Emerging Technologies in Emerging Technologies in Smart Device Smart Device

CommunicationsCommunications

Mary Ann IngramMary Ann IngramSchool of Electrical and Computer EngineeringSchool of Electrical and Computer Engineering

Georgia Institute of TechnologyGeorgia Institute of Technology

TR-50 MeetingTR-50 MeetingApril 12, 2010April 12, 2010

Overview

Objective Cooperative Transmission Energy Harvesting and Storage RFID and Sensors Wake-up Radios Conclusions How Georgia Tech can help

Objective

Consider some emerging technologies that would impact the standard, because they impact the Data Link Layer (DLL)

Identify some DLL issues for each technology

Cooperative Transmission (CT) Overview

Definition and SNR advantage Transmit time synchronization Range extension Application to the energy hole in

wireless sensor networks Application to broadcasts in dense

networks DLL issues

Definition of CT A protocol where multiple, neighboring

radio platforms cooperate in the physical layer to send a single message

I’ll help you if you will help me!

OK!

That way, if your channel is faded,

maybe mine will be better!

Working together, our signals can go

farther and we may save energy!

J. Laneman, D. Tse, and G. Wornell, “Cooperative diversity in wireless networks: Efficient protocols and outage behavior,” IEEE Trans. on Information Theory, vol. 50, no. 12, pp. 3062–3080, Dec. 2004.

CT Gives an SNR Advantage

Diversity gain– >13 dB in Rayleigh-fading channels

Array gain (e.g. 6 dB with 4 nodes)– When all nodes transmit with same power

Extra SNR can be used for range extension, TX power reduction, lower PER, higher data rate

cc NNDivADVANTAGESNR 10log10)(dB)in (

Concurrent CT (CCT) Cooperating nodes transmit at the same time

– A commonly received packet provides reference for synchronization

The alternative is time-division CT – Nodes transmit in non-overlapping time periods

CCT is better for range extension– Receivers get the benefit of diversity gain for

synchronization– Less SNR loss from imperfect RX synchronization

Concurrent CT (CCT) Transmit Time Synchronization

Non-coherent FSK

Median rms TX time spread for -5 dBm TX pwr – First hop

60 ns– Other hops

~125 ns Will see in the

demoChang and Ingram, "Convergence Property of Transmit Time Pre-Synchronization for Concurrent Cooperative Communication," submitted to Globecom 2010

CCT Range Extension – Dispersed Cluster

Light grey = 2-hop non-CT coverage Light + dark grey = 2-hop CT coverage

– 80% increase

Jung, Chang and Ingram, "Comparison of Two Cluster Topologies for Cooperative Transmission Range Extension in the 2.4GHz Band," submitted to Globecom 2010

CCT Range Extension – Tight Cluster

CT yields 280% area increase But total 2-hop dispersed CT area is 55% larger than tight

Jung, Chang and Ingram, "Comparison of Two Cluster Topologies for Cooperative Transmission Range Extension in the 2.4GHz Band," submitted to Globecom 2010

The Energy Hole Problem in The Energy Hole Problem in Battery-Driven WSNsBattery-Driven WSNs

The nodes near The nodes near the sink have to the sink have to relay the data relay the data from the rest of from the rest of the network, and the network, and die earlydie early

Sink

A single sink

Conventional Approaches to Conventional Approaches to Mitigate the Energy HoleMitigate the Energy Hole

Non-uniform distribution Non-uniform distribution [Wu08][Wu08]– Placing more nodes in the area close to Placing more nodes in the area close to

the sinkthe sink– The extra sensors can significantly raise The extra sensors can significantly raise

the costthe cost Using mobile sensors Using mobile sensors [Wang05][Wang05]

– May not be possible depending on the May not be possible depending on the environment and the hardwareenvironment and the hardware

[Wu08] X. Wu, G. Chen, and S. K. Das, “Avoiding energy holes in wireless sensor networks with nonuniform node distribution,” IEEE Trans. Parallel Distrib. Syst., vol. 19, no. 5, pp. 710–720, 2008.[Wang05] W. Wang, V. Srinivasan, and K.-C. Chua, “Using mobile relays to prolong the lifetime of wireless sensor networks,” in Proc. IEEE MobiCom, 2005.

Using CT to Extend Network LifeUsing CT to Extend Network Life Extend range with CT to Extend range with CT to jump overjump over

heavily-loaded nodesheavily-loaded nodes Simulation: Factor of 8X lifetime extension Simulation: Factor of 8X lifetime extension

Conventional (Non-CT) Routing Cooperative Routing

Non-CT FlowVirtual MISO Link

Jung and Ingram, "Residual-Energy-Activated Cooperative Transmission (REACT) to Avoid the Energy Hole," ICC CoCoNet Wkshp, 2010.

Opportunistic Large Array

A group of nodes that, without coordinating with each other, transmit the same message at approximately the same time in response to a signal received from another transmitter or OLA

Uses:– Fast, contention-free, reliable broadcasts– Complexity is independent of density for high-

density networks– Simple OLA-based unicasting is available

* A. Scaglione, and Y. W. Hong, IEEE Trans.Signal Processing, 2003.

OLA 1

Decoding Level 1 (DL1)

DL1

DL2

DL3

OLA 2

Decoding Level 2 (DL2)

Decoding Level 3 (DL3)

In more energy efficient versions, only subsets transmit

OLA Broadcasting

* A. Scaglione, and Y. W. Hong, IEEE Trans.Signal Processing, 2003.

