Collaborative OBSS interference mitigation for wireless communication systems

This application claims priority under 35 U.S.C. § 119 from U.S. Provisional Application No. 62/085,267, entitled “Collaborative OBSS Interference Mitigation for 11ax,” filed on Nov. 27, 2014, the subject matter of which is incorporated herein by reference.

The disclosed embodiments relate generally to wireless network communications, and, more particularly, to OBSS interference mitigation in wireless communication systems.

IEEE 802.11 is a set of media access control (MAC) and physical layer (PHY) specification for implementing wireless local area network (WLAN) computer communication in the Wi-Fi (2.4, 3.6, 5, and 60 GHz) frequency bands. The standards and amendments provide the basis for wireless network products using the Wi-Fi frequency bands. For example, IEEE 802.11ac is a wireless networking standard in the 802.11 family providing high-throughput WLANs on the 5 GHz band. Significant wider channel bandwidths (20 MHz, 40 MHz, 80 MHz, and 160 MHz) were proposed in the IEEE 802.11ac standard. The High Efficiency WLAN study group (HEW SG) is a study group within IEEE 802.11 working group that will consider the improvement of spectrum efficiency to enhance the system throughput in high-density scenarios of wireless devices. Because of HEW SG, TGax was formed and tasked to work on IEEE 802.11ax standard that will become a successor to IEEE 802.11ac.

IEEE 802.11ax is seeking throughput enhancement in the dense deployed environment. Specifically, uplink (UL) and downlink (DL) user aggregation has been introduced to increase network efficiency. UL orthogonal frequency division multiple access (OFDMA) has significantly higher spectral density (narrower bandwidth), which might increase overlapping basic service set (OBSS) interference. For OFDMA and multi-user multiple input and multiple output (MU-MIMO), UL timing synchronization is required and the duration of UL and DL PPDU TX times are controlled by the access point (AP) of each BSS.

For APs deployed in managed network or by carrier operator, it is possible to have AP-AP information exchange. Possible AP-AP information exchange methods may include wired communication between AP-AP such as through Internet or in a wired local network; APs co-located with LTE base stations are connected through the cellular backbone network; and AP-AP connected via wireless link in different bands/channels.

There are different BSS overlapping geometries and hence different types of OBSS interferences. Based on the different OBSS interference types, a solution for OBSS interference mitigation is sought.

A method of mitigating inter-BSS interference via synchronized inter-BSS operation is proposed. By aligning the transmission of UL and DL transmission in two overlapping OBSSs, the inter-BSS STA-to-STA and inter-BSS AP-to-AP types of interference can be eliminated. Furthermore, based on the BSS overlapping geometry and the STA location, different interference mitigation schemes can be applied to mitigate inter-BSS AP-to-STA and inter-BSS STA-to-AP types of interference.

In one embodiment, a first AP 1 serving a first BSS 1 performs uplink and downlink timing synchronization with a second AP 2 serving a second BSS 2 in a wireless communication network. AP 1 divides wireless stations (STAB) in BSS 1 into a first group and a second group. The transmissions by STAB in the first group do not interfere with AP 2 reception. The transmissions by STAB in the second group interfere with AP 2 reception. In one example, STA 1 in BSS 1 and STA 2 in BSS 2 are both located within an overlapping coverage area of BSS 1 and BSS 2 and belong to the second group. AP 1 coordinates between BSS 1 and BSS 2 for STAB in BSS 1 and BSS 2 to communicate based on a group status and a predetermined rule to mitigate interference between BSS 1 and BSS 2 .

Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.

FIG. 1 illustrates a wireless communications network with OBSS interference mitigation in accordance with one novel aspect.

FIG. 2 is a simplified block diagram of a wireless transmitting device and a wireless receiving device in accordance with one novel aspect.

FIG. 3 illustrates three different types of BSS overlapping geometries in a wireless communication system, which includes AP-AP overlap, BSS-BSS overlap, and STA-STA overlap.

