Transcript Emerging technologies in Smart Device communications

Emerging Technologies in
Smart Device
Communications
Mary Ann Ingram
School of Electrical and Computer Engineering
Georgia Institute of Technology
TR-50 Meeting
April 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!
That way, if your
channel is faded,
maybe mine will be
better!
OK!
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
SNR ADVANTAGE( in dB)  Div( N c )  10 log10 N c

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
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
demo
Chang 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
Battery-Driven WSNs
 The
nodes near
the sink have to
relay the data
from the rest of
the network, and
die early
A single sink
Sink
Conventional Approaches to
Mitigate the Energy Hole
 Non-uniform
distribution [Wu08]
– Placing more nodes in the area close to
the sink
– The extra sensors can significantly raise
the cost
 Using
mobile sensors [Wang05]
– May not be possible depending on the
environment 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 Life
Extend range with CT to jump over
heavily-loaded nodes
Non-CT
Flow lifetime extension
 Simulation: Factor
of 8X

Virtual MISO Link
Conventional (Non-CT) Routing
Cooperative Routing
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 highdensity networks
– Simple OLA-based unicasting is available
* A. Scaglione, and Y. W. Hong, IEEE Trans.Signal Processing, 2003.
OLA Broadcasting
Decoding Level 1
(DL1)
OLA 1
Decoding Level 2
(DL2)
DL3
DL2
DL1
OLA 2
Decoding Level 3 (DL3)
In more energy efficient versions, only subsets
transmit
Faster than multi-hop because no contention
* A. Scaglione, and Y. W. Hong, IEEE Trans.Signal Processing, 2003.
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
 Supercapacitor
(SC)
–
–
–
–
Low energy density
High peak power
High leakage
Million recharge
cycles
– Variable voltage
– Not sensitive to
depth of discharge
19
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
mW to 170 mW 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 dutycycling 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!