A Waveform Design Method for a Unipolar Light Source in Visible Light Communication

Abstract

This invention provides a method for designing waveforms of a unipolar light source for visible light communication, belonging to the field of visible light communication technology. This design method involves designing a unipolar code group, utilizing the ideal autocorrelation characteristic of the pilot sub-codes within the code group as the spreading code for the pilot light source. By leveraging the characteristics between the pilot sub-codes and communication sub-codes in the code group, the communication sub-codes are combined in pairs to serve as the spreading code for the communication light source. At the transmitting end, the pilot light source and the communication light source are combined to jointly transmit multi-source VLC-CDMA optical signals. At the receiving end, a demodulation method is provided for the transmitted light source waveforms, enabling demodulation of communication signals from various users without cross-correlation interference. This invention can meet the needs of simultaneous data transmission by multiple users under the current limitations of image sensor hardware, improving the communication performance of multi-source VLC-CDMA systems.

Description

Technical Field

[0001] This invention belongs to the field of visible light communication technology and relates to the design of light source emission waveforms, specifically to a method for designing waveforms of a unipolar visible light source for image sensors. Background Technology

[0002] Visible Light Communication (VLC), as a novel wireless communication technology, transmits information using high-speed on / off signals emitted by light-emitting diodes (LEDs). It boasts advantages unmatched by traditional wireless communication technologies, such as abundant spectrum resources, no electromagnetic radiation, and environmental friendliness. Furthermore, for multi-user applications, using Code Division Multiple Access (CDMA) as the system's multiple access method to construct a VLC-CDMA system fully leverages the advantages of CDMA technology, enabling multiple users to simultaneously transmit data.

[0003] In multi-source VLC-CDMA systems based on image sensors, since the signal loaded onto the LED must be a unipolar signal, two different system structures can be defined based on the polarity of the spreading code used by the light source. If the spreading code used by the light source in the system is a unipolar code, compared to using a bipolar code, the spread signal can be directly loaded onto the LED without polarity conversion within the system, thereby reducing system complexity and facilitating the construction and implementation of visible light communication systems.

[0004] A review of existing literature revealed that Chang He, Lie-liang Yang, and Pei Xiao et al. published "DS-CDMA Assisted Visible Light Communications Systems" at the IEEE International Workshop on Computer Aided Modelling and Design of Communication Links and Networks (CAMAD) in 2015. This paper analyzed the bit error rate performance of Optical Orthogonal Code (OOC), which is widely used in unipolar codes. Due to its excellent correlation characteristics, OOC's communication performance is significantly better than bipolar codes such as Gold and Walsh sequences. However, OOC suffers from problems such as overly complex code generation and insufficient user capacity. Furthermore, when the number of users in the communication system is large, demodulating a single user's signal at the receiver can result in severe cross-correlation interference from other users, thus affecting the overall communication performance of the VLC-CDMA system. Summary of the Invention

[0005] The technical problem this invention aims to solve is that when using the widely used unipolar code as the spreading code for the light source in a multi-source VLC-CDMA system, there are problems such as complex code generation and insufficient number of users, which seriously limit the communication performance of the VLC-CDMA system.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for designing waveforms of a unipolar visible light communication source is disclosed. This method involves designing a unipolar code group, utilizing the ideal autocorrelation characteristic of the pilot sub-codes within the code group as the spreading code for the pilot light source, and combining the communication sub-codes in pairs to form the spreading code for the communication light source. At the transmitting end, the pilot and communication light sources are combined to transmit multi-source VLC-CDMA optical signals. At the receiving end, a demodulation method is provided for the transmitted light source waveforms. This method enables demodulation of communication signals from various users without cross-correlation interference, thereby meeting the requirement for simultaneous data transmission by multiple users under the current limitations of image sensor hardware and improving the communication performance of multi-source VLC-CDMA systems. The design method provided by this invention is as follows:

[0008] Assuming that a multi-source VLC-CDMA system needs to provide data communication functions for K users, the unipolar light source waveform design method is divided into two parts: unipolar code group design and system structure design.

[0009] Part 1: Unipolar Code Design

[0010] Step 1: Determine the number of subcodes and the code length within the code group.

[0011] Based on the number of users K in the VLC-CDMA system, the number of subcodes N in a unipolar code group is:

[0012] N = 2K + 1 (1)

[0013] The code length L of each subcode within the code group is:

[0014] L=MN (2)

[0015] Where M is a multiple of the code length and must satisfy M≥K.

