©2002 skycross. all rights reserved. antenna design for wireless products kerry greer vice...
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©2002 SkyCross. All Rights Reserved.
Antenna Design For Wireless Products
Kerry GreerVice President of Engineering
SkyCross Inc.February 25, 2002
Page 2leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 2
Topics
• Antenna Evolution
• Changing Wireless Marketplace
• Issues Driving Antenna Design
• Antenna Physics
• Antenna Performance Specifications
• External Antennas
• Embedded Antennas
• Advanced Wireless Device Antenna Concepts
Page 3leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 3
Antenna Evolution
• Antennas Have Always Been the Part That Makes a Wireless Device Wireless
• Have Traditionally Been External, Connectorized Components- Misunderstood, considered “black magic”
- Gangly, obtrusive
- Added on at the end of the design
• Antennas for Mobile Devices Have Evolved Since Their Introduction- Whips Retractables Stubbies Embedded
Page 4leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 4
Changing Market
• Antennas are Slowly Becoming More and More Integral, as More and More Devices are Adding Wireless Capability
• OEMs/Consumers Are Demanding More from Their Wireless Devices- Smaller
- More functionality
- More power
- Improved performance
• FCC Mandate That All Cell Phones Have E-911 Capability
Page 5leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 5
Wired/Wireless Networks of Today
Mobile DataDevices
PC / Server
BroadbandInternetServicePCS
MobileNetwork
Ethernet
Cable
GPS
DigitalCamera
DVD/TV/VCR
Printer
Real-TimeVideo
PCSDevice
Auto
DSL
USB
RCA
Mobile Devices
Wired Devices
Page 6leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 6
Future of the Wired/Wireless World
Mobile DataDevices
PC / Server
BroadbandDigital Data(Fiber/FWA)
BTWPAN
3GWWN
802.11WLAN
OC3WAN
GPS
DigitalCamera
DVD / HDTV
PrinterReal-Time
Video
Auto
ConsumerPOS
SODA
3G MobileDevices
Black: Multi-ModeOrange: Single Mode
Wireless Legend
Page 7leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 7
Issues Facing Antenna Design
• Traditional Cost vs. Performance Tradeoffs
• Three Different Groups Have Three Different Sets of Priorities (OEMs Service Providers Consumers)
• Antenna Considerations (OEM Priority)1) Cost
2) Size
3) Performance
4) Multiple operation modes
Page 8leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 8
How Much is Better Performance Worth?
Churn in the US exceeds 200% of new subscribers
83,924
103,418
123,740
144,804
166,158
187,679
25,17734,127
43,30953,577
64,80176,948
0
20000
40000
60000
80000
100000
120000
140000
160000
180000
200000
1999 2000 2001 2002 2003 2004
Churning Total Subscribers
Cost to sign-up new customer $350 - $400
43% of Subscribers Change Carrier Because of Coverage Problems*
Source: Cahners in-stat
Poor Coverage is worth: (43,309)*($350)*(0.43) = $6.5M / yearPoor Coverage is worth: (43,309)*($350)*(0.43) = $6.5M / year
* Source: Yankee Group
20 million newsubscribers
Page 9leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 9
1. Cost
• Antennas are Viewed as Components, with Traditional Price Points:- External whip/monopole assemblies: $1 - $2
- Internal/embedded antennas: ~$0.50
• OEMs Very Reluctant to Consider Higher-priced Antennas for Existing Applications- Regardless of Performance Gains
• Alternative: Provide Multiple Functionality- Example: 3 antennas in 1 (Cell/PCS/ISM)
- $3 for 1 multi-band antenna is better overall choice than 3 separate antennas for $1 each
Page 10leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 10
2. Reduced Size
• With the Evolution of Wireless Devices the Antenna has Been Forced to Reduce in Size
• Some Size Reduction Has Been Natural Result of Physics as Frequencies Increase- Example: commercial radio broadcast television
analog cellular digital PCS wireless data
• But Further Reduction in Size Introduces New Complexities- Must continue to increase performance
- But, must maintain a minimum certain size in order to meet bandwidth and energy requirements
Page 11leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 11
3. Performance
• Better Performance is Usually Achieved by Increased S/N in the Wireless Link- Performance improvements can be realized
by higher gain antenna (if beam is properly focused)
• Example: Want horizontal beam for cell phone, zenith beam for GPS
