KR101334270B1 - Neighbor Assisted WiFi beacon detection method using ZigBee - Google Patents

Abstract

Using a ZigBee interface, a mobile device that discovers a WiFi access point can provide the surrounding device with channel information of the WiFi access point to discover the WiFi access point. According to an embodiment of the present invention, a method for discovering an access point may include channel information of a WiFi access point from a second mobile device in which a first mobile device that has not found a WiFi access point is located around the first mobile device and discovers the WiFi access point. And receiving the WiFi access point using the channel information received by the first mobile device.

Description

Neighbor Assisted WiFi beacon detection method using ZigBee}

The present invention relates to a method for searching for an access point for a wireless Internet connection, and more particularly, to a method for efficiently searching for an access point using a low power ZigBee interface.

Recently, many wireless interfaces (WiFi, Bluetooth, Cellular, Zigbee, etc.) are becoming common in smartphones and small mobile devices capable of wireless communication. In such an environment, wireless Internet access through the WiFi interface has become a great concern. have. Wireless Internet access via the WiFi interface provides higher throughput than the Bluetooth and Zigbee interfaces, but also has the advantage of being less affected by Cross Technology Interference with other standards in the 2.4 GHz band.

In particular, due to the popularity of smart phones, more WiFi infrastructures are being installed to provide wireless Internet access in mobile environments. Through this, users who use small mobile devices can access the Internet network using the WiFi interface and receive various services (email checking, file transfer, web search, etc.) necessary every moment. However, since there are a limited number of places where WiFi can be used, it is necessary to discover a nearby WiFi AP from time to time, or find a new WiFi AP and establish a connection when it is out of the applicable radius of the currently connected WiFi AP.

A general method for periodically checking a WiFi AP is to scan a channel in which the WiFi AP exists periodically by using a WiFi interface. However, turning on the WiFi interface for periodic WiFi AP discovery consumes a lot of energy. Small mobile devices capable of wireless communication have a battery-operated feature, so increasing energy efficiency and throughput is one of the most important challenges.

Various methods have been proposed to utilize low-power wireless communication interface to solve the problem that energy consumption is increased by leaving the WiFi interface in the idle state to discover the WiFi network. Primarily, the WiFi interface is used only for actual communication and is a technique for reducing energy consumption of the WiFi interface by periodically sensing a low power wireless communication interface.

The WiFi interface shows higher throughput, transmission range, and energy efficiency (the amount of energy required to transmit a certain amount of data) for data transmission, but the idle state with no traffic, and the energy efficiency of finding a new AP. In comparison, the Zigbee interface supports lower data rates than the Bluetooth interface, but consumes less energy in transmit, receive, and idle states. Therefore, it is more efficient from an energy standpoint to find a WiFi AP available around you using the Zigbee interface instead of using the WiFi interface. However, since the WiFi interface and the Zigbee interface have different reception sensitivity and use different bandwidths (WiFi 20Mhz, Zigbee 2Mhz), there may be a problem in detecting a WiFi AP as a Zigbee interface.

SUMMARY OF THE INVENTION The present invention has been made in view of the above technical background, and an object of the present invention is to provide a method for efficiently searching for a WiFi AP using a low power Zigbee interface.

Another object of the present invention is a method for discovering an AP with the help of a neighboring Zigbee node in an area where the Zigbee interface cannot discover the WiFi AP by using different bandwidths between the WiFi and the Zigbee interface. It is to provide.

In order to solve such a problem, the present invention enables a mobile device that discovers a WiFi access point using a Zigbee interface to discover the WiFi access point by providing channel information of the WiFi access point to other devices around it.

That is, the method for searching for an access point according to an aspect of the present invention is a method in which a mobile device searches for a WiFi access point by using a Zigbee interface, wherein a first mobile device that does not find a WiFi access point is located around the first mobile device. Receiving channel information of the WiFi access point from a second mobile device that is located at and discovers the WiFi access point, and discovering the WiFi access point using the channel information received by the first mobile device It includes.

Here, it is preferable that the second mobile device is a device which has found the WiFi access point directly.

In addition, the channel information may include information of the discovery, not found, unknown for each channel for the entire WiFi channel.

According to the present invention, it is possible to expand the WiFi AP discovery area while maintaining high energy efficiency of the mobile device with high WiFi usage by utilizing the Zigbee interface with low power consumption. As a result, more clients can effectively discover WiFi APs through cooperative communication techniques.