Faster than multi-hop because no contention

Some DLL Issues CT is not supported by current protocols Cooperators have to be selected

– Can be done autonomously in sufficiently dense networks

CCT requires a commonly received packet to provide a synchronization reference– Need a field in the header to command the CCT

CCT node transmissions must not include their addresses– Received signal must appear to have come from a single

node through a multi-path channel Distributed ARQ required for OLA transmissions

(OLA has no cluster heads) For energy balancing, the set of all potential

cooperators must be informed of Sink Nav

Overview

Objective Cooperative Transmission Energy Harvesting and Storage RFID and Sensors Wake-up Radios Conclusions How Georgia Tech can help

Energy Harvesting and Storage

Success of embedded/pervasive devices depends on success of energy harvesting

Device technologies are developing fast– Harvester prototypes for almost every kind of

energy– Amounts of energy harvested differ by orders

of magnitude (solar highest; RF lowest)– Storage technologies are also developing fast

MAC/routing research is slowed because of inadequate system-level models for harvester and storage

M.A. Ingram et al., “Energy Harvesting Wireless Sensor Networks,” in Globalisation of Mobile and Wireless Communications: Today and in 2020, R. Prasad, ed., Springer, to appear Spring 2009.

Two Energy Storage Options- Opposites in Many Respects

Rechargeable battery (RB) – High energy density– Low peak power– Low leakage– Few hundred

recharge cycles– Constant voltage– Sensitive to depth of

discharge

19

Supercapacitor (SC)– Low energy density– High peak power– High leakage – Million recharge

cycles– Variable voltage– Not sensitive to

depth of discharge

Harvester Power Matching

Harvester has low efficiency if storage device is not impedance matched to source

Source impedance varies Optimal matching circuits have to

track the source, but can consume too much energy themselves

DLL Issues

Harvesting takes time- – Devices can be unavailable for minutes

to hours while harvesting Duty cycling becomes problematic

because of random node availability– Adaptive control theory proposed for

aperiodic energy sources

Overview

Objective Cooperative Transmission Energy Harvesting and Storage RFID and Sensors Wake-up Radios Conclusions How Georgia Tech can help

RF Tag Review Mature technology Optical bar code replacement Passive RF Tag

– No battery on board– Processor activated by a reader’s RF signal– Reader has to be close (few meters) – limited by

activation– Modulated backscatter for transmission

Eliminates power amplifier Semi-passive RF Tag

– Battery on board, runs the processor– Modulated backscatter for transmission– Read range longer- limited by mod backscatter

New: RF Tag + Sensor + Energy Harvesting

Two-tier network– Top tier: mesh network of readers

Access power mains– Bottom tier: low-power semi-passive sensors

Powered by harvested ambient energy Communicate with top tier by modulated backscatter

Dedicated source can provide RF for energy harvesting (PowerCast)– This source need not be a communicating node– Powers nodes where there is no ambient

energy Behind walls, low-light areas, above the ceiling

Clark et al., "Towards Autonomously-Powered CRFIDs," Workshop on Power Aware Computing and Systems (Hot-Power ’09), October 2009

A. Sample and J. R. Smith, "Experimental Results with two Wireless Power Transfer Systems," 2009.

DLL Issues

MAC needed for reader transmissions Multiple readers can collide trying to

read the same tag Reader TX power for modulated

backscatter is higher than traditional radio- larger interference range

Must provide time for power-up delay

Waldrop et al, “Colorwave: A MAC for RFID Reader Networks,” 2003

G. P. Joshi, S.W. Kim, “Survey, Nomenclature and Comparison of Reader Anti-collision Protocols in RFID,” IETE Tech Review [serial online’ 2008.

Overview

Objective Cooperative Transmission Energy Harvesting and Storage RFID and Sensors Wake-up Radios Conclusions How Georgia Tech can help

Wake-Up Radios A well known significant source of energy

drainage is radio idle listening Traditional power management uses duty

cycling– Nodes periodically wake up to check if they are

needed. Most of the time they are not needed. An ultra low-power radio can be used to

trigger a remote interrupt at the sleeping device when communication with the device is required

Enables more efficient utilization for event-based and on-demand applications

Van der Doorn et al, “A prototype low-cost wakeup radio for the 868 MHz band,” Int. J. Sensor Networks, Vol. 5, No. 1, 2009

Some Example Wakeup Radio Numbers

~20 W to 170 W consumption -75 dBm sensitivity @ 915 MHz 0.5 m to 2 m range from 0 dBm

transmitter

Le-Huy and Roy, “Low-Power Wake-Up Radio for Wireless Sensor Networks,” Mobile Networks and Applications, April 2010.

Van der Doorn et al, “A prototype low-cost wakeup radio for the 868 MHz band,” Int. J. Sensor Networks, Vol. 5, No. 1, 2009

Comparison to RFID

Wake-up radio operates like passive RFID

Wake-up radio is less complex than RFID and requires less power to be energized

Comparison to “Wake-On Radio”

In wake-on radio, the radio wakes up periodically to listen for incoming packets without microcontroller interaction

Wake-up radio does not wake up periodically

Lu et al, “A Wake-On Sensor Network,” Sensys, 2009

DLL Issues

How often should wake up signals be sent?

Wake up signal wakes all nodes in the neighborhood

Collisions can happen between nodes sending wake up signals

Conclusions

CT offers many advantages for networks of highly energy-constrained radios; CT is not supported by current protocols

Energy harvesting and storage makes duty-cycling difficult – good models lacking

Wake-up radios can make duty cycling much easier; current technology is short-range

RFID with energy harvesting is sustainable, but needs reader

All these technologies would significantly impact the standard

How Georgia Tech Can Help With Standards Development

Objective technology assessment Determine protocol changes to

support specific technologies Prototype development and

prototype testing Channel and device modeling Certification test design

Thank You!