CN109450534B - Visible light wireless local area network based on image sensor - Google Patents

Visible light wireless local area network based on image sensor Download PDF

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CN109450534B
CN109450534B CN201811144132.6A CN201811144132A CN109450534B CN 109450534 B CN109450534 B CN 109450534B CN 201811144132 A CN201811144132 A CN 201811144132A CN 109450534 B CN109450534 B CN 109450534B
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access request
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CN109450534A (en
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迟学芬
王爽
钟苏华
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Jilin University
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    • 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
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    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N23/00—Cameras or camera modules comprising electronic image sensors; Control thereof
    • H04N23/70—Circuitry for compensating brightness variation in the scene
    • H04N23/73—Circuitry for compensating brightness variation in the scene by influencing the exposure time

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Abstract

The invention relates to a visible light wireless local area network based on an image sensor, wherein a plurality of terminals of the local area network are communicated with an AP; a terminal LED sends an access request and data to be transmitted to AP equipment, and a terminal light detector receives feedback information of the AP equipment; an AP end optical detector receives data transmitted by terminal equipment; the image sensor receives an access request transmitted by the terminal side and transmits acquired image information to the processor, and feedback information of the processor is sent to the terminal side through the AP side LED; the communication link for receiving the optical signal by the terminal photodetector and the AP end photodetector belongs to a VLC link, and the communication link for receiving the optical signal by the image sensor belongs to an OCC link; the frequency of sending access requests over the OCC link is much less than the frequency of feedback information and data transmitted over the VLC link. The invention alleviates the access collision problem of the wireless local area network, thereby effectively reducing the energy consumption and the access time delay of the terminal.

Description

Visible light wireless local area network based on image sensor
Technical Field
The invention belongs to the technical field of visible light communication, and relates to a visible light wireless local area network based on an image sensor.
Background
VLC becomes one of the concerned emerging communication technologies with the advantages of rich spectrum resources, green energy conservation, low cost, high data transmission rate and the like. Nowadays, the conventional wireless communication based on the radio frequency technology faces the problems of spectrum resource shortage and the like, and cannot meet the requirements of high data transmission rate, ultra wide bandwidth and the like of future terminals, and the unprecedented VLC can be just used as the effective supplement of the existing wireless communication to solve the problem of spectrum resource shortage.
The most commonly used photodetectors in visible light communications are PIN photodiodes, avalanche diodes, and the like. With the development of CMOS technology, CMOS image sensors are embedded in many electronic devices, such as mobile phones and monitoring devices. In order to realize the wide application of VLC, a special visible light communication technology-OCC comes from now. The OCC uses an image sensor as a light receiving device, and has a main advantage over VLC in that the image sensor has spatial resolution and can naturally support a Multiple Input Multiple Output (MIMO) technology, thereby easily realizing diversity and multiplexing and enhancing the effectiveness and reliability of a communication system.
In recent years, research on the theory of visible light communication has been around, and most of research results are concentrated on the physical layer, and the research results on the optical network link layer are few. However, with the exponential increase of the number of wireless terminals, a large number of terminals need to access a limited shared wireless channel, so bandwidth contention conflict becomes a bottleneck in the development of wireless communication systems. How to efficiently and fairly allocate limited bandwidth resources and alleviate bandwidth contention conflicts is a key problem to be solved by a Media Access Control (MAC) layer of a wireless communication system.
Disclosure of Invention
The invention aims to solve the technical problem of providing a visible light wireless local area network based on an image sensor, which can relieve bandwidth competition conflict and effectively reduce the energy consumption and the access time delay of a terminal.
In order to solve the technical problem, the visible light wireless local area network based on the image sensor of the invention is characterized in that a plurality of terminals communicate with an AP; on the terminal side, each terminal device is provided with a terminal LED and a terminal light detector; on the AP side, an AP end LED and an AP end light detector are arranged on the AP equipment; the terminal LED is used for sending an access request and data to be transmitted to the AP equipment, and the terminal optical detector is used for receiving feedback information of the AP equipment; the AP equipment is also provided with an image sensor, a processor and a controller; the AP end optical detector is used for receiving data transmitted by the terminal equipment; the image sensor receives an access request transmitted by a terminal side and transmits acquired image information to the processor, and the processor processes the image information to obtain feedback information; the feedback information is sent to the terminal side through the AP side LED 22; the controller controls the effective receiving time of the image sensor and the AP end light detector under the indication of the clock circuit; the communication link for receiving the optical signal by the terminal optical detector and the AP end optical detector belongs to a VLC link, and the communication link for receiving the optical signal by the image sensor belongs to an OCC link; the frequency of sending access requests over the OCC link is much less than the frequency of feedback information and data transmitted over the VLC link.