1 shows a channel configuration of the 802.15.4 and 802.11 standards.
2 and 3 show noise levels measured by a Zigbee interface mounted on a client when the experimental WiFi AP is not turned on in an indoor lab environment. FIG. 2 shows external signals and obstacles (iron gates) with the measurement client in the laboratory. The measurement results in the disconnected situation, Figure 3 is the measurement results in a situation affected by the ambient interference with the measurement client outside the laboratory.
4 is a diagram illustrating a difference in reception sensitivity between a WiFi interface and a Zigbee interface.
FIG. 5 is a diagram illustrating a red zone in which a Zigbee interface mounted in a mobile device and a gray zone in which no AP is found when one AP exists.
6 shows a configuration of a cooperative communication message in which each node indicates the presence of a WiFi AP.
7 shows a 1-hop gain in the AP discovery method according to an embodiment of the present invention.
8 illustrates a gain ratio of one-hop cooperative communication according to an embodiment of the present invention.
9 illustrates a gray zone according to a result of sensing a signal of a WiFi AP through a Zigbee interface.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. In the present specification, the singular form includes plural forms unless otherwise specified in the specification. It is noted that the terms "comprises" and / or "comprising" used in the specification are intended to be inclusive in a manner similar to the components, steps, operations, and / Or additions.

Hereinafter, with reference to the accompanying drawings will be described in detail with respect to the access point search method with the help of the neighbor Zigbee node according to an embodiment of the present invention.

ZigBee and WiFi are defined in the 802.15.4 and 802.11 standards, respectively. The 802.15.4 and 802.11 standards are proposed to be applied in the unlicensed 2.4 Ghz band. 802.15.4 defines 16 2 Mhz channels and the 802.11b / g standard defines 13 20 Mhz channels.

1 shows a channel configuration of the 802.15.4 and 802.11 standards.

As shown in Figure 1, one 802.11 channel overlaps four 802.15.4 channels. Therefore, four 802.15.4 channels for one 802.11 channel have an opportunity to detect a transmission signal.

In the case of typical sensor node TelosB with 802.15.4, channels are sampled by taking an average of 8 symbols (128us) every 32us. This can provide enough time for multiple sensing, even for small beacon frames in the range of 80 to 200 Bytes.

Even among wireless devices that follow the same standard (802.15.4), various reception sensitivity may be different according to the type of antenna and the characteristics of the antenna. It may have a signal strength difference of 10 dB or more and different power spectral density (PSD). In addition, the difference between the band pass filters of the transmitting and receiving nodes may further reduce the strength of the received signal.

Energy sensing of the WiFi AP signal using the Zigbee interface may result in a difference in reception sensitivity similar to the above, and the inaccuracy of the RSSI itself may further deepen the difference. That is, this causes a gray zone in which a WiFi AP cannot be found as a Zigbee interface.

In order to confirm the existence of the gray zone experimentally, the TelosB node was connected to a Linux-based laptop capable of using the WiFi interface, and energy sensing was performed using the WiFi interface and the Zigbee interface.

Recently proposed ZiFi defines Threshold as -90dBm because it determines that the connection between client and AP is impossible or communication efficiency is not good when the signal of WiFi AP is smaller than -90dBm.

2 and 3 show the noise level measured by the Zigbee interface mounted on the client when the experimental WiFi AP is not turned on in the indoor laboratory environment. FIG. 2 is a measurement result in a situation where the measurement client is placed inside the laboratory and disconnected from external signals and obstacles (iron gate), and FIG. 3 is a measurement result in a situation where the measurement client is outside the laboratory and affected by ambient interferences. As shown in FIG. 2 and FIG. 3, it can be seen that a change in the noise level occurs according to the surrounding environment.

In order to find out the difference of RSSI sensed by WiFi interface and Zigbee interface when detecting WiFi AP signal, we experimented with various cases. Case 1 is a case where a wireless AP and a client are placed as line-of-sight (LOS) in a corridor with few obstacles outside the laboratory, and the distance between the two nodes is about 63m, and Case 2 has a large number of obstacles. In this case, the WiFi AP and the client are placed as non-line-of-sight (NLOS) inside the lab, and the distance between the two nodes is about 13m. Case 3 is a case where a WiFi AP is placed inside a laboratory and a client is located inside another laboratory about 37 meters away from the laboratory, and two nodes must pass through two sturdy steel gates for communication between the two nodes. In this experiment, the WiFi AP set the Linksys WRT54G AP to operate in 802.11b mode and measured the signal strength with the WiFi interface and the Zigbee interface.

4 shows the results of these experiments.

As shown in FIG. 4, it can be seen that the measurement results of the WiFi interface and the Zigbee interface differ by about 8 to 10 dB. In other words, by measuring the signal of the WiFi AP lower than the Zigbee interface below a certain level, there is a gray zone that cannot be found despite the presence of the WiFi AP.

Since the ZigBee interface consumes less power than the WiFi interface, you can significantly increase energy efficiency by turning off the WiFi interface and using the ZigBee interface to discover WiFi APs. However, as described above, when the gray zone occurs, there is a problem in that the opportunity to use WiFi is greatly reduced. In an embodiment of the present invention, coordination of neighboring Zigbee nodes is used to solve such a gray zone phenomenon.

FIG. 5 is a diagram illustrating a red zone in which a Zigbee interface mounted in a mobile device and a gray zone in which no AP is found when one AP exists.

As shown in FIG. 5, a plurality of mobile devices exist around one AP 100, and a Zigbee interface mounted on the mobile device relatively close to the AP 100 may discover an AP. The zone 310 and the gray zone 320 where the Zigbee interface mounted on the mobile device cannot discover the AP appear separately.