[0016] Step 2: Generation of subcodes within the code group

[0017] Based on the number of subcodes N and the subcode length L obtained in step one, construct an N-row, L-column matrix A consisting entirely of zeros. Let a denote the position in matrix A where the n-th row and l-th column undergoes a 0-1 transition. nl And a nl For the expression ∈{1,2,...,L}, the 01 is converted to change the value of 0 to the value of 1. Specifically:

[0018] 2.1) First, generate pilot subcodes. For the first row of all-zero values ​​in matrix A, perform 0-1 conversion at positions 1+mN, where m∈{0,1,2,...,M-1}, and denote the converted pilot subcode as s. p At this point, the pilot code contains M 1s and M(N-1) 0s.

[0019] 2.2) Next, generate the communication subcode. For the 2K rows from row 2 to row N of matrix A, all 0 values, the criteria for 0-1 conversion are as follows: (1) The total number of positions for 0-1 conversion in each row matrix must be equal to M; (2) The position a for 0-1 conversion in each row matrix nl (2) No repetition, meaning that the set of all transformed positions in matrix A after the transformation must be equal to {1,2,...,L}; (3) The difference between any two transformed positions in each row matrix cannot be a multiple of N. The 2K row matrices transformed by the above transformation criteria are used as communication subcodes, denoted as s1,s2,...,s 2K .

[0020] 2.3) Finally, the matrix A after the 0-1 conversion is re-recorded as code group A, which contains 1 pilot subcode and 2K communication subcodes. The subcodes in code group A have the following characteristics: (1) The pilot subcode has ideal autocorrelation characteristics, that is, the autocorrelation function has only one impulse function at zero shift, and the function value at other shifts is 0; (2) The pilot subcode and each communication subcode have excellent cross-correlation characteristics, that is, the function value of the cross-correlation function between the pilot subcode and each communication subcode at each shift is less than or equal to 1; (3) The positions of 1 in each subcode in code group A are independent and there is no repetition.

[0021] Part Two: System Structure Design

[0022] The structural design of a multi-source VLC-CDMA system can be divided into two aspects: the transmitter and the receiver. The transmitter transmits VLC-CDMA signals using a pilot light source and a communication light source. The receiver uses a CMOS image sensor to receive the superimposed signals from the pilot and communication light sources, and demodulates the transmitted data from each user, enabling communication transmission among K users. The specific steps of the system structural design are as follows:

[0023] Step 1: Determine the number of pilot light sources and communication light sources in the transmitting end.

[0024] Based on the number of users K in the VLC-CDMA system and the code length multiple M in code group A, the number P of pilot light sources that need to be deployed at the transmitter is:

[0025]

[0026] in, This means rounding down the result of M / K, i.e., achieving synchronous communication for K users using P pilot light sources. Simultaneously, K communication light sources need to be deployed, meaning data communication for K users is achieved through K communication light sources.

[0027] Step 2: Signal transmission from the pilot light source at the transmitting end

[0028] Let the data information transmitted by the P pilot light sources be identical and all be d. p , where d p ∈{1}, based on the code group A generated in the first part, using the pilot subcode s within it. p Using the spreading code as the basis for all pilot light sources, and spreading the transmitted data with the spreading code, the unipolar baseband signal of a single pilot light source after spreading modulation is:

[0029] T p =d p s p (4)

[0030] Then the baseband signal T p By applying on-off keying (OOK) modulation to each pilot light source, the VLC-CDMA superimposed signal transmitted by the P pilot light sources is:

[0031]

[0032] Among them, A Pi This represents the optical power amplitude value of the signal transmitted by the i-th pilot light source.

[0033] Step 3: Signal transmission from the communication light source in the transmitting end

[0034] Let the data information transmitted by K communication light sources be d1, d2, ... d K Where d1, d2, ... d K ∈{0,1}, based on the code group A generated in the first part, divide the 2K communication subcodes from the 2nd to the Nth row into pairs, that is, (s1,s2),(s3,s4),...,(s 2K-1 ,s 2K Group them into pairs as S1, S2, ..., S K These are used as spreading codes for the K communication light sources, respectively. The first spreading code in each group is designated to handle spreading modulation when the data information is 1, and the second spreading code is designated to handle spreading modulation when the data information is 0. Therefore, the baseband signal of the kth communication light source after spreading modulation according to the above rules is:

[0035]

[0036] Where k∈{1,2,...,K}. Then the baseband signal T k By applying OOK modulation to each communication light source, the VLC-CDMA superimposed signal transmitted by the K communication light sources is:

[0037]

[0038] Among them, A Kk This represents the optical power amplitude value of the signal transmitted by the k-th communication light source.

