Broadband MIMO Antennas for MIMO Radar Systems

Chan-Chieh Lin*, Tzu-Chun Tang, Ken-Huang Lin

Department of Electrical Engineering National Sun Yat-Sen University

Kaohsiung, Taiwan Email: dragonnox@pcs.ee.nsysu.edu.tw, a565656kimo@pcs.ee.nsysu.edu.tw, khlin@mail.nsysu.edu.tw

Abstract—Methods of reducing both electric and magnetic field coupling for enhancing the isolation in broadband MIMO antenna system is presented. The proposed MIMO antenna consists of two coplanar waveguide (CPW) fed self-complementary antennas (SCAs) and a U-shaped slot is etched on the ground plane. The protruding structure from the ground plane behaves as both the part of the SCA and an isolator when two SCA protruding structures are connected to each other. Furthermore, the U-shaped slot is introduced for isolation enhancement. The proposed MIMO antenna covers the impedance bandwidth of 4-8 GHz (|S11| ≤ -10 dB) and reaches the good isolation (|S21| ≤ -18 dB) in the band of interest to radar systems.

I. INTRODUCTION Nowadays, development of multiple-input multiple-output

(MIMO) technology is widely adopted in wireless communications and radar systems. The requirements of a radar system are quite different from those of wireless communications [1]. With regard to MIMO antenna design, the isolation between two closely antenna elements should be considered as a significant criterion of MIMO antenna performance, and the broadband operation is preferred in the MIMO radar system for lowering the detection error rates and achieving better resolution than traditional phase array antenna based radar systems [2].

According to Babinet’s principle [3], SCA structure possesses the constant antenna input impedance of 188.5 Ω, which was firstly introduced by Mushiake [4] for broad impedance bandwidth antenna design. However, an ideal SCA always requires an infinite complementary ground plane to obtain characteristics of constant input impedance. In reality, planar microstrip antenna faces the challenges of limited ground plane area available in finite substrate, and the finite ground plane area will cause some degradation in antenna bandwidth performance.

Furthermore, the proposed MIMO antenna combines two decoupling mechanisms together to reduce electromagnetic coupling between antenna elements, reaching broadband operation for both impedance and isolation bandwidths.

II. ANTENNA ELEMENT DESIGN The proposed antenna element is shown in Fig. 1(a), which

is based on SCA concept. It is a monopole antenna structure with complementary slot built on the 50 Ω CPW-fed ground

plane, a quarter-circle was cut from the rectangular patch for impedance matching. The S11 derived from Ansoft HFSS 13.0 is shown in Fig. 1(b), and the design parameters of the antenna are shown in Table I.

(a)

(b)

Figure 1: Antenna element (a) antenna structure (b) S11 (dB)

Table I: Geometrical parameters of the antenna (unit: mm) L1=20 w1=10 w3=9 Quarter-circle radius r=6.5 L2=27 w2=9 FR4 substrate height=1.6

III. MIMO ANTENNA STRUCTURE

A. Performance Analysis In Fig. 2, MIMO antenna was formed by considering two

different situations. The first structure is shown in Fig. 2(a), causing the degradation in S11 performance because of asymmetric antenna structure. In Fig. 2(b), the protruding structure is connected to each other, generating a branch between two antenna elements, and the protruding branch from ground plane enhances the isolation [5]. However, this method encounters the limitation in isolation bandwidth. To overcome the problem, the following section will introduce an

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idea of using U-shaped slot on the ground for further isolation enhancement.

(a)

(b)

Figure 2: MIMO structure: (a) two elements align in same direction (b) two elements align in opposite direction

To investigate the effects of branch protruding from

ground plane, Fig. 3(a) removes the branch from two antenna elements. Because of removal of the branch, it causes some degradation in S21 performance from 3.5 GHz to 9 GHz, as shown in Fig. 3(b).

(a)

(b)

Figure 3: Branch effect (a) branchless structure (b) S21 (dB)

B. Broadband Operation To achieve broadband operation in MIMO antenna system,

a U-shaped slot is etched on the ground plane as shown in Fig. 4(a), and in Fig. 4(b), a good improvement of broadband

isolation |S21| ≤ -18 dB from 4 GHz to 8 GHz is achieved because of change in ground plane current distribution. In some frequencies, two reverse surface currents appear in both sides of edge of the U-shaped slot providing ability to reduce magnetic coupling between antenna elements. Therefore, the isolation will be enhanced resulting in broadband operation.

(a)

(b)

Figure 4: Proposed MIMO antenna (a) complete structure (b) S21 (dB)

IV. CONCLUSION We proposed a MIMO antenna which achieves satisfactory

broadband operations in terms of both the impedance (|S11| ≤ -10 dB) and isolation (|S21| ≤ -18 dB) characteristics in the band of 4 GHz to 8 GHz. The proposed MIMO antenna possesses the good performance for MIMO radar applications.

REFERENCES

[1] J. Li and P. Stoica, MIMO Radar Signal Processing. Wiley, 2008. [2] X. H. Wu, A. A. Kishk, and A. W. Glisson, “Antenna effects on a

monostatic MIMO radar for direction estimation , a cramer-rao lower bound analysis,” IEEE Trans. Antennas Propag., vol. 59, no. 6, pp. 2388–2395, Jun. 2011.

[3] C. B. Balanis. Antenna Theory, Analysis and Design. Second edition, 1997.

[4] Y. Mushiake, “Self-complementary antenna,” IEEE Antenna and Propagat. Magazine, vol. 34, no. 6, pp. 23-29, Dec. 1992.

[5] T. Y. Wu, S. T. Fang and K. L. Wong, “Printed diversity monopole antenna for WLAN operation,” Electron. Lett., vol. 38, no. 25, pp. 1625-1626, Dec 2002.

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