1. Enabling Coexistence of Heterogeneous Wireless Systems: Case for ZigBee and WiFi The University of Michigan Kang G. Shin kgshin@eecs.umich.edu Xinyu Zhang xyzhang@eecs.umich.edu

2. Coexistence between ZigBee and WiFi Spatial coexistence: ZigBee (monitoring & control) WiFi (Internet access) 20MHz Frequency-domain coexistence (spectrum sharing):

3. Current scheme for managing coexistence Built-in MAC protocols: Is the built-in CSMA/CA effective? CSMA/CA  Listen before you talk Versions used by both ZigBee and WiFi Some small scale measurement studies: Evidence from the real-world: In a 90-node ZigBee building energy monitoring network. 50+% ZigBee nodes suffer connection loss during WiFi peak hours Severe collision occurs under moderate to high WiFi traffic [C-J. M. Liang, et al., “Surviving Wi-Fi Interference in Low Power ZigBee Networks,” SenSys 2010, November 2010]

4. Why CSMA fails Scheduling mode: ZigBee allows TDMA mode Problem: direct collision (no carrier sensing) Transmit power: WiFi: 15dBm; ZigBee: < 0 dBm Problem: asymmetric interference Heterogeneity challenges coexistence: WiFi transmission range ZigBee transmission range Asymmetric interference region

5. Why CSMA fails Communication barrier: Time resolution: WiFi: OFDM; ZigBee: DSSS e.g., WiFi: 9 us; ZigBee: 320us Problem: preemption: Problem: lack of ability to negotiate

6. New solution: Cooperative Busy Tone (CBT) Principles of CBT: Make ZigBee visible to WiFi, without interfering with ZigBee Allow ZigBee to coexist and contend with WiFi in frequency, spatial, and temporal domains Preserve carrier-sensing-based spectrum etiquette

7. CBT Overview WiFi AP WiFi client ZigBee TX signaler WiFi TX ZigBee TX ZigBee signaler DATA ACK DATA ACK CCA, backoff switching time A separate node (ZigBee signaler) emits a busy-tone to make WiFi aware of ZigBee transmission Busy tone harbingers the data packet and continues throughout the DATA-ACK transmission to prevent WiFi preemption

8. Challenges How can the busy-tone be prevented from interfering with the ZigBee data packet? When should the signaler begin and end sending the busy-tone?

9. Signaler “frequency flip” Avoids signaler interfering with ZigBee data packet: Busy tone Transmitter sends data packet on some channel Signaler sends busy-tone on an adjacent channel Return to the original channel after sending the busy-tone

10. Busy-tone scheduler objectives Schedule the signaler’s busy-tone so as to: reduce WiFi preemption of ZigBee transmissions minimize the potential influence on WiFi performance be able to protect both the TDMA and CSMA modes of ZigBee

11. Busy-tone scheduler: TDMA mode CCA attempts frequency flip Harbinger time too large: busy-tone wastes channel time too large: busy-tone wastes channel time too small: no idle slot can be sensed, busy-tone aborted too small: no idle slot can be sensed, busy-tone aborted Key parameter: harbinger time Analytical framework: relate to network performance

12. Busy-tone scheduler: CSMA mode backoffCCAswitching frequency flip too large: busy-tone wastes channel time too large: busy-tone wastes channel time too small: data/ACK may not be protected too small: data/ACK may not be protected Key parameter: busy-tone duration Analytical framework: relate to network performance Enlarge by extending busy-tone time by extra slots

13. Performance analysis and parameter optimization Network model: Traffic: Poisson, arrival rate and, respectively Topology: co-located ZigBee and WiFi networks, (ZigBee signaler within range of WiFi transmitter) Parameters: Traffic intensity and Transmit power and Topology: or (ZigBee transmitter within range of WiFi transmitter, or not) Using legacy ZigBee or CBT

14. Performance analysis and parameter optimization Performance metrics: Normalized throughput: and Approach: Analyze collision probability under each parameter setting Analyze throughput based on collision probability: Focus primarily on temporal collision probability Incorporate spatial collision probability (includes node locations and capture effect) Assume ZigBee does not affect WiFi traffic (low power and low duty cycle)

15. ZigBee TDMA mode with WiFi Collision probability of legacy ZigBee: Tag an arbitrary packet from, and calculate the collision probability with randomly arrived packets (assuming ZigBee does not affect WiFi traffic) Collision probability of CBT: Relate CCA failure rate to harbinger time (derived in Proposition 1 in paper) Relate collision probability to the CCA failure rate: Derive collision probability as a function of,,, etc.

