CN104768232A - A method, device and system for realizing VLC dynamic access - Google Patents

Abstract

The invention discloses a method, device and system for realizing VLC dynamic access. A user node generates and sends a reservation request frame for reserving an uplink channel; after the reservation is successful, the user node sends data in the reserved time slot.

Description

A method, device and system for realizing VLC dynamic access

technical field

The present invention relates to the field of optical communication, in particular to a method, device and system for realizing dynamic access of visible light communication (VLC).

Background technique

In recent years, light-emitting diodes (LEDs), known as "green lighting", have developed rapidly. LEDs have the advantages of low power consumption, long life, green environmental protection, good modulation performance, and high response sensitivity. At present, while using the LED as the lighting source, the signal can also be modulated onto the visible light beam of the LED for transmission to realize VLC.

According to the environment, VLC can be divided into two types: indoor application and outdoor application. Outdoor applications mainly include: LED intelligent transportation systems, mines, underwater communication systems, etc. Indoor applications use white LED lighting equipment instead of wireless LAN base stations. Communication can only be performed within the range of LED lights, forming a visible-light communication personal area network (VPAN). When the user exceeds the light coverage, it will be terminated. Communications are highly confidential. In addition, because VLC completely avoids the electromagnetic interference of radio frequency access, it is also very suitable for radio frequency sensitive fields such as airplanes and hospitals. The communication speed of VLC can reach tens to hundreds of megabytes per second, coupled with the limited indoor transmission distance (<10m), and the small optical pulse delay, it can provide high-speed data transmission capabilities. Therefore, VLC provides a new high-speed data access method for indoor communication, and has great development prospects.

The indoor VLC system belongs to the star network topology. The VLC access point sends data to each user node by broadcasting downlink, and each user node is connected to the VLC access point through a point-to-point optical link. The characteristic of this topology is that it is easy to add new nodes in the network. The security and priority of data are easy to control, and it is easy to realize network monitoring. When multiple users have data transmission requests, in order to make better use of channel resources, it is necessary to reasonably allocate uplink channels, and at this time, it is necessary to apply Multi-Access (MA) technology.

At present, the multiple access modes stipulated in the MAC protocol are divided into: fixed (resource) access, dynamic (resource) access and hybrid access. Fixed access methods include time division multiple access (TDMA), frequency division multiple access (FDMA) and code division multiple access (CDMA), which can provide a reliable and continuous network, but when the business is bursty or there is no packet transmission for a long time In this case, channel resources will be wasted. Dynamic access can be divided into random contention access and control access. Representatives of random contention access methods include ALOHA and carrier sense protocol (CSMA), which are suitable for bursty service networks. When the number of users or business volume is small, the time delay is small and the channel utilization rate is high. Controlling access can effectively avoid transmission conflicts, but when the number of users is small, large overhead is generated and channel utilization is low. The hybrid access mode is mainly reservation-based on-demand access, which can be divided into two forms: single-service packet transmission and multi-service packet transmission.

In the indoor VLC system, due to the limited number of user nodes under the VPAN in the illumination coverage area, each user is independent of each other, sends transmission data randomly, and cannot detect each other's uplink signals, and at the same time, the indoor transmission distance is short and the transmission delay can be can be ignored. Therefore, it is more appropriate to use random contention access as the uplink access method of the system. The ALOHA protocol in the random contention access method is simple to implement and supports bursty data services. Compared with the fixed access method, it can reduce unnecessary overhead, but ALOHA will blindly occupy channel resources, resulting in transmission conflicts. Low utilization. The CSMA protocol is a random access method based on monitoring the channel state, which can reduce conflicts and improve channel resource utilization, but the access delay is large and the communication quality is affected.

Contents of the invention

In view of this, the present invention provides a method, device and system for realizing VLC dynamic access, so as to reduce the probability of overlapping collisions.

Technical scheme of the present invention is realized like this:

A method for realizing dynamic access of visible light communication VLC, the method comprising:

The user node generates and sends a reservation request frame for reserving an uplink channel; after the reservation is successful, the user node sends data in the reserved time slot.

A user node sends one reservation request frame at a time, and when the reservation request frames sent by more than two user nodes collide, the conflicting user nodes retry to send the reservation request frame.

The reservation request frame is sent by means of ALOHA competition.

