Optimal multiple antenna design for compact MIMO terminals with ground plane excitation

Abstract

ABSTRACT:The compactness of mobile terminals complicates the design of multiple antennas, since coupling among the antennas increases when they are placed in proximity of one another. While it is possible to mitigate coupling between closely spaced antennas, a tradeoff in bandwidth is required. In this paper, we highlight ground plane excitation as an additional dimension to consider in the design of multiple antenna terminals. This is because a compact ground plane, especially in mobile application, can interact with the antenna elements and contribute significantly to their radiation characteristics. Our results show that several design parameters, namely element locations, spacing between elements, and radiation characteristics of individual elements, must be jointly considered in order to achieve the optimal MIMO performance tradeoff for compact multiple antenna terminals.

Antenna design challenges and solutions for compact MIMO terminals

“…However, in the case of compact MIMO antennas on a shared ground plane, a more dominant coupling mechanism occurs when more than one antenna element efficiently exploit the ground plane as a radiator [2], [11], [12]. This phenomenon of ground plane radiation has been studied primarily in the context of single antennas using different approaches, including the use of equivalent circuit and theory of characteristic mode.…”

“…Then, we outline strategies to mitigate the problem of coupling, i.e., decoupling the antennas [5]- [8] or matching the antenna to the environment [9], [10]. This is followed by the use of the theory of characteristic mode to analyze the negative impact of ground plane excitation on the performance of MIMO antennas [11], [12]. Finally, we present several practical techniques that may be used to mitigate performance degradation resulting from ground plane excitation [11]- [13].…”

Angle and polarization diversity in compact dual-antenna terminals with chassis excitation

“…The intrinsic radiation properties of the chassis are evaluated based on the theory of characteristic mode introduced by Harrington [5]. The first characteristic value (resonant frequency) of the chassis is found to be 1.35 GHz [4], which is near the 900 MHz band for mobile communications. The normalized characteristic electric and magnetic fields on a plane 5 mm above the chassis at 1.35 GHz are shown in Figure 1.…”

“…This is despite that the bigger challenge is in decoupling terminal antennas for frequency bands below 1 GHz, where the isolation is typically lower than 6 dB. The poor isolation is mainly because the chassis can play an important role in determining the mutual coupling between the antennas [4]. For low frequency bands, the chassis can act not only as a common ground plane, but also as a radiator shared by more than one antenna.…”

“…Thus, the radiation patterns can be severely modified by the chassis, such that they are less orthogonal to one another in both angular and polarization properties. In [4], the mutual coupling is reduced for a dual-antenna prototype by optimizing the position of one antenna, with the second antenna located at one short edge of the 100 mm  40 mm chassis. It is found that coupling is minimized when the first antenna is located at the center of the chassis, which corresponds to the position of least chassis excitation for that antenna.…”