Carbon Nanotube Antennas for Wireless Communications

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Carbon Nanotube Antennas for Wireless
Communications
Jack Winters Jack Winters Communications,
LLC jack_at_jackwinters.com www.jackwinters.com
NJ Coast Section Meeting Sponsored by the
ElectroMagnetic Compatibility/Vehicular
Technology/Antennas Propagation Chapter March
18, 2010
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Outline

• Overview of Wireless Trends
• Carbon Nanotube Antennas
• Applications to Wireless Communications
• Conclusions

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Overview

• Goal Wireless communications, anywhere, in any
  form
• Means Standard-based heterogeneous networks,
  since no one wireless network is best in all
  cases
• Centralized networks cellular/LTE, WiMax
• Decentralized systems WLANs, Bluetooth, sensor
  networks RFID
• Multi-mode terminals
• Small, ubiquitous devices (RFID, smart dust)

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Wireless System Evolution

• Cellular
• 2G GPRS 56-114 kbps
• 2.5G - EDGE up to 400 kbps (Evolved EDGE 1
  Mbps)
• 3G
• HSPA 7.2 Mbps (ATT completed 2009)
• HSPA 21/42 Mbps
• LTE/WiMAX/IMT-Advanced 100 Mbps and higher
• LTE 50 Mbps UL, 100 Mbps DL (deployment in 2012
  by ATT)

(From IEEE Comm. Mag. 1/10)
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Wireless System Evolution

• WLAN
• 802.11n
• gt100 Mbps in MAC
• gt3 bits/sec/Hz
• 802.11ac (lt 6GHz) and 802.11ad (60 GHZ)
• gt500 Mbps link throughput
• gt1 Gbps multiuser access point throughput
• gt7.5 bits/sec/Hz
• (Network throughput is not addressed)

• RFID
• Active and passive tags
• Read ranges with omni-directional antennas
• Active tags (433 MHz) - 300 feet
• Passive tags (900 MHz) - 9 feet

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Techniques for Higher Performance

• Smart Antennas (keeping within standards)
• Range increase
• Interference suppression
• Capacity increase
• Data rate increase using multiple
  transmit/receive antennas (MIMO)
• Radio resource management techniques
• Dynamic channel/packet assignment
• Adaptive modulation/coding/platform (software
  defined radio)
• Cognitive radio (wideband sensing)

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Smart Antennas
Adaptive Antenna Array
Switched Multibeam Antenna
SIGNAL OUTPUT
Smart antenna is a multibeam or adaptive antenna
array that tracks the wireless environment to
significantly improve the performance of wireless
systems. Switched Multibeam versus Adaptive Array
Antenna Simple beam tracking, but limited
interference suppression and diversity gain,
particularly in multipath environments Adaptive
arrays are generally needed for devices and when
used for MIMO
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Key to Higher Data RatesMultiple-Input
Multiple-Output (MIMO) Radio

• With M transmit and M receive antennas, can
  provide M independent channels, to increase data
  rate M-fold with no increase in total transmit
  power (with sufficient multipath) only an
  increase in DSP. Peak link throughput increase
• Indoors up to 150-fold in theory
• Outdoors 8-12-fold typical

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MIMO

• LTE/WiMAX/802.11n 2X2, 4X2, 4X4 MIMO
• 802.11ad (60 GHz)
• 10 to 100 antennas
• Phased array
• On chip
• 802.11ac (lt6 GHz)
• 8X4 or 16X2 MIMO gt multiple access
  point/terminal antennas
• 80-100 MHz bandwidth gt cognitive radio (large
  networks)

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RFID Adaptive Arrays for Readers and Tags

• Active and passive tags
• Read ranges with omni-directional antennas
• Active tags (433 MHz) - 300 feet
• Passive tags (900 MHz) - 9 feet
• Reader can use scanning beam to transmit,
  adaptive array to receive
• Tag can use adaptive array to receive, then use
  same weights to transmit

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Issues

• Large arrays at access point/base
  station/terminal
• Diversity (for MIMO) in small size
• 700 MHz
• Low cost/power signal processing
• 802.11n up to 4 on card/computer, but only 1 or
  2 at handset
• Multiplatform (MIMO) terminals, and the need for
  multi-band/conformal/embedded antennas, increase
  the problem
• Cognitive radio cross-layer with
• MIMO
• Wide bandwidth

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Adaptive Arrays for RFID Tags

• Tags can be very small devices (single chip),
  making multiple antenna placement an issue
• At 900 MHz, half-wavelength spacing is 6 inches.

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Diversity Types
Spatial Separation only ¼ wavelength needed at
terminal (but cant do at 700 MHz) Polarization
Dual polarization (doubles number of antennas in
one location Pattern Allows even closer than ¼
wavelength gt 16 or more on a handset
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Multiplatform Devices with Smart Antennas

• Most systems consider only 2 antennas on devices
  (4 antennas in future) because of costly A/Ds and
  size of antennas.

