KR20100115991A - Handover method considering interference in wireless lan systems - Google Patents

Abstract

The present invention relates to a method for performing handover in consideration of interference in a WLAN system, the communication terminal measuring received signal strength of neighboring APs, and for each candidate AP whose received signal strength is greater than or equal to a preset reference value, respectively. Calculating a signal-to-interference ratio of a; calculating an interference margin of each candidate AP using the signal-to-interference ratio calculated above; and subtracting the interference margin from the received signal strength of each candidate AP, Acquiring a selection criterion value and selecting an AP having the largest selection criterion value as a new AP to handover to.

Description

Handover method considering interference in Wireless LAN systems

The present invention relates to a method for performing handover in a wireless LAN system. In particular, in a wireless LAN system, a handover is performed in consideration of interference when a terminal performs handover between APs in an environment where a plurality of APs exist and neighboring APs interfere with each other. It is about technology to do.

At present, IEEE 802.11-based wireless LAN system using an unlicensed frequency band (2.4GHz, 5GHz) to enable an effective service at a low cost. In addition, it uses a well-designed physical layer and Medium Access Control (MAC) to provide high data rates up to 54Mbps (IEEE 802.11a / g).

The WLAN system includes a plurality of basic service sets (BSSs), and each service set (BSS) includes an access point (AP) connected to a wired network and a plurality of communication terminals.

If the communication terminal is wired through the AP, and the service areas of different BSSs overlap each other, the APs of the BSS use different channels so that even if the communication terminals move, handover between the APs is seamless. You can get the service.

At this time, the communication terminal performs a handover when the received signal strength of the currently connected AP (current AP) is less than the set threshold. In this case, since each channel of the frequency domain provided by IEEE 802.11 overlaps with each other, quality may be greatly degraded due to an interference effect when communicating with other communication terminals using the same channel or adjacent channel in a neighboring AP. .

Therefore, when selecting an AP (new AP) to move based only on the strength of the received signal, a phenomenon may occur in which quality is greatly degraded by interference after accessing a new AP.

According to the present invention, a handset considering interference in a WLAN system capable of suppressing deterioration in quality by selecting a new AP in consideration of interference of neighboring APs using the same channel or adjacent channel when the communication terminal hands over in a WLAN system. This is to provide a method of performing over.

Technical problems to be achieved by the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned above may be clearly understood by those skilled in the art from the following description. There will be.

Technical method of the present invention for achieving the above object, the step of measuring the received signal strength of each of the peripheral AP; Calculating respective signal-to-interference ratios for candidate APs whose received signal strength is equal to or greater than a preset reference value; Calculating an interference margin of each candidate AP using the signal-to-interference ratio calculated above; And subtracting interference margins from the received signal strength of each candidate AP to obtain selection criteria of each candidate AP, and selecting the AP having the largest selection criteria as the new AP to hand over. It is done.

In detail, the signal-to-interference ratio is a ratio of received signal strengths of adjacent APs interfering with a specific AP, and when calculating the signal-to-interference ratio, interference of the same channel and adjacent channels is considered in consideration of the frequency overlapping of the channels. It is characterized by obtaining using the interference factor for the effect of.

The interference factor is set differently according to the degree of adjoining the reference channel, and the interference factor is set to the same value as the reference channel and the first adjacent channel to the first value, and the second adjacent channel is smaller than the first value because the overlapping factor is small. The second value is set to a small second value, and the remaining channels are set to a third value smaller than the second value.

If there is one neighboring AP interfering with the candidate AP, and the neighboring AP is adjacent to the candidate AP, the signal-to-interference ratio may be obtained as a difference in received signal strengths between the two APs.

The reference value is a value obtained by adding a preset delta value to a preset threshold value, wherein the threshold value is a determination reference value for the received signal strength of the current AP, which causes the communication terminal to request the emission of probe signals from neighboring APs. It is done.

As described above, in the present invention, the method for performing handover in consideration of interference in the WLAN system may select an AP having less interference since the new AP is selected in consideration of the interference margin as well as the signal strength. There is an advantage that can maintain good communication quality.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention. Like elements in the figures are denoted by the same reference numerals wherever possible. In addition, detailed descriptions of well-known functions and configurations that may unnecessarily obscure the subject matter of the present invention will be omitted.

