4G MIMO ANTENNA DESIGN & Verification - Agilent Technologies

4G MIMO ANTENNA DESIGN & Verification

Using Genesys And Momentum GX To Develop MIMO Antennas

© Copyright 2008 Agilent Technologies, Inc.

Agenda • 4G Wireless Technology • Review Of Patch Technology • Review Of Antenna Terminology • Design Procedure In Genesys • Verifying Antenna Performance • Using Genesys To Determine Multi-Element Patterns • Verifying Method With Momentum GX • Conclusion


4G Wireless: LTE, WiMAX, Mobile WiMAX, 802.11n Fourth Generation Wireless Infrastructure: • Higher Data Rates – Up to 150 Mbs downlink, 50Mbs uplink • Multi-Data Formats – Edge, GSM, FTE, UMTS etc. • Speedy Mobiles 100 km/hr


Antenna Parameters Selection criteria for antenna type: • Beam pattern • Gain • Power handling capability • Directivity • Bandwidth • Manufacturability • Cost


Patch Antenna Characteristics Patch antenna topologies: • Advantages – Ease of manufacture – Form complicated antenna patterns – Flexible substrates – Variety of shapes and structures – Weight – Cost

• Disadvantages – Substrate material limits efficiency – Lossy, lower radiation efficiency means increased transmit power – Power limited


Patch Antenna Shapes There is an almost endless number of antenna feed topologies: • Rectangular, Circular, Arrays • Shapes affect bandwidth, radiation patterns and polarization • Spacing and phase affect directivity, gain and radiation pattern Patch Patterns

Parallel Feed Series Feed


Radiation Patterns Fields defined by E-theta and E-phi • Etotal is the vector sum of both components • Etheta sweeps from the North Pole 0o to 90o • Ephi sweeps from 0o to 180o around the North Pole Eθ

Eφ

Note relationship of the field to the X and Y axis of the circuit board


Orientation In Antenna Patterns Patch antennas Rarely Have Symmetrical Pattern • Due to current distribution on patch(s)

Phi=0o

Phi=90o


Antenna Design Procedure Use linear analysis to evaluate physical dimensions Verify design with Momentum GX • Determine additional matching circuitry using MATCH • Examine prototype with far field analysis

Design and verify a steer-able beam array • Develop a mathematical model of the far field pattern • Apply procedure to dual antenna pattern • Verify multi-element pattern with Momentum


Patch Design Start with a rectangular design • Resonance is determined by length along the feed axis – Length is approximately

λ 2

– Width is loosely equal to length, however maximum efficiency is given for width by (1) W=

ν0 2 ∗ fr

2 ε r +1

W  + 0.3)⋅  + 0.264 ∆L h  = 0.412∗ h (εeff − 0.258)⋅  Wh + 0.8  

(ε

eff

ε eff =

ε r + 1 ε r −1  2

+

h ∗ 1+12∗  2  W ∆L

L=

ν0 2 ⋅ f r ⋅ ε eff

L

∆L

− 2 ⋅ ∆L W

Fringe Effect


Patch Design Frequency requirements for LTE band II • Approximately 7.5% bandwidth • Transmit band is 60 MHz wide, 1850-1910 MHz • Receive band is 60 MHz wide, 1930-1990 MHz

The design will then center at 1920 MHz • Start a patch Length =1440 mils, with a Width =1860 mils • Substrate is FR4 Er=4.5, height = .059 inches


Using Linear Modeling Start with simple transmission line model to verify the length


Using Momentum GX First simulation establishes resonant frequency

Markers show band edges for the transmit and receive bands Of course transmission line does not model radiation


Reducing Patch Width And Optimizing Length Reducing patch width has small effect on response but reduces footprint Width =1200 mils Length =1434.5 mils


Evaluating Matching Structures Using Genesys MATCH we can determine the optimum matching structure Start with settings dialog we set the frequency band of match • The settings represent the full band 1850-1990 MHz with 50 pts