FIG. 4 illustrates four different types of inter-BSS interferences in a wireless communication system, which includes inter-BSS AP-AP interference, inter-BSS STA-STA interference, inter-BSS AP→STA interference, and inter-BSS STA→AP interference.

FIG. 5 is a diagram of one embodiment of OBSS interference mitigation, wherein two different APs communicate to its STAB in overlapping coverage area using non-overlapping service periods.

FIG. 6 is a diagram of one embodiment of OBSS interference mitigation, wherein two different APs communicate to selected STAB with good received signal strength in overlapping coverage area using the same service period.

FIG. 7 is a diagram of one embodiment of OBSS interference mitigation, wherein an AP communicates with its STAB employing beamforming while nulls to other STAB in overlapping coverage area.

FIG. 8 is a diagram of one embodiment of OBSS interference mitigation, wherein two different APs communicate to its STAB over different sub-channel using the same service period.

FIG. 9 illustrates one example of synchronized inter-BSS operation by adjusting end of DL frames of two overlapping BSSs.

FIG. 10 illustrates another example of synchronized inter-BSS operation by adjusting end of UL frames of two overlapping BSSs.

FIG. 11 illustrates one embodiment of a synchronization method between two APs serving two overlapping BSSs.

FIG. 12 is flow chart of a method of OBSS mitigation in accordance with one novel aspect.

Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings.

FIG. 1 illustrates a wireless communications network 100 with overlapping basic service set (OBSS) interference mitigation in accordance with one novel aspect. Wireless communications network 100 comprises a first access point AP 1 serving a first basic service set BSS 1 having a plurality of wireless stations STA 11 , STAl 2 , and STA 13 (collaboratively referred to as STA 1 ). Wireless communications network 100 also comprises a second access point AP 2 serving a second basic service set BSS 2 having a plurality of wireless stations STA 21 , STA 22 , and STA 23 (collaboratively referred to as STA 2 ). In the example of FIG. 1 , BSS 1 and BSS 2 are overlapping BSSs (OBSS) having overlapping coverage area. In IEEE 802.11ax, uplink (UL) and downlink (DL) user aggregation has been introduced to increase network efficiency in the dense deployed environment. UL orthogonal frequency division multiple access (OFDMA) has significantly higher spectral density (narrower bandwidth), which might increase inter-BSS interference.

In one novel aspect, mitigation of inter-BSS interference can be achieved via synchronized inter-BSS operation. By aligning the transmission of UL and DL transmission in two OBSSs, the inter-BSS STA→STA and inter-BSS AP→AP types of interference can be eliminated. Based on the BSS overlapping geometry and the STA location, different interference mitigation schemes can be applied to mitigate inter-BSS AP→STA (e.g., AP 1 →STA 22 ) and inter-BSS STA→AP (e.g., STA 13 →AP 2 ) types of interference. In the example of FIGS. 1 , AP 1 and AP 2 synchronizes with each other for all the UL and DL packet transmission. As a result, inter-BSS AP 1 ←→AP 2 and inter-BSS STA 1 ←→STA 2 types of interference are eliminated. Furthermore, for STAB (STA 13 and STA 22 ) that are located within the overlapping area of BSS 1 and BSS 2 , AP 1 and AP 2 coordinate with each other to mitigate inter-BSS AP→STA interference and STAAP interference.

FIG. 2 is a simplified block diagram of wireless stations 201 and 211 in accordance with a novel aspect. For wireless device 201 , antenna 207 transmits and receives radio signals. RF transceiver module 206 , coupled with the antenna, receives RF signals from the antenna, converts them to baseband signals and sends them to processor 203 . RF transceiver 206 also converts received baseband signals from the processor, converts them to RF signals, and sends out to antenna 207 . Processor 203 processes the received baseband signals and invokes different functional modules to perform features in wireless device 201 . Memory 202 stores program instructions and data 208 to control the operations of the wireless device.

