Wireless Communications

Small Terminal Multi Antenna Systems based on the Theory of Characteristic Modes

Research grant by DFG (German Research Foundation)

Timeframe: 01.12.2010 - 30.11.2013

DFG MA 4981/1-1:

MIMO (Multiple Input Multiple Output) is a technique to utilize multi antenna systems to increase the capacity of modern communication systems. The integration of multiple low correlated antennas is especially challenging if we look at small terminal, such as mobile phones. The Theory of Characteristic Modes describes the current distribution on a conducting body as a superposition of several orthogonal modes. Our research aims at utilizing these modes on the chassis of a small terminal to realize a built-in multi-port antenna system. Such a concept is highly desired for next generation LTE (Long Term Evolution) mobile terminals and WLAN IEEE802.11.n devices.

Therefore, the surface current distribution J (the entire radiation pattern |E|) of an small terminal can be decomposed into a set of orthogonal characteristic modes Jn (orthogonal radiation pattern of the modes |EJn|):

mimo_modes1

Within this project we already designed a 3-port MIMO (Multiple Input Multiple Output) antenna on a small terminal, such as smart phone based on the selective excitation of orthogonal chassis modes. The selective excitation of these modes is realized by sets of non-resonant inductive coupling elements integrated at predefined positions into the PCB (Printed Circuit Board) layout. Stripline feed networks, also integrated within the seven-layer PCB layout care for a good impedance match to 50 Ω at an operating frequency f = 2.5 GHz. The concept presents a straight forward application of the TCM and is therefore of general interest for the design of small terminal antennas. Furthermore, due to the electrically small couplers it requires only a small area at the outer edge of the PCB. The concept represents an interesting solution with the clear potential to be extended to more ports.

mimo_modes2

Further research will be directed in a practical realization of this 3-port MIMO antenna system and its measurements in our anechoic chamber.  (link)

 

Publications:
 
  1. Manteuffel, D. , "MIMO Antenna Design Challenges", In Antennas Propagation Conference, 2009. LAPC 2009. Loughborough., nov., 2009. , pp. 50 -56. DOI
    [Abstract]
     
    This paper provides a brief introductions into the challenges involved when designing antenna for MIMO (Multiple Input Multiple Output) systems on small terminals. The investigation is based on the fundamental parameter of cross-correlation of the antenna transfer functions. Some considerations on the correlation of antennas implemented on small terminals are undertaken. It is shown that the correlation of the elements of a multiple antenna system which is implemented on a small terminal is significantly affected by the integration impairments and the platform itself.
    [BibTeX]
     
    @inproceedings{5352587,
      author = {Manteuffel, D.},
      title = {MIMO antenna design challenges},
      booktitle = {Antennas Propagation Conference, 2009. LAPC 2009. Loughborough},
      year = {2009},
      pages = {50 -56},
      doi = {10.1109/LAPC.2009.5352587}
    }

  2. Martens, R. and Manteuffel, D. , "Element Correlation of MIMO Antennas on Small Terminals", In Antennas and Propagation (EuCAP), 2010 Proceedings of the Fourth European Conference on., april, 2010. , pp. 1 -5. DOI
    [Abstract]
     
    This paper aims at investigating some significant integration aspects involved in daily practical mobile terminal design and their impact on the correlation coefficient of multiple antenna elements. While the antenna elements itself are kept as simple as possible the influence of its location on the terminal platform is investigated systematically. The investigations show, that the influence of the integration is significant on the one hand. On the other hand the general physical behaviour can be easily estimated and can also be used to find simple design rules for closely spaced de-correlated antennas on small terminals.
    [BibTeX]
     
    @inproceedings{5505858,
      author = {Martens, R. and Manteuffel, D.},
      title = {Element correlation of MIMO antennas on small terminals},
      booktitle = {Antennas and Propagation (EuCAP), 2010 Proceedings of the Fourth European Conference on},
      year = {2010},
      pages = {1 -5}
    }

  3. Martens, R., Safin, E. and Manteuffel, D. , "On the Relation Between the Element Correlation of Antennas on Small Terminals and the Characteristic Modes of the Chassis", In Antennas and Propagation Conference (LAPC), 2010 Loughborough., nov., 2010. , pp. 457 -460. DOI
    [Abstract]
     
