Istvan J. Szini

On Small Terminal MIMO Antennas, Harmonizing Characteristic Modes With Ground Plane Geometry

multiple input-multiple output (MIMO) antenna systems are defined by fundamental figure of merits (FoM), such as branch imbalance, total efficiency, mean effective gain (MEG), and the correlation coefficient. Those FoM requirements are challenging, specially when applied to electrically small mobile devices, i.e., smart-phones, due to the form factor-reduced dimensions and multiplicity of adjacent frequency bands of operation. Uncorrelated antennas are especially important for MIMO antenna systems; other than few exceptional cases, the reduction of magnitude of complex correlation coefficient will increase the system capacity and data throughput. This work proposes an MIMO antenna system for mobile terminals, based on a realistic form factor and packaging implementation, with a very low magnitude of the complex correlation coefficient and an impressive isolation. An isolation better than 20 dB has been achieved using a folded monopole and a commonly adopted planar inverted F antenna (PIFA) on a platform with a very small form factor (100 × 50 × 10 mm). The proposed implementation is based on the synergy of two known techniques that consists in the: 1) reference ground plane geometry manipulation and 2) in the application of the characteristic mode theory to obtain orthogonal radiation modes. Since MIMO antenna systems at frequencies higher than 1.7 GHz are naturally proper isolated and decorreleated, this work demonstrates the proposed antenna topology enabling higher isolation and uncorrelated antenna system at 750 MHz, which is more difficult to achieve in form factors smaller than 1λ, while maintaining high total efficiency and adequate gain imbalance. In this paper, a simulation model and prototype (1/4λ long) measurement results are presented, demonstrating the implementation feasibility of such antenna system in realistic mobile device embodiment.

The enhanced bandwidth folded inverted conformal antenna (EB FICA) for mufti-band cellular handsets

An internal multi-band antenna for mobile phones featuring wide fractional bandwidth is presented. The antenna is realized by a flat conformal conductor above the ground plane, featuring a feed and ground connections to the PCB. A slot is cut in the flat conductor, which is open only in between the ground and feeding connections of the flat conductor, thus forming a slit. This structure is called enhanced bandwidth folded inverted conformal antenna (EB-FICA), with reference to its precursor FICA (Di Nallo and Faraone, 2005). The EB-FICA can be designed to be penta-band, covering the four GSM bands and the UMTS Rx band, and has been adopted in several Motorola products.

Design and verification of MIMO 2×2 reference antennas

The development and initial discussion of a reference MIMO 2×2 antenna concept has been presented in [1]. The reference antenna concept has been created to eliminate the uncertainties linked to the unknown antenna performance of the few LTE MIMO 2×2 reference devices or golden standards currently available for evaluating radiated data throughput measurement methodologies and test facilities. The proposed concept is based on simple antennas with a well-known Figure of Merit (FoM) and controllable performance. In this paper we present the recent developments on the antenna concept and report on the first measured performance at uniform incoming power distribution, figures and correlations between different measurement labs.

On small terminal MIMO antenna correlation optimization adopting characteristic mode theory

This work proposes a MIMO antenna system for mobile terminals, based on a compact form factor and packaging implementation, with a very low magnitude of the complex correlation coefficient and an impressive isolation. An isolation better than 20 dB has been achieved using a folded monopole and a commonly adopted PIFA (planar inverted F antenna) on a platform with a very small form factor (100×50 mm). The proposed implementation is based in the application of the characteristic mode theory to obtain orthogonal radiation modes. The adopted topology enables uncorrelated antenna system at 750 MHz, while maintaining high total efficiency and adequate gain imbalance. In this present paper simulation model as well as prototype measurement results are presented, demonstrating the implementation feasibility of such antenna system in compact mobile device embodiment.

Channel Spatial Correlation Reconstruction in Flexible Multiprobe Setups

This letter discusses one aspect of over-the-air (OTA) testing for multiple-input-multiple-output (MIMO) capable terminals in flexible multiprobe setups. Two techniques to obtain weights as well as angular locations for the OTA probes are proposed for accurate reconstruction of the channel spatial correlation at the receiver side. Examples show that with a small number of probes in a flexible setup, accurate spatial correlation can still be achieved within the test zone.

MIMO Reference Antennas Performance in Anisotropic Channel Environments

The new generation of cellular wireless communications, long term evolution (LTE), requires multiple antennas at both the transmit and receive ends of the radio link to improve system performance. This use of the radio link is termed multiple input-multiple output (MIMO), and in this work, the focus is on the downlink (base station transmit and mobile receive). Efforts to define over-the-air (OTA) downlink MIMO measurement methodologies are still being carried out within several standards bodies and are based on the evaluation of so-called good, nominal, and bad MIMO reference antennas. The reference antennas were necessarily defined independently of the channel environment, which is still under discussion. The reference antenna figures of merit, for an isotropic channel environment, were chosen as efficiency, branch power ratio (BPR), and correlation. The objective of this work is to evaluate the radiated performance of the MIMO reference antennas using anisotropic (nonuniform power angular spread) channel environments. The performance of the reference antennas in anisotropic environments is different to that in an isotropic environment, due to the antenna orientation in the three-dimensional space relative to the channel. This leads to a statistical description of the branch power ratio (BPR), correlation, and efficiency, which is expressed in terms of mean effective gain (MEG). The outcome of this new anisotropic evaluation is that the reference antennas are seen to vary significantly for BPR and mean effective gain (MEG) and somewhat less for correlation, relative to the isotropic environment.

On Antenna Polarization Discrimination, Validating MIMO OTA Test Methodologies

The radiated performance of a multiple-input-multiple-output (MIMO) antenna system is commonly evaluated based on a set of antenna-centric figures of merit (FoM). These generally include the antennas total efficiency, branch imbalance, and magnitude of the complex (or envelope) correlation coefficient. On the other hand, to validate the over-the-air (OTA) performance of the MIMO system, including both antennas and other parts of the real transceivers, test methodologies were created based on a single FoM: the absolute data throughput. While an antenna-centric FoM provides a preliminary overall insights into the radiated performance of the MIMO antenna system, these FoMs are limited in their overall applicability. Each of these FoMs is based on far-field antenna measurements and assuming a uniform distribution of the incoming power, meaning that passive antenna measurements are made in single-input-single-output (SISO) anechoic chambers. Furthermore, without the implementation of channel models with controllable spatial characteristics, it is impossible to isolate the effect of polarization changes on MIMO performance. In this letter, the radiated performance of MIMO antenna systems will be compared in terms of data throughput using different channel models both with and without controllable cross-polarization ratio and other spatial characteristics.