Enabling Coexistence ofHeterogeneous Wireless

Systems:

Case for ZigBee and WiFi

The University of Michigan

Kang G. Shinkgshin@eecs.umich.edu

Xinyu Zhangxyzhang@eecs.umich.edu

Coexistence between ZigBee and WiFi

Spatial coexistence:

ZigBee (monitoring & control)

WiFi (Internet access)

20MHz

Frequency-domain coexistence (spectrum sharing):

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]

Why CSMA fails

Scheduling mode: ZigBee allows TDMA modeProblem: direct collision (no carrier sensing)

Transmit power: WiFi: 15dBm; ZigBee: < 0 dBm

Problem: asymmetric interference

Heterogeneity challenges coexistence:

WiFitransmissionrange

ZigBee transmissionrange

Asymmetric interferenceregion

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

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

CBT Overview

WiFi AP

WiFi client

ZigBee TX

signaler

WiFi TX

ZigBee TX

ZigBee signaler

DATA ACK

DATA ACKCCA, backoff

switching time

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

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?

Signaler “frequency flip”

Avoids signaler interfering with ZigBee data packet:

Busy tone

Transmitter sends data packet on some channelSignaler sends busy-tone on an adjacent channelReturn to the original channel after sending the busy-tone

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

Busy-tone scheduler: TDMA mode

CCA attempts frequency flip

Harbinger time

zzms JCKH

too large: busy-tone wastes channel timesH

too small: no idle slot can be sensed, busy-tone abortedsH

Key parameter: harbinger time

Analytical framework: relate to network performancesH

Busy-tone scheduler: CSMA mode

backoff CCA switching frequency flip

bT

too large: busy-tone wastes channel time

too small: data/ACK may not be protected

Key parameter: busy-tone duration

Analytical framework: relate to network performance

bT

bT

bT

bKEnlarge by extending busy-tone time by extra slotsbT

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

z wtz WS

z w

Transmit power and z w

Topology: or

(ZigBee transmitter within range of WiFi transmitter, or not)tt WZ tt WZ

Using legacy ZigBee or CBT

Performance analysis and parameter optimization

Performance metrics:

Normalized throughput: and

Approach:

Analyze collision probability under each parameter setting

Analyze throughput based on collision probability:

z w

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)

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)

tZ

tW

Collision probability of CBT:

Relate CCA failure rate to harbinger time (derived in Proposition 1 in paper)

sH

Relate collision probability to the CCA failure rate:

Derive collision probability as a function of , , , etc. sHzw

ZigBee TDMA mode with WiFi (cont’d)

Network performance:

Model transmission attempt of as a renewal reward process tZ

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 tt WZ

Prob.[no collision] data packet size

ZigBee CSMA mode with WiFi

Performance of legacy ZigBee:

Derive mean service time, based on a Markov chain model

txP : Transmission probability (after CCA)

dP : Data packet collision probability

aP : ACK packet collision probability

These depend on WiFi traffic intensity

: i-th backoff & CCA stageiBS

ZigBee CSMA mode with WiFi (cont’d)

Performance of CBT:

Also depends on key parameter: busy tone duration bT

bT

bT

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

Spatial collision probability

Probability that a packet cannot be decoded, given that temporal collision already occurs(Account for capture effect)

eI

Approximate ina random topology:

eI

Details in the paper

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

Temporal collision probability

Analysis matches simulation

CBT significantly reduces the collision rate for both data and ACK packets

Markers = simulation results; lines = analytical results

td

Spatial-temporal collision probability

Probability that ZigBee cannot decode collided packet

(accounting for capture effect and random node locations)

tt WZ tt WZ Out of interference range

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

Impact of harbinger time in TDMA mode

Larger larger more overhead, buthigher ZigBee throughput under high WiFi interference

mK sH

Under low duty-cycle ZigBee traffic (below 0.05), WiFi throughput is virtually unaffected by harbinger time

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

Testbed results: Collision probability (TDMA mode)

For randomly selected links:

CBT reduces collision rate by 60+% for most links

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:

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

Appendix

Normalized throughput

CSMA mode

Impact of busy-tone duration in CSMA mode

CSMA mode