1 is a block diagram illustrating a general wireless LAN system.

The WLAN system is composed of a plurality of Basic Service Sets (BSS) as shown in FIG. 1, and each of the plurality of Service Sets BSS1 to BSS3 has overlapping service areas.

Each service set includes an access point (AP) connected by a wired network, and a plurality of communication terminals in which a communication channel is formed with each access node (AP). That is, each communication terminal (T1 ~ T5) is connected to the wired network through the AP.

The communication channels of the service sets are formed of different channels from the adjacent service sets.

Each AP of the adjacent service sets uses a different channel, so that when a communication terminal moves a service area or moves to a location where the service areas overlap, a handover between APs may be performed to receive a seamless service. Here, the communication terminal performs a handover when the received signal strength of the currently connected AP (current AP) is less than or equal to the set threshold.

In the case of a wireless LAN, a frequency band of 2.4 GHz or 5 GHz is used, and each frequency band is composed of a plurality of channel sets. Although the number of channels of a wireless LAN varies according to countries and regions, 13 channels are used in a domestic case.

FIG. 2 is a diagram illustrating a general 2.4 GHz channel arrangement. As shown, in the case of IEEE 802.11, the interval of each channel is 5 MHz, but the bandwidth of each channel is 22 MHz based on 30 dBm attenuation.

At this time, the two channels, that is, four channels in total, overlap with each other before and after the center of the specific channel. For example, the center of channel 6 (CH 6) overlaps the frequency bands of channel 4 (CH 4), channel 5 (CH 5), channel 7 (CH 7) and channel 8 (CH 8). .

Ideally, if APs are allocated channels without overlapping frequencies, for example, if AP1 is assigned to channel 1, AP2 to channel 6, and AP3 to channel 11, interference due to frequency overlap is considered. no need. However, since the number of channels that can be allocated without overlapping frequencies is limited to three or less in the 2.4 GHz band, commercial WLAN systems often allocate randomly to APs, so interference by frequency overlap should be considered.

In addition, IEEE 802.11 uses a carrier sensing multiple access / collision avoidance (CSMA / CA) scheme to prevent collision (or interference) of communication terminals using the same AP. When the communication terminal detects whether another terminal is using the same channel when transmitting data, if it is used, the other terminal transmits data when the channel is not used after the channel is used. In the case of wired LAN system, data collision is detected and retransmitted after a certain time.However, in case of wireless, it is difficult to detect data collision, so the communication terminal transmits a short frame called RTS to AP before transmitting data. The channel is reserved and the AP emits CTS frames to other terminals. Other terminals wait for the reserved time and then transmit data to avoid collisions. Therefore, when the same channel is used in the neighboring AP, only the data rate may be reduced or the voice delay may be increased without being affected by the interference.

However, in actual commercial WLAN systems, CSMA / CA is often not implemented correctly (CTS transmission from AP is required in IEEE 802.11, but RTS transmission is optional in communication terminals). Interference may be affected.

Therefore, since each channel of the frequency band provided by IEEE 802.11 overlaps with each other, the quality may be greatly degraded due to the interference effect when communicating with other communication terminals using the same channel or adjacent channel in the neighboring AP. have.

3 and 4 are flowcharts illustrating a handoff process of a communication terminal in a WLAN system according to an exemplary embodiment of the present invention, which will be described with reference to FIGS. 1 and 2. In the present embodiment, a description will be given on the assumption that the communication terminal T1 moves from the current AP (AP1) to another service area or to an overlapping service area in the WLAN system.

First, the communication terminal T1 periodically measures the received signal strength which is the strength of the signal received from the AP AP1 in a state of being connected to the current AP AP1 in the WLAN system (S10). In general, Received Signal Strength Indication (RSSI) is expressed in dBm.

Subsequently, the communication terminal T1 compares the average received signal strength measured for several seconds with a preset threshold value to determine whether the average received signal strength is less than or equal to the threshold value (S20). If the average received signal strength is less than or equal to the threshold value, the communication terminal T1 emits a probe request signal to neighboring APs AP2, AP3, ... (S30). Of course, if the average received signal strength is greater than or equal to the threshold, the communication terminal T1 continues to form a communication channel with the current AP (S40).

