CN110289938A

CN110289938A - More passive reflective label access systems and control method based on CDMA

Info

Publication number: CN110289938A
Application number: CN201910516968.2A
Authority: CN
Inventors: 杨盘隆; 李向阳; 宓楠浣
Original assignee: University of Science and Technology of China USTC
Current assignee: University of Science and Technology of China USTC
Priority date: 2019-06-14
Filing date: 2019-06-14
Publication date: 2019-09-27
Anticipated expiration: 2039-06-14
Also published as: CN110289938B

Abstract

The invention discloses a kind of more passive reflective label access systems and control method based on CDMA, comprising: multiple passive reflective labels, driving source and receiver；Wherein, each passive reflective label by the way that transmitted data are successively carried out envelope frame, coding and modulation after, sent through the back-scattered signal to driving source to the receiver, in each coding comprising corresponding passive reflective label have by oneself unique PN sequences；The receiver, the coded data that can be sent to received passive reflective label is decoded, and distinguishes the coded data that different passive reflective labels are sent according to the PN sequence that decoding obtains.The system realizes in the passive reflective communication system comprising multiple passive reflective labels, and multiple node simultaneous transmissions are realized with CDMA.

Description

Multi-passive reflection label access system based on code division multiple access and control method

Technical Field

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

Background

While conventional 802.11 wireless networks have met with great success in providing higher and higher transmission rates for each device, more and more internet of things devices have recently brought new requirements and challenges to future network paradigm designs. By 2020, it is expected that there will be 30 billion internet of things devices, a number that increases at a rate of 20% per year. These numerous devices will be deployed around people and even physically to provide various types of perception to improve people's quality of life. Unlike traditional laptops and smart phones, which require high transmission rates, these internet of things devices typically transmit data at low rates or in bursts. The main problems faced in this way are two, the first is the energy requirement, where the tiny amount of power required by these internet of things devices without power plugs comes from. Second, the number of connected IoT devices will be orders of magnitude larger.

In the past few years, backscatter communications have attracted much attention due to their low power consumption and ease of deployment characteristics. And most of the requirements of low power consumption are met by adopting a backscattering technology. However, existing work focuses on single node (label) scenarios, and multiple nodes (labels) cannot communicate simultaneously, severely limiting scalability to accommodate the large number of internet of things devices in the future.

In order to achieve high capacity, the current methods are all based on a scheme for avoiding collision, and mainly include two multiplexing modes: frequency division multiple access multiplexing and time division multiple access multiplexing. Wherein in frequency division multiple access multiplexing, different tags (tag) are assigned different frequency channels to communicate with a receiver. The tag 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 allocate a frequency band as a control node. Furthermore, the available bandwidth is extremely limited, which makes frequency division multiple access multiplexing a very expensive solution for large scale deployment. Time division multiple access multiplexing is the most popular multiplexing method for backscatter technology. The medium access scheme may be deterministic, typically a tree search based scheme, or probabilistic. The framework is based on aloha (fsa) approach. However, the receiver acts as a slotted ALOHA centralized control node, which coordinates the frame size in the network. Therefore, it cannot satisfy the requirements of the distributed scheme.

Disclosure of Invention

Based on the problems existing in the prior art, the invention aims to provide a multi-passive reflection tag access system based on code division multiple access and a control method, which can realize simultaneous transmission of a plurality of nodes on passive reflection signals by using code division multiple access.

The purpose of the invention is realized by the following technical scheme:

the embodiment of the invention provides a multi-passive reflection label access system based on code division multiple access, which comprises:

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 can distinguish the coded data sent by different passive reflection tags according to different PN sequence decoding areas.

The embodiment of the invention also provides a multi-passive reflection label access control method based on the code division multiple access, and the multi-passive reflection label access system based on the code division multiple access comprises the following steps:

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

and decoding the received coded data transmitted by the passive reflection tags by the receiver, and decoding according to the different PN sequences of each passive reflection tag so as to distinguish the coded data transmitted by different passive reflection tags.

