interference alignment by motion swarun kumar fadel adib, omid aryan, shyamnath gollakota and dina...
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Interference Alignment By Motion
Swarun Kumar
Fadel Adib, Omid Aryan, Shyamnath Gollakota and Dina Katabi
Major Advances in MIMO
E.g. Interference Alignment
Significant gains in throughput
Single-Antenna Devices
Single Antennas, due to limits on power and sizeLargely left out of these MIMO benefits
Single Antennas, due to limits on power and sizeLargely left out of these MIMO benefits
Bring MIMO Benefits to Single Antenna Devices
“Interference Alignment”
Goal
Interference Alignment
antenna 1
C1
C2 C3
antenna 2
AP 1
C1
interfere interfere
2-antenna node can decode only 2 signals
1 2
C2 C3
Interference Alignment
antenna 1
C1
C2 C3
antenna 2
AP 1
C1
interfere interfere
1 2
C2 C3
2-antenna node can decode only 2 signals
Interference Alignment
antenna 1
C1
C2 C3
antenna 2
AP 1
C1
interfere interfere
1 2
C2 C3
“align”
2-antenna node can decode only 2 signals
Interference Alignment
antenna 1
C1
C2
C3
antenna 2
AP 1
C1
interfere interfere
1 2
C2 C3
“align”
2-antenna node can decode only 2 signals
Interference Alignment
antenna 1
C1 antenna 2
C1
interfere interfere
1
C2 C3
one unwanted interferer
2
AP 1
“align”
2-antenna node can decode only 2 signals
Single-Antenna Devices
C1
interfere interfere
1
C2 C3
antenna 1
C1
C2 C3
antenna 2
Can we still perform interference alignment?
Signals from all clients will change
2
AP 1
Single-Antenna Devices
C1
interfere interfere
1
C2 C3
antenna 1
C1
C2 C3
antenna 2Signals from all clients will change
2
AP 1
Perform Interference Alignment purely at the AP• Eliminates feedback/cooperation with clients• Brings benefits of alignment to new devices
Perform Interference Alignment purely at the AP• Eliminates feedback/cooperation with clients• Brings benefits of alignment to new devices
Can we still perform interference alignment?
MoMIMO
• Moves the AP’s antenna to positions that achieve interference alignment
• Needs to only displace antenna by up to 2 inches
• Achieves 1.98x gain in throughput over 802.11n
1. How do we “find” positions of alignment?
2. How does it impact general wireless networks?
Feasibility of “Alignment by Motion”
Record antenna displacement for interference to drop below noise
AP 12 inch radius
21
interferedesired
C1 C2 C3
Feasibility of “Alignment by Motion”
Feasibility of “Alignment by Motion”
Why is the required displacement small?Why is the required displacement small?
Median: 0.3 inch
90th Percentile: 1 inch
A Simple Example
antenna 1
C1
ante
nna
2align ReferenceReference
C1
1 2
AP 1
A Simple Example
antenna 10align ReferenceReference
C1
1 2
AP 1
Goal: Minimize signal from C1 to antenna 2 Goal: Minimize signal from C1 to antenna 2
C1 ante
nna
2
• Paths combine constructively or destructively based on phase
Indoor Environments Rich in Multipath
High signal @2(poor alignment)
C1
1 2
AP 1
Indoor Environments Rich in Multipath
Paths differ by extra 2”
• Paths combine constructively or destructively based on phase
• For Wi-Fi, 2” ≈ λ/2
λ0° 360°C1
1 2
AP 1
Indoor Environments Rich in Multipath
• Paths combine constructively or destructively based on phase
• For Wi-Fi, 2” ≈ λ/2
• In-phase paths now out-of-phase!
Low Signal @2 (good alignment)
0° 180°λ2C1
1 2
AP 1
Paths differ by extra 2”
• Small displacement suffices for alignment• Generalizes to many reflectors, any alignment• Small displacement suffices for alignment• Generalizes to many reflectors, any alignment
{{
How Can We Find Good Alignment?We must quantify goodness of alignment
antenna 1
C1
C2
antenna 2
interference
C1
C2 interference
Poor
GoodC1
C2
interference ≈ 0
Goal: Find antenna location that minimizes interference
Goal: Find antenna location that minimizes interference
Naïve solution: Random walk
• Simulated the spatial profile of interference
• Ten reflectors placed in randomly chosen locations
• Applied standard multipath models
Does not work!