• Increased Gain Can be Used in Different Ways- Better cell coverage area
• Increase cell size / range• Given all mobiles at max power, then
less dropouts- Less battery power
• Given strong signal area, then reduced Tx Battery
• Especially critical in CDMA networks- Some combination of above
Reduce Battery Size
R1
Reduce Dropouts
R1R2
Increase Cell Coverage
Page 12leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 12
4. Multiple Operating Frequencies/ Modes
• RF Spectrum is Allocated by Governments Worldwide
• Operating Modes and Frequencies Are Not the Same Thing- Example: US “Tri-mode” cell phone is really a
dual-mode/dual-frequency phone• AMPS at 800, digital at 800, and digital at 1900 MHz
GPS
1550 1610
ISM
2400 2500
PCS/IMT-2000
1710 1990 22002110
ISM2
5725 5875
UNII
5150 53502335
AudioCellular/GSM
806 960470
Dig TV
Page 13leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 13
Antenna Physics
• Antenna is Fundamentally a Transmission Line- Electrical energy is converted to Electromagnetic
Radiation
• f = c/- as frequency goes up, wavelength gets shorter
• Loosely Speaking, Radiation Occurs Anywhere There is a Change in an Electric Current’s Velocity (Speed and/or Direction)- Consequently, antennas come in all forms of
curved, bent and folded metal shapes designed to alter current velocity or density
Page 14leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 14
Antenna Performance Specifications
• Antenna is Traditionally Evaluated for Performance Under These CriteriaA. FrequencyB. Gain / directivityC. Return Loss / VSWRD. Bandwidth E. Impedance
• Today Additional Antenna Performance Parameters Must be EvaluatedF. EfficiencyG. Volumetric size
Page 15leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 15
A. Frequency of Operation
• Electromagnetic Spectrum Is Measured in Terms of Frequency
• Most Antennas Transceive Over a Narrow Frequency Range Which Is Usually ~10% of the Center Frequency
• For Antennas, This Includes Both the Uplink and Downlink Frequencies (if duplex)- Example: (US PCS Tx and Rx) = 1850 MHz to 1990
MHz
Page 16leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 16
B. Gain & Directivity
• Combined parameter that characterizes the actual performance achievable in a “real” antenna
• Antenna gain is expressed as the ratio of the power transmitted by an antenna in a given direction and the power that would be transmitted in that direction by a perfectly efficient isotropic radiator (spherical) in that direction- Isotropic is uniform 0dBi, by
definition
Perfectly Efficient Isotropic Radiator
(3D Sphere)
Page 17leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 17
Units of Antenna Gain - dBi
<0 dBi
> 0 dBi
• Ratios of power are often expressed in decibels (dB) and are a comparison to some other known reference value. For instance, power referred to 1 watt is indicated as dBw. In the case of antennas, the unit is dBi since the reference measurement isrelative to an isotropic radiator (i for isotropic)
• dBi = 10 x log(Pa/Pi), where Pa is the power transmitted by the antenna in a given direction,and Pi is the power that would be transmittedby a perfectly efficient isotropic (spherical)radiator in that same direction
• Gain values >0dBi indicate that the antenna emits more power in that direction than the theoretically perfect isotropic radiator. Values < 0 dBi indicate that the antenna emits less power in the given direction
Page 18leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 18
Peak Gain
• Peak gain is the value of the gain in the direction in which the most power per unit area (flux) at a fixed distance or the most power per unit of solid angle is radiated by the antenna
• A narrow beam antenna will have a peak gain much greater than 0dBi, assuming it is a highly efficient radiator
• A broad beam or omni-directional antenna will have a peak gain around 0 dBi, because its power is spread out over a large area of the radiation sphere
Peak
Page 19leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 19
Average Gain
• As with peak gain, average gain for an antenna is also expressed in dBi, because it is referenced to a perfectly efficient isotropic radiator
• The highest possible average gain for an antenna is 0 dBi, because the total power emitted by an antenna can never be greater than the total power emitted by a perfectly efficient radiator. This assumes the averaging is done over the entire 4 steradian volume—the entire sphere