The method also includes:

The uplink channel is divided into N+1 time slots, and one of the time slots is used as a reservation time slot for sending a reservation request frame, and the other N time slots are used as transmission packet time slots for transmitting packet data.

The reserved time slot includes information about the length of time the user node will occupy and the number of groups.

A method for realizing VLC dynamic access, the method comprising:

The VLC access point notifies the current channel status and the time when uplink random signals can be sent through downlink broadcasting; and, according to the received reservation request frame, the VLC access point dynamically allocates the number N of time slots for transmitting packets.

The method also includes:

The uplink channel is divided into N+1 time slots, and one of the time slots is used as a reservation time slot for sending a reservation request frame, and the other N time slots are used as transmission packet time slots for transmitting packet data.

A device for realizing VLC dynamic access, the device is a user node; the user node is used for:

Generate a reservation request frame for reserving the uplink channel and send it; after the reservation is successful, send data in the reserved time slot.

The user node is used to send one reservation request frame at a time, and when the reservation request frames sent by more than two user nodes collide, the conflicting user nodes are used to retry sending the reservation request frame.

The user node is configured to compete to send the reservation request frame in an ALOHA manner.

The user node is also used for:

The uplink channel is divided into N+1 time slots, and one of the time slots is used as a reservation time slot for sending a reservation request frame, and the other N time slots are used as transmission packet time slots for transmitting packet data.

The reserved time slot includes information about the length of time the user node will occupy and the number of groups.

A device for realizing VLC dynamic access, the device is a VLC access point; the VLC access point is used for:

Notify the status of the current channel and the time when uplink random signals can be sent through downlink broadcasting; and dynamically allocate the number N of time slots for transmitting packets according to the received reservation request frame.

The VLC access point is also used for:

The uplink channel is divided into N+1 time slots, and one of the time slots is used as a reservation time slot for sending a reservation request frame, and the other N time slots are used as transmission packet time slots for transmitting packet data.

A system for realizing VLC dynamic access, the system includes a VLC access point and a user node; wherein,

The VLC access point is used to notify the state of the current channel through downlink broadcasting, and the time when uplink random signals can be sent; and, according to the received reservation request frame, dynamically allocate the number N of time slots for transmitting packets;

The user node is used to generate and send a reservation request frame for reserving an uplink channel; after the reservation is successful, send data in the reserved time slot.

The invention reduces the probability of overlapping collisions.

Description of drawings

Fig. 1 is the network topology schematic diagram of indoor VLC system;

Fig. 2 is a schematic diagram of the relationship (M=10) between the reserved ALOHA throughput S and the network load G of the embodiment of the present invention;

FIG. 3 is a schematic diagram of the relationship (M=10) between the reservation type ALOHA normalized delay and the network load G according to the embodiment of the present invention;

Fig. 4 is a schematic diagram of the relationship between the packet loss rate and the network load according to the embodiment of the present invention;

5 is a schematic diagram of the relationship between the blocking rate and the normalized access density according to the embodiment of the present invention;

FIG. 6 is a simplified flow chart of implementing VLC dynamic access according to an embodiment of the present invention.

Detailed ways

In general, a VLC network may include a VLC access point and at least one user node, and dynamic access may be applied to a downlink broadcast channel and an uplink channel.

Both the VLC access point and the user node can divide the uplink channel into N+1 time slots, and use one of the time slots as the reservation time slot for sending the reservation request frame, and the other N time slots as the time slot for transmitting packet data. To transmit packet time slots, a simple TDMA (simple-TDMA) protocol can be used. All user nodes share the N time slots, and the use of each time slot is controlled by the VLC access point.

The VLC access point notifies the user node of the current channel status and the time when the user node can send an uplink random signal through downlink broadcasting.

When the user node has uplink data to send, the user node generates a reservation request frame (which can be a packet length) and sends a reservation application containing the reservation request frame to the VLC access point to reserve the uplink channel. The reservation application can use the ALOHA method Competitive send. After the reservation is successful, the user node can send data in the reserved time slot until the data transmission is completed.

The reserved time slot in the present invention contains the information of the length of time that the user node will occupy and the number of groups.

A user node sends one reservation request frame at a time. When the reservation request frames sent by two or more user nodes overlap in the reservation time slot (that is, a collision occurs), the conflicting user nodes will wait for a random period of time and then retry the reservation request. frame transmission.

