CN108631832B - Index modulation combined multi-user MIMO-OOFDM visible light communication method - Google Patents

Index modulation combined multi-user MIMO-OOFDM visible light communication method Download PDF

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CN108631832B
CN108631832B CN201810455353.9A CN201810455353A CN108631832B CN 108631832 B CN108631832 B CN 108631832B CN 201810455353 A CN201810455353 A CN 201810455353A CN 108631832 B CN108631832 B CN 108631832B
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CN108631832A (en
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江明
蔡鲲翼
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National Sun Yat Sen University
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H04B7/0452—Multi-user MIMO systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
    • H04B10/114—Indoor or close-range type systems
    • H04B10/116—Visible light communication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H04B7/0456—Selection of precoding matrices or codebooks, e.g. using matrices antenna weighting
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0056—Systems characterized by the type of code used
    • H04L1/0071—Use of interleaving
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2602—Signal structure
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2626—Arrangements specific to the transmitter only
    • H04L27/2627—Modulators
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2697—Multicarrier modulation systems in combination with other modulation techniques

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Abstract

The invention considers the high-speed transmission VLC scene with stronger indoor reflection, excavates the potential technical combination points and innovation points, and designs the multi-user MIMO-OOFDM visible light communication system based on IM by introducing the IM concept and combining the OOFDM technology and the indoor multi-user scene. By means of the IM technology, each user can simultaneously use the symbol information and the carrier index information on the carrier to perform data transmission, so that a multi-user system obtains a more stable transmission link in a typical indoor environment, and the overall Bit Error Rate (BER) performance of the system is effectively improved.

Description

Index modulation combined multi-user MIMO-OOFDM visible light communication method
Technical Field
The invention provides a novel indoor Multi-User Precoding (MUP) Multi-Input Multi-Output (MIMO) Optical Orthogonal Frequency Division Multiplexing (OOFDM) VLC Communication method based on Index Modulation (IM) in the field of Visible Light Communication (VLC).
Background
With the development of the era and the wide application of Light-Emitting diodes (LEDs) in daily life, VLC technology based on LED lamps is gradually favored by researchers in various countries, and a great deal of research is being conducted thereon. VLC technology is mainly based on intensity modulation and direct detection, and it uses the rapidly changing light intensity, negligible to the human eye, for information transmission. The MIMO technology widely applied to the traditional wireless communication is also widely applied to the VLC technology by virtue of the advantages of the MIMO technology in the aspects of system capacity expansion and the like, and can effectively and simultaneously utilize a plurality of LED lamps to realize indoor data transmission and improve the transmission rate of the VLC system. Therefore, the combination of MIMO technology and VLC technology is also becoming a research hotspot of current VLC communication systems.
Multi-User (MU) MIMO technology has recently received much attention from researchers as an extension and application of MIMO technology. Unlike the case where all Photodetectors (PDs) belong to the same user terminal in the conventional MIMO-VLC system, the multi-user MIMO-VLC system supports multiple user terminals, each of which includes one or more PDs. The Multi-User MIMO system eliminates inter-User Interference (MUI) through a certain Interference elimination algorithm, so that a plurality of User terminals can simultaneously communicate with each other without mutual Interference by using the same LED lamp array.
Orthogonal Frequency Division Multiplexing (OFDM) is a Multi-subcarrier Multiplexing technology developed by Multi-Carrier Modulation (MCM) and can effectively suppress inter-symbol Interference (ISI) caused by a fading channel, so the OOFDM technology developed by the OFDM technology is also increasingly used in high-speed VLC systems to overcome negative effects caused by indoor light scattering during high-speed transmission of signals, thereby supporting indoor VLC signal transmission at higher speed. Currently, the research on a high-speed multi-user MIMO visible light communication system in an environment with strong indoor reflection is less by combining with OOFDM. The introduction of new wireless technologies and the development of potential application combination points are a common idea for designing novel indoor high-speed multi-user VLC systems. Orthogonal Frequency Division Multiplexing with Index Modulation (OFDM-IM) technology is a new OFDM concept that has emerged in recent years, and it utilizes a new dimension of Index information to transmit information, thereby improving the transmission performance of the system. Specifically, the IM concept further simultaneously utilizes the index information dimension of the subcarrier activation combination to realize information transmission on the basis of transmitting information by utilizing subcarrier modulation symbols. Compared with the traditional OFDM technology, the OFDM-IM can obtain certain performance gain by utilizing the potential transmission reliability of index information in the IM concept in low-order modulation such as Binary Phase Shift Keying (BPSK) and Quadrature amplitude Keying (QPSK), and has potential combination points and innovation spaces with various technologies.
Disclosure of Invention
The invention considers the high-speed transmission VLC scene with stronger indoor reflection, excavates the potential technical combination points and innovation points, and designs the multi-user MIMO-OOFDM visible light communication method based on IM by introducing the IM concept and combining the OOFDM technology and the indoor multi-user scene. By means of the IM technology, each user can simultaneously use the symbol information and the carrier index information on the carrier to perform data transmission, so that a multi-user system obtains a more stable transmission link in a typical indoor environment, and the overall Bit Error Rate (BER) performance of the system is effectively improved.
In order to realize the purpose, the technical scheme is as follows:
the multi-user MIMO-OOFDM visible light communication method combined with index modulation comprises the following steps:
s1, for a multi-user MIMO-OOFDM system, B bits input by each data stream of each user are firstly divided into G groups
Figure GDA0002636352450000021
The G index modulated IM sub-blocks into which the available sub-carriers are divided correspond to NFThe number of points of the fast Fourier transform and the inverse fast Fourier transform; definition of NUThe number of subcarriers N contained in each IM sub-block is the total number of subcarriersSIs composed of
Figure GDA0002636352450000022
S2. N of each IM subblockSOnly L in the sub-carriersSThe subcarriers are selected by the index selector to be activated, and the remaining NS-LSThe sub-carriers are not activated and set to 0; so that the combination of subcarrier activations can represent in p bits
Figure GDA0002636352450000023
A bit wherein
Figure GDA0002636352450000024
Represents the total combination number of b elements in a set of a elements for carrying out combination number operation,
Figure GDA0002636352450000025
represents a round-down operation; thus sharing in common
Figure GDA0002636352450000026
Different subcarrier activation combinations are recorded, and the set of all C possible subcarrier activation combinations in each IM subblock is recorded as
ΩU＝{U1,U2,...,UC} (2)
Wherein
Figure GDA0002636352450000031
c＝1,2,...,C，
Figure GDA0002636352450000032
Denotes the l th in the c th activation combinationsThe activated subcarriers index, and for all C1, 2s＝1,2,...,LSAre all provided with
Figure GDA0002636352450000033
The remainder of p2＝Lf log2(M) bits will pass through M-order constellation diagram omegaMMapping as LfM-th constellation symbols, denoted as s ═ s (1), s (2)f)] (3)
Wherein for ls＝1,2,...,LfAre all provided with s (l)s)∈ΩMThey will be modulated in IM subblock LfOn the activated subcarriers;
s3, using rjData stream number, definition, representing the jth user
Figure GDA0002636352450000034
For user jth obtained by the above IM methodjG (1 XN) th on stripe data streamS) An IM subblock of dimensions, wherein G ═ 1, 2.., G; r isj＝1,2,...,Rj；j＝1,2,...,J；
Figure GDA0002636352450000035
Denotes the user jth rjNth in the g IM sub-block on the stripe data streamsSignal on sub-carriers within an IM sub-block, ns＝1,2,...,NS；
S4, in each OOFDM symbol time, the total transmission rate of the multi-user system is
Figure GDA0002636352450000036
Wherein N isCPLength of cyclic prefix of OFDM symbol;
s5, define
Figure GDA0002636352450000037
For the user jth rjAll G (1 XN) pieces of strip data streamS) Vitamin IM subblock