Subsequently, the communication terminal T1 receives a response signal for a probe request from adjacent neighbor APs AP2, AP3, ... (S50). The probe response signal includes channel information used by each AP (AP2, AP3, ...). Subsequently, the communication terminal T1 obtains received signal strength and corresponding channel information through the response signals received from each AP (AP2, AP3, ...) (S60).

The communication terminal T1 compares the received signal strengths of the respective APs AP2, AP3, ... with a predetermined reference value and checks and extracts whether there are candidate APs whose received signal strength exceeds the reference value (S70). ). The reference value is a value obtained by adding up the threshold value and a preset delta value (threshold value + delta value).

For example, if the preset threshold is -75 dBm and the delta is 8 dB, the reference value is -67 dBm. Since the reference value is set to -67 dBm, APs having a received signal strength greater than -67 dBm will be candidate APs.

If there are a plurality of candidate APs, an optimal AP is selected in consideration of the received signal strength of each candidate AP and interference parameters of neighboring APs related to the candidate AP at the same time (S80).

When there is no candidate AP whose received signal strength of the neighboring APs exceeds the reference value, the communication terminal T1 maintains a communication channel with the current AP AP1 and radiates probe requests back to the neighboring APs.

As the interference parameter for each AP, a signal-to-interference ratio (SIR), an interference factor, an interference margin, and the like may be considered. The larger the value, the better the signal-to-interference ratio, and the smaller the value of the interference factor and the interference margin.

First, the communication terminal T1 calculates signal-to-interference ratios for respective candidate APs as shown in FIG. 4 (S81). At this time, the signal-to-interference ratio uses an interference factor.

Here, the signal-to-interference ratio refers to the ratio of the received signal strength of the adjacent AP that interferes with a specific AP, for example, the signal-to-interference ratio (SIR (k)) of the k-th AP is the interference factor as shown in Equation 1 below. Can be obtained using

Here, RSSI (k) is the received signal strength of the k-th AP, RSSI (i) is the received signal strength of the i-th AP affecting the k-th AP, and a (k, i) is the k-th AP. Interference factor for interference on the AP.

Here, the interference factor a (k, i) may be determined by the frequency overlap between the channel used by the AP (k) and the channel used by the AP (i), that is, by FIG. 2. The influence of the interference may consider about two channels that are adjacent to the same channel. The interference factor is set differently according to the degree of adjoining the reference channel. The interference factor is set to the same value as the reference channel and the first adjacent channel to the first value, and the first adjacent channel in the reference channel has a small overlap. The second channel may be set to a second value smaller than the value, and the remaining channels may be set to a third value smaller than the second value. For example, the interference channel is set to 1 because the reference channel, the same channel, and the first adjacent channel have a large overlap, which greatly affects interference, and the interference factor is set to 0.2 because the second adjacent channel has a small overlap. The remaining channels can set the interference factor to zero.

As such, the interference factor of the interference of AP (i) on AP (k) is shown in a table for each channel as shown in FIG. 5. For example, if AP1 uses channel 3, if AP2 uses channel 4, the interference factor is 1 for interference from AP2 to AP1. If AP3 uses channel 1, the interference factor for interference from AP3 to AP1 is used. Can be set to 0.2.

On the other hand, when the received signal strength is expressed in units of dB, the calculation may be complicated by Equation 1. When the received signal strength is expressed in dB, it can be calculated by approximating Equation 1. For example, when one neighboring AP interferes with a specific AP and the interference factor is 1, the signal-to-interference ratio (SIR) can be easily obtained as a difference in received signal strength between two APs as shown in Equation 2 below.

Here, RSSI (k) is the received signal strength of the k-th AP, RSSI (i) is the received signal strength of the i-th AP affecting the k-th AP.

The interference margin of each AP is calculated using the signal-to-interference ratio obtained as described above. In this case, the communication terminal T1 may have a different method of obtaining the interference margin depending on whether the signal-to-interference ratio exceeds a preset interference reference value. The interference margin is calculated using the signal-to-interference ratio as a weight for the received signal strength to consider the influence of the interference of the neighboring AP (S83).