16. ZigBee TDMA mode with WiFi (cont’d) Network performance: Model transmission attempt of as a renewal reward process ZigBee throughput = mean reward rate = Average amount of data sent within an attempt Mean service time of a data packet Includes retransmission, ACK, and switching time WiFi throughput approximated using simpler model (in paper): Depends on whether or not Prob.[no collision] data packet size

17. ZigBee CSMA mode with WiFi Performance of legacy ZigBee: Derive mean service time, based on a Markov chain model : Transmission probability (after CCA) : Data packet collision probability : ACK packet collision probability These depend on WiFi traffic intensity : i-th backoff & CCA stage

18. ZigBee CSMA mode with WiFi (cont’d) Performance of CBT: Also depends on key parameter: busy tone duration If = data packet duration + max backoff&CCA duration, then collision probability 0 Otherwise, the collision probability is bounded: Bound depends on (derived in Proposition 2 in paper) Similar Markov chain model

19. Spatial collision probability Probability that a packet cannot be decoded, given that temporal collision already occurs (Account for capture effect) Approximate in a random topology: Details in the paper

20. Simulation and testbed evaluation Based on ns-2 ZigBee model Simulation: Testbed experiments: CBT (TDMA mode): implementation of signaler in GNURadio, running on USRP2 software radio Legacy ZigBee: Based on openzb in TinyOS, running on MICAz motes Synchronize USRP signaler to ZigBee coordinator using short notification messages Modeled CBT (TDMA and CSMA mode) in ns-2

21. Temporal collision probability Analysis matches simulation CBT significantly reduces the collision rate for both data and ACK packets Markers = simulation results; lines = analytical results

22. Spatial-temporal collision probability Probability that ZigBee cannot decode collided packet (accounting for capture effect and random node locations) Out of interference range

23. Normalized throughput: TDMA mode CBT gives about 2 ZigBee throughput improvement under moderate to high WiFi traffic Negligible degradation of WiFi throughput, compared with legacy ZigBee CBT may have lower throughput than legacy ZigBee under light WiFi traffic (a sweet spot exists) Sweet spot

24. Impact of harbinger time in TDMA mode Larger larger more overhead, but higher ZigBee throughput under high WiFi interference Under low duty-cycle ZigBee traffic (below 0.05), WiFi throughput is virtually unaffected by harbinger time

25. Experimental testbed configuration Node locations: Nodes A and B are WiFi All other nodes are ZigBee (MICAz motes) Only TDMA mode implemented CBT signaler implemented in GNURadio on USRP2 software radio

26. Testbed results: Collision probability (TDMA mode) For randomly selected links: CBT reduces collision rate by 60+% for most links

27. Testbed results: Impact on WiFi (TDMA mode) CBT and legacy ZigBee have similar effects on WiFi performance WiFi performance essentially unaffected when ZigBee traffic load < 2% WiFi packet delay:

28. Conclusion Traditional CSMA fails in heterogeneous networks: Due to disparate MAC/PHY properties CBT resolves collision between ZigBee and WiFi: Frequency flip: preventing signaler/transmitter interference Stochastic models for performance analysis and optimization Busy-tone scheduler: ensure busy-tones protect data packets Possible future work: Extension to other heterogeneous networks, such as WiFi/Bluetooth (802.15.3), WiFi/WiMax (802.11y), and whitespace networks Simulation as well as measured testbed performance

29. Appendix

30. Normalized throughput CSMA mode

31. Impact of busy-tone duration in CSMA mode CSMA mode