Figure GDA0002636352450000038
Of
Figure GDA0002636352450000039
Dimension IM Total data Block, rj＝1,2...,Rj(ii) a J ═ 1, 2.., J, expressed as follows
Figure GDA00026363524500000310
S6, interleaving the sub-carrier elements among different data streams of the same user, regarding all the data streams of the user as a virtual long data stream, and then interleaving to define XjAll R for user j derived from equation (5)jR corresponding to strip data streamjTotal block of IM data
Figure GDA00026363524500000311
Of
Figure GDA00026363524500000312
The dimension to be interleaved blocks are as follows:
Figure GDA00026363524500000313
Figure GDA0002636352450000041
wherein J is 1, 2.. times.j; through the interleaving operation, the method can obtain
Figure GDA0002636352450000042
Interleaved data of dimension
Figure GDA0002636352450000043
The following were used:
Figure GDA0002636352450000044
s7, interweaving the fibers in the formula (7)
Figure GDA0002636352450000045
Dimensional data vector
Figure GDA0002636352450000046
Repartition into RjAn
Figure GDA0002636352450000047
The dimension data block is used as a data block actually transmitted by each data stream of the user j; after repartitioning, the r-th user of the j-th userjOn a striped data stream
Figure GDA0002636352450000048
Dimension data block
Figure GDA0002636352450000049
Is composed of
Figure GDA00026363524500000410
Wherein r isj＝1,2,...,Rj；j＝1,2,...,J；
S8, based on the total subcarrier number being NFLet the subcarrier number nfIs taken from 0 to NF-1; defining the nth frequency domainfOn sub-carriers (R × N)T) The channel matrix of all user frequency domains is
H(nf)＝[H1(nf)T,H2(nf)T,...,HJ(nf)T]T (9)
Wherein Hj(nf) Denotes the nth userfOn sub-carrier (R)j×NT) A dimensional frequency domain channel matrix; in asymmetric clipped light orthogonal frequency division multiplexing ACO-OFDM,
Figure GDA00026363524500000411
the available sub-carriers correspond to sub-carrier numbers
Figure GDA00026363524500000412
Only the BD pre-coding operation of block diagonalization is needed to be carried out on the frequency domain channel matrixes corresponding to the subcarrier numbers;
s9. for
Figure GDA00026363524500000413
Using BD precoding method to H (n)f) Calculating to obtain a precoding matrix of each user on the frequency domain point; definition of nfOn sub-carriers except for the jth user ((R-R)j)×NT) The dimension user channel complement matrix is:
Figure GDA0002636352450000051
to pair
Figure GDA0002636352450000052
The SVD decomposition step of (A) is written as
Figure GDA0002636352450000053
Wherein ((R-R)j)×(R-Rj) Dimension matrix
Figure GDA0002636352450000054
Contains all left singular vectors, ((R-R)j)×NT) Dimension matrix
Figure GDA0002636352450000055
Representing a matrix of singular values; definition of
Figure GDA0002636352450000056
Figure GDA0002636352450000057
Dimension matrix
Figure GDA0002636352450000058
Comprises a front part
Figure GDA0002636352450000059
The number of right singular vectors,
Figure GDA00026363524500000510
dimension matrix
Figure GDA00026363524500000511
The remaining right singular vectors are included,
Figure GDA00026363524500000512
middle isotropic position
Figure GDA00026363524500000513
Within the null space of (a); it can be assumed that the channel is of full rank in general, having
Figure GDA00026363524500000514
Thereby obtaining the n-thfThe equivalent channel matrix for user j on a subcarrier is
Figure GDA00026363524500000515
For equivalent channel matrix
Figure GDA00026363524500000516
The steps for proceeding with the SVD decomposition are written as:
Figure GDA00026363524500000517
wherein Λj(nf) Is (R)j×Rj) Diagonal matrix of singular values of dimension, Uj(nf) Is for finally demodulating the signal (R)j×Rj) Dimensional unitary matrix, (R)j×Rj) Dimension matrix
Figure GDA00026363524500000518
Right singular vectors are included; finally, the nth user of the jth user is obtainedfOn sub-carriers (N)T×Rj) Dimensional precoding matrix
Figure GDA00026363524500000519
S10, defining F (n)f) For n after precodingfOn sub-carriers (N)TX 1) dimensional frequency domain data vector; for the
Figure GDA00026363524500000520
This is achieved by
Figure GDA00026363524500000521
One available subcarrier of ACO-OFDM, having
Figure GDA00026363524500000522
Wherein u isj(nf) For the jth user nfOn sub-carrier (R)jX 1) dimensional precoding data vector consisting of transmission data block in equation (8)
Figure GDA00026363524500000523
Inner corresponding element
Figure GDA00026363524500000524
I.e. to countAccording to block
Figure GDA00026363524500000525
Mapping the elements in the vector to corresponding positions according to the mapping rule of the available subcarriers of the ACO-OFDM, and then carrying out precoding;
due to the zero padding operation of ACO-OFDM, when nfWhen it is an even number, there is (N)TX 1) dimensional data vector F (0) ═ F (2) · F (N)F-2) ═ 0; symmetrically operated by Hermite conjugation, with F (n)f)＝F*(NF-nf)，
Figure GDA0002636352450000061
Finally constituting (N) of J usersT×NF) Dimensional ACO-OFDM frequency domain matrix FinputIs composed of
Figure GDA0002636352450000062
S11, obtaining a corresponding time domain real number signal after IFFT processing, and recording the time domain real number signal after IFFT on the ith LED as x0,i(t), the signal being a real signal; according to the principle of ACO-OFDM, clipping the negative real-valued signal to obtain the final transmission signal of
Figure GDA0002636352450000063
In a VLC system using intensity modulation and direct detection techniques, defining the LED electrical-to-optical conversion coefficient as mu, the mathematical expectation of the light signal emitted by the ith LED is the average emitted light power P of the lampopt,i＝E{μxi(t) }; the average emitted light power P of the ith LED can be known by combining the time domain signal characteristics of the ACO-OFDMopt,iProportional to the electrical power of the pre-coded frequency domain data of the ith LED; generally, after each frequency domain subcarrier is subjected to precoding matrix processing, the electric power of frequency domain data on each LED is different, so that the emitted light power of each LED is different; in practice, the LEDs are usually symmetrically arranged on the indoor ceiling, and in order to ensure uniform illumination in the room, the LEDs are required to be arranged on the indoor ceilingAverage luminous power P emitted by each LEDopt,i，i＝1,2,...,NTMay be the same, therefore, in a multi-user MIMO-OOFDM visible light communication system, an extra dc bias should be added to the LED with smaller average optical power to ensure uniform illumination, note NTThe maximum desired optical power in each LED is
Figure GDA0002636352450000064
The extra DC offset that should be added to the ith LED is
Figure GDA0002636352450000065
After the direct current bias adjustment, the aim of transmitting VLC information is achieved, and meanwhile the requirement of indoor uniform illumination can be met; after the uniform illumination adjustment, the emission signal on the ith LED
Figure GDA0002636352450000066
Is composed of
Figure GDA0002636352450000067
They satisfy that the average light power emitted by all the LEDs is uniform, namely
Figure GDA0002636352450000068
S12, at the user terminal side, a light detector PD receives light information from free space; after transmission through the VLC channel, the time-domain received signal at the r PD can be represented as
Figure GDA0002636352450000071
Wherein h isr,i(t) represents the VLC time domain channel impulse response between the ith LED and the ith PD,
Figure GDA0002636352450000072
representing the time-domain light signal emitted at the ith LED, nr(t) represents time-domain zero-mean real additive white Gaussian noise on the r-th PD, gamma represents the photoelectric conversion coefficient of the PD,
Figure GDA0002636352450000073
a convolution operation representing a time domain signal;
s13, after the received optical signal is subjected to light intensity detection and photoelectric conversion processing of PD, the optical signal is converted into an electric signal, and ACO-OFDM demodulation is carried out on the time domain received electric signal to obtain frequency domain data; according to the BD precoding reception principle
Figure GDA0002636352450000074
Nth of jth userfEquivalent frequency domain received signal Y on subcarrierj(nf) Is composed of
Figure GDA0002636352450000075
Wherein n isj(nf) Denotes the nth userfA corresponding frequency domain zero mean additive white Gaussian noise AWGN vector on the subcarrier; matrix U generated using equation (12)j(nf) Y obtained by the conjugate transpose pair formula (22)j(nf) Processing to obtain the nth userfProcessed on sub-carriers (R)jX 1) dimensional vector
Figure GDA0002636352450000076
As shown below
Figure GDA0002636352450000077
S14. for all J ═ 1,2j＝1,2,...,RjReception vector on available subcarriers of ACO-OFDM obtained by equation (23)
Figure GDA0002636352450000078
Figure GDA0002636352450000079
The user jth r is shown as followsjOn a striped data stream
Figure GDA00026363524500000710
Received data blocks of a dimension
Figure GDA00026363524500000711
Figure GDA00026363524500000712
Wherein r isj＝1,2,...,Rj，j＝1,2,...,J，
Figure GDA0002636352450000081
Figure GDA0002636352450000082
Representing diagonal matrix from singular values Λj(2lc-1) taking the (r) thj,rj) The operation of the individual elements is carried out,
Figure GDA0002636352450000083