• Often average gain is expressed only over a portion of the sphere, such as a 2D planar cut in azimuth
Page 20leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 20
C. Return Loss / VSWR
• Difference between the power input to and the power reflected back from a discontinuity in a transmission circuit
• In a perfectly matched transmission system, there are no standing waves and the Voltage Standing Wave Ratio (a ratiometric measure of the crest to null of the voltage standing on the line) is 1:1
• Antennas having VSWR less than 3:1 are acceptable for receive applications and low power transmission,with 2:1 being very good
Page 21leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 21
D. Bandwidth of Operation
• Amount of Spectrum Needed for a Particular Communications Channel or Group of Channels - Defined in units of frequency and is computed as the
difference between an upper and lower band edge limit- Example: (PCS)
• 1990 – 1850 = 140 MHz BW• 140 / 1920 (center) = 7.3 % BW
- Channel Bandwidth is usually much less than total allocated bandwidth
• Narrowband Antennas - Operate only on the band of frequencies for which the device
was intended• Broadband Antennas
- Tend to perform less effectively than narrowband antennas but provide multiple frequency integration
Page 22leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 22
E. Input Impedance
• Antenna Input Impedance Is Traditionally Specified at 50 Ohms for Most Antenna Devices- Many not necessarily be optimal, but provides
easy test / debug via standard coax test equipment
- OEMs have begun to consider lower impedances, primarily driven to better match the output of the transmitting power amplifier (which is where most of the battery power is consumed)
• 75 Ohms Typical for Video Equipment
Page 23leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 23
F. Efficiency
• Direct Measure of How Well an Antenna Transforms Onboard Electrical Energy Into Transmitted Signal Energy - 100% efficient antenna would theoretically convert
all input power into radiated power, with no loss to resistive or dielectric elements
- Most all antennas in use exhibit at least 50% efficiency, with 70%-80% being very good designs
Page 24leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 24
G. Volumetric size
• Volumetric Size of the Best-designed Antenna Is Ultimately Limited by Theoretical Considerations That Depend on the Maximum Bandwidth Over Which the Antenna Must Operate - Some antennas exhibit a smaller occupied
volume for a given degree of performance
Page 25leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 25
Wireless Device Antenna Choices
Loop
Patch Monopole MLA
PIFA
Page 26leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 26
Monopole Type Patterns
This Antenna Has Gain of About 1 dBi Gain From 75 to 120 Degrees of Elevation (Referenced From Zenith
Above)
• Monopole Pattern Is a Torus-shape
• PIFA and Loop Patterns Are Similar, Only Beam Is Directed in Different Ways
Page 27leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 27
Patch Pattern
This Antenna Has Gain of About 4 dBi Peak Gain Focused in One
Direction
• Patch Pattern Is Uni-directional
• Projects a Cone Upward From the Surface of the Patch
Page 28leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 28
MLA and Other Patterns
• Meander Line Antenna (MLA) - Has both monopole and patch radiating modes,
depending on operating frequency
• Monopole, Loop, PIFA and MLA May All Have Similar Beam Patterns- Depending on the size of the antenna relative to
the wavelength of the signal, loop
Page 29leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 29
Embedded Antenna Considerations
• Include Antenna Designer as Part of the Design Team from Beginning
• Critical Considerations- Identify all frequency bands (GPS later ?)- Identify multiple-frequency RF front-end architecture
• Number of antenna feed points• Diplexers, Tx/Rx paths, filtering needed
- Identify orientation• Desired beam pattern under consumer usage scenario• Problem: PDA is used as both handheld and desktop
- Plan and allocate for internal volume needed to achieve desired level of performance
- Identify upfront the grounding scheme and adjacent metal surfaces
Page 30leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 30
Multi-band or Wideband?
