Werner L. Schroeder

Numerical analysis of characteristic modes on the chassis of mobile phones

This paper explores the application of the concept of characteristic modes on the chassis of a mobile phone to purposeful design of its radiation characteristics. Focus is on the resonant chassis modes which dominate the overall radiation properties. Modal radiation quality factors are derived from characteristic mode eigenvalues. Antenna ¿ chassis coupling, previously only considered in terms of equivalent circuits, is discussed from a field theoretic point of view. A numerical approach for characteristic mode analysis is presented which is an eigenmode solver extension to the well known NEC2 code. Numerical results are given for wire grid models of a bar-type and a folder-type phone chassis.

MIMO antenna system based on orthogonality of the characteristic modes of a mobile device

A novel concept is introduced to realize a multiple antenna system on a small mobile device by coupling to its characteristic modes. According to the theory, the current density on the chassis of a mobile device is the superposition of characteristic mode current densities which have unique resonance frequencies. There are some interesting frequency ranges where more than a single mode is near resonance, i.e. may be appreciably excited and contributes to the overall radiation. For a selective excitation of the characteristic modes near to their resonant frequencies, an arrangement of 4 capacitive couplers and a feed network can be introduced. This type of excitation circumvents crosstalk between the input ports and ensures mutually orthogonal radiation patterns. The main problem to be solved is the simultaneous matching of all modes. The easiest approach is the method of a balanced mismatch (minimization of the maximum mode reflection coefficient). Advanced methods combine this idea with a systematic modification of the parameters of the characteristic modes.

Optimal antenna location on mobile phones chassis based on the numerical analysis of characteristic modes

This paper contributes to the solution of the antenna-chassis coupling problem for handheld devices in mobile communication. That problem, up to now addressed in terms of equivalent circuits, is discussed here from a field theoretical point of view. A numerical analysis of characteristic modes on the chassis of the device provides a resonant mode which dominates the overall radiation properties. The modal excitation coefficient which relates the above mentioned chassis dominant wavemode to that of a given antenna element (exciter) should be maximized to achieve an optimal modal coupling. This paper establishes conditions to achieve maximum modal excitation coefficient which lead to an optimal placement of the antenna element on the chassis. A numerical approach for the evaluation of the optimal location of the exciter (electric and magnetic) on the chassis is performed. In this paper a theoretical approach based on some assumptions of the evaluation of the modal excitation coefficient is presented for the cases of an electric and a magnetic exiter. Numerical results are given for a bar-type and a folder-type phone chassis.

Matching network synthesis for mobile MIMO antennas based on minimization of the total multi-port reflectance

An algorithm for MIMO antenna matching based on minimization of the single figure of merit called total multi-port reflectance or equivalently maximization of Total Multi-Port Return Loss (TMRL) is presented. A general approach for matching of an N-port MIMO antenna system is shown and for the special case of a symmetric two-port MIMO antenna a case-specific, simplified and explicit algorithm is described. As an example, a symmetric two-port antenna for the 1710–1880 MHz band is designed. The matching network topology and element values for the structure are given together with S-parameters, TMRL and Power Envelope Correlation Coefficient (PECC), in order to prove the usefulness of the proposed matching concept.

Estimation of Maximum Attainable Antenna Bandwidth in Electrically Small Mobile Terminals

An estimate for the maximum attainable antenna bandwidth in electrically small mobile terminals is derived for the case that the operating band is below the chassis resonance frequency. The study is based on the double resonator model. Optimization is used to estimate the maximum bandwidth. The results are combined with numerically calculated radiation quality factors for plates of different aspect ratio to present maximum attainable bandwidth as a function of chassis length and aspect ratio

Direct Measurement of Small Antenna Radiation Efficiency by Calorimetric Method

A novel method for radiation efficiency measurement of small-size radiators such as mobile phones and other handheld wireless devices is presented. The method is based on the accurate determination of the power which is dissipated in the antenna under test using a calorimetric approach. The principle of the method, some of its variants, and the experimental setup are described. Application of the method is demonstrated for several test objects with different efficiencies. Accuracy is validated by comparison of calorimetric radiation efficiency against numerical estimates and independent measurements using the reflection efficiency method, a generalized variant of the Wheeler cap method, whose basic limitations are briefly discussed. Due to the calorimetric principle employed in the proposed new method, it qualifies equally well as a highly accurate reference method for radiation efficiency measurements and as a low-cost alternative to three-dimensional pattern integration and Wheeler cap type efficiency estimates

Design of a Wideband, Tunable Four-Port MIMO Antenna System With High Isolation Based on the Theory of Characteristic Modes

Cognitive radio (CR) systems, which are intended for opportunistic spectrum access over a large portion of the mobile radio spectrum, require highly isolated and wideband multiport antennas. The subject of this paper is an approach for mapping characteristic modes (CMs) to radiation modes (RMs) to build a multiple-input multiple-output (MIMO) antenna system meeting the requirements of isolation and bandwidth (BW). A set of mutually orthogonal RMs of the antenna system is constructed from superpositions of CMs of the chassis of the device. By exploitation of orthogonality properties and permutation relations between the couplers due to symmetry, the complexity of the matching network (MN) and the number of tunable reactances are reduced. The approach is applied to the design of a four-port antenna system for a femto-cell form factor device operating in the [470, 790] MHz range. We proceed to the computation of the CMs of the device, the selection of most efficient ones, and their excitation with couplers whose locations respect the symmetry of the structure and of the current density distributions of the chosen modes. Theoretical considerations and simulations are compared against measured results for our hardware prototype.

Electrically tunable mode decomposition network for 2-port MIMO antennas

Typically, geometrically symmetric 2-port Multiple Input - Multiple Output (MIMO) antennas for LTE-enabled small mobile wireless devices feature radiation modes with significantly different radiation quality factors. It is difficult to precisely match radiation modes over the desired frequency band. Therefore, a solution for low-loss electrical tuning is of interest. The new solution, reported in the present paper, uses electrically tunable Mode Decomposition Network (MDN). Both radiation modes can be tuned individually by means of mutually independent mode-specific matching networks to the desired frequency bands while maintaining high modal efficiencies. The usable tuning range is restricted by parameters of commercially available components. An investigation of the practically attainable tuning range is shown.

Metrics and Methods for Evaluation of Over-The-Air Performance of MIMO User Equipment

Commercial User Equipment (UE) testing and certification has become more complex for state-of-the-art mobile communication standards such as 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) due to the extensive use of Multiple Input-Multiple Output (MIMO) transmission techniques. The variety of different MIMO operating modes and the almost unlimited choice of possible multipath channel conditions under which UE performance may be evaluated are not accounted for by established Single Input-Single Output (SISO) Over-The-Air (OTA) performance metrics like Total Isotropic Sensitivity (TIS) and Total Radiated Power (TRP). As pointed out in this contribution, meaningful metrics and cost-effective, low-complexity measurement methods can, nevertheless, be derived by focusing on characterization of the physical attributes of UE and by adopting statistical metrics. Starting from a brief review of the most important MIMO operating modes in the 3GPP LTE standard, the relation between UE properties and UE performance, which is observed in these operating modes, is discussed. Two complementary metrics and corresponding measurement procedures for evaluation of MIMO OTA performance are presented in order to address the diversity of possible propagation scenarios. Measurement results from preliminary implementations of the two proposed measurement procedures, including comparison between different LTE devices, are presented.

Equivalent circuit model for closely coupled symmetrical two-port MIMO antennas in small volume

This paper proposes an equivalent circuit model for handset electrically small symmetrical two-port MIMO antennas. In limited volume of mobile devices only exploition of orthogonal modes of radiation can lead to low coupling and low correlation between antennas. This paper explains the principle of operation for small MIMO antennas by means of common and differential mode excitation. A circuit model is introduced, validated based on full-wave simulations of an antenna prototype and used to aid MIMO antenna design for PCS band (1850–1990 MHz).