overcoming the antennas-per-ap throughput limit in mimo shyamnath gollakota samuel david perli and...
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Overcoming the Antennas-Per-AP Throughput Limit in MIMO
Shyamnath Gollakota
Samuel David Perli and Dina Katabi
MIMO LANs
Today, MIMO delivers as many concurrent packets as the antennas on the AP
Talk presents a practical technique to double the concurrent packets in MIMO
LANs
Talk presents a practical technique to double the concurrent packets in MIMO
LANs
MIMO Primer
11h
12h
22h21h1p
2p
APBob
111 ph
112 ph
112
11 ph
h
222
21 ph
h
Antenna 1
Antenna 2
hij is the channel from antenna i to antenna j
AP
1p
2p
AP receives the sum of these vectors
MIMO Primer
11h
12h
22h21h
112
11 ph
h
222
21 ph
h
Bob
AP
1p
2p
AP projects on a direction orthogonal to interference
p2
p1
How does the AP decode each packet?Current MIMO decodes as many
concurrent packets as there are antennas per AP
Current MIMO decodes as many concurrent packets as there are antennas
per AP
MIMO Primer
11h
12h
22h21h
112
11 ph
h
222
21 ph
h
Bob
Can We Get More Concurrent Packets?
Bob AP
1p
2p
3p
p3
2p
1p p3
No direction is orthogonal to all interference AP can’t decode
All current MIMO LANs are limited by number of antennas-per-AP
All current MIMO LANs are limited by number of antennas-per-AP
1p
2p
Alice
Let the APs Coordinate Over the EthernetNaive solution: Emulate 4-antenna AP by sending every signal sample over Ethernet
Let the APs Coordinate Over the Ethernet
Impractical Overhead,
Naive solution: Emulate 4-antenna AP by sending every signal sample over Ethernet
1p
2p
3p
2p
1p p3
1p
2p
Eth
ern
et
Raw samples
E.g., a 3 or 4-antenna system needs 10’s of Gb/s
Can we leverage the Ethernet with minimal overhead?
Can we leverage the Ethernet with minimal overhead?
Eth
ern
et
p1
Bob AP11p
2p 2p
1p
3p
p3
2p
1p p3
• Align P3 with P2 at AP1
• AP1 broadcasts P1 on Ethernet • AP2 subtracts/cancels P1 decodes P2, P3
p1
p2 p3
AP2Alice
AP1 decodes P1 to its bits
Interference Alignment and Cancellation (IAC)
• IAC overcomes the antennas-per-AP throughput limit
• In IAC, a packet is decoded, then broadcasted once on the Ethernet minimal overhead
•
• IAC overcomes the antennas-per-AP throughput limit
• In IAC, a packet is decoded, then broadcasted once on the Ethernet minimal overhead
•
Contributions
• First MIMO LAN to overcome the antennas-per-AP limit
• IAC synthesizes interference alignment and cancellation
• Proved that IAC almost doubles MIMO throughput
• Implemented IAC in software radios showing practical throughput gains
How to Change Packet Direction?
10
1pH
H h11 h12h21 h22
11hClient AP
12h
22h21h1p
2p
h11h21
p1
h12h22
p221
0pH
10
1p
H21
0p
21 6.
4.
5.
5.pp
21 1
0
5.
5.pp
2211 pvpv
21
0pH
How to Change Packet Direction?
10
1pH
Client AP
H
15.
5.pH
26.
4.pH
Sender controls packet direction by multiplying with a vector
11pvH
12 pvH
How Do We Align?
Bob AP1 1111 pvH
11H
12H
21H
22H
2211 pvpv
33 pv
3321 pvH
2211 pvH
211321 vHvH
Alice AP2
How Does Alignment Work in Presence of Modulation?
Real
Imaginary
Modulated samples are complex numbers with different phases
Real
Imaginary
Sample in P3
Sample in P2
Alignment is in the antenna domain not the modulation domain
Antenna 1
Antenna 2
Alignment works independent of modulation phases
Alignment works independent of modulation phases
How Does AP2 Subtract Interference from P1?