That is, the interference margin (Margin_ _{interference)} is a signal to interference ratio is equal to or greater interference margins than the interference reference value as shown in Equation 3 below to 0dB, and the signal-to-interference ratio when less than the interference reference value in the interference reference value obtained by subtracting the signal-to-interference ratio (dB) Let the value be the interference margin. For example, the interference reference value may be set to about 3dB to 7dB. Here, the interference reference value may be determined as a value of the signal-to-interference ratio that determines that the influence of the interference can be ignored.

SIR ≥ Ref_ _interference , Margin_ _interference = 0dB

SIR <Ref_ _interference , Margin_ _interference = Ref_ _interference -SIR [dB]

Here, SIR is the signal-to-interference ratio of each AP, Ref_ _interference is the interference pre-set reference value.

If the signal-to-interference ratio is equal to or greater than the interference reference value, no degradation is caused by interference of neighboring APs. If the signal-to-interference ratio is smaller than the interference reference value, the degradation is caused by interference of neighboring APs. Therefore, if the signal-to-interference ratio is smaller than the interference reference value, the interference margin increases in proportion to the signal-to-interference ratio, so that the interference margin (dB) can be used as a weight to increase the probability of selecting an AP with less interference.

Subsequently, the communication terminal T1 subtracts the calculated interference margin of each AP from the received signal strength to calculate a selection reference value (received signal strength-interference margin) of each AP (S85), and the communication terminal T1 each The AP having the largest value among the AP selection criteria values is determined as a new AP (S87), and handover is performed to the determined new AP (S90).

Here, when performing the handover considering the interference, the advantage of using the selection reference value instead of the signal-to-interference ratio (received signal strength-interference margin (dB)) is less affected by the interference prediction error.

An embodiment of selecting a new AP using the obtained interference margin is shown in FIG. 6.

That is, it is assumed that the current communication terminal T1 is connected to AP1, the start condition of handover is -75dBm, and the selection condition of the candidate AP is -67dBm. Since the received signal strength of AP1 is -80dBm lower than -75dBm, the probe request signal is radiated to the neighboring AP for handover.

As a result of analyzing the probe response signals of the neighboring APs, APs AP2, AP3, AP4 and AP5 were found to have received signal strengths greater than -67dBm, which is a selection condition of the candidate AP. At this time, AP2 uses channel 2, AP3 uses channel 9, AP4 uses channel 6, and AP5 uses channel 5. Here, AP5 may be selected using the conventional method because AP5 is the largest as -61dBm when comparing only the received signal strength. However, since AP5 uses channel 5 and AP4 uses an adjacent channel, when AP4 performs handover to AP5 and accesses AP5, communication quality may be degraded due to adjacent channel interference from AP4.

However, when the interference reference value is set to 5 dB and the interference margin is calculated using Equations 1 to 3, the interference margin is subtracted from the received signal strength of each AP to calculate the selection reference value (received signal strength-interference margin). Here, AP3 may be selected as a handover target because AP3 has the largest selection criterion value of −63 dBm. Since AP3 uses channel 9 and there is no interference from neighboring APs using the same channel and adjacent channels (channels 7 to 11), AP3 has lower signal strength than AP5, but better communication quality can be obtained.

As such, the communication terminal T1 determines an AP having the best selection criterion value as a new AP and performs a handover to the determined new AP.

The above embodiments are merely preferred examples, and may be variously changed as necessary.

For example, in the case of the signal-to-interference ratio, the received signal strength can be obtained by dividing the received signal strength by the interference amount (interference factor ㅧ interference received signal strength), as shown in Equation 1, but as shown in Equation 2, the interference amount (interference factor ㅧ interference received signal strength) You can also subtract).

In addition, as an approximation method of signal-to-interference ratio, there may be various ways as follows depending on the situation.

First, when there is no interfering AP around the reference AP (A), the signal-to-interference ratio can be obtained using the equation SIR (A) = RSSI (A)-100 dBm (noise).