The invention realizes the multi-packet reception of the terminal access request by utilizing the spatial resolution of the image sensor, thereby relieving the access collision problem of the wireless local area network and effectively reducing the energy consumption and the access time delay of the terminal.
Further, for any terminal i, the distance between the terminal i and the AP cannot exceed a limit value Umax；
Figure BDA0001816427270000021
In the formula (10), f is the focal length of the image sensor, L is the diameter of the terminal LED, and f1iSending access request frequency for a terminal i, wherein n is the number of pixel lines which can be exposed by an image sensor each time, q is the bit number contained in an access request frame, rho is the effective pixel density of an image acquired by the image sensor, and feIs the image sensor exposure frequency.
The distance between the terminal and the AP does not exceed a limit value UmaxAnd the access request frame transmitted by each terminal can be completely received by the AP.
Each terminal reflects the priority of the terminal by the frequency of the access request, and the smaller the frequency of the access request is, the higher the priority of the terminal is; on the AP side, the processor obtains the priority of each terminal according to the unit stripe width W of each light spot in the light spot image collected by the image sensor.
The processor plans a reasonable access probability for each terminal through an optimization algorithm according to the priority information of each terminal, the distance between each terminal and the AP and the communication light source incidence angle of each terminal, and sends the reasonable access probability serving as feedback information to the terminal side through an AP side LED; the distance U between the terminal and the AP and the light source incidence angle psi of the terminal LED are as follows:
Figure BDA0001816427270000031
Figure BDA0001816427270000032
wherein f is the focal length of the image sensor, L is the diameter of the terminal LED, rho is the effective pixel density of the image collected by the image sensor, L 'is the number of pixels occupied by the diameter of the light spot imaged by the terminal LED, and s' is the distance from the center of the light spot imaged by the terminal LED to the center of the whole image.
The image sensor preferably selects a panoramic image sensor, and receives an access request by adopting rolling shutter exposure. The terminal priority is embodied by using the rolling effect of the image sensor and different signal transmission frequencies, so that the access request frame does not need to contain a priority field, and the communication system overhead is saved.
Furthermore, at the terminal side, the information sending equipment consists of a clock circuit, a frequency adjusting device, an encoder, a modulator, a driving circuit, a switching circuit and a terminal LED; the demodulator, decoder and terminal light detector constitute information receiving equipment; the frequency adjusting device processes a clock signal output by the clock circuit to obtain a pulse signal corresponding to a sending access request frequency and a sending data frequency; an access request sent by a user or data to be transmitted is subjected to Manchester coding through a coder, the coded information is subjected to on-off keying modulation through a modulator, and then the information is transmitted through the on-off state of the LED12 of the drive circuit and the on-off circuit control terminal; the pulse signal output by the frequency adjusting device is used for controlling the switch circuit to be switched on and off according to the frequency corresponding to the access request and the sending data, so that the on-off state of the terminal LED12 is controlled to transmit the access request and the sending data; the terminal optical detector receives the optical signal fed back by the AP side and carries out optical/electrical conversion on the optical signal, and the electrical signal is decoded into original feedback information by a demodulator and a decoder;
at the AP side, an encoder, a modulator, a switch circuit, a drive circuit and an AP end LED form information sending equipment; the AP end optical detector, the demodulator and the decoder form a receiving device; the AP end optical detector receives data transmitted by the terminal end, and then the demodulator and the decoder analyze the data; the encoder and the modulator carry out Manchester encoding and OOK modulation on the feedback information, the on-off state of the AP side LED is controlled through the switching circuit and the driving circuit to transmit a feedback signal, and the feedback signal is fed back to the terminal light detector through the VLC link.
At the AP side, the processor plans the access probability of the non-highest priority terminal to be 0, calculates the access probability of the highest priority terminal, and then sends the access probability obtained by planning to each terminal side through the information sending equipment; the access probability calculation method of the terminal with the highest priority is as follows:
firstly, calculating the optical power P received by the AP end optical detector according to the formulas (11) and (12)or
Por＝H(0)×Pot (11)
Figure BDA0001816427270000041
Where H (0) is the channel DC gain, PotFor the average emitted optical power of the terminal LED, a represents the physical receiving area of the AP-side photodetector, m represents the lambertian radiation order, U represents the distance between the terminal LED and the AP-side photodetector,
Figure BDA0001816427270000044
indicating the source radiation angle of the terminal LED, # indicating the source incidence angle of the terminal LED, TsDenotes the AP-side photodetector filter gain, g (-) denotes the gain of the AP-side photodetector optical concentrator, ψFOVRepresenting the field angle of the AP-end photodetector;
secondly, calculating the noise power N of the VLC system according to the formulas (13), (14) and (15)totalIn which N isshotRepresenting the power of shot noise, NthermaltRepresenting the thermal noise power.