[0039] Step 4: Reception of superimposed optical signals at the receiving end

[0040] Using a CMOS image sensor as the receiver in a VLC-CDMA system, P+K superimposed VLC-CDMA signals from the pilot and communication light sources are received via a rolling shutter exposure mode. After grayscale processing of the CMOS sensor's image, the unipolar superimposed grayscale value sequence representing the P+K superimposed VLC-CDMA signals is y. GIts sequence length is L G Its expression is:

[0041]

[0042] Among them, y GP Let y represent a sequence of unipolar superimposed gray values ​​from P pilot light sources. GK G represents a sequence of unipolar superimposed grayscale values ​​from K communication light sources. Pi G represents the grayscale value of the bright stripe in the image of the i-th pilot light source. Kk Let G represent the grayscale value of the bright stripe of the k-th communication light source, and G... Pi With G Kk All are related to the optical power amplitude value A of the pilot light source Pi The optical power amplitude value A of the communication light source Kk The relationship is linear.

[0043] Step 5: Determine the start and end points of demodulation.

[0044] According to the spreading code s of the pilot light source p And its code length L, the sequence y G The first L position and the last L position are respectively related to s p Perform cross-correlation calculations, then perform maximum value detection on the results of the two correlation calculations, and record the positions a1 and a2 of the two maximum values ​​respectively, where a1 corresponds to the first L positions and s. p The maximum value after cross-correlation, a2 corresponds to the last L bits and s p The maximum value after cross-correlation; then for the received superimposed grayscale value sequence y G The demodulation begins at position a. begin for:

[0045] a begin =a1 (9)

[0046] The position where demodulation ends (a) end for:

[0047] a end =L G -(L-a2)-1 (10)

[0048] Based on the start and end demodulation positions, the unipolar superimposed grayscale value sequence y is... G The first to the (a1-1)th and the Lth G -(L-a2) to the Lth G Remove each bit individually to obtain the middle sequence y after removing the left and right ends. cut Its sequence length is L cut .

[0049] Step Six: Demodulate the transmitted signal of the communication light source

[0050] The sequence y obtained from step five cut For the demodulation of the signal transmitted by the k-th communication light source, its corresponding spreading code S is used. k The spreading code s in 2k-1 and s 2k , respectively for y cut Perform despreading to obtain the demodulated sequence when the responsible data information is 1. and the demodulation sequence when the data information is 0 Will and Perform a subtraction operation on the sequences and then decide the result: if... Then the data information at the corresponding position will be set to 1. Then the data information at the corresponding position is set to 0, thus obtaining the data information d recovered by the kth communication light source. k The data information d1',d2',...,d transmitted by all K communication light sources can be recovered using the demodulation method described above. K Ultimately, this enables cross-correlation-free data transmission between K users in a multi-source VLC-CDMA system.

[0051] The beneficial effects of this invention are as follows:

[0052] Compared to VLC-CDMA systems that use traditional unipolar codes as spreading codes for light sources, this invention designs a simple unipolar code group that can meet the needs of a specified number of users. The pilot sub-codes and communication sub-codes within the code group are used as spreading codes for the pilot and communication light sources, respectively. At the receiving end, the advantages of each sub-code are fully utilized. The excellent autocorrelation characteristics of the pilot sub-codes provide synchronization for the communication light source. Simultaneously, a demodulation method designed using the characteristics between the sub-codes in the code group enables demodulation without cross-correlation interference for each user, thereby improving the communication performance of the multi-source VLC-CDMA system. Attached Figure Description

[0053] Figure 1 This is a flowchart illustrating the specific process of transmitting optical signals at the transmitting end of the visible light communication unipolar light source waveform design method of the present invention. In the diagram: 1 represents the transmission data of two pilot light sources; 2 represents the spreading code of the pilot light source; 3 represents the spreading operation of the pilot light source and the process of loading the signal onto the pilot light source to transmit the VLC-CDMA optical signal; 4 represents the transmission data of three communication light sources; 5 represents the spreading code of the communication light source; and 6 represents the process of spreading and modulating the transmission data of the communication light sources in a prescribed manner and loading the signal onto the communication light source to transmit the VLC-CDMA optical signal.