represents the demodulated frequency domain AWGN signal; combined type (15) with
Figure GDA0002636352450000084
Thus, it is possible to provide
Figure GDA0002636352450000085
Is that
Figure GDA0002636352450000086
A corresponding received data vector; due to the subcarrier interleaving operation of equations (6) and (7), the symbols in each IM subblock are dispersed among the correlation coefficientsLow different sub-carriers, and cannot be transmitted at this time
Figure GDA0002636352450000087
Directly recovering the r-th of the user jjEach IM subblock to be demodulated on each data stream needs to extract symbols and equivalent channel singular values corresponding to the symbols scattered in the IM subblocks of each data stream of the user j and reconstruct the symbols into the IM subblocks to be demodulated;
specifically, first, all R of the users j in the formula (24)jObtained on a striped data stream
Figure GDA0002636352450000088
Dimension to be demodulated data block
Figure GDA0002636352450000089
Composition of
Figure GDA00026363524500000810
Data vector of dimension
Figure GDA00026363524500000811
j＝1,2,...,J
Figure GDA00026363524500000812
S15, reconstructing the r-th channel by the formula (25) at the receiving end of the user j according to the subcarrier interleaving rules defined in the formula (6) and the formula (7) by the formula (25)jG (1 XN) th on stripe data streamS) Dimension to be demodulated IM sub-block
Figure GDA00026363524500000813
Figure GDA00026363524500000814
For all ns＝1,2,...,NSIs provided with
Figure GDA00026363524500000815
Secondly, due to the IM subblocks to be demodulated
Figure GDA00026363524500000816
Wherein each element was distributed throughout multiple data streams of the same user for transmission, thereby completing the pairing
Figure GDA00026363524500000817
Before demodulation, it is also necessary to
Figure GDA00026363524500000818
Each element of
Figure GDA00026363524500000819
Extracting and reconstructing corresponding singular values of the experienced actual channel according to the same rule; user jth rjOn a stripe data stream
Figure GDA00026363524500000820
Corresponding singular values on the available subcarriers may be sorted into
Figure GDA00026363524500000821
Singular value vector of dimension
Figure GDA00026363524500000822
rj＝1,2,...,Rj，j＝1,2,...,J
Figure GDA00026363524500000823
Wherein
Figure GDA0002636352450000091
Representing a matrix of singular values, Λ, taken from equation (23)j(2lcR in 1)jCorresponding singular values of the strip data stream; all R of user jjCorresponding to a strip data stream
Figure GDA0002636352450000092
Composition of
Figure GDA0002636352450000093
Singular value vector λ of dimensionj，j＝1,2,...,J
Figure GDA0002636352450000094
Similarly, the r-th of the user j can be reconstructed by the formula (28)jG received (1 XN) on stripe data streamS) Dimension to be demodulated IM sub-block
Figure GDA0002636352450000095
Corresponding demodulated singular value vector
Figure GDA0002636352450000096
Figure GDA0002636352450000097
For all ns＝1,2,...,NSIs provided with
Figure GDA0002636352450000098
S16, through the above operation, the IM subblock to be demodulated, which is deinterleaved in each data stream belonging to each user, can be obtained at the user terminal
Figure GDA0002636352450000099
And corresponding user equivalent channel singular values
Figure GDA00026363524500000910
S17, according to the principle of IM concept, for each IM subblock to be demodulated
Figure GDA00026363524500000911
Demodulation is mainly based on two criteria, one is based on a Maximum Likelihood (ML) criterion, and the other is based on a log-likelihood ratio (LLR) criterion;
in the selection ofWhen demodulating based on the ML criterion, for each IM subblock, all possible transmission vector combinations need to be searched; in particular, the set of all possible combinations of transmit vectors is defined as ΩXIM subblocks estimated according to maximum likelihood definition
Figure GDA00026363524500000912
Can be produced by the following formula
Figure GDA00026363524500000913
Wherein
Figure GDA00026363524500000914
Representing by vectors
Figure GDA00026363524500000915
An operation of generating a corresponding diagonal matrix; substituting (30) all possible IM sub-block transmission vectors into a joint solution to obtain an estimated IM sub-block transmission vector
Figure GDA00026363524500000916
The index information and the symbol information of the p bits can be recovered at the same time; it is clear that the ML demodulation complexity varies with the order M of the modulation symbol constellation carried on the subcarrier and the number L of subcarriers activated per IM sub-blockSBut has an exponential rising trend;
demodulation based on the LLR criterion is a linear demodulation method, which avoids the demodulation complexity of exponential rise; has the same performance as ML; therefore, the demodulation of the system is a better choice by adopting the LLR criterion; in the method, each IM subblock to be demodulated is calculated first
Figure GDA00026363524500000917
N insLLR values for subcarriers
Figure GDA00026363524500000918
Wherein n iss＝1,2,...,NSAs shown below
Figure GDA0002636352450000101
Wherein s isχIs the x modulation symbol mapped on the M-th order constellation diagram,
Figure GDA0002636352450000102
is the corresponding frequency domain AWGN power used for the calculation of the LLR ratio; thus, according to
Figure GDA0002636352450000103
The subcarriers within all IM sub-blocks can be calculated, i.e., total NSLLR values for the subcarriers; next, in connection with the definition of the subcarrier activation combination in equation (2), for all C ═ 1,2
Figure GDA0002636352450000104
Corresponding subcarrier activation combining
Figure GDA0002636352450000105
Sum of LLR of
Figure GDA0002636352450000106
According to all calculated
Figure GDA0002636352450000107
LLR is summed, and the decision is as follows
Figure GDA0002636352450000108
I.e. the second one with the largest LLR and corresponding
Figure GDA0002636352450000109
A set
Figure GDA00026363524500001010
As IM subblocks
Figure GDA00026363524500001011
Activates the optimal solution of the combination to obtain the corresponding index information bit group p1(ii) a Then according to
Figure GDA00026363524500001012
For the lsAn activated sub-carrier, and the constellation symbol on the activated sub-carrier is solved
Figure GDA00026363524500001013
Finally obtain LfSymbol information bit group p on activated sub-carrier2。
Drawings
FIG. 1 is a general block diagram of an IM-based indoor multi-user MIMO-OOFDM visible light communication system
FIG. 2 is a schematic diagram of an exemplary operation of subcarrier interleaving between user data streams of an IM data block
FIG. 3 is a general flowchart of a multi-user MIMO-OOFDM visible light communication method based on IM
Figure 4 graph comparing BER performance for different systems for two position examples at 2M
Figure 5 graph comparing BER performance for different systems for two position examples at 4M
Figure 6 graph comparing BER performance for different systems for two position examples at 8M
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the patent;
the invention is further illustrated below with reference to the figures and examples.
Example 1
Fig. 1 shows a novel indoor multi-user MIMO-OOFDM visible light communication system model based on IM. Suppose that N is uniformly distributed on the ceiling of an indoor roomTAn LED, J user terminals in the room, the J user terminal equipped with RjThe number of data streams Q that the PD is working and needs to supportjCan be set as Qj＝Rj. The core of the multi-user precoding algorithm is to process the signal at the transmitting end to eliminate MUI, and the Block Diagonalization (BD) is a simple linear precoding method, which has low decoding complexity and can be conveniently applied to small-sized receiving devices to reduce energy consumption. The basic principle of the BD pre-coding method is to design a pre-coding matrix for eliminating the interference among users, and under the limitation of the BD pre-coding method, the number Q of data streams transmitted by the jth user when the signal is required to be transmittedj≤RjAnd is
Figure GDA0002636352450000111
For convenience, no assumption is made
Figure GDA0002636352450000112
Therefore it has the advantages of
Figure GDA0002636352450000113
The bit information of each data stream of each user is processed by a respective IM module to generate frequency domain IM modulation data; IM modulation data of the same user is subjected to subcarrier interleaving processing of the user, and finally, frequency domain data to be transmitted of each user is formed. In order to eliminate MUI and simultaneously meet the transmission requirement of a VLC channel, frequency domain data generate a non-negative real time domain signal after corresponding pre-coding operation and OOFDM processing, and finally the signal is loaded on NTAnd sent out on each LED.
At the receiver side, after the PD of each user terminal receives the optical signal, frequency domain data is restored through corresponding OOFDM demodulation, then IM modulation data on each data stream of each user is extracted according to a multi-user demodulation method and corresponding inverse IM operation, and finally information restoration of each user is completed by Maximum Likelihood (ML) or Log-Likelihood Ratio (Log-Likelihood Ratio, LLR) demodulation.
