1. Month Year
doc.: IEEE yy/xxxxr0
May 2010
Interference Management Using Beamforming Technique in OBSS Environment
Authors:
Yusuke Asai (NTT)
John Doe, Some Company

2. May 2010
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
May 2010
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
5190
5230
5250
5270
5310
5350[MHz]
40MHz channel
80MHz channel
5470
5510
5550
5590
5630
5670
5710
5725[MHz]
Yusuke Asai (NTT)

4. Possible Interference Management [4]
May 2010
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
May 2010
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. May 2010
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. May 2010
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 C
Data for STA2 BA 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
May 2010
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
STA1
STA2
STA1
STA2
Yusuke Asai (NTT)

9. Throughput Evaluation
May 2010
Throughput Evaluation
Parameters
Parameter
Value
# of APs 2
# of Tx/Rx antennas per AP 8
# of Tx/Rx antennas per STA
# of STAs per AP
1 / 2 / 3 / 4
# of subcarrier per spatial stream
234
Frequency bandwidth
80 MHz
MCS
64QAM, R=5/6
MSDU size
1500 Byte
A-MPDU size
64kByte
Yusuke Asai (NTT)

10. Frame sequence without interference management (for comparison)
May 2010
Frame sequence without interference management (for comparison)
AP1 and AP2 shares a channel based on DCF
AP1
Data for STA1
(medium busy) BA
STA1
(medium busy)
AP2
(medium busy)
Data for STA2 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
May 2010
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. May 2010
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. May 2010
References
[1] Yasushi Takatori, “Importance of Overlapped BSS issue in ac,” Doc. IEEE /0630r1.
[2] Peter Loc and Minho Cheong, “TGac Functional Requirements and Evaluation Methodology,” Doc. IEEE /0451/r12.
[3] Yuichi Morioka, “Two Levels of OBSS Control in 11ac ,” Doc.IEEE /0833/r0.
[4] Yasushi Takatori, “Consideration on Interference Management in OBSS” Doc. IEEE /0839r1.
[5] Graham Smith, “TGaa OBSS Background,” Doc. IEEE /0762/r0.
[6] Graham Smith, “20/40MHz Channel Selection,” Doc. IEEE /0740/r0.
[7] Graham Smith, “Overlapping BSS Proposed Solution,” Doc. IEEE /0457/r3.
[8] Kentaro Nishimori, "Measurement results for OBSS in home network scenarios,"Doc /1031r0
Yusuke Asai (NTT)