successive interference cancellation: a back of the
TRANSCRIPT
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
ThresholdSignal = Noise
SNR =
T1
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Interferer
What if parallel transmissions?
Decoding successful only if:
T1AP
T2
Signal = Interference + Noise
SINR = Threshold
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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
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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 ...
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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
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(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?
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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