Each client turns off the WiFi interface and periodically monitors the existence of AP for each WiFi channel through the ZigBee channel close to each channel using the Zigbee interface. At this time, a node that directly discovers an AP in at least one WiFi channel becomes a red node 210_1, 210_2, and 210_3 which periodically informs the neighboring nodes of the presence of the AP. Nodes that determine that the AP does not exist in the channel being monitored become gray nodes 220_1 and 220_2, and if there are red nodes 210_1 and 210_2 in the vicinity, they are not directly discovered by the help of the red nodes 210_1 and 210_2. Wi-Fi AP is found by receiving channel information about the failed AP. In the case of the gray node, if the AP information received from the red node is additionally delivered to other gray nodes, wrong information may be transmitted to the nodes of the region where the real AP does not exist, because the AP information may be transmitted to the other gray nodes. The strategy is to forward only the Red nodes that find the AP directly.

Each node informs the presence of the WiFi AP is configured as shown in FIG.

For the entire WiFi channel, if a WiFi AP is found for each channel, 11 (detected), 10 (not detected) if not found, 00 (unknown) if not yet determined. In the case of 802.11 b / g, 26 bits of data are periodically transmitted over 13 channels.

Since the Zigbee interface transmits less energy than the received energy due to the characteristics of the chip, periodically transmitting a cooperative communication message indicating whether a WiFi AP exists does not reduce energy efficiency. In addition, a quorum-based monitoring and transmission scheme is applied to hint exchange of WiFi APs through the Zigbee interface.

According to an embodiment of the present invention, a 1 hop cooperative communication technique of a Zigbee interface is used to increase the WiFi AP discovery radius. In this case, the gain that can be obtained by the 1-hop cooperative communication scheme is an embodiment of the present invention among the clients existing within the 1-hop communication distance (TX _R to TX _R + TX _Z ) of the Zigbee interface from the region outside the Red Zone for the WiFi AP. It can be represented by the ratio of the nodes that can discover the WiFi AP through the method according to.

As shown in FIG. 7, an area of nodes that may help clients existing between TX _R and TX _R + TX _Z may be defined as A _I , and this area is represented by Equation 1 below. Is expressed.

가 된다.Assuming that node placement in the region follows a Poisson point process of density l, the average probability that nodes in region A _I can assist clients between TX _R and TX _R + TX _Z is given by 2]. Where Pr (A _I ) is the probability that at least one cooperating node exists in the region A _I

.

The ratio of nodes that can be assisted when there is a particular client between TX _R and TX _R + TX _Z according to the number of nodes present from the 1 hop perspective of the client is shown in FIG. 8. Even when there are 20 nodes, it can be seen that about 81% of clients can additionally discover the WiFi AP through the 1-hop cooperative communication technique.

Path-loss exponents in indoor free spaces can vary between 1.6 and 3.5, depending on the environment. In the case of open corridors, there is a tendency to appear small due to the presence of wavelength guardian, and in the case of a large number of furniture in the room, various paths of multipath fading may occur in addition to the loss of free space, resulting in a large path-loss exponent.

Considering this environment, the result of sensing the signal of the WiFi AP through the WiFi interface can be obtained through a simplified path-loss model (Equation 3) in the free space. In Equation 3, K is determined by the characteristics of the antenna and the average channel attenuation, g is a path-loss exponent, and d ₀ is a reference distance to the antenna wave field. In addition, the result of sensing the signal of the WiFi AP to the Zigbee interface is shown as shown in Figure 9 in consideration of the error in the reception sensitivity obtained by the experiment in addition.

Since the effective WiFi communication distance is generally about 100m, the gray zone that occurs below 100m affects the actual WiFi AP discovery. In the path-loss exponent 3 environment, the Zigbee interface becomes a threshold of -90dBm at which the Zigbee interface recognizes the WiFi AP. Thus, one hop cooperation of the Zigbee interface can cover most areas. It is out of range. That is, with the Zigbee interface, the WiFi AP can be found up to about 29m, but the signal delivery distance of the actual WiFi AP is about 50m.

Although the present invention has been described above with reference to preferred embodiments, the apparatus and method of the present invention are not necessarily limited to the above-described embodiments, and various modifications and variations can be made without departing from the spirit and scope of the invention. Accordingly, the appended claims are intended to embrace all such modifications and variations as fall within the true spirit of the invention.

Claims (3)

A method for a mobile device to discover a WiFi access point using a Zigbee interface,
Periodically transmitting, by a first mobile device, a Zigbee cooperative communication message indicating whether the WiFi access point exists;
Receiving a Zigbee cooperative communication message from the first mobile device by a second mobile device that does not find the WiFi access point; And
And discovering the WiFi access point using channel information included in the Zigbee cooperative communication message received by the second mobile device. The method of claim 1,
And the first mobile device is a device for which the first mobile device has found the WiFi access point directly. The method of claim 1, wherein the channel information,
A method for discovering an access point that includes discovery, not found, and unknown information for each channel for all WiFi channels.

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