    In this paper we present some basic investigations to show the relation between the element correlation of antennas on a common small terminal and the characteristic modes on the chassis excited by those antennas. The results show, that the characteristic of an antenna array is heavily affected by the weighted characteristic current modes of the chassis. The actual mode and its weight, however, depend on the dimensions of the chassis and the locations of the antenna elements for a given frequency. In case of a two antenna systems the effect of the common ground plate on the element correlation can be minimized by locating the antennas in such a way that both excite the same mode 180 #x00B0; out of phase.
    [BibTeX]
     
    @inproceedings{5666206,
      author = {Martens, R. and Safin, E. and Manteuffel, D.},
      title = {On the relation between the element correlation of antennas on small terminals and the characteristic modes of the chassis},
      booktitle = {Antennas and Propagation Conference (LAPC), 2010 Loughborough},
      year = {2010},
      pages = {457 -460},
      doi = {10.1109/LAPC.2010.5666206}
    }

  4. Manteuffel, D. and Martens, R. , "A Concept for MIMO Antennas on Small Terminals Based on Characteristic Modes", In Antenna Technology (iWAT), 2011 International Workshop on., march, 2011. , pp. 17 -20. DOI
    [Abstract]
     
    In this paper, we present a concept for a MIMO antenna system on a small terminal platform. The concept is based on the selective excitation of different orthogonal characteristic modes on the chassis of the small terminal. Thereby the chassis of the mobile terminal itself can be effectively used as the MIMO antenna similar to the concept commonly applied to single port (single mode) antennas on mobile terminals. The different modes generate orthogonal radiation patters and are therefore suited to radiate de-correlated signals fed to the different antenna ports. Such modes can be excited selectively by certain sets of couplers distributed around the outer edges of the chassis. First stage numerical investigations show that coupling between the modes is low.
    [BibTeX]
     
    @inproceedings{5752386,
      author = {Manteuffel, D. and Martens, R.},
      title = {A concept for MIMO antennas on small terminals based on characteristic modes},
      booktitle = {Antenna Technology (iWAT), 2011 International Workshop on},
      year = {2011},
      pages = {17 -20},
      doi = {10.1109/IWAT.2011.5752386}
    }

  5. Martens, R., Safin, E. and Manteuffel, D. , "Selective Excitation of Characteristic Modes on Small Terminals", In Antennas and Propagation (EUCAP), Proceedings of the 5th European Conference on., april, 2011. , pp. 2492 -2496. DOI
    [Abstract]
     
    In this paper we present a concept for the selective excitation of different orthogonal modes of a small terminal chassis. The concept can be used to realize an efficient multi port antenna system in order to enable MIMO (Multiple Input Multiple Output) for small terminals. The idea is based on the TCM (Theory of Characteristic Modes). The selective excitation of these modes is achieved by sets of couplers that are located at specific positions on the terminals' PCB (Printed Circuit Board). Passive matching circuits care for a good impedance match to 50 #x03A9;. As a result it is demonstrated that nearly perfect de-correlation of the ports of the multi antenna system can be achieved.
    [BibTeX]
     
    @inproceedings{5782054,
      author = {Martens, R. and Safin, E. and Manteuffel, D.},
      title = {Selective excitation of characteristic modes on small terminals},
      booktitle = {Antennas and Propagation (EUCAP), Proceedings of the 5th European Conference on},
      year = {2011},
      pages = {2492 -2496}
    }

  6. Martens, R., Safin, E. and Manteuffel, D. , "Inductive and Capacitive Excitation of the Characteristic Modes of Small Terminals", In Antennas and Propagation Conference (LAPC), 2011 Loughborough., nov., 2011. , pp. 1 -4. DOI
    [Abstract]
     
    In this paper we present concepts for the selective excitation of orthogonal modes of small terminals by inductive and capacitive coupling elements. The concepts can be used to implement an efficient MIMO (Multiple Input Multiple Output) antenna system based on the TCM (Theory of Characteristic Modes). Both coupling concepts are discussed with respect to their ability to excite specific modes separately. An important result obtained is that inductive couplers offer advantages over capacitive couplers in exciting the desired modes purely even when they are not close to their modal resonance.
    [BibTeX]
     
    @inproceedings{6114141,
      author = {Martens, R. and Safin, E. and Manteuffel, D.},
      title = {Inductive and capacitive excitation of the characteristic modes of small terminals},
      booktitle = {Antennas and Propagation Conference (LAPC), 2011 Loughborough},
      year = {2011},
      pages = {1 -4},
      doi = {10.1109/LAPC.2011.6114141}
    }

  7. Safin, E., Martens, R. and Manteuffel, D. , "Modal Source Reconstruction Based on Radiated Far-Field for Antenna Design", In Antennas and Propagation (EUCAP), 2012 6th European Conference on., march, 2012. , pp. 1645 -1649. DOI
    [Abstract]
     