[0054] Figure 2This is a flowchart illustrating the receiving end portion of the visible light communication unipolar light source waveform design method of the present invention, which receives and demodulates the superimposed signal. In the figure: 1 is the unipolar superimposed gray value sequence after grayscale processing; 2 is the sequence to be demodulated by truncating the left and right ends of the sequence according to the start and end positions of demodulation; 3 is the process of demodulating the communication signal of one user to recover its transmitted data. Detailed Implementation

[0055] The present invention will be further described below with reference to specific embodiments.

[0056] Assuming that a multi-source VLC-CDMA system needs to provide data communication functions for 3 users, the unipolar light source waveform design method is divided into two parts: unipolar code group design and system structure design.

[0057] Part 1: Unipolar Code Design

[0058] Step 1: Determine the number of subcodes and the code length within the code group.

[0059] Since the number of users K in the VLC-CDMA system is 3, and the code length multiple M is 4, the number of subcodes N and the code length L in the code group are calculated to be 7 and 28 respectively by equation (1) and equation (2).

[0060] Step 2: Generation of subcodes within the code group

[0061] Based on the number of subcodes N and the subcode length L obtained in step one, construct a 7-row, 28-column matrix A consisting entirely of zeros. Let a denote the position in matrix A where the n-th row and l-th column undergoes a 0-1 transition. nl And a nl For the expression ∈{1,2,...,28}, the 01 is converted to change the value of 0 to the value of 1. Specifically:

[0062] First, pilot subcodes are generated. For the first row of all zeros in matrix A, a 0-1 transition is performed at position 1+7m, where m∈{0,1,2,3}, and the pilot subcode after the transition is denoted as s. p At this point, the pilot code contains four 1s and 24 0s.

[0063] Next, the communication subcode is generated. For the 6 rows (rows 2 to 7) of matrix A containing all zeros, the 0-1 conversion positions in rows 2 to 7 are generated according to the prescribed conversion criteria, including the conversion positions of the already generated pilot subcode. Therefore, the conversion positions in rows 1 to 7 of matrix A are (1,8,15,22), (2,3,4,5), (6,9,11,12), (7,10,13,16), (14,17,18,19), (20,21,23,24), and (25,26,27,28). The final matrix A generated from these conversion positions is:

[0064]

[0065] Finally, the matrix A after the 0-1 conversion is re-denoted as code group A, where the first row of code group A represents the pilot subcode s. p Lines 2 to 7 represent communication subcodes s1, s2, s3, s4, s5, s6, and the subcodes in code group A have the following characteristics: (1) Pilot subcode s p It has ideal autocorrelation characteristics, that is, its autocorrelation function has a value of 4 at zero shift and a value of 0 at other shifts; (2) pilot subcode s p It has excellent cross-correlation characteristics with each communication subcode, that is, the cross-correlation function between the pilot subcode and each communication subcode has a value of less than or equal to 1 at each shift. (3) The positions of 1 in each subcode in code group A are independent and there is no repetition.

[0066] Part Two: System Structure Design

[0067] The structural design of a multi-source VLC-CDMA system can be divided into two aspects: the transmitter and the receiver. The transmitter transmits VLC-CDMA signals using a pilot light source and a communication light source. The receiver uses a CMOS image sensor to receive the superimposed signals from the pilot and communication light sources, and demodulates the transmitted data from each user, enabling communication transmission between the three users. The specific steps of the system structural design are as follows.

[0068] Step 1: Determine the number of pilot light sources and communication light sources in the transmitting end.

[0069] Since the number of users K and the code length multiple M in code group A in the VLC-CDMA system are 3 and 4 respectively, according to equation (3), the number of pilot light sources P that need to be arranged at the transmitter is 2. Therefore, 2 pilot light sources need to be arranged in the system to realize the synchronous communication function of 3 users. At the same time, the number of communication light sources needs to be arranged is 3, that is, the data communication function of 3 users is realized through 3 communication light sources.

