case study 4: wlans and aperture antennas · case study 4: wlans and aperture antennas increasing...

1 ECE422 / Prof. S. V. Hum

Case Study 4: WLANs and Aperture Antennas

Increasing the range of your WLAN

Agenda

•  Background: WLAN standards (WiFi) •  EIRP limitations •  Circular waveguide theory •  Design of a open-end circular waveguide

aperture •  “Mod”-ing an 802.11b/g access point •  Does it work?


WiFi WLAN Background •  Governed by the 802.11

standard established by the Institute of Electrical and Electronics Engineers (IEEE)

•  Standard describes over-the-air (OTA) modulation techniques for wireless LANs –  Operating frequency –  Protocol


Capacity

•  Higher throughputs possible through data compression and source coding

•  View of legacy channel assignments at 2.4 GHz (courtesy Wikipedia)


Older 802.11x Protocols Draft Release

date Frequency Throughput Protocol Indoor

Range 802.11 (obsolete)

1997 2.4 GHz 1-2 Mbit/s FHSS, DSSS

?

802.11a Oct. 1999 5 GHz 27 Mbit/s OFDM 35 m 802.11b Oct. 1999 2.4 GHz 5 Mbit/s FHSS,

DSSS 35 m

802.11g June 2003 2.4 GHz 22 Mbit/s OFDM 38 m 802.11n Oct. 2009 2.4/5 GHz ≤ 600 Mbit/s * 70 m


•  *802.11n uses multiple-input multiple-output (MIMO) antenna technology to build on throughput of earlier standards, as well at 40 MHz channels

802.11ac

•  Mandatory 80 MHz channels, extendable to 160 MHz

•  8 spatial MIMO streams (vs. 4 in 802.11n) •  Downlink multi-user MIMO (MU-MIMO),

for handling more users •  Higher spectral efficiency: 256 QAM (vs 64

QAM in 802.11n) •  Single-user capacity up to 1.7 Gbps


Other Features •  Many older access points

use two antennas for “diversity” combining of radio signals –  For combatting fading

•  Newest generation equipment uses multiple antennas for MIMO / MU-MIMO –  Each TX/RX pair constitute

an “antenna stream” that is uncorrelated from other streams


Discussion on Performance

•  Operation at 5 GHz gives (gave) higher throughput due to higher carrier frequency

•  All things equal, the fractional bandwidth leads to higher channel bandwidths at higher carrier frequencies

•  802.11a was better for throughput reasons –  2.4 GHz ISM band also heavily congested

•  802.11b was vastly more popular due to lower hardware costs, and comparable throughputs with 802.11g extension


Range Limitations •  Many objects attenuate

signals more at higher frequencies, making 2.4 GHz more preferable for indoor applications than 5 GHz

•  Regardless, regulatory bodies restrict the allowable EIRP from 802.11 devices which limits the ultimate range of WLANs


Range Limitations for Mobile LANs

•  Limitations to control co-channel interference •  If transmit antenna gain < 6 dBi

–  Maximum EIRP is 1 W (+30 dBm) –  Example: 100 mW into 3 dBi antenna = 200 mW

(compliant) –  Most “stock” WLAN antennas fall into this category

•  If transmit antenna gain > 6 dBi –  1 dB reduction in transmit power required for each

additional 3 dB of antenna gain past 6 dBi –  Thus, small increases in TX power are permissible


Custom Antennas for WLANs

•  Practically we are limited in the EIRP that can be produced –  Hardware (power amplifier) limitations –  Physical size of aperture that would be needed

•  Let’s look at a simple “homebrew” medium-gain aperture antenna

•  We need: –  Aperture “current” distribution, so that we can find far

field pattern –  Input impedance, for matching purposes


Can-antennas (cantennas) •  Simple design proposed

by amateurs for increasing WLAN range

•  Based on a circular aperture antenna

•  Essentially is an open-ended circular waveguide: how do we analyze that?


Circular Waveguide Theory •  Consider the equation

•  Expanding the Laplacian,

•  Solution can be expressed as a product (using separation of variables)


h2

Circular Waveguide Solutions

where Jn is a Bessel function of the first kind of nth order


•  TM and TE modes are supported in circular waveguides

•  We can have transverse field variation in the radial [R(r)] and the axial directions [Φ(f)]

•  Mode designation: TMmn and TEmn, where m is the # of half-wave field variations in φ-direction, n is the # of half-wave field variations in the r-direction

TE11 Mode (dominant mode) •  We are concerned with

the TE mode for antenna purposes

•  Boundary conditions and Maxwell’s equations to find the remaining field components

•  The transverse field distribution is important for computing the far-field


TE11 Mode (dominant mode) •  The electric and magnetic

fields across the aperture compose “equivalent currents” in a continuous 2D array

•  The Fourier transform of these equivalent currents gives the far-field pattern


Making a Cantenna 1.  Get a large can of

appropriate diameter so that we are above the cutoff frequency

2.  Consume the contents 3.  Remove one end of the

can to form the aperture

a = 53 mm length = 180 mm


“Back of the envelope” directivity calculation: 4π/λ2 * Aap = 7.10 = 8.51 dBi

fc11 = 1.658 GHz

Making a Cantenna 4.  Insert a coaxially-fed

probe a short distance away from the shorted end of the waveguide

•  The location and length of the feed probe are optimized to maximize coupling into the TE11 mode

–  Optimized numerically in a CAD package

–  Fine-tuned experimentally


Impedance Match to 50Ω


Computed Radiation Pattern •  Highly directive beam in

E-plane and H-plane •  Directivity: 8.55 dBi at 2.4

GHz •  Exceeds the 6 dBi gain

limit imposed on EIRP calculations – need to scale back TX power by 1 dB

•  Since we have no control over that, we are breaking the rules


“Back of the envelope” directivity calculation: 8.51 dBi – pretty close!

Modifying the Access Point •  Starting point: Linksys

WUSB854G •  Stock antenna: integrated λ/2 dipole

•  What kind of range improvement does the cantenna provide?

•  Need to connect antenna port on device to SMA connector on cantenna


Replacing Stock Antenna with UFL “pigtail”

Assembly hex screw Exposed innards


•  Device not generally user serviceable!

Replacing Stock Antenna with UFL “pigtail”

Front side w/antenna Replacement UFL pigtail


Observations •  Significantly more access

points are visible with new antenna

•  RSSI of existing stations increases by about 6 dB

•  Theoretical improvement should yield a minimum of double the range as before


Conclusions and Extensions •  Aperture antennas can be

easily fabricated and used to “improve WLAN link budgets” using household items

•  Aperture / waveguide theory helps guide the design

•  Many more access points can be picked up by the cantenna compared to a standard integrated laptop antenna

•  Existing stations can be picked up over longer distances

•  Extensions: larger antenna (e.g. “woktenna”)


Acknowledgement

•  Krishna Kumar Kishor, for simulating the cantenna design in SEMCAD, and for testing the final antenna


case study 4: wlans and aperture antennas · case study 4: wlans and aperture antennas increasing...

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