The system is focused on the construction and design of the novel structure, namely the whole process from signal transmission, signal transmission to signal receiving processing, and the following specific description and explanation are carried out according to the sequence. Meanwhile, for convenience of description, the present invention describes a system by taking asymmetric Clipped Optical Orthogonal Frequency Division Multiplexing (ACO-OFDM) as an example, however, the present invention indicates that the scheme is also applicable to other OOFDM systems, such as DC Biased Optical OFDM (DCO-OFDM), Unipolar OFDM (U-OFDM), and so on.
Suppose NFIs the number of Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) points, i.e. the total number of subcarriers of one OOFDM symbol. OOFDM requires zero padding and Hermite conjugate symmetric operation to generate non-negative real time domain signal satisfying VLC channel transmission condition, so N in ACO-OFDMFOf the sub-carriers, only
Figure GDA0002636352450000121
One available subcarrier. If other OOFDM modulation technology is adopted in the system, corresponding preprocessing can be carried out according to the characteristics of the OOFDM modulation technology so as to meet the requirement of the same non-negative real number signal.
For a conventional multi-user MIMO-OOFDM system, it is assumed that each user inputs B bits per data stream in each OOFDM symbol, and the B bits are all mapped to corresponding Quadrature Amplitude Modulation (QAM) constellation symbols and modulate all the data streams in ACO-OFDM
Figure GDA0002636352450000122
On one available subcarrier. However, in the IM concept, only a part of the B bits is represented by using the constellation symbols, and the other part of the B bits is represented by the specific activated subcarrier index information. Therefore, in the present system, the B bits per user per data stream input are first divided into G groups
Figure GDA0002636352450000123
The G IM sub-blocks into which the available sub-carriers are divided correspond. Wherein, each IM subblock comprises the number of subcarriers NSIs composed of
Figure GDA0002636352450000124
Thus, each IM subblock corresponds to
Figure GDA0002636352450000125
And (4) a bit.
Different from the traditional OOFDM mode in which all the available sub-carriers are activated, the N of each IM sub-block is determined according to the design principle of the IM conceptSOnly L in the sub-carriersSThe subcarriers are selected by the index selector to be activated, and the remaining NS-LSThe subcarriers are not activated and set to 0. So that the combination of subcarrier activations can represent in p bits
Figure GDA0002636352450000126
A bit wherein
Figure GDA0002636352450000127
Represents the total combination number of b elements in a set of a elements for carrying out combination number operation,
Figure GDA0002636352450000128
indicating a rounding down operation. In other words, it shares
Figure GDA0002636352450000129
Different subcarrier activation combinations are provided, and the specific subcarrier activation combination can be determined by a table look-up mode. The set of all C possible subcarrier activation combinations in each IM subblock is denoted as
ΩU＝{U1,U2,...,UC} (2)
Wherein
Figure GDA00026363524500001210
Figure GDA00026363524500001211
Denotes the l th in the c th activation combinationsThe activated subcarriers index, and for all C1, 2s＝1,2,...,LSAre all provided with
Figure GDA00026363524500001212
The remainder of p2＝LSlog2(M) bits will pass through M-order constellation diagram omegaMMapping as LSA constellation symbol of M order
s＝[s(1),s(2),...,s(LS)] (3)
Wherein for ls＝1,2,...,LSAre all provided with s (l)s)∈ΩMThey will be modulated in IM subblock LSOn the activated sub-carriers.
For convenience of description, r is used in the present inventionj(J ═ 1, 2.., J) denotes the data stream number of the jth user. Definition of
Figure GDA0002636352450000132
For user jth obtained by the above IM methodjG (1 XN) th on stripe data streamS) IM subblocks of dimensions, wherein
Figure GDA0002636352450000133
Denotes the user jth rjNth in the g IM sub-block on the stripe data streamsThe signal on the sub-carrier within an IM sub-block may be either a certain symbol in the constellation diagram or a zero symbol set on the non-activated zero sub-carrier.
With NS＝4，LSTable 1 gives an example of a table lookup implementation of an IM subblock, for example 3. In this example, the number of bits per IM subblock index information portion per user per data stream is p1＝2。
TABLE 1NS＝4，LS＝3，p1IM subblock example of 2
Index bit combination (p)1＝2) Indexing of active subcarrier combinations IM sub-block
[0 0] {1 2 3} [s(1) s(2) s(3) 0]
[0 1] {1 2 4} [s(1) s(2) 0 s(3)]
[1 0] {1 3 4} [s(1) 0 s(2) s(3)]
[1 1] {2 3 4} [0 s(1) s(2) s(3)]
Thus, the total transmission rate of the multi-user system is, per OOFDM symbol time, as
Figure GDA0002636352450000134
Wherein N isCPIs the length of the cyclic prefix of the OFDM symbol.
Definition of
Figure GDA0002636352450000135
For the user jth rjAll G (1 XN) pieces of strip data streamS) Vitamin IM subblock
Figure GDA0002636352450000136
Of
Figure GDA0002636352450000137
Total data Block of dimension IM, expressed as follows
Figure GDA0002636352450000138
In order to fully exert the potential of the IM technology, the system adopts a subcarrier interleaving mode to design a multi-user VLC system, namely, the interleaving operation is carried out on subcarrier elements among different data streams of the same user. Specifically, all data streams of the user are regarded as a virtual long data stream, and then an interleaving operation is performed. In particular, X is definedj(J ═ 1, 2.., J) is all R of user J obtained from formula (5)jR corresponding to strip data streamjTotal block of IM data
Figure GDA0002636352450000141
Of
Figure GDA0002636352450000142
Dimension to be interleaved block as follows
Figure GDA0002636352450000143
Through the interleaving operation, the method can obtain
Figure GDA0002636352450000144
Interleaved data of dimension
Figure GDA0002636352450000145
As follows
Figure GDA0002636352450000146
For convenience of explanation, FIG. 2 showsSchematic diagram of user j data stream subcarrier interleaving. The figure assumes that users j share R j2 data streams, NF＝32，
Figure GDA0002636352450000147
NSEach data stream has 2IM subblocks, G.
As can be seen from fig. 2, after interleaving, the symbols originally belonging to the same IM subblock are not adjacently placed, but are scattered into the whole frequency domain. Therefore, the symbols belonging to the same IM subblock can be transmitted on the subcarrier channel with the lowest possible correlation, increasing the channel selectivity experienced by the signal transmission, and thus obtaining a frequency selective diversity gain, which will improve the same group p corresponding to each IM subblock1BER performance of individual IM bits, thereby improving overall performance.
It is specifically pointed out that fig. 2 is only an interleaving scheme showing the design concept, and is not the only scheme. In an actual system, a matched interleaving scheme can be reasonably selected according to the fading condition of a channel.
After the interleaving operation is finished, the invention interleaves the data in the formula (7)
Figure GDA0002636352450000148
Dimensional data vector
Figure GDA0002636352450000149
Repartition into RjAn
Figure GDA00026363524500001410
And the dimension data block is used as a data block actually transmitted by each data stream of the user j. After repartitioning, the r-th user of the j-th userjOn a striped data stream
Figure GDA00026363524500001411
Dimension data block
Figure GDA00026363524500001412
Is composed of
Figure GDA0002636352450000151
Based on the total number of subcarriers being NFLet the subcarrier number nfIs taken from 0 to NF-1. Defining the nth frequency domainfOn sub-carriers (R × N)T) The channel matrix of all user frequency domains is
Figure GDA0002636352450000152
Wherein Hj(nf) Denotes the nth userfOn sub-carrier (R)j×NT) A dimensional frequency domain channel matrix. In the case of the ACO-OFDM,
Figure GDA0002636352450000153
the available sub-carriers correspond to sub-carrier numbers
Figure GDA0002636352450000154
Only the BD precoding operation needs to be performed on the frequency domain channel matrices corresponding to these subcarrier numbers.
