Decoupling network synthesis for
improving MIMO antenna isolation
April 26, 2016
Jaakko Juntunen, Optenni Ltd.
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MIMO Antenna Challenge
• Antenna-to-antenna isolation is an important concern in MIMO
antenna systems
– Matched antennas exhibit typically the worst isolation when operating at the
same band
– In portable devices the antennas are close to each other, making the problem
worse
• Poor isolation causes
(a) power loss in the coupled ports, and
(b) increased correlation between the received signals leading to lower data rates
• Antenna isolation can be improved using Optenni Lab
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Outline of Decoupling Process
• We follow and extend the technique developed by Chen et al.1
• The idea is to transform cross-admittance term Y21 to purely imaginary
value, and to cancel it by a single reactive component
• The limiting assumptions are
(a) perfect inherent matching
(b) single frequency condition for
achieving purely imaginary Y21
(c) single frequency condition for
Y21 cancellation
(d) availability of purely reactive
lumped components
Chen, Y.-S. Wang and S.-J. Chung, “A Decoupling Technique for Increasing the Port
Isolation Between Two Strongly Coupled Antennas”, IEEE Trans. Antennas Propag., vol. 56,
no. 12, pp. 3650-3658, Dec. 2008.
1 S.-C.
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Initial Y21 at fc
Y21 after adding
short microstrips
to input ports,
Re(Y21)=0
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Improving Decoupling with Optenni Lab
• To tackle the limitations (a)-(d), we can formulate the problem generally
in terms of
– input port efficiencies, and
– port-to-port isolation
• Input port efficiency takes into account the losses due to reflection,
discrete component losses, and power lost to other port(s) by finite
isolation
• Successful decoupling enables thus better efficiency, too
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Example Case
• We use a symmetric antenna pair configuration, and
target at band 2.4-2.5GHz
• Transforming Y21 purely imaginary is easy by adding
transmission lines in Schematic
• Preparing for the admittance cancellation by a discrete
component, we introduce a differential port for the
shunt component (port 3 in the picture)
• This example is challenging
from the isolation perspective
– The antennas are close to each other
– They have the same polarization
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Cancel Y21
•
It is easy to calculate from cancellation criterion jC=Y21 that a capacitor
C=1.35pF should do the decoupling at 2.45GHz
–
•
The optimal value will be a bit different because the shunt port is not exactly at input ports
The 3-port model is sent to Optenni Lab (Macros\Matching Circuits\Launch
Optenni Lab), and adding and tuning a capacitor at the shunt port, we indeed see
that the ports get neatly decoupled  but the matching gets distorted 
without decoupling
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with decoupling
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EM Isolation of Original vs. Decoupled System
• Optenni Lab’s EM Isolation analysis calculates the worst-case isolation
between the signal ports (“guaranteed isolation”), by assuming perfect
simultaneous match of the ports at each frequency
• We can observe a very significant
improvement:
– Original worst case isolation was 2.1dB
– Single decoupling component provides
10.5dB worst case isolation
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Rematch Antennas
• It remains to rematch the antennas (and to fine-tune the decoupling
component)
• Assuming a 2-component matching circuit at each input, we get a nice
result
– The optimized circuit is slightly
asymmetrical, because the simulation
data is not perfectly symmetrical
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Enhancing Decoupling
• We will next consider the possible enhancement of the isolation by using
additional decoupling components, so we prepare a T-topology layout
• A useful merit of the technique of Chen et al. is the conceptual
separation of decoupling (Y21 cancellation) from the impedance matching
• So in the design flow, we first optimize isolation with decoupling
components, and then correct the matching with matching components
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Enhancing Decoupling
• The five-port EM model is sent into Optenni Lab, and components at
ports 3-5 are synthesized and optimized for a few isolation targets
Design candidate A
Design candidate C
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Design candidate B
Design candidate D
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Ranking Design Candidates
• Optenni Lab provides Bandwidth potential analysis, which is a measure
of ”easiness” of matching for given center frequency
• Let us analyze both EM Isolation and Bandwidth potential of the design
candidates, before considering the matching circuit synthesis
• We see that a wideband decoupling is possible, but the antennas are
hard to match for more than 100MHz – candidate A seems to be the best
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Combining Decoupling and Matching
• We can freeze the decoupling components of candidate A at ports 3-5
and synthesize a 2-component matching circuit to ports 1 and 2
• From EM Isolation analysis we know in advance that the isolation is
guaranteed to about 20.4dB
This circuit provides 21.8dB isolation and -0.7dB
efficiency
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Summary
• The single-component decoupling method improves the efficiency
from 38% to 83% and reduces the coupled power from 61% to 8.3%
• The three-component decoupling method improves the efficiency
even more to 85% and reduces the coupled power down to 0.7%
• EM Isolation and Bandwidth potential analyses enable ranking of
multi-antenna systems for good isolation and matching, before
synthesizing the matching circuits
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©2016 Optenni Ltd. All rights reserved.
www.optenni.com