Search

A low-profile wideband tri-polarized multi-input multi-output (MIMO) antenna is proposed in this communication. Based on characteristic mode analysis (CMA) of a circular patch, four slots are etched symmetrically on the patch to transform the high-order monopole-like modes into broadside modes with lower resonant frequencies. Four sets of shorting pins are also applied to simultaneously adjust the resonant frequencies of the monopole-like modes and broadside fundamental modes of the patch. The effects of the slots and the shorting pins on these modes are analyzed in detail. Three dual-resonance ports with high isolation are achieved by exciting three pairs of orthogonal modes of the patch. The proposed antenna is wideband with respect to its profile (bandwidth/height = 216), as facilitated by the dual-resonance property of each port. Experimental results show that the proposed antenna features high port isolation (>35 dB) and high gain (>5.58 dB). The proposed low-profile wideband tri-port antenna can be applied in highly integrated MIMO systems. [ABSTRACT FROM AUTHOR]

Massive multiple-input, multiple-output (MIMO) arrays are becoming critical elements in cellular base station infrastructure. We utilize the theory of characteristic modes (TCM) to design a novel trimodal broadside antenna element that is suitable for constructing compact massive MIMO arrays. The proposed antenna element consists of three ports, formed by reviewing an existing compact two-port Y-shaped patch antenna from a TCM perspective. Using this perspective, the Y-shaped antenna is modified into a snowflake-shaped patch antenna and excited by three ports via capacitive coupling. The advantage of this design approach is that the size of the three-port antenna is approximately the same as a conventional dual-polarized patch antenna, hence allowing 50% more antenna elements for the same array aperture. By cutting the ground plane into a hexagonal shape, a multiple of the proposed three-port canonical antennas are concatenated together to form 21- and 102-port massive MIMO arrays with all radiation patterns pointing in the broadside direction. Key performance characteristics of the massive MIMO arrays, i.e., the Hermitian product of the simulated massive MIMO channel and mutual coupling, validate the effectiveness of the compact design. [ABSTRACT FROM AUTHOR]

In this communication, a compact pattern reconfigurable dual-antenna array is designed at 2.65 GHz for multiple-input multiple-output (MIMO) terminal applications, such as laptops. Each antenna is composed of two layers, i.e., the monopole layer and the planar inverted-F antenna (PIFA) layer. By switching the p-i-n diodes on each layer, its radiation patterns can be varied between conical and boresight patterns. Two such identical antennas are placed side by side on a common ground plane for MIMO application, providing altogether four operating modes. The center-to-center separation between the antennas is 0.25 wavelength, to keep the overall array compact. To facilitate an isolation of above 17 dB over all the modes for this compact array, two decoupling techniques based on the decoupling slits and the shielding wall are effectively combined. The envelope correlation coefficients (ECCs) of the far-field patterns in the four modes are below 0.05 in free space, and below 0.1 for most cases in the indoor and outdoor scenarios. The only exception is the monopole-PIFA mode in the outdoor scenario, but the ECC is still below 0.3 for this case. The measured efficiencies of the antennas are between −1.7 (68%) and −0.7 dB (85%) for all the modes. Therefore, good diversity and MIMO performances are expected for the proposed design. [ABSTRACT FROM AUTHOR]

Measuring multiple-input multiple-output (MIMO) specific absorption rate (SAR) is an important part of mobile handset evaluation, following the introduction of uplink MIMO transmission schemes in long-term evolution advanced systems. However, the measurement of MIMO-SAR is complex and time consuming. In this communication, six unique dual-antenna mobile handsets, including both typical and novel designs, are analyzed to determine the dependence of MIMO-SAR on antenna configuration and correlation. It is found that, for certain antenna configurations, the location of the maximum MIMO-SAR and the corresponding relative phase between the ports can be predicted. This can be applied to drastically reduce measurement time. In addition, dual antennas with low envelope correlation coefficients in the radiation patterns also offer near-orthogonal electric near fields. This leads to smaller MIMO-SAR than stand-alone SAR (S-SAR) over all relative phases, making MIMO-SAR measurement unnecessary. The results also provide guidelines for designing multi-antenna handsets with low SARs. For verification, the S-SAR and MIMO-SAR at a relative phase of 0° were measured for several prototypes. The measured SAR distributions showed good agreement with the simulated ones. [ABSTRACT FROM AUTHOR]

