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Citation information: DOI 10.1109/TAP.2015.2398111, IEEE Transactions on Antennas and Propagation IEEE TRANSACTION ON ANTENNA AND PROPAGATION 1 On Small Terminal MIMO Antennas, Harmonizing Characteristic Modes with Ground Plane Geometry Istvan Szini, Alexandru Tatomirescu, and Gert Frølund Pedersen Abstract—MIMO antenna systems are defined by fundamen- tal 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. smartphones, due to the form factor reduced dimensions and multiplicity of adjacent frequency bands of operation. Uncorrelated antennas are especially important for Multiple Input - Multiple Output (MIMO) antenna systems, other than few exceptional cases, the reduction of magnitude of complex correlation coefficient will increase system capacity and data throughput. This work proposes a 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 PIFA (planar inverted F antenna) on a platform with a very small form factor (100x50x10 mm). The proposed implementation is based in the synergy of two known techniques where the first technique consists in the reference ground plane geometry manipulation and the other technique consists 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 it is more difficult to achieve in form factors smaller than 1 λ , while maintaining high total efficiency and adequate gain imbalance. In this paper a simulation model as well as prototype (1/4λ long) measurement results are presented, demonstrating the implementation feasibility of such antenna system in realistic mobile device embodiment. Index Terms—MIMO, correlation coefficient, low-band, an- tenna, handset. I. I NTRODUCTION T HE widespread use of MIMO in current and future wireless communications systems has spurred consid- erable attention in the research community. Over the past years, small antenna array design has been the subject of numerous research articles [1]–[21]. Receiver diversity and MIMO have improved the reliability and data rate of wireless links enabling 4G communications standards see e.g. [1]–[3]. Ideally, by taking advantage of the multi-path properties of the wireless channel, MIMO uses parallel data streams to linearly increase capacity with the number of array elements I. Szini is with Motorola Mobility LLC., Chicago, USA; email: Ist- van.Szini@motorola.com. I. Szini, A. Tatomirescu and G. F. Pedersen are with the Antennas, Propagation and Radio Networking section at the Department of Electronic Systems, Faculty of Engineering and Science, Aalborg University, Denmark; email: {ijs; ata; gfp}@es.aau.dk [2]. In practice, the increase in system capacity is strongly depended on the channel proprieties governed by the various wireless channel propagation characteristics such as the power imbalance between the multiple links or their correlation [4], [5]. 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 [22]. 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 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 [6]. Therefore, MIMO antenna systems adopted in handsets size devices need to rely on radiation pattern diversity and isolation enhanced techniques to enable somewhat adequate MIMO performance. The MIMO antenna system introduced in this paper, defines realistic antenna placements, independent excitation modes and feeding mechanisms, manipulation of ground plane ge- ometry to control ground surface current and application of characteristic mode theory. The synergy of these techniques enables the design of highly isolated and uncorrelated antennas at low frequency bands, with adequate gain imbalance and total efficiency, within the constrains of realistic implemen- tation in small terminals. This work is presented starting with an investigation of the most adopted techniques to reach similar results, then a discriminative characterization of the concept proposed in this paper, the proof of this concept through simulation and measurement results, and finally the conclusion. There is vast amount of literature demonstrating MIMO antenna system design techniques to improve antennas iso- lation and decorrelation [8]–[21], [23]–[25]. Guo et al [7] demonstrated that at higher frequencies (above 1.7GHz), a PIFA antenna system design can achieve envelope correlation coefficient < 0.5. Using PIFA antennas placed on a uniform ground plane, and small distance between antennas, the overall envelope correlation coefficient between them remains suitable for MIMO applications. Kuonanoja [8], presents a solution which addressed partially the correlation coefficient and iso- lation in lower bands, i.e. bellow 960MHz. His work is based on ground clearance underneath both antennas, and dynamic