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  • 1.
    Li, Hui
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Decoupling and Evaluation of Multiple Antenna Systems in Compact MIMO Terminals2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Research on multiple antenna systems has been a hot topic in recent years due to the demands for higher transmission rate and more reliable link in rich scattering environment in wireless communications. Using multiple antennas at both the transmitter side and the receiver side increases the channel capacity without additional frequency spectrum and transmitted power. However, due to the limited space at the size-limited terminal devices, the most critical problem in designing multiple antennas is the severe mutual coupling among them. The aim of this thesis is to provide compact, decoupled and efficient multiple antenna designs for terminal devices. At the same time, we propose a simple and cost effective method in multiple antenna measurement. All these efforts contribute to the development of terminal devices for the fourth generation wireless communication.

    The background and theory of multiple antenna systems are introduced first, in which three operating schemes of multiple antenna systems are discussed. Critical factors influencing the performance of multiple antenna systems are also analyzed in details. To design efficient multiple antenna systems in compact terminals, several decoupling methods, including defected ground plane, current localization, orthogonal polarization and decoupling networks, are proposed. The working mechanism and design procedure of each method are introduced, and their effectiveness is compared. Those methods can be applied to most of the terminal antennas, reducing the mutual coupling by at least 6dB.

    In some special cases, especially for low frequency bands below 1GHz, the chassis of the device itself radiates like an antenna, which complicates the antenna decoupling. Thus, we extend the general decoupling methods to the cases when the chassis is excited. Based on the characteristic mode analysis, three different solutions are provided, i.e., optimizing antenna locations, localizing antenna currents and creating orthogonal modes. These methods are applied to mobile phones, providing a more reliable link and a higher transmission rate, which are evaluated by diversity gain and channel capacity, respectively.

    In order to measure the performance of multiple antenna systems, it is necessary to obtain the correlation coefficients. However, the traditional measurement technique, which requires the phase and polarization information of the radiation patterns, is very expensive and time consuming. In this thesis, a more practical and convenient method is proposed. Fairly good accuracy is achieved when it is applied to various kinds of antennas.

    To design a compact and efficient multiple antenna system, besides the reduction of mutual coupling, the performance of each single antenna is also important. The techniques for antenna reconfiguration are demonstrated. Frequency and pattern reconfigurable antennas are constructed, providing more flexibility to multiple antenna systems.

  • 2.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A compact reconfigurable antenna with pattern diversity2012In: Antennas and Propagation Society International Symposium (APSURSI), 2012 IEEE, IEEE , 2012, p. 6348776-Conference paper (Refereed)
    Abstract [en]

    A compact pattern reconfigurable antenna is proposed, in which the beam pattern can be switched between a monopole like radiation and a broadside radiation. Two layers and three PIN diodes are employed in the antenna. A single coaxial feed is utilized to excite both monopole mode and PIFA mode at a center frequency of 2.62 GHz. The total efficiencies of the antenna are 72% and 95% for monopole and PIFA modes, respectively. The envelope correlation coefficient calculated from the two patterns is 0.28, indicating good diversity performance of the antenna in wireless communications.

  • 3.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Lau, B. K.
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Angle and polarization diversity in compact dual-antenna terminals with chassis excitation2011In: URSI Gen. Assem. Sci. Symp., URSIGASS, 2011Conference paper (Refereed)
    Abstract [en]

    For multiple antennas on a compact terminal chassis, chassis current excitation can severely increase mutual coupling between the antennas. This is because the same characteristic mode of the chassis may be strongly excited by more than one antenna. Here, we propose to use one antenna that exploits chassis excitation and introduce a second antenna that minimizes chassis excitation. In this way, angle and polarization diversity can be realized. The concept is illustrated and verified through the design of a practical dual-antenna prototype at 920 MHz, which achieves an isolation of over 30 dB and total efficiencies of above 80%.

  • 4.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. Department of Electrical and Information Technology, Lund University, Sweden .
    Lau, B. K.
    Tan, Y.
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ying, Z.
    Impact of current localization on the performance of compact MIMO antennas2011In: Proceedings of the 5th European Conference on Antennas and Propagation, 2011, p. 2423-2426Conference paper (Refereed)
    Abstract [en]

    In this paper, we study the influence of current localization on the isolation between two antennas in a compact terminal setting. The two antennas are chosen to be a monopole and a PIFA. The degree of current localization is controlled by the permittivity value of the PIFA's dielectric loading. Both lossless and lossy cases are simulated in order to ascertain the underlying performance impact from current localization and its potential use in real implementation. Our results show that significant isolation enhancement is achieved with more localized currents. Moreover, the technique improves the terminal's diversity and capacity performance both at the center frequency and over a given bandwidth. In addition, PIFAs with dielectric loadings of higher permittivity values and more localized currents are physically smaller. Antenna prototypes are fabricated, and the measured results agree well with the simulated results.

