Skip-Correlation for Multi-Power Wireless Carrier Sensing
Romil Bhardwaj, Krishna Chintalapudi, Ramachandran Ramjee
Motivation
AP1 1W
C1
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80
TCP
Th
rou
ghp
ut
(Mb
ps)
Time (s)
1W AP1 -> C1 100mW AP2 -> C2
C2
AP2100 mW
Why does this happen?
• Devices listen before they talk
• Listening sensitivity?
• 802.11 defines Carrier Sensing
Threshold (CST) at -82 dBm
AP1 1W
AP2100 mW
-78 dBm
-88 dBm
AP2 detects AP1AP1 does not detect AP2
Why is this important?
vs
A New Sensing Technique – Skip Correlation
“A senses B if and only if B senses A”(Carrier Sensing Symmetry)
No “Collateral” DamageCauses no new, unnecessary interactions
Backward CompatibleWorks with legacy devices, minor change to existing circuits
Use a lower CST?
AP1 1W
AP2100 mW
-82 dBm -92 dBm
Now Sensing @-92 dBm CST
Now Sensing @-92 dBm CST
Use a lower CST?
AP1 1W
AP2100 mW
-82 dBm -92 dBm
Now Sensing @-92 dBm CST
Now Sensing @-92 dBm CST
AP3100 mW
Now Sensing @-92 dBm CST
Collateral Damage
High Power APs use lower CST?
AP1 1W
AP2100 mW
-82 dBm
Now Sensing @-92 dBm CST
AP3100 mW
AP4 1W
Collateral Damage
Now Sensing @-92 dBm CST
Key Insights
P1= 30 dBm P2=20 dBm
-82 dBm
(-82-10)= -92 dBm
Should lower CST to -92 dBm
Key Insights
P1= 30 dBmP3=26 dBm
-82 dBm
(-82-4)= -88 dBm
Should lower CST to -88 dBm
CST(𝑷𝟏,𝑷𝟐) = ቊ−𝟖𝟐, 𝑷𝟏 ≤ 𝑷𝟐
−𝟖𝟐 − 𝑷𝟏 − 𝑷𝟐 , 𝑷𝟏 > 𝑷𝟐
Counter-Intuitive Insight
Dynamic CST is dependent on the receiver’s AND transmitter’s transmit powers!
Determining Transmitter’s Power is Hard!
• Cannot encode power in the SIGNAL Field
STS Preamble LTS Preamble SIGNAL Field (OFDM) DATA
Back-off Decision
20µs8µs8µs
Skip Correlation
STS STS STS STS STS STS STS STS STS STS
16 32 48 64 80 96 112 128 144 1600
Carrier Sensing 101
AutoCorrelation =
𝑖=1
𝐿2
𝑆𝑟𝑒𝑐𝑣 𝑖 𝑆𝑟𝑒𝑐𝑣 𝑖 +𝐿
2
0
0.2
0.4
0.6
0.8
1
1.2
Correlation Running Sum
Correlation
16
Wi-Fi STS Preamble
Threshold
STS STS STS STS STS STS STS STS STS STS
16 32 48 64 80 96 112 128 144 1600
Changing the Correlation Length
AutoCorrelation =
𝑖=1
𝐿2
𝑆𝑟𝑒𝑐𝑣 𝑖 𝑆𝑟𝑒𝑐𝑣 𝑖 +𝐿
2
0
0.5
1
1.5
2
2.5
Correlation Running Sum
Correlation L Correlation 2L
32
Wi-Fi STS Preamble
0 L
Correlation Window2L
𝐶𝑜𝑟𝑟𝑒𝑙𝑎𝑡𝑖𝑜𝑛 𝑆𝑁𝑅 ∝ 𝐿𝑒𝑛𝑔𝑡ℎ
Threshold
Correlation Length & Packet Detection
Doubling L is same as lowering CST by 3 dB!
