Antenna design challenges and solutions for compact MIMO terminals

The design of antennas for compact multiple-input multiple-output (MIMO) terminals is a challenging feat, due to the space constraint and the physical structure of the terminal. The space constraint forces the antennas to be closely spaced, which degrades MIMO performance due to high signal correlation and low antenna efficiency. Moreover, the terminals are equipped with ground planes which further complicate the implementation of antennas for good MIMO performance. This paper elaborates on the two aforementioned challenges in MIMO antenna design and surveys recent techniques that are developed to address these challenges

Multiplexing efficiency of MIMO antennas

A simple and intuitive metric of multiplexing efficiency is proposed for evaluating the performance of MIMO antennas in the spatial multiplexing mode of operation. Apart from gaining valuable insights into the impact of antenna efficiency, efficiency imbalance and correlation on multiplexing performance, the metric is particularly useful for antenna engineers whose goal is to achieve the optimum antenna system design. Experimental results involving prototype mobile terminals highlight the effectiveness of our proposal

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

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

Performance of a multiband diversity antenna with hand effects

The implementation of multiband diversity antennas in compact mobile handsets, for the purpose of increasing transmission quality, is a topic of current interest in the mobile phone industry. In order to achieve the expected performance improvement in typical operating conditions, we not only have to contend with the challenges of designing multiple multiband antennas, which are closely spaced within the compact handset and thus strongly interacting with one another electromagnetically [1], we also need to keep in check the electromagnetic interaction between the whole antenna system (i.e., the handset) and the user. Previous studies have concluded that the presence of a user degrades the mean effective gain (MEG) of the diversity antennas significantly [1]-[3]. Different results have been presented on the effect of the user on the correlation coefficient, no effect [1] and a significant increase of the correlation [2], [3], have been pointed out. However, these studies have been performed on simple single band antennas in talk position. In this paper a more realistic approach is presented by choosing the diversity antenna system to comprise compact versions of PIFA and monopole antennas which cover three WCDMA bands: WCDMA850, WCDMA1800 and UMTS [4]. Such compact antennas are easily conformable for small mobile phone products [4]. The choice of the bands, as well as the evaluation of the diversity performance for the data mode position, is derived from the increasing demand on HSDPA applications in the mobile phone market. The investigation of user interaction presented in this paper focuses on the comparison between the free space and data mode diversity performance of a tri-band “stick” phone size prototype in the uniform 3D propagation environment. A state-of-the-art phantom hand from IndexSAR [5] is used to hold the diversity prototype in the data mode position

Characterization of MIMO antennas with multiplexing efficiency

A simple and intuitive metric of multiplexing efficiency is proposed for evaluating the performance of MIMO antennas in the spatial multiplexing mode of operation. The metric is particularly useful for antenna engineers whose goal is to achieve the optimum antenna system design. Experimental results involving prototype mobile terminals highlight the effectiveness of our proposal

Decoupling of multiple antennas in terminals with chassis excitation using polarization diversity, angle diversity and current control

Excitation of the chassis enables single-antenna terminals to achieve good bandwidth and radiation performance, due to the entire chassis being utilized as the main radiator. In contrast, the same chassis excitation phenomenon complicates the design of multiple antennas for MIMO applications, since the same characteristic mode of the chassis may be effectively excited by more than one antenna, leading to strong mutual coupling and severe MIMO performance degradation. In this paper, we introduce a design concept for MIMO antennas to mitigate the chassis-induced mutual coupling, which is especially relevant for frequency bands below 1 GHz. We illustrate the design concept on a dual-antenna terminal at 0.93 GHz, where a folded monopole at one chassis edge excites the chassis’ fundamental electric dipole mode and a coupled loop at the other chassis edge excites its own fundamental magnetic dipole mode. Since the two radiation modes are nearly orthogonal to each other, an isolation of over 30 dB is achieved. Moreover, we show that the antenna system can be conveniently modified for multiband operation, such as in the 900/1800/2600 MHz bands. Furthermore, by controlling the phase of the feed current on the folded monopole, the two antennas can be co-located on the same chassis edge with an isolation of over 20 dB. The co-located dual antenna prototype was fabricated and verified in the measurements

Actual diversity performance of a multiband diversity antenna with hand and head effects

Using the metric actual diversity gain (ADG), diversity performance is investigated for a compact mobile terminal prototype with two internal, triple frequency band antennas in four different cases of user interaction. ADG is presented as a preferred alternative to apparent diversity gain and effective diversity gain. Absorption due to user proximity causes degradation and imbalance in mean effective gain of the antennas over the frequency bands, contributing to a degradation in diversity performance. However, user-induced changes in the antenna patterns cause a decrease in correlation in the low frequency band, which facilitates increased diversity gain. The study reveals that a significant net diversity gain, i.e., ADG of 5-8 dB compared to a single antenna prototype, can be achieved using multiband antennas in the proximity of a user, even at low frequencies for antennas with high mutual coupling

A compact six-port dielectric resonator antenna array: MIMO channel measurements and performance analysis

MIMO systems ideally achieve linear capacity gain proportional to the number of antennas. However, the compactness of terminal devices limits the number of spatial degrees of freedom (DOFs) in such systems, which motivates efficientantenna design techniques to exploit all available DOFs. In this contribution, we present a compact six-port dielectric resonator antenna (DRA) array which utilizes spatial, polarization and angle diversities. To evaluate the proposed DRA array, a measurement campaign was conducted at 2.65GHz in indoor officescenarios for four 6 × 6 multiple antenna systems. Compared to the reference system of monopole arrays which only exploit spatial diversity, the use of dual-polarized patch antennas at the transmitter enriches the channel’s DOF in the non-line-of-sight scenario. Replacing the monopole array at the receiver withthe DRA array that has a 95% smaller ground plane, the 10% outage capacity evaluated at 10 dB reference signal-to-noise ratio becomes equivalent to that of the reference system, due to the DRA’s rich diversity characteristics. In the line-of-sight scenario, the DRA array gives a higher DOF than the monopole array asthe receive counterpart to the transmit patch array. However, the outage capacity is 1.5 bits/s/Hz lower, due to the DRA array’s lower channel gain