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Design and Experimental Evaluation of Multi-User Beamforming in Wireless LANs
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Transcript of Design and Experimental Evaluation of Multi-User Beamforming in Wireless LANs
Design and Experimental Evaluation of Multi-User Beamforming in Wireless LANs
Theodoros SalonidisTechnicolor
ACM MobiCom 2010
Edward KnightlyRice
Narendra AnandRice
Ehsan AryafarRice
MIMO LANs
Ehsan Aryafar Rice Networks Group
Ehsan Aryafar Rice Networks GroupRx Rx Rx RxRx
• MIMO increases throughput with antenna arrays at transmitter and receiver• However, real world client devices have fewer antennas than APs due to cost
and space • MUBF allows for APs to leverage antennas belonging to group of nodes
Tx
Rx
Tx
We present the design and experimental evaluation of the first MUBF platform for
WLANs
Xirrus 16 ant AP
Crash Course on Beamforming
• Omni– Fixed vs ant
selection
Ehsan Aryafar Rice Networks Group
p1p2
AP
Crash Course on Beamforming
• Omni– Fixed vs ant
selection
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p1p2
AP
• Adaptive Beam (SUBF)– Higher coverage– Higher SNR
Multi-User Beamforming: Throughput Increase
Ehsan Aryafar Rice Networks Group
s1s2
AP
• MUBF sends the contents to both receivers at the same time• Each user’s data stream is weighted at the transmitter
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w1s1w'1 s1
++
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w2s2w'2 s2
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y1 = h1w1s1+ h1w2s2+noise
desired signal
inter-user interference
• Appropriate weights can reduce or eliminate the amount of inter-user interference
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w1
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w2
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h1
Ehsan Aryafar Rice Networks Group
s1s2
AP
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w1s1w'1 s1
+
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w2s2w'2 s2
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y1 = h1w1s1+ h2w2s2+z1
desired signal
inter-user interference
• Zero-Forcing beamforming (ZFBF)– weights are selected such that the amount of
inter-user interference is zero
Multi-User Beamforming: Throughput Increase
0
Multi-User Beamforming: Interference Reduction
Ehsan Aryafar Rice Networks Group
• A user can obtain an interference-free channel by sharing its channel information
Client 1Client 2
User affected by AP’sinterference
Channel InformationAP
Outline
• Background
• System Implementation
• Experimental Evaluation
• Conclusion
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Methodology• Unified Implementation Platform
– First Implementation and experimental evaluation of different beamforming algorithms on a common platform
• Experimental Characterization of System Performance– Compare against single-user TDMA schemes– Use repeatable controlled channels and– Real-time indoor channels
• Evaluation Metric– SNR or the corresponding Shannon capacity
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WARPLab Research Framework
• WARP is clean-slate MAC and PHY– Off-the-shelf platforms: Limited
programmability/observability
• WARPLab brings together WARP and MATLAB– Manage network communication of up to
16 WARP nodes – Baseband signals are generated in
MATLAB and downloaded to WARP nodes– WARP nodes send/receive the RF signals
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Virtex-II Pro FPGA
Implementation
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Tx Rx Rx Rx
( )
BF W
eigh
ts
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MUBF Data (OTA)6
Rx Training Feedback
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Rx RSSI Readings
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Log RSSI Data (End of Cycle)
8H Matrix and Weight Calculation
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Trai
ning
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Training (OTA)2
For more information about our testbed and implementation please attend our
demo!
Experimental Design• Multiplexing Gain
– Receiver separation distance– User selection algorithm– User population size
• Channel Variation– Environmental variation– User mobility
• Spatial Reuse– Location based interference– Multi-point interference reduction– Network throughput
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Impact of Receiver Separation
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• Issue: How does receiver separation distance affect spatial multiplexing gain?
Impact of Receiver Separation
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λ - 4λ/2 - 5λ/4 - 6
TX
R1
2.85m
2.85m
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11123
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R2
• Issue: How does receiver separation distance affect spatial multiplexing gain?