Despite its long history, the theory of characteristic modes (TCMs) has only been utilized in antenna design for perfect electric conductors (PECs). This is due to computational problems associated with dielectric and magnetic materials. In particular, the symmetric form of the Poggio–Miller–Chan–Harrington–Wu–Tsai (PMCHWT) surface formulation for the method of moments (MoM) solves for both external (real) and internal (nonreal) resonances of a structure. The external resonances are the characteristic modes (CMs), whereas the internal resonances are not. This paper proposes a new postprocessing method capable of providing unique and real CMs in all physical mediums, including lossy magnetic and dielectric materials. The method removes the internal resonances of a structure by defining a minimum radiated power, which is found through utilizing the physical bounds of the structure. The CMs found using the proposed method are verified through the use of an MoM volume formulation, time domain antenna simulations, and experiments involving multiple antenna prototypes. [ABSTRACT FROM AUTHOR]

It is well known that user proximity introduces absorption and impedance mismatch losses (MLs) that severely degrade multiple-input multiple-output (MIMO) performance of handset antennas. In this work, we experimentally verified the potential of adaptive impedance matching (AIM) to mitigate user interaction effects and identified the main AIM gain mechanism in realistic systems. A practical setup including custom-designed CMOS silicon-on-insulator (SOI) impedance tuners implemented on a MIMO handset was measured in three propagation environments and ten real user scenarios. The results indicate that AIM can improve MIMO capacity by up to 42% equivalent to 3.5 dB of multiplexing efficiency (ME) gain. Taking into account the measured losses of 1 dB in the integrated tuners, the maximum net ME gain is 2.5 dB suggesting applicability in practical systems. Variations in ME gains of up to 1.5 dB for different hand-grip styles were mainly due to differences in impedance mismatch and tuner loss distribution. The study also confirmed earlier results on the significant differences in mismatch and absorption between phantoms and real users in which the phantoms underestimated user effects and, therefore, AIM gains. Finally, propagation environments of different angular spreads were found to give only minor ME gain variations. [ABSTRACT FROM PUBLISHER]

Using the metrics of channel capacity and multiplexing efficiency, the adaptive impedance matching (AIM) performances of two multiple-input multiple-output (MIMO) terminals with different antenna designs were evaluated and compared. The evaluation was performed in LTE Band 18 Downlink (860–875 MHz) under realistic usage conditions of two measured user handgrips and simulated propagation channels with different angular spreads (ASs). The results provide potential performance gains from AIM based on realistic MIMO terminal prototypes, and the underlying mechanisms by which the gains were achieved, which can serve as antenna and AIM circuit design guidelines. In particular, the evaluation revealed that ideal uncoupled AIM networks can increase the capacity by up to 52% relative to 50\text-\Omega terminations. However, the observed gains depend heavily on the antenna design, the user scenario, and the channel’s AS. For example, the wideband design in different user cases experienced capacity gain of 4%–9% from AIM in uniform 3-D channels, in contrast to the 1.3%–44% gain seen in a conventional narrowband design. In nonuniform channels with small ASs, the AIM gain for different mean incident angles depends on the absolute mean effective gain (MEG) and the change in correlation due to AIM; in cases where AIM has little impact on correlation, the mean incident angles with high AIM gains were close to those with high MEGs. [ABSTRACT FROM AUTHOR]

Correlation coefficient of received signals across a pair of antennas is a key performance indicator for multiple-input–multiple-output (MIMO) systems. For multipath environments with uniform 3-D angular power spectrum, the signal correlation between two antennas can be exactly calculated from their 3-D radiation patterns. When radiation patterns are unavailable, a simplified approach that only requires the antennas' scattering parameters can be used instead. However, the simpler method assumes lossless antennas and thus only works well for antennas with high radiation efficiencies. To take into account the antenna loss, the idea of equivalent circuit approximation is used in this paper to analytically separate the lossy components (resistance or conductance) from the lossy antenna arrays, using known scattering parameters and radiation efficiencies. The simplified method using S parameters can then be applied to obtain the correlation coefficient of the equivalent lossless antennas. The effectiveness of the method has been verified on antennas operating at a single mode, such as dipole or patch at its lowest resonant frequency. Good results were also obtained for the measured case of a dual-antenna mobile terminal, consisting of a monopole and a PIFA. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

