Trapezoidal Monopole Antenna and Array for UWB-MIMO Applications

Xue-Song Yang1, Jussi Salmi2, Andreas F. Molisch3, Shi-Gui Qiu1, Seun Sangodoyin3,

and Bing-Zhong Wang1 1Institute of Applied Physics, University of Electronic Science and Technology of China, Chengdu, 610054, China

2 Department of Signal Processing and Acoustics, SMARAD CoE, Aalto University, Espoo, Finland 3Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA

Abstract- Ultra-wideband (UWB) antenna arrays are

important for UWB communications, positioning, and radar. Antenna efficiency as well as omnidirectionality of the antenna pattern are important for these applications. This paper introduces and investigates a new printed array consisting of trapezoidal monopole antennas. The antenna elements have good impedance matching and omni-directional radiation pattern in the frequency range of interest, namely 2.2 GHz -7.5 GHz. Since a uniform linear array (ULA) of these elements shows severe pattern distortions at higher frequencies, we propose an interleaved uniform rectangular array (iURA). The iURA achieves a lower mutual coupling and thus improved omni-directionality of the radiation pattern. Furthermore, by comparing the ambiguity functions of a virtual iURA with that of a virtual ULA, it is found that, even though without considering the mutual coupling, the iURA has a better angular resolution performance than the ULA. We conclude that the iURA is more suitable than the ULA for UWB-MIMO applications. 1

Key words- antenna array, ultra-wideband (UWB), multiple-input multiple-output (MIMO), interleaved uniform rectangular array, ambiguity function.

I. INTRODUCTION

Ultra-wideband (UWB) signaling has received, over the past 10 years, great attention from both the academic and the industrial community. The large bandwidth enables the transmission of high data rates, and the inherent robustness of UWB to multipath propagation provides high reliability [1]. Furthermore, the high signal bandwidth also makes UWB uniquely suited for precision localization systems [7]. These advantages can be further enhanced by the use of multiple antenna techniques. Multiple-input multiple-output (MIMO) systems, which have multiple antenna elements at both link ends, can fully utilize the multipath components to improve the performance of wireless systems [2].

UWB combined with MIMO technology (UWB-MIMO) is thus a feasible way to achieve extremely high data rates for wireless communications [3]. UWB is often applied to short-range and hence mainly indoor communications, where the environment is characterized by dense multipath propagation. For this type of environment, MIMO system

This work was supported in part by the Fundamental Research Funds for the Central Universities (ZYGX2009J039) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (200806141039). The work of X.-S. Yang was supported by the Chinese Scholarship Council.

brings a significant increase of spectral efficiency by utilizing the inherent array gain and spatial multiplexing gain of the system. Furthermore, UWB-MIMO is well suited to obtain high-precision localization and enhanced radar imaging.

As for all wireless transmission systems, the antenna is a key component of UWB signaling; consequently UWB antennas have attracted a lot of research interest in academic and industrial fields [4], [5]. For some applications, directional antennas, which can radiate energy in a fixed direction with high gain, are important. However, omnidirectional radiation characteristics are usually more desirable. This is due to unique requirements of UWB systems – in particular the limits on equivalent isotropically radiated power EIRP imposed by many spectrum regulators – as well as the need of applications such as ranging and radar to work well with signals that arrive from any (unpredictable) direction. Thus, a new but important design requirement of UWB antennas is a large pattern bandwidth, which is defined as the bandwidth over which the radiation pattern remains approximately constant, e.g., omnidirectional. Most of the existing UWB antennas have ultra-wide impedance matching bandwidth, but not pattern bandwidth. However, in [5], one of us designed planar trapezoidal monopole antennas using a Genetic Algorithm (GA), aiming to provide omni-directional pattern and uniform gain over the ultra-wide frequency range.

In this paper, one of the pattern-optimized antennas is used as the building block for antenna arrays for UWB-MIMO systems. The array structure has important implications on the performance of MIMO systems, such as fading correlation (and thus capacity) of a communication system, as well as ability to resolve directions of multipath components. While uniform linear arrays (ULAs) are the most popular array structure, we show that in our context they have significant drawbacks in particular related to mutual coupling between the array elements. We thus propose an interleaved uniform rectangular array (iURA), and analyze its performance.

The remainder of the paper is organized as follows: in Section II, the structure and performance of the planar trapezoidal monopole antenna are introduced. Section III describes the performance of a ULA and an iURA. Section

978-1-4673-2185-3/12/$31.00 ©2012 IEEE

IV provides the ambiguity functions of the two arrays, and Section V gives the conclusions.

