1. Beyond Co-existence: Exploiting WiFi White Space for ZigBee Performance Assurance
Jun Huang 1,  Guoliang Xing 1,  Gang Zhou 2, Ruogu Zhou 1  
1 Michigan State University, 
2 College of William and Mary

2. ZigBee Networks Low communication power (10~50 mw) Application domains
Smart energy, healthcare IT, Industrial/home automation, remote controls, game consoles….
Ex: 10 million smart meters installed in the US by 2010
Smart thermostat (HAI )
Smart electricity meter (Elster)
Industrial sensor networks
(Intel fabrication plant)

3. Challenge & State of the Art
Interference in open radio spectrum
Numerous devices in 2.4 GHz band: WiFi, bluetooth…
AT&T public WiFi usage: 300% up Q1/09~Q1/10 [1]
Multi-channel assignment
WiFi interferes with 12 of total 16 ZigBee channels
Co-existence on same/overlapping channels
Carrier sense multiple access (CSMA)
[1]

4. Empirical Study of Coexistence
WiFi interferer:
802.11g
Change WiFi node location
Measure ZigBee sending rate
WiFi interference on sender
Measure ZigBee packet delivery ratio
WiFi interference on receiver
Interference
link
Data link
ZigBee sender and recver
TelosB with CC2420
Method: change WiFi interferer position to create diversity
Measure sending rate for evaluating the impact of WiFi interference on ZigBee Sender, packet delivery ratio for impact on ZigBee recver
WiFi sending rate doesn’t change during the experiment
WiFi Interferer Position

5. WiFi Hidden Terminals Don’t trigger backoff at ZigBee sender
Corrupt packets at ZigBee receiver
WiFi Interferer Position

6. WiFi Exposed Terminals
Defer ZigBee sender’s transmissions
Not strong enough to corrupt ZigBee packets
WiFi Interferer Position

7. WiFi Blind Terminals Interfere both ZigBee sender and receivers
Severe packet loss on ZigBee link
WiFi sending rate not affected

8. Why Blind Terminals ? Power asymmetry Heterogeneous PHY layers
WiFi only senses de- modulatable signals
Energy-based sensing?
ZigBee
tx range
ZigBee sender
ZigBee recver
WiFi interferer
WiFi tx range 8

9. White Space in Real-life WiFi Traffic
Large amount of channel idle time
WiFi frames are clustered
white space: cluster gaps that can be utilized by ZigBee
Frame intervals: MAC layer intervals caused by DIFS and backoff. Usually less then 50 micro seconds. The PHY and MAC layer header of ZigBee requires nearly 1 milliseconds.
Inter-cluster intervals: Application layer intervals

10. Self-Similarity of Cluster Arrivals
Variance is similar at different time scales
Rigorously tested via rescaled range statistics and periodogram-based analysis
# clusters/5s
# clusters/s

11. Modeling WiFi White Space
Length of white space follows iid Pareto distri.
Implementation
Collect white space samples in a moving time window
Generate model by Maximum Likelihood Estimation
α = 1ms
shorter intervals are not usable for ZigBee
Modeling method: Collect white space samples during a time window. Use Maximum Likelihood Estimation for model generation.

12. Pareto Model: Goodness of Fit
Point represents the test result of a trace file.
Divide the trace files into equal sized time windows. Drive model for each window.
X(Y) shows the proportion of windows that passes the K-S(Independent) test.
OSDI ’06 traces
SigCOMM’08 traces
Pareto model is accurate when modeling window < 100ms
Sampling frequency is about 200Hz  20 samples are enough!

13. Outline Motivation Blind Terminal Problem WiFi White Space Modeling
WISE: WhIte Space-aware framE adaptation
Experimental Results 13

14. Basic Idea of WISE Sender splits ZigBee frame into sub-frames
Fill the white space with sub-frames
Receiver assembles sub-frames into frame
WiFi frame cluster
ZigBee sub-frames
Modeling method: Collect white space samples during a time window. Use Maximum Likelihood Estimation for model generation.
ZigBee
Time
sampling window
ZigBee frame pending

15. Maximum ZigBee frame size
Frame Adaptation
Collision probability
Sub-frame size optimization
Sub-Frame size
White space age
ZigBee data rate
250Kbps
Collision Threshold
Maximum ZigBee frame size 15

16. Experiment Setting ZigBee configuration WiFi configuration
TelosB with ZigBee-compliant CC2420 radios
Good link performance without WiFi interference
WiFi configuration
802.11g netbooks with Atheros AR9285 chipset
D-ITG for realistic traffic generation
Baseline protocols
B-MAC and Opportunistic transmission (OppTx)
Evaluation metrics
Modeling accuracy, sampling frequency, delivery ratio, throughput, overhead 16

17. Frame Delivery Ratio
Broadcast
Unicast with 3 retx 17

18. Conclusions Empirical study of WiFi and ZigBee coexistence
Blind terminal problem
WiFi white space modeling
Rigorous statistic analysis on real WiFi traffic
WISE: White space aware frame adaptation
Implemented in TinyOS 2.x on TelosB
Significant performance gains over B-MAC and OppTx 18

19. Throughput Overhead 19

20. Throughput

21. WiFi Interference Summary
Hidden terminal
The WiFi node is located within the interference range of ZigBee receiver, but outside the CCA range of ZigBee sender.
Exposed terminal
The WiFi node is located within the CCA range of ZigBee sender, but outside the interference range of ZigBee receiver.
Blind terminal
The WiFi node is located within both the CCA range of ZigBee sender and the interference range of ZigBee receiver.
Design flaw
of CSMA
CSMA supposed to work.
Why blind terminals? 21

22. Self-Similarity of WiFi Frame Clusters
Arrival process of frame cluster is self-similar
Variance is similar at different time scales

23. WISE Protocol Design Original ZigBee frame Sub-frame layout
WISE treat each MAC layer frame as a session
MAC protocol independent
Protocol overhead?
Small sub-frames have low collision probability
Large sub-frames are transmission efficient
Payload
PHY Hdr
MAC Hdr
CRC
PHY Hdr
MAC Hdr ID
PHY Hdr ID
Payload
PHY Hdr ID
Payload
CRC
WISE works between MAC and PHY. To make WISE independent with MAC, we didn’t implement retx in WISE. So there is only one CRC for each MAC layer frame 23

24. Frame Adaptation Optimal sub-frame size λ and ρ are measured on-line
Average white space lifetime
λ and ρ are measured on-line 24

25. Measure the White Space Model
WiFi white space sampling
Sampling the interrupt on CCA pin of CC2420: sampling frequency 4K~8KHz
Record white space sample if
Signal cannot be decoded
Interval between signals is longer than 1ms
Impact of ZigBee interference 25

26. Effect of Sampling Frequency26

27. CSMA is NOT White Space Aware
Collisions
CCA
Transmission
ZigBee
WiFi channel trace
Modeling method: Collect white space samples during a time window. Use Maximum Likelihood Estimation for model generation.
Time

28. ZigBee Link Performance Analysis
What’s the prob. of colliding w/ WiFi packets?
Analytical collision probability model
ZigBee carrier sensing model
White space model

29. Choose random waiting time T between [1, CW]
Why Blind Terminals ?
Heterogeneous PHY layer
backoff algorithm
No modulated
packet
in channel No
Choose random waiting time T between [1, CW]
Carrier Sense
T=0?
Count down T
modulated packet
detected
Yes
Data ready
Increase T
by the packet duration
Send
ZigBee In-friendly 29