CN110289938A - Multi-passive reflective tag access system and control method based on code division multiple access - Google Patents

Abstract

The invention discloses a multi-passive reflective tag access system and control method based on code division multiple access, including: a plurality of passive reflective tags, an excitation source and a receiver; After the transmitted data is framed, coded and modulated in turn, it is sent to the receiver through the backscatter signal of the excitation source, and each code contains the unique PN sequence of the corresponding passive reflective tag; The receiver can decode the coded data sent by the received passive reflective tag, and distinguish the coded data sent by different passive reflective tags according to the PN sequence obtained by decoding. The system realizes simultaneous transmission of multiple nodes by using code division multiple access in a passive reflective communication system including multiple passive reflective tags.

Description

Multi-passive reflective tag access system and control method based on code division multiple access

technical field

The present invention relates to the field of passive communication, in particular to a multi-passive reflective tag access system and control method based on code division multiple access.

Background technique

Although traditional 802.11 wireless networks have achieved great success in providing each device with higher and higher transmission rates, the recent increasing number of IoT devices has brought new requirements and challenges to future network paradigm designs . By 2020, there are expected to be 3 billion IoT devices, a number that is growing at a rate of 20% per year. These plethora of devices will be deployed around and even on people's bodies to provide various types of perception to improve people's quality of life. Unlike traditional laptops and smartphones, which require high transfer rates, these IoT devices typically transfer data at low rates or in bursts. There are two main problems faced in this way. The first one is the problem of energy demand. Where does the tiny power required by these IoT devices without power plugs come from? The second is that the number of connected IoT devices will be orders of magnitude larger.

Over the past few years, backscatter communication has attracted a lot of attention due to its low power consumption and ease of deployment. Using backscatter technology, most of them meet the low power consumption requirements. However, existing works focus on a single node (tag) scenario, and multiple nodes (tags) cannot communicate simultaneously, severely limiting the scalability to accommodate a large number of IoT devices in the future.

In order to achieve high capacity, the current methods are all based on collision avoidance schemes, mainly including two multiplexing methods: frequency division multiple access and time division multiple access. Wherein, in frequency division multiple access technology, different tags are assigned different frequency channels to communicate with receivers. Tags should be able to freely adjust the transmission frequency within the bandwidth. In this case, the cost of the tag increases, and the receiver should act as the control node to allocate the frequency band. Furthermore, the available bandwidth is extremely limited, making FDMA a very expensive solution for large scale deployments. And time division multiple access technology is the most popular multiplexing method of backscattering technology. Media access schemes can be deterministic, typically tree search based, or probabilistic. Framework-based ALOHA (FSA) program. However, the receiver acts as a slotted ALOHA centralized control node, which coordinates the frame size in the network. Therefore, it cannot meet the requirements of distributed schemes.

Contents of the invention

Based on the problems existing in the prior art, the object of the present invention is to provide a multi-passive reflective tag access system and control method based on code division multiple access, which can realize multiple simultaneous transmission of nodes.

The purpose of the present invention is achieved through the following technical solutions:

The embodiment of the present invention provides a multi-passive reflective tag access system based on code division multiple access, including:

Multiple passive reflective tags, excitation sources, and receivers; where,

Each passive reflective tag sends the transmitted data to the receiver through the backscattering signal of the excitation source after frame sealing, coding, energy control and modulation in sequence, and each coded data contains the corresponding The passive reflective tag has its own unique PN sequence;

The receiver can decode the coded data sent by the received passive reflective tag, and can distinguish the coded data sent by different passive reflective tags according to different PN sequence decoding.

The embodiment of the present invention also provides a CDMA-based multi-passive reflective tag access control method, using the CDMA-based multi-passive reflective tag access system described in the present invention, including the following steps:

After each passive reflective tag performs frame sealing, encoding, energy control and modulation on the sent data in turn, it sends the backscatter signal to the receiver through the excitation source, and each encoded data contains the corresponding The passive reflective tag has its own unique PN sequence;

The receiver decodes the coded data sent by the received passive reflective tag, and decodes according to the different PN sequence of each passive reflective tag to distinguish the coded data sent by different passive reflective tags.