In actual application, the channel can be divided into frames according to time according to the traffic volume of user nodes in the network, and each frame is divided into several reserved time slots with a length of T1 and transmission packet time slots with a length of T2.

Each user node can compete for the reserved time slot at random, and only the user nodes that do not collide in the reserved time slot can occupy the transmission group time slot, so no collision will occur in the group transmission time slot.

According to the reservation application of the user node, the VLC access point dynamically allocates the number N of time slots for transmitting packets.

If no collision occurs in the reserved time slot, the packet data is sent in the corresponding time slot for transmitting the packet. Obviously, the reserved ALOHA method can greatly improve the throughput S, and the theoretical extreme value is S=N/(N+1), where N is the number of time slots for transmitting packets in each frame.

According to the characteristics of the indoor VLC system, the following conditions can be provided:

(1). The channel is considered error-free, that is, the packet loss rate only involves the probability of transmission failure due to collision;

(2). The reserved groups in the reserved time slots of the newly arrived network conform to the Poisson process, and the average arrival rate is the average arrival of S groups in the unit time T;

(3). The whole process of reservation packets arriving at the network (including the arrival process of newly arrived and retransmitted reservation packets) is a Poisson process, and within a unit time T, an average of G packets arrive;

(4). Each user node has at most one reservation group (including any reservation group that has previously conflicted) ready to send;

(5). Since the indoor VLC network topology is star-shaped, as shown in Figure 1, the coverage area is small, the transmission distance is short, and the optical pulse delay is small, so the transmission delay is ignored when considering the conflict;

(6). It can be assumed that there are 10 user nodes under a VLC access point, M=10 (the indoor VLC communication range is limited, assuming that there are only 10 users under a lighting lamp).

Throughput:

The reserved ALOHA frame length is T0, which is divided into N+1 time slots. The length of the reservation time slot used for reservation is T1, and the length of the transmission time slot for transmitting data packets is T2. Then there are T1=T0/(1+N), T2=N×T1, and 1<=N<=9 (number of user nodes<10).

The throughput in reserved slots is discussed separately first. Taking T as the unit time and λ as the arrival rate of reserved groups, according to the Poisson formula, the probability of any group other than K reserved groups appearing within time T is:

At this time, the newly arrived reservation group is g=λT, and the average number of reservation groups is G (including the number of newly arrived and retransmitted reservation groups). F The probability that the reservation packet can be successfully transmitted is P(K=0)=e ^-GT , and the retransmission probability is:

P _r =1-P(K=0)=1-e ^-GT ;

When the VLC system includes a limited number of user nodes M, each user node randomly sends frames independently, and the length of one time slot can send one frame. Let Si be the probability that user node i successfully transmits a frame in any time slot, then 1-Si is the probability that user node i does not transmit successfully (failure to send or not send at all) in any time slot.

Gi and 1-Gi are respectively the probability that user node i sends and does not send a frame in any time slot. Therefore, for all i, Si≤Gi, and each user node sends frames independently, so we get:

${\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}\underset{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; \&NotEqual;</span>\mathrm{<span\; class="notranslate">\; j</span>}}{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; \&Pi;</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; G</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ;</span>$

Assuming that the statistical characteristics of each user node are the same, that is, Si=S/M and Gi=G/M, and S and G are the throughput and network load of the entire system, respectively, it can be simplified as:

S=G(1-G/M)M-1;

When the reservation packet is transmitted successfully, the data packet can be correctly transmitted. That is to say, successfully sending a data packet needs to send 1/S reservation packets on average, and the probability of no conflict in each reservation packet is S, the time occupied by the packet data is T2, and the number of user nodes M=10, so the throughput can be Approximately expressed as:

$\mathrm{<span\; class="notranslate">\; S</span>}\frac{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; S</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; NT</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; NT</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 9</span>}}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; NG</span>}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 9</span>}}}{\mathrm{<span\; class="notranslate">\; NG</span>}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 9</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ;</span>$

as shown in picture 2.

real-time:

Delayed time = ALOHA delay D1 + reservation request frame collision retransmission delay D2 + packet transmission delay D3.