A closely-packed ultrawideband (UWB) multiple- input multiple-output (MIMO)/diversity antenna (of two elements) with a size of 25 mm by 40 mm is proposed for USB dongle applications. Wideband isolation can be achieved through the different patterns and polarizations of the two antenna elements. Moreover, the slot that is formed between the monopole and the ground plane of the half slot antenna is conveniently used to further enhance the isolation at the lower frequencies and to provide an additional resonance at one antenna element in order to increase its bandwidth. The underlying mechanisms of the antenna's wide impedance bandwidth and low mutual coupling are analyzed in detail. Based on the measurement results, the proposed antenna can cover the lower UWB band of 3.1–5.15 GHz, and within the required band, the isolation exceeds 26 dB. The gains and total efficiencies of the two antenna elements are also measured. Furthermore, a chassis mode can be excited when a physical connection is required between the ground planes of the two antenna elements. Without affecting the performance of the half slot element, the monopole can now cover the band of 1.78–3 GHz, apart from the UWB band. The proposed antenna structure is found to provide good MIMO/diversity performance, with very low envelope correlation of less than 0.1 across the UWB band. [ABSTRACT FROM PUBLISHER]

Multiple-antenna handheld terminals are an integral part of the latest mobile communication systems, due to the adoption of multiple-input multiple-output (MIMO) technology. In this contribution, MIMO performances of three multiple-antenna configurations for smart phones are investigated in an urban macrocellular environment, based on an extensive MIMO measurement campaign at 2.65 GHz. The smart phones were held in a two-hand user grip position and both downlink noise- and interference-limited scenarios were evaluated. Our results show that, overall along the test route, the capacity performances are dictated by the power of the communication links. Nevertheless, locally, the multipath richness of the communication channel can have a significant impact on the capacity performances. In addition to the terminal antenna configurations, spatial and cross polarization references were also utilized to further illustrate the importance of multipath richness in both the desired and the interference channels. Moreover, the differences in the local capacity performances along the route for the three antenna configurations give a first indication of the potential benefit in implementing reconfigurable multiple antennas. If switching for maximum capacity is applied between the terminal antenna configurations in the two-hand grip position, average capacity improvements of up to 17% and 30% for local 20 m route sections are observed in the noise- and the interference-limited scenarios, respectively. [ABSTRACT FROM PUBLISHER]

It has been postulated that six colocated orthogonally polarized electrical and magnetic dipoles can offer up to six degrees of freedom (DOFs) in a multipath scattering environment. In other words, a sixfold increase in channel capacity can be achieved in the context of multiple-input–multiple-output (MIMO) systems. However, due to the complexity in designing and measuring such a six-port antenna, to our knowledge, no experimental verification has yet been successfully performed. In this paper, the six DOFs hypothesis is experimentally verified at 77 MHz. The experiment involves the design and fabrication of two six-port arrays. In particular, one array is made compact with colocated antenna elements enclosed in a small cubic volume of (0.24 \lambda)^3. Using the two arrays, MIMO channel measurements are performed in an RF shielded laboratory in order to measure a multipath scattering environment. Analysis on eigenvalues of the measured channel shows that six DOFs are attained due to the orthogonality in radiation patterns of the two arrays. [ABSTRACT FROM PUBLISHER]

An efficient technique is introduced to reduce mutual coupling between two closely spaced PIFAs for MIMO mobile terminals. The proposed mutual coupling reduction method is based on a T-shape slot impedance transformer and can be applied to both single-band and dual-band PIFAs. For the proposed single-band dual PIFAs, the 10 dB impedance bandwidth covers the 2.4 GHz WLAN band (2.4–2.48 GHz), and within the WLAN band an isolation of over 20 dB is achieved. Moreover, the dual-band version covers both the WLAN band and the WiMAX band of 3.4–3.6 GHz, with isolations of over 19.2 dB and 22.8 dB, respectively. The efficiency, gain and radiation patterns of the two-PIFA prototypes are verified in measurements. Due to very low pattern correlation and very good matching and isolation characteristics, the capacity performances are mainly limited by radiation efficiency. The single-band and dual-band PIFAs are also studied with respect to their locations on the ground plane. An eight-fold increase in the bandwidth of one PIFA is achieved, when the single-band PIFAs are positioned at one corner of the ground plane, with the bandwidth of the other PIFA and the good isolation unchanged. [ABSTRACT FROM PUBLISHER]

We study the potential benefits of cooperative multiple-input multiple-output signaling from multiple coherent base stations with one or more mobile stations in an urban macrocellular environment at 2.66 GHz. The analysis uses fully-coherent measurements of the channel from three base stations to a single mobile station equipped with four antennas. The observed channels are used to explore the gains in capacity enabled by cooperative base station signaling for point-to-point and multi-user communications. The analysis shows that for point-to-point links, the average capacity for cooperative signaling is 53% higher than that achieved for a single base station. For downlink and uplink communication with three mobile users, cooperative signaling yields average sum rate increases of 91% and 63%, respectively. [ABSTRACT FROM PUBLISHER]

An introduction is presented to articles in the journal which discuss antenna arrays in multiple-input multiple-output (MIMO) wireless communication systems including Zheng et al., Qin et al. and Nishimori et al.

