Walking down the STAIRS: Efficient Collision Resolution with Constructive Interference

Xiaoyu Ji, Yuan He, Jiliang Wang, Wei Dong, Xiaopei Wu and Yunhao Liu

INFOCOM, 2014, Toronto

Hong Kong University of Science and Technology

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Motivation• Wireless Sensor Networks (WSNs)

– Event-driven mode– Low duty cycle operating– Large number of nodes

• CSMA-like protocols– Limitations – Backoff...

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The Recent Art- COMA• COMA- Contend before data transmission

– Contention packets reserve channel for real data packets

– The drawback: dedicated contention packets in each round

DATA 1Receiver

Sender3

Sender2

Sender1

Collision Contention Data

DATA 1

DATA 2

DATA 2

DATA 3

DATA 3

DATA

DATA

Data DataContention Contention

DATA

DATA

Can we resolve the collision in just one round!

Ref: F. Osterlind, et. al, Strawman: resolving collisions in bursty low-power wirelessnetworks,” in IEEE/ACM IPSN, 2012

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One-round Collision Resolution• The problem:

– Count, identify and schedule– And of course in one round!

• Approach– Active contention– Virtual ID– Fast identification

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Our weapon: RSSI Stair Pattern• The observation

– Signals can constructively collide– Requirements of Constructive Interference (CI)

• 0.5 μs• Identical signal waveform

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The Principle

( ) ( 1) 10 20 log , 21CI k CI k

kRSSI k

k

Proposition: Given the superposed signal CI(k) under CI, let A1 = A2 = … = A be the amplitude and τ1 = τ2= … = B denoting the phase offset with respect to the first signal generated by transmitter i = 1. Consider one IEEE 802.15.4 standard based communication system, RSSICI(k) is equal to:

( )1

20log cos cC i

k

I k ii

RSSI A

Where ωc is a constant and τ1 =0

D1 CR

CR

D3 CR

SP

S1

R

S3

Period 1Collision

D2 CRS2

SP D1

Period 2Contention

CP1

CP2

CP3

D2SP

SP

SP

D3

D1

Period 3Data transmission

Time

SP

SP .........

... RSSI ValueD2 SP

SP

D3

CR Contention Request

CP Contention Packet

SP Schedule Packet

D Data

Design of STAIRS• Overview

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Through intentional contention, senders can be identified from the stair-like pattern of RSSI in one round.

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Design Challenges• Challenge 1: Synchronization

– Requirement of CI: Δ≤0.5μs• Challenge 2: Falling edge detection

– CP packets with the same length– External interference, e.g., WiFi signals

CP1CP2

CP3

(1) False negatives

False falling edges

(2) False positives

Alignment for CP packets• Receiver-initiated (CR)

– Triggering transmissions of CP packets– Serving as ACK/NACK– Coping with hidden terminals

• Parallelizing receiving and reading

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S-CUSUM Edge Detection• Discrete lengths of CP packets

– Total sender number N, maximum packet size L, increase step ΔL, length of CP is:

• A paradox- how to find a good ΔL?

, 2 ,3 ...l CP L L L m L

Less false edges

Larger CP space

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• Finding the optimal ΔL– p=1/m: choose any of the m lengths– α: the probability of false positives

• Three cases for a schedule:

1

(1 )

(1 )

1

Ni

Ns

c i s

P p

P Np p

P P P

argmax , ,opti s c

L

L f P P P

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Implementation• STAIRS

– A plug-in between APP and MAC layer– Invoked when collision happens– Three main components

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Evaluation• Micro-benchmark

– Synchronization – Edge detection

• Multi-hop testbed– Completion time– Efficiency– Duty cycle

• Large-scale simulation

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Micro-benchmark

• Offset among arriving packets less than 0.25 μs!• S-CUSUM increases detection efficiency.• Average detection accuracy is > 85%.

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Testbed Settings

• Multi-flow-multi-hop environment.• ΔL is set to 10 bytes.• 20 TelosB sensor nodes

Flow number

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Compared with Strawman

• STAIRS beats Strawman, especially with large number of senders.

• Contention overhead of STAIRS is amortized.

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Duty Cycle Evaluation

• Both sender and receiver duty cycles are improved, as contention time is reduced.

• Energy efficiency is therefore improved.

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Simulation• Settings

– Up to 50 senders– Linear backoff (CSMA-L)– Exponential backoff (CSMA-E)

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Results

• No degraded performance• CSMA-L beats CSMA-E after the threshold• Backoff time dominates!

Pkt_size = 50 bytes Pkt_size = 100 bytes

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Summary• Collision resolution with active contention • Observing the RSSI stair-like pattern, we then

look into its principle• Design STAIRS based on the stair pattern and

solve challenges like synchronization and finding optimal ΔL

• Evaluation in both real testbed and large-scale simulation

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Thank you！