II. PLANAR TRAPEZOIDAL MONOPOLE ANTENNA

The planar trapezoidal monopole antenna, corresponding to the antenna A in [5], is shown in Fig. 1. It is fabricated on a sheet of Rogers Duroid 6002 substrate, which has a thickness of 0.03” and dielectric constant of 2.94. The other structure parameters of the antenna are listed in Table I in [5].

In this work, the antenna is both simulated and measured. The simulation is carried out by an electromagnetic simulation software HFSS, which is based on the Finite Element Method (FEM). The measurements of the S-parameters are done by means of a Vector Network Analyzer, model Agilent 8720ET, to verify the simulated results. The simulated and measured |S11| parameters are shown in Fig. 2. From the results, it can be seen that the impedance bandwidth of -10 dB |S11| of the antenna is from 2.5 GHz up to more than 8 GHz.

We furthermore performed measurements of the antenna patterns in the anechoic chamber located at the UltraLab at

(a)

(b)

Figure 1. (a) Structure of the planar trapezoidal monopole antenna, (b) the fabricated planar trapezoidal monopole antenna.

2 3 4 5 6 7 8-60

-50

-40

-30

-20

-10

0

S p

aram

eter

s (d

B)

Frequency (GHz)

S11_mea (dB) S11_sim (dB)

Figure 2. Measured and simulated S parameters of the antenna.

the University of Southern California. In a transmitting/receiving antenna pair measurement, two identical antennas are placed in the anechoic chamber, arranged in a face-to-face, back-to-back, and side-by-side configuration. The distance between the antennas is 5.4 meters, thus ensuring that far-field conditions are satisfied. The gains at the three orientations of each antenna are extracted from the measurements and shown in Fig. 3. From the results, it can be seen that the gains of the antenna at three orientations are very stable in the frequency range from 2.2-7.5 GHz, implying a very good omni-directional pattern, as well as a very stable gain within this frequency band. We note that many frequency regulators allow UWB transmission in the frequency range starting at 3.1 GHz. However, providing a lower bandedge allows the antenna to also function for transmission in the ISM band at 2.45 GHz. Such dual-band capability is desirable for many applications

III. UNIFORM LINEAR ARRAY AND INTERLEAVED UNIFORM RECTANGULAR ARRAY

The proposed trapezoidal monopole antenna can be used to form arrays for UWB MIMO applications; here we analyze a uniform linear array (ULA), the most popular array structure. In MIMO systems, the spacing between neighboring elements of the antenna array should be approximately half the free space operating wavelength, providing a compromise between signal decorrelation (for spatial multiplexing) and uniqueness of direction-of-arrival determination. Since the array element has good performance over the frequency range of 2.2-7.5 GHz, the distance between adjacent elements of the array is selected to be 50 mm, which corresponds to the free space half wavelength at 3 GHz.

When antenna elements are placed close to each other (as occurs in an array), mutual coupling between the elements can lead to a distortion of the element patterns [8]. The simulated S parameters and radiation patterns of a five-element ULA (ULA-5) are shown in Figs. 4 and 5, respectively. The middle element is named element 1, and the left two elements are named elements 2 and 3. S11, S21 and S31 are given to show the impedance matching and mutual coupling of the array. It is clear that the patterns, especially at higher frequencies, are distorted significantly, due to the strong mutual coupling.

2 3 4 5 6 7 8 9 10 11 12-70

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0

10

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Gai

n (d

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Frequency (GHz)

face to face back to back side by side

Figure 3. Measured gains of the antenna at three orientations.

2 3 4 5 6 7 8-50

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-5

S pa

ram

eter

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|S11| iURA-5 |S11| ULA-5 |S21| iURA-5 |S21| ULA-5 |S31| iURA-5 |S31| ULA-5

Figure 4. Simulated S parameters of the two arrays ULA-5 and iURA-5.

2.5 GHz 4.5 GHz 6.5 GHz

Figure 5. Simulated radiation patterns of the middle element in a

five-element uniform linear array (ULA-5), with inter-element space 50 mm.

In order to decrease the mutual coupling and maintain a

higher angular resolution, we propose an interleaved uniform rectangular array (iURA), as shown in Fig. 6. In order to maintain a compact size of the array, the patches of elements are oriented towards the outer side of the array, while the feedlines are close to each other. Each element is fed at the end of the microstrip feedline through a via from the bottom of the substrate.