Similar configuration exists in wireless device 211 where antenna 217 transmits and receives RF signals. RF transceiver module 216 , coupled with the antenna, receives RF signals from the antennae, converts them to baseband signals and sends them to processor 213 . The RF transceiver 216 also converts received baseband signals from the processor, converts them to RF signals, and sends out to antenna 217 . Processor 213 processes the received baseband signals and invokes different functional modules to perform features in wireless device 211 . Memory 212 stores program instructions and data 218 to control the operations of the wireless device.

The wireless devices 201 and 211 also include several functional modules and circuits to perform certain embodiments of the present invention. In the example of FIG. 2 , wireless device 211 is a wireless communications station (e.g., a non-AP STA) that includes an encoder 204 for encoding and transmitting a frame to device 201 , a decoder 205 for receiving and decoding a frame from device 201 , and a measurement module 209 for measuring channel qualities and estimating channel conditions. Wireless device 201 is another wireless communications station (e.g., an AP) that includes an OBSS interference mitigation module 220 . The OBSS interference mitigation module 220 further comprises an OFDMA/MU-MIMO handler 221 for scheduling uplink OFDMA or MU-MIMO for multiple STAB, a divider 222 for partitioning STAB into groups based on their geographic location, a coordinator 223 for coordinating UL and DL transmission service periods with neighbor APs with overlapping coverages, and a synchronization circuit 224 for performing UL and DL transmission timing synchronization with neighbor APs with overlapping coverages. OBSS interference mitigation module 220 may further comprises sounding and feedback modules for channel state information (CSI) and beamformer and other FDMA/CDMA circuits to mitigate OBSS interference. AP 201 also comprises the functional modules and circuits of a non-AP STA. The different functional modules and circuits can be configured and implemented by software, firmware, and hardware, or any combination thereof. The function modules and circuits, when executed by the processors 203 and 213 (e.g., via executing program codes 208 and 218 ), allow wireless stations 201 and 211 to perform certain embodiments of the present invention.

FIG. 3 illustrates three different types of BSS overlapping geometries in a wireless communication system, which includes AP-AP overlap, BSS-BSS overlap, and STA-STA overlap. FIG. 3( a ) depicts STA-STA overlapping geometry, where the communication range of some STAB overlapping with each other. For example, the communication range of STA 13 and the communication range of STA 22 overlap with each other. FIG. 3( b ) depicts BSS-BSS overlapping geometry, where the coverage of two BSSs overlapping with each other. For example, the coverage of BSS 1 and the coverage of BSS 2 overlap with each other. FIG. 3( c ) depicts AP-AP overlapping geometry, where the communication range of two BSS APs overlapping with each other. For example, the communication range of BSS 1 AP 1 and the communication range of BSS 2 AP 2 overlap with each other. Typically, the level of inter-BSS interference are highest in AP-AP overlap geometry, and lowest in STA-STA geometry. In a managed network, APs are typically deployed in “BSS-BSS overlap” and “STA-STA overlap” geometry or they have completely no overlapping.

FIG. 4 illustrates four different types of inter-BSS interferences in a wireless communication system, which includes inter-BSS AP→AP interference, inter-BSS STA→STA interference, inter-BSS AP→STA interference, and inter-BSS STA→AP interference. In a BSS-BSS overlap geometry as depicted by FIG. 4 , there are inter-BSS AP→STA interference (e.g., from AP 1 to STA 22 ), STA→AP interference (e.g., from STA 13 to AP 2 ), and STA→STA interference (e.g., STA 13 to STA 21 ). However, there is no inter-BSS AP→AP interference (e.g., from AP 1 to AP 2 or from AP 2 to AP 1 ). In a STA-STA overlap geometry, there is only inter-BSS STA-STA interference.

