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

ThresholdSignal = Noise

SNR =

T1

3

Interferer

What if parallel transmissions?

Decoding successful only if:

T1AP

T2

Signal = Interference + Noise

SINR = Threshold

4

Collision

Collision

Decoding fails because:

Interferer

T1AP

T2

Signal = Interference + Noise

SINR = 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

1. Decode strongest signal first

2. Model and subtract

3. Decode as ifsimple transmission

6

Capacity with SIC

SNR =

Rblue = Sblue

noiselog 1 +

SINR =

R*green = Sgreen

Sblue + noiselog 1+

T1T2

Interferer

AP

Rate of green signal far lessRate of blue signal remains same

Strong signal penalized, weak signal gets all the benefitsRSIC = Sblue + Sgreen

noiselog 1+

+

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Channel Capacity w/o SIC

SNR =

Rblue = Sblue

noiselog 1 +

T1T2

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 when equal signal strengths

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 let’s verify that ...

11

SIC: A Packet Perspective

n Weaker blue packet can be at a high rate

MAC Layer throughput can actually suffer

T1T2

Interferer

AP

HOLE

n Stronger green packet has to be at low rate

Packet Transmission Time

Rate

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Mathematically ...

T1T2

InterfererAP

Packet Transmission Time

TimeSIC =

TimewoSIC =

Packet Transmission Time

LRblue

LR*green

max ,=

LRblue

LRgreen

+=

SIC Gain =

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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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Why is This Happening?

T1T2

InterfererAP

> T > T

=

Rgreen

Rblue

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Capacity Vs. Throughput

n We expected:§ Maximizing SIC capacity will maximize throughput

n Reality:§ Equal signal strengths maximize capacity but not throughput

Capacity HOLE

by reducing size of the hole ...

Can’t we improve MAC layer throughput with SIC

We study SIC enabled throughput in two scenarios

1. Two transmitters transmitting to a common receiver

2. Two transmitters transmitting to distinct receivers

We begin with

1. Common receiver

We consider:

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

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(1) Power Control

n 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

22

(2) Client Pairing

T1 T2

T4

T3

T1, T3 T2, T4

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(3) MultiRate Packetization

n 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

n Packet Packing§ Send multiple packets to fill up the hole§ Hard because stronger signal modeling becomes difficult

R*green

Rblue

- Perform Monte Carlo Simulations

How Does Adaptation Help?

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Considerable Improvement with Adaptation

Upload Performance with MAC Modifications

SIC

Packet Packing

PowerControl

Rate

2. Distinct Recievers

T1

T2 R1R2

Situations much less favorable to SIC

2. Distinct Recievers

T1

T2 R1R2

Main Concern:

• T1 will transmit at best possible bit-rate to R1

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

2. Distinct Recievers

T1

T2 R1R2

Thus, necessary (but not sufficient) condition:

• R2’s interferer(T1) must be closer than its own transmitter(T2)

• T1’s own receiver(R1) must be further than interfered receiver(R2)

T1

T2 R1R2

Gains available when all conditions hold

T1T2

R1

R2

Gains available when all conditions hold

How often do these SIC permissible topologies occur?

32

Gain with SIC in less than 10% of the cases

Monte Carlo Simulation

(AP Transmission Range)

Can protocol adaptation help?

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

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Not many topologies support SIC …thus limited scope for protocols

Does MAC Adaptation Help?

Implication on Network Architecture?

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

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

T1T2

R1

R2

T1T2

R1

R2

Of course:

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

However, rate adaptation schemes such as SoftRate and AccuRate 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 knownZigZag, ANC, CSMA/CN, Full Duplex ...

This paper investigates the case ofUnknown Interference Cancellation

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Conclusion

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

n Throughput gain is not immediate from SIC§ Permissible bit rates impact the length of packet transmission times§ Creates under-utilization of the channel

n Protocol adaptations possible to cope with problem§ Some gains available for common receiver scenario§ However, limited gains for networks with distinct receivers

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

Questions, comments?

Thank you

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