Ntotal＝Nshot+Nthermal (13)
Nshot＝2qeRePorBN+2qeIbgI2BN (14)
Figure BDA0001816427270000042
The following parameters in equations (13), (14), and (15) are all intrinsic parameters of the AP-side photodetector, where q iseIs the charge quantity of electrons, ReIndicates the photoelectric conversion efficiency of the photodetector, BNRepresenting the equivalent noise bandwidth, IbgRepresents the background current, I2And I3Is the noise bandwidth factor, k is the Boltzmann constant, TkRepresenting absolute temperature, G representing open-loop voltage gain, η being the fixed capacitance per unit area of the photodetector, GmRepresenting the transconductance of the field effect transistor, and gamma representing the channel noise factor of the field effect transistor;
thirdly, calculating the signal-to-noise ratio gamma corresponding to each terminal on the AP side according to a formula (16):
Figure BDA0001816427270000043
fourthly, calculating the throughput of the terminal i according to the formulas (17), (18) and (19):
Ri＝Blog2(1+γi) (17)
Figure BDA0001816427270000051
Figure BDA0001816427270000052
Rifor the AP to the service rate of terminal i, B is the channel bandwidth, γiSignal-to-noise ratio gamma for terminal i on AP sidei，pi sFor access success probability, piIs the access probability of terminal i, N is the number of terminals, SiIs the throughput of terminal i;
fifthly, calculating the fairness FI among the terminal devices according to a formula (20);
Figure BDA0001816427270000053
and sixthly, obtaining the access probability of the highest terminal of each priority through iterative update search of the particle swarm algorithm, namely obtaining the corresponding access probability of the highest terminal of each priority when the fairness FI is the maximum.
Figure BDA0001816427270000054
Wherein p ═ p1,p2,…,pi,…,pN-1,pN]T。
The invention has the advantages that:
1. the invention realizes the multi-packet reception of the terminal access request by utilizing the spatial resolution of the image sensor, thereby relieving the access collision problem of the wireless local area network and effectively reducing the energy consumption and the access time delay of the terminal.
2. The invention utilizes the rolling effect of the image sensor to reflect the priority of the terminal by different signal sending frequencies, thereby leading the access request frame not to contain the priority field and saving the expense of a communication system.
3. The invention reasonably plans the access probability for each terminal by using the access probability optimization algorithm, thereby realizing the fair access of the terminals with the same priority.
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The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
Fig. 1 is a schematic diagram of a visible light wireless local area network based on an image sensor.
Fig. 2 is a block diagram of a terminal device configuration.
Fig. 3 is a block diagram of an AP device configuration.
Fig. 4 is an access mechanism workflow diagram.
Fig. 5 is a diagram of a slot structure.
Fig. 6 is a schematic diagram of global exposure.
Fig. 7 is a schematic view of a rolling shutter exposure.
Fig. 8 is a diagram of an access request frame format.
Fig. 9 is a schematic diagram of light spots collected by the image sensor in the rolling shutter exposure operation mode.
Fig. 10 is an imaging schematic diagram of an image sensor.
Fig. 11 is a simulation diagram of system fairness performance of a novel media access control mechanism of a visible light wireless local area network based on an image sensor.
Detailed Description
As shown in fig. 1, in the visible light wireless local area network based on the image sensor of the present invention, a plurality of terminals communicate with an AP; each terminal device 1 is provided with a terminal LED12 and a terminal photodetector 11, and the AP device 2 is provided with an AP-side LED22, an AP-side photodetector 21 and an image sensor 23; the terminal LED12 is configured to send an access request and data to be transmitted to the AP device 2, and the terminal optical detector 11 is configured to receive information fed back by the AP device 2; the AP side LED22 is used to send feedback information to each terminal device 1, the image sensor is used to receive an access request from the terminal device 1, and the AP side optical detector is used to receive data transmitted by the terminal device 1.
Among them, the communication link of the light signal received by the terminal photodetector 11 and the AP-side photodetector 21 belongs to the VLC link, and the communication link of the light signal received by the image sensor 23 belongs to the OCC link. The image sensor is preferably a panoramic image sensor, and the 360-degree field angle can multiply the coverage range of the network.
As shown in fig. 2, in the existing terminal device 1, an information transmitting device is composed of a clock circuit, an encoder, a modulator, a driving circuit, a switching circuit, and a terminal LED 12; the demodulator, decoder, terminal light detector 11 make up the information receiving equipment; the invention adds a frequency adjusting device and an access probability adjusting device in the information sending equipment, and realizes the two functions by embedding corresponding program codes in the inherent processor of the terminal.