Therefore, for
Figure GDA0002636352450000155
Using BD precoding method to H (n)f) And calculating the precoding matrix of each user on the frequency domain point. Note that here the operation is performed on the frequency domain channel matrix corresponding to each used subcarrier. Definition of nfOn sub-carriers except for the jth user ((R-R)j)×NT) The user channel is maintained with a complementary matrix of
Figure GDA0002636352450000156
To pair
Figure GDA0002636352450000157
The SVD decomposition step of (A) is written as
Figure GDA0002636352450000158
Wherein ((R-R)j)×(R-Rj) Dimension matrix
Figure GDA0002636352450000159
Contains all left singular vectors, ((R-R)j)×NT) Dimension matrix
Figure GDA00026363524500001510
Representing a matrix of singular values. Definition of
Figure GDA00026363524500001511
Figure GDA00026363524500001512
Dimension matrix
Figure GDA00026363524500001513
Comprises a front part
Figure GDA00026363524500001514
The number of right singular vectors,
Figure GDA00026363524500001515
dimension matrix
Figure GDA00026363524500001516
The remaining right singular vectors are included,
Figure GDA00026363524500001517
middle isotropic position
Figure GDA00026363524500001518
Within the null space of (a). It can be assumed that the channel is of full rank in general, having
Figure GDA00026363524500001519
Thereby obtaining the n-thfEquivalent information for user j on sub-carrierThe channel matrix is
Figure GDA00026363524500001520
For equivalent channel matrix
Figure GDA00026363524500001521
The step of proceeding with SVD decomposition is written as
Figure GDA00026363524500001522
Wherein Λj(nf) Is (R)j×Rj) Diagonal matrix of singular values of dimension, Uj(nf) Is for finally demodulating the signal (R)j×Rj) Dimensional unitary matrix, (R)j×Rj) Dimension matrix
Figure GDA00026363524500001523
Right singular vectors are included. Finally, the nth user of the jth user is obtainedfOn sub-carriers (N)T×Rj) Dimension precoding matrix Pj(nf)
Figure GDA0002636352450000161
In addition, F (n) is definedf) For n after precodingfOn sub-carriers (N)TX 1) dimensional frequency domain data vector. For the
Figure GDA0002636352450000162
This is achieved by
Figure GDA0002636352450000163
One available subcarrier of ACO-OFDM, having
Figure GDA0002636352450000164
Wherein u isj(nf) For the jth user nfSub-unitOn a carrier (R)jX 1) dimensional precoding data vector consisting of transmission data block in equation (8)
Figure GDA0002636352450000165
Inner corresponding element
Figure GDA0002636352450000166
I.e. data blocks
Figure GDA0002636352450000167
Mapping the elements in the vector to corresponding positions according to the mapping rule of the available subcarriers of the ACO-OFDM, and then carrying out precoding.
Second, due to the zero-padding operation of ACO-OFDM, when n isfWhen it is an even number, there is (N)TX 1) dimensional data vector F (0) ═ F (2) · F (N)F-2) 0. By hermitian conjugate symmetry, and
Figure GDA0002636352450000168
finally constituting (N) of J usersT×NF) Dimensional ACO-OFDM frequency domain matrix FinputIs composed of
Figure GDA0002636352450000169
Obtaining a corresponding time domain real number signal after IFFT processing, and recording that the time domain real number signal after IFFT on the ith LED is x0,i(t), the signal being a real signal. According to the principle of ACO-OFDM, clipping the negative real-valued signal to obtain the final transmission signal of
Figure GDA00026363524500001610
In a VLC system using intensity modulation and direct detection techniques, defining the LED electrical-to-optical conversion coefficient as mu, the mathematical expectation of the light signal emitted by the ith LED is the average emission of the lampOptical power Popt,i＝E{μxi(t) }. The average emitted light power P of the ith LED can be known by combining the time domain signal characteristics of the ACO-OFDMopt,iIs proportional to the electrical power of the pre-coded frequency domain data of the ith LED. Generally, after each frequency domain subcarrier is processed by the precoding matrix, the electrical power of the frequency domain data on each LED is different, and therefore the emitted optical power of each LED is different. In practice, the LEDs are usually symmetrically arranged on the indoor ceiling, and in order to ensure uniform illumination in the room, the average light power P emitted by each LED is requiredopt,i(i＝1,2,...,NT) As identical as possible, therefore, in the multi-user MIMO-OOFDM visible light communication system, an extra dc bias should be added to the LED with smaller average light power to ensure uniform illumination, note NTThe maximum desired optical power in each LED is
Figure GDA0002636352450000179
The extra DC offset that should be added to the ith LED is
Figure GDA0002636352450000171
After the direct current bias adjustment, the aim of VLC information transmission is achieved, and meanwhile the requirement of indoor uniform illumination can be met. After the uniform illumination adjustment, the emission signal on the ith LED
Figure GDA0002636352450000172
Is composed of
Figure GDA0002636352450000173
They satisfy that the average light power emitted by all the LEDs is uniform, namely
Figure GDA0002636352450000174
In addition, the center subcarrier does not carry any data in the OOFDM system, so the time domain direct current component added for realizing uniform illumination in the OOFDM system in the invention only appears on the DC subcarrier after the receiving end is processed by the FFT, and the demodulation of any data signal is not influenced.
At the user terminal side, the PD receives optical information from free space. After transmission through the VLC channel, the time-domain received signal at the r PD can be represented as
Figure GDA0002636352450000175
Wherein h isr,i(t) represents the VLC time domain channel impulse response between the ith LED and the ith PD,
Figure GDA0002636352450000176
representing the time-domain light signal emitted at the ith LED, nr(t) represents time-domain zero-mean real Additive White Gaussian Noise (AWGN) on the r-th PD, γ represents a photoelectric conversion coefficient of the PD,
Figure GDA0002636352450000177
representing a convolution operation of the time domain signal.
After the received optical signal is subjected to light intensity detection and photoelectric conversion processing of the PD, the optical signal is converted into an electric signal, and the ACO-OFDM demodulation is carried out on the time domain received electric signal to obtain frequency domain data. According to the BD precoding reception principle
Figure GDA0002636352450000178
Nth of jth userfEquivalent frequency domain received signal Y on subcarrierj(nf) Is composed of
Figure GDA0002636352450000181
nj(nf) Denotes the nth userfA corresponding frequency domain zero mean AWGN vector on the subcarriers. Matrix U generated using equation (12)j(nf) Y obtained by the conjugate transpose pair formula (22)j(nf) Processing to obtain the nth userfProcessed on sub-carriers (R)jX 1) dimensional vector
Figure GDA0002636352450000182
As shown below
Figure GDA0002636352450000183
J and corresponding r for all J ═ 1,2j＝1,2,...,RjReception vector on available subcarriers of ACO-OFDM obtained by equation (23)
Figure GDA0002636352450000184
The user jth r is shown as followsjOn a striped data stream
Figure GDA0002636352450000185
Received data blocks of a dimension
Figure GDA0002636352450000186
Figure GDA0002636352450000187
Wherein
Figure GDA0002636352450000188
Figure GDA0002636352450000189
Representing diagonal matrix from singular values Λj(2lc-1) taking the (r) thj,rj) The operation of the individual elements is carried out,
Figure GDA00026363524500001810
representing the demodulated frequency domain AWGN signal. Combined type (15) with
Figure GDA00026363524500001811
Thus, it is possible to provide
Figure GDA00026363524500001812
Is that
Figure GDA00026363524500001813
The corresponding received data vector. Due to the subcarrier interleaving operation of the equation (6) and the equation (7), the symbols in each IM subblock are transmitted scattered on different subcarriers with low correlation, and cannot be transmitted by the subcarriers with low correlation
Figure GDA00026363524500001814
Directly recovering the r-th of the user jjEach IM subblock to be demodulated on each data stream needs to extract symbols and equivalent channel singular values corresponding to the symbols scattered in the IM subblocks of each data stream of the user j and reconstruct the symbols into the IM subblocks to be demodulated.
Specifically, first, all R of the users j in the formula (24)jObtained on a striped data stream
Figure GDA00026363524500001815
Dimension to be demodulated data block
Figure GDA0002636352450000191
Composition of
Figure GDA0002636352450000192
Data vector of dimension
Figure GDA0002636352450000193
Figure GDA0002636352450000194
Then, according to the subcarrier interleaving rules defined in the formulas (6) and (7), at the receiving end of the user j, the r-th channel is reconstructed by the formula (25) according to the following formulajG (1 XN) th on stripe data streamS) Dimension to be demodulated IM sub-block
Figure GDA0002636352450000195
Figure GDA0002636352450000196
For all ns＝1,2,...,NSIs provided with
Figure GDA0002636352450000197
Secondly, due to the IM subblocks to be demodulated
Figure GDA0002636352450000198
Wherein each element was distributed throughout multiple data streams of the same user for transmission, thereby completing the pairing
Figure GDA0002636352450000199
Before demodulation, it is also necessary to
Figure GDA00026363524500001910
Each element of
Figure GDA00026363524500001911
And extracting and reconstructing corresponding singular values of the experienced actual channel according to the same rule. User jth rjOn a stripe data stream
Figure GDA00026363524500001912
Corresponding singular values on the available subcarriers may be sorted into
Figure GDA00026363524500001913
Singular value vector of dimension
Figure GDA00026363524500001914
Figure GDA00026363524500001915
Wherein
Figure GDA00026363524500001916
Representing a matrix of singular values, Λ, taken from equation (23)j(2lcR in 1)jCorresponding singular values of the strip data stream. All R of user jjCorresponding to a strip data stream
Figure GDA00026363524500001917
Composition of
Figure GDA00026363524500001918
Singular value vector λ of dimensionj(j＝1,2,...,J)
Figure GDA00026363524500001919
Similarly, the r-th of the user j can be reconstructed by the formula (28)jG received (1 XN) on stripe data streamS) Dimension to be demodulated IM sub-block
Figure GDA00026363524500001920
Corresponding demodulated singular value vector
Figure GDA00026363524500001921
Figure GDA00026363524500001922
For all ns＝1,2,...,NSIs provided with
Figure GDA00026363524500001923
Through the above operations, the IM subblock to be demodulated, de-interleaved on each data stream belonging to each user, can be obtained at the user terminal
Figure GDA0002636352450000201
And corresponding user equivalent channel singular values
Figure GDA0002636352450000202
According to the principle of IM concept, for each IM subblock to be demodulated
Figure GDA0002636352450000203
Demodulation is based primarily on two criteria, one is based on the ML criterion and one is based on the LLR criterion.
