418 J. GUTERMAN, A. MOREIRA, C. PEIXEIRO, Y. RAHMAT-SAMII, ANTENNA-USER INTERACTION IN MIMO-ENABLED LAPTOPS Antenna-User Interaction in MIMO-Enabled Laptops Jerzy GUTERMAN .1 , António MOREIRA 1 ,Custódio PEIXEIRO 1 , Yahya RAHMAT-SAMII 2 1 Instituto de Telecomunicações, Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal 2 University of California, Los Angeles, 420 Westwood Plaza, Los Angeles, CA 90095-1594, USA jerzy.guterman@ieee.org, antonio_moreira@ieee.org, custodio.peixeiro@lx.it.pt, rahmat@ee.ucla.edu Abstract. The operation of wireless personal communica- tion terminals very close to the user inherently faces the problem of electromagnetic (EM) coupling between the device and the biological tissues. In this paper the effects of the electromagnetic antenna-human interaction is studied for a laptop MIMO antenna system, where four integrated antenna elements can operate simultaneously. Two points of view are considered: antenna performance and EM dosimetry. The first one addresses not only the degradation of the antenna performance but includes also the effect of the human proximity on the antenna charac- teristics, namely scattering matrix, Total Active Reflection Coefficient (TARC), radiation efficiency and envelope correlation between port signals. The exposure of the human tissues to EM radiation is expressed in terms of Specific Absorption Rate (SAR). These characteristics are evaluated as a function of the array excitation scheme (including phased array approach and MIMO-like signal- ing) and compared to simple scenarios where all the power is radiated only by one antenna element. Keywords Laptop antennas, MIMO antenna systems, TARC, SAR, EM dosimetry. 1. Introduction In wireless personal communications the mobile ter- minals inherently operate very close to the user. The cou- pling between the antenna and the human, often located in the antenna near-field zone, affects the antenna parameters and causes microwave energy absorption in the user’s body. In the beginning of the 90s, when handsets became widely popular, coupling between antennas and the user's head has been investigated [1]. Nowadays, as the variety of portable units equipped with wireless interfaces are grow- ing, the antenna-human interaction should be thoroughly investigated in a wide range of scenarios. Moreover, use of new Multiple-Input Multiple-Output (MIMO) techniques [2] requires the simultaneous operation of several terminal antenna elements which affects the way the terminal inter- acts with the human. In the present paper the effects of electromagnetic interaction between a laptop integrated MIMO antenna system operating in the popular ISM 2.4 GHz band (de- scribed in Section 2) and the user (human model described in Section 3) are studied numerically with the aid of CST Microwave Studio software tool [3]. The human effects on the antenna performance and the antenna operation effect on the human are addressed in Sections 4 and 5, respec- tively. Simulation results obtained with CST Microwave Studio software tool have been validated by experimental results in many and varied printed antenna configurations [4-8]. Therefore, although only CST simulation results are presented the authors have confidence in their validity. 2. MIMO Antenna System of Back-to- Back E-Shaped Patches 2.1 Back-to-Back E-Shaped Patch Element The back-to-back E-shaped patch element has been introduced in [4]. This novel element consists of two E- shaped sections (printed on a dielectric substrate) and a connecting strip, constituting a continuous patch con- formably embracing the ground plane edge. The detailed antenna geometry and the inverted microstrip feeding line are presented in [5]. In the numerical simulations the metallic parts are modeled as thin PEC layers whereas the dielectric is assumed to be lossless. The antenna operates in the ISM 2.4 GHz and UNII 5.2 GHz bands and despite integration into the electrically large laptop, assures almost omnidirectional total gain horizontal plane pattern [5]. Due to limited computation resources all simulations including the presence of the electrically large human model (see Section 3) are limited to 2.4 GHz. 2.2 4 Element Linear Array Integrated in the Laptop A four-element linear array of back-to-back E-shaped patches (Fig. 1) has been introduced in [6]. The radiators are integrated along the top screen rim with spacing D = 81 mm, which corresponds to 0.66λ (@2.44 GHz) as shown in Fig. 2a. In the numerical simulations the laptop