Using Antenna Data For Match In Sections Tab We Point MATCH To The Momentum Data Set As The Terminating Impedance The Type Of Matching Structure Is Selected Next • We will try to use distributed matching for incorporation into the layout


Stepped Impedance Network Stepped impedance provides a good match at band center but the band edges are not improved


Quarter Wave Matching Line A simple quarter wave provides improvement at band center but again the band edges are not improved


Match For Transmit Band The patch antenna chosen is inherently narrow band • Focus on matching for the transmit frequencies since a poor match can result in watts of power loss • Re-center resonant frequency for transmit band center 1880 Mhz

Slight increase in antenna length decreases center frequency


Synthesize Matching Network Using MATCH Again To Find Best Structure • In this case a simple quarter wave transmission provides an adequate match at band center and edges Length =1434.5 mils


Final Momentum Analysis Final analysis places center frequency at ~1880 MHz • Quarter wave matching line gives us -36 dB return loss at ~1880 • Transmit band edges provide ~-10 dB return loss • Receive band has the worst match of -6.5 -> -3 db, possible second antenna


Plotting Field Patterns We must have performed a Momentum simulation first!


Etotal Compared To Phi Select antenna graph measurement then select phi cut • Radiation pattern is dependent upon rotation around phi axis


Antenna Patterns Field pattern is a function of

φ

φ =0

φ = 90

θ

φ = 00

θ

φ = 900


MIMO Networks Require Agile Antennas Standards affect antennas for base stations and mobiles devices Base stations need to provide data to multiple users while compensating for multi-path and delay Mobiles also need to compensate for multi-path and fading


Networks Require Agile Antennas Variety of antenna function and types

Omni Directional

Diversity Steer-able Array

Patch Antenna Types Switched or Multi-Beam


MIMO- Steerable Antenna Use Genesys to develop MIMO Antennas Design and evaluation of steerable MIMO Antennas • We use the results of our antenna design to predict the contributions from an array • If the patch antennas are reasonably isolated Smn~ 0, then linear superposition can be used to plot the far field contributions (2)


Determining Far Fields Far Field value is the superposition of each radiator

FAR FIELD FAR FIELD

Fp1 = ( R * sin(θ )) 2 + ( R * cos(θ ) − d ) 2

Fp1

Fp2

Fp 2 = ( R * sin(θ )) 2 + ( R * cos(θ ) + d ) 2

R

R * sin(θ )

R

θ

A∠α

θ

B∠β R * cos(θ ) − d

R * cos(θ ) + d d =λ/4


Mathematically Generated Pattern Superposition of Fields k := 0.. 360

θ k := k⋅

λ := 1 α := 0deg

β := 0deg

π

A := .5

180

n := 1

R := 100⋅ λ

(

k

D := 2⋅ K

4

) )1.0 − .5 cos (θ k − π)

(

Ant := .5⋅ cos θ k − π k

2

E :=

K := n ⋅

B := .5 λ

 R⋅ cos  θ − π  + K +  R⋅ sin  θ − π     k     k  2 2      

2

2

F := k

 R⋅ cos  θ − π  − K +  R⋅ sin  θ − π     k     k  2 2      

2

γ k := α − 2π⋅ E

k

δk := β − 2π⋅ F

k

(

( )

( ))

(

( )

( ))

v1 := Ant ⋅ A ⋅ cos γ k + A ⋅ sin γ k ⋅ i k k v2 := Ant ⋅ B⋅ cos δk + B⋅ sin δk i k

k

V := v1 + v2 k

105

90

k

k

75

120

105 60

135

45

0.8 0.6

150 165

30

0.6

0

195

345

210

330 225

315 240

300 270 θk

30

0.4 165

15 0.2

0

255

45

0.8

150 15

0.2

180

75 60

135

0.4 Ant k

90

120

285

Vk

180

0

0

195

345

210

330 225

315 240

300 255

270 θk

285


Superposition Of A Two Element Array Far Field plot of two omni-directional antennas • Driven with equal amplitude and in phase • Note the new directionality 105

