SWB MIMO Antenna with Dual Band Rejection Characteristics and Polarization Diversity for UWB Applications

: In this paper a super wide band 2-element MIMO antenna with dual band-notched characteristics and high isolation is presented for use in broadband polarization diversity communication systems. Total dimensions of the fabricated antenna is 34 × 60 mm 2 and achieves a super wide impedance matching of more than 15 GHz from 2.3 to 18 GHz due to the simulated and measured results which covers L, S, C, X, Ku bands and some parts of K band with two notched frequency bands around 3.1 – 3.7 GHz for WiMAX and 5.1 – 5.8 GHz for WLAN which realized by etching two half-wavelength elliptic single complementary split-ring resonators (ESCSRRs) of different dimensions on the radiating patches. This antenna comprises two identical elements which placed orthogonally to each other and fed by two 50 Ω coplanar waveguide (CPW) lines, therefore an isolation of more than 20 dB is obtained autonomously without utilizing any other special technique. Diversity performance in terms of envelop correlation coefficient (ECC < 0.0035 ), multiplexing efficiency ( (cid:2015) > -1 dB) and diversity gain (DG > 9.99 dB) are also calculated, showing satisfactory MIMO characteristics.


Introduction
Microstrip antennas are considered as the most common types of antennas during the past few years due to their obvious advantages of light weight, low cost, low profile, planar configuration, superior portability, easy for fabrication, and easy integration with microwave monolithic integrated circuits (MMICs) [1] and ever since the Federal Communication Commission (FCC) assigned ultra-wide band (UWB) frequency range from 3.1 to 10.6 GHz, UWB technology have gained so much attention [2].
Meanwhile SWB antennas become prevalent which cover both long-range and short-range communication that includes the UWB range and is an integral part in advanced wireless communication. Along with all the above advantages, multipath fading, reliability limitations and interfering some existing narrowband communication systems such as WiMAX, WLAN and X-band, are some important problems that should be solved well for UWB and SWB systems. In this regard numerous researches have been allocated and it has been confirmed that UWB-MIMO wireless communication combining UWB technology with multiple input multiple output (MIMO) technology can effectively increase the channel capacity and reliability of the system, reduce the channel error rate, increase the working bandwidth, and overcome multipath fading of UWB communication systems without requiring any additional bandwidth consumption therefore UWB-MIMO antenna design in the field of wireless communications has received more and more attention [3][4][5][6]. The UWB-MIMO systems require compact antennas with good isolation between the ports. As regards with that, several techniques have been studied and utilized to improve the isolation between the elements of MIMO antennas while maintaining their small electrical lengths. For example to reduce the mutual coupling between the ports of the proposed 2-element MIMO antennas in [7] and [8] a ε-negative metasurface superstrate and a fractal isolator which is an electromagnetic band gap (EBG) structure based on metamaterial is introduced between the elements respectively and isolation of more than 25 dB and 31 dB is obtained in each case. Also in [9] and [10] by using the neutralization line technique, the mutual coupling between the radiating patches is reduced to less than -22 dB in each case. In [11][12][13] elements of the MIMO antennas are placed orthogonal to each other and the polarization or pattern diversity cause high isolation of more than 20 dB for each antenna.
Also to suppress the interferences of the mentioned narrowband systems, different methods such as etching inverted L-shape [14], U-shape [15] and T-shape [16] slots of different dimensions on the ground plane, on the feed line and on the radiating patch have been proposed, respectively. Embedding different types of split-ring resonators on the radiating patch [17] and [18], placing various strips near the feed-line [19], or near the ground plane [20], utilizing parasitic elements such as bended dual L-shaped branches on the patch [21] and using substrate integrated waveguide (SIW) based metallic via integration of hexagonal and semi-hexagonal parasitic slot [22], have been utilized to obtain UWB antennas with single or several band-notched characteristics. In [23] three open ended slots are introduced into the radiating patch to filter WiMAX, WLAN and X-band interferences and in [24] a monopole antenna with six band-notched characteristics is presented based on SRRs and U-shaped parasitic strips and notched bands around 2.96-3.33 GHz, 3.73-3.88 GHz 4.43-4.53 GHz, 5.37-5.57 GHz, 7.02-7.30 GHz and 7.56-8.06 GHz have been obtained. Also in [25] a dual band-notched MIMO antenna is proposed with a total size of 25 × 39 mm 2 and impedance bandwidth from 2.6 to 12.5 GHz. In this work, to reduce mutual coupling to less than -20 dB the elements placed perpendicular to each other and to filter the interferences of WLAN and X-band, two L-shaped slots are etched on each radiating patches.
In this paper a compact single-layer dual band-notched SWB MIMO antenna with high isolation and polarization diversity is proposed. The total size of this design is about 34 × 60 × 1.6 mm 3 which is small compare to some previous researches [26] and [27] and consist of two identical monopole antennas. Each monopole has a simple circular patch which fed by a 50 Ω CPW line and placed perpendicular to each other with shared ground at the small spatial distance of only 2 mm. Towards realizing rejection bands, two ESCSRRs of different dimensions are embedded on each radiating patched for WiMAX and WLAN bands which operate between 3.3 to 3.7 GHz and 5.1 to 5.825 GHz for each bands respectively. Also impedance bandwidth of the antenna broadened by introducing two large elliptic slots on the shared ground plane and the bandwidth of more than 153% is achieved from 2.3 to 18 GHz. Even though the distance between the elements is too small, the mutual coupling of less than -20 dB is accomplished autonomously thanks to polarization diversity. It is noteworthy to mention that each element separately, is a single input single output (SISO) SWB antenna with dual band rejection characteristics having the size of 31 × 31 × 1.6 mm 3 . The designed MIMO antenna provides omnidirectional radiation pattern at xoz plane and near 6 dBi of peak gain for each port. Also the value of ECC, multiplexing efficiency and diversity gain are better than 0.035 and -1 dB and 9.99 dB respectively. The salient feature in this design is to provide SWB impedance matching for each port with two rejection bands for WLAN and WiMAX, along with the simple structure and small dimensions.

