Port Isolation Enhancement via Active Integration for a UWB MIMO Antenna System Sagar K. Dhar and Mohammad S. Sharawi
Fadhel M. Ghannouchi
Department of Electrical Engineering King Fahd University of Petroleum and Minerals Dhahran, 31261, Saudi Arabia Email:
[email protected];
[email protected]
Department of Electrical and Computer Engineering University of Calgary Calgary, Canada Email:
[email protected]
I.
INTRODUCTION
II.
5
Multiple-input-multiple-output (MIMO) antenna systems received significant research interest in recent years because of their exotic features like high data throughput, better transmission quality and increased coverage due to their spatial multiplexing gain, diversity gain and array gain. However, designing a MIMO antenna is highly challenging considering the compact size of mobile terminals and different MIMO antenna parameters that needs to be satisfied especially port isolation and envelop correlation coefficient [1].
In this work, an RF front-end based antenna co-design approach is introduced for port isolation enhancement of a UWB MIMO antenna. As an example, a semi-ring two element UWB monopole MIMO antenna is designed together with a power amplifier (PA). The co-designed active integrated antenna (AIA) shows improvement in port isolation and antenna matching over the operating band of 1.5-6GHz which subsequently provides improved total efficiency and realized gain of the antenna.
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90
Y Top Z
GND
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50
(a)
(b)
Fig. 1. Antenna geometry: (a) model and (b) fabricated prototype 20
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0 S-parameters
Different isolation enhancement examples are found in literature using antenna placement and orientation [2], decoupling networks [3], parasitic elements [4], defected ground structure [5], neutralization line [6] and metamaterials [7]. However, all existing approaches generally design antennas assuming 50Ω port impedance. But from a system point of view, the radio frequency (RF) components following the antenna do not have exactly 50Ω especially when designed for ultra-wideband (UWB) systems. Such mismatch between the antenna and RF front–ends deteriorates the system performance in terms of antenna matching which results in reduced total radiation efficiency and realized gain. Moreover, port isolation techniques in [2]-[7] requires tedious optimization trials.
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ACTIVE INTEGRATED UWB MIMO ANTENNA
The UWB monopole MIMO antenna model and its fabricated prototype are shown in Fig. 1. The antenna is designed in CST Microwave Studio (MS) on a low cost FR4 substrate having the size of 50×90×1.52mm3 and dielectric constant of 4.0 which is suitable for standard mobile terminals. A UWB PA (GVA 63+) is selected from Mini-Circuits. Its measured S-parameters and stability parameters when soldered on a test board are shown in Fig. 2. It can be seen that the amplifier is stable and shows input and output matching less than -10dB over the band 1-6GHz.
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S12 (dB) S21(dB) S11(dB) S22(dB)
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|Δ | k
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Stability Parameters |Δ| and k
Abstract—In this paper, a new approach of port isolation for UWB MIMO antennas is investigated through active integration with a power amplifier (PA). Instead of 50Ω based design, a codesign approach based on the complex impedance of the PA and the antenna is adopted for better MIMO antenna matching and port isolation. Such approach does not require extra decoupling networks. The integrated antenna system covers 1.5-6GHz and shows minimum port isolation improvement of 5.5dB. The minimum efficiency and highest envelop correlation coefficient are found to be 62% and 0.115 respectively over the band 1.56GHz.
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Fig. 2. PA test board and characteristics
AP-S 2015
Γ ant (int) =
Γin , Zin
RFin
Input MN
* Z ant − Z MN Z ant + Z MN
0
Γant , Zant
Output MN
Reflection Co-eff. (Standalone Antenna) Reflection Co-eff. (Directly Loaded Antenna) Reflection Co-eff. (Codesigned Antenna) S21 (Co-designed Antenna) S21 (Standalone Antenna)
-5
(2)
ΓL , ZL
Amplifier
6GHz. Such high gain is because of the addition of the amplifier gain to the antenna gain. The envelop correlation coefficient (ECC) is also calculated from simulated far field radiation patterns and found to be less than 0.115 over the frequency range 1.5-6GHz.
