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					           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
 SIGNAL




                  BEAMFORMER
                                BEAM    SIGNAL                                      SIGNAL
                               SELECT   OUTPUT                                      OUTPUT


                                                 INTERFERENCE


                                                                       BEAMFORMER
                                                        INTERFERENCE
                                                                         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                    7
MIMO
              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 multi-
     band/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
                 • One-atom-thick graphite rolled up into cylinder
           R//
   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   1.5 mm array   Scanning     One MWCNT
silicon                   electron     antenna – 24 nm
                          microscope   outer, 10 nm
                          image        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         top view
          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
                                                             150 mm                            • Radiation efficiency: 83%
56 mm




                                 Return loss                                                                Gain
               0                                                                       10

               -2

               -4                                                                       5




                                                                  Realized gain (dB)
               -6
   S11 (dB)




                                                                                        0
               -8
   dB




                                                                         dB
              -10
                                                                                        -5
              -12
                                                                                                            Measured CNTs patch
              -14                                                                      -10                  Simulated PEC patch
              -16                                                                                           Simulated CNTs patch

              -18                                                                      -15
                1.5          2                     2.5   3                               1.5        2                     2.5      3
                                 Frequency (GHz)                                                        Frequency (GHz)                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 non-
aligned SWCNT sheets).


                                                             35

				
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