  • 5.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. Department of Electrical and Information Technology, Lund University, Sweden .
    Lau, B. K.
    Ying, Z.
    Optimal multiple antenna design for compact MIMO terminals with ground plane excitation2011In: Final Program Book Abstr. - iWAT: IEEE Int. Workshop Antenna Technol.: Small Antennas, Novel Struct. Innovative Metamaterials, 2011, p. 218-221Conference paper (Refereed)
    Abstract [en]

    The compactness of mobile terminals complicates the design of multiple antennas, since coupling among the antennas increases when they are placed in proximity of one another. While it is possible to mitigate coupling between closely spaced antennas, a tradeoff in bandwidth is required. In this paper, we highlight ground plane excitation as an additional dimension to consider in the design of multiple antenna terminals. This is because a compact ground plane, especially in mobile application, can interact with the antenna elements and contribute significantly to their radiation characteristics. Our results show that several design parameters, namely element locations, spacing between elements, and radiation characteristics of individual elements, must be jointly considered in order to achieve the optimal MIMO performance tradeoff for compact multiple antenna terminals.

  • 6.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Lau, Buon Kiong
    Tan, Yi
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A pattern reconfigurable antenna array for multiple antenna systemsArticle in journal (Other academic)
  • 7.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Lau, Buon Kiong
    Ying, Zhinong
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Decoupling of Multiple Antennas in Terminals With Chassis Excitation Using Polarization Diversity, Angle Diversity and Current Control2012In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 60, no 12, p. 5947-5957Article in journal (Refereed)
    Abstract [en]

    Excitation of the chassis enables single-antenna terminals to achieve good bandwidth and radiation performance, due to the entire chassis being utilized as the main radiator. In contrast, the same chassis excitation phenomenon complicates the design of multiple antennas for MIMO applications, since the same characteristic mode of the chassis may be effectively excited by more than one antenna, leading to strong mutual coupling and severe MIMO performance degradation. In this paper, we introduce a design concept for MIMO antennas to mitigate the chassis-induced mutual coupling, which is especially relevant for frequency bands below 1 GHz. We illustrate the design concept on a dual-antenna terminal at 0.93 GHz, where a folded monopole at one chassis edge excites the chassis' fundamental electric dipolemode and a coupled loop at the other chassis edge excites its own fundamental magnetic dipole mode. Since the two radiationmodes are nearly orthogonal to each other, an isolation of over 30 dB is achieved. Moreover, we show that the antenna system can be conveniently modified for multiband operation, such as in the 900/1800/2600MHz bands. Furthermore, by controlling the phase of the feed current on the folded monopole, the two antennas can be co-located on the same chassis edge with an isolation of over 20 dB. The co-located dual-antenna prototype was fabricated and verified in the measurements.

  • 8.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Lin, XianQi
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Lau, B. K.
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Calculating signal correlation in lossy dipole arrays using scattering parameters and efficiencies2013In: 2013 7th European Conference on Antennas and Propagation, EuCAP 2013, EurAAP , 2013, p. 519-523Conference paper (Refereed)
    Abstract [en]

    Correlation coefficient, as a critical performance metric of multiple antenna systems, can be calculated from the 3D radiation patterns of the antennas for multipath environments with uniform 3D angular power spectrum. A simpler, faster and lower cost approach uses the antennas' scattering parameters to determine correlation coefficient. However, the method assumes lossless antennas, and hence only achieves good accuracy for antennas with high radiation efficiencies. In this work, a method for calculating correlation coefficients in lossy dipole arrays is proposed, using only the scattering parameters and radiation efficiencies. The method is based on the equivalent circuit of antennas, and it gives a significantly better estimate of the correlation coefficient than the existing methods. The proposed method is also applied to two folded monopole antennas in a compact terminal to demonstrate its effectiveness.