The design of multiple antennas in compact mobile terminals is a significant challenge, due to both practical and fundamental design tradeoffs. In this paper, fundamental antenna design tradeoffs of multiple antenna terminals are presented in the framework of characteristic mode analysis. In particular, interactions between the antenna elements and the characteristic modes and their impact on design tradeoffs are investigated in both theory and simulations. The results reveal that the characteristic modes play an important role in determining the optimal placement of antennas for low mutual coupling. Moreover, the ability of antenna elements to localize the excitation currents on the chassis can significantly influence the final performance. To demonstrate the effectiveness of the proposed approach, a dual-band, dual-antenna terminal is designed to provide an isolation of over 10 dB for the 900 MHz band without additional matching or decoupling structures. A tradeoff analysis of bandwidth, efficiency, effective diversity gain and capacity is performed over different antenna locations. Finally, three fabricated prototypes verify the simulation results for representative cases. [ABSTRACT FROM PUBLISHER]

Impedance matching for coupled antenna arrays has received considerable attention in the research community. Because optimal matching requires implementation of a coupled network, leading to high complexity and often narrow operation bandwidth, new research has focused on the development of uncoupled matching networks with good performance. This paper explores traditional uncoupled impedance matching techniques for coupled arrays, specifically matching to the array active and passive impedances, within the context of multiple-input multiple-output communication. The concept of matching to the array active impedance is extended to the case where the propagating field is specified stochastically, and the performance of this solution is compared to that of traditional solutions using simulations. While emphasis is placed on matching for maximum power transfer, the paper concludes with a discussion on matching for minimum amplifier noise figure. [ABSTRACT FROM PUBLISHER]

We analyze the impact of the matching network on compact multiple-input multiple-output systems. Existing studies have found that the matching network has a significant influence on the performance of multiple antenna systems when the antennas are in close proximity. However, none has examined the wide-band case. In this paper, we investigate the wide-band performance of four different matching networks for multiple dipole antennas. The performance of the matching networks is given in terms of the bandwidths of correlation and matching efficiency, which are extensions of the single-antenna concept of bandwidth to multiple antenna systems. We also investigate the impact of the propagation conditions on the matching and bandwidth. For a uniform two-dimensional (2-D) angular power spectrum, we find that while individual-port matching can achieve in excess of 3% fractional correlation bandwidth for envelope correlation of 0.5 at an antenna separation of 0.01lambda, multiport matching is required for efficiency bandwidth to exist for a return loss of -6 dB. Moreover, even with multiport matching, both correlation and efficiency bandwidths decrease drastically at small antenna separations. At 0.01lambda, the correlation and efficiency bandwidths are 0.4% and 0.2%, respectively. Similar evaluations were performed for measured outdoor-to-indoor channels with moderate to small 2-D angular spreads. We find that the efficiency advantage of multiport matching over individual-port matching diminishes with decreasing angular spread [ABSTRACT FROM PUBLISHER]

The Theory of Characteristic Modes (TCM) had its humble beginnings in the early 1970's. The beauty of TCM lies in its ability to fully characterize the radiation and scattering properties of an arbitrary object based only on the object's geometry and material properties. This ability provides valuable insights into an antenna's behavior independent of the feeding arrangement as well as providing information about how desirable radiation modes can be excited. This feature has led to its use to design integrated antennas in the High Frequency (HF) band for land vehicles, ships and aircraft. However, TCM had largely remained a specialist field until it was rediscovered for aiding the design of mobile handset antennas about a decade ago. In particular, TCM provides a powerful tool to understand and exploit excitation of the terminal chassis to enhance antenna performance. Another powerful feature of TCM is that multiple characteristic modes at a given frequency facilitate orthogonal radiation patterns, which provide effective Multiple-Input Multiple-Output (MIMO) antennas. [ABSTRACT FROM AUTHOR]