    We present a mathematical technique to reconstruct the modal current distribution on arbitrary antennas from the radiated far-field and general knowledge about the modes involved. The method leads to good results if the radiation mechanism of the actual antenna can be approximated by the current distribution on a simplified structure. The current distribution of the simplified structure is decomposed into its characteristic modes and the related modal far-field is calculated. Assuming that the far-field of the actual antenna contains the same modes, their weighting coefficients are calculated by comparing the far-field of the actual antenna to the modal far-field of the simplified structure. It can be shown that the weighting coefficients of all significant modes can be reconstructed with good accuracy.
    [BibTeX]
     
    @inproceedings{6205814,
      author = {Safin, E. and Martens, R. and Manteuffel, D.},
      title = {Modal source reconstruction based on radiated far-field for antenna design},
      booktitle = {Antennas and Propagation (EUCAP), 2012 6th European Conference on},
      year = {2012},
      pages = {1645 -1649},
      doi = {10.1109/EuCAP.2012.6205814}
    }

  8. Martens, R. and Manteuffel, D. , "A Feed Network for the Selective Excitation of Specific Characteristic Modes on Small Terminals", In Antennas and Propagation (EUCAP), 2012 6th European Conference on., march, 2012. , pp. 1842 -1846. DOI
    [Abstract]
     
    We investigate different strategies to realize a feed network for a set of ICEs (Inductive Coupling Elements) on a small terminal for the selective excitation of characteristic modes. The inductive excitation is realized by embedding small rectangular loops into the PCB (Printed Circuit Board) of the terminal. Due to the small dimensions of the ICEs the input impedance of the elements is highly reactive. Therefore, additional matching networks are required. The trade-offs of lumped elements versus distributed transmission lines to match and combine the elements of each set of couplers are investigated. An important result obtained is that the feed network with double-stub tuning offers advantages compared to the feed network with lumped SMD (Surface Mountable Device) components with respect to the efficiency of the entire antenna systems.
    [BibTeX]
     
    @inproceedings{6205813,
      author = {Martens, R. and Manteuffel, D.},
      title = {A feed network for the selective excitation of specific characteristic modes on small terminals},
      booktitle = {Antennas and Propagation (EUCAP), 2012 6th European Conference on},
      year = {2012},
      pages = {1842 -1846},
      doi = {10.1109/EuCAP.2012.6205813}
    }

  9. R. Martens, D. Manteuffel , "2-Port Antenna Based on the Selective Excitation of Characteristic Modes", In APS 2012 - IEEE International Symposium on Antennas and Propagation 2012. Chicago, July, 2012. DOI
    [Abstract]
     
    In this paper we present a two-port MIMO (Multiple Input Multiple Output) - antenna based on the selective excitation of two orthogonal modes on a small terminal. The concept applies the TCM (Theory of Characteristic Modes). The excitation of the chassis modes are realized by two sets of ICEJn (Inductive Coupling Elements) at predefined positions on a commonly used PCB. Feed Networks (FNJn) care for a good impedance match to 50 Ohm with double-stub tuning and phase shifters realized in stripline technology. Numerical results show a good de-correlation of the antenna ports and a high quality of mode excitation.
    [BibTeX]
     
    @inproceedings{,
      author = {R. Martens, D. Manteuffel},
      title = {2-Port Antenna Based on the Selective Excitation of Characteristic Modes},
      booktitle = {APS 2012 - IEEE International Symposium on Antennas and Propagation 2012},
      year = {2012},
      note = {accepted for Publication}
    }

  10. Manteuffel, D. and Martens, R. , "Improved Radiation Pattern Measurements of MIMO Handheld Mobile Terminals", In Antennas and Propagation (EUCAP), 2013 7th European Conference on., April 2013.
    [Abstract]
     
    In this paper we report on our experience made when measuring different radiation patterns of a multi-antenna handheld terminal. In order to avoid unwanted effects from surface waves on a connecting cable we equipped the terminal with a programmable wireless transmit module. Parasitic effects due to the presence of the device holder of the positioner in the anechoic chamber are analyzed by numerical simulations resulting in recommendations for optimal positioning. }

  11. Safin, E., Martens, R., Capek M., Eichler, J., Hazdra, P., Manteuffel, D., and Mazanek M., "Discussion and Evaluation of Modal and Radiation Q-Factors for MIMO Antennas Based on Theory of Characteristic Modes", COST IC1102, 3. WG Meeting & Technical Workshop 2012, Thessaloniki, June, 2013. DOI
    [Abstract]
     