Single element SWB dual band-notched antenna
Where is the total length of the elliptic slots which should be approximately equal to half of the guided wavelength at the desired notch frequencies and used for the calculation of the circumference of each ellipse with ellipticity = / . For n = 1 and n=2 WiMAX and WLAN band rejection were obtained respectively. The thickness of each ESCSRRs is = 0.3 , is the central frequency of the notched bands and is the speed of light in free space. Also ε can obtain from the following equation [18].
Where is the relative permittivity of substrate that is about 4.4 for FR-4, ℎ and are the thickness of the substrate and width of the microstrip feed line which is about 1.6 mm and 2.8 mm in this case, respectively. Fig. 1(b) shows the location of the ESCSRRs on the circular patch.  Table 1.

Proposed MIMO SWB dual band-notched antenna
Designed single element antenna that presented in previous section, is converted to a 2-port MIMO antenna by placing the SISO elements perpendicular to each other with shared ground at the spatial distance of only 2 mm as shown in Fig. 3(a), therefore, a super wide impedance matching and low mutual coupling can be achieved through the whole operating bandwidth. Fig. 3    S-Parameters (dB) S11 S22 S12 S21

Current distribution and notch performance
In this section notched mechanism of the antenna is analyzed in detail from the surface current distributions and parametric study.  VSWR L S1 = 9.9 mm L S1 = 10.5 mm L S1 = 11.1 mm