As shown in fig. 3, in the existing AP device 2, an information transmitting device is composed of an encoder, a modulator, a switching circuit, a driving circuit, and an AP-side LED 22; and the AP end light detector 21, the demodulator and the decoder. The invention adds image sensor, processor, controller and clock circuit in the information sending device; wherein, the processor and the controller are both AP inherent devices.
On the terminal side, for the information sending device, the access request or the data to be transmitted is firstly subjected to manchester encoding through an encoder, the encoded information is subjected to on-off keying (OOK) modulation through a modulator, and then the on-off state of the terminal LED12 is controlled through a driving circuit and a switching circuit to transmit the information. The access request of the terminal device 1 is transmitted through the OCC link, the data to be transmitted by the terminal device 1 is transmitted through the VLC link, and in consideration of the difference of the sampling frequencies of the light receiving devices (image sensor and photodetector) of the two links, the frequency adjusting device is required to generate pulse signals with different frequencies to control the switch circuit, so that two types of information with different frequencies, namely the access request and the data, are sent. For the information receiving apparatus, the optical signal of the AP side LED22 is received by the terminal optical detector 11 and is subjected to optical/electrical conversion, and the electrical signal is demodulated by a demodulator and a decoder to obtain the original feedback information.
On the AP side, for the information receiving apparatus, the controller controls the effective reception time of the image sensor and the AP-side photodetector 21 under the instruction of the clock circuit. The image sensor receives an access request sent by the terminal LED12, and transmits acquired image information to the processor, and the processor delivers a processing result as feedback information to the information sending equipment. For the information sending equipment, the encoder and the modulator carry out Manchester encoding and OOK modulation on the feedback information, and the feedback information is fed back to each terminal equipment through the switching circuit, the driving circuit, the AP end LED and the VLC link; the AP-side optical detector 21 receives the data transmitted by the terminal LED12, and then the demodulator and the decoder analyze the data.
The reason why the encoding method is manchester encoding is adopted is that, first, the cutoff frequency f of human eyescGreater than 100Hz, if the frequency of the LED transmission information is less than fcThe human eye can perceive the flickering of the LED. Manchester encoding makes the maximum length of continuous same logic bit not more than 2, if this encoding mode is adopted, the length of continuous same logic bit is not more than 2In theory, as long as the frequency of the transmitted information exceeds 200Hz, visible light communication which is not perceived by human eyes can be realized. Second, error detection can be achieved using the invalid encoding types (00 and 11) of Manchester encoding without increasing the encoding overhead.
A novel media access control mechanism is adopted in a visible light wireless local area network based on an image sensor, the working process is shown in figure 4, and firstly, a terminal i takes a frequency f1iSending an access request; the AP processes the access request received by the image sensor using the processor and then uses the frequency f2Sending feedback information of the access request; the terminal i adjusts the self access probability p according to the feedback information of the APiThen at a frequency f2According to probability piAnd accessing the data to the network. Taking into account the difference in achievable sampling frequency between the photodetector and the image sensor, f is typically the case2>>f1i. The time slot structure divided according to the above process is shown in fig. 5, and the clock circuit at the terminal side will precisely control the working time of each stage of the system according to the above time slot structure.
The work flow of the image sensor is mainly divided into two steps of exposure and data reading, wherein the exposure is divided into two modes of global exposure and rolling shutter exposure. The principle of global exposure is shown in fig. 6, all pixels of an image are exposed at the same time (the exposure time is very short and can be approximated to a time), so that an image exposed in this way can only record one state of the photographed object at the exposure time. The principle of rolling shutter exposure is shown in fig. 7, in which the pixels of an image are exposed line by line, i.e. the exposure time of the pixels of different lines is different, so that an image exposed in this way can record a plurality of states of the object at different exposure times. Obviously, the time-sharing exposure feature using the rolling shutter exposure can double the system capacity of the OCC.
At terminal i, the user selects the frequency f1iSending the access request, the frame format of the access request is shown in fig. 8, the access request frame is composed of q bits, wherein the first five bits are frame synchronization codes for positioning the head of the access request frame during demodulation, 11110 is convenient to distinguish with the address field which is Manchester coded thereafterAnd (4) dividing. The address field is a destination address for providing feedback information to the AP, and an address of each terminal is unique. The end flag 0 separates the address field of the current frame from the frame synchronization code field of the next frame.
If the AP side image sensor adopts rolling shutter exposure to receive the access request, the image sensor can expose n rows of pixels each time, and the exposure frequency is fe. Then, the schematic diagram of the image sensor collecting light spots of each terminal LED12 is shown in fig. 9, where a black light spot represents that the terminal to which this terminal LED12 belongs does not send an access request, that is, the terminal does not participate in contention during this access process; the light spots alternating with black and white stripes represent the access requests sent by the terminal to which the terminal LED12 belongs, each terminal having a different transmission frequency f1iWhen the access request is sent, the unit stripe widths W of the light spots are different from each other, and the expression of W is shown as (1).