When using ML-based criteria demodulation, all possible combinations of transmit vectors need to be searched for each IM subblock. In particular, the set of all possible combinations of transmit vectors is defined as ΩXIM subblocks estimated according to maximum likelihood definition
Figure GDA0002636352450000204
Can be produced by the following formula
Figure GDA0002636352450000205
Wherein
Figure GDA0002636352450000206
Representing by vectors
Figure GDA0002636352450000207
An operation of generating a corresponding diagonal matrix. Substituting (30) all possible IM sub-block transmission vectors into a joint solution to obtain an estimated IM sub-block transmission vector
Figure GDA0002636352450000208
I.e. the index information and symbol information of p bits can be recovered simultaneously. It is clear that the ML demodulation complexity varies with the order M of the modulation symbol constellation carried on the subcarrier and the number L of subcarriers activated per IM sub-blockSBut has an exponential upward trend.
Demodulation based on the LLR criterion is a linear demodulation method that avoids the exponential rise demodulation complexity. Has the same performance as ML. The use of the LLR criterion is a better choice for demodulation in the present system. In the method, each is first calculatedAn IM subblock to be demodulated
Figure GDA0002636352450000209
N insLLR values for subcarriers
Figure GDA00026363524500002010
Wherein n iss＝1,2,...,NSAs shown below
Figure GDA00026363524500002011
Wherein s isχIs the x modulation symbol mapped on the M-th order constellation diagram,
Figure GDA00026363524500002012
is the corresponding frequency domain AWGN power used for the calculation of the LLR ratio. Thus, according to
Figure GDA00026363524500002013
The subcarriers within all IM sub-blocks can be calculated, i.e., total NSLLR values for the subcarriers. Next, in connection with the definition of the subcarrier activation combination in equation (2), for all C ═ 1,2
Figure GDA00026363524500002014
Corresponding subcarrier activation combining
Figure GDA00026363524500002015
Sum of LLR of
Figure GDA00026363524500002016
According to all calculated
Figure GDA00026363524500002017
LLR is summed, and the decision is as follows
Figure GDA00026363524500002018
I.e. the second one with the largest LLR and corresponding
Figure GDA0002636352450000211
A set
Figure GDA0002636352450000212
As IM subblocks
Figure GDA0002636352450000213
Activates the optimal solution of the combination to obtain the corresponding index information bit group p1. Then according to
Figure GDA0002636352450000214
For the lsAn activated sub-carrier, and the constellation symbol on the activated sub-carrier is solved
Figure GDA0002636352450000215
Finally obtain LSSymbol information bit group p on activated sub-carrier2。
Finally, the present invention provides a general flow chart of the system of the present invention in fig. 3.
Example 2
To more fully illustrate the advantages of the present invention, the following description is provided in conjunction with simulation results and analysis to further illustrate the effectiveness and advancement of the present invention.
In this embodiment, the simulation system selects a typical indoor room model, the dimensions of the room are 6 mx 3m, the midpoint of the room is the origin of coordinates, and N are symmetrically arranged on the ceilingTThe 4 LED lamps are respectively positioned in the centers of the four quadrants. The number of user terminals J equals 2, each equipped with 2 PDs and supporting two data streams, i.e. Q1＝Q2＝R1＝R 22. In order to meet the requirement of miniaturization of the device, the distance between 2 PDs of the same user terminal is set to 10cm in the classical model,and in order to reduce the channel correlation of PDs in the miniaturized device, 2 PDs of the same user terminal use different field of View (FOV) as an example of implementation of reducing the channel correlation, respectively set to 70 ° and 50 °, while assuming that the user terminals are all located on a usage plane 0.85m high from the ground. An indoor VLC channel in simulation adopts a classical ray tracing model, the Signal-to-Noise Ratio (SNR) adopted by simulation is consistent with the literature, and the SNR of an LED transmitting end is defined as
Figure GDA0002636352450000216
Wherein P isTIs the total luminous average light intensity of all the LEDs in the room,
Figure GDA0002636352450000217
is the time domain AWGN power.
The invention selects BPSK, QPSK, 8QAM and 16QAM with M2, M4 and M8 as modulation schemes of the traditional multi-user MIMO-OOFDM system without IM. To maintain a peak transmission rate of the system consistent with the comparative system scheme, the following table 2 setting was used as a specific example of the present system:
table 2 table of parameters of embodiments of the present system
Figure GDA0002636352450000218
Figure GDA0002636352450000221
Assuming that the user terminal 1 is located in the middle of the room (0,0,0.85), the user terminal 2 picks two locations as simulation examples of the system, setting location 1 to (0.2,0.2,0.85) and location 2 to (2.7,2.7, 0.85). In particular, to better present the system characteristics and the importance of subcarrier interleaving in the present system, the present invention compares together the system scheme that does not use subcarrier interleaving but employs IM, in addition to the conventional multi-user MIMO-OOFDM system. Figures 4 to 6 show a comparison of BER performance for different values of M for each system.
As shown in fig. 4 to 6, in the simulation scenario, when the ue 2 is located at the location 1 where two users are very close to each other, the performance of all systems is significantly degraded due to the very strong channel correlation. It can also be seen from the figure that in both exemplary scenarios, due to the particularity of the indoor VLC channel, the final performance of the multi-user MIMO visible light communication system is deeply affected by the specific location of the user terminal, but the laws of the difference between the relative performances between different systems are similar. Therefore, the performance behavior of the different systems in position 2 will be discussed as an example.
As can be seen from fig. 4 to 6, when M is 2, the BER of the multi-user IM visible light communication system using subcarrier interleaving is 10-5To 10-6Compared with the traditional system, the gain of the multi-user IM visible light communication system is about 3dB, when M is 4, the multi-user IM visible light communication system using subcarrier interleaving has better performance than the traditional OOFDM system when the SNR is 146dB, and the BER is 10-5To 10-6There is a gain of about 1.2 dB. The new system gain using subcarrier interleaving drops when M is 8, but still has a gain of about 0.6dB at high SNR.
Therefore, under the indoor VLC environment adopted by the invention, the multi-user IM visible light communication system using subcarrier interleaving can obtain certain performance gain under different low-order modulations, thereby improving the transmission reliability of the communication system as much as possible. According to the IM technology concept, after the system is reasonably designed by using IM, the overall performance of the system is improved due to the improvement of the reliability of the information transmitted by using the subcarrier index combination under the higher SNR.
In contrast, a multi-user MIMO-OOFDM-IM visible light communication system that does not use subcarrier interleaving can hardly exhibit good performance gain under all settings compared to a conventional MIMO-OOFDM visible light communication system. In most cases, the BER performance of the system only approaches or slightly exceeds that of the traditional system at high SNR, which further embodies the importance of subcarrier interleaving in playing the potential of the IM-based multi-user MIMO-OOFDM visible light communication system.
It should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.