• How Many Antennas are Needed for a 4-Band Device?- Ex: 800, 1575, 1900,
2450 MHz• Multiple Band Antennas
- A 2-band is feasible, 3-band or more is difficult to achieve adequate BW in all needed bands
• Wideband Antennas- More aesthetic- Easier to manufacture- Needs detailed systems
work to determine filtering,interference and isolationissues
Wideband Antenna VSWR
Page 31leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 31
Advanced Concepts in Antenna Design
• Higher Data Rate Wireless Links
• Diversity Combining
• Multiple Input Multiple Output (MIMO) Systems
2G 2.5G 3G
GSM
TDMA
CDMA
GPRS
1xRTT HDR CDMA 2000
N-CDMA
EDGE
Page 32leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 32
• The Convergence of Wireless Access and Broadband Internet Surfing Has Created a Demand for Low BER, High Datarate Wireless Systems
• Interference Dominates System Capacity Implying That Increasing Radiated Power at the Source Is Not the Solution
• The Electromagnetic Spectrum Is Limited and Efficient Usage Is Required From a Economic Perspective
Higher and Higher Datarates
Page 33leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 33
The Road Ahead
• Improved Spectral Efficiency Is Achievable in Environments of Interest for Commercial Wireless Services- Multipath characteristics of the channel determine the
improvement in spectral efficiency- Published test results indicate that very small separation
provides measurable improvements at mobile terminals
• As a Result, Multiple Antenna Diversity Is Being Introduced Into the Standards for Mobile 3G Systems (UMTS, EDGE) As Well As Indoor Systems (802.11 WLAN)
Page 34leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 34
Diversity Methods
• To Mitigate These Problems, Diversity Techniques are Being Developed- Spatial Diversity is a widespread technique based on the use of an
array of sufficiently spaced antennas at either transmitter or receiver- Polarization Diversity takes advantage of existence of statistical
independence of different polarization states as a wave is scattered in the environment
- Pattern Diversity uses the antenna itself to select angularly diverse components of the scattered wave to mitigate fading and increase SNR
- Temporal Diversity exploits the multidimensionality of the channel to improve SNR
- MIMO uses antenna arrays at both transmitter and receiver—multipath propagation can actually be exploited to establish multiple parallel channels
Page 35leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 35
Diversity Improvements
• There is Strong Evidence That One or More Forms of Diversity can Improve the Channel Performance of a Radio by Several dB- Cho, et al. 7 to 8 dB gain due to polarization
• A combination of height (spatial) and polarization diversity provide a robust scheme
- Braun, et al. 9 dB with a two antenna configuration- Dietrich, Stutzman,et al. Also 5 to 7 dB- Many other references in
• Bibliography at GLOMO Project VPI
Page 36leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 36
Directional (Pattern)
• To Date, WLAN OEMs Have Not Exploited Directional- Traditional WLAN systems have relied on spatial only- Why not use 2 different WLAN antennas with higher peak
gains focused in different directions?• Example: 1 antenna omni-directional, 1 antenna patch diversity
combines to better total hemi-spherical coverage• “Smart Antennas”
- Discriminate multi-path components- Process them separately
• Butler Matrix- If the signals from different directions can be processed
separately, co-channel interference can be suppressed- Improve transmission quality and/or capacity- Can reduce delay spread
Page 37leading wireless innovation™ COMPANY CONFIDENTIAL©2002 SkyCross. All Rights Reserved.leading wireless innovation™ Page 37
Wireless Device Considerations for MIMO
• Multiple-Input, Multiple-Output (MIMO)- Diversity Transmit and Diversity Receive
• Size Limited, Two Antennas Is About the Maximum Allowable in a Handheld Device
• Can Use Embedded Antenna as the Second One• Can Exploit Polarization Diversity
- Second polarization for the whip antenna- Dual polarization antenna for the hidden antenna
• Treat Polarization Terms as Spatial Term• Beam-formed Elements Again Treat Them as
Spatial Terms and Process the Same Way