    TThe effort to integrate one or more antennas (e.g. for the realization of a MIMO antenna systems) in a small terminal is a challenge that is barely to cope. Due to the spatial limited structure some antenna performance measures, such as the Q-factor is affected by the integration impairments of the small terminal. The TCM (Theory of Characteristic Modes) provides one possible approach in order to understand the impact of the small terminal and the effects on the overall radiation. The experiences of CAU (Kiel) are related to the selective excitation of modes concerning MIMO applications and CTU Prague has a fundamental knowledge on the Q-factor calculation. Both participants (CAU and CTU Prague) have developed different in-house tools to calculate the characteristic modes of a radiating structure. The combination of both inhouse tools can be used to characterize the entire performance (Q-factor) of the MIMO antennas based on the TCM.
    [BibTeX]
     
    author={Safin, E., Martens, R., Capek M., Eichler, J., Hazdra, P., Manteuffel, D., and Mazanek M.},
      booktitle={COST IC1102, 3. WG Meeting & Technical Workshop 2013},  
      title={Discussion and Evaluation of Modal and Radiation Q-Factors for MIMO Antennas Based on Theory of Characteristic Modes},
      year={2013},
      month={June},
      volume={},
      number={},
      pages={},
      keywords={},
      doi={},
      ISSN={},}
  12. Martens, R., Holopainen, J. , Safin, E., and Manteuffel, D., "Small Terminal Multi-Antenna System Based on Two Different Types of Non-Resonant Coupling Elements", COST IC1102, 4. WG Meeting & Technical Workshop 2013, Ghent, Sept. 2013. DOI
    [Abstract]
     
    This paper introduces a 2.6-GHz MIMO antenna system for mobile terminals. The antenna
    structures consists of a broadband main antenna (operation frequencies 0.7-0.96 and 1.7-3.5 GHz),
    and a secondary 2.6-GHz MIMO antenna. The main antenna consists of a traditional monopole-type
    capacitive coupling element (CCE) placed at the end of the terminal, and the secondary MIMO
    antenna consists of two out-of-phase fed inductive coupling elements (ICE) placed on the long edges
    of the chassis.
    A Characteristic Mode [1] analysis using an in-house software [2] shows that the operation of the CCE
    at the low-band is based on the effective excitation of the lowest-order (half-wave) characteristic
    wavemode of the chassis, which radiates more than 90% of the total radiation of the main antenna
    below 1 GHz. Furthermore, the location of the feeding pin was optimized in such a way that it creates
    two self-resonances at the higher band. Then, the secondary MIMO antenna is designed in such a way
    that it does not excite effectively the dominant wavemodes of the main antenna structure. Five of the
    most dominant characteristic wavemodes excited by the main antenna at 2.6 GHz. Those five modes
    are responsible for up to 80% of the total radiation of the main antenna. The remaining 20% is
    radiated by the "minor" characteristic modes, from which the sixth mode radiates 6%, the seventh 3%,
    the eighth 1%, and the rest modes less than 1% each. As a general finding it can be concluded that the
    characteristic modes of the chassis are significantly affected by the loading caused by the different
    coupling elements and has to be taken into account in the optimization of the overall sytem.
    Both coupling elements require additional matching networks to be matched to 50 Ω in the desired
    frequency bands. In addition the set of inductive couplers requires a feed network combining its
    elements and adjusting their phased excitation. The impedance matching for the main (CCE) antenna
    is implemented with the help of a dual-resonant matching circuitry for the low-band operation (LB),
    and with the combination of the L-section matching circuitry and self-resonances for the high-band
    operation (HB).
    Low antenna-to-antenna coupling as well as a low envelope correlation of the antenna trasfer
    functions can be obtained due to the concept based on orthogonal wave mode excitation. However,
    the matching networks degrade the radiation efficiency of the antennas. It can be observed that this is
    less severe for the capacitive coupling element using low cost SMD elements.
    [BibTeX]
     
    author={Martens, R., Holopainen, J. , Safin, E., Manteuffel, D.},
      booktitle={COST IC1102, 4. WG Meeting & Technical Workshop 2013},  
      title={Small Terminal Multi-Antenna System Based on Two Different Types of Non-Resonant Coupling Elements},
      year={2013},
      month={Sept.},
      volume={},
      number={},
      pages={},
      keywords={},
      doi={},
      ISSN={},}