Received signal strength indication for use as a baseband processor qualifier in high density WLAN environments

A station or access point has a list of associated stations including a BSSID. During an observation period, measurements of RSSI are made for each BSSID, including a maximum RSSI and a minimum RSSI. After the observation interval, an RSSI_threshold is computed which is below the weakest RSSI of a station which is on the list of associated stations, and also above the weakest RSSI of a station which is not on the list of associated stations. During packet reception, packet acquisition starts when the receiver signal level is detected to be above the RSSI_threshold. During packet transmission, a clear channel assessment (CCA), which ordinarily prevents transmission when signal energy is detected, is overridden if the measured RSSI is below the RSSI_threshold value, enabling earlier transmission of the packet than if the transmitter were to wait for CCA to be asserted.

The present invention relates to the operation of wireless Local Area Networks (WLAN) in dense environments. In particular, it relates to the computation of a Received Signal Strength Indicator (RSSI) threshold for use with a baseband processor to allow more efficient utilization of a congested wireless channel.

BACKGROUND OF THE INVENTION

FIG. 1shows the diagram of a prior art distributed access point deployment, such as a sports stadium. Many wireless users may congregate in the stadium, and enhance their sport viewing experience using portable wireless devices which include a WLAN capability. In the prior art, a plurality of access points AP1102, AP2104, AP3106, AP4108, AP5110, AP6112, AP7114, AP8116are placed around the perimeter of the stadium, or in any manner which provides adequate WLAN coverage over the stadium. Each of the access points (AP) advertises a BSSID (Basic Service Set Identifier), which is typically the MAC address of the access point.FIG. 2shows the wireless links202and210between access points AP1 and AP5, respectively, to station STA1118, all of which operate together on the using shared media access on channel 1, where each channel represents a 20 Mhz band of subcarrier frequencies to prevent interference with other channels. There may be many stations such as STA1118, which associated with nearby AP1102on the same channel, and AP5110operates on the same channel and can be a source of remote interference, as it is part of a shared media channel which is not in direct communication with STA 1118. Although a small number of frequency channels may be available for all of the stations of the WLAN, it is desired to separate as much as possible each of the access points operating on a particular frequency from other access points operating on the same channel, as shown for channel 1 AP1102and AP5110. A separate set of stations and access points may operate independently on channel 6, shown as STA2120and STA 3122near AP 8116, and STA4124and STA5126which are near AP4108. Other access points AP2104and AP6112are also operative on channel 6. In the example of a stadium, where multipath losses are low, one issue that arises is that the access points which share the same channel of operation such as CH6, shown inFIGS. 1 and 2, interact in a manner which reduces the overall bandwidth even though they are physically separated and the respective BSSIDs are associated with different stations.

WLAN communications systems which operate according to IEEE standard 802.11b or 802.11g operate with each station (STA) associated with a particular access point (AP), such that a plurality of stations120and122may associate with AP8 operative in CH6, and a different plurality of stations such as STA4124and STA5126may be associated with an access point AP4 which is also operative in CH6, and yet another plurality of stations (not shown) may be associated with access points AP2 and AP6. Under the IEEE 802.11b and 802.11g WLAN standards, the simultaneous transmission by access points and stations is known as a collision, and the AP and STA will re-transmit the packet when the intended recipient of the corrupted packet fails to acknowledge receipt by detecting the missing sequence number of the corrupted packet in the received packet stream. The transmitter will reduce the likelihood of collision through use of the detection of a clear channel assessment (CCA) signal. For the indoor WLAN environment with multi-path reflection, it is desired to operate in the manner, as a station may be associated with an access point through a multi-path reflection environment which produces a weak signal at the associated station or access point.

