DYNAMIC FREQUENCY SELECTION SCHEME FOR IEEE 802.11 WLANs

Dynamic frequency selection scheme for IEEE 802.11 WLANs

The present invention relates to a dynamic frequency selection (DFS) mechanism in an IEEE 802.11(h) wireless local area network (WLAN), wherein the operating channel of the WLAN is determined by an access point (AP) based on a channel quality report. It is dynamically selected according to the channel decision made by

Physical properties for wireless local area networks (WLANs) supporting medium access control (MAC) and physical layer units are described in the international standard ISO/IEC 8802-11 "Information Technologuey--Telecommunications and It is specified in the IEEE 802.11 standard specified in "information exchange area networks", 1999 Edition. The standard specifies two variants of WLAN: infrastructure-based and ad-hoc type. In earlier networks, communication typically only occurs between wireless nodes called stations (STAs) and access points (APs), not directly between wireless nodes as in later networks. STAs and AP that are within the same radio coverage are known as Basic Service Set (BSS).

When two adjacent basic service sets (BSS), called overlapping BSSs, are located close to each other and operate on the same channel, the quality of service (QoS) required due to possible mutual interference between the overlapping BSSs. ) is difficult to support. Additionally, other co-located systems close to a particular STA (eg, a HIPERLAN/2 device as disclosed in the European Radio Communications Committee (ERC) regulations) may cause reception interference. Avoiding interference by carefully planning channel assignments to BSSs prior to WLAN deployment is not always possible, especially in a home/office environment where other WLAN devices operate independently in the vicinity, eg in neighboring houses or offices.

Accordingly, there is a need for a dynamic frequency selection (DFS) scheme that can be incorporated into the IEEE 802.11 standard that allows an access point (AP) to select a channel for all stations (STA) associated with its basic service set (BSS). exist. To achieve this, the present invention provides 802.11 medium access control (MAC) and 802.11a physical layer (PHY) (IEEE 802.11 WLAN operation in 5 GHz unlicensed bands) that enable dynamic selection of a frequency channel for operation of the network. for) to introduce changes to This facilitates meeting the requirements imposed by the European Radio Communications Committee (ERC) and enhances the performance of 802.11 WLAN operation in the 5 GHz band.

The present invention relates to a method and system for dynamic frequency selection in a wireless local area network (WLAN), wherein each channel is dynamically selected according to criteria determined by an access point (AP).

According to an aspect of the present invention, a method is provided for dynamically selecting a communication channel between an access point (AP) and a plurality of stations (STA) located within a coverage area of a basic service set (BSS). The method includes determining whether a new channel used by a plurality of wireless STAs is required, requesting, by an AP, at least one of a plurality of STAs to measure a channel quality, channel quality of a plurality of frequency channels transmitting a report from the requested STAs to the AP, wherein the channel quality report is a clear channel assessment (CCA) busy period of all channels measured by a plurality of STAs and a received signal strength indication (RSSI) the transmitting step, determining whether a signal from an adjacent BSS is received by a plurality of STAs, according to a channel quality report, regulation requirements and channel for use in communication between the plurality of STAs and the AP according to the channel quality report. Selecting a new channel based on quality, communicating information about the new channel to the plurality of STAs from the AP, and switching all STAs to the new channel.

According to another aspect of the present invention, there is provided a system for dynamically selecting a communication channel between an AP and a plurality of STAs located within a coverage area of a BSS in a WLAN. The system includes means for determining whether a new channel used by a plurality of STAs is required, means for requesting, by an AP, a channel signal quality measurement to at least one of the plurality of STAs, one of the plurality of STAs. Means for transmitting a channel quality report of a plurality of frequency channels between at least one and an AP, the channel quality report comprising clear channel assessment (CCA) busy periods of all channels measured by a plurality of STAs and a received signal means for transmitting, including a strength indication (RSSI), means for determining whether a frame from an adjacent BSS is received by a plurality of STAs, if an adjacent BSS signal or periodicity is detected, the AP and the plurality of means for selecting a new channel based on least degradation of channel quality for use in communication between STAs; and means for communicating information about the new channel from the AP to the plurality of STAs.

A more complete understanding of the method and apparatus of the present invention may be obtained by reference to the following detailed description taken in conjunction with the accompanying drawings.

A more complete understanding of the method and apparatus of the present invention may be obtained by reference to the following detailed description taken in conjunction with the accompanying drawings.