Figure BDA0001816427270000081
If the terminals are classified into different priorities according to the delay requirement, each terminal can transmit the frequency f of the access request1iTo express its priority, f1iThe smaller the terminal priority. The AP side can know the priority of each terminal through the unit stripe width W of each light spot in the light spot image collected by the image sensor (the wider W is, the higher the priority of the corresponding terminal is), and uniformly sends the priority information to the processor. (in fig. 9, if the terminal sends 1010 … … information, the light spot has the same width of strip, i.e. one black, one white, two black if we send random information 1001011 … …). According to the light spot image received by the image sensor, the number of pixels l' occupied by each light spot diameter can be known, and the formula (2) ensures that the access request frame transmitted by each terminal can be completely received by the AP. The unit of the spot diameter can be converted from pixel to micron by equation (3), where the unit is μm/pixel.
Figure BDA0001816427270000091
l＝ρ×l' (3)
q is the number of bits contained in an access request frame.
An imaging schematic diagram of the image sensor is shown in fig. 10, where O is a lens center, F is a focal point, F is a focal length, L is a diameter of a terminal LED, L is a spot diameter imaged by the terminal LED, U is an object distance, and U is an image distance. The relationship among the object distance, the image distance and the focal length is shown as formula (4), formula (5) can be obtained according to the similar relationship between delta ABO and delta CDO, and the expression of U can be obtained by bringing formula (3) and formula (5) into formula (4) as formula (6). The diameter L of the terminal LED is known, the diameter L' of the light spot imaged by the terminal LED is also known according to the light spot image received by the image sensor, and the distance U between the terminal and the AP can be calculated by the formula (6).
Figure BDA0001816427270000092
Figure BDA0001816427270000093
Figure BDA0001816427270000094
If the diameters of the LEDs of the terminals are the same, in order to ensure that the access request frame transmitted by each terminal can be completely received by the AP, the distance between the terminal i and the AP cannot exceed a limit value UmaxU can be obtained by substituting the formula (1) or (6) for the formula (2)maxExpression (7) of (1).
Figure BDA0001816427270000095
According to the light spot images received by the image sensor, the distance S' from the center of each light spot to the center of the whole image can be known, and an equation (8) can be obtained by analogy to equation (5), wherein S is the horizontal distance between the terminal LED and the image sensor. The expression (9) of S can be obtained by substituting the formula (8) for the formula (4).
Figure BDA0001816427270000101
Figure BDA0001816427270000102
In this case, the light source incidence angle ψ of each terminal LED can be obtained from equation (10) based on the distance U and the horizontal distance S between the terminal LED and the image sensor.
Figure BDA0001816427270000103
Assuming that the average emitted light power of the terminal LED of each terminal is PotThe AP receives the optical power P through the AP end optical detectororIs shown in (11), where H (0) is the channel DC gain. The expression of H (0) is shown in (12), A represents the physical receiving area of the AP-side photodetector, U represents the distance between the terminal LED and the AP-side photodetector,
Figure BDA0001816427270000105
indicating the source radiation angle of the terminal LED, # indicating the source incidence angle of the terminal LED, #FOVDenotes the angle of view of the photodetector, m denotes the Lambert radiation order, Ts(. cndot.) represents the filter gain of the AP-side photodetector, and g (. cndot.) represents the gain of the optical concentrator of the AP-side photodetector.
Por＝H(0)×Pot (11)
Figure BDA0001816427270000104
The noise of the photodetector in the VLC system mainly includes shot noise and thermal noise. Due to, among other things, the discrete nature of the carriersThe noise generated by the resulting current fluctuations is shot noise, while the noise generated by the jitter due to the thermal interaction of free electrons in the conductor medium with the vibrating ions is thermal noise. Both of them are modeled here as white gaussian noise and are uncorrelated, from which the noise power N of the VLC system can be derivedtotalIs shown in (13), wherein the shot noise power NshotAnd thermal noise power NthermaltThe expressions (2) are shown in (14) and (15), respectively. q. q.seIs the charge quantity of electrons, ReIndicates the photoelectric conversion efficiency of the photodetector, BNRepresenting the equivalent noise bandwidth, IbgRepresents the background current, I2And I3Is the noise bandwidth factor, k is the Boltzmann constant, TkRepresenting absolute temperature, G representing open-loop voltage gain, η being the fixed capacitance per unit area of the photodetector, GmThe transconductance of the field effect transistor is represented, and the gamma represents the channel noise factor of the field effect transistor. R is as defined abovee、BN、Ibg、I2、I3、G、η、gmAnd gamma is the inherent parameter of the AP end optical detector.
