International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 12 (2017) pp. 3243-3247 © Research India Publications. http://www.ripublication.com 3243 Antenna Array with Low Mutual Coupling for MIMO-LTE Applications Eduardo Rodríguez Araque 1 , Ezdeen Elghannai 2 , Roberto G. Rojas 3 and Roberto Bustamante 4 1 Foundation Universitary Cafam (Unicafam), Bogotá, Colombia. 2 Ohio State University, USA, 3MIT Lincoln Laboratory, USA, 4University of Los Andes, Bogotá, Colombia. 1 ORCID: 0000-0003-0229-7920 Abstract In this work we present a compact MIMO array platform that operates in the 2.6 GHz for Long Term Evolution (LTE) band and wireless communication systems. The array consists of four compact patch antennas on a dielectric substrate with total dimensions of 125 mm  62.5 mm  1.27 mm. Modifications on the ground plane along with systematic placement and orientation of each antenna on top of the substrate plays a key role in reducing the mutual coupling which normally degrades the MIMO array performance. The performance of this designed MIMO array is assessed through simulations and measurements of the scattering parameters, radiation patterns, and correlation coefficients, including an evaluation of the capacity in indoor and outdoor-to-indoor scenarios with outstanding results. Keywords: Antenna array, Capacity, Characteristic Modes, Correlation, Diversity, Multiple-Input-Multiple-Output (MIMO) systems, mutual coupling INTRODUCTION The operation of MIMO array systems is based on a technique that allows us to increase the data rate and link reliability when operating in rich scattering environments. This technique normally generates low-correlation parallel sub-channels between elements of the array, thus incrementing the system capacity by several orders of magnitude [1]. An important factor that has a deleterious effect on the MIMO system performance is the mutual coupling between antennas. A high level of mutual coupling can increase the correlation between the signals received by each antenna, which in turn, reduces the number of independent parallel sub-channels [2]. There are different techniques to reduce the mutual coupling between the elements of an array; one way is to increase the spacing between antenna elements of the array. However, if the antenna array is built within small mobile phones, expanding the spacing is not realistic. Several antenna designs have been proposed to reduce the mutual coupling of MIMO arrays in small mobile phones. Corrugated ground plane techniques are presented in [3-4] to achieve high isolation. A T-shaped ground plane slot is implemented to reduce the mutual coupling between antennas [5]. In [6], T-shaped and dual inverted are inserted to reduce the mutual coupling. Additionally, neutralization techniques are also used to increase the port-to-port isolation between two closely placed antennas [7]. In this work, we propose a MIMO array designed to operate at 2.6 GHz (4G-LTE) band. The antenna array consists of four compact patch antennas on a PCB/dielectric substrate. Modification of the ground plane (GND) with corrugated slots in addition to a systematic placement and orientation of each antenna on top of the PCB plays a key role to increase the isolation; therefore, reducing the correlation between antennas. The design of the antenna elements, modifications of the GND, placement, and orientation of each antenna on the array was done based on the insights provided by the method of Characteristic Modes. The Method of Characteristic Modes (CMs) is a modal analysis scheme that provides physical insight into the potential resonant characteristic of a structure by finding and rigorously examining the natural current modes of the structure [8-9]. We used the original CMs [9] to design the antenna elements as well as to study the currents induced on the ground plane of the MIMO array. Another version of this method is the Network Characteristic Mode (NCM) technique, in this application; the network is formed by the 4 input ports of the antenna elements. DESIGN OF MIMO ARRAY The goal is to design and build a four-element array within a compact mobile chassis and with high isolation between the antennas. The first step was the initial design of a single antenna that operates at 2.6 GHz. Thanks to miniaturization techniques, we were able to design an electrically small antenna by properly inserting slots as depicted in Fig. 1. Note that this antenna has a radiation efficiency of 63 % after miniaturization.