Secondly, if there is one interfering AP (AP (B)) around the reference AP (A), and the interfering AP (B) uses the adjacent first channel or the same channel, the equation SIR (A) = RSSI (A) − Signal-to-interference ratio can be obtained using RSSI (B), and if the interfering AP (B) uses an adjacent second channel, the equation SIR (A) = RSSI (A)-(RSSI (B) + interference factor [ dB]) can be used to obtain the signal-to-interference ratio. If the interference factor is 0.2, -7dB, 0.4 is -5dB, 0.6 is -2.2dB, 0.8 is -1dB, and 1 is 0dB.

Third, when there are two interfering APs (AP (B) and AP (C)) around the reference AP (A), and RSSI (B)> RSSI (C), the equation RSSI (B) -RSSI ( According to C), signal-to-interference ratio can be approximated.

Preferred embodiments of the present invention are disclosed for purposes of illustration, and those skilled in the art will be able to make various modifications, changes, and additions within the spirit of the present invention. Therefore, such modifications, changes and additions should be determined not only by the claims below, but also by equivalents to those claims.

1 is a block diagram illustrating a general wireless LAN system.

2 is an exemplary view showing an arrangement of a typical 2.4 GHz band channel.

3 and 4 are flowcharts illustrating a handoff process of a communication terminal in a WLAN system according to an embodiment of the present invention.

5 is a diagram illustrating an interference factor according to the degree of overlap of each channel according to an embodiment of the present invention.

6 is a diagram illustrating parameters for selecting a new AP by a communication terminal connected to AP1 according to an embodiment of the present invention.

Claims (9)

Measuring received signal strength of neighboring APs; Calculating respective signal-to-interference ratios for candidate APs whose received signal strength is equal to or greater than a preset reference value; Calculating an interference margin of each candidate AP using the signal-to-interference ratio calculated above; And Subtracting the interference margin from the received signal strength of each candidate AP to obtain selection criteria of each candidate AP, and selecting the AP having the largest selection criteria as the new AP to handover to; A method for performing handover considering interference in the network. The method of claim 1, The signal-to-interference ratio is a ratio of the received signal strength of the adjacent AP that interferes with a specific AP handover performing method in consideration of interference in a WLAN system. The method of claim 1, The method for performing handover in consideration of interference in a WLAN system, wherein the signal-to-interference ratio is calculated using an interference factor on the influence of interference between the same channel and adjacent channels in consideration of the frequency overlapping degree of the channel. The method of claim 3, wherein The interference factor is a method for performing handover in consideration of interference in a WLAN system, characterized in that differently set according to the degree adjacent to the reference channel. The method of claim 3, wherein The interference factor is the same channel as the reference channel and the first adjacent channel is set to the first value, the second adjacent channel is set to a second value smaller than the first value because the overlap is small, the remaining channels are smaller than the second value A method for performing handover in consideration of interference in a WLAN system, characterized in that set to a third value. The method of claim 1, If one neighboring AP interferes with the candidate AP, and the neighboring AP is adjacent to the candidate AP, the signal-to-interference ratio is obtained by considering the interference in the WLAN system, characterized in that the difference in the received signal strength between the two APs. How to do over. The method of claim 1, The signal-to-interference ratio (SIR (k)) is a method for performing handover in consideration of interference in a wireless LAN system, characterized in that it is obtained using the interference factor as shown in Equation 1 below. Equation 1

However, RSSI (k) is the received signal strength of the k-th AP, RSSI (i) is the received signal strength of the i-th AP affecting the k-th AP, and a (k, i) is the k-th AP. Interference factor for interference on the AP. The method of claim 1, The interference margin Margin_ _interference is calculated using Equation 2 below. Equation 2 SIR ≥ Ref_ _interference , Margin_ _interference = 0dB SIR <Ref_ _interference , Margin_ _interference = Ref_ _interference -SIR [dB] However, SIR is the signal-to-interference ratio of each AP, and Ref_ _interference is a preset interference reference value. The method of claim 1, The reference value is a value obtained by adding a preset delta value to a preset threshold value, wherein the threshold value is a determination reference value for the received signal strength of the current AP, which causes the communication terminal to request the emission of probe signals from neighboring APs. Handover method considering interference in a WLAN system.

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