The technical solution provided by the present invention shows that the cdma-based multiple passive reflective tags access method provided by the embodiment of the present invention has the following beneficial effects:

the coded data coded by each passive reflection tag comprises the unique PN sequence of the corresponding passive reflection tag, so that the receiver can distinguish the coded data sent by different passive reflection tags according to different PN sequences when decoding the received coded data, the simultaneous transmission of a plurality of nodes is realized by using code division multiple access in a passive reflection communication system comprising a plurality of passive reflection tags, and the system performance is improved by performing energy control on the passive reflection tags.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to these drawings without creative efforts.

Fig. 1 is a block diagram of a cdma-based multiple passive reflective tag access system according to an embodiment of the present invention;

fig. 2 is a schematic diagram of a cdma-based multi-passive reflective tag access system according to an embodiment of the present invention;

fig. 3 is a schematic diagram of signal transmission of a cdma-based multiple passive reflection tag access system according to an embodiment of the present invention;

fig. 4 is a diagram of an application example of a cdma-based multiple passive reflective tag access system according to an embodiment of the present invention;

fig. 5 is a flowchart of a cdma-based multiple passive reflective tag access control method according to an embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention are clearly and completely described below with reference to the specific contents of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention. Details not described in the embodiments of the present invention belong to the prior art known to those skilled in the art.

As shown in fig. 1, an embodiment of the present invention provides a multiple passive reflection tag access system based on code division multiple access, which is characterized by including:

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.

The above system further comprises: 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.

The above system further comprises: the receiver sends feedback data packets for energy control to the passive reflection tags, and the passive reflection tags 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 from the passive reflection tags to the receiver does not exceed a set value.

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

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.

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

after each passive reflection tag successfully sends the coded data, the passive reflection tag can receive the feedback data packet sent back from the receiver, and can adjust the power of the passive reflection tag according to the receiving rate of the received feedback data packet to carry out energy control.

In the system, the way that each passive reflection tag adjusts 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.

In the above system, the way of increasing the power of the passive reflective tag is as follows:

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

In the system, if a certain passive reflection tag cannot meet the requirement of data transmission after increasing the power of the passive reflection tag, a new passive reflection tag with the power meeting the requirement is reselected from the plurality of passive reflection tags to replace the passive reflection tag, and data transmission is performed.

In the above system, if a certain passive reflection tag increases its own power and still cannot satisfy the requirement of transmitting data, the following steps are performed:

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.

In the above system, the manner of reselecting a new passive reflective tag with satisfactory power from a plurality of passive reflective tags to replace the passive reflective tag is as follows:

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. As the number of selections increases, the probability decreases and tends to zero after multiple selections.

As shown in fig. 5, an embodiment of the present invention further provides a cdma-based multiple passive reflective tag access control method, where the cdma-based multiple passive reflective tag access system includes 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.

The method further comprises the following steps: 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.

The method further comprises the following steps: the receiver sends feedback data packets for energy control to the passive reflection tags, and the passive reflection tags 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 from the passive reflection tags to the receiver does not exceed a set value.

In the above method, the way for the receiver to send the feedback data packet for energy control to each passive reflection tag is as follows:

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

In the method, the way that each passive reflection tag adjusts its own power according to the receiving rate of the received feedback data packet to perform energy control is as follows:

In the above method, the passive reflective tag increases its own power by:

In the method, if a certain passive reflection tag cannot meet the requirement of data transmission after increasing its own power, a new passive reflection tag with power meeting the requirement is reselected from the plurality of passive reflection tags to replace the passive reflection tag, and data transmission is performed.

In the above method, if the power of a certain passive reflection tag cannot meet the requirement of transmitting data by itself, the following steps are performed:

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.