It can be seen from the above-mentioned technical solutions provided by the present invention that the multi-passive reflective tag access method based on code division multiple access provided by the embodiment of the present invention has the beneficial effects of:

By including the unique PN sequence of the corresponding passive reflective tag in the coded data encoded by each passive reflective tag, the receiver can distinguish different passive tags according to different PN sequences when decoding the received coded data. The coded data sent by the source reflective tag realizes the simultaneous transmission of multiple nodes by using code division multiple access in a passive reflective communication system including multiple passive reflective tags, and through energy control on the passive reflective tag Thereby improving system performance.

Description of drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the accompanying drawings that need to be used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For Those of ordinary skill in the art can also obtain other drawings based on these drawings on the premise of not paying creative work.

FIG. 1 is a composition diagram of a CDMA-based multi-passive reflective tag access system provided by an embodiment of the present invention;

FIG. 2 is a schematic diagram of the principle of a multi-passive reflective tag access system based on code division multiple access provided by an embodiment of the present invention;

Fig. 3 is a schematic diagram of signal transmission of a multi-passive reflective tag access system based on code division multiple access provided by an embodiment of the present invention;

Fig. 4 is an application example diagram of a CDMA-based multi-passive reflective tag access system provided by an embodiment of the present invention;

FIG. 5 is a flow chart of a method for controlling the access of multiple passive reflective tags based on code division multiple access provided by an embodiment of the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the specific content of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention. The content not described in detail in the embodiments of the present invention belongs to the prior art known to those skilled in the art.

As shown in Figure 1, the embodiment of the present invention provides a multi-passive reflective tag access system based on code division multiple access, which is characterized in that it includes:

Multiple passive reflective tags, excitation sources, and receivers; where,

Each passive reflective tag sends the transmitted data to the receiver through the backscattering signal of the excitation source after frame sealing, coding, energy control and modulation in sequence, and each coded data contains the corresponding The passive reflective tag has its own unique PN sequence;

The receiver can decode the coded data sent by the received passive reflective tag, and distinguish the coded data sent by different passive reflective tags according to the PN sequence obtained through decoding.

The above system further includes: after the receiver decodes the coded data, broadcasts the ID of the correctly decoded passive reflective tag to all passive reflective tags according to the PN sequence obtained through decoding.

The above system also includes: the receiver sends a feedback data packet for energy control to each passive reflective tag, and each passive reflective tag adjusts its own power to perform energy control according to the reception rate of the received feedback data packet, so that each passive reflective tag The transmission power difference between the passive reflective tag and the receiver does not exceed a set value.

In the above system, the way the receiver sends the feedback data packet for energy control to each passive reflective tag is as follows:

Down-sample the received coded data, and when the receiver detects the preamble of a certain passive reflective tag, it sends the ACK feedback packet with the passive reflective tag ID back to the passive reflective tag.

In the above system, the energy control mode of each passive reflective tag is:

After successfully sending encoded data, each passive reflective tag can receive the feedback data packet sent back from the receiver, and can adjust its own power to perform energy control according to the receiving rate of the received feedback data packet.

In the above system, each passive reflective tag adjusts its own power according to the reception rate of the received feedback data packet to perform energy control:

When the reception rate of the feedback data packet received by the passive reflective tag is lower than 70%, it is confirmed that the power of the backscatter signal of the passive reflective tag is so low that it cannot be detected by the receiver, then the passive reflective tag Increase your own power.

In the above system, the way for the passive reflective tag to increase its own power is:

The passive reflective tag changes its own antenna impedance to increase its own power.

In the above system, if a passive reflective tag still cannot meet the requirements of data transmission after increasing its own power, a new passive reflective tag whose power meets the requirements is reselected from multiple passive reflective tags to replace the passive reflective tag. for data transfer.

In the above system, if a passive reflective tag increases its own power and still cannot meet the requirements of data transmission:

If the reception rate of the feedback data packets received by the receiver is still lower than 70% after a certain passive reflective tag increases its own power, or the transmission error rate is higher than 20%, it is confirmed that the passive reflective tag cannot meet the data transmission requirements. Require.