[0070] Step 2: Signal transmission from the pilot light source at the transmitting end

[0071] Let the data information transmitted by the two pilot light sources be the same and both be d. p , where d p ∈{1}, as shown in the appendix Figure 1 As shown at point 1. Based on the code group A generated in the first part, the pilot subcode s within it is used. p As the spreading code for all pilot light sources, as shown in the appendix Figure 1 As shown in section 2, and by using equation (4) to spread spectrum modulate the transmitted data with the spreading code, the unipolar baseband signal of a single pilot light source after spread spectrum modulation is T. p Then the baseband signal T p The OOK modulation is applied to each pilot light source separately, as shown in the attached figure. Figure 1 As shown in section 3. Then, according to equation (5), the VLC-CDMA superimposed signal transmitted by the two pilot light sources is y. P , where y P =A P1 T p +A P2 T p .

[0072] Step 3: Signal transmission from the communication light source in the transmitting end

[0073] Let the data information transmitted by the three communication light sources be d1, d2, d3, where d1, d2, d3 ∈ {0, 1}, as shown in the attached figure. Figure 1 As shown in section 4. Based on code group A generated in the first part, the six communication subcodes from row 2 to row 7 are divided into pairs, that is, (s1,s2), (s3,s4), and (s5,s6) are grouped into pairs S1, S2, and S3, respectively, and used as spreading codes for the three communication light sources, as shown in the attached figure. Figure 1 As shown in section 5. It is stipulated that the first spreading code in each group is responsible for spreading modulation when the data information is 1, and the second spreading code is responsible for spreading modulation when the data information is 0. Then, from equation (6), the baseband signals of the three communication light sources after spreading modulation are T1, T2, and T3, respectively. Then, the baseband signals T1, T2, and T3 are loaded onto each communication light source through OOK modulation, as shown in the attached figure. Figure 1 As shown in section 6. The VLC-CDMA superimposed signal transmitted by the three communication light sources is obtained from equation (7) as y. K , where y K =A K1 T1+A K2 T2+A K3 T3.

[0074] Step 4: Reception of superimposed optical signals at the receiving end

[0075] Using a CMOS image sensor as the receiver of the VLC-CDMA system, five VLC-CDMA signals (2+3) are received via a rolling shutter exposure mode. After grayscale processing of the image from the CMOS sensor, the unipolar superimposed grayscale value sequence of the five VLC-CDMA superimposed signals is obtained by equation (8) as y. G As attached Figure 2 As shown at point 1. Where, sequence y... G The sequence length is L G , and y G =y GP +y GK =G P1 T p +G P2 T p +G K1 T1+G K2 T2+G K3 T3.

[0076] Step 5: Determine the start and end points of demodulation.

[0077] According to the spreading code s of the pilot light source p And its code length L, the sequence y G The first 28 and last 28 bits are respectively related to s p Perform cross-correlation operations, then perform maximum value detection on the results of the two correlation operations, and record the positions a1 and a2 of the two maximum values ​​respectively, where a1 corresponds to the first 28 bits and s. p The maximum value after cross-correlation, a2 corresponds to the last 28 bits and s p The maximum value after cross-correlation. Based on equations (9) and (10), the superimposed grayscale value sequence y is obtained. G The start demodulation position a begin With end demodulation position a end a1 and L respectively G -(28-a2)-1, and the sequence y G The first to the (a1-1)th and the Lth G -(28-a2) to the Lth G Remove each bit individually to obtain the middle sequence y after removing the left and right ends. cut Its sequence length is L cut As attached Figure 2 As shown in point 2.

[0078] Step Six: Demodulate the transmitted signal of the communication light source

[0079] The sequence y obtained from step five cutFor demodulating the transmitted signals from the three communication light sources, the spreading codes (s1,s2), (s3,s4), and (s5,s6) in the spreading code groups S1, S2, and S3 corresponding to each light source are used to demodulate the sequence y. cut Perform a despreading operation to obtain the sequence when the data information is 1 and 0. and and and Perform subtraction operations on the sequences separately and determine the result: if the subtraction result is greater than 0, set the data information at the corresponding position to 1; if the subtraction result is less than 0, set the data information at the corresponding position to 0. Since the demodulation steps are the same for all users, the signal demodulation process for one user is as follows: Figure 2 As shown in section 3, the data information d1', d2', and d3' recovered from the three communication light sources are obtained, ultimately enabling data transmission between the three users in the multi-light source VLC-CDMA system without cross-correlation interference.

