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Overcoming the Antennas-Per-AP Throughput Limit in MIMO Overcoming the Antennas-Per-AP Throughput Limit in MIMO

Overcoming the Antennas-Per-AP Throughput Limit in MIMO - PowerPoint Presentation

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Overcoming the Antennas-Per-AP Throughput Limit in MIMO - PPT Presentation

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

mimo iac packets antenna iac mimo antenna packets throughput ethernet uplink antennas concurrent interference downlink aps gain ap2 client

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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