Fabrication and measured results
The fabricated prototype of the antenna with the optimized dimensions mentioned in Section 2, was measured to validate the accuracy of simulated results. Fig. 7 illustrates the simulated and measured VSWR at each port and amplitude of parameter.
The proposed antenna offers good impedance bandwidth (VSWR < 2) from 2.3 to more than 18 GHz (IBW 153%) with band notches at 2.9 to 3.8 GHz and 5.1 to 6.1 GHz for each port due to both simulated and measured results as demonstrated in Fig. 7(a).
Also the amplitude of simulated and measured mutual coupling ( ) values are less than -20 dB in the entire operating band as shown in Fig. 7 (b). Good agreement is identified between the simulated and measured results from this figure.  MIMO diversity performance characteristics such as Envelope Correlation Coefficient (ECC), diversity gain (DG) and multiplexing efficiency ( ) of the suggested MIMO antenna have been studied and can obtain from the following equations.
ECC utilized to evaluate correlation between radiation patterns. Uncorrelated radiation patterns is one of the fundamental requirements for elements of a MIMO antenna to be effectively utilized in pattern diversity.
ECC can be evaluated using S-parameters by equation (4) [28]: Where and are the reflection coefficients of the two ports and and are the coupling between two ports. Another important parameter for MIMO diversity performance is diversity gain (DG) which is the figure of merit and used to quantitatively evaluate the performance level of various diversity techniques and defined with equation (5) [28]. Also multiplexing efficiency ( ) is an advantageous metric to optimize channel capacity which define the imbalance between antenna efficiency and correlation and calculated by relation (6) [29].
Where and are the total efficiencies of port 1 and port 2 respectively and | | is the square root of magnitude of the complex correlation between the two elements of MIMO antenna and it is almost equal to ECC.
Simulated and measured peak gain for port 1 and 2 and simulated multiplexing efficiency of the suggested MIMO antenna are depicted in Fig. 9. It can be observed from this figure that peak gain of the antenna is about 6.3 dBi for each port and decreases sharply at the center of notched frequencies. Also multiplexing efficiency is less than -1 dB excluding the two rejected bands and decrease to less than -5 and -4 dB for WiMAX and WLAN rejection bands respectively as it is clear from this figure. ECC and DG also plotted in Fig. 10. For uncorrelated diversity antenna the ideal value of ECC should be zero but practical limit for that, is less than 0.5. The ECC of the proposed antenna is less than 0.004 through the entire frequency band including the notched bands which indicates such an excellent diversity performance and very low correlation between the radiators. Also DG which is another performance metric of MIMO antenna is better than 9.99 dB as shown in Fig. 10.
xoz plane Co-polar yoz plane Co-polar xoz plane Cross-polar yoz plane Cross-polar The comparison between the proposed antenna and eight other designed MIMO antennas are given in Table. 2. Size, working band width, the number of notched bands, multiplexing efficiency, the number of elements, isolation and ECC are contrasted in this table for various antennas. It can be concluded that the proposed antenna has better radiation properties.

Conclusion
In this work a dual band-notched SWB MIMO antenna with high isolation is designed and fabricated for polarization diversity communication systems. The total size of the antenna is 31× 65 × 1.6 mm 3 covering more than 15 GHz impedance bandwidth from 2.3 to 18 GHz at each port except at the two notched bands from 2.9 to 3.9 GHz for WiMAX (3.3-3.7GHz) and 5 to 6.1 GHz for WLAN (5.1-5.825GHz) which appears in the existence of two ESCSRRs on the two radiating patches. Also an isolation of more than 20 dB is obtained in the entire bandwidth. The presented antenna provides an excellent polarization diversity performance and peak gain of about 6.3 dBi at each port, ECC of less than 0.01, DG better than 9.99 dB, multiplexing efficiency above -1 dB and nearly omnidirectional radiating pattern. Comparison results in Table 2 show that the proposed antenna has better performance for UWB and SWB applications yet the structure is simple and easy to fabricate. [    The SWB dual band-notched MIMO antenna, (a) Antenna con guration, (b) fabricated prototype.

Figure 6
Surface current distribution of the band-notched antenna (a) 3.6 GHz, (b) 5.8 GHz.

Figure 7
Simulated and measured (a) VSWR parameters for each port, (b) amplitude of mutual coupling (S12).

Figure 9
Simulated peak gain and multiplexing e ciency of the designed SWB MIMO antenna.

Figure 10
Simulated DG and ECC of the designed SWB dual band-notched MIMO antenna.