R1
02468
10Omni SUBF ZFBF
Sum
Cap
acity
(b
ps/H
z)
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101520253035
Per-
link
SNR
(dB)
Impact of Receiver Separation
• Issue: How does receiver separation distance affect spatial multiplexing gain?
• ZFBF doubles capacity compared to Omni
• Similar capacity up to λ/2 Separation distance
• ZFBF at λ/4:– 6 dB decrease in per-link SNR
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Location ID: 2 3 4(λ) 5(λ/2) 6(λ/4) 7
Location ID: 2 3 4(λ) 5(λ/2) 6(λ/4) 7
Experimental Design• Multiplexing Gain
– Receiver separation distance– User selection algorithm– User population size
• Channel Variation– Environmental variation– User mobility
• Spatial Reuse– Location based interference– Multi-point interference reduction– Network throughput
Ehsan Aryafar Rice Networks Group
User Mobility
Ehsan Aryafar Rice Networks Group
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h1
€
h'1
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h2
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h'2
• Issue: Evaluate impact of outdated channel information due to user mobility
User Mobility
• Issue: Evaluate impact of outdated channel information due to user mobility
• Repeatable channel conditions– 802.11n Task Group channel
model
• Required channel update rate– Channel must be updated at
(λ/8) movement– Equal to 10 msec update rate
for a typical pedestrian speed (3 mph)
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Per-link SNR
Aggregate Capacity
SNR
(dB)
bps/
Hz
Similar experiments can be done for static receivers (in paper). The required
channel rate for a typical residential environment is 100 msec.
Experimental Design• Multiplexing Gain
– Receiver separation distance– User selection algorithm– User population size
• Channel Variation– Environmental variation– User mobility
• Spatial Reuse– Location based interference– Multi-point interference reduction– Network throughput
Ehsan Aryafar Rice Networks Group
Multi-Point Interference Reduction
Ehsan Aryafar Rice Networks Group
• Issue: Evaluate a sender’s ability to reduce transmission footprint at multiple locations– Interference reduction at
unintended receivers– Impact on the QoS of the
served user
p1
Interference Reduction Points
Multi-Point Interference Reduction
Ehsan Aryafar Rice Networks Group
• Issue: Evaluate a sender’s ability to reduce transmission footprint at multiple locations
• Interference Reduction:– Interference reduction
capability does not depend on the location/number of unintended receivers
Multi-Point Interference Reduction
Ehsan Aryafar Rice Networks Group
• Issue: Evaluate a sender’s ability to reduce transmission footprint at multiple locations
• Interference Reduction:– Interference reduction
capability does not depend on the location/number of unintended receivers
• Increase in number of unintended receivers, can significantly drop the QoS of the currently served users
SNR difference at the intended receiver
Prior Work
• Theoretical Work on MU-MIMO– DPC (Costa’83) and its optimality (CS’03)– ZFBF (YG’06 and WES’08)
• Practical Protocols– IAC (GPK’09) and SAM (TLFWZCV’09)
Ehsan Aryafar Rice Networks Group
We present the design and experimental evaluation of a MUBF platform for wireless LANs
In Summary• Design and implementation of the first MUBF platform for WLANs and found
via experimental evaluation:
• Users can simultaneously receive data down to a half of wavelength from one another
• ZFBF can tolerate channel variations due to environmental variation, however, is strongly affected by user mobility
• ZFBF can efficiently eliminate interference at undesired locations. This does not depend on the location/number of unintended receivers, however, can significantly reduce the QoS for the currently served users
WARP: http://warp.rice.edu RNG: http://networks.rice.edu
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Back Up
Ehsan Aryafar Rice Networks Group
iburst
Ehsan Aryafar Rice Networks Group
Patented technology for
concurrent transmissionSuitable for
outdoor channels
Crash Course on Beamforming
• Omni– Fixed vs ant
selection
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p1p2
AP
• Switched Beam– Fixed beam– High coverage
• Adaptive Beam– Higher range– SUBF
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h1121h
Weight Selection Algorithms
• Zero-Forcing beamforming (ZFBF)– Condition: => – Heterogeneous link qualities through power
allocation• Regularized Channel Inversion– Increase system performance– Does not easily allow for heterogeneous link
qualities due to non-zero inter-user interference
Ehsan Aryafar Rice Networks Group
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hkw j = 0 ∀ j≠k
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Pkhkwksk + z k
Multi-Point Interference Reduction
Ehsan Aryafar Rice Networks Group
• Issue: Evaluate a sender’s ability to reduce transmission footprint at multiple locations
• Interference Reduction:– SUBF’s interference could be
significantly higher/lower than Omni
– ZFBF’s interference reduction capability does not depend on the location/number of unintended receivers
Weight Selection Zero Forcing Beamforming (ZFBF)
• Assume 4 Tx Antennas and 3 single-antenna receivers
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H hk's – H for each recv.
