Carbon Nanotube Antennas for Wireless
Communications
Jack Winters
Jack Winters Communications, LLC
jack@jackwinters.com
www.jackwinters.com
NJ Coast Section Meeting
Sponsored by the ElectroMagnetic Compatibility/Vehicular Technology/Antennas & Propagation Chapter
March 18, 2010
1
Outline
• Overview of Wireless Trends
• Carbon Nanotube Antennas
• Applications to Wireless Communications
• Conclusions
2
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)
3
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)
4
(From IEEE Comm. Mag. 1/10)
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 tags
Read ranges with omni-directional antennas:
Active tags (433 MHz) - 300 feet
Passive tags (900 MHz) - 9 feet
5
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)
6
Smart Antennas
Switched Multibeam Antenna
Adaptive Antenna Array
SIGNAL
BEAMFORMER
SIGNAL
BEAM
SELECT
SIGNAL
OUTPUT
SIGNAL
OUTPUT
INTERFERENCE
INTERFERENCE
BEAMFORMER
WEIGHTS
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
7
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
8
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)
9
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
10
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 multiband/conformal/embedded antennas, increase the problem
• Cognitive radio – cross-layer with
– MIMO
– Wide bandwidth
11
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.
12
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
13
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.
Antenna
Location
14
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)
15
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
16
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
40 x 15mm
4 dBi
http://www.rayspan.com/pdfs/Metarray_n_data_sheet_032607.pdf
Netgear has implemented metamaterial antennas in their WLANs
17
Closely-Spaced Antennas - Solutions
1) Metamaterials (cont.):
-
1/50th of a wavelength demonstrated
(http://www.physorg.com/news183753164.html):
18
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
19
Closely-Spaced Antennas (cont.)
3) Superconductivity
Can “pull” transmitted power to receiver (requires
large currents)
20
4) Carbon Nanotube Antennas
Basic features
R//
• One-atom-thick graphite rolled up into cylinder
L
D
• 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
21
Carbon Forms
([1] D. Mast – Antenna Systems Conference 2009)
22
Carbon Nano-Forms [1]
23
SWCNT [1]
• 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)
http://en.wikipedia.org/wiki/File:Kohlenstoffnanoroehre_Animation.gif
24
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)
25
Structure of SWCNTs [1]
26
Implementation
SWCNT pillars – connect with array electronics
http://www.ou.edu/engineering/nanotube/
27
Arrays
Graphene electronics:
• 2 orders of magnitude higher electron
mobility than silicon
• >30 GHz transistors demonstrated
http://arstechnica.com/science/2010/02/graphe
ne-fets-promise-100-ghz-operation.ars
Antenna Weights
28
SWCNT Radio [1]
29
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)
30
Multi-Walled Carbon Nanotubes – Threads [1]
31
MWCNT Thread in Radio [1]
32
Non-Aligned Carbon Nanotube Antennas
High conductivity and flexibility
([2] Zhou, Bayram, Volakis, APS2009)
• Non-aligned CNT sheet [3]
• Sheet resistivity: ~ 20 /
cross section
view
top view
• CNT length: ~200 μm
• CNT spacing distance: ~ 100 nm
• CNT tips are entangled (touching),
giving rise to high conductivity
33
Polymer-CNT Patch Antenna Performance [2]
MCT-PDMS
substrate, 5 mm
31 mm
• CNT patch: 0.9 Ohm/square
CNTs sheet
• Patch antenna: 5.6 dB gain
(compared to 6.4 dB of PEC patch)
8 mm
• Radiation efficiency: 83%
150 mm
56 mm
Return loss
Gain
10
0
-2
5
dBgain (dB)
Realized
dB (dB)
S11
-4
-6
-8
-10
-12
0
-5
Measured CNTs patch
-14
-10
Simulated PEC patch
-16
-18
1.5
Simulated CNTs patch
2
2.5
Frequency (GHz)
3
-15
1.5
2
2.5
Frequency (GHz)
3
34
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 nonaligned SWCNT sheets).
35