Claims (1)

1. The multi-user MIMO-OOFDM visible light communication method combined with index modulation is characterized in that: the method comprises the following steps:
s1, for a multi-user MIMO-OOFDM system, B bits input by each data stream of each user are firstly divided into G groups
Figure FDA0002636352440000011
The G index modulated IM sub-blocks into which the available sub-carriers are divided correspond to NFThe number of points of the fast Fourier transform and the inverse fast Fourier transform; definition of NUThe number of subcarriers N contained in each IM sub-block is the total number of subcarriersSIs composed of
Figure FDA0002636352440000012
S2. N of each IM subblockSOnly L in the sub-carriersSThe subcarriers are selected by the index selector to be activated, and the remaining NS-LSThe sub-carriers are not activated and set to 0; so that the combination of subcarrier activations can represent in p bits
Figure FDA0002636352440000013
A bit wherein
Figure FDA0002636352440000014
Representing the total combination of b elements in a set of a elements for the operation of the number of combinationsThe number of the first and second groups is,
Figure FDA0002636352440000015
represents a round-down operation; thus sharing in common
Figure FDA0002636352440000016
Different subcarrier activation combinations are recorded, and the set of all C possible subcarrier activation combinations in each IM subblock is recorded as
ΩU＝{U1,U2,...,UC} (2)
Wherein
Figure FDA0002636352440000017
Figure FDA0002636352440000018
Denotes the l th in the c th activation combinationsThe activated subcarriers index, and for all C1, 2s＝1,2,...,LSAre all provided with
Figure FDA0002636352440000019
The remainder of p2＝Lflog2(M) bits will pass through M-order constellation diagram omegaMMapping as LfM-th constellation symbols, denoted as s ═ s (1), s (2)f)] (3)
Wherein for ls＝1,2,...,LfAre all provided with s (l)s)∈ΩMThey will be modulated in IM subblock LfOn the activated subcarriers;
s3, using rjData stream number, definition, representing the jth user
Figure FDA00026363524400000110
For user jth obtained by the above IM methodjG (1 XN) th on stripe data streamS) An IM subblock of dimensions, wherein G ═ 1, 2.., G; r isj＝1,2,...,Rj；j＝1,2,...,J；
Figure FDA00026363524400000111
Denotes the user jth rjNth in the g IM sub-block on the stripe data streamsSignal on sub-carriers within an IM sub-block, ns＝1,2,...,NS；
S4, in each OOFDM symbol time, the total transmission rate of the multi-user system is
Figure FDA0002636352440000021
Wherein N isCPLength of cyclic prefix of OFDM symbol;
s5, define
Figure FDA0002636352440000022
For the user jth rjAll G (1 XN) pieces of strip data streamS) Vitamin IM subblock
Figure FDA0002636352440000023
Of
Figure FDA0002636352440000024
Dimension IM Total data Block, rj＝1,2...,Rj(ii) a J ═ 1, 2.., J, expressed as follows
Figure FDA0002636352440000025
S6, interleaving the sub-carrier elements among different data streams of the same user, regarding all the data streams of the user as a virtual long data stream, and then interleaving to define XjAll R for user j derived from equation (5)jR corresponding to strip data streamjTotal IM data block Xj,rjOf
Figure FDA0002636352440000026
The dimension to be interleaved blocks are as follows:
Figure FDA0002636352440000027
Figure FDA0002636352440000028
wherein J is 1, 2.. times.j; through the interleaving operation, the method can obtain
Figure FDA0002636352440000029
Interleaved data of dimension
Figure FDA00026363524400000210
The following were used:
Figure FDA00026363524400000211
s7, interweaving the fibers in the formula (7)
Figure FDA00026363524400000212
Dimensional data vector
Figure FDA00026363524400000213
Repartition into RjAn
Figure FDA00026363524400000214
The dimension data block is used as a data block actually transmitted by each data stream of the user j; after repartitioning, the r-th user of the j-th userjOn a striped data stream
Figure FDA00026363524400000215
Dimension data block
Figure FDA00026363524400000216
Is composed of
Figure FDA0002636352440000031
Wherein r isj＝1,2,...,Rj；j＝1,2,...,J；
S8, based on the total subcarrier number being NFLet the subcarrier number nfIs taken from 0 to NF-1; defining the nth frequency domainfOn sub-carriers (R × N)T) The channel matrix of all user frequency domains is
H(nf)＝[H1(nf)T,H2(nf)T,...,HJ(nf)T]T (9)
Wherein Hj(nf) Denotes the nth userfOn sub-carrier (R)j×NT) A dimensional frequency domain channel matrix; in asymmetric clipped light orthogonal frequency division multiplexing ACO-OFDM,
Figure FDA0002636352440000032
the available sub-carriers correspond to sub-carrier numbers
Figure FDA0002636352440000033
Only the BD pre-coding operation of block diagonalization is needed to be carried out on the frequency domain channel matrixes corresponding to the subcarrier numbers;
s9. for
Figure FDA0002636352440000034
Using BD precoding method to H (n)f) Calculating to obtain a precoding matrix of each user on the frequency domain point; definition of nfOn sub-carriers except for the jth user ((R-R)j)×NT) The dimension user channel complement matrix is:
Figure FDA0002636352440000035
to pair
Figure FDA0002636352440000036
The SVD decomposition step of (A) is written as
Figure FDA0002636352440000037
Wherein ((R-R)j)×(R-Rj) Dimension matrix
Figure FDA0002636352440000038
Contains all left singular vectors, ((R-R)j)×NT) Dimension matrix
Figure FDA0002636352440000039
Representing a matrix of singular values; definition of
Figure FDA00026363524400000310
Figure FDA00026363524400000311
Dimension matrix
Figure FDA00026363524400000312
Comprises a front part
Figure FDA00026363524400000313
The number of right singular vectors,
Figure FDA00026363524400000314
dimension matrix
Figure FDA00026363524400000315
The remaining right singular vectors are included,
Figure FDA00026363524400000316
middle isotropic position
Figure FDA00026363524400000317
Within the null space of (a); it can be assumed that the channel is full in generalRank of, having
Figure FDA00026363524400000318
Thereby obtaining the n-thfThe equivalent channel matrix for user j on a subcarrier is
Figure FDA00026363524400000319
For equivalent channel matrix
Figure FDA00026363524400000320
The steps for proceeding with the SVD decomposition are written as:
Figure FDA00026363524400000321
wherein Λj(nf) Is (R)j×Rj) Diagonal matrix of singular values of dimension, Uj(nf) Is for finally demodulating the signal (R)j×Rj) Dimensional unitary matrix, (R)j×Rj) Dimension matrix
Figure FDA00026363524400000322
Right singular vectors are included; finally, the nth user of the jth user is obtainedfOn sub-carriers (N)T×Rj) Dimensional precoding matrix
Figure FDA0002636352440000041
S10, defining F (n)f) For n after precodingfOn sub-carriers (N)TX 1) dimensional frequency domain data vector; for the
Figure FDA0002636352440000042
This is achieved by
Figure FDA0002636352440000043
One available subcarrier of ACO-OFDM, having
Figure FDA0002636352440000044
Wherein u isj(nf) For the jth user nfOn sub-carrier (R)jX 1) dimensional precoding data vector consisting of transmission data block in equation (8)
Figure FDA0002636352440000045
Inner corresponding element
Figure FDA0002636352440000046
I.e. data blocks
Figure FDA0002636352440000047
Mapping the elements in the vector to corresponding positions according to the mapping rule of the available subcarriers of the ACO-OFDM, and then carrying out precoding;
due to the zero padding operation of ACO-OFDM, when nfWhen it is an even number, there is (N)TX 1) dimensional data vector F (0) ═ F (2) · F (N)F-2) ═ 0; symmetrically operated by Hermite conjugation, with F (n)f)＝F*(NF-nf)，
Figure FDA0002636352440000048
Finally constituting (N) of J usersT×NF) Dimensional ACO-OFDM frequency domain matrix FinputIs composed of
Figure FDA0002636352440000049
S11, obtaining a corresponding time domain real number signal after IFFT processing, and recording the time domain real number signal after IFFT on the ith LED as x0,i(t), the signal being a real signal; according to the principle of ACO-OFDM, clipping the negative real-valued signal to obtain the final transmission signal of
Figure FDA00026363524400000410
In a VLC system using intensity modulation and direct detection techniques, defining the LED electrical-to-optical conversion coefficient as mu, the mathematical expectation of the light signal emitted by the ith LED is the average emitted light power P of the lampopt,i＝E{μxi(t) }; the average emitted light power P of the ith LED can be known by combining the time domain signal characteristics of the ACO-OFDMopt,iProportional to the electrical power of the pre-coded frequency domain data of the ith LED; generally, after each frequency domain subcarrier is subjected to precoding matrix processing, the electric power of frequency domain data on each LED is different, so that the emitted light power of each LED is different; in practice, the LEDs are usually symmetrically arranged on the indoor ceiling, and in order to ensure uniform illumination in the room, the average light power P emitted by each LED is requiredopt,i，i＝1,2,...,NTMay be the same, therefore, in a multi-user MIMO-OOFDM visible light communication system, an extra dc bias should be added to the LED with smaller average optical power to ensure uniform illumination, note NTThe maximum desired optical power in each LED is
