International Journal of Antennas and Propagation

Volume 2015, Article ID 412127, 16 pages

http://dx.doi.org/10.1155/2015/412127

## Study of the Colocated Dual-Polarized MIMO Capacity Composed of Dipole and Loop Antennas

Department of Communication Engineering, Communication University of China, Beijing 100024, China

Received 7 January 2015; Revised 12 May 2015; Accepted 17 May 2015

Academic Editor: Angelo Liseno

Copyright © 2015 Dazhi Piao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The colocated dual-polarized dipole (DPD) and dual-polarized loop (DPL) MIMO channel performances are compared. Computation results show that, for the ideal electric and magnetic dipoles, the dual-polarized MIMO systems have identical channel capacity. But the contour plots of the capacity gain of the realistic DPD and DPL are different, due to the difference in antenna patterns. The cumulative distribution function (CDF) of the capacity gain in the two-mirror (TM) channel shows that, for small distance, the capacity gain obtained by the DPD is obviously smaller than that of the DPL, but, with the increase of the distance, the difference gets smaller. A DPL with low mutual coupling is fabricated. Measured results show that high MIMO capacities can be obtained by this DPL in both the anechoic chamber (AC) and the realistic office room. The capacity gain of the DPL antenna is 1.5–1.99, which basically coincides with the theoretical and numerical results. Furthermore, the capacity of the virtual DPL antenna with no mutual couplings is also investigated. It is shown that, in the AC, the mutual coupling will generally decrease the dual-polarized MIMO capacity; however, in the office room, the effect of mutual coupling is not always negative.

#### 1. Introduction

Multipolarized antenna can be used to detect vector electromagnetic field components, which is very attractive to wireless communication, localization, and navigation. The potential of utilizing all the three electric dipoles and three magnetic dipoles for communication which are colocated and orthogonal has been theoretically evaluated in [1–5]. To obtain high polarization degrees of freedom with the colocated multipolarized antennas, high port isolations between the antennas are generally required. However, it is a challenging task to avoid the mutual coupling between the colocated antennas. Thus, most of the analysis about the colocated multipolarized MIMO system is theory oriented, which is based on the ideal electric and magnetic dipoles with no mutual coupling.

There are also some field test studies using multiple colocated, orthogonal electric dipoles, and it has been verified that multiple independent subchannels could be obtained [6, 7]. However, due to the constraints of feeding and mutual coupling, the colocated loops are difficult to realize; thus, the measurement based characteristics of the dual-polarized multiple-input multiple-output (MIMO) channel using colocated loop antennas are seldom seen and whether similar MIMO performance can be obtained by the multipolarized loops to that of the multipolarized dipoles has not been verified. Thus, in this paper, firstly, the MIMO capacity of the dual-polarized loop (DPL) is compared with that of the dual-polarized dipole (DPD) by theoretical computations and numerical simulations; then, the colocated DPL antenna is fabricated and its MIMO performance is measured.

A colocated dual-polarized loop antenna has been presented in [8] using a modified classic Kandoian loop, in which the saw tooth interlaced structure is cleverly used to colocate the orthogonal loops. However, this structure introduces a comparatively high coupling between the two loops, and the isolation is about −13 dB at the working frequency. In [9], we presented a MIMO antenna composed of two colocated, orthogonal DPLs, which has a constant current distribution along each loop and has a simulated mutual coupling lower than −30 dB. Furthermore, this DPL is compared with another two structures of DPL [10], which have different current distributions and isolations. Simulated results in [10] show that the DPL with constant current distribution and comparatively low mutual coupling has a larger and more robust MIMO capacity. Thus, in this paper, the proposed DPL antenna is fabricated, which has a measured isolation performance better than −25 dB. Furthermore, in order to study the influence of multipath richness on the information carrying ability of the DPL antenna, the experiments are conducted under an ordinary office environment and an anechoic chamber (AC), individually. The effects of the communication distance, the transmitting (Tx) and receiving (Rx) antenna heights, and the azimuthal angle between the Tx and Rx antennas on the 2 × 2 MIMO channel characteristics are also evaluated. Furthermore, to study the effect of mutual coupling of the antennas on the dual-polarized MIMO capacity, the measurements based on a virtual DPL antenna are also conducted, which is composed of two single loops having the same polarizations with that of the colocated DPL, but with no mutual couplings. Meanwhile, the MIMO performances of the virtual DPL antennas with other combination of polarizations are also studied, by which the other magnetic components can be utilized.

#### 2. MIMO Capacity Comparison between the DPL and the DPD

The richness of the multipath generally has an important effect on the capacity of a MIMO system. Thus the MIMO performance of the DPL and the DPD antenna is evaluated in free space (FS) and a parallel perfect electric conductor (PEC) two-mirror (TM) channel, respectively. The reason is that, in the FS channel, there is only one multipath, but, in the TM channel, the multipath reflections are infinity; thus two cases of the sparse and rich-multipath environment can be illustrated. Furthermore, for the ideal electric and magnetic dipole, in the FS channel, the exact solution to Maxwell’s equation can be found, and, in the parallel PEC TM channel, the electromagnetic (EM) field can be obtained by the image theory [5]. In fact, for the antenna composed of ideal electric and magnetic dipoles, the MIMO system will have similar performance, but, for the antenna composed of realistic electric and magnetic dipoles, the MIMO system may have different behaviours due to the difference in the radiation patterns. Thus, the dual-polarized MIMO channel performance is evaluated by both the theoretical computations considering the ideal electric and magnetic dipoles and the numerical simulations considering the realistic DPD and DPL antenna using the commercial software FEKO.

##### 2.1. Construction of the DPL and DPD Antenna

For the colocated DPL, assume Tx_{1} is located in the plane (-polarized loop) and Tx_{2} is located in the plane (-polarized loop), as shown in Figure 1(a). For the colocated DPD, assume Tx_{1} is located along the axis and Tx_{2} is located along the axis , as shown in Figure 1(b). Here, is the distance from the centre of the antenna to the axis . In the TM channel, there are two parallel infinite PEC planes vertical to the axis with a separation .