In the above-mentioned system, the method of reselecting a new passive reflective tag whose power meets the requirements from multiple passive reflective tags to replace the passive reflective tag is as follows:

Randomly select a passive reflective tag that has not sent data, and then calculate the difference between the theoretical received signal strength of the original passive reflective tag and the new passive reflective tag, if the difference is less than 0, replace the original passive reflective tag with this new passive reflective tag reflective label;

If the new passive reflective tag makes the signal intensity received by the receiver greater than the original passive reflective tag, use the new passive reflective tag, otherwise, receive some other new passive reflective tags with a selection probability less than 1, up to multiple times After selection, the selection probability tends to zero. As the number of choices increases, this probability will decrease, and the probability will tend to zero after multiple choices.

As shown in Figure 5, the embodiment of the present invention also provides a CDMA-based multi-passive reflective tag access control method, using the above-mentioned CDMA-based multi-passive reflective tag access system, including the following step:

Each passive reflective tag will frame, code, energy control and modulate the transmitted data in turn, and then send the backscattered signal to the excitation source to the receiver, and each coded data contains the corresponding wireless The unique PN sequence of the source reflection tag itself;

The receiver decodes the coded data sent by the received passive reflective tag, and distinguishes the coded data sent by different passive reflective tags according to the PN sequence obtained through decoding.

The above method also includes: after the receiver decodes the coded data, broadcast the ID of the correctly decoded passive reflective tag to all passive reflective tags according to the PN sequence obtained through decoding.

The above method further includes: the receiver sends a feedback data packet for energy control to each passive reflective tag, and each passive reflective tag adjusts its own power to perform energy control according to the reception rate of the received feedback data packet, so that each passive reflective tag The transmission power difference between the passive reflective tag and the receiver does not exceed a set value.

In the above method, the way that the receiver sends the feedback data packet for energy control to each passive reflective tag is:

The received data is down-sampled, and when the receiver detects the preamble of a passive reflective tag, the feedback packet is sent back to the passive reflective tag.

In the above method, each passive reflective tag adjusts its own power according to the reception rate of the received feedback data packet to perform energy control:

When the reception rate of the feedback data packet received by the passive reflective tag is lower than 70%, it is confirmed that the power of the backscatter signal of the passive reflective tag is so low that it cannot be detected by the receiver, then the passive reflective tag Increase your own power.

In the above method, the way for the passive reflective tag to increase its own power is:

The passive reflective tag changes its own antenna impedance to increase its own power.

In the above method, if a passive reflective tag still cannot meet the requirements of data transmission after increasing its own power, a new passive reflective tag whose power meets the requirements is reselected from multiple passive reflective tags to replace the passive reflective tag , for data transmission.

In the above method, if increasing the power of a passive reflective tag itself still cannot meet the requirements of data transmission:

If the reception rate of the feedback data packet sent back by the receiver is still lower than 70% after a certain passive reflection tag increases its own power, or the error rate of transmission is higher than 20%, then confirm the passive reflection The label cannot meet the requirements for transmitting data.

In the above method, the method of reselecting a new passive reflective tag whose power meets the requirements from multiple passive reflective tags to replace the passive reflective tag is as follows:

Randomly select a passive reflective tag that has not sent data, and then calculate the difference between the theoretical received signal strength of the original passive reflective tag and the new passive reflective tag, if the difference is less than 0, replace the original passive reflective tag with this new passive reflective tag reflective label;

If the new passive reflective tag makes the signal intensity received by the receiver greater than the original passive reflective tag, then use the new passive reflective tag, otherwise, receive some other new passive reflective tags with a selection probability less than 1, until The selection probability tends to zero after multiple selections. As the number of choices increases, this probability will decrease, and the probability will tend to zero after multiple choices.

In the system and method of the present invention, code division multiple access is used to realize simultaneous transmission of multiple nodes on passive reflected signals, and correlation-based detection is used to solve the problem of asynchrony between multiple signals, which is greatly affected by distance The energy of each node will affect the performance of the entire system, and the problem of poor system performance is solved by using energy control on each passive reflective tag.