(1). ALOHA delay refers to the observation time before the reservation request frame is sent. Although the arrival of each packet data is approximately a Poisson process, due to the different channel traffic, there are different packet data arrival rates, so each user node waits for T2/2 on average before sending packet data, and then starts sending reservation requests frame, that is:

D ₁ =T ₂ +T ₂ /2=3T ₂ /2;

(2). The transmission delay factor is R, that is, after sending a reservation request frame, it takes RT1 to receive the confirmation message and send the next frame. Therefore, it takes time T1(1+R) to transmit one reservation request frame.

When a collision occurs and retransmission is necessary, it is necessary to wait for T1 times n times (n is a random positive integer). After the retransmission is complete, the acknowledgment message can only be received after RT1. Therefore, it takes time T1(1+R+n+R) to retransmit one time.

The average number of retransmissions is related to n. The smaller n is, the greater the probability of frame collision during retransmission is, and the number of retransmissions will also increase; the larger n is, the probability of collision decreases, but the average delay of sending a frame increases. Theoretical analysis shows that choosing n=5 is a better compromise. In this case, the number of retransmissions NR has little relationship with n. It can be concluded that:

${\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; R</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; S</span>}}{\mathrm{<span\; class="notranslate">\; S</span>}}<span\; class="notranslate">\; ;</span>$

To sum up, it can be seen that the average delay of collision retransmission of reservation request frame is:

${\mathrm{<span\; class="notranslate">\; D.</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; R</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ;</span>$

(3). The minimum delay D ₃ =2RT ₁ for transmitting packet data; therefore, the average delay of packet data is:

$\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; +</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; RT</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ;</span>$

Similarly, in the case of a limited number of user nodes M, let R=0, n=5, T2=N×T1, T1=T0/(1+N), so the normalized time delay is:

$\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; [</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; G</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 9</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; ;</span>$

As shown in Figure 3.

Packet loss rate:

The packet loss rate is the probability that the reservation request frame collides within the reservation time slot, and the request still fails after multiple retransmissions and is rejected by the system. Assuming that the maximum number of retransmissions of the reservation request frame is M, the value of M increases by 1, and the delay increases by at least R+T1. Because the retransmission probability of the reservation request frame is Then the average number of retransmissions is:

$\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}}^{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; e</span>}}^{{\mathrm{<span\; class="notranslate">\; GT</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; ;</span>$

so $S={\mathrm{TT}}_1={\mathrm{Ge}}^{-{\mathrm{GT}}_1}\&CenterDot;T_1=\frac{\ln \stackrel{\&OverBar;}{m}}{\stackrel{\&OverBar;}{m}};$

Assuming that the retransmission fails 16 times, the frame is discarded, indicating that the transmission failed and reported to the upper layer protocol. Therefore, if the retransmission fails 16 times, the transmission fails, and the probability of transmission failure is:

${\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; A</span>}}<span\; class="notranslate">\; =</span>{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}}^{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; GT</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; G</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 17</span>}}<span\; class="notranslate">\; ;</span>$

As shown in Figure 4.

Blocking rate:

For the VLC system, when the number of traffic channels N is less than the number of users, and the user node successfully requests the reservation request frame and can access the system, it happens that all available resources are in use, and congestion will occur at this time. Obviously, the more available resources provided by the system, the smaller the blocking probability. Assuming that the number of indoor users remains unchanged, the blocking rate is closely related to the number N of data transmission channels.

The Irish B formula points out the relationship between the number N of available channel resources, the number ρ of each channel being accessed, and the blocking rate as follows:

$\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; \&rho;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}^{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; !</span>}{\underset{\mathrm{<span\; class="notranslate">\; i</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}^{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; !</span><span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; ;</span>$

ρ is the normalized access density of the unit channel, ρ=λ/μ, λ is the arrival rate of user access requests, and μ is the service rate of the access process. The above formula assumes that λ obeys Poisson distribution and μ obeys exponential distribution.

As shown in Figure 5.

Through the calculation and image analysis of the performance of the multiple access method based on reserved ALOHA, the conclusions are drawn: (1). The more channel divisions, the greater the throughput and the greater the relative delay, but the blocking rate will be reduced; (2) ). The channel division is less, the throughput performance is reduced, and the delay performance is improved at the same time, but the blocking performance is reduced.

It can be seen that the present invention combines fixed allocation and dynamic allocation, and the VLC access point in the VLC system divides the number of channels correspondingly according to the user's specific service requirements, so that the allocation is flexible.