The performance of a five-element iURA (iURA-5) is simulated with the Ansoft HFSS program. The chosen parameters for inter-element spacing are dh = 5 cm and dv = 10 cm is simul, where dh and dv are the horizontal and vertical distance between the patch centers of two neighboring elements in the array. The simulated S parameters of the array and radiation patterns of the middle element ele_1 are shown in Figs. 4 and 7, respectively. Because the antenna array is symmetric with respect to ele_1, only a subset of the S parameters is given.

From Fig. 4, it can be seen that the reflection coefficient is below -10 dB after 2.4 GHz, and the mutual coupling between neighboring elements are lower than -17.5 dB over the same frequency band. The mutual coupling between the neighboring elements of the iURA is much lower than that of the ULA.

As for the radiation pattern, it can be seen from Fig. 7 that the three dimensional radiation pattern of element ele_1 at 2.5 GHz is close to omnidirectional, and that at 4.5 GHz is better than that of the ULA. However, the pattern at 6.5 GHz shows some significant distortions, similar to the ULA.

IV. AMBIGUITY FUNCTIONS OF THE ARRAYS

The ambiguity function is a parameter used to evaluate the angular resolution performance of an array in MIMO channel measurement, MIMO communications, MIMO radar, etc.. It is defined as [6]

(a) (b)

Figure 6. (a) Scheme of a five-element interleaved uniform rectangular array (iURA-5) and (b) antenna element in the iURA.

2.5 GHz 4.5 GHz 6.5 GHz

Figure 7. Simulated radiation patterns of the middle element in the five-element iURA-5, with inter-element space dh=5 cm (Δy=5 cm and

dv=10 cm (Δz=10 cm).

( ( )) ( ( ))( , )

|| ( ( )) || || ( ( )) ||B B

B B

Hi j

i ji F j F

vec vecC

vec vec� �

� �� �

��

� (1)

where /( ) T R fM Mi C� ��B is the complex-valued

frequency response (transfer function) at angle i� of an antenna array with MT/R ports, sampled at Mf frequency points, operation ( )vec � stacks all the elements of the

matrix (tensor) into a column vector, || ||F� denote the

Frobenius norm operation, and superscript H� denote complex Hermitian transpose. In this paper, the complex frequency response of the array is obtained using the virtual antenna array model [6], i.e. the measured response of a single antenna shifted according to the location and/or rotation of the elements. The benefit of using virtual array is that no mutual coupling effect exists between elements.

The ambiguity functions of a 10-element ULA and a 10-element iURA are plotted in Figs. 8 and 9. The narrower the main lobe of ambiguity function, the better the angular resolution of the array. The smaller the side lobes of the ambiguity function, the lower the angular ambiguities. From the figures, it is clear that both ULA-10 and iURA-10 have very high quality ambiguity functions in the azimuth angle (elevation angle equals to 0) and they are nearly the same. However, the ambiguity function in the elevation angle (azimuth angle equals to 0) of the iURA-10 is much better that of the ULA-10. The reason for the improvement in the ambiguity function of the iURA-10 is the expansion of the array size along the elevation direction. That means with the use of the iURA, the elevation angular resolution is improved while the azimuth angular resolution is maintained.

V. CONCLUSION

This paper investigated arrays of trapezoidal monopole UWB antennas. The antenna elements have good impedance matching, omni-directional radiation pattern, and stable gain over the 2.2 GHz -7.5 GHz

y z

(a)

(b)

Figure 8. Ambiguity functions of antenna array ULA-10, (a) ambiguity function in the azimuth angle and (b) ambiguity function in the elevation

angle.

frequency range. Based on this antenna, two uniform UWB arrays have been constructed, i.e., a ULA and an iURA. With the use of the interleaved arrangement, the mutual coupling between adjacent elements is decreased and good omni-directional radiation pattern performance is achieved. Furthermore, by comparing the ambiguity functions of the virtual iURA with that of the virtual ULA, it is found that, even though without considering the mutual coupling, the iURA has an improved angular resolution performance compared to the ULA. This characteristic makes the iURA especially suitable for UWB-MIMO applications, such as high-data-rate communications, channel sounding, precision localization, and radar.

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(a)

(b)

Figure 9. Ambiguity functions of antenna array iURA-10, (a) ambiguity function in the azimuth angle and (b) ambiguity function in the elevation

angle.

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