Enabling Coexistence of Heterogeneous Wireless Systems: Case for ZigBee and WiFi

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

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

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

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

5. Why CSMA fails Time resolution: e.g., WiFi: 9 us; ZigBee: 320us Problem: preemption: Communication barrier: WiFi: OFDM; ZigBee: DSSS 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 switching time WiFi client A separate node(ZigBee signaler) emits abusy-tone to make WiFi aware of ZigBee transmission Busy tone harbingersthe data packet and continues throughout theDATA-ACK transmission to prevent WiFi preemption ACK DATA ZigBee TX ZigBee signaler signaler WiFi TX ZigBee TX CCA, backoff DATA ACK WiFi AP

8. Challenges How can the busy-tone be prevented from interferingwith 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: 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 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 too large: busy-tone wastes channel time too small: no idle slot can be sensed, busy-tone aborted Key parameter: harbinger time Analytical framework: relate to network performance Harbinger time

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

13. Performance analysis and parameter optimization Network model: Parameters: Traffic: Poisson, arrival rate and , respectively Topology: co-located ZigBee and WiFi networks, (ZigBee signaler within range of WiFi transmitter) 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: Approach: Focus primarily on temporal collision probability Incorporate spatial collision probability (includes node locations and capture effect) Normalized throughput: and Analyze collision probability under each parameter setting Analyze throughput based on collision probability: Assume ZigBee does not affect WiFi traffic (low power and low duty cycle)

15. ZigBee TDMA mode with WiFi Collision probability of legacy ZigBee: Collision probability of CBT: Tag an arbitrary packet from , and calculate the collision probability with randomly arrived packets(assuming ZigBee does not affect WiFi traffic) 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 = WiFi throughput approximated using simpler model (in paper): Prob.[no collision] data packet size Average amount of data sent within an attempt Mean service time of a data packet Includes retransmission, ACK, and switching time Depends on whether or not

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

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

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

20. Simulation and testbed evaluation Simulation: Testbed experiments: Based on ns-2 ZigBee model 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, buthigher 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 Nodes A and B are WiFi All other nodes are ZigBee (MICAz motes) Node locations: Only TDMA mode implemented CBT signaler implemented in GNURadio on USRP2 software radio

26. Testbed results: Collision probability (TDMA mode) CBT reduces collision rate by 60+% for most links For randomly selected 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 Frequency flip: preventing signaler/transmitter interference Stochastic models for performance analysis and optimization Busy-tone scheduler: ensure busy-tones protect data packets 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 CBT resolves collision between ZigBee and WiFi: Possible future work:

29. Appendix

30. Normalized throughput CSMA mode

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