[0080] The reason for achieving cross-correlation-free communication transmission between users in this invention is as follows: First, based on the ideal autocorrelation characteristics of the pilot subcode in code group A and the excellent cross-correlation characteristics of the pilot subcode and the communication subcode, when the pilot subcode is used to perform cross-correlation operation on the superimposed signal of the pilot and the communication light source at the receiving end, the autocorrelation peak value of the pilot subcode is always significantly greater than the cross-correlation value between the pilot subcode and the communication subcode. Therefore, the start and end positions of demodulation can be quickly determined. Second, based on the characteristic that the positions of 1s among the subcodes in code group A are independent and non-repeating, when any subcode is despread with itself, the absolute value of the despreading result is equal to the code length multiple M. However, when any subcode is despread with other subcodes, the despreading result is 0. Therefore, when the received superimposed sequence is despread using the communication subcode, its despreading result will not be affected by other subcodes, thereby achieving cross-correlation-free demodulation between the signals of each communication user.

[0081] The embodiments described above are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A method for designing waveforms of a unipolar light source for visible light communication, characterized in that, The design method described above designs a unipolar code group. Utilizing the ideal autocorrelation characteristic of the pilot sub-codes within the unipolar code group, it uses them as spreading codes for the pilot light source. Taking advantage of the characteristics between the pilot sub-codes and communication sub-codes in the unipolar code group, the communication sub-codes are combined in pairs to serve as spreading codes for the communication light source. At the transmitting end, the pilot light source and communication light source are combined to jointly transmit multi-source VLC-CDMA optical signals. At the receiving end, a demodulation method is provided for the transmitted light source waveforms. This enables demodulation of communication signals from various users without cross-correlation interference at the receiving end, thereby meeting the requirement for simultaneous data transmission by multiple users under the current limitations of image sensor hardware and improving the communication performance of the multi-source VLC-CDMA system. Includes the following steps: Assuming that in a multi-source VLC-CDMA system it is necessary to provide If a user provides data communication functionality, the unipolar light source waveform design method is divided into two parts: the unipolar code group design part and the system structure design part. Part 1: Unipolar Code Design Step 1: Determine the number of subcodes and code length within the unipolar code group. Based on the number of users in the VLC-CDMA system The number of subcodes within a unipolar code group for: (1) Code length of each subcode within a unipolar code group for: (2) in, It must be a multiple of the code length of the subcode within the unipolar code group and must satisfy the following conditions: ; Step 2: Generation of subcodes within a unipolar code group Based on the number of subcodes obtained in step one Subcode length , build a OK A matrix of all zeros ; to matrix The Middle Line 1 The position where the column is converted from 0 to 1 is denoted as and The 01 is converted to change the 0 value to the 1 value; Step two in the first part specifically refers to: 2.1) First, generate pilot subcodes for the matrix. The first row of values ​​is all zeros. Perform a 0-1 transition at the position, where And record the converted pilot subcode as At this time, there are a total of pilot subcodes 1 and One zero; 2.2) Next, generate the communication sub-code, for the matrix The second line to the third line Total lines For rows with all zeros, the criteria for 0-1 conversion are as follows: (1) The total number of positions in each row matrix that undergo 0-1 conversion must be equal to the number of zeros in the row matrix. (2) The positions where 0 and 1 transformations are performed in each row matrix. No duplicates, meaning the matrix after transformation must be unique. The set of all transformation positions in the equation must be equal to (3) The difference between any two transformation positions in each row matrix cannot be equal to 1. Multiples of; after transformation according to the above conversion criteria The row matrix is ​​used as the communication sub-code, denoted as . ; 2.3) Finally, the matrix after 0-1 transformation... Rewritten as unipolar code groups It contains a total of 1 pilot subcode and One communication subcode, and simultaneously a unipolar code group The subcodes in the pilot code have the following characteristics: (1) The pilot subcode has ideal autocorrelation characteristics, that is, the autocorrelation function has only one impulse function at zero shift, and the function value at other shifts is 0; (2) The pilot subcode and each communication subcode have excellent cross-correlation characteristics, that is, the cross-correlation function between the pilot subcode and each communication subcode has a function value of less than or equal to 1 at each shift; (3) Unipolar code group The positions of 1 in each sub-code are independent and there is no repetition; Part Two: System Structure Design The structural design of a multi-source VLC-CDMA system includes two aspects: the transmitter and the receiver. The transmitter transmits VLC-CDMA signals through pilot and communication light sources. The receiver receives the superimposed signals from the pilot and communication light sources using a CMOS image sensor and demodulates the transmitted data