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 ID: 725935
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Slide1
Overcoming the Antennas-Per-AP Throughput Limit in MIMO
Shyamnath GollakotaSamuel David Perli and Dina KatabiSlide2
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 LANsSlide3
MIMO Primer
AP
Bob
Antenna 1
Antenna 2
h
ij
is the channel from antenna
i
to antenna
j
Slide4
AP
AP receives the sum of these vectors
MIMO Primer
BobSlide5
AP
AP projects on a direction
orthogonal
to interference
p
2
p
1
How does the AP decode each packet?
Current MIMO decodes
as many concurrent packets as there are antennas per AP
MIMO Primer
BobSlide6
Can We Get More Concurrent Packets?Bob
AP
p
3
p
3
No direction is orthogonal to all interference
AP can’t decode
All current MIMO LANs are
limited by number of antennas-per-AP
AliceSlide7
Let the APs Coordinate Over the Ethernet
Naive solution: Emulate 4-antenna AP by sending every signal sample over EthernetSlide8
Let the APs Coordinate Over the Ethernet
Impractical Overhead,
Naive solution
: Emulate 4-antenna AP by sending every signal sample over Ethernet
p
3
Ethernet
Raw samples
E.g., a 3 or 4-antenna system needs
10’s of
Gb
/s
Can we leverage the Ethernet with
minimal overhead?Slide9
Ethernet
p1
Bob
AP1
p
3
p
3
Align
P3 with P2 at AP1
AP1 broadcasts P1 on Ethernet
AP2 subtracts/
cancels
P1
decodes
P2, P3
p
1
p
2
p
3
AP2
Alice
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
Slide10
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 gainsSlide11
How to Change Packet Direction?Slide12
How to Change Packet Direction?
Client
APSlide13
How to Change Packet Direction?
Client
AP
Sender controls packet direction by multiplying with a vectorSlide14
How Do We Align?Bob
AP1
Alice
AP2Slide15
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 Slide16
How Does AP2 Subtract Interference from P1?
Can’t subtract the bits in packetNeed 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 signalChannel estimation in the presence of interference as in [ZigZag, SIGCOMM’08]
Subtract! Slide17
How Does IAC Generalize to M-Antenna MIMO?Slide18
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 downlinkSlide19
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 uplink
18 packets on downlink
For a large M,
IAC doubles MIMO throughputSlide20
What if There is a Single Client?Client
AP1
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 clientsSlide21
IAC MACRequirements Allow concurrent packetsClients are oblivious to each other
Works even when channel changes (i.e., the matrix H changes)Slide22
IAC MAC Leverages 802.11 PCF modeClients are simple: APs compute
v vectors and send them to clients in the Grant messageIAC adapts to changing channels because APs get a new channel estimate from each ACK packet
CF- End
Contention-free
Contention
Downlink
Uplink
. . . . .
. . . . .
ACKs
Grant
P4
P5
P6
P1
P2
P3
Time
Slide23
PerformanceSlide24
Implementation
GNURadio software 2-antenna MIMO USRP
nodes
Carrier Freq: 2.4GHzSlide25
Testbed
20-node testbed
All nodes within radio range of each other
Each run randomly picks APs and clientsSlide26
Gain =
Client throughput in IACClient throughput in current MIMO
MetricSlide27
Uplink Gain
CDF of RunsPer-Client Throughput Gain Slide28
Uplink GainCDF of Runs
Per-Client Throughput Gain
On uplink, IAC’s median gain is 2.1x
Gain is partially due to diversity but more to concurrencySlide29
Downlink GainCDF of Runs
Per-Client Throughput Gain
On downlink, IAC’s median gain is 1.5xSlide30
Gains as a Function of SNRSlide31
Gains as a Function of SNRSNR in dB
Uplink Throughput GainIAC is beneficial across the operational range of SNRs Slide32
Related WorkInterference 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 Slide33
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 practiceSlide34
IAC MAC Leverages 802.11 PCF modeAPs compute and send
v vectors in Grant Clients are oblivious to each otherAPs can track channels, i.e., H, from using ACKs
CF-
End
Contention-free
Contention
Downlink
Uplink
. . . . .
. . . . .
ACKs
Grant
P4
P5
P6
P1
P2
P3
Time
Slide35
Ethernet
Uplink:
for M=2 antennas, IAC delivers 2M=4 packets
Clients
APs
p
1
p
2
p4
p3Slide36
APs
Clients
p
1
p
2
p
3
Downlink:
- Clients can’t coordinate over Ethernet
- For M=2 antennas, IAC delivers 3M/2 = 3 packetsSlide37
IAC’s concurrency increases capacity bound
C = d log(SNR) + o(log(SNR))IAC increases degrees of freedom
d
is degrees of freedom
Interference cancellation does not increase degrees of freedom but provides a better use of them