90

75

120

105 60

135

45

0.8 0.6

150

Ant k

135 30

0.6

0

195

345

210

330 225

315 240

300 270 θk

30

0.4 165

15 0.2

0

255

45

0.8

150 15

0.2

180

75 60

0.4 165

90

120

285

Single Antenna Pattern

Vk

180

0

0

195

345

210

330 225

315 240

300 255

270 θk

285

Result Of Far Field Superposition


Antenna Interference An intuitive look at interference vs. spacing • Like colors or phases add while unlike colors or phases subtract

A

B + +

+

+


Front Sided Antenna Little or no backward radiation • Pattern becomes narrower with little side lobe radiation • Typical of patch antenna 105

90

75

120

105 60

135

0.6

150

60

135 30

0.6

150

Ant k

180

0

195

345

210

330 225

315 240

300 270 θk

165

15 0.2

0

255

30

0.4

15

0.2

45

0.8

0.4 165

75

120 45

0.8

90

285

Vk

180

0

0

195

345

210

330 225

315 240

300 255

270 θk

285


Changing The Feed Phase Varying the phase and amplitude of the elements • Results in controlling the tilt or angle of maximum radiation 105

90

75

120

60

135

45

0.8 0.6

150

30

0.4 165

15 0.2

Vk 105

90

180

60

195

0.6

210

15 0.2 0

195

345

210

330 225

315 240

300 270 θk

240

285

β := −60deg

270 θk

75 45

0.8 0.6

150

30

0.4 165

300 285

90

60

315 255

0

255

330 225

0.4

180

105

120

345

30

165 Vk

β := −90deg 135

45

0.8

150

0

75

120 135

0

15 0.2

Vk

180

0 β := −180deg

0

195

345

210

330 225

315 240

300 255

270 θk

285


Pattern Array Field patterns for an array of antenna elements can be analyzed or synthesized by*…. 1) Knowing the single element radiation pattern 2) The amplitude and phase of the sources driving each element 3) Knowing the spacing or separation between elements

Method may be extended to multiple elements d

A∠α

d

B∠β

d

C∠χ

D∠δ

*Interference or coupling between elements is zero or nearly zero


Array Design In Genesys Applying the same trigonometry within Genesys • We start with the single patch antenna from before • Using Momentum far field data we obtain the element pattern • Genesys’ rich set of math functions allows us to project far field data from the captured single element pattern • This method may be extended to two or more element arrays • The ability to tune parameters such as feed amplitude and phase as well as antenna distance gives full control over the far field • When applied to a large number of elements, optimization reduces the time and effort


Extracting The Element Pattern Run a Momentum GX analysis of the proposed antenna Extract Momentum E-field dataset values for single element

New Data Vector With Field Values


Using Genesys Math Functions Trigonometric equations relating far-field value to element pattern characteristics SUPER POSITION OF FIELDS k := 0 .. 360

(

θ k := k⋅

π

A := .5

180

) )1.0 − .5cos (θ k − π)

(

λ := 1

B := .5 n := 1

D := 2⋅ K K := n⋅

Ant := .5⋅ cos θ k − π k

E := k

 R⋅ cos  θ −   k  

π

2

    + K  +  R⋅ sin  θ k − 2   

π 

2

 2 

F := k

α := 0deg

λ

R := 100⋅ λ

4

 R⋅ cos  θ −   k  

β := 0deg

π

2

    − K  +  R⋅ sin  θ k − 2   

π 

2

 2 

γ k := α − 2π⋅ E

k

δk := β − 2π⋅ F

k

(

( )

( ))

(

( )