1 is a diagram showing the structure of a wireless communication system to which embodiments of the present invention are applied.

2 is a simplified block diagram of each station (STA) and access point (AP) within a specific basic service set (BSS) in accordance with an embodiment of the present invention;

Fig. 3 is a flow diagram illustrating operational steps for selectively switching to a new channel in accordance with an embodiment of the present invention;

4 is a diagram illustrating a format of a channel measurement request frame that may be used to transmit information from an AP to an STA according to an embodiment of the present invention;

5A and 5B are diagrams illustrating a format of channel measurement method information elements that may be used to transmit information from an AP to STAs according to an embodiment of the present invention;

6A-6E are diagrams illustrating a format of channel measurement report information elements that may be used to transmit information from a plurality of STAs in a BSS to an AP in accordance with an embodiment of the present invention;

7 is a flow diagram illustrating a process for determining a new channel by an AP based on a channel quality report in accordance with an embodiment of the present invention;

8 is a diagram illustrating a format of a frame body used to transmit announcement data from an AP to a plurality of STAs according to an embodiment of the present invention;

In the following detailed description, for purposes of explanation and not limitation, specific details are set forth, such as specific structures, interfaces, techniques, etc., in order to provide a thorough understanding of the present invention. For simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the detailed description of the present invention with unnecessary detail.

1 shows a representative network to which embodiments of the present invention are applied. In accordance with the principles of the present invention, an access point (AP) selects a channel based on channel quality reports for all stations (STA) associated with its basic service set (BSS) which also reduces the interface with other co-located systems. A dynamic frequency selection (DFS) scheme is provided that allows for selection. Note that the network shown in Fig. 1 is made small for the sake of illustration. In practice, most networks include a much larger number of mobile STAs.

The present invention has applications for wireless local area networks (WLANs) by allowing an AP to provide a new radio link to all stations (STAs) associated with its BSS. For example, BSS₁STA₃is the neighboring BSS₂may be in an overlapping region with₂STA at₂experiences contentions from Alternatively, STA₃may experience interference from nearby non-802.11 compliant devices belonging to other authorized operators, such as satellites and radar systems. To this end, the present invention introduces changes to the 802.11 MAC and 802.11 PHY specifications that enable dynamic selection of a frequency channel for operation of the network. This facilitates meeting the requirements imposed by the European Radiocommunications Commission (ERC), which enhances the performance of 802.11 WLAN operation in the 5 GHz band or other band range, ie 2.4 GHz. It will be apparent to those skilled in the art that the present invention can be readily extended to other frequency bands such as 2.4 GHz using different physical layer specifications such as the IEEE 802.11b PHY specification.

Referring to FIG. 2 , each STA and AP in the WLAN shown in FIG. 1 may include a system having the structure shown in the block diagram of FIG. 2 . Both AP and STA have display 20 , CPU 22 , transmitter/receiver 24 , input device 26 , storage module 28 , random access memory (RAM) 30 , read-only memory (ROM) ) 32 and a common bus 40 . Although the detailed description refers to terms commonly used to describe particular computer systems, the present detailed description and concepts are equally applicable to other processing systems, including systems having structures not similar to those shown in FIG. applies. A transmitter/receiver 24 is coupled to an antenna (not shown) to transmit the desired data, and the receiver converts the received signals into corresponding digital data. The CPU 22 operates under the control of an operating system contained in the ROM 32 and operates under the control of a wireless local area network (WLAN) by enabling the AP to provide a new channel or radio link to all stations (STAs) associated with its BSS. ), use the RAM 30 to perform frequency selection.

Now, the principle of the operating steps according to the invention in the selection of a new channel for all stations (STA) by the AP is described below.

Referring to FIG. 3 , the steps of the present invention include the following steps, monitoring of channels 100 , selecting a new channel by the AP 200 , sending a channel switch announcement 300 , and and switching (400) to a channel. Monitoring 100 of channels includes three sub-steps: (1) channel measurement by the AP, (2) a request for channel measurement by the AP, and (3) measurement report by STAs.

<u>Monitoring of channels (step 100 in FIG. 3 )</u>

In the following cases (but not necessarily limited thereto), (1) a specific base-service-set (BSS) is newly formed by the AP (step 401), and (2) the AP is operating a given BSS without any associated STA, (3) one or more STAs in the AP and/or BSS experience a permanently poor communication channel, (4) overlapping of the BSSs causes channel interference, ( 5) Channel monitoring may be initiated when one of the occurrences of detection of other authorized operators occurs. Should any of these cases occur, the AP may dynamically select a new radio link to operate its BSS. Thus, before switching to the best radio link, the AP needs to know the current and status of other channels as well as the presence of other authorized operators, either by detecting channel conditions directly or by requesting channel conditions from associated STAs.