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Signal Processing Analog/Switching (RF) or
Digital

• Analog Advantages
• Digital requires M complete RF chains, including
  M A/D and D/A's, versus 1 A/D and D/A for analog,
  plus substantial digital signal processing
• The cost is much lower than digital (see, e.g.,
  R. Eickhoff, et al, Developing Energy-Efficient
  MIMO Radios, IEEE VT magazine, March 2009)
• Switched antennas have even lower cost
• Digital Advantages
• Slightly higher gain in Rayleigh fading (as more
  accurate weights can be generated)
• Temporal processing can be added to each antenna
  branch much easier than with analog, for higher
  gain with delay spread
• Needed for spatial processing with MIMO
• gt Use RF combining where possible, minimizing
  digital combining (limit to number of spatial
  streams)

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Combination of Switching, RF, and Digital
Combining (Hybrid)
Capacity and Complexity Trade-offs in MIMO
AnalogDigital Combining Systems, Xin Zhou, Jack
Winters, Patrick Eggers, and Persefoni Kyritsi,
Wireless Personal Communications, July 24, 2009.
RF combining in addition to digital combining
provides added gain for higher data rates over
larger area with reduced cost
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Closely-Spaced Antennas - Solutions

• Metamaterials
• Closer spacing with low mutual coupling but good
  diversity (pattern) and smaller size with
  directivity (active antennas)
• Ex Rayspan MetarrayTM
• 1/6 wavelength spacing
• 1/10 wavelength antenna length
• http//www.rayspan.com/pdfs/Metarray_n_data_sheet_
  032607.pdf
• Netgear has implemented metamaterial antennas in
  their WLANs

40 x 15mm 4 dBi
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Closely-Spaced Antennas - Solutions

• Metamaterials (cont.)
• 1/50th of a wavelength demonstrated
  (http//www.physorg.com/news183753164.html)

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Closely-Spaced Antennas (cont.)
2) Active antennas Use of MEMs with
metamaterial antennas and carbon nanotube
antennas on graphene substrates Frequency
agility, reducing the number of
antennas Bandwidth/polarization/beampattern
adaptation Low cost, small size/form factor
solution
http//wireless.ece.drexel.edu/publications/pdfs/P
iazza_ElecLtr06.pdf
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Closely-Spaced Antennas (cont.)
3) Superconductivity Can pull transmitted
power to receiver (requires large currents)
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4) Carbon Nanotube Antennas

• Basic features
• Wave velocity is 1 of free space
• 1.7 mm (vs. 17 cm) half-wavelength spacing at
  900 MHz
• 10,000 antennas in same area (106 antennas in
  same volume) as standard antenna
• gt Very low antenna efficiency but have pattern
  diversity
• gt Much stronger than steel for given weight
• Can be integrated with graphene circuitry for
  adaptive arrays

• One-atom-thick graphite rolled up into cylinder

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Carbon Forms (1 D. Mast Antenna Systems
Conference 2009)
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Carbon Nano-Forms 1
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SWCNT 1

• Length to width of 108
• Current density gt metal (3 orders of magnitude
  greater than copper)
• Strength gt Steel (2 orders of magnitude stronger
  by weight)
• Thermal Conductivity gt Diamond (1 order of
  magnitude greater than copper)

http//en.wikipedia.org/wiki/FileKohlenstoffnanor
oehre_Animation.gif
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SWCNT Issues 1

• Small diameter (usually no larger than 2 nm)
• Short length (usually less than 100 microns)
• 1/3 metallic and 2/3 semiconductor (without
  control of which kind)
• Full scale, low cost production
• Electrical contact to electronics (graphene
  electronics)

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Structure of SWCNTs 1
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Implementation
SWCNT pillars connect with array
electronics http//www.ou.edu/engineering/nanotube
/
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Arrays

• Graphene electronics
• 2 orders of magnitude higher electron mobility
  than silicon
• gt30 GHz transistors demonstrated
• http//arstechnica.com/science/2010/02/graphene-f
  ets-promise-100-ghz-operation.ars

Antenna Weights
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SWCNT Radio 1
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Multi-Walled Carbon Nanotubes 1
Array on silicon
1.5 mm array
Scanning electron microscope image
One MWCNT antenna 24 nm outer, 10 nm inner
diameter (transmission electron microscope image)
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Multi-Walled Carbon Nanotubes Threads 1

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MWCNT Thread in Radio 1
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Non-Aligned Carbon Nanotube Antennas
High conductivity and flexibility (2 Zhou,
Bayram, Volakis, APS2009)

• Non-aligned CNT sheet 3

• Sheet resistivity

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Polymer-CNT Patch Antenna Performance 2

• CNT patch 0.9 Ohm/square
• Patch antenna 5.6 dB gain (compared to 6.4 dB
  of PEC patch)
• Radiation efficiency 83

Return loss
Gain
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Summary and Conclusions

• Communication systems increasingly need
  electrically small, active antennas
  multiplatform devices with MIMO, small RFIDs
• Carbon nanotube antennas have unique properties
  including strength, current density, wave
  velocity, and thermal conductivity.
• They can be connected directly to graphene
  electronics (with high electron mobility) for
  dense adaptive arrays of SWCNT.
• Many issues to be resolved, but substantial
  innovation opportunity (examples including MWCNT
  threads and non-aligned SWCNT sheets).