• Calculate weights with pseudo-inverse
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• “Zero Interference” Condition
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Implementation - WARPLab
• All baseband processing performed on Host PC• Processed signals are downloaded to buffers in FPGA on transmitting WARP node• HostPC sends Transmit/Receive trigger signals to WARP nodes• Data is transmitted over the air, stored in buffers on receiving node’s FPGA• Data/RSSI readings uploaded to HostPC for data processing/logging
User Population Size
Door
Door
Door Door Door
Door
2.85m
2.85m
Tx/Rx
Tx/Rx
Tx/Rx
Rx
RxRx
1
2
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5 6
02468
101214
2 3 4
bps/
hz
# of Receivers
Omni SUBF ZFBF
0
5
10
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25
2 3 4
SIN
R (d
b)
# of Receivers
Omni SUBF ZFBF
Aggregate Capacity Average Per-User SINR
• Q: How does the number of concurrently served users affect performance?
• A: Capacity increases and saturates while per-user SINR drops significantly.
User Selection (Link Quality Difference)
Door
Door
Door Door Door
Door
2.85m
2.85m
Tx/Rx
Tx/Rx
Tx/Rx
Rx
RxRx
1
2
34
5 6• Q: How do link quality differences between receivers affect system performance?
• A: Link quality differences between concurrently served users do not affect each user’s SINR.
Environmental Variation
10 50 100 5002
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bps/
hzTime (ms)
Omni-T SUBF-T ZFBF-T
Omni-R SUBF-R ZFBF-R
10 50 100 5000
5
10
15
20
25
SIN
R (d
B)
Time (ms)
Omni-T SUBF-T ZFBF-TOmni-R SUBF-R ZFBF-R
Aggregate Capacity
Average Per-User SINR
-802.11n Task Group model for indoor residential environment- (T) : Typical –Fading rate of 1.157 Hz- (R) : Rapid –Fading rate of 2.778 Hz
• Q: How does performance vary with channel update rate in typically/rapidly varying channels?
• A: Assuming a link can suffer up to a 3dB decrease in SNR below Omni, 100ms and 50ms update rates are necessary for typically/rapidly varying channels, respectively.
• Q: How does MUBF’s interference reduction capability vary with the location of the unintended receiver?
• A: The location of the unintended receiver does not affect the interference reduction performance of MUBF (when #Rx < DOF).
Interference Reduction (Location)
Door
Door
2.85mDoor Door Door
2.85m
Door
Door
1
2
345
6
78
910
R
TX
λ/2
W
Stairs
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5
10
15
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25
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1 2 3 4 5 6 7 8 9 10
Inte
rfer
ence
(db)
Location ID
Omni ZFBFInterference at W
Channel Variation
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Testbed
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Channel Estimation
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Network Throughput
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