(1) Channel measurement by AP

When the AP directly performs channel measurement, the measurement is performed in such a way that service interruption can be minimized. This can be achieved by measuring the channel quality during a contention free period (CFP) or during a contention free burst (CFB). CFP is part of the current 802.11 standard, while CFB is expected to be part of the upcoming 802.11e standard. CFP and CFB are useful because the AP is designed to be ready to receive frames at any time unless it is transmitting frames. In operation, by announcing aCFMaxDuration greater than the value required to support the STAs and consistent quality of service (QoS), the AP may measure the current channel and/or other channels for the resulting remaining period. CFB may be used in a similar manner in 802.11e compliant WLANs. That is, during the collision period CP, the AP may initiate a CFB by polling itself (ie, sending a QoS CF-Poll addressed to itself). During the CFB period determined by the duration field found in the QoS CF-Poll, all STAs remain silent, and the AP may measure the current channel and/or other channels during this period. Alternatively, the AP may use a Clear-to-Send (CTS) frame to measure the channel without service interruption. By sending a CTS frame with a self-address as the receiver address (RA), the AP will force all STAs receiving this CTS frame to remain silent for a specified period during which the AP can measure the channel. can

(2) Request for channel measurement by AP

When the AP requests channel measurement to the set of STAs associated with its BSS, the AP transmits a channel measurement request frame as shown in FIG. 4 . The transmission of the request for channel quality measurement to STAs may be a single cast (unicast), multicast, or broadcast. The request frame will specify (1) when starting the measurement, (2) the channel to measure, (3) the measurement period, and (4) the measurement method. As shown in Figure 4, the channel measurement frame has four fields, "action code", "Activation Delay", "Dialog Token", and "channel. measurement method element". "Active Delay" The field specifies when to start the channel measurement procedure. "Conversation Token" is a single octet field that distinguishes different measurement requests from each other. "Channel Measurement Method Element" The field indicates the set of channels to be measured, where each octet specifies the number of channels, as shown in FIGS. 5A and 5B, and takes two forms, "Basic Channel Measurement Method" Alternatively, it may be one of "CF channel measurement methods.

Referring to FIG. 5A, "basic channel measurement frame" is "element ID" and "length" In addition to the fields it contains three fields: "Measurement Period", "Numbers of Channels", and "Reporting Time Limit". The "measurement period" (0) field indicates the duration of each channel measurement performed by the requested STA. "Number of channels" The field indicates the set of channels to be measured, where each octet specifies the number of channels. The "reporting time limit" (0) field indicates a time period during which the requested STA must report the measurement result back to the AP.

Referring to FIG. 5B, "CF channel measurement frame" is "element ID" and "length" In addition to the fields, there are 5 fields: "Measurement Period", "Measurement Offset", "Non-Measurement Period", "Numbers of Channels" and "reporting time limit". The "measurement period" (0) field indicates, in the number of collision-free period (CFP) repetition intervals (CFPRI's), the time period that the requested STA spends for measurement of each channel. "Measurement Offset" and "non-measurement period" The fields indicate each non-CFPRI time period during which the requested STA must not be away from the current channel for measurement of the remote channel. For example, during CFPRI[0, CFPRI] starting from the target beacon transmission time (TBTT) starting from the CFP, the STA sets the period [CFPRI*MO/256, CFPRI*(MO+NMD)/256]. Apart from the current channel for the measurement of the remote channel, where MO denotes the value of "measurement offset" and NMD denotes the value of "non-measurement period", respectively. "Number of channels" The field indicates the set of channels to be measured, where each octet specifies the number of channels. The "reporting time limit' (0) field indicates a time period during which the requested STA must report the measurement backwards to the AP.

(3) measurement report by stations (STA)

When receiving a request to measure a channel by the AP as described in the previous section, or when measured voluntarily, each STA will transmit a channel measurement report frame. 6A shows the format of a channel measurement report frame. It is noted that the channel measurement report frame may be transmitted without being requested by the AP via the channel measurement request frame. In such a case, the "conversation token" The value of the field will be set to zero.

