1 Carbon Nanotube Antennas for Wireless Communications Jack Winters Jack Winters Communications, LLC [email protected] NJ Coast.

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Carbon Nanotube Antennas for Wireless Communications

Jack WintersJack Winters Communications, LLC

[email protected]

www.jackwinters.com

NJ Coast Section MeetingSponsored 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

3

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 (AT&T 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 AT&T)

(From IEEE Comm. Mag. 1/10)

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Wireless System Evolution

WLAN:

802.11n:

>100 Mbps in MAC

>3 bits/sec/Hz

802.11ac (< 6GHz) and 802.11ad (60 GHZ)

>500 Mbps link throughput

>1 Gbps multiuser access point throughput

>7.5 bits/sec/Hz

(Network throughput is not addressed)

RFID:Active and passive tagsRead 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

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

SIGNAL OUTPUT

SIGNAL

INTERFERENCE

INTERFERENCEBEAMFORMER

WEIGHTS

SIGNAL OUTPUT

BEAM SELECT

SIGNAL

BE

AM

FOR

ME

R

Adaptive Antenna ArraySwitched Multibeam Antenna

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Key to Higher Data Rates:Multiple-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 (<6 GHz)

– 8X4 or 16X2 MIMO => multiple access point/terminal antennas

– 80-100 MHz bandwidth => 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 can’t do at 700 MHz)

Polarization: Dual polarization (doubles number of antennas in one location

Pattern: Allows even closer than ¼ wavelength

=> 16 or more on a handset

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

Multiplatform Devices with Smart Antennas

Antenna Location

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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

=> 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 Analog–Digital 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

1) 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 15mm4 dBi

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Closely-Spaced Antennas - Solutions

1) 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/Piazza_ElecLtr06.pdf

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Closely-Spaced Antennas (cont.)

3) SuperconductivityCan “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=> Very low antenna efficiency – but have pattern diversity=> Much stronger than steel for given weight Can be integrated with graphene circuitry for adaptive arrays

L

R//

D

• 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]

http://en.wikipedia.org/wiki/File:Kohlenstoffnanoroehre_Animation.gif

• Length to width of 108

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

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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 electronicshttp://www.ou.edu/engineering/nanotube/

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Arrays

Antenna Weights

Graphene electronics: • 2 orders of magnitude higher electron mobility than silicon• >30 GHz transistors demonstrated

http://arstechnica.com/science/2010/02/graphene-fets-promise-100-ghz-operation.ars

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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

• Non-aligned CNT sheet [3]

• Sheet resistivity: ~ 20 /

High conductivity and flexibility([2] Zhou, Bayram, Volakis, APS2009)

• CNT length: ~200 μm

• CNT spacing distance: ~ 100 nm

• CNT tips are entangled (touching), giving rise to high conductivity

cross section view

top view

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Polymer-CNT Patch Antenna Performance [2]

1.5 2 2.5 3-15

-10

-5

0

5

10

Frequency (GHz)

dB

Measured CNTs patch

Simulated PEC patch

Simulated CNTs patch

Rea

lize

d ga

in (

dB)

1.5 2 2.5 3-15

-10

-5

0

5

10

Frequency (GHz)

dB

Measured CNTs patch

Simulated PEC patch

Simulated CNTs patch

Measured CNTs patch

Simulated PEC patch

Simulated CNTs patch

Rea

lize

d ga

in (

dB)

1.5 2 2.5 3-18

-16

-14

-12

-10

-8

-6

-4

-2

0

dB

Frequency (GHz)

S11

(dB

)

1.5 2 2.5 3-18

-16

-14

-12

-10

-8

-6

-4

-2

0

dB

Frequency (GHz)

S11

(dB

)

31 mm

56 mm

8 mm

MCT-PDMS substrate, 5 mm

CNTs sheet

150 mm

Return loss Gain

• CNT patch: 0.9 Ohm/square

• Patch antenna: 5.6 dB gain (compared to 6.4 dB of PEC patch)

• Radiation efficiency: 83%

• CNT patch: 0.9 Ohm/square

• Patch antenna: 5.6 dB gain (compared to 6.4 dB of PEC patch)

• Radiation efficiency: 83%

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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).