INFOCOM, 2014, Toronto
Walking down the STAIRS: Efficient Collision
Resolution with Constructive Interference
Xiaoyu Ji, Yuan He, Jiliang Wang,
Wei Dong, Xiaopei Wu and Yunhao Liu
Hong Kong University of
Science and Technology
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
Collision
Sender1
Sender2
Sender3
Receiver
Contention
Data
Contention
Data
Contention
Data
Can we resolve the collision
in just one round!
DATA
DATA 1
DATA
DATA
DATA 3
DATA
DATA 2
DATA 1
DATA 2
DATA 3
Ref: F. Osterlind, et. al, Strawman: resolving collisions in bursty low-power wireless
networks,” 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
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:
RSSI CI ( k )
 k

 20 log   A i cos c i  
 i 1

RSSI CI ( k ) CI ( k 1)  20 log10
Where ωc is a constant and τ1 =0
k
, k  2
k 1
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Design of STAIRS
• Overview
Period 2
Contention
Period 1
Collision
S1
D1
CR
S2
D2
CR
S3
D3
CR
CP1
CP2
CP3
Period 3
Data transmission
SP
SP
SP
D
D1
D2
SP
SP
SP
D3
...
...
...
Data
Contention
CR Request
Contention
CP Packet
Schedule
SP Packet
R
CR
SP
D2
SP
D1
SP
D3
...
RSSI Value
Time
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
CP1
CP2
CP3
False falling edges
(1) False negatives
(2) False positives
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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:
l CP  L,2L,3L...mL
• A paradox- how to find a good ΔL?
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:
Pi   (1  p ) N
Ps  Np (1  p ) N 1
Pc  1  Pi  Ps
Lopt  arg max f  Pi , Ps , Pc 
L
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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
Pkt_size = 50 bytes
Pkt_size = 100 bytes
• No degraded performance
• CSMA-L beats CSMA-E after the threshold
• Backoff time dominates!
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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！
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