FIG. 5 is a diagram of one embodiment of OBSS interference mitigation, wherein two different APs communicate to its STAB in overlapping coverage area using non-overlapping service periods. In the BSS 1 -BSS 2 overlap geometry as depicted in FIG. 5 , an STA can be in one of the three areas: Area A where STAB can only hear AP 1 serving BSS 1 , Area B where STAB can hear both AP 1 and AP 2 , and Area C where STAB can only hear AP 2 serving BSS 2 . AP 1 and AP 2 reception of UL transmission can be interfered by transmission from OBSS STA in Area B. Similarly, STA reception in Area B can be interfered by transmission from OBSS AP (AP 1 or AP 2 ).

In accordance with one novel aspect, to mitigate the OBSS interference, AP 1 and AP 2 first establish UL and UL synchronization. STA 13 or STA 22 in Area B reports its list of visible APs and its BSS ID to those visible APs. When AP 1 communicates to STAB in Area A and AP 2 communicates to STAs in Area C, OBSS interference can be avoided. For communication to STAs in Area B, AP 1 and AP 2 needs to schedule non-overlapping service periods of BSS 1 and BSS 2 . For example, during service period 1 , AP 1 communicate to STA 13 and BSS 2 AP 2 keeps quiet during period 1 , during service period 2 , AP 2 communicates to STA 22 and BSS 1 AP 1 keeps quiet during period 2 .

FIG. 6 is a diagram of one embodiment of OBSS interference mitigation, wherein two different APs communicate to selected STAs with good received signal strength in overlapping coverage area using the same service period. In the BSS 1 -BSS 2 overlap geometry as depicted in FIG. 6 , an STA can be in one of the three areas: Area A where STAs can only hear AP 1 serving BSS 1 , Area B where STAs can hear both AP 1 and AP 2 , and Area C where STAs can only hear AP 2 serving BSS 2 . AP 1 and AP 2 reception of UL transmission can be interfered by transmission from OBSS STA in Area B. Similarly, STA reception in Area B can be interfered by transmission from OBSS AP (AP 1 or AP 2 ).

In accordance with one novel aspect, to mitigate the OBSS interference, AP 1 and AP 2 first establish UL and UL synchronization. STA 13 or STA 22 in Area B reports its list of visible APs and the corresponding received signal strength indicators (RSSIs), and its BSS ID to those visible APs. AP 1 and AP 2 records RSSIs from STAs (STA 13 and STA 22 ) in Area B. When AP 1 communicates to STAs in Area A and AP 2 communicates to STAs in Area C, OBSS interference can be avoided. For communication to STAs in Area B, AP 1 and AP 2 selects a number of STA pairings (each pair contains one STA from BSS 1 and one STA from BSS 2 that have acceptable signal to interference plus noise ratio (SINR) in both UL and UL when this pair of STAB use the medium at the same time) to communication in service period 1 . For the rest of STAB in Area B not belonging to the pairing, AP 1 and AP 2 needs to schedule non-overlapping service periods of BSS 1 and BSS 2 . For example, during service period 2 , AP 1 communicate to STA 13 and BSS 2 AP 2 keeps quiet during period 2 , during service period 3 , AP 2 communicates to STA 22 and BSS 1 AP 1 keeps quiet during period 3 .

FIG. 7 is a diagram of one embodiment of OBSS interference mitigation, wherein an AP communicates with its STAB employing beamforming while nulls to other STAB in overlapping coverage area. The BSS 1 -BSS 2 overlap geometry depicted in FIG. 7 is similar to FIG. 5 and FIG. 6 . To mitigate the OBSS interference, AP 1 and AP 2 first establish UL and UL synchronization. STA 13 or STA 22 in Area B reports its list of visible APs and its BSS ID to those visible APs. AP 1 and AP 2 both perform sounding and feedback with STAB in Area B to obtain channel state information (CSI) and derive precoding matrix (beamforming weights). When AP 1 communicates to STAB in Area A and AP 2 communicates to STAB in Area C, OBSS interference can be avoided. For communication to STAB in Area B, AP 1 employs beamforming to transmit to the STAB in BSS 1 while nulls to the STAB in BSS 2 . AP 2 employs beamforming to transmit to the STAB in BSS 2 while nulls to the STAB in BSS 1 . Note that simultaneous beamforming and nulling can be obtained using the precoding matrix as in the DL MU-MIMO. Precoding matrix allows a spatial stream to be transmitted (beamformed to) one STA while nulls to other STAB. Each AP needs to inform the other AP which STA to communicate to in advance (either through back channel such as wired link or other means). Both APs could select a service period to communicate to STAB in Area B.

