Sic Hotnets Talk

8/13/2019 Sic Hotnets Talk

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Successive Interference Cancellation:A Back of the Envelope Perspective

Souvik Sen, Naveen Santhapuri,

Romit Roy Choudhury, Srihari Nelakuditi


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Simple Case of Wireless Transmission

Decoding successful if:

AP

Threshold

Signal =Noise

SNR=

T1


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Interferer

What if parallel transmissions?

Decoding successful only if:

T1AP

T2

Signal =

Interference + NoiseSINR =

Threshold


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Collision

Collision

Decoding fails because:

Interferer

T1AP

T2

Signal =

Interference + NoiseSINR = Threshold


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Successive Interference Cancellation

Interferer

T1AP

T2

Thus, it is as if SIC can uncollide signals,

resulting in two successful transmissions


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Capacity with SIC

SNR =

Rblue =Sblue

noiselog 1 +

SINR =

R*green =Sgreen

Sblue +noiselog 1+

T1

T2

Interferer

AP

Strong signal penalized, weak signal gets all the benefits

RSIC =Sblue +Sgreen

noiselog 1+

+


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Channel Capacity w/o SIC

SNR =

Rblue =Sblue

noiselog 1 +

T1

T2

Interferer

AP

SNR =

SgreenRgreen =

noiselog 1 +

Gainsic =

Rsic =Sblue +Sgreen

noiselog 1+

RwoSIC = max( Rblue, Rgreen )


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SIC PHY Capacity Gain


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SIC PHY Capacity Gain

Max SIC gain whenequal signal strengths


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We were tempted to schedule packet

transmissions of similar signal strengths ...

As protocol designers ...

Our interpretation was that ...

maximizing SIC capacity will maximize throughput

But lets verify that ...


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SIC: A Packet Perspective

!Weaker blue packet can be at a high rate

MAC Layer throughput can actually suffer

T1T2

Interferer

AP

HOLE

!Stronger green packet has to be at low rate

Packet Transmission Time

Rate


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Mathematically ...

T1T2

InterfererAP


TimeSIC =

TimewoSIC =


L

Rblue

L

R*greenmax ,=

L

Rblue

L

Rgreen

+=

SIC Gain=


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SIC Throughput Gain


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SIC Throughput Gain

Max throughput gain whensignal strengths are 2:1


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Why is This Happening?

T1T2

InterfererAP

> T> T =

Rgreen

Rblue


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Capacity Vs. Throughput

!We expected:" Maximizing SIC capacity will maximize throughput

!Reality:

"Equal signal strengths maximize capacity but not throughput

Capacity HOLE


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by reducing size of the hole ...Cant we improve MAC layer throughput with SIC


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We study SIC enabled throughput in two scenarios

1. Two transmitters transmitting to a common receiver

2. Two transmitters transmitting to distinct receivers


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We begin with

1. Common receiver

We consider:

1. Power control

2. Scheduling

3. Multirate packetization

4. Packet packing


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(1) Power Control

!Reduce power of blue Tx such that

SINR*green=Rgreen

Rblue

= 2 *

Reduce


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(2) Client Pairing

T1 T2

T4

T3

T1, T2 T3, T4


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(2) Client Pairing

T1 T2

T4

T3

T1, T3 T2, T4


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(3) MultiRate Packetization

!Multirate Packetization" Send the strong packet at high rate after weak packet has finished

R*green

RblueRgreen

Rblue


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(4) Packet Packing

!Packet Packing" Send multiple packets to fill up the hole

" Hard because stronger signal modeling becomes difficult

R*green

Rblue


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- Perform Monte Carlo Simulations

How Does Adaptation Help?


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Considerable Improvement with Adaptation

Upload Performance with MAC Modifications

SIC

PacketPacking

Power

Control

Rate


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2. Distinct Recievers

T1

T2 R1R2

Situations much less favorable to SIC


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T1

T2 R1R2

Main Concern:

T1 will transmit at best possible bit-rateto R1

R2 has to decode T1s signal at this bit rate...-Despite the presence of T2s signal


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T1

T2 R1R2

Thus, necessary (but not sufficient) condition:

R2s interferer(T1) must be closer than its own transmitter(T2)

T1s own receiver(R1) must be further than interfered receiver(R2)


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T1

T2 R1R2

Gains available when all conditions hold


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T1

T2

R1

R2

Gains available when all conditions hold

How often do these SIC permissible

topologies occur?


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Gain with SIC in less than 10% of the cases

Monte Carlo Simulation

(AP Transmission Range)


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Can protocol adaptation help?

1. Power control

2. Scheduling

3. Multirate packetization

4. Packet packing


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Not many topologies support SIC

thus limited scope for protocols

Does MAC Adaptation Help?


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Implication on Network Architecture?


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Residential WLANs:

Neighbors AP may be strongerSome SIC scenarios possible

Enterprise WLANs:

Clients likely to associate with stronger APSuch scenarios unlikely

T1

T2

R1

R2


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T1

T2

R1

R2

Of course:

SIC throughput gains will increase if bit rate T1->R1 is sub optimal

However, rate adaptation schemes such as SoftRate andAccuRate approaching optimality ... little room for SIC


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Mismatch with Literature?

Interference cancellation has been shown to be beneficial ...Our observations are not in conflict

In existing schemes, interfering bits are known

ZigZag, ANC, CSMA/CN, Full Duplex ...

This paper investigates the case ofUnknown Interference Cancellation


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Conclusion

!Successive Interference Cancellation" A PHY layer capability to uncollide transmissions

!Throughput gain is not immediate from SIC

" Permissible bit rates impact the length of packet transmission times" Creates under-utilization of the channel

!Protocol adaptations possible to cope with problem

" Some gains available for common receiver scenario" However, limited gains for networks with distinct receivers


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SIC aware protocol design fraught

with pitfalls

Consider doing a back-of-the-envelope calculation

before plunging into system design

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Takeaway Message


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Questions,

comments?Thank you

Duke SyNRG Research Grouphttp://synrg.ee.duke.edu

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