The embodiments of the present invention will be further described in detail below.

The embodiment of the present invention provides a multi-passive reflective tag access system based on code division multiple access, and its main components are an excitation source, multiple passive reflective tags and a receiver. The excitation source can use monotone signals or other signals (such as Wifi, Bluetooth), and these signals are relatively common, so no special instructions are given here. These signals need to be used as carrier signals to provide energy to the tag side.

Only the specific structural diagram of the tag and the receiver is given below (see Figure 5).

On the tag side (ie passive reflective tag, tag) is mainly composed of frame sealing, coding, energy control, on/off modulation and spectrum shifting.

First of all, the data sent by the tag needs to be encapsulated into a frame format, including a known preamble of one byte, one byte used to display the frame length, up to 126 bytes of valid data and two bytes of cyclic check code.

The next step is the encoding process, which is to encode the frame-encapsulated data, and each label has its own specific PN sequence. Moreover, the orthogonality of these PN sequences is relatively good, which can ensure that each label is distinguished by the PN sequence of each label during decoding, and the data between each label will not interfere with each other.

Next is energy control. Based on the nature of CDMA, the best performance is when each tag arrives at the receiver at the same energy level. This property is verified by doing pre-experiments. The experimental scene is shown in Figure 2, and some experimental results are shown in Table 1.

Table 1:Error Rate vs the power difference.

A coordinate system is established, as shown in Figure 2. Points labeled A and B represent excitation source 'Es' and receiver 'Rx'. The point marked O is the origin of this coordinate system. Then the receiver places the excitation source and the receiver at (-D, 0) and (D, 0) positions respectively (D=50cm in this implementation). Meanwhile, (x1, y1) and (x2, y2) are represented as the positions of "label 1" and "label 2" in one test. For each test, 2 of the 5 labels (indicated by 1, 2, 3, 4 and 5 in Table 1) are selected and placed randomly. Due to space constraints, only some of the results are presented in Table 1. The difference is calculated as the ratio between the power difference and the greater power of the two tags. And the error rate is calculated as the number of packets lost over the number of packets transmitted. It was found that when the power difference is below 10% (both tags have similar power), the error rate is much lower than otherwise. Therefore, these results can be used for power control to improve the overall performance. A power control scheme is proposed and details are presented in the next section.

Finally, data is loaded onto the reflected signal using on/off modulation. The specific process of how to upload data is shown in Figure 3. If the tag wants to send a '1', it causes the square wave to control the state of the antenna for a symbol time, otherwise, the tag stays quiet and does nothing. Specifically, there are two layers of modulation to enable communication on tags. The first layer is to generate spectrum shift by sending a square wave with frequency Δf. The other is on/off (OOK) modulation to enable the tag to send its own data. In this modulation, a Δf square wave acts as a carrier whose presence for a certain duration represents a binary 1 and absence for the same duration represents a binary 0. Therefore, at the receiver end, the received signal can be decoded as a '1', and no signal can be decoded as a '0'. When the bit rate transmitted by the tag is f0, the '1' bit is transmitted to reflect the signal of the Δf/f0 period of the square wave. It should be noted that f0 is smaller than Δf. In fact, first sample the data with frequency f0 to the frequency of Δf, and then perform 'AND' operation with the square wave of Δf, as shown in Figure 3. Therefore, it is convenient to adjust the bitrate. To change the bit rate, the only thing that changes is the time period during which the tag reflects or absorbs the signal.

Next, the receiver is introduced, which mainly consists of frame synchronization, user detection, decoding and ACK.

Frame synchronization is achieved using sliding window energy detection. Specifically, a moving average filter is first performed on the received energy levels with a window size Wn. The filtered sequence then passes through a comparator to determine whether a new frame has been received by comparing the current power level with the filtered power level. A decision threshold Pth is used, which is configured to be 3dB above the filtered power level.

Before decoding, you first need to know which PN sequences are included, so user detection must be performed first. User detection is performed using the orthogonality property in the PN sequence. Specifically, each PN sequence is used to perform cross-correlation with the preamble of the received frame. If the correlation value of the PN sequence is greater than the threshold set by the implementation, it is considered that the user with the PN sequence is sending data.

Then, use the PN sequence corresponding to the detected user to perform cross-correlation on the entire frame to realize decoding. After user detection, a cross-correlation is performed with the received frames using the detected user's PN sequence. If the correlation with the PN sequence representing '1' is higher than the correlation with the PN sequence representing '0', the chip is decoded as '1' and vice versa.

The last is the ACK part, which is the receiver broadcasts the ID of the tag it decoded correctly to each tag for feedback, which is an important part of energy control.

The most important design is energy control, which can be mainly divided into the selection of tag impedance and the selection of tags.

Choice of tag impedance: Receive backscattered signals in IQ space: I(t) and Q(t). The power of the received signal is Since the sampling rate of is higher than the bit rate, the received data is first down-sampled. Each tag has its own PN code. When the receiver detects a tag's preamble, it sends an ACK packet back to that tag. Therefore, when the tag receives few ACK feedback packets, it is considered that most transmitted packets will be lost by this mark. The reason is that the power of the backscattered signal is too low to be detected by the receiver. In order to improve transmission performance, power must be increased. As mentioned above, the antenna impedance can be changed to tune the reflection coefficient Γ* to achieve the purpose of power increase. The pseudocode of the power control algorithm is proposed, which will be in the later part, omitting the downsampling and decoding process. In this experiment, the power control was looped to try every possible power level. In order to prevent the power control scheme from falling into an infinite loop, and because the size of the tag is relatively small, there are not many energy selection levels above, so the current method is to traverse all possible energy levels.

Tag selection: However, even with power control, some tags may still not be able to receive ACK signals, or the error rate of the system may be too high. There are two reasons. First, the backscatter signal is much weaker than the excitation signal. If some tags are far away from the receiver, their power will be too weak to be detected at the receiver, even if the tag power is set to the highest possible value by adjusting the impedance. Second, when two tags are close in space, they will interfere with each other, resulting in poor system performance. In this case, increasing the power does not help much. To address the limitations of power control schemes, another optimization scheme, label selection, is proposed. If the system performance cannot meet the expectations of power control, those tags whose ACK feedback information is lower than 70% will be replaced. Therefore, the main problem is how to select new labels. In the inventive system, the communication distance depends on two factors: (i) the distance between the excitation source and the tag and (ii) the distance between the tag and the receiver. The signal strength of the receiver Pr can be expressed by the Friis path loss model as follows

According to this equation, the theoretical result of the received signal strength at each location can be obtained. Therefore, a greedy algorithm (see the following algorithm pseudo code) is used to select the tag so that the tag moves continuously in the direction as the received signal strength increases, to select labels for locations with high receiver signal strength. When there are many tags distributed in the environment, some of them are selected as a group to transmit data. Bad tags must be discarded after each round of power control, first randomly select a tag that has not sent data, and then calculate the difference in theoretical received signal strength between the original tag and this new tag. If the difference is less than 0, replace this new label with the old one. However, to avoid clustering of these tag locations transmitting data, those tags that are nearby other working tags are discarded. However, instead of picking a label at a better location, one is chosen at random. If the new tag improves the received signal strength, the new tag is used. Otherwise, some new labels are received with a probability less than 1, which decreases as the number of selections increases. In addition, when there are not many tags to choose from in the environment, some tags must be relocated to improve system performance.

The pseudo code of the above algorithm is as follows:

This method can be used to implement code division multiple access based on reflected signals, and an implementation scene diagram is shown in FIG. 4 .

The system of the present invention supports up to 10 backscatter tags and backscatters data in a reliable and efficient manner. This system enables multiple tags to transmit concurrently with a simple on-tag power control scheme and can be decoded using any commercially available WiFi NIC while leaving the original WiFi communication unaffected. In fact, due to the custom code design of the tags, our system can be deployed efficiently and can work friendly with commodity WiFi devices. The design details of the inventive system are described, and a prototype is built using an FPGA and an existing WiFi device. The system of the present invention realizes a multi-label bit rate up to 8Mbps, and the receiver distance can reach up to 5m. Compared to single-node solutions, the present system can increase backscatter throughput by more than 10 times in challenging indoor scenarios with obstacles and interference.

Example

(a) First, multiple passive reflective tags are required, as shown in Figure 4, each tag sends different data to realize code division multiple access multiplexing in backscatter communication, and uses a square wave signal to control the on-off of the antenna Realize signal reflection and spectrum shifting.

(b) An excitation source ES is required, which can emit a single-tone sinusoidal signal or a WiFi signal.

(c) A receiver is needed again, which can receive the reflected signal, and the receiver processes the received data, and runs the model and algorithm of the present invention.

Those of ordinary skill in the art can understand that all or part of the processes in the methods of the above embodiments can be completed by instructing related hardware through a program, and the program can be stored in a computer-readable storage medium. During execution, it may include the processes of the embodiments of the above-mentioned methods. Wherein, the storage medium may be a magnetic disk, an optical disk, a read-only memory (Read-Only Memory, ROM) or a random access memory (Random Access Memory, RAM) and the like.

The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person familiar with the technical field can easily conceive of changes or changes within the technical scope disclosed in the present invention. Replacement should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims (18)

1. A cdma-based multiple passive reflective tag access system, comprising:

a plurality of passive reflective tags, an excitation source and a receiver; wherein,

each passive reflection tag sequentially carries out framing, coding, energy control and modulation on the transmitted data, and then transmits the data to the receiver through a backscattering signal of an excitation source, wherein each coded data comprises a unique PN sequence owned by the corresponding passive reflection tag;

the receiver can decode the received coded data sent by the passive reflection tags, and distinguishes the coded data sent by different passive reflection tags according to the PN sequences obtained by decoding.

2. The cdma-based multi-passive reflective tag access system of claim 1, further comprising: and after the receiver decodes the coded data, the receiver broadcasts the correctly decoded ID of the passive reflection tag to all the passive reflection tags according to the PN sequence obtained by decoding.

3. The cdma-based multi-passive reflective tag access system according to claim 1 or 2, further comprising: the receiver sends feedback data packets for energy control to the passive reflection tags, and the passive reflection tags can adjust the power of the passive reflection tags according to the receiving rate of the received feedback data packets to perform energy control, so that the difference of the sending power of the passive reflection tags to the receiver is not more than a set value.

4. The cdma-based multiple passive reflective tag access system of claim 3, wherein the receiver sends the feedback data packet for power control to each passive reflective tag in a manner that:

and downsampling the received coded data, and when a receiver detects the preamble of a certain passive reflection tag, sending an ACK feedback data packet with the ID of the passive reflection tag back to the passive reflection tag.

5. The cdma-based multiple passive reflective tag access system of claim 3, wherein the manner in which each passive reflective tag can adjust its own power according to the reception rate of the received feedback data packet to perform energy control is as follows:

when the receiving rate of the feedback data packet received by the passive reflection tag is lower than 70%, the power of the backscattering signal of the passive reflection tag is confirmed to be low enough to be detected by the receiver, and then the power of the passive reflection tag is increased.

6. The cdma-based multiple passive reflective tag access system of claim 5, wherein the passive reflective tag increases its power by:

the passive reflecting label changes the self antenna impedance to increase the self power.

7. The cdma-based multiple passive reflective tag access system of claim 5 or 6, wherein if a certain passive reflective tag cannot meet the requirement for data transmission after increasing its own power, a new passive reflective tag with a power meeting the requirement is reselected from the other passive reflective tags to replace the passive reflective tag for data transmission.

8. The cdma-based multiple passive reflective tag access system of claim 7, wherein if a passive reflective tag increases its own power, it still cannot satisfy the requirement of transmitting data:

if the receiving rate of the feedback data packet of the receiving receiver is still lower than 70% after the power of a certain passive reflection tag is increased, or the error rate of transmission is higher than 20%, it is determined that the passive reflection tag cannot meet the requirement of transmitting data.

9. The cdma-based multiple passive reflective tag access system of claim 7, wherein the new passive reflective tag with satisfactory power is reselected from the plurality of passive reflective tags to replace the passive reflective tag by:

randomly selecting a passive reflection tag which does not send data, then calculating the difference of the theoretical received signal strength of the original passive reflection tag and the new passive reflection tag, and if the difference is less than 0, replacing the original passive reflection tag with the new passive reflection tag;

and if the new passive reflection tag enables the signal intensity received by the receiver to be larger than that of the original passive reflection tag, using the new passive reflection tag, otherwise, receiving other new passive reflection tags with the selection probability smaller than 1 until the selection probability tends to zero after multiple selections.

10. A cdma-based multiple passive reflective tag access control method, wherein the cdma-based multiple passive reflective tag access system of any one of claims 1 to 9 is adopted, and the method comprises the following steps:

each passive reflection tag sequentially carries out framing, coding, energy control and modulation on the transmitted data, and then transmits the data to a receiver through a backscattering signal of an excitation source, wherein each coded data contains a unique PN sequence owned by the corresponding passive reflection tag;

and the receiver decodes the received coded data sent by the passive reflection tag and distinguishes the coded data sent by different passive reflection tags according to the PN sequence obtained by decoding.

11. The cdma-based multi-passive reflective tag access control method of claim 10, further comprising: and after the receiver decodes the coded data, broadcasting the correctly decoded ID of the passive reflection tag to all the passive reflection tags according to the PN sequence obtained by decoding.

12. The cdma-based multiple passive reflective tag access control method of claim 10 or 11, further comprising: the receiver sends feedback data packets for energy control to the passive reflection tags, and the passive reflection tags can adjust the power of the passive reflection tags according to the receiving rate of the received feedback data packets to perform energy control, so that the difference of the sending power of the passive reflection tags to the receiver is not more than a set value.

13. The cdma-based multiple passive reflecting tags access control method according to claim 12, wherein the receiver sends the feedback data packet for energy control to each passive reflecting tag in a manner that:

and downsampling the received coded data, and when a receiver detects the preamble of a certain passive reflection tag, sending an ACK feedback data packet with the ID of the passive reflection tag back to the passive reflection tag.

14. The cdma-based multiple passive reflecting tags access control method according to claim 12, wherein in the method, the way for each passive reflecting tag to adjust its own power according to the receiving rate of the received feedback data packet to perform energy control is as follows:

when the receiving rate of the feedback data packet received by the passive reflection tag is lower than 70%, the power of the backscattering signal of the passive reflection tag is confirmed to be low enough to be detected by the receiver, and then the power of the passive reflection tag is increased.

15. The cdma-based multiple passive reflection tag access control method of claim 14, wherein in the method, the passive reflection tag increases its own power by:

the passive reflecting label changes the self antenna impedance to increase the self power.

16. The cdma-based multiple passive reflective tags accessing control method of claim 14, wherein in the method, if a certain passive reflective tag cannot meet the requirement of transmitting data after increasing its own power, a new passive reflective tag with a power meeting the requirement is reselected from the multiple passive reflective tags to replace the passive reflective tag, so as to perform data transmission.

17. The cdma-based multiple passive reflecting tags access control method of claim 16, wherein if the power of a certain passive reflecting tag is increased, the requirement that the transmitted data cannot be satisfied is as follows:

if the receiving rate of the feedback data packet sent back by the receiver after the power of a certain passive reflection tag is increased is still lower than 70%, or the error rate of transmission is higher than 20%, it is determined that the passive reflection tag cannot meet the requirement of transmitting data.

18. The cdma-based multiple passive reflecting tags accessing control method of claim 16, wherein the method of reselecting a new passive reflecting tag with satisfactory power from the multiple passive reflecting tags to replace the passive reflecting tag comprises the following steps:

randomly selecting a passive reflection tag which does not send data, then calculating the difference of the theoretical received signal strength of the original passive reflection tag and the new passive reflection tag, and if the difference is less than 0, replacing the original passive reflection tag with the new passive reflection tag;

and if the new passive reflection tag enables the signal intensity received by the receiver to be larger than that of the original passive reflection tag, using the new passive reflection tag, otherwise, receiving other new passive reflection tags with the selection probability smaller than 1 until the selection probability tends to zero after multiple selections.