S-parameter Results (dB)
The block diagram of an amplifier integrated active antenna system is shown in Fig. 3. The co-design approach starts with deciding on the value of ΓS from the gain and noise circles for their optimum value after which Γout is calculated from (1). Then an output matching network (MN) is designed looking at the antenna complex impedance Zant and Zout so that the integrated antenna shows optimum matching over the operating band 1.5-6GHz. Similarly, an input MN is designed to see 50Ω input port as ZS. The integrated antenna reflection co-efficient can be calculated from (2). The finalized power amplifier based two element antenna schematic is shown in Fig. 4 and is simulated in Advanced Design System (ADS). S S Γ Γ out = S22 + 12 21 S (1) 1 − S11Γ S
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Antenna
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Fig. 5. S-parameter results
ZS, ΓS
Zout, Γout
ZMN , ΓMN
IV.
Fig. 3. AIA system block diagram L1=10mm, L2=3mm, L3=3mm and W=3.1mm P1
TL1 C1=0.4pF
P2
TL1
L1=1.5nH
TL2
PA S-para mete rs
C2=0.6pF L1
C1
C3 =0.5pF TL2
C2
TL3
PA S-para mete rs
TL3
L2=1nH C4=0.5pF
A PA based new port isolation enhancement is investigated of a UWB MIMO antenna. The active integrated MIMO antenna system offers -10dB bandwidth over the band 1.56GHz and a minimum isolation enhancement of 5.5dB is observed. Active antenna system realized gain is found to be more than 19.5dB and ECC is found to be lower than 0.115.
Antenna Port1 MIMO Anten n a
L2
C3
C4
Antenna Port2
ACKNOWLEDGMENT The work was supported by DSR at KFUPM, project no. IN131027.
Fig. 4. Two element active integrated MIMO antenna schematic
III.
CONCLUSION
RESULTS AND DISCUSSION
The reflection coefficient and isolation curves of the UWB active integrated MIMO antenna are shown in Fig. 5. It can be seen that co-designing improves the integrated antenna reflection co-efficient compared to a standalone and directly PA loaded antenna (in which both PA and antenna are designed assuming 50Ω interface and cascaded) over the operating range 1.5-6GHz. Minimum port isolation (S21 or S12) is observed to be 11dB which is around 5.5dB improvement compared to the standalone antenna isolation. Here, port isolation is achieved via the loop gain from port1 to port2 and a trade-off between amplifier gain and isolation is observed. To see the radiation characteristics of the active integrated MIMO antenna, a co-simulation with CST MS and CST Design Studio (DS) is performed. This way, CST combines results from circuit and electromagnetic simulators and shows integrated antenna radiation characteristics. Thus, the realized gain, total radiation efficiency and envelop correlation coefficient are obtained. The realized gain and total antenna radiation efficiency of the integrated antenna system are found to be more than 19.5dB and 62% over the operating band 1.5-
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[4]
[5]
[6]
[7]
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M. S. Sharawi, “Printed Multi-Band MIMO Antenna Systems and Their Performance Metrics [Wireless Corner],” IEEE Antennas and Propagation Magazine, vol. 55, no. 5, pp. 218–232, Oct. 2013. J. Ren, W. Hu, Y. Yin, and R. Fan, “Compact Printed MIMO Antenna for UWB Applications,” IEEE Antennas and Wireless Propagation Letters, vol. 13, pp. 1517–1520, 2014. C.-H. Wu, G.-T. Zhou, Y.-L. Wu, and T.-G. Ma, “Stub-Loaded Reactive Decoupling Network for Two-Element Array Using Even-Odd Analysis,” IEEE Antennas and Wireless Propagation Letters, vol. 12, pp. 452–455, 2013. Z. Li, Z. Du, M. Takahashi, K. Saito, and K. Ito, “Reducing Mutual Coupling of MIMO Antennas With Parasitic Elements for Mobile Terminals,” IEEE Transactions on Antennas and Propagation, vol. 60, no. 2, pp. 473–481, Feb. 2012. M. S. Sharawi, A. B. Numan, M. U. Khan, and D. N. Aloi, “A DualElement Dual-Band MIMO Antenna System with Enhanced Isolation for Mobile Terminals,” IEEE Antennas and Wireless Propagation Letters, vol. 11, pp. 1006–1009, 2012. C. H. See, R. A. Abd-Alhameed, Z. Z. Abidin, N. J. McEwan, and P. S. Excell, “Wideband Printed MIMO/Diversity Monopole Antenna for WiFi/WiMAX Applications,” IEEE Transactions on Antennas and Propagation, vol. 60, no. 4, pp. 2028–2035, Apr. 2012. D. A. Ketzaki and T. V. Yioultsis, “Metamaterial-Based Design of Planar Compact MIMO Monopoles,” IEEE Transactions on Antennas and Propagation, vol. 61, no. 5, pp. 2758–2766, May 2013.