Ntotal＝Nshot+Nthermal (13)
Nshot＝2qeRePorBN+2qeIbgI2BN (14)
Figure BDA0001816427270000111
In summary, the expression of the snr γ corresponding to each terminal on the AP side is shown in (16).
Figure BDA0001816427270000112
Service rate R of AP to terminal iiIs shown in (17), where B is the channel bandwidth. From equation (17), the signal-to-noise ratio γ of each terminal on the AP side can be knowniThe larger, the service rate R that the AP is servingiThe larger.For the terminal i, successful access in one access process means that only the terminal i is accessed independently on the premise that other terminals are not accessed, and the access success probability p of the terminal i isi sIs shown in (18). The expression of the throughput of terminal i thus obtained is shown in (19).
Ri＝Blog2(1+γi) (17)
Figure BDA0001816427270000113
Figure BDA0001816427270000114
The distance U between the terminal LED of each terminal and the AP-side photodetector, and the light source incident angle ψ of the terminal LED are generally different from each other. From the above derivation, the access probability p of each terminaliMeanwhile, the farther away from the AP, the larger the light source incidence angle, the smaller the throughput of the terminal, and such an access mechanism is obviously unfair for the terminal. Therefore, an access probability p for planning each terminal is needed at the AP sideiTo improve the fairness of the system.
To illustrate the fairness problem of the system more clearly, Jain' S Fair Index (FI) is used to describe the fairness among the terminal devices, and the expression of FI is shown as (20), where N is the number of terminals and S isiIs the throughput of terminal i. FI is a value of 0-1, and as fairness increases, the value of FI increases.
Figure BDA0001816427270000115
On the basis of the equation (20), we will establish an optimization problem with system fairness as the optimization target as shown in equation (21). And obtaining the access probability of the terminal with the highest priority through iterative update search of the particle swarm algorithm, namely the access probability of each corresponding terminal with the highest priority when the fairness FI is the maximum. In fig. 11, the solid line indicates the fairness that can be achieved by using the access mechanism system proposed in the present invention on the premise that all terminals have the same priority; the dotted line represents the fairness that can be achieved by the system when all terminals access the network with probability 1/N under the same premise, and fig. 11 shows that the access mechanism proposed by the present invention is a more fair access method.
Figure BDA0001816427270000121
Wherein p ═ p1,p2,…,pi,…,pN-1,pN]T。
The invention provides a novel media access control mechanism to relieve the access collision problem of a wireless local area network, thereby effectively reducing the energy consumption and the access time delay of a terminal. In addition, the terminal access probability optimization algorithm in the mechanism can improve the system fairness and realize the fair access of the terminals with the same priority. The novel media access control mechanism of the visible light wireless local area network based on the image sensor mainly comprises:
(1) each terminal sends an access request frame to the AP through the OCC link at a certain frequency, different time delay requirements of the terminals correspond to different sending priorities, and different priorities correspond to different access request sending frequencies.
(2) And the image sensor at the AP side receives an access request of the terminal by adopting a rolling exposure working mode and then transmits the acquired light spot image to the processor.
(3) The AP side processor firstly resolves the priority of each terminal according to the unit stripe width of each light spot frame synchronization field in the image and resolves the address information of the terminal through the address field. And then estimating the distance between the terminal and the AP and the light source incidence angle of the LED of the terminal according to the spot size and the position of the highest priority terminal. And finally, the access probability of the non-highest priority terminal is planned to be 0, and the access probability of the highest priority terminal is obtained by establishing an optimization problem which takes the optimal system fairness as a target and searching by adopting a particle swarm algorithm.
(4) The AP sends feedback information of the access request to each terminal through the VLC link, namely, the result of the access probability planning algorithm is fed back to each terminal.
(5) Each terminal adjusts the self access probability p according to the information fed back by the APiThen according to the probability piThe network is accessed through a VLC link.
The invention designs an access request frame suitable for the media access control mechanism. The frame consists of q bits, wherein the first five bits are the frame synchronization code used for positioning the head of the access request frame during demodulation, and the 11110 form is convenient to distinguish from the address field which is Manchester encoded thereafter. The address field is a destination address for providing feedback information to the AP, and an address of each terminal is unique. The end flag 0 separates the address field of the current frame from the frame synchronization code field of the next frame.