  • 9.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Lin, Xianqi
    Lau, Buon Kiong
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Calculation of signal correlation in lossy antenna arrays using scattering matrixArticle in journal (Other academic)
  • 10.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Tan, Yi
    Lau, Buon Kiong
    Ying, Zhinong
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Characteristic Mode Based Tradeoff Analysis of Antenna-Chassis Interactions for Multiple Antenna Terminals2012In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 60, no 2, p. 490-502Article in journal (Refereed)
    Abstract [en]

    The design of multiple antennas in compact mobile terminals is a significant challenge, due to both practical and fundamental design tradeoffs. In this paper, fundamental antenna design tradeoffs of multiple antenna terminals are presented in the framework of characteristic mode analysis. In particular, interactions between the antenna elements and the characteristic modes and their impact on design tradeoffs are investigated in both theory and simulations. The results reveal that the characteristic modes play an important role in determining the optimal placement of antennas for low mutual coupling. Moreover, the ability of antenna elements to localize the excitation currents on the chassis can significantly influence the final performance. To demonstrate the effectiveness of the proposed approach, a dual-band, dual-antenna terminal is designed to provide an isolation of over 10 dB for the 900 MHz band without additional matching or decoupling structures. A tradeoff analysis of bandwidth, efficiency, effective diversity gain and capacity is performed over different antenna locations. Finally, three fabricated prototypes verify the simulation results for representative cases.

  • 11.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Tan, Yi
    Lau, Buon Kiong
    Ying, Zhinong
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Isolation enhancement of compact MIMO antennas with current localizationArticle in journal (Other academic)
  • 12. Li, Hui
    et al.
    Xiong, J.
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Extremely compact dual-band PIFAs for MIMO application2009In: Electronics Letters, ISSN 0013-5194, E-ISSN 1350-911X, Vol. 45, no 17, p. 869-U20Article in journal (Refereed)
    Abstract [en]

    A MIMO structure consisting of two extremely compact planar inverted-F antennas (PIFAs) (lambda/60 x lambda/20 x lambda/6) is proposed for GSM 900 MHz and 2.4 GHz wireless local area network applications. The miniaturisation of each antenna is achieved through a spiral-shaped PIFA and a capacitive load, forming a LC resonator. Both antennas have a good impedance bandwidth of 0.88-0.945 and 2.39-2.48 GHz for 0.9 and 2.4 GHz band, respectively. The two PIFA antennas are polarised orthogonally to each other, and isolation better than 228 dB is achieved in both bands without any structure etched on the ground. The predicted results are compared with the measured data and good agreement is found.

  • 13. Li, Hui
    et al.
    Xiong, J.
    Ying, Z.
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Compact and low profile co-located MIMO antenna structure with polarisation diversity and high port isolation2010In: Electronics Letters, ISSN 0013-5194, E-ISSN 1350-911X, Vol. 46, no 2, p. 108-109Article in journal (Refereed)
    Abstract [en]

    A compact co-located MIMO structure with high port isolation is proposed for 2.4 GHz WLAN application. The antenna structure consists of a proximity-coupled square ring patch antenna, and a probe fed planar inverted F antenna co-located inside. The operating frequency of the two radiators is designed to be at the same frequency, and a high port isolation below -25 dB is achieved with an extremely narrow edge to edge inter-antenna space of 1 mm (0.008 lambda). The total lateral size of the co-located radiators is only 0.28 lambda x 0.28 lambda, i.e. only about 25% of the size of a half-wavelength dual-port patch antenna. The MIMO structure gives high gains (> 4.0 dB), low correlation (< 0.2), and high diversity gain (about 10 dB).

  • 14. Li, Hui
    et al.
    Xiong, Jiang
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A Compact Planar MIMO Antenna System of Four Elements With Similar Radiation Characteristics and Isolation Structure2009In: IEEE Antennas and Wireless Propagation Letters, ISSN 1536-1225, E-ISSN 1548-5757, Vol. 8, p. 1107-1110Article in journal (Refereed)
    Abstract [en]

    A compact planar multiple-input-multiple-output (MIMO) antenna system of four elements with similar radiation characteristics is proposed for the whole 2.4-GHz WLAN band. It consists of two proximity-coupled fed microstrip square ring patch antennas and two lambda/4 microstrip slot antennas of the same linear polarization. These two types of antennas are printed on different sides of the substrate to reduce mutual coupling. With a novel isolation structure etched on the ground plane of the FR4 substrate, high port isolation (below -25 dB) and good MIMO performance are achieved. The overall lateral size of the MIMO system is only 0.64 lambda X 0.48 lambda, and good impedance matching (S11 < 10 dB) is achieved across the operating band for all the antenna elements. Full spherical radiation patterns are measured for the MIMO system, showing similar radiation characteristics, and the gains are above 2.3 dB across the operating band.

  • 15.
    Li, Hui
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Xiong, Jiang
    Yu, Yufeng
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A Simple Compact Reconfigurable Slot Antenna With a Very Wide Tuning Range2010In: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 58, no 11, p. 3725-3728Article in journal (Refereed)
    Abstract [en]

    A simple and compact slot antenna with a very wide tuning range is proposed. A 25 mm (roughly equal to lambda(H)/8, where lambda(H) corresponds to the highest frequency of the tuning range) open slot is etched at the edge of the ground. To achieve the tunability, only two lumped elements, namely, a PIN diode and a varactor diode are used in the structure. By switching the PIN diode placed at the open end of the slot, the slot antenna can resonate as a standard slot (when the switch is on) or a half slot (when the switch is off). Continuous tuning over a wide frequency range in those two modes can be achieved by adjusting the reverse bias (giving different capacitances) of the varactor diode loaded in the slot. Through optimal design, the tuning bands of the two modes are stitched together to form a very wide tuning range. The fabricated prototype has a tuning frequency range from 0.42 GHz to 1.48 GHz with S11 better than -10 dB, giving a frequency ratio (f(R) = f(u)/f(L)) of 3.52:1. The measured full spherical radiation patterns show consistent radiation characteristics of the proposed antenna within the whole tuning range.

  • 16.
    Lin, XianQi
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Li, Hui
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Fan, Y.
    A decoupling technique for increasing the port isolation between two closely packed antennas2012In: Antennas and Propagation Society International Symposium (APSURSI), 2012 IEEE, IEEE , 2012, p. 6349240-Conference paper (Refereed)
    Abstract [en]

    A general decoupling technique for increasing the port isolation between two closely packed antennas is proposed in this paper. The two antennas are first modeled by a π-shaped circuit from its S-parameters. Then, the detailed decoupling method is discussed to increase the port isolation where an optimization objective of maximum energy unbalance is adopted. Taking two closely parked patch antennas for example, good agreements between the theoretical analysis and full wave simulation are obtained. The present technique can increase the port isolation, thus the total efficiency, correlation coefficient and diversity gain are improved. Such a general decoupling technique can also be applied to any other antenna types.

  • 17. Xiong, J.
    et al.
    Wang, B. -Z
    Zhao, M.
    Li, Hui
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ying, Z.
    Efficient radiating omnidirectional loops for collocated polarization diversity MIMO dipoles2012In: Antennas and Propagation (APCAP), 2012 IEEE Asia-Pacific Conference on, IEEE , 2012, p. 19-20Conference paper (Refereed)
    Abstract [en]

    Design and implementation of efficient radiating omnidirectional loops have been studied. A modified shielded loop with four feeding sections has been proposed. Measured results show that good impedance matching, omnidirectional radiation pattern, and good radiation efficiency have been achieved. A combination of the proposed loop and a half-wavelength E-dipole in a collocated position has also been examined. Extremely low correlation achieved indicates that this antenna can be used as a reference for evaluating the MIMO performance of future collocated polarization diversity based antennas, and a measurement antenna for measuring the polarization at the mobile as well.

  • 18. Xiong, Jiang
    et al.
    Zhao, Mingyu
    Li, Hui
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ying, Zhinong
    Wang, Bingzhong
    Collocated Electric and Magnetic Dipoles With Extremely Low Correlation as a Reference Antenna for Polarization Diversity MIMO Applications2012In: IEEE Antennas and Wireless Propagation Letters, ISSN 1536-1225, E-ISSN 1548-5757, Vol. 11, p. 423-426Article in journal (Refereed)
    Abstract [en]

    In this letter, a magnetic dipole (M-dipole) and an electric dipole (E-dipole) are designed to form a collocated polarization-diversity-based dual-antenna system. The M-dipole is a modified shielded loop with multiple feed, and the E-dipole is a traditional lambda/2 dipole. Measured results show that both radiators have ideal impedance matching and port isolation and identical omnidirectional pattern with perfect orthogonal polarization at 860 MHz. The upper bound of the envelope correlation coefficient rho(emax), with the measured antenna efficiency taken into account, is only 0.018, and it is the lowest value when compared to some previous publications. Excellent effective diversity gain and channel capacity are also achieved at the operating frequency. The proposed dual-antenna system can mainly be used as a reference antenna for evaluating the performance of future collocated polarization-diversity-based multiple-input-multiple-output (MIMO) antennas.

  • 19. Yu, Yufeng
    et al.
    Xiong, Jiang
    Li, Hui
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    An Electrically Small Frequency Reconfigurable Antenna With a Wide Tuning Range2011In: IEEE Antennas and Wireless Propagation Letters, ISSN 1536-1225, E-ISSN 1548-5757, Vol. 10, p. 103-106Article in journal (Refereed)
    Abstract [en]

    An electrically small frequency-reconfigurable antenna with a very wide tuning band is proposed. Three varactor diodes are used to achieve the tunable capacitance. With a modified feeding structure, the measured tuning range of the fabricated antenna reaches from 457.5 to 894.5 MHz, and the tuning band enhancement is also explained through an equivalent circuit analysis.

1 - 19 of 19
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