from each user, thus realizing... Communication transmission for individual users; the specific steps in the system architecture design are as follows: Step 1: Determine the number of pilot light sources and communication light sources in the transmitting end. Based on the number of users in the VLC-CDMA system and the unipolar code group generated in the first part Multiples of code length The number of pilot light sources that need to be installed at the transmitting end for: (3) in, Indicates will The result is rounded down, that is, by A pilot light source is implemented Synchronous communication function for individual users; the number of communication light sources that need to be deployed simultaneously is... That is, through A communication light source is implemented Data communication functions for individual users; Step 2: Signal transmission from the pilot light source at the transmitting end make The data information transmitted by each pilot light source is the same and is all... ,in Based on the unipolar code group generated in the first part , and the pilot subcode therein Using the spreading code as the basis for all pilot light sources, and spreading the transmitted data information with the spreading code, the unipolar baseband signal of a single pilot light source after spreading modulation is: (4) Then the baseband signal The modulation is applied to each pilot light source separately through on-off keying (OOK) modulation. The VLC-CDMA superimposed signal transmitted by each pilot light source is: (5) in, Indicates the first The optical power amplitude value of the signal transmitted by the pilot light source; Step 3: Signal transmission from the communication light source in the transmitting end make The data information sent by each communication light source is ,in Based on the unipolar code group generated in the first part Take the second line to the third line. Total lines The communication subcodes are divided into pairs and grouped together. Grouped into pairs and respectively as The spreading codes for a communication light source are specified such that the first spreading code in each group is responsible for spreading modulation when the data information is 1, and the second spreading code is responsible for spreading modulation when the data information is 0. Then the... The baseband signal after the data information of the communication light source is spread spectrum modulated according to the above specifications is: (6) in, Then the baseband signal The signal is modulated by OOK and applied to each communication light source. The VLC-CDMA superimposed signal transmitted by each communication light source is: (7) in, Indicates the first The optical power amplitude value of the signal transmitted by the communication light source; Step 4: Reception of superimposed optical signals at the receiving end A CMOS image sensor is used as the receiver in the VLC-CDMA system, and the pilot light source and communication light source are received through a rolling shutter exposure mode. The signals superimposed by VLC-CDMA, after grayscale processing of the CMOS sensor's image, represent... The unipolar superimposed grayscale value sequence of a VLC-CDMA superimposed signal is as follows: Its sequence length is Its expression is: (8) in, express A sequence of unipolar superimposed gray values ​​of a pilot light source. express A sequence of unipolar superimposed gray values ​​of a communication light source. Indicates the first The grayscale value of the bright stripes on the image of a pilot light source. Indicates the first The size of the grayscale value of the bright stripes of each communication light source, and and All are related to the optical power amplitude value of the pilot light source Optical power amplitude value of communication light source The relationship is linear; Step 5: Determine the start and end points of demodulation. Based on the spreading code of the pilot light source and its code length , convert the sequence The former Position and after The positions are respectively with Perform cross-correlation calculations, detect the maximum value of the results after the two correlation calculations, and record the positions of the two maximum values. and ,in Corresponding to the front Position and The maximum value after cross-correlation operation. After corresponding Position and The maximum value after cross-correlation; then for the received superimposed grayscale value sequence. The position where demodulation begins for: (9) The position where demodulation ends for: (10) Based on the start and end demodulation positions, the unipolar superimposed grayscale value sequence is... The 1st to the 1st Position and First To the Remove each bit individually to obtain the middle sequence after removing the left and right ends. Its sequence length is ; Step Six: Demodulate the transmitted signal of the communication light source The sequence obtained from step five For the first Demodulation of signals transmitted by individual communication light sources, utilizing their corresponding spreading code groups spreading codes in and , respectively Perform despreading to obtain the demodulated sequence when the responsible data information is 1. and the demodulation sequence when the data information is 0 ,Will and Perform a subtraction operation on the sequences and then decide the result: if... If so, the data information at the corresponding position will be set to 1. If the data at the corresponding position is zero, then the result is obtained. Data information recovered by a communication light source All data were recovered using the demodulation method described above. Data information transmitted by a communication light source Ultimately, this will enable the multi-source VLC-CDMA system to achieve... Data from individual users is transmitted via communication without cross-correlation interference.

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