odd carrier signal taken by the upper branch circuit to obtain the following formula (11):
Figure BDA0002308809780000118
then N-point FFT operation and judgment are carried out to obtain X for eliminating carrier noiseDCOAnd signals, wherein the signals X and N are frequency domain signals, and the signal X' is a time domain signal.
Reconstructing DCO-OFDM in the step 6) comprises the following steps:
x to be outputDCOCarrying out N-point IFFT operation again, adding direct current offset and cyclic prefix to obtain a reconstructed time domain signal X'DCOAnd (even), subtracting the received signal of the lower branch circuit to obtain a signal as shown in formula (12):
Figure BDA0002308809780000119
then N-point FFT is carried out, the odd number part is taken to be enlarged by 2 times, and the final signal X is obtainedACO。

Claims (6)

1. A visible light communication system of MIMO-ADO-OFDM comprises a transmitting end and a receiving end, and is characterized in that the transmitting end is provided with K LED units with the same structure, the receiving end is provided with K receiving units with the same structure, wherein K is an integral multiple of 2, the transmitting end and the receiving end of an MIMO array are both in a transmitting and receiving state with a theoretically optimal angle, and one LED unit of the transmitting end comprises sequentially connected LED units
A serial-parallel conversion unit: the serial-parallel conversion unit converts the input serial structure data into parallel structure data;
a constellation mapping unit: the constellation mapping unit carries out QAM mapping on the signal, conjugate data are added to the complex signal subjected to QAM constellation mapping, so that the signal forms a Hermitian conjugate symmetric structure, and the output of IFFT is ensured to be a real number;
an ACO-OFDM signal generation unit: the ACO-OFDM signal generating unit divides the information vector X into odd subcarriers and even subcarriersCarrier wave of wherein XMagic cardRepresenting QAM constellation points on odd subcarriers, with even subcarrier modulation zeroed, XMagic card＝[0,X1,0,X3,...,0,XN-1]Wherein X isNRepresenting the frequency domain data symbol modulated to the Nth subcarrier after mapping, performing zero value amplitude limiting after IFFT operation to generate an ACO-OFDM signal
Figure FDA0002308809770000011
Wherein n isACOIs the clipping noise of the ACO-OFDM signal, the signal obtained after FFT operation exists in the even carrier wave, xMagic cardIs XMagic cardObtaining a time domain signal after IFFT operation;
DCO-OFDM signal generation unit: x in DCO-OFDM signal generating unitDollRepresenting QAM constellation points on even carriers, zero setting of odd subcarrier modulation data, XDoll＝[X0,0,X2,...,XN-2,0]，XDollThrough IFFT operation and DC offset addition, DCO-OFDM signal X is generatedDCO；
A cyclic prefix adding unit: a cyclic prefix adding unit adds XACO、XDCOAdding the signals into a cyclic prefix CP after adding;
a parallel-serial conversion unit: the parallel-serial conversion unit converts the parallel data into serial data to form a new ACO-OFDM modulation signal X ═ XACO+XDCOFinally, an optical signal is sent out through the LED;
each receiving unit of the receiving end comprises 2 branches, and the first branch is provided with a plurality of sequentially connected branches
First photodetector PD: the optical signal is used for receiving the optical signal sent by the light source;
a first A/D conversion and serial-parallel conversion unit: the first A/D conversion and serial-parallel conversion unit carries out analog-to-digital conversion and serial-parallel conversion on an optical signal received by the photoelectric detector PD to enable the signal to be a parallel digital signal;
first STO estimation and de-CP unit: the first STO estimation and CP removal unit is used for measuring and calculating a timing deviation (STO value) by adopting the similarity between a Cyclic Prefix (CP) of a signal and a corresponding sequence at the tail of a symbol;
a first bidirectional maximum likelihood detection unit: the first bidirectional maximum likelihood detection unit establishes a candidate emission vector set capable of bidirectional detection to reduce the search range of a maximum likelihood vector (ML vector), so that the maximum likelihood vector can be obtained in limited searches during signal detection;
a DCO-OFDM signal demodulation and reconstruction modulation unit: the DCO-OFDM signal demodulation and reconstruction modulation unit has the function of obtaining the DCO-OFDM signal X on the even carrierDCO；
The second branch being provided with connections in series
The second photodetector PD: the optical signal is used for receiving the optical signal sent by the light source;
a second A/D conversion and serial-parallel conversion unit: the second A/D conversion and serial-parallel conversion unit performs analog-to-digital conversion and serial-parallel conversion on the optical signal received by the photoelectric detector PD to enable the signal to be a parallel digital signal;
second STO estimation and de-CP unit: the second STO estimation and CP removal unit is used for measuring and calculating a timing deviation (STO value) by adopting the similarity between a Cyclic Prefix (CP) of a signal and a corresponding sequence at the tail of a symbol;
a second bidirectional maximum likelihood detection unit: the second bidirectional maximum likelihood detection unit establishes a candidate emission vector set capable of bidirectional detection to reduce the search range of a maximum likelihood vector (ML vector), so that the maximum likelihood vector can be obtained in limited searches during signal detection;
an ACO-OFDM signal demodulation and reconstruction modulation unit: the DCO-OFDM signal demodulation and reconstruction modulation unit is connected with the ACO-OFDM signal demodulation and reconstruction modulation unit to output to obtain a final signal XACO。
2. A receiving method of a MIMO-ADO-OFDM visible light communication system, comprising the MIMO-ADO-OFDM visible light communication system of claim 1, the method comprising the steps of:
1) the receiving end divides the optical signal into an upper branch and a lower branch to respectively receive the optical signal, the optical signal is converted into an electric signal by adopting a first photoelectric detector PD and a second photoelectric detector PD respectively, and then the visible light analog signal is converted into a digital signal by a first A/D conversion and serial-parallel conversion unit and a second A/D conversion and serial-parallel conversion unit;