FIG. 8 is a diagram of one embodiment of OBSS interference mitigation, wherein two different APs communicate to its STAB over different sub-channel using the same service period. The BSS 1 -BSS 2 overlap geometry depicted in FIG. 8 is similar to FIG. 5 and FIG. 6 . In this embodiment, AP 1 communicates to STAB in Area A and AP 2 communicates to STAB in Area C freely with any bandwidth restriction. For communications to STAB in Area B, AP 1 communicates to STAB in Area B (STA 13 ) using sub-channel 1 , while AP 2 communicates to STAB in Area B (STA 22 ) using sub-channel 2 . Sub-channel 1 and sub-channel 2 are not overlapping. AP 1 can optionally send to STA in Area A using sub-channel 2 at the same time that it transmits to STA in Area B. For example, AP 1 chooses beamforming (BF) pattern for sub-channel 2 that has good CSI for STA in Area A and properly null for Area B. Assume that STA 13 has reported that STA 22 is its nearby interference STA using sub-band 2 , or AP 1 has other method to obtain such information. AP 1 also needs to obtain the CSI result of STA 11 and STA 13 for several BF pattern in sub-band 2 (amplitude info). AP 1 might be able to generate these results from one CSI feedback and apply BF pattern to get the expected CSI response. Similarly, AP 2 can optionally send to STA in Area C using sub-channel 1 at the same time that it transmits to STA in area B. In an alternative embodiment, other methods such as employing different spread codes (CDMA) can be used.

There are different methods for performing UL and DL timing synchronization for IEEE 802 wireless networks. Please refer to “IEEE Std. P802.1AS”, IEEE Standard for Local and Metropolitan Area Networks—Timing and Synchronization for Time-Sensitive Applications; the contents are incorporated herein by in entirety. In the precision timing standard 802.1AS in which Nano-second scale timing information to propagate over an 802.1 network (Ethernet switches and hubs) so that ports can be synchronized. The respective AP (BSS) can then use this network time to synchronize their own clocks—and thus their own TSF. This will enable beacon intervals and UL and DL transmission timing or other timing related information of these OBSSs to participate in the mechanism described in the current invention. In 802.11ax, UL aggregation (UL MU-MIMO and UL OFDMA) starts with AP transmitting trigger frame carried STA IDs and UL PPDU transmit duration. When AP knows the neighbor's AP DL and UL transmission duration, it can synchronize the DL/UL transmission in its BSS by using the same DL transmission duration and setting the same UL PPDU transmit duration in the trigger frame, allowing its UL STA's transmission to end at the same time as its neighbor BSS. The information of the DL and UL transmission duration can be exchanged or distributed among neighboring BSSs.

In an alternate embodiment, for OBSS APs to establish UL and DL timing synchronization, the following method can be used. First, select one AP (AP 1 in BSS 1 ) as the timing master and one AP (AP 2 in BSS 2 ) as the timing slave. STAB in BSS 1 and BSS 2 report the observed OBSS APs to their own AP. APs also observe the OBSS STAB in range. Next, AP 1 indicates synchronization operation when it transmits to STAB in area B. In the simplest synchronization operation, AP 1 uses the fixed TX time for its subsequent DL transmission and assign fixed TX time to STA's UL transmission. STAB belong to BSS 1 in area B signals the synchronizing operation and UL and DL TX times to OBSS AP, and OBSS AP can also observe the STA transmission in area B to figure out the UL and DL TX times. AP 2 then adjusts the subsequent UL and DL start time to synchronize to BSS 1 . Note AP 2 adjusts the start time of UL and DL by increasing or decreasing UL and DL TX times. Note that more flexible UL and DL TX time is allowed (instead of fixed TX time) in BSS 1 as long as the TX times are communicated to AP 2 in advance. The communication between AP 1 and AP 2 can be via a back channel (wired channel, different wireless networks, etc.) instead of via STAB in Area B. Method of selecting timing master and slave are vendor specific or beyond the scope of this invention. It can also depend on which BSS is transmitting to the medium first.