( ))

v1 := Ant ⋅ A ⋅ cos γ k + A ⋅ sin γ k ⋅ i k

k

v2 := Ant ⋅ B⋅ cos δk + B⋅ sin δk i k

k

V := v1 + v2 k

k

k


Tuning For Phase And Levels Antenna parameters are made tunable • Instant visualization on far field pattern

0o Phase Antenna A level Phase Difference Antenna B level

Antenna spacing in half wavelengths


Result Of Phase Offsets Antenna beam steers, side-lobes and beam width change

~42o ~28o

60o Phase 90o Phase


Additional Degrees Of Freedom Pattern is influenced by drive levels and element separation Added elements offer improved control over beam

Element spacing 1.3 half wavelengths

Element spacing 3 half wavelengths


Verifying Predicted Pattern Layout two element patch antenna modeled after single element previously designed and simulated Use Momentum to generate the combined far-field with appropriate voltages and phase Review the far-field pattern to verify the predicted performance


Verifying Isolation At band center, 1880 MHz isolation is -43 dB

-43 dB=50 millionths


Setting Source Values Under Momentum’s far-field options The source levels and relative phase

Note that the phase is set at 180o …Why?


More Features For Momentum GX 3D Field Viewer


Comparing Predicted Field Comparison of far-field predicted and Momentum GX • Both show a half power beam width of 290 PREDICTED

Momentum

Momentum 3D View Single Element Pattern


Adding Phase Shift At Ports Result of 28o difference in phase between sources • Note identical beam values at -3dB of 38O from beam center PREDICTED

Momentum 28 Degrees

Note: The current version of Momentum plots half beam


Using Two Evaluations Plotting both halves of Momentum field requires two phase evaluations • Note values are equal between predicted and Momentum! Momentum

PREDICTED 60 Degrees


Elements Driven Opposite The extreme for two element antenna is 180 phase difference Difference in magnitude due to re-normalizing in Momentum 180 Degrees

PREDICTED

Momentum


Orientation Of Beam Relative To Board Major cut was through phi = 0 Swept pattern steers along X-axis

φ = 00


Field Pattern In Genesys Extending the equations to four elements a narrow beam is achieved


Optimization Of Beam Pattern Beyond two elements selecting the correct feed-phase is burdening • We use the optimization features of Genesys to aide in finding the best set of feed amplitude and phases

Goal = 30 deg


Beam Amplitude Optimization Additionally we optimize the feed amplitudes to compensate for beam power as a result of steering a = 0.268 ∠1.139 b = 0.268 ∠ − .374 c = 0.271∠ 2.688 d = 0.265 ∠ − 1.886

Angles in radians


Other Sources Of Antenna Data Single antenna element information can be measured and imported via TestLink


Conclusion An antenna design procedure was presented A rectangular patch was designed and verified with Momentum 3D-Planar EM Field Simulator A modified antenna was optimized for a LTE band and matching network incorporated The single patch field pattern was then used to model or predict the effect of an array of two or more elements Verification of this technique was established with Momentum field solver Optimization aides in extending this procedure to larger arrays


Agilent Genesys product bundles start at about $4K USD The modules used to complete the synthesis, design and verification of MIMO antenna system presented in this paper can be found in the Genesys Non-Linear Pro GX (W1426L) for about $16.6K USD


References Antenna Theory Analysis and Design, Constantine Balanis, Wiley, second edition, Pgs 727-736 Ibid, Pgs 249-261 Fundamentals of Applied Electromagnetics, Fawwaz Ulaby, Prentice Hall,1997, Pgs 316-365 Agilent AN note “3GPP Long Term Evolution”, doc 5989-8139EN Agilent AN “Mobile WiMAX PHY Layer Operation and Measurement”, doc 59898309EN Agilent AN “MIMO Channel Modeling and Emulation Test Challenges”, doc 59898973EN Agilent AN “MIMO Wireless LAN PHY Layer RF Operation & Measurement”, doc 5989-3443EN