In the method, a new passive reflective tag with satisfactory power is reselected from the plurality of passive reflective tags to replace the passive reflective tag in the following manner:

The system and the method of the invention realize the simultaneous transmission of a plurality of nodes by using code division multiple access on the passive reflection signals, solve the problem of asynchronism among the plurality of signals by adopting the detection based on the correlation, and solve the problem of poor system performance by adopting an energy control mode on each passive reflection label for the problem that the energy of each node greatly influenced by the distance can influence the performance result of the whole system.

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

The multi-passive reflection tag access system based on the code division multiple access provided by the embodiment of the invention mainly comprises an excitation source, a plurality of passive reflection tags and a receiver. The excitation source may be a single tone signal or other signals (such as Wifi, bluetooth), which are common, and therefore, no specific description is given here. These signals need to be carrier signals that provide energy to the tag end.

Only a detailed block diagram of the tag and receiver is given below (see fig. 5).

At the tag end (i.e., a passive reflective tag, tag) is mainly composed of framing, encoding, energy control, on/off modulation and spectrum shifting.

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

The encoding process follows, that is, the framed data is encoded, and each tag has its own specific PN sequence. And the orthogonality of the PN sequences is better, so that each label can be distinguished through the PN sequence of each label during decoding, and data among each label cannot interfere with each other.

Energy control is then performed, based on the nature of CDMA, with the best performance being when each tag arrives at the receiver with an energy level that is the same. This property was verified by making preliminary experiments. The experimental scenario is shown in fig. 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 fig. 2. The points labeled a and B represent the excitation source 'Es' and the receiver 'Rx'. The point marked O is the origin of the coordinate system. The receiver then places the excitation source and 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 "tag 1" and "tag 2" in one test. For each test, 2 of the 5 tags (denoted 1,2,3,4 and 5 in table 1) were selected and placed randomly. Only partial results are presented in table 1 under space constraints. The difference is calculated as the ratio between the power difference and the larger 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% (the power of the two tags is similar), the error rate is much lower than otherwise. Therefore, power control can be performed using these results to improve the overall performance. A power control scheme is proposed and detailed information is presented in the next section.

Finally, data is loaded onto the reflected signal using on/off modulation. The process of how to upload the data is detailed as shown in fig. 3. If the tag wants to send a '1', it enables the square wave to control the state of the antenna for a period of the symbol time, otherwise the tag remains quiet and does nothing. Specifically, there are two layers of modulation to enable communication on the tag. The first layer is to generate spectral shifts by sending a square wave at frequency Δ f. The other is on/off (ook) modulation to enable the tag to send its own data. In this modulation, the af square wave acts as a carrier whose presence for a certain duration represents a binary 1 and whose absence for the same duration represents a binary 0. Thus, at the receiver end, the received signal may be decoded to '1' and no signal may be decoded to '0'. When the bit rate transmitted through the tag is f0, the '1' bit is transmitted to reflect the signal of Δ f/f0 cycles of the square wave. Note that f0 is less than Δ f. In practice, data at frequency f0 is first up-sampled to a frequency Δ f AND then 'AND' ed with a square wave of Δ f, as shown in FIG. 3. Therefore, it is convenient to adjust the bit rate. To change the bit rate, the only thing to change is the time period that the tag reflects or absorbs the signal.

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

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

Before decoding, it is first necessary to know which PN sequences are included, so user detection is first performed. User detection is performed using orthogonal properties 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 value set by the implementation, the user with the PN sequence is considered to be transmitting data.

Then, the detected PN sequence corresponding to the user is used for carrying out cross correlation on the whole frame, and decoding is realized. After user detection, cross-correlation is performed with the received frame using the detected user's PN sequence. If the correlation with the PN sequence representing '1' is higher than that representing '0', the chip is decoded to '1', and vice versa.

The ACK part is the part where the receiver broadcasts the ID of the tag that he correctly decoded to each tag for feedback, which is an important part of implementing energy control.