The invention provides a scheme for embodying the priority of each terminal by using the rolling effect of an image sensor and different information sending frequencies. The scheme divides the terminals into different priorities according to the time delay requirement, and each terminal can transmit the frequency f of the access request1iTo express its priority, f1iThe smaller the terminal priority. The AP side adopts the image sensor of the roller shutter exposure to receive the access request, the image sensor can expose n rows of pixels each time, the exposure frequency is feAnd the AP can know the priority of each terminal according to the unit stripe width of each light spot in the light spot image acquired by image sensing. When the diameter of each terminal LED is the same, in order to ensure that the access request frame transmitted by each terminal can be completely received by the AP, the distance between the terminal and the AP can not exceed a limit value UmaxThe expression is shown as formula (10), where f is the focal length of the image sensor, ρ is the effective pixel density of the image collected by the image sensor, q is the number of bits contained in the access request frame, L is the diameter of the terminal LED, and f1iThe frequency of transmission of the request is accessed for terminal i.
Figure BDA0001816427270000131
The invention provides a method for calculating the distance between a terminal and an AP (access point) and the incident angle of an LED (light emitting diode) light source of the terminal based on the size and position information of each light spot in a light spot image acquired by an image sensor. The diameter L of the terminal LED is known. According to the light spot image received by the image sensor, the light spot diameter l 'of the terminal LED image and the distance s' from the center of each light spot to the center of the whole image are also known. According to the imaging principle of the image sensor and the triangle similarity theorem, formulas for calculating the distance U between the terminal and the AP and the incident angle psi of the LED light source of the terminal can be deduced as shown in (6) and (22).
Figure BDA0001816427270000132
Figure BDA0001816427270000133
The invention provides a terminal access probability optimization algorithm, which realizes fair access of terminals with the same priority level on the premise of considering terminal time delay requirements. The algorithm firstly sets the access probability of the terminal with the non-highest priority to 0 according to the priority information of each terminal, and then the access probability of the terminal with the highest priority is obtained by establishing an optimization problem with the maximum Jain's Fair Index (FI) as a target and searching by adopting a particle swarm algorithm.

Claims (6)

1. A visible light wireless local area network based on an image sensor is characterized in that a plurality of terminals are communicated with an AP; on the terminal side, each terminal device (1) is provided with a terminal LED (12) and a terminal light detector (11); on the AP side, an AP end LED (22) and an AP end light detector (21) are arranged on the AP equipment (2); the terminal LED (12) is used for sending an access request and data to be transmitted to the AP equipment (2), and the terminal optical detector (11) is used for receiving feedback information of the AP equipment (2); the AP equipment is characterized in that an image sensor (23), a processor and a controller are also arranged on the AP equipment (2); the AP end optical detector is used for receiving data transmitted by the terminal equipment (1); image sensor receiving terminalThe access request transmitted by the end side transmits the acquired image information to the processor, and the processor processes the image information to obtain feedback information; the feedback information is sent to the terminal side through an AP side LED (22); the controller controls the effective receiving time of the image sensor and the AP end light detector under the indication of the clock circuit; wherein, the communication link of the light signal received by the terminal photodetector (11) and the AP end photodetector (21) belongs to a VLC link, and the communication link of the light signal received by the image sensor (23) belongs to an OCC link; the frequency of sending the access request through the OCC link is far less than the frequency of the feedback information and data transmitted through the VLC link; for any terminal i, the distance between it and the AP cannot exceed a limit value Umax；
Figure FDA0002891262790000011
In the formula (10), f is the focal length of the image sensor, L is the diameter of the terminal LED, and f1iSending access request frequency for a terminal i, wherein n is the number of pixel lines which can be exposed by an image sensor each time, q is the bit number contained in an access request frame, rho is the effective pixel density of an image acquired by the image sensor, and feIs the image sensor exposure frequency.
2. The visible light wireless local area network based on the image sensor as claimed in claim 1, wherein each terminal represents the priority by the frequency of sending the access request, and the lower the frequency of sending the access request is, the higher the priority of the terminal is; on the AP side, the processor obtains the priority of each terminal according to the unit stripe width W of each light spot in the light spot image collected by the image sensor.
3. The visible light wireless local area network based on the image sensor as claimed in claim 2, wherein the processor plans a reasonable access probability for each terminal through an optimization algorithm according to the priority information of each terminal, the distance between each terminal and the AP, and the light source incidence angle of each terminal, and sends the reasonable access probability as feedback information to the terminal side through an AP side LED; the distance U between the terminal and the AP and the light source incidence angle psi of the terminal LED are as follows:
Figure FDA0002891262790000021
Figure FDA0002891262790000022
wherein f is the focal length of the image sensor, L is the diameter of the terminal LED, rho is the effective pixel density of the image collected by the image sensor, L 'is the number of pixels occupied by the diameter of the light spot imaged by the terminal LED, and s' is the distance from the center of the light spot imaged by the terminal LED to the center of the whole image.
4. The image sensor-based visible light wireless local area network of claim 1, wherein the image sensor is preferably a panoramic image sensor, and the access request is received by using rolling shutter exposure.