In general, a channel measurement report frame has the following broad forms: (i) detection of other BSSs, (ii) measurement of Clear Channel Assesment (CCA) busy periods, and (iii) received signal strength It will include the results of measurements made in the measurement of statistics.

(i) detection of other BSSs

The detection of other BSSs on the requested frequency channel is a "scan" This may be done by using an existing MAC Sublayer Management Entity (MLME) service known as a service and/or variants thereof. This service is requested by a station management entity (SME) present in each STA in the MLME through a management primitive MLME-SCAN.request to request detection of existing BSSs in the number of channels. After that, the primitive MLME-SCAN.confirm returns to the SME the scan results containing a complete description of all BSSs found. It is noted that this service is stipulated from the beginning in 802.11 for the STA to investigate potential BSSs that may later be elected to perform a handoff. When a BSS is detected, the STA sends a frame with a set of fields "To DS" (if the frame is transmitted in the AP direction) and/or "From DS" /or specifies whether beacon frames have been received.

(ii) measurement of CCA busy periods;

In addition, a measurement of interference level or noise by 802.11 non-compliant devices, eg ETSI BRAN HIPERLAN/2 devices or satellite systems, is detected and reported to the AP. The presence of such a device is detectable as co-channel interference and not as BSS.

The STA keeps track of the CCA busy periods to report back the fractional period during which the CCA is busy outside the full measurement period. that the CCA is busy by (1) the beginning of an effective OFDM transmission at a receiver level equal to or greater than -82 dBm with a greater than 90% probability within 4 usec, and (2) any signal above -62 dBm; Note what is indicated. Therefore, the partial period cannot be zeroed even when no BSS is detected.

The STA also attempts to determine the characteristics of periodic bursts by keeping track of the following information. Each STA may keep track of the number of consecutive CCA busy periods observed, with each busy period being defined as a CCA busy indication for one slot time. At the same time, each STA can keep track of non-zero intervals in the slot times between successive busy periods, so that these two parameters can separate two or more times several times. Upon successive matching to tolerance, the detected signal can be configured to come from a temporal source (some non-802.11 compliant devices exhibit temporal characteristics) and reported to the AP, as shown in Figure 6b. Also, if two or more consecutive busy and idle periods match by some tolerance, it may indicate the presence of radar type signals, and this information is communicated to the AP.

6B illustrates a format of a channel measurement report information element used by a STA to report a channel quality measurement back to an AP in accordance with the present invention. As shown in Figure 6b, the length of the frame depends on the number of channels. Referring to Figure 6c, one octet "Measurement Summary" The field is a BSS field to specify that at least one valid MAC header was received during channel measurement, "QBSS" As a field, this bit may be set only when STA reporting is 802.11e MAC enabled, the "QBSS" field, "Periodicity" indicating that at least two consecutive CCA busy on/off patterns are periodic. field, "Beacon" specifying that at least one beacon was received during the measurement. field and "to DS" fields and "from DS" fields "to DS" (or "to AP") and "from DS" The channel measurement report information element as shown in FIG. 6B is copied from the 4 bits of "Transmitted Power" in the SERVICE field of the measurement request frame from the AP, "Own AP Transmitted Power" Power)" field, indicating the energy observed at the antenna used to receive the PLCP preamble of the measurement request frame from the AP, and encoded according to FIG. Received Signal Strength Indicator)" It contains more fields. In addition, "CCA to specify the fraction of time that CCA is busy, calculated according to the following equation, CCA Busy Part = Cap(255 x [CCA Busy Period]/[Total Measurement Period in Channel]) Busy part" field, "CCA Busy Period" indicating the number of consecutive time slots in which CCA Busy was detected. field and "CCA Busy Interval" indicating the time interval in time slots repeating the CCA Busy Indicators. fields are included. "CCA Busy Period" and "CCA Busy Interval" Both fields are only valid if "periodicity" is set in the measurement summary field.

(iii) measurement of received signal strength statistics

Moreover, measurements of the strength of the received signal that can be used to determine the channel condition are reported to the AP as shown in FIGS. 6D and 6E . Referring to FIG. 6E , a parameter called Received Signal Strength Range Index (RSSRI), which ranges from 0 to 7, is used to indicate the energy level observed at each station's antenna. Depending on the value of the 'BSS' field in FIG. 6B , the 4-octet Received Signal Strength Range Index (RSSRI) statistics field will indicate two different measurement results/indicators. When the BSS field is set, i.e. equal to 1, it indicates statistics of the energy level measured during reception of the PLCP preamble for each received frame, whereas when it is not set, i.e. zero, it indicates that the antenna Shows statistics of the instantaneous energy level observed in . It is sampled periodically.