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

Selection of tag impedance: receiving the backscatter signal in I-Q space: i (t) and Q (t). The power of the received signal isSince the sampling rate is higher than the bit rate, the received data is first down-sampled. Each tag has its own PN code. When the receiver detects the preamble of a tag, it sends an ACK packet back to the tag. Therefore, when the tag receives few ACK feedback packets, it is considered that most of the transmitted packets are lost by this flag. 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 may be varied to tune the reflection coefficient Γ for power augmentation purposes. Pseudo code for the power control algorithm is proposed, which will omit the down-sampling and decoding process in the latter part. In this experiment, power control was performed cyclically to try each possible power level. To avoid the power control scheme falling into an infinite loop and because the tag size is relatively small, the number of energy selection levels above is not so large, so the current approach is to traverse all possible energy levels through one pass.

And (3) selecting a label: however, even with power control, some tags may still not be able to receive the ACK signal or the error rate of the system may be too high. The reasons are two. First, the backscatter signal is much weaker than the excitation signal. If some tags are far from the receiver, their power is still 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 in close spatial proximity, they can interfere with each other, resulting in poor system performance. In this case, it is not useful to increase the power. To address the limitations of the power control scheme, another optimization scheme, tag selection, is proposed. Tags with ACK feedback information below 70% are replaced if system performance fails to meet the expectations of power control. Therefore, the main problem is how to select a new tag. In the system of the present invention, the communication distance depends on two factors: (i) a distance between the excitation source and the label and (ii) a distance between the label and the receiver. The signal strength of the receiver Pr can be expressed by Friis path loss model as follows

From this equation, a theoretical result of the received signal strength for each location can be obtained, and therefore, a greedy algorithm (see pseudo code for the algorithm below) is used to select tags that move continuously in direction as the received signal strength increases, so as to select tags for locations where the receiver signal strength is high. When many tags are 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, a tag that has not transmitted data is randomly selected first, and then the difference between the theoretical received signal strength of the original tag and the new tag is calculated. If the difference is less than 0, the new tag is replaced with the old tag. However, to avoid the situation where these tag locations transmitting data are clustered, those tags that have other tags in the vicinity that are operating are discarded. However, rather than picking the tags in a better location, one is chosen randomly. If the new tag improves the received signal strength, the new tag is used. Otherwise, some new tags are received with a probability less than 1, which also decreases as the number of selections increases. In addition, when there are not many tags available in the environment, the location of some of the tags must be changed to improve system performance.

The algorithm pseudo code is as follows:

the code division multiple access based on the reflected signal can be realized by the method, and an implementation scene diagram is shown in fig. 4.

The system of the present invention supports up to 10 backscatter tags and backscatter data in a reliable and efficient manner. The system allows multiple tags to transmit concurrently through a simple power control scheme on the tag and can decode using any commercially available WiFi NIC while leaving the original WiFi communication unaffected. Indeed, due to the custom code design of the tag, the system can be deployed efficiently and can be friendly to collaborate with commodity WiFi devices. The design details of the system of the present invention are described and prototypes are constructed using an FPGA and existing WiFi equipment. The system of the invention realizes the multi-label bit rate of 8Mbps, and the farthest distance of the receiver can reach 5 m. The present system can improve backscatter throughput by more than a factor of 10 in challenging indoor scenarios with obstructions and interferences compared to single node solutions.

Examples

(a) First, a plurality of passive reflection tags are needed, as shown in fig. 4, each tag transmits different data to implement code division multiple access multiplexing in backscatter communication, and a square wave signal is used to control the on/off of an antenna to implement signal reflection and spectrum shifting.

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

(c) A receiver is also required to receive the reflected signal, process the received data, and run the models and algorithms of the present invention.

Those of ordinary skill in the art will understand that: all or part of the processes of the methods for implementing the embodiments may be implemented by a program, which may be stored in a computer-readable storage medium, and when executed, may include the processes of the embodiments of the methods as described above. The storage medium may be a magnetic disk, an optical disk, a Read-Only Memory (ROM), a Random Access Memory (RAM), or the like.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

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

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:

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:

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

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:

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:

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:

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:

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:

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:

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:

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: