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