2) the first STO estimation and CP removal unit and the second STO estimation and CP removal unit respectively perform STO estimation on symbol timing deviation STO existing in the ADO-OFDM electric signal to obtain a synchronous signal and remove a cyclic prefix CP;
3) the first bidirectional maximum likelihood detection unit and the second bidirectional maximum likelihood detection unit respectively carry out maximum likelihood ML detection, detect and separate complex signals in a multi-input multi-output (MIMO) system;
4) the signal output by the first bidirectional maximum likelihood detection unit is divided into two paths, the main path signal is demodulated and reconstructed and modulated by ACO-OFDM, the bypass is the odd subcarrier of the signal separated after the maximum likelihood detection in the step 3), the odd subcarrier is subtracted from the signal demodulated and reconstructed by the main path, and then N-point FFT operation and judgment are carried out to obtain a DCO-OFDM modulated signal X without carrier noise interferenceDCO；
5) Modulating the DCO-OFDM modulation signal X in the step 4)DCOPerforming DCO-OFDM reconstruction modulation, subtracting the signal output by the second bidirectional maximum likelihood detection unit in the second branch in the step 3), performing N-point FFT, taking odd carrier, and finally expanding by two times to obtain the final signal XACO。
3. The receiving method of the visible light communication system of MIMO-ADO-OFDM as claimed in claim 2, wherein the symbol timing offset STO in step 2) is estimated as:
calculating timing deviation (STO) value by using similarity of signal Cyclic Prefix (CP) and tail corresponding sequence of symbol, calculating square difference of data with first CP length, adding a subsequent square difference and subtracting the square difference of the front CP of the sequence each time to obtain the square difference of the corresponding part of the second CP, copying the last part of the data content in an OFDM symbol, setting two sliding windows (W)1And W2The length of the sliding window is the same as that of the CP and the distance is one symbol length, two sliding windows are used for sliding measurement in the two windowsSimilarity of data, then when the content in sliding window one is CP, sliding window one W1And a sliding window II W2The received signal in time domain can be represented as y n when the timing error STO is delta]＝x[x+δ]The corresponding frequency domain signal is:
Figure FDA0002308809770000031
the STO value is calculated as in equation (4):
Figure FDA0002308809770000032
wherein: n is a radical ofgDenotes the length of the CP, ylAnd representing the l-th received signal in the time domain, after finding the STO, subtracting the first and the last incomplete OFDM symbols, and estimating the rest part, wherein after the STO is estimated, the cyclic prefix CP of the signal is removed.
4. The receiving method of the visible light communication system of MIMO-ADO-OFDM as claimed in claim 2, wherein the bidirectional maximum likelihood detection ML in step 3) is:
firstly, a candidate transmitting vector set capable of being detected in two directions is established, the search range of the ML vector is reduced through the vector set, and therefore the maximum likelihood vector is obtained in the limited search, and the steps are as follows:
1) solving a set S of candidate transmission vectors that can be detected bidirectionallyMLIn a 2 × 2 MIMO system, since
y＝Hx+n＝h1x1+h2x2+n (5)，
Wherein h is1,h2Two column vectors of H matrix, when x1When known, the following can be obtained:
Figure FDA0002308809770000033
the set of candidate transmit vectors is therefore formed as:
Figure FDA0002308809770000034
all the same reason is x2When known, can obtain
Figure FDA0002308809770000041
So that the set of candidate transmit vectors is formed as
Figure FDA0002308809770000042
That is, all constellation points in the constellation diagram are x respectively1From x1The corresponding most probable signal x can be determined2If from the x2Can reversely push out x1', and x1′＝x1Then, the vector x can be determined as [ x ]1,x2]TIs two-way detectable and is otherwise discarded, and therefore a set of candidate transmit vectors can be constructed:
Figure FDA0002308809770000043
wherein x1、x2The corresponding most likely transmitted signals are found for the constellation points in the constellation diagram and,
Figure FDA0002308809770000044
omega' is x2The set of components is composed of a plurality of groups,
Figure FDA0002308809770000045
2) in a candidate send vector set SMLIn which a maximum likelihood search is performed according to equation (10):
Figure FDA0002308809770000046
Figure FDA0002308809770000047
in the set SMLThe maximum likelihood vector can be obtained by limited searching.
5. The receiving method of the visible light communication system of MIMO-ADO-OFDM as claimed in claim 2, wherein the process of reconstructing ACO-OFDM in step 4) comprises:
the upper branch takes the odd carrier signals as:
Figure FDA0002308809770000048
the signal after the lower branch performs the ACO-OFDM demodulation on the signal is:
Figure FDA0002308809770000049
after the ACO-OFDM signal is reconstructed, the odd carrier signal taken by the upper branch is subtracted, and the result is expressed by formula (11):
Figure FDA00023088097700000410
then N-point FFT operation and judgment are carried out to obtain X for eliminating carrier noiseDCOAnd signals, wherein the signals X and N are frequency domain signals, and the signal X' is a time domain signal.
6. The receiving method of the visible light communication system of MIMO-ADO-OFDM as claimed in claim 2, wherein the reconfiguration of DCO-OFDM in step 6) is:
x to be outputDCOCarrying out N-point IFFT operation again, adding direct current offset and cyclic prefix to obtain a reconstructed time domain signal X'DCOAnd (even), subtracting the received signal of the lower branch circuit to obtain a signal as shown in formula (12):
Figure FDA0002308809770000051
then N-point FFT is carried out, the odd number part is taken to be enlarged by 2 times, and the final signal X is obtainedACO。
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