FIG. 3illustrates one of the throughput problems associated with having several stations and access points within reception range of each other, such as in a low multi-path reflection high user density environment such as the open stadium shown inFIGS. 1 and 2. In an actual system, the BSSID would be the 48 bit MAC address of the access point, however, for clarity in understanding the invention, each access point of the present example advertises a BSSID in the form of the access point name provided as an underscore suffix. For example, the BSSID associated with AP4 is shown as BSSID_AP4 and the BSSID associated with AP8 is shown as BSSID_AP8. Each station utilizes the well-known 802.11 association protocol to “join” with a particular BSSID. The problem is that although the individual stations have joined a particular BSSID, the channel media is still shared as far as packet acquisition (detection of a packet) and packet transmission (holdoff from transmit when another station or AP is using the shared media) are concerned.FIG. 3shows an example of this from the perspective of STA2120, which is associated with BSSID_AP8, and on the same shared media channel 6 as “other” BSSID_AP4. “Other BSSID”302indicates packet326being transmitted by BSSID_AP8, during which time the “clear channel assessment” CCA308is unasserted starting at time314. The local station STA2120has a transmit request310with associated packet ready to transmit at time316, however as shared media, the CCA308signal unassertion causes the station to wait until indication of clear channel at time317, after which time data322and associated packet328is transmitted. By comparison, at time320when transmit request310is asserted, the data324is immediately sent as packet330. A similar problem occurs during packet acquisition, as shown inFIG. 4. From the perspective of STA2120, a packet from “other” BSSID_AP4 arrives, shown in plot402as packet404, and is subject to packet acquisition process by the baseband processor, which performs a preamble detect and acquisition414, header recovery416and begins demodulating the packet payload416. In this case, the BSSID which will be recovered will be undesired BSSID_AP4 of the “other” station. During this interval, a packet for “our” BSSID_AP8 arrives406as shown408, which has stronger signal level as shown in RSSI410waveform, and the coincident stronger signal only serves to corrupt418the packet404that STA2 had started to acquire.

Therefore, a problem occurs in high density user environments where a large number of stations share a channel number and a plurality of BSSIDs are present, where local station transmission is deferred while awaiting a remote sender on a different BSSID to complete, and reception of a low signal level packet from a remote BSSID interferes with reception of a high signal level packet from a nearby BSSID.

It is desired to provide a mechanism to reduce interference from remote BSSIDs in transmission and reception of packets from a local station, and thereby increase throughput in a high density station environment with multiple access points sharing a particular wireless channel.

OBJECTS OF THE INVENTION

A first object of the invention is the computation of an RSSI_threshold during a listening interval when the RSSI of stations associated with the current access point (AP) and the RSSI of stations not associated with the current access point (AP) are saved for use in computing the RSSI_threshold, the RSSI_threshold used to transmit a current packet during an interval when a station not associated with the current AP is also transmitting.

A second object of the invention is the computation of an RSSI_threshold during a listening interval when the RSSI of stations associated with the current access point (AP) and the RSSI of stations not associated with the current access point (AP) are saved for use in computing the RSSI_threshold, the RSSI_threshold used to determine when to start the packet acquisition process for packets directed to the input of a baseband processor.

SUMMARY OF THE INVENTION

In a high station and access point density environment, where multi-path reflection and attenuation is low, a station which has associated with a particular BSSID enters an observation interval, where a table of BSSIDs and received signal strength indicator (RSSI) minimum and maximum levels for each is taken from observed BSSIDs during the observation interval. Each table entry consists at least of a BSSID, a minimum observed RSSI, and a maximum observed RSSI. After exclusion of BSSID associated with the present station, the min_RSSI of the “other” APs are examined to determine an RSSI_threshold value for use by the baseband processor. The baseband processor defers acquisition of a packet until an RSSI level occurs which exceeds the RSSI_threshold value, after which the baseband processor starts packet acquisition. During station packet transmit, a clear channel assessment (CCA) signal is used to prevent transmission by the station only when the RSSI exceeds the RSSI_threshold. A different RSSI_threshold may be used for receive events compared to transmit events, or an increase in RSSI may be used to re-start the packet acquisition processor.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 5shows a flowchart for generating a table of BSSID entries, the table for use in generating an RSSI_threshold value for use by the baseband processor for qualifying the transmission and reception of packets. Upon entry of the process500, in step502, a BSSID with associated min_RSSI representing a minimum observed RSSI value, and a max_RSSI representing a maximum observed RSSI value, is received. If the BSSID is not found in the table504, it is added in steps510and512, or alternatively, the minimum and maximum RSSI values are updated or averaged into an existing table entry in step502. Step506represents the observation time for generation of the BSSID table600, and step508computes the RSSI_threshold value.

In one embodiment of the invention, the RSSI_threshold is a value set between the min_RSSI for the BSSID the instant station is associated with, and the max_RSSI for an “other” BSSID, such as the remote access points used by other stations. The RSSI_threshold value may be computed any number of different ways, but the computation relies on the current BSSID of station association having a stronger signal level than a remote BSSID, which is consistent with distance separation from the associated AP as shown inFIG. 2.

