Successive Interference Cancellation: A Back of the Envelope Perspective

Souvik Sen, Naveen Santhapuri, Romit Roy Choudhury, Srihari Nelakuditi

2

Simple Case of Wireless Transmission

Decoding successful if:

AP

Signal Noise

SNR =

T1

> Threshold=

3

Interferer

What if parallel transmissions?

T1AP

T2

Decoding successful only if:

Signal Interference + Noise

SINR = > Threshold=

4

Collision

Collision Interferer

T1AP

T2

Decoding fails when:

Signal Interference + Noise

SINR = < Threshold=

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Successive Interference Cancellation

Interferer

T1AP

T2

1. Decode strongest signal first

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Successive Interference Cancellation

Interferer

T1AP

T2

1. Decode strongest signal first

2. Model and subtract

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Successive Interference Cancellation

Interferer

T1AP

T2

3. NormalDecode

1. Decode strongest signal first

2. Model and subtract

It is as if SIC can “uncollide” signals, resulting in two successful transmissions

8

Capacity with SIC

SNR =

Rblue = Sblue

noiselog 1 +

SINR =

R*green = Sgreen

Sblue + noiselog 1+

T1T2

Interferer

AP

RSIC = Sblue + Sgreen

noiselog 1+

Green bit rate has to be far less Blue bit rate remains same

Strong signal penalized, weak signal gets all the benefits

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Channel Capacity w/o SIC

SNR =

Rblue = Sblue

noiselog 1 +

T1T2

Interferer

AP

SNR =

SgreenRgreen =

noiselog 1 +

RSIC = Sblue + Sgreen

noiselog 1+

RwoSIC = max( Rblue, Rgreen )

Gainsic =

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SIC Capacity Gain

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SIC PHY Capacity Gain

Max SIC capacity gain when equal signal strengths

We were tempted to schedule packet transmissions of similar signal strengths ...

As MAC protocol designers ...

Our interpretation was that ...

maximizing SIC capacity will maximize throughput

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SIC: A Packet Perspective

MAC Layer throughput can actually suffer

T1T2

Interferer

AP

HOLE

Stronger green packet has to be at low rate

Weaker blue packet can be at a high rate

Packet Transmission Time

Rate

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Mathematically ...

T1T2

Interferer

AP

TimeSIC = LRblue

LR*green

max ,=

Transmission Time

TimewoSIC = L

Rblue

LRgreen

+=

Transmission Time

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Mathematically ...

T1T2

Interferer

AP

GainSIC =

TimeSIC = LRblue

LR*green

max ,=

Transmission Time

TimewoSIC = L

Rblue

LRgreen

+=

Transmission Time

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SIC Throughput Gain

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SIC Throughput Gain

Max throughput gain when signal strengths are 2:1

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Capacity Vs. Throughput

We expected: Maximizing SIC capacity will immediately maximize throughput

Reality: Equal signal strengths maximize capacity Disparate signal strengths (2:1) maximize throughput

Capacity

by reducing size of the hole?

Can’t we improve MAC layer throughput with SIC

Certainly possible:

1. Power control2. Scheduling3. Multirate packetization4. Packet packing

by reducing size of the hole?

Can’t we improve MAC layer throughput with SIC

Certainly possible:

1. Power control2. Scheduling3. Multirate packetization4. Packet packing

But at what cost?

We study SIC enabled throughput in two scenarios

1. Common receiver

2. Distinct receivers

We begin with

1. Common receiver

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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

T2 T3

T4

T1

T1, T2 T3, T4

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(2) Client Pairing

T2 T3

T4

T1

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

Rblue RgreenRblue

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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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Monte Carlo Simulations

SIC

RatePowerControl

Packing

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Considerable Improvement with Adaptation

Monte Carlo Simulations

SIC

RatePowerControl

Packing

2. Distinct receivers

2. Distinct receivers

Main Concern:

• Bit Rate of T1R1 is optimal

• R2 has to decode T1’s signal at this bit rate• Despite the presence of T2’s signal

T1

R2T2

R1

Gains available when several topological constraints hold:

T1

R2T2

R1

How often do these SIC permissible topologies occur?

33

Gain with SIC in less than 10% of the cases

Monte Carlo Simulations

(AP Transmit Range)

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Not many topologies support SIC …thus limited scope for protocols

Does MAC Adaptation Help?

Implication on Network Architectures?

T1

R2

T2

R1

Enterprise WLANs:• Clients likely to associate with stronger AP• Such scenarios unlikely

Residential WLANs:• Neighbors AP may be stronger• Some SIC scenarios possible

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Conclusion

Successive Interference Cancellation A PHY layer capability to “uncollide” transmissions

Throughput gain 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 scenarios However, limited gains for networks with distinct receivers

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Take Away Message:

SIC aware protocol design fraught with pitfalls …

Consider doing a back-of-the-envelope calculation before plunging into system design

Questions, comments?

Thank you

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

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