Characterization of Small Antennas & Wireless Devices for MIMO Systems in Multipath & Line-of-Sight

Doktorsavhandling, 2014

The current thesis investigates over-the-air (OTA) performance characterization of small antennas and wireless terminals in rich isotropic multipath (RIMP) environment, as well as in pure line-of-sight (LOS) environment when taking into account the random positions and orientations of the mobile terminals caused by the realistic user behavior. The latest mobile wireless terminals such as smart phones and tablets can be used in any random orientation thereby making even a received LOS-component appear as a random voltage at the antenna ports over time. The terms "RIMP" and "Random-LOS" are coined to represent the two limiting environments for OTA performance characterization of mobile terminals. In practice, RIMP environment can be emulated using reverberation chambers while the LOS environment can be emulated using anechoic chambers. Mobile terminals are mostly used in indoor multipath environments, e.g. homes, offices and shopping malls. Therefore the main focus of this thesis is on measuring and simulating OTA performance of small antennas and wireless devices in RIMP environment. The paper [A1] is related to using RIMP as a reference environment, and contains a study of the comparison between 2D and 3D multipath environments for single-port and multi-port antennas. Recently, the OTA testing of 4G Long Term Evolution (LTE) devices has become very important in order to characterize the implemented Multiple-Input Multiple-Output (MIMO) and Orthogonal Frequency Division Multiplexing (OFDM) technologies. Features such as adaptable modulation scheme, system bandwidth, coding rate, and diversity have made communication system more robust and adaptable to the environment. We introduced in 2011 a new theory based on an ideal threshold receiver in order to model the Total Isotropic Sensitivity (TIS) and the OTA throughput of LTE devices including their diversity gains due to MIMO and OFDM. This model was later tested for different LTE system bandwidths and coherence bandwidths in papers [B1-B3], and it has shown to give very good agreement with measurements in RIMP. Measuring and modeling the OTA performance of the latest wireless commercial devices have always been interesting. However, such commercial devices are often a black box and most of the technical details are kept as business secrets. Under such circumstances, we benefit from the Universal Software Radio Peripheral (USRP) which is an inexpensive and flexible Software Defined Radio (SDR). It provides a radio platform to design a reliable communication link where all important settings such as modulation, frequency, sampling rate, antenna gain, etc. are defined on software by the user. The USRPs are good alternatives to both devices-under-test (DUTs) as well as measurement instruments. For the first time, we demonstrate that USRPs can measure both active and passive OTA performance inside the reverberation chamber in papers [C1, C2]. The OTA performance characterization of single-port mobile terminals has been studied for several years now. Today, LTE mobile terminals are equipped with multiport antennas which must be characterized for OTA environments. The study of OTA performance characterization of a two-port mobile phone mockup on both sides of the head following standardized talk positions is presented in papers [D1, D2]. At Chalmers University, a unique multiport ultra-wideband (UWB) self-grounded bow-tie antenna has been designed and developed during the last few years. The OTA performance characterization of this antenna has been completed by measurements and simulations in RIMP environment. The results are presented in papers [E1, E2]. The OTA performance of compact UWB antennas designed for mobile phone applications at Linköping University, Sweden, and for body-centric wireless communications at École polytechnique fédérale de Lausanne (EPFL), Switzerland are presented in papers [F1, F2]. A unique study comparing drive test measurements and reverberation chamber measurements is presented in paper [F3]. Another interesting paper which introduces a new real-life OTA measurement method to improve cellular network performance is appended as paper [F4]. During the last decade, several multipath environment models and simulation tools have been developed. Similarly, Rayleigh-lab and ViRM-lab are developed at Chalmers University to study convergence and validation of efficiency, correlation, and diversity gain measurements in reverberation chamber. Moreover, we can also compare different multipath environments as well as different devices in different environments. This thesis presents some results based on these simulation tools which show excellent agreement with the measurements. The description of these simulation tools with some examples is presented in papers [G1, G2].