FIG. 6shows an example BSSID table for stations on channel 6 of the diagram ofFIG. 2, with the suffix associated with the associated access point (AP). As described previously, a typical BSSID is the 48 bit MAC address of the access point, and the RSSI is a numerical value indicating signal strength received at the instant station creating the table.FIG. 7shows an arrangement of values on an RSSI amplitude scale for the example shown inFIGS. 1 and 2. The most distant station AP4 generates the lowest amplitude range of values from min_RSSI_AP4 to max_RSSI_AP4, and the second highest RSSI range is associated with AP2, which is the second closest station, and the highest RSSI value is associated with nearby AP8, with a large RSSI gap in table entries located between the min_RSSI_AP8 and max_RSSI_AP2. By setting the RSSI_threshold in this range as shown, it is now possible to discriminate between access points for “this” BSSID and other stations which are “other” BSSIDs. In this manner, it is possible to distinguish AP the instant a packet is received, and to use this information in deciding what to do with the packet, or whether to holdoff a pending transmission by the station.

FIG. 8shows a transmit sequence using the RSSI_threshold805computed inFIG. 5. A packet826is transmitted by an “other” BSSID such as AP4 at time814, which results in an RSSI806which is below the RSSI_threshold value805, and although the CCA808is not asserted, indicating the channel is in use, upon assertion of a transmit request810at time816, the RSSI value806below RSSI_threshold805causes the transmit baseband processor to override CCA assertion808and respond at time816with transmission of data822and STA2 transmit packet828during the time that distant AP4108is still transmitting its packet826. The CCA waveform808which indicates the presence of wireless packet energy, and the RSSI value806and RSSI_threshold805are shown as separate waveforms for clarity in understanding the invention. The CCA808can be any signal detection method known in the prior art, including the detection of wireless signal energy, whereas the computation of RSSI_threshold is done using the values from a recent observation period, which may be updated from time to time. In an alternative embodiment of the invention, the combination of RSSI_threshold805and RSSI806may be used to override or de-assert CCA808to achieve the result of early transmit timing804after request810as is presently shown inFIG. 8. Transmit request810is again asserted at time818when CCA808indicates the channel is not available, but transmit holdoff is honored because the RSSI806value exceeds threshold805, indicating a transmit by “our” BSSID AP8. The transmit processor waits until completion of the BSSID_AP8 packet until time820, when the STA2 packet832is sent. The advantage of the invention is that in a dense environment with many access points, the distant access point will not corrupt the local packet, and with many transmissions and access points operating at the same time, the effective shared media access would be reduced to a negligible value, and local stations would experience a very high rate of CCA unassertion from all the surrounding traffic, and consequently would be unable to transmit data. With the present system, the use of CCA in combination with RSSI_threshold computed from the observed RSSI and BSSID allows the station to discriminate between “our” BSSID for which it should wait until completion, and “other” BSSIDs for which it can ignore for transmit delay, and the local BSSID station for which it should conform to CCA.

FIG. 9shows a local station receive process using the RSSI_threshold value909. At time950, a packet from AP4 is received, and the baseband processor912defers packet acquisition until time952, when a packet from nearby AP8908arrives and generates a higher RSSI value which crosses RSSI_threshold909. RSSI910exceeding RSSI_threshold909at time952results in the preamble acquisition914of the baseband processor starting, followed by header recovery916and payload recovery918. Without the use of RSSI_threshold computed from the surrounding BSSID signal strength values, packet acquisition would have otherwise started incorrectly at time950, with the packet acquisition of undesired packet904corrupted by the AP8 packet908we seek to acquire. In another embodiment of the invention, packet acquisition may start at time950ignoring the RSSI910which is below the RSSI_threshold909, and then restarted at time952when the RSSI910jumps above RSSI_threshold909. The invention therefore improves the utilization of the shared media by using RSSI as an indicator of when to begin packet acquisition, based on the relative strengths of surrounding access point received RSSI.

In the described embodiments, the description of example shows the access point (AP) has made a table of RSSI values for stations which are associated with the AP, and also a set of entries for RSSI values of stations which are not associated with the AP. It is also possible for each station (STA) of the WLAN to use the method described herein and similarly make a table of entries using the method described forFIG. 5with table entries as shown inFIG. 6, where each STA and AP of a particular BSSID is indicated as “this” BSSID802and the “other” BSSIDs604are used in the manner described to compute the RSSI_threshold value as shown inFIG. 7and used to override transmission events of “other” BSSIDs as shown inFIG. 8or to initiate or restart packet acquisition as shown inFIG. 9.