Each octet of the RSSI statistics field is expressed as follows.

<table id="1" cols="2"><row><entry he="130" wi="3403" rb="1" re="1" cb="1" ce="1" al="l">Bits: 3</entry><entry he="130" wi="4663" rb="1" re="1" cb="2" ce="2" al="l"> 5</entry><row><entry he="130" wi="3403" rb="2" re="2" cb="1" ce="1" al="l">RSSRI</entry><entry he="130" wi="4663" rb="2" re="2" cb="2" ce="2" al="l">density</entry></row></row></table>

The 3-bit Received Signal Strength Range Index (RSSRI) is a function as shown in Figure 6e,

(1) the BSS field setting, i.e., in the case of 1, the energy level observed during reception of the PLCP preamble of the received frame, or

(2) BSS field non-setting, i.e., in the case of zero, is defined as the instantaneous energy level observed at the antenna.

During frequency channel measurement, the STA keeps track of the number of measured samples corresponding to each RSSRI. A 5-bit density field is

Density (RSSRI) = defined as the upper limit (431 x [number of samples corresponding to RSSRI]/[total number of samples]).

The 4 RSSRIs with the largest density values will be selected and included in the RSSRI Statistics field.

<u>Decision by AP (step 200 in Fig. 3)</u>

After obtaining the channel quality reports in step 100, the AP now determines a new channel to be used for communication between the AP and the stations. A new channel selection method according to the present invention is shown in the flowchart of FIG.

In step 8, the process of selecting a channel from the supported channels is started. Then, it is determined whether the presence of another BSS is detected by each STA based on the signal received in step 10 . If not, it is determined in step 12 whether periodicity of the received signal is detected. If periodicity is detected, in step 14 the corresponding channel is excluded from the candidate channel list. Here, the candidate channel list includes the number of channels through which the AP can consider switching of all STAs for communication.

On the other hand, if another BSS is detected in step 10, in step 16 "from DS" It is determined whether the field is set. If so, the corresponding channel is excluded from the candidate channel list in step 18 because interference from the AP in the other BSS is highly undesirable. In step 16, "from DS" If the field is not set, or if no periodicity is detected in step 12, then in step 20 the AP includes them in the candidate channel list. Thereafter, in step 22 it is determined whether all channels are scanned. If so, the channel with the minimum RSSRI and/or CCA value is selected. That is, after obtaining the number of potential candidate channels, the AP can determine the specific channel with minimal interference to other co-located systems, e.g., HIPERLAN/2 devices, as well as other STAs with BSS. have. Finally, the AP determines the channel selected in step 24 as a new channel to which all STAs should switch.

<u>Channel switch announcement by AP (step 300 in Fig. 3)</u>

After selecting a new channel to switch to, the AP transmits new channel information via beacon transmission to switch all STAs to the selected channel based on its own measurements as well as channel measurements received from STAs associated with this particular BSS. . The AP will repeatedly transmit beacon frames having information indicating the time and channel at which switching is performed. 8 shows an announcement frame that can be used for this channel switch announcement, with Channel-to-Switch indicating the number of frequency channels to switch to, and before the channel switch when BSS occurs. Contains "Channel Switch Count" indicating the number of beacons (including the current frame) appearing.

<u>Switching to a new channel (step 400 in Fig. 3)</u>

Eventually, the movement to a new channel is implemented by changing the carrier frequency of the 802.11a OFDM PHY. In an embodiment, the switching preferably occurs just before the target beacon transmission time (TBTT) by both the STAs and the AP.

As is apparent from the foregoing, the present invention has the advantage that a dynamic frequency selection (DFS) mechanism can be obtained with some minor modifications in the current 802.11 specifications. Although this specification is limited to infrastructure-based 802.11 WLANs with the AP as the centralized decision-maker of the DFS in the BSS, the present invention supports an ad hoc mode of WLANs. can be easily extended to

Thus, although preferred embodiments of a dynamic frequency selection (DFS) method for determining a channel for use within a WLAN system have been described, it will be apparent to those skilled in the art that certain advantages of the system are achieved. What has been described above has been construed only as an exemplary embodiment of the present invention. Those skilled in the art can readily devise alternative devices providing functions similar to those of the present embodiment without departing from the basic principles and scope of the present invention.