Figure FDA0002636352440000051
The extra DC offset that should be added to the ith LED is
Figure FDA0002636352440000052
After the direct current bias adjustment, the aim of transmitting VLC information is achieved, and meanwhile the requirement of indoor uniform illumination can be met; after the uniform illumination adjustment, the emission signal on the ith LED
Figure FDA00026363524400000510
Is composed of
Figure FDA0002636352440000053
They satisfy that the average light power emitted by all the LEDs is uniform, namely
Figure FDA0002636352440000054
S12, at the user terminal side, a light detector PD receives light information from free space; after transmission through the VLC channel, the time-domain received signal at the r PD can be represented as
Figure FDA0002636352440000055
Wherein h isr,i(t) represents the VLC time domain channel impulse response between the ith LED and the ith PD,
Figure FDA0002636352440000056
representing the time-domain light signal emitted at the ith LED, nr(t) represents time-domain zero-mean real additive white Gaussian noise on the r-th PD, gamma represents the photoelectric conversion coefficient of the PD,
Figure FDA0002636352440000057
a convolution operation representing a time domain signal;
s13, after the received optical signal is subjected to light intensity detection and photoelectric conversion processing of PD, the optical signal is converted into an electric signal, and ACO-OFDM demodulation is carried out on the time domain received electric signal to obtain frequency domain data; according to the BD precoding reception principle
Figure FDA0002636352440000058
Nth of jth userfEquivalent frequency domain received signal Y on subcarrierj(nf) Is composed of
Figure FDA0002636352440000059
Wherein n isj(nf) Denotes the nth userfA corresponding frequency domain zero mean additive white Gaussian noise AWGN vector on the subcarrier; matrix U generated using equation (12)j(nf) Y obtained by the conjugate transpose pair formula (22)j(nf) Processing to obtain the nth userfProcessed on sub-carriers (R)jX 1) dimensional vector
Figure FDA0002636352440000061
As shown below
Figure FDA0002636352440000062
S14. for all J ═ 1,2j＝1,2,...,RjReception vector on available subcarriers of ACO-OFDM obtained by equation (23)
Figure FDA0002636352440000063
The user jth r is shown as followsjOn a striped data stream
Figure FDA0002636352440000064
Received data blocks of a dimension
Figure FDA0002636352440000065
Figure FDA0002636352440000066
Wherein r isj＝1,2,...,Rj，j＝1,2,...,J，
Figure FDA0002636352440000067
Figure FDA0002636352440000068
Representing diagonal matrix from singular values Λj(2lc-1) taking the (r) thj,rj) The operation of the individual elements is carried out,
Figure FDA0002636352440000069
represents the demodulated frequency domain AWGN signal; combined type (15) with
Figure FDA00026363524400000610
Thus, it is possible to provide
Figure FDA00026363524400000611
Is that
Figure FDA00026363524400000612
A corresponding received data vector; due to the subcarrier interleaving operation of the equation (6) and the equation (7), the symbols in each IM subblock are transmitted scattered on different subcarriers with low correlation, and cannot be transmitted by the subcarriers with low correlation
Figure FDA00026363524400000613
Directly recovering the r-th of the user jjEach IM subblock to be demodulated on each data stream needs to extract symbols and equivalent channel singular values corresponding to the symbols scattered in the IM subblocks of each data stream of the user j and reconstruct the symbols into the IM subblocks to be demodulated;
specifically, first, all R of the users j in the formula (24)jObtained on a striped data stream
Figure FDA00026363524400000614
Dimension to be demodulated data block
Figure FDA00026363524400000615
Composition of
Figure FDA00026363524400000616
Data vector of dimension
Figure FDA00026363524400000617
Figure FDA0002636352440000071
S15, reconstructing the r-th channel by the formula (25) at the receiving end of the user j according to the subcarrier interleaving rules defined in the formula (6) and the formula (7) by the formula (25)jG (1 XN) th on stripe data streamS) Dimension to be demodulated IM sub-block
Figure FDA0002636352440000072
Figure FDA0002636352440000073
For all ns＝1,2,...,NSIs provided with
Figure FDA0002636352440000074
Secondly, due to the IM subblocks to be demodulated
Figure FDA0002636352440000075
Wherein each element was distributed throughout multiple data streams of the same user for transmission, thereby completing the pairing
Figure FDA0002636352440000076
Before demodulation, it is also necessary to
Figure FDA0002636352440000077
Each element of
Figure FDA0002636352440000078
Extracting and reconstructing corresponding singular values of the experienced actual channel according to the same rule; user jth rjOn a stripe data stream
Figure FDA0002636352440000079
Corresponding singular values on the available subcarriers may be sorted into
Figure FDA00026363524400000710
Singular value vector of dimension
Figure FDA00026363524400000711
rj＝1,2,...,Rj，j＝1,2,...,J
Figure FDA00026363524400000712
Wherein
Figure FDA00026363524400000713
Representing a matrix of singular values, Λ, taken from equation (23)j(2lcR in 1)jCorresponding singular values of the strip data stream; all R of user jjCorresponding to a strip data stream
Figure FDA00026363524400000714
Composition of
Figure FDA00026363524400000715
Singular value vector λ of dimensionj，j＝1,2,...,J
Figure FDA00026363524400000716
Similarly, the r-th of the user j can be reconstructed by the formula (28)jG received (1 XN) on stripe data streamS) Dimension to be demodulated IM sub-block
Figure FDA00026363524400000717
Corresponding demodulated singular value vector
Figure FDA00026363524400000718
Figure FDA00026363524400000719
For all ns＝1,2,...,NSIs provided with
Figure FDA00026363524400000720
S16, through the above operation, the IM subblock to be demodulated, which is deinterleaved in each data stream belonging to each user, can be obtained at the user terminal
Figure FDA00026363524400000721
And corresponding user equivalent channel singular values
Figure FDA00026363524400000722
S17, according to the principle of IM concept, for each IM subblock to be demodulated
Figure FDA0002636352440000081
Demodulation is mainly based on two criteria, one is based on a Maximum Likelihood (ML) criterion, and the other is based on a log-likelihood ratio (LLR) criterion;
when the ML-based criterion demodulation is selected, all possible emission vector combinations are required to be searched for each IM subblock; in particular, the set of all possible combinations of transmit vectors is defined as ΩXIM subblocks estimated according to maximum likelihood definition
Figure FDA0002636352440000082
Can be produced by the following formula
Figure FDA0002636352440000083
Wherein
Figure FDA0002636352440000084
Representing by vectors
Figure FDA0002636352440000085
An operation of generating a corresponding diagonal matrix; substituting (30) all possible IM sub-block transmission vectors into a joint solution to obtain an estimated IM sub-block transmission vector
Figure FDA0002636352440000086
The index information and the symbol information of the p bits can be recovered at the same time; it is clear that the ML demodulation complexity varies with the order M of the modulation symbol constellation carried on the subcarrier and the number L of subcarriers activated per IM sub-blockSBut has an exponential rising trend;
demodulation based on the LLR criterion is a linear demodulation method, which avoids the demodulation complexity of exponential rise; has the same performance as ML; therefore, the demodulation of the system is a better choice by adopting the LLR criterion; in the method, each IM subblock to be demodulated is calculated first
Figure FDA0002636352440000087
N insLLR values for subcarriers
Figure FDA0002636352440000088
Wherein n iss＝1,2,...,NSAs shown below
Figure FDA0002636352440000089
Wherein s isχIs the x modulation symbol mapped on the M-th order constellation diagram,
Figure FDA00026363524400000810
is the corresponding frequency domain AWGN power used for the calculation of the LLR ratio; thus, according to
Figure FDA00026363524400000811
The subcarriers within all IM sub-blocks can be calculated, i.e., total NSLLR values for the subcarriers; next, in connection with the definition of the subcarrier activation combination in equation (2), for all C ═ 1,2
Figure FDA00026363524400000812
Corresponding subcarrier activation combining
Figure FDA00026363524400000813
Sum of LLR of
Figure FDA00026363524400000814
According to all calculated
Figure FDA00026363524400000815
LLR is summed, and the decision is as follows
Figure FDA00026363524400000816
I.e. the second one with the largest LLR and corresponding
Figure FDA00026363524400000817
A set
Figure FDA00026363524400000818
As IM subblocks
Figure FDA00026363524400000819
Activates the optimal solution of the combination to obtain the corresponding index information bit group p1(ii) a Then according to
Figure FDA0002636352440000091
For the lsAn activated subcarrier, the activated subcarrier is solvedConstellation symbols on waves
Figure FDA0002636352440000092
Finally obtain LfSymbol information bit group p on activated sub-carrier2。
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