FIG. 9 illustrates examples of synchronized inter-BSS operation by adjusting end of DL frames of two overlapping BSSs. In the examples of FIG. 9 , AP 1 of BSS 1 is the timer master and AP 2 of BSS 2 is the timer slave. In the simplest synchronization operation, AP 1 uses the fixed TX time for its subsequent DL transmission and assign fixed TX time to STA's UL transmission as depicted. In a first example, AP 2 learns the BSS 1 TX time during DL frame 911 and UL MU frame 912 . AP 2 then gains TXOP for DL frame 923 . AP 2 adjusts DL frame 923 TX time such that the end of DL frame 913 in BSS 1 and the end of DL frame 923 in BSS 2 are the same. As a result, the subsequent UL and DL transmissions of BSS 1 and BSS 2 are synchronized. In a second example, AP 2 learns the BSS 1 TX time during DL frame 914 and UL MU frame 915 . STA 2 then gains TXOP for UL frame 925 , which is followed by DL frame 926 . AP 2 adjusts DL frame 926 TX time such that the end of DL frame 916 in BSS 1 and the end of DL frame 926 in BSS 2 are the same. As a result, the subsequent UL and DL transmissions of BSS 1 and BSS 2 are synchronized.

FIG. 10 illustrates another example of synchronized inter-BSS operation by adjusting end of UL frames of two overlapping BSSs. In the example of FIG. 10 , AP 1 of BSS 1 is the timer master and AP 2 of BSS 2 is the timer slave. In the simplest synchronization operation, AP 1 uses the fixed TX time for its subsequent DL transmission and assign fixed TX time to STA's UL transmission as depicted. AP 2 learns the BSS 1 TX time during DL frame 1011 and UL MU frame 1012 . AP 2 then gains TXOP for DL frame 1023 , which is followed by UL MU frame 1024 . Instead of adjusting DL frame 1023 , AP 2 adjusts UL MU frame 1024 TX time such that the end of UL MU frame 1014 in BSS 1 and the end of UL MU frame 1024 in BSS 2 are the same. As a result, the subsequent UL and DL transmissions of BSS 1 and BSS 2 are synchronized.

FIG. 11 illustrates one embodiment of a synchronization method between two APs (AP 1 and AP 2 ) serving two overlapping BSSs (BSS 1 and BSS 2 ). AP 1 communicates to STAB in Area B, which has visibility to both AP 1 and AP 2 . An STA belongs to BSS 1 in Area B signals the synchronization operation and UL and DL TX times to AP 2 . For example, AP 1 can signal information to AP 2 via the STA by piggy backing (e.g., using an inter-BSS information exchange IE) on the DL frame 1111 or the UL MU frame 1112 .

FIG. 12 is flow chart of a method of OBSS mitigation in accordance with one novel aspect. In step 1201 , a first AP 1 serving a first BSS 1 performs uplink and downlink timing synchronization with a second AP 2 serving a second BSS 2 in a wireless communication network. In step 1202 , AP 1 divides wireless stations (STAB) in BSS 1 into a first group and a second group. The transmissions by STAB in the first group do not interfere with AP 2 reception. The transmissions by STAB in the second group interfere with AP 2 reception. In one example, STA 1 in BSS 1 and STA 2 in BSS 2 are both located within an overlapping coverage area of BSS 1 and BSS 2 and belong to the second group. In step 1203 , AP 1 coordinates between BSS 1 and BSS 2 for STAB in BSS 1 and BSS 2 to communicate based on group status and a predetermined rule to mitigate interference between BSS 1 and BSS 2 .

Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.