Oct 27, 2011 - hand in data-mode grip and two hands in browsing-mode grip, is pre- ... antenna elements at every channel realisation, achieves 2 dB higher.
Multi-antenna mobile terminal diversity performance in proximity to human hands under different propagation environment conditions A.A.H. Azremi, V. Papamichael and P. Vainikainen The effective diversity gain (EDG) of a four-element planar inverted-F antenna array, which operates at 3500 MHz in the proximity to one hand in data-mode grip and two hands in browsing-mode grip, is predicted using a recently developed hybrid stochastic-electromagnetic methodology. The dependency of the EDG results on the distorted radiation patterns due to the hands and on the cross-polarisation power ratio and incident wave parameters of the propagation environment is demonstrated. The results indicate that the generalised selection combining technique, i.e. by using only the best two out of four antenna elements at every channel realisation, achieves 2 dB higher EDG than the selection combining and only 1 dB less than the maximal ratio combining techniques.
mutual coupling is obtained between any two elements out of the fourelement structure. The studied multi-antenna structure is oriented as in Figs. 1c and d according to the hand grip scenarios. Results: The simulation results of the scattering parameters for all antenna elements are well-matched at 3500 MHz with the impedancematching criterion S11 ≤ 26 dB, and they exhibit good isolation levels between 248 and 217 dB among all antenna elements. In the proximity of the human hands, the individual element radiation efficiencies are degraded significantly owing to the absorption by the hand as shown in Table 1, and the gain patterns are distorted (Fig. 2). The hands were disembodied to neglect the effects of a human body, and free from any additional conductors that might affect the amount of electromagnetic field radiated from the multi-antenna structures. The shadowing impact on the antenna gain is observed around 3308 in data-mode grip, and around 908 and 2708 in browsing-mode grip. (+x)
f
Introduction: Implementation of multi-antenna elements in small terminal devices has become a significant challenge [1]. Instead of increasing the spectral efficiency, antenna designers also have to deal with mutual coupling that is proportional to the number of antenna elements, as well as considering the variable effect of the hand on the mobile terminal performance metrics [2– 4]. The radiation patterns are significantly distorted owing to mutual coupling, and even more affected when the terminal is in the presence of a human hand. The grip of the hand is of great importance [5], especially with the current trend of people holding a terminal using either one or two hands at a time. Furthermore, the propagation environment conditions and polarisation, which are diverse in mobile communication systems, also have to be taken into consideration. Many studies have been conducted, either numerically or experimentally, on clarifying the effects of the hand on multi-antenna performance metrics, such as envelope correlation, mean effective gain, power imbalance, diversity gain and multipleinput multiple-output (MIMO) channel capacity [2 – 4, 6], but relatively little research provides essential information on several diversity combining schemes with different propagation environment conditions in the proximity of the realistic human hand grips. short
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Fig. 2 Normalised azimuthal gain patterns when antenna element 1 is excited ——— gain (horizontal) --------- gain (vertical) a No hand b Data-mode grip c Browsing-mode grip
Table 1: Element radiation efficiency results for different scenarios at 3500 MHz Element No hand (%) Data-mode (%) Browsing-mode (%) 1 96.0 38.7 25.8 2 96.0 55.7 29.1 3 96.0 20.7 25.8
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single antenna Rayleigh MRC no hand GSC no hand SC no hand MRC data-mode GSC data-mode SC data-mode MRC browsing-mode GSC browsing-mode SC browsing-mode
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mv-mh=75º sv-sh=30º XPR=0 dB
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MRC no hand GSC no hand SC no hand MRC data-mode GSC data-mode SC data-mode MRC browsing-mode GSC browsing-mode SC browsing-mode
a Multi-antenna configuration with location of feeding ports and shorting strips b Dimension of structure in millimetres c Multi-antenna with hand in data-mode grip d Multi-antenna (rotated 908 towards xy-plane) with hands in browsing-mode grip
Multi-antenna structure: In this study, a four-element planar inverted-F antenna (PIFA) array structure which operates at 3500 MHz with a size of 9.8 × 9.8 mm, was designed and evaluated. The height between the ground plane and the radiating plate is 5 mm and the separation between shorting and feeding plates is 1 mm. The size of the ground plane is a typical 100 × 40 mm mobile terminal chassis size. The configuration and dimensions are shown in Figs. 1a and b. The PIFA array elements were located away from the chassis edges to improve antenna isolation, with D ¼ 0.74l, S ¼ 0.14l, X ¼ 0.05l, G ¼ 0.11l and Y ¼ 0.75l. The element locations were optimised such that an inherent stopband of
effective diversity gain, dB
Fig. 1 Multi-antenna under test and hand models 14 12 10 8
mv-mh=60º, sv-sh=15º mv-mh=80º, sv-sh=15º mv-mh=80º, sv-sh=45º
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Fig. 3 Diversity combining performance of multi-antenna structure with different hand operations a Cumulative distribution functions with XPR ¼ 0dB, mv ¼ mh ¼ 758, and sv ¼ sh ¼ 308 b Diversity gain against XPR, mean values and angular spreads
ELECTRONICS LETTERS 27th October 2011 Vol. 47 No. 22
The method adopted in order to evaluate the diversity performance of the mobile terminal is a recently developed one, which has been presented in [7]. Here, instead of generating a uniform propagation environment, we assume a uniform distribution for the azimuth angle w, a Gaussian distribution for the elevation angle u and several values for the cross-polarisation power ratio (XPR). Fig. 3a shows the cumulative distribution function (CDF) of relative signal-to-noise ratio (SNR) for all hand scenarios with chosen typical indoor and outdoor propagation incident wave parameters [8]. In the proximity of the human hand, the highest performance from investigated diversity schemes for all hand scenarios at 1% probability level is obtained through the maximal ratio combining (MRC) technique, with 1 dB higher than the generalised selection combining (GSC) technique. However, only with the use of the best two antenna elements at every channel realisation, the GSC technique achieves 2 dB higher than the SC technique for all cases. Therefore, with less use of antenna elements even in the two hands browsing-mode grip, an EDG of at least 8.6 dB over a Rayleigh single antenna reference can be obtained. In Fig. 3b, the EDG for all hand scenarios are estimated as a function of XPR. Generally for the case of the multi-antenna mobile terminal in free space and in the data-mode grip, the EDGs are decreased according to the increase of absolute XPR value. However, this is not the case for the browsing-mode grip, where the EDG increased and reached the same performance as the data-mode grip at high XPR value, i.e. at XPR ¼ 10 dB. This is due to the dominating distorted vertical-polarised gain pattern in the browsing-mode grip. Therefore, it is of paramount importance to take into account the distorted radiation patterns due to the human hands before the estimation of the diversity performance. The effects of mean value and angular spread across all XPR values are minimal and only up to a few tenths of a dB. Conclusions: The effective diversity gain under SC, MRC and GSC of a four-element PIFA array with high isolation between antenna elements has been evaluated using a hybrid stochastic-electromagnetic methodology in different realistic hand grip scenarios under various propagation environment conditions. The human hands have a negative influence on EDGs, which is mostly attributed to the dramatic drop of the antenna radiation efficiencies. Moreover, it was found that the XPR value has important influence on EDGs owing to the power imbalance between the partial power gain patterns. On the other hand, the effect of the incident wave parameters of the propagation environment on the EDG performance is almost negligible.
# The Institution of Engineering and Technology 2011 16 September 2011 doi: 10.1049/el.2011.2936 One or more of the Figures in this Letter are available in colour online. A.A.H. Azremi and P. Vainikainen (Department of Radio Science and Engineering, Aalto University School of Electrical Engineering, Aalto 00076, Finland) E-mail: abdullah.azremi@aalto.fi V. Papamichael (Department of Electrical and Computer Engineering, University of Patras, Patras 26500, Greece) A.A.H. Azremi: Also with the School of Computer and Communication Engineering, Universiti Malaysia Perlis (UniMAP), Malaysia. References 1 Vaughan, R.G., and Andersen, J.B.: ‘Antenna diversity in mobile communications’, IEEE Trans. Veh. Technol., 1987, 36, pp. 149– 186 2 Green, B.M., and Jensen, M.A.: ‘Diversity performance of dual-antenna handsets near operator tissue’, IEEE Trans. Antennas Propag., 2000, 48, pp. 1017–1024 3 Plicanic, V., Lau, B.K., Derneryd, A., and Ying, Z.: ‘Actual diversity performance of a multiband diversity antenna with hand and head effects’, IEEE Trans. Antennas Propag., 2009, 57, pp. 1547– 1556 4 Harryson, F., Medbo, J., Molisch, A.F., Johansson, A.J., and Tufvesson, F.: ‘Efficient experimental evaluation of a MIMO handset with user influence’, IEEE Trans. Wirel. Commun., 2010, 9, pp. 853– 863 5 Li, C.-H., Ofli, E., Chavannes, N., and Kuster, N.: ‘Effects of hand phantom on mobile phone antenna performance’, IEEE Trans. Antennas Propag., 2009, 57, pp. 2763– 2770 6 Okano, Y., and Cho, K.: ‘Dependency of MIMO channel capacity on XPR around mobile terminals for multi-band multi-antenna’. European Conf. on Antennas and Propagation, Berlin, Germany, November 2007, pp. 1 –6 7 Papamichael, V., and Soras, C.: ‘Generalized selection combining diversity performance of multi-element antenna systems via a stochastic electromagnetic-circuit model’, IET Microw. Antennas Propag., 2010, 4, pp. 837– 846 8 Plicanic, V.: ‘Antenna diversity studies and evaluation’, MSc Thesis, Department of Electroscience, Sweden, Lund University, May 2004
ELECTRONICS LETTERS 27th October 2011 Vol. 47 No. 22