5. The visible light wireless local area network based on the image sensor according to claim 1, further characterized in that, on the terminal side, the information sending device is composed of a clock circuit, a frequency adjusting device, an encoder, a modulator, a driving circuit, a switch circuit, and a terminal LED; the demodulator, decoder and terminal light detector constitute information receiving equipment; the frequency adjusting device processes a clock signal output by the clock circuit to obtain a pulse signal corresponding to a sending access request frequency and a sending data frequency; an access request sent by a user or data to be transmitted is subjected to Manchester coding through a coder, the coded information is subjected to on-off keying modulation through a modulator, and then the information is transmitted through the on-off state of the LED12 of the drive circuit and the on-off circuit control terminal; the pulse signal output by the frequency adjusting device is used for controlling the switch circuit to be switched on and off according to the frequency corresponding to the access request and the sending data, so that the on-off state of the terminal LED12 is controlled to transmit the access request and the sending data; the terminal optical detector receives the optical signal fed back by the AP side and carries out optical/electrical conversion on the optical signal, and the electrical signal is decoded into original feedback information by a demodulator and a decoder;
at the AP side, an encoder, a modulator, a switch circuit, a drive circuit and an AP end LED form information sending equipment; the AP end optical detector, the demodulator and the decoder form a receiving device; the AP end optical detector receives data transmitted by the terminal end, and then the demodulator and the decoder analyze the data; the encoder and the modulator carry out Manchester encoding and OOK modulation on the feedback information, the on-off state of the AP side LED is controlled through the switching circuit and the driving circuit to transmit a feedback signal, and the feedback signal is fed back to the terminal light detector through the VLC link.
6. The visible light wireless local area network based on the image sensor as claimed in claim 1, wherein at the AP side, the processor plans the access probability of the non-highest priority terminal to 0, calculates the access probability of the highest priority terminal, and then sends the access probability obtained by the planning to each terminal side through the information sending device; the access probability calculation method of the terminal with the highest priority is as follows:
firstly, calculating the optical power P received by the AP end optical detector according to the formulas (11) and (12)or
Por＝H(0)×Pot (11)
Figure FDA0002891262790000031
Where H (0) is the channel DC gain, PotFor the average emitted optical power of the terminal LED, a represents the physical receiving area of the AP-side photodetector, m represents the lambertian radiation order, U represents the distance between the terminal LED and the AP-side photodetector,
Figure FDA0002891262790000032
indicating the source radiation angle of the terminal LED, phiLight source incident angle, T, of terminal LEDsDenotes the AP-side photodetector filter gain, g (-) denotes the gain of the AP-side photodetector optical concentrator, ψFOVRepresenting the field angle of the AP-end photodetector;
secondly, calculating the noise power N of the VLC system according to the formulas (13), (14) and (15)totalIn which N isshotRepresenting the power of shot noise, NthermaltRepresenting the thermal noise power;
Ntotal＝Nshot+Nthermal (13)
Nshot＝2qeRePorBN+2qeIbgI2BN (14)
Figure FDA0002891262790000033
the following parameters in equations (13), (14), and (15) are all intrinsic parameters of the AP-side photodetector, where q iseIs the charge quantity of electrons, ReIndicates the photoelectric conversion efficiency of the photodetector, BNRepresenting the equivalent noise bandwidth, IbgRepresents the background current, I2And I3Is the noise bandwidth factor, k is the Boltzmann constant, TkRepresenting absolute temperature, G representing open-loop voltage gain, η being the fixed capacitance per unit area of the photodetector, GmRepresenting the transconductance of the field effect transistor, and gamma representing the channel noise factor of the field effect transistor;
thirdly, calculating the signal-to-noise ratio gamma corresponding to each terminal on the AP side according to a formula (16):
Figure FDA0002891262790000034
fourthly, calculating the throughput of the terminal i according to the formulas (17), (18) and (19):
Ri＝Blog2(1+γi) (17)
Figure FDA0002891262790000041
Figure FDA0002891262790000042
Rifor the AP to the service rate of terminal i, B is the channel bandwidth, γiSignal-to-noise ratio gamma for terminal i on AP sidei，pi sFor access success probability, piIs the access probability of terminal i, N is the number of terminals, SiIs the throughput of terminal i;
fifthly, calculating the fairness FI among the terminal devices according to a formula (20);
Figure FDA0002891262790000043
sixthly, obtaining the access probability of the highest terminal of each priority through iterative update search of a particle swarm algorithm, namely obtaining the corresponding access probability of the highest terminal of each priority when the fairness FI is the maximum;
Figure FDA0002891262790000044
wherein p ═ p1,p2,…,pi,…,pN-1,pN]T。
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