Correlation Length L -> 90% detection probability @ -82 dBm
Correlation Length 2L -> 90% detection probability @ -85 dBm
Skip Correlation
CST(𝑃𝑎, 𝑃𝑏) = ቊ−82, 𝑃𝑎 ≤ 𝑃𝑏−82 − 𝑃𝑎 − 𝑃𝑏 , 𝑃𝑎 > 𝑃𝑏
Transmitter Receiver
Skip CorrelateDependent on Receiver's Tx
Power
High Power – Short preambles
Low Power – Long preambles
Skip Correlation Example - 4 Power Levels
20 dBm AP20
Transmit PreamblesL/4 L/2 3L/4 L0
23 dBm AP23
26 dBm AP26
29 dBm AP29
20 dBm AP20
Receive Correlation SequenceL/4 L/2 3L/4 L0
23 dBm AP23
26 dBm AP26
29 dBm AP29
L/4 L/2 3L/4 L0
L/4 L/2 3L/4 L0
23 dBm AP23
26 dBm AP26
29 dBm AP29
23 dBm AP23
26 dBm AP26
29 dBm AP29
AP20-> AP20
20 dBm AP20
Transmit Preambles
20 dBm AP20
AP20-> AP20
Correlation
Correlation Value = CCST = -82 dBm
CST(𝑷𝟏,𝑷𝟐) = ቊ−𝟖𝟐, 𝑷𝟏 ≤ 𝑷𝟐
−𝟖𝟐 − 𝑷𝟏 − 𝑷𝟐 , 𝑷𝟏 > 𝑷𝟐
20 dBm AP20
L/4 L/2 3L/4 L0
L/4 L/2 3L/4 L0
23 dBm AP23
26 dBm AP26
29 dBm AP29
23 dBm AP23
26 dBm AP26
AP20-> AP29
20 dBm AP20
Transmit Preambles
AP20-> AP29
Correlation
Correlation Value = 8CCST = -91 dBm
29 dBm AP29
CST(𝑷𝟏,𝑷𝟐) = ቊ−𝟖𝟐, 𝑷𝟏 ≤ 𝑷𝟐
−𝟖𝟐 − 𝑷𝟏 − 𝑷𝟐 , 𝑷𝟏 > 𝑷𝟐
29 dBm AP29
20 dBm AP20
20 dBm AP20
L/4 L/2 3L/4 L0
L/4 L/2 3L/4 L0
26 dBm AP26
29 dBm AP29
23 dBm AP23
AP23-> AP26Transmit Preambles
AP23-> AP26
Correlation
Correlation Value = 2CCST = -85 dBm
26 dBm AP26
23 dBm AP23
CST(𝑷𝟏,𝑷𝟐) = ቊ−𝟖𝟐, 𝑷𝟏 ≤ 𝑷𝟐
−𝟖𝟐 − 𝑷𝟏 − 𝑷𝟐 , 𝑷𝟏 > 𝑷𝟐
Skip Correlation
20 dBm AP20
Transmit PreamblesL/4 L/2 3L/4 L0
23 dBm AP23
26 dBm AP26
29 dBm AP29
20 dBm AP20
Receive Correlation SequenceL/4 L/2 3L/4 L0
23 dBm AP23
26 dBm AP26
29 dBm AP29
CST(𝑷𝟏,𝑷𝟐) = ቊ−𝟖𝟐, 𝑷𝟏 ≤ 𝑷𝟐
−𝟖𝟐 − 𝑷𝟏 − 𝑷𝟐 , 𝑷𝟏 > 𝑷𝟐
All Tx-Rx Pairs
20 dBm 23 dBm 26 dBm 29 dBm
20 dBm L, -82 L, -82 L, -82 L, -82
23 dBm 2L, -85 L, -82 L, -82 L, -82
26 dBm 4L, -88 2L, -85 L, -82 L, -82
29 dBm 8L, -91 4L, -88 2L, -85 L, -82
Transmitter Tx Power
Re
ceiv
er
TxP
ow
er
Effective Correlation Length (L) and CST (dBm) using Skip Correlation
CST(𝑷𝟏,𝑷𝟐) = ቊ−𝟖𝟐, 𝑷𝟏 ≤ 𝑷𝟐
−𝟖𝟐 − 𝑷𝟏 − 𝑷𝟐 , 𝑷𝟏 > 𝑷𝟐
Not Just 4 Power Levels..Tr
ansm
itR
ece
ive
𝚼
𝑷𝒌−𝒊
⋮
𝑷𝑵
𝚼𝑷𝑵
𝑷𝟏
𝑷𝟏
𝑷𝒌
⋮𝑷𝑵
𝚼𝑷𝑵
𝑷𝒌−𝒊+𝟏𝑷𝒌−𝒊+𝟏
𝑷𝟏
𝝀𝒌,𝒊
⋮
𝝀𝒌,𝒊+𝟏
𝚼𝑷𝑵
𝑷𝒌−𝒊⋮ 𝝀𝒌,𝒌−𝟏
𝑷𝟐
𝝀𝟐,𝟏
⋮
⋮𝝀𝒌,𝟏
𝑷𝒌𝚼
𝑷𝑵
𝑷𝒌
⋮
𝑷𝟐
𝚼𝑷𝑵
𝑷𝟐
Minimal Change in Silicon
Regular Wi-Fi Auto-Correlator Skip Correlator
12 adders = 0.03% increase in FPGA utilization
Implementation
• Implemented on the WARP v3 FPGA
• Supports 4 power levels, 0 to 9 dB in
increments of 3 dB
• Backward compatible design
Throughput Experiments
Nexus 5x
Samsung Galaxy S3
TP-Link Archer C7 AP 802.11n
WARP APA B B can sense A
23dBm 26dBm
20dBm
Without Skip Correlation
With Skip Correlation
Sensing Symmetry Experiments
Det
ect
ion
P
rob
abili
ty
Log10(C)
Skip Correlation Summary
• Carrier Sense Threshold
• Dynamic
• Tx Rx power level dependent
• Skip Correlation
• Realizes dynamic CST
• Leverages L and CST Relationship
• Experiments
• Sensing symmetry, even with legacy devices
• Simple implementation