Wireless Communications

KoAMo - Cognitive Antenna Systems for mobile Applications

Research grant by BMBF (Bundesministerium für Bildung und Forschung)

Timeframe: 01.03.2012 - 28.02.2015

FKZ 01BU1231:

KoAMo is a collaborative research project within the SME (Small and Medium Enterprises) Initiative of the BMBF. Consortium Partners are IMST GmbH, CAU and KIT.

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KoAMo aims at developing new concepts of antenna systems for mobile applications such as small terminal and automotive platforms. Tasks of CAU include the design and integration of antenna elements for multi-standard and MIMO applications.


 Results Achieved

Antenna Design for Handheld Device

In the first project phase, the handheld device is preliminary investigated to find the optimal shape and location for the antennas. The Theory of Characteristic Modes is used to analyze the radiation mechanism with the aid of the chassis wave modes. These wave modes are used for the initial modification of the chassis.
The picture below shows the eigenvalues of the significant modes. From the basic theory, it is known that the eigenvalue are direct related to the reactive power of modes. Hence, the zero eigenvalue point determines the resonance modes. Any modification of the chassis or the antenna is aimed on the resonance of certain modes in the desire frequency band. In the initial example, a single resonance of the mode J1 can be found around f=1 GHz. In order to enable a multi-port antenna system, a second resonance (e.g. mode J2) in the same frequency band is required.

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The first mode J1 is excited with a capacitive coupling element placed along the minor edge of the chassis. This coupling element interacts with the chassis wave modes and reveals a resonance shift, as denoted in the figure below.  In order to excite mode J2 and to displace this mode into the low frequency band (LTE: 700 MHz – 960 MHz), a careful additional modification of the chassis is required. If the size and shape of the second coupling element is well-chosen the resonance of mode J2 can be shifted down in frequency. In this case, mode J2 can be excited with a high dominance. For that, the second coupling element is placed along the left side of the chassis, as depicted in the figure below.

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The two coupling elements have a high influence on the eigenvalue behavior of the respective modes, as shown in the next picture. The second wave mode J2 reveals a resonance in the low frequency band and a high gradient. From the basic theory it is known that a high gradient of the eigenvalues typically results into a high Q-Factor and, therefore, a narrow bandwidth.

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These preliminary investigations are used in a further project phase to design a tuneable impedance matching network. The first coupling element is matched with a Pi-network consists out of a static capacitor and two tuneable elements (viz. capacitor and inductor). Both elements are LTCC (Low Temperature Cofired Ceramics) components placed near the coupling element port. The second port is matched with a tuneable T-network, as denoted below. Here, a single tuneable inductor is used to impedance match the coupling element at the low frequency band.

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The simulation results are shown in the two figures below. It can be observed that the two feeding ports are impedance matched in the low frequency band. Owing to the selective excitation of the respective chassis wave modes, the two feeding port are decoupled.

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The final simulation model is shown in the next figure. The model includes the tuneable components and two microcontrollers for the respective feeding port.

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Antenna Design for Automotive Application

The second phase is aimed on the MIMO system design for the automotive application. Some portions of the car body are used as the chassis for antenna design. Two antenna locations are found, which supposed promising results. The first position for the MIMO system is the rearview mirror located within the car. The MIMO system is design to operate in the LTE frequency bands (700-960 MHz, 1.7–2.6 GHz). The second position is chosen carefully for the MIMO system integration aiming on the LTE and C2C (Car-to-Car: 5.9 GHz) communication standard. Owing to the available space, the left side mirror is used for the mode excitation.
The antenna concept of the rearview mirror is somehow different from the above explained hand-held device. Here, the chassis wave modes are excited in combination with four capacitive coupling elements (CCE) placed at the four edges of the mirror. Since the mirror contains metallic particle, it is suitable for the MIMO antenna integration. 

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Components of a rearview mirror. The mirror itself is used a radiating element.

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Position of the rearview mirror within a car body.

The next figure shows the antenna concept for the rearview mirror with two feeding ports. Two CCEs are feed in combination with a 180° hybrid-power divider, in order to excite specific chassis wave modes. Owing to the symmetry of the feeding concept, both ports and the respective CCEs excite the same wave modes with a well-defined phase shift. Hence, a low mutual coupling can be observed in the entire frequency range. Please see the next figure below.

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Antenna concept with four CCEs for the rearview mirror.

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Both ports have similar reflection coefficients and they can be adjusted with a tuneable impedance matching network. A single LTCC inductor is used for the tuning in the low and high LTE frequency band. The simulated far field patterns for the low (800 MHz) and the middle (1800 MHz) frequency show, more or less, an omni-directional radiation. The far field pattern of the high (2600 MHz) frequency band shows a high radiation toward the side direction.

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The antenna integration into a side mirror requires an investigation of the mirror construction. Although the mirror contains metal particle, the outer frame is more preferred for the antenna integration, as it is made out of aluminium. Therefore, the metal chassis is used for the mode analysis, as shown below.

