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Interference Management Using Beamforming Technique in OBSS Environment

Interference Management Using Beamforming Technique in OBSS Environment. Authors:. Background. It is expected that the number of the OBSSs in TGac becomes more than existing standards (11a/n) because of both frequency bandwidth extension and increase in the number of WLAN devices.

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Interference Management Using Beamforming Technique in OBSS Environment

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  1. Interference Management Using Beamforming Technique in OBSS Environment Authors: Yusuke Asai (NTT)

  2. Background • It is expected that the number of the OBSSs in TGac becomes more than existing standards (11a/n) because of both frequency bandwidth extension and increase in the number of WLAN devices. • TGac agrees that it is important to see how 11ac devices behave in OBSS [1] and has defined the usage scenarios with OBSSs [2]. • Interference management has a potential to achieve further throughput improvement in OBSS environment [3][4]. It is also addressed in TGaa [5][6][7]. Yusuke Asai (NTT)

  3. OBSS Issue in TGac • OBSS issue is important in TGac because frequency channel shortage is expected. • Only four non-overlapped channels are available for 80MHz mode. • In addition, TGac is studying to use bandwidth of 160MHz and multi-channel transmission (contiguous / non-contiguous channel usage.) • Two Levels of interference management in OBSS environment are discussed in [3]. • Level 1: “OBSS Tolerance” Appropriately set the NAV at the OBSS • Level 2: “OBSS Coordination” Effectively coordinate between 11ac OBSSs to maximize SDMA throughput Frequency band allocation in Japan 20MHz channel • 5150 • 5250 • 5350[MHz] • 5190 • 5230 • 5270 • 5310 40MHz channel 80MHz channel • 5470 • 5725[MHz] • 5510 • 5550 • 5590 • 5630 • 5670 • 5710 Yusuke Asai (NTT)

  4. Possible Interference Management [4] • Frequency domain: • Access mechanism that enables frequency channel assignment would be devised to use 80MHz bandwidth for QoS/non-QoS traffic.(For 11aa case, see [5]) • Falling back to narrowband mode,e.g. 20/40MHz mode, can be one of the practical solutions. (For 11aa case, see [6]) • Spatial domain.: • Transmission power control (TPC) might be effective to decrease the number of OBSSs. (TGh addressed TPC to avoid interference to other systems. Although it is effective in an apartment scenario, TPC might cause range limitation in other scenarios [7].) • Beamforming might be also useful because 11ac devices already have beamforming capability to enable MU-MIMO. (TGac APs supporting DL-MU-MIMO inherently have beamforming function. ) Yusuke Asai (NTT)

  5. OBSS environment in apartment scenario • In apartment scenario: • Experimental results show the density of OBSS becomes high [8]. The effect of frequency channel assignment may be limited due to shortage of available channels. • Many applications which requires broadband traffic with QoS exist in home network (ex. wireless video transmission.) [2] Falling back to narrowband mode may cause shortage of bandwidth. • Distance between two OBSSs is small. The effect of TPC also may be limited. • How about beamforming? Yusuke Asai (NTT)

  6. Basic concept of interference management using beamforming in OBSS environment • Some degrees of freedom on antennas at AP can be used to mitigate interference to the STAs associated on other BSSs to form null to them. • When two APs cooperatively works with each other, spatial multiplexing between two Aps is possible. AP1 AP2 STA1 STA2 Null steering to the STA on the other BSS Yusuke Asai (NTT)

  7. Example of frame sequence using beamforming for OBSS interference management • Implicit feedback is assumed in this sequence. Frame sequence: • Sequence initiation • CSI acquisition • NAV setting • data transmission (spatially multiplexing using beamforming) • BA transmission (scheduled) • Medium is released. AP1 (initiator) R C Data for STA1 S BA STA1 AP2 (responder) R Data for STA2 BA C S STA2 1. 2. 3. 4. 5. 6. R: RTS frame / C: CTS frame / S: Sounding Frame Data: A-MPDU Data Frame (beamformed) / BA: BlockAck Yusuke Asai (NTT)

  8. Example of beamforming for interference management in OBSS • Two APs case: • AP1/2 obtains complete MIMO channel. • AP1/2 calculates DL-MU-MIMO steering matrices and prepares transmit signals. Null steering to direction of STA2/1 is set by transmission of no signal to the direction. • AP1 and AP2 transmit data frames simultaneously. 1. 2. 3. AP1 AP1 AP2 S: Steering matrices t1 :Transmit signals for AP1 t2: Transmit signals for AP2 STA2 STA1 STA2 STA1 Yusuke Asai (NTT)

  9. Throughput Evaluation • Parameters Yusuke Asai (NTT)

  10. Frame sequence without interference management (for comparison) • AP1 and AP2 shares a channel based on DCF Data for STA1 AP1 (medium busy) BA STA1 (medium busy) Data for STA2 AP2 (medium busy) BA STA2 (medium busy) Data transmission (from AP1 to STA1) Data transmission (from AP2 to STA2) Channel access phase based on DCF Yusuke Asai (NTT)

  11. Throughput Evaluation • Interference management using beamforming enhances throughput performance due to spatial multiplexing between two APs. (up to 70% improvement) • When the number of terminals per BSS is 4, there is no degrees of freedom for antenna. Normalized throughput The number of terminals per BSS Yusuke Asai (NTT)

  12. Summary • OBSS issue is important in TGac because frequency channel shortage is expected. • To improve throughput performance in OBSS environment, several interference management scheme can be applied. • Frequency domain approach: • Spatial domain approach: • In dense OBSS environment, beamforming technique is attractive. • An example of interference management using transmit beamforming is introduced, which is one of useful technique to improve throughput performance in OBSS environment. Yusuke Asai (NTT)

  13. References [1] Yasushi Takatori, “Importance of Overlapped BSS issue in 802.11ac,” Doc. IEEE802.11-09/0630r1. [2] Peter Loc and Minho Cheong, “TGac Functional Requirements and Evaluation Methodology,” Doc. IEEE802.11-09/0451/r12. [3] Yuichi Morioka, “Two Levels of OBSS Control in 11ac ,” Doc.IEEE802.11-09/0833/r0. [4] Yasushi Takatori, “Consideration on Interference Management in OBSS” Doc. IEEE802.11-09/0839r1. [5] Graham Smith, “TGaa OBSS Background,” Doc. IEEE802.11-09/0762/r0. [6] Graham Smith, “20/40MHz Channel Selection,” Doc. IEEE802.11-09/0740/r0. [7] Graham Smith, “Overlapping BSS Proposed Solution,” Doc. IEEE802.11-08/0457/r3. [8] Kentaro Nishimori, "Measurement results for OBSS in home network scenarios,"Doc. 11-09/1031r0 Yusuke Asai (NTT)

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