In conjunction with the above description, it can be seen that the operation of realizing VLC dynamic access in the present invention can represent the flow process as shown in Figure 6, and the flow process includes the following steps:

Step 610: the user node generates and sends a reservation request frame for reserving an uplink channel;

Step 620: After the reservation is successful, the user node sends data in the reserved time slot.

To sum up, it can be seen that no matter it is a method, a device or a system, the present invention combines the fixed allocation mode TDMA and the dynamic allocation mechanism. The time slot for transmitting packets is composed of N time slots with a length of T2, and the simple TDMA method can provide reliable service and high real-time performance. Reservation time slots are sent using the ALOHA protocol competition, which can reduce the probability of overlapping collisions and make full use of channel resources.

In addition, it avoids disorderly contention for channel resources caused by packet transmission competition, and can reasonably allocate channel resources according to the actual business needs of each user node, effectively improving network throughput and reducing transmission delay.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention.

Claims (15)

1. A method for realizing dynamic access of visible light communication VLC, characterized in that the method comprises: The user node generates and sends a reservation request frame for reserving an uplink channel; after the reservation is successful, the user node sends data in the reserved time slot. 2. The method of claim 1, wherein, A user node sends one reservation request frame at a time, and when the reservation request frames sent by more than two user nodes collide, the conflicting user nodes retry to send the reservation request frame. 3. The method according to claim 1, wherein the reservation request frame is sent in an ALOHA manner. 4. The method according to any one of claims 1 to 3, characterized in that the method further comprises: The uplink channel is divided into N+1 time slots, and one of the time slots is used as a reservation time slot for sending a reservation request frame, and the other N time slots are used as transmission packet time slots for transmitting packet data. 5 . The method according to claim 4 , wherein the reserved time slot includes information about the length of time the user node will occupy and the number of groups. 6. A method for realizing VLC dynamic access, characterized in that the method comprises: The VLC access point notifies the current channel status and the time when uplink random signals can be sent through downlink broadcasting; and, according to the received reservation request frame, the VLC access point dynamically allocates the number N of time slots for transmitting packets. 7. The method according to claim 6, characterized in that the method further comprises: The uplink channel is divided into N+1 time slots, and one of the time slots is used as a reservation time slot for sending a reservation request frame, and the other N time slots are used as transmission packet time slots for transmitting packet data. 8. A device for realizing VLC dynamic access, the device is a user node; it is characterized in that, the user node is used for: Generate a reservation request frame for reserving the uplink channel and send it; after the reservation is successful, send data in the reserved time slot. 9. The device of claim 8, wherein: The user node is used to send one reservation request frame at a time, and when the reservation request frames sent by more than two user nodes collide, the conflicting user nodes are used to retry sending the reservation request frame. 10. The device according to claim 8, wherein the user node is configured to compete to send the reservation request frame in an ALOHA manner. 11. The device according to any one of claims 8 to 10, wherein the user node is further used for: The uplink channel is divided into N+1 time slots, and one of the time slots is used as a reservation time slot for sending a reservation request frame, and the other N time slots are used as transmission packet time slots for transmitting packet data. 12 . The device according to claim 11 , wherein the reserved time slot includes information about the length of time the user node will occupy and the number of groups. 13 . 13. A device for realizing VLC dynamic access, the device is a VLC access point; it is characterized in that, the VLC access point is used for: Notify the status of the current channel and the time when uplink random signals can be sent through downlink broadcasting; and dynamically allocate the number N of time slots for transmitting packets according to the received reservation request frame. 14. The device according to claim 13, wherein the VLC access point is also used for: The uplink channel is divided into N+1 time slots, and one of the time slots is used as a reservation time slot for sending a reservation request frame, and the other N time slots are used as transmission packet time slots for transmitting packet data. 15. A system for realizing VLC dynamic access, characterized in that, the system includes a VLC access point and a user node; wherein, The VLC access point is used to notify the state of the current channel through downlink broadcasting, and the time when uplink random signals can be sent; and, according to the received reservation request frame, dynamically allocate the number N of time slots for transmitting packets; The user node is used to generate and send a reservation request frame for reserving an uplink channel; after the reservation is successful, send data in the reserved time slot.