30
20
10
0
-10 Inte
rfer
ence
(dB)
Naïve solution: Random walk
High interference
x (in)
y (in
)
3
1
2
0
-1
-2
-332
1
-1-2-3 0
3
1
2
0
-1
-2
-332
1
0-1-2-3
30
20
10
0
-10 Inte
rfer
ence
(dB)
Naïve solution: Random walk
Low interference
Goal: Find blue spotsy
(in)
x (in)
3
1
2
0
-1
-2
-332
1
0-1-2-3
30
20
10
0
-10
Naïve solution: Random walk
x (in)
y (in
)
Blue spots of low interference are small Hard to stumble upon in a random walk
Goal: Find blue spots
Inte
rfer
ence
(dB)
3
1
2
0
-1
-2
-332
1
0-1-2-3
30
20
10
0
-10
Key Observation: Interference is smooth
• Wireless channels are continuous and smooth functions over space
x (in)
y (in
)
Inte
rfer
ence
(dB)
3
1
2
0
-1
-2
-332
1
0-1-2-3
30
20
10
0
-10
Solution: A Hill Climbing Algorithm• Move in random direction and track interference
x (in)
y (in
)
Inte
rfer
ence
(dB)
3
1
2
0
-1
-2
-332
1
0-1-2-3
30
20
10
0
-10
Solution: A Hill Climbing Algorithm• Move in random direction and track interference– If interference : continue in that direction
x (in)
y (in
)
Inte
rfer
ence
(dB)
3
1
2
0
-1
-2
-332
1
0-1-2-3
30
20
10
0
-10
Solution: A Hill Climbing Algorithm• Move in random direction and track interference – If interference : continue in that direction
x (in)
y (in
)
Inte
rfer
ence
(dB)
3
1
2
0
-1
-2
-332
1
0-1-2-3
30
20
10
0
-10
Solution: A Hill Climbing Algorithm• Move in random direction and track interference– If interference : continue in that direction– If interference : continue in opposite direction
x (in)
y (in
)
Inte
rfer
ence
(dB)
Algorithm converges to spot of minimum interferenceGuides antenna to find positions of alignment
Algorithm converges to spot of minimum interferenceGuides antenna to find positions of alignment
1. How do we “find” positions of alignment?
2. How does it impact general wireless networks?
Interference Alignment
AP 1
C1 C2 C3
AP 2 AP 3
AlignC2 and C3
Interference Alignment
AP 1
C1 C2 C3
AP 3
AlignC1 and C3
AP 2
Interference Alignment
AP 1
C1 C2 C3
AlignC1 and C2
AP 2 AP 3
Interference Alignment
AP 1
C1 C2 C3
AP 2 AP 3
• 3 concurrent streams Gain in throughput!N antenna APs enable N+1 concurrent uplink streams• 3 concurrent streams Gain in throughput!N antenna APs enable N+1 concurrent uplink streams
What about downlink traffic?
AP 1
C1 C2 C3
AP 2 AP 3
What about downlink traffic?
AP 1
C2 C3
AP 1 has 2 antennas
null ??
2 antenna node can null interference at up to 1 antenna
Nothing!
C2 & C3 alignedat AP 1
AP 1
C2 C3
AP 1 has 2 antennas
null
2 antenna node can null interference at up to 1 antenna
C2 & C3 alignedat AP 1
AP 1
C2C3
null for free!
Uplink Wireless Channels
AP 1
C2 C3
h1
h2 h3 h4
antenna 1ante
nna
2 (h1, h2) (h3, h4)
h1
h2
h3
h4
=
Downlink Wireless Channels
AP 1
C2 C3
h1
h2 h3 h4
Channel Reciprocityx
h1x + h2αx
null
αx
Downlink Wireless Channels
AP 1
C2 C3
h1
h2 h3 h4
Channel Reciprocityx αx
h1x + h2αx = 0
null
Downlink Wireless Channels
AP 1
C2 C3
h1
h2 h3 h4
Channel Reciprocityx αx
α =
null
-h1
h2
α = -h3
h4
h1
h2
h3
h4
=
null
Alignment on the uplink enables nulling on the downlink, with no extra movement
Alignment on the uplink enables nulling on the downlink, with no extra movement
Downlink Traffic
AP 1
C1 C2 C3
AP 2 AP 3
Downlink Traffic
AP 1
C1 C2 C3
AP 2 AP 3
Downlink Traffic
AP 1
C1 C2 C3
AP 2 AP 3
Downlink Traffic
AP 1
C1 C2 C3
AP 2 AP 3
• 3 concurrent streams on the downlinkMoMIMO provides gains to uplink & downlink traffic• 3 concurrent streams on the downlink
MoMIMO provides gains to uplink & downlink traffic
Experimental Results
MoMIMO Implementation
• Implemented on USRP N210
• Mounted antenna on Roomba to emulate sliding antennas
• Compare MoMIMO with 802.11n, n+
TestbedRandomly assign nodes to red locations
Class RoomOffice Space
Can Alignment Reduce Interference?CD
F
Interference (dB)
Can Alignment Reduce Interference?CD
F
802.11nMoMIMODownlink
Median: -2.5dB
Interference (dB)
ThroughputHeterogeneous mix of 1 & 2-antenna nodes
CDF
Network Throughput (Mbps)
ThroughputHeterogeneous mix of 1 & 2-antenna nodes
1.98x
CDF
Network Throughput (Mbps)
802.11nMoMIMO
ThroughputHeterogeneous mix of 1 & 2-antenna nodes
1.31xCDF
Network Throughput (Mbps)
802.11n
n+MoMIMO
Conclusion
• Performs Interference Alignment purely by moving an antenna of the AP
• Displaces antenna by up to 2 inches
• New applications at intersection of networkingand robotics