Compact Electronically Reconfigurable WiMAX Band-Notched Ultra-wideband MIMO Antenna

A low-profile electronically reconfigurable WiMAX band-notched dual port multiple-input multipleoutput (MIMO) antenna design for ultra-wideband application has been presented. The two symmetrical MIMO antenna elements proposed in this work exhibit a good impedance match (VSWR ≤ 2) over frequency band of 3 to 12 GHz, while offering high isolation. The decoupling structure is used to enhance the isolation level above 25 dB over the entire UWB spectrum. The reconfigurable band notch characteristic in MIMO design is achieved by inserting PIN diodes along the filtering Ω-shaped slotted structure in main radiators. Notch appears for WiMAX 3.5 GHz (3.2–3.8 GHz) frequency band by switching the PIN diode to ‘ON’ state. The proposed antenna is fabricated and measured, the results suggest its appropriateness for UWB applications where WiMAX band notch characteristics may be desired on-demand.


Introduction
Wireless technologies have gained much research attention over the last few decades.The prime focus of efforts put in these technologies are related to resource optimization, cost cutting and achieving high data rates along with little interference to other existing wireless communication standards.A communication system with an absolute bandwidth of more than 500 MHz is considered as an Ultra-Wide Band (UWB) system.The UWB spectrum from 3.1 to 10.6 GHz offers bandwidth of 7.5 GHz which is very attractive for future wireless technologies, mainly due to its license-free usage and possibility of achieving high data rate [1], [2].By incorporation of MIMO technology in UWB systems, high data rates and improved channel capacities can be achieved.However, miniaturization is a significant challenge in UWB-MIMO antenna design.Due to unwanted mutual coupling caused by miniaturization, effectiveness of MIMO system is compromised.Therefore, an efficient decoupling structure is necessary in MIMO systems to provide high isolation between antenna elements without compromising the size.Numerous antennas have been reported in the existing literature, in which the decoupling/isolation structures are applied to reduce mutual coupling without affecting the compactness.The decoupling structure mainly enhances the isolation by reflecting and/or absorbing radiation from the radiators.Decoupling structure prevents the interference of the radiations of antenna elements in MIMO system.For isolation purpose slotted structures are placed between antenna elements.Frequency Selective Surface (FSS) based decoupling structures and Defected grounds are used in [5] to enhance the isolation between antenna elements.A decoupling structure is placed at the rear of the substrate to eliminate undesired mutual coupling between the antenna elements [7].In [15] a long ground slot is introduced vertically on the T-shaped ground to provide better isolation between the two input ports.
Generally, different wireless standards, WiMAX being one of them, overlap the frequency spectra 3.1 to 10.6 GHz dedicated for UWB communication [2].Therefore, it is highly likely that a UWB communication system will create distortion in other narrow-band systems working in this band or vice versa.The straightforward solution to this issue is to design an antenna having band notch characteristics.In this regard several band notch antennas have been reported in the existing literature.In one study reported in [8], a parasitic structure pair is etched in a ground plane to notch WLAN band.Integration of a slot resonator is presented in [9].In [10], inverted parasitic strips for interference mitigation at 5.2/5.8GHz are reported.A combination of slotