1. On small terminal MIMO antenna correlation optimization adopting characteristic mode theory
Istvan Szini
Alexandru Tatomirescu
Gert Pedersen
Aalborg University
Faculty of Engineering and Science
Department of Electronic Systems
DK-9220 Aalborg, Denmark

2. Table of contents Background Test Vehicle Simulation Model
Simulation Results
Measurement Results
Conclusions
July 2014
IEEE APS 2014

3. Background
Any conducting object has natural resonant frequencies at which its entire structure oscillates sinusoidally. From an antenna perspective, maximum radiation is achieved when the structures is exited at these frequencies. They can be linked to the physical shape of the object by describing the radiant mechanism with a set of oscillating modes. Modal analysis has been used extensively in the past and is well established in the electromagnetic analysis of closed, bounded structures such as waveguides and cavities, for which it is straight forward to arrive at close form solutions. However, open radiating structures, such as antennas and scattering problems, pose a different challenge which is more computationally demanding.
The theory of characteristic modes or eigen functions of conducting bodies, introduced by Garbacz in 1968 and improved by Harrington and Mautz, provides a set of orthogonal spherical field distributions in the far-field. These field modes, also known as characteristic fields, have the useful property that they are radiated from an orthogonal set of current distributions running along the surface of the conducting body of arbitrary shapes. These current distributions, the modal or characteristic currents, are real or equiphasal currents that diagonalize the generalized impedance matrix for the surface for which they were defined
July 2014
IEEE APS 2014

4. Background, cont
The fields radiated by eigen currents have the important property that they are orthogonal over the sphere at infinite. In other words, different modes will have orthogonal far-field radiation patterns which will lead to a zero correlation according to the equation bellow, ideal for the design of MIMO antennas. However when it comes to compact phones, in the low bands, the ground plane is the main radiator which has only one efficient mode.
July 2014
IEEE APS 2014

5. Test Vehicle
In classical array design, the distance between the elements has been kept to λ/2 to minimize the unwanted coupling between the elements of an array and to minimize the spatial correlation. However, the size of even the modern smart phones does not allow for this criterion to be satisfied if we consider the low bands standardized and implemented by some mobile operators for Long Tern Evolution (LTE), the total array size is much smaller that the requirement of λ/2 and at these lower frequencies (e.g. Band 13) the antennas rely on the shared electrically small Printed Circuit Board (PCB) ground plane for efficient radiation. Consequently, the antennas will have a very strong electromagnetic coupling and high far-field pattern correlation thus a limited capacity.
The form factor 100x50 mm, is a typical lower bound for a modern phone size that supports MIMO. In the low band used for communications (LTE band 13, MHz), the phone is considerably smaller than the wavelength of operation and thus designing two decoupled and efficient radiators is not straightforward. For a compact phone the ground plane of the Printed Circuit Board (PCB) is the main radiator and hence, the characteristic modes of the ground plane offer insight into the required design of the exciting elements in order to obtain decoupled radiation modes.
July 2014
IEEE APS 2014

6. Simulation Model
The simulation model of the antenna system topology, was built combining the manipulation of the GND plane geometry, and further optimizing the current distribution on the radiated structure applying modal analysis.
Fig 1 Test vehicle simulation model
July 2014
IEEE APS 2014

7. Simulation Results, Return Loss, isolation and impedance characteristic
Fig 2 Return Loss and impedance characteristic
July 2014
IEEE APS 2014

8. Simulation Results, 3D radiation patterns @ 751MHz
ρ ≤ 0.01
Fig 3 Simulated 3D farfield radiation patterns, main antenna (left) and secondary antenna (right).
July 2014
IEEE APS 2014

9. Simulation Results, current distribution
The current distribution of the first two characteristic modes of two types of ground planes, a realistic C shaped with space for the battery and a classical rectangular shape, have been plotted in Fig. 4 and Fig. 5, respectively. Only the fist two modes have been shown because the modal significance of the higher modes at low frequency is very small resulting in a negligible contribution to total radiation. The modes have been computed by eigen mode decomposition of the impedance matrix.
Fig 4 First characteristic dipole mode
Fig 5 Second characteristic cross-polar mode
July 2014
IEEE APS 2014

10. Simulation Results, modal analysis
The current distributions shown in the previous slide suggest that for both ground plane types the first mode, the dipole mode, the radiated structure can be easily exited by an element that capacitive couples to the GND with an element at the top or bottom second mode, the cross polar dipole mode, can be excited be placing a similar element in the middle of the sides.
Fig 6 The modal significance of the tree most important modes
July 2014
IEEE APS 2014

11. Measurement Results, Return Loss, Isolation and Total Efficiency
ρ ≤ 0.01
Fig 7 Measurement Results, Return Loss, Isolation and Total Efficiency
July 2014
IEEE APS 2014

12. Conclusions
This investigation demonstrates that commonly adopted antennas following an application of characteristic mode theory, and manipulation of the GND and optimal placement, can achieve strong uncorrelation between low band antennas in electrically small devices;
A significant reduction of the magnitude of complex correlation coefficient was achieved while maintaining the total antenna efficiency and BPR in realistic antenna volume and device form factors;
Simulation results confirmed that an antenna system with commonly used PIFA antennas can be configured to excite the handset chassis in orthogonal modes, maintaining high isolation between antennas and orthogonal radiation patterns, therefore enabling radiation pattern uncorrelation;
Uncorre-lated antennas will result in lower magnitude of complex correlation coefficient, therefore higher system capacity fulfilling the Long Term Evolution (LTE) requirements towards handsets with fourth generation capabilities and beyond.
July 2014
IEEE APS 2014

13. Thank you, for your attention!
July 2014
IEEE APS 2014