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Schematic construction of the side mirror.

The mode analysis is shown in the next figure. Two basic wave modes can be found for the excitation. The modal surface currents of the two modes J1 and J2 reveal two positions for the antenna integration, the left and the lower corner.

 

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In the next step, two capacitive coupling elements are design, as denoted in the next figure. Both antennas contain static impedance matching network to match the antenna impedance at the LTE frequency bands and the C2C frequency.

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A prototype is fabricated and measured. The measurement results show a good impedance matching of the two antenna ports at the aimed LTE frequency bands. The first port shows an additional impedance matching at C2C frequency band (5.9 GHz).

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Radiation patterns of the side mirror at different frequencies.

The next table shows the measured total efficiency of the ports. Port 1 observes a high efficiency in the low LTE frequency band and a gain of 5.4 dBi. The second port has an overall efficiency of 50% in the middle and high LTE frequency band. The total efficiency at 5.9 GHz is low, as denoted in the table. Owing to the high wavelength, compare to the outer dimension of the side mirror, high lossies are unavoidable.

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Publications:

  1. Safin E. and Manteuffel, D. , "Resonance Behaviour of Characteristic Modes Due to the Presence of Dielectric Objects", In Antennas and Propagation (EUCAP), 2013 7th European Conference on., April 2013.
    [Abstract]
     
    In this paper we present a modal analysis of dielectric bodies of arbitrary shape. The problem of a dielectric body is reformulated into equivalent electric and magnetic surface currents. Using the weighed eigenvalue equation the characteristic modes of the equivalent problem are calculated. A physical interpretation of the eigenvalue of the dielectric characteristic modes is presented and discussed.
  2. Martens, R., Holopainen, J. , Safin, E., Ilvonen, J., and Manteuffel, D., "Optimal Dual-Antenna Design in a Small Terminal Multi-Antenna System," IEEE Antennas and Wireless Propagation Letters, 2014. (accepted for publication) DOI 
    [Abstract]
     
    This letter introduces a novel 2.6-GHz multiple-input and multiple-output (MIMO) antenna system for mobile terminals. The antenna structures consist of a broadband main antenna covering most of the LTE-A bands, and a narrowband second antenna operating at the 2.6 GHz band. The main antenna is a traditional monopole-type capacitive coupling element (CCE) placed on the short edge of the terminal. The second antenna consists of two - out-of-phase fed inductive coupling elements (ICE) placed on the long edges of the chassis. The main purpose of the paper is to demonstrate that this kind of antenna system offers good performance in terms of electromagnetic (EM) isolation and envelope correlation between the antennas. This has been experimentally verified and is originated on the fact that both antennas excite effectively different orthogonal wavemodes, which is studied with the help of the theory of characteristic wavemodes.
    [BibTeX]
     
    @null{6692896,
    author={Martens, R. and Holopainen, J. and Safin, E. and Ilvonen, J. and Manteuffel, D.},
    journal={Antennas and Wireless Propagation Letters, IEEE}, 
    title={Optimal Dual-Antenna Design in a Small Terminal Multi-Antenna System}, 
    year={2014},
    volume={PP},
    number={99},
    pages={1-1},
    abstract={This letter introduces a novel 2.6-GHz multipleinput and
    multiple-output (MIMO) antenna system for mobile terminals. The antenna
    structures consist of a broadband main antenna covering most of the 
    LTE-A bands, and a narrowband second antenna operating at the 2.6 GHz 
    band. The main antenna is a traditional monopole-type capacitive 
    coupling element (CCE) placed on the short edge of the terminal. The 
    second antenna consists of two - out-of-phase fed inductive coupling 
    elements (ICE) placed on the long edges of the chassis. The main purpose
    of the paper is to demonstrate that this kind of antenna system offers 
    good performance in terms of electromagnetic (EM) isolation and envelope
    correlation between the antennas. This has been experimentally verified
    and is originated on the fact that both antennas excite effectively 
    different orthogonal wavemodes, which is studied with the help of the 
    theory of characteristic wavemodes.},
    keywords={Capacitive coupling element;MIMO;characteristic modes theory;inductive coupling element;mobile antennas;multi-antenna system},
    doi={10.1109/LAWP.2013.2296147},
    ISSN={1536-1225},}
  3. Safin E. and Manteuffel, D. , "Influence of the Impedance Matching on the Chracteristic Wave Modes", In Antennas and Propagation (EUCAP), 2014 8th European Conference on., April 2014.
  4. Safin E., Valkonen R., and Manteuffel D., "Reconfigurable LTE MIMO Automotive Antenna System Based on the Characteristic Mode Analysis", in Antennas and Propagation (EUCAP), 2015 9th European Conference on., Lisbon, Portugal, April 13-17, 2015.