Can’t subtract the bits in packet• Need to subtract interference signal as received by
AP2
Solution
• AP2 Re-modulate P1’s bits
• AP2 estimate and apply the channel P1 traversed to itself on modulated signal– Channel estimation in the presence of
interference as in [ZigZag, SIGCOMM’08]
• Subtract!
Theorem
In a M- antenna MIMO system, IAC delivers
• 2M concurrent packets on uplink
• max{2M-2, 3M/2} concurrent packets on downlink
How Does IAC Generalize to M-Antenna MIMO?
E.g., M=2 antennas 4 packets on uplink 3 packets on downlink
Theorem
In a M- antenna MIMO system, IAC delivers
• 2M concurrent packets on uplink
• max{2M-2, 3M/2} concurrent packets on downlink
How Does IAC Generalize to M-Antenna MIMO?
E.g., M=10 antennas 20 packets on uplink18 packets on downlink
For a large M,
IAC doubles MIMO throughput
For a large M,
IAC doubles MIMO throughput
What if There is a Single Client?
ClientAP1
1p
2p
2p
1p 2p
1p
Current MIMO exploits diversity and pick best of two APs
Can’t have more than 2 concurrent packets, but …
IAC can pick the best antenna pair across APs
AP2
•IAC provides higher diversity than Current MIMO
•Diversity gain applies to one or more clients
•IAC provides higher diversity than Current MIMO
•Diversity gain applies to one or more clients
IAC MAC Leverages 802.11 PCF mode
• Clients are simple: APs compute v vectors and send them to clients in the Grant message
• IAC adapts to changing channels because APs get a new channel estimate from each ACK packet
CF- End
Contention-free Contention
Downlink
Uplink
. . . . .
. . . . . ACKs
Gra
nt
P4 P5
P6
P1
P2 P3 Time
Testbed
• 20-node testbed
• All nodes within radio range of each other
• Each run randomly picks APs and clients
Uplink GainCD
F of
Run
s
Per-Client Throughput Gain
• On uplink, IAC’s median gain is 2.1x
• Gain is partially due to diversity but more to concurrency
• On uplink, IAC’s median gain is 2.1x
• Gain is partially due to diversity but more to concurrency
Downlink GainCD
F of
Run
s
Per-Client Throughput Gain
On downlink, IAC’s median gain is 1.5xOn downlink, IAC’s median gain is 1.5x
Gains as a Function of SNR
SNR in dB
Upl
ink
Thro
ughp
ut G
ain
IAC is beneficial across the operational range of SNRs
IAC is beneficial across the operational range of SNRs
Related Work
• Interference Alignment [AMK’08,JS’08]• Interference Cancellation [GC’80,HWA’08]• MU-MIMO [NJ’06]
IAC provably provides more throughput, and doubles the number of concurrent packets
Conclusion
• First MIMO LAN to overcome the antennas-per-AP limit
• IAC synthesizes interference alignment and cancellation
• Proved that IAC almost doubles MIMO throughput
• Implemented IAC in software radios showing that it works in practice
IAC MAC Leverages 802.11 PCF mode
• APs compute and send v vectors in Grant Clients are oblivious to each other
• APs can track channels, i.e., H, from using ACKs
CF- End
Contention-free Contention
Downlink
Uplink
. . . . .
. . . . . ACKs
Gra
nt
P4 P5
P6
P1
P2 P3 Time
Eth
ern
et
1p
2p
Uplink: for M=2 antennas, IAC delivers 2M=4 packets
Clients APs
3p
4p
p1
p2
1p
2p 3p 4p
1p
2p
3p 4p
3p 2p4p
1pp4p3
1pAPs Clients
2p
3p
p1
p2
1p
2p 3p
2p
1p3p
3p2p 1p
p3
Downlink: - Clients can’t coordinate over Ethernet - For M=2 antennas, IAC delivers 3M/2 = 3
packets