network and systems laboratory nslab.ee.ntu.edu.tw branislav kusy, christian richter, wen hu,...

Network and Systems Laboratorynslab.ee.ntu.edu.tw

Radio Diversity for Reliable Communication in WSNs

Branislav Kusy, Christian Richter, Wen Hu, Mikhail Afanasyev, Raja Jurdak, Michael Brunig, David Abbott, Cong Huynh, and Diethelm Ostry

CSIRO ICT Centre, Australia

Archiang2011/06/13


OutlineIntroductionTheoretical Basis of DiversityHardware PlatformImplementationEvaluationConclusions


IntroductionLow power wireless mesh is the most common

communication architecture for WSNs today.State-of–the-art data collection protocols react quickly to

changes in network connectivity and repair their routing state.Network congestion and increased packet collisions

Link failures become more expensive to repair with increasing network size as the routing state may need to be propagated network-wide.


IssuesThis paper improves primary radio link reliability to

construct large reliable and efficient WSNs spanning regions.

Selection of node locations to achieve a robust wireless mesh is a challenging problem.Cost increased by requiring additional hardware or time-

consuming iterative deployment adjustment procedures.


Proposed ArchitectureA new network architecture is proposed based on this

radio communication diversity.A new platform with two independent radios operating at well-

separated frequencies and spatially separated antennas.2 IEEE 802.15.4 radio chips, operating in the 900MHz and

2400MHz bands

DisadvantageLower energy efficiency (33% more energy)Loss of compatibility with existing hardwareAdditional components cost


Signal LossMulti-path propagation

Spatially Selective fading The far scattering model The near scattering model

Frequency selective fadingBounds on spatial and frequency fading

Obstruction and environmental fadingThe line of sight propagation path between nodes is obstructed.Radius of Fresnel zone

Interference2400MHz: WiFi networks.900MHz: Telemetry networks and cordless telephones.


The far scattering model The near scattering model


Frequency selective fadingFrequency selective fading is caused by differences in the times

of arrival of multiple radio signals traveling along different path.

Bounds on spatial

and frequency fading


Diversity DesignFor multi-path

In 802.15.4(2MHz BW) or WiFi(20MHz BW) An antenna separation: 1m Frequency separation: 100MHz

For interferenceFrequency diversity is robust against interference if the sources

of interference are uncorrelated at the multiple frequencies.


Hardware PlatformEach board was configured to use a separate antenna

and thus has its own RF connector.Antennas provide a 5dBi gain on 2400 MHz and 2.2dBi

gain on 900 MHz.


ImplementationDual-band radio driver

Separate the two radio stacks Execute the two drivers in parallel

Link estimation layer 4 bit link estimator(4BLE)

Data collection layer CTP-multi, the extension of CTP to

multiple radio bands

Low power listening Time of idle listening is double Energy consumption increases 3%

- 33%


Characterization of DiversityOutdoor open space experiment

Line-of-sight deployment, and no in-band interference 900m transmission range, 10 packets per second,

and 250kbps data rate Improve PRR of these links by as much as 84%

Indoor office space experiment Evaluate the benefits of frequency diversity Verify equal RSSI in both bands 100 locations 10 packets per second


Channel diversity Channel diversity is insufficient in a number of scenarios Channel hopping provides limited benefits

when multipath fading affects the radio signals

and is far less useful in avoiding external

interference.


Data Collection Performance30 nodes in 400m X 450mMax distance between

neighbor nodes is 80m90 bytes per packet, 4kbps5 seconds inter-packet interval


Comparison of dual to single

band networks


Evaluation of network diversity


Energy Overhead of Multiple BandsLow power listeningCurrent

Sleep: 300uAActive: 14.1mA (RF212)

19.1mA (RF230)

CTP uses unicast transmissions to deliver data packets to the base station and control traffic is a small fraction of the data traffic.

Energy overhead


ConclusionsThis work improves reliability of primary radio links by

radio diversity.Multiple antennas are mounted on each node to gain

additional resilience against multipath fading.Using low power listening, energy overhead reduces to

less than 33%.The future work includes improvement of the energy

consumption of the diversity radio stack. Turning off the radios can decrease the overhead of idle listening.

network and systems laboratory nslab.ee.ntu.edu.tw branislav kusy, christian richter, wen hu,...

Documents

network congestion

network connectivity

scattering model network

frequency fading network

increasing network size

propagated networkwide

radio communication

powerpoint radio diversity