successive interference cancellation: a back of the envelope perspective
DESCRIPTION
Successive Interference Cancellation: A Back of the Envelope Perspective. Souvik Sen, Naveen Santhapuri, Romit Roy Choudhury , Srihari Nelakuditi. Simple Case of Wireless Transmission. AP. T1. Decoding successful if: . Signal Noise. > Threshold. SNR = . = . - PowerPoint PPT PresentationTRANSCRIPT
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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
Signal Noise
SNR =
T1
> Threshold=
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Interferer
What if parallel transmissions?
T1AP
T2
Decoding successful only if:
Signal Interference + Noise
SINR = > Threshold=
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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
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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
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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
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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
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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?
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We study SIC enabled throughput in two scenarios
1. Common receiver
2. Distinct receivers
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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
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2. Distinct receivers
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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
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Gains available when several topological constraints hold:
T1
R2T2
R1
How often do these SIC permissible topologies occur?
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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?
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Implication on Network Architectures?
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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
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Questions, comments?
Thank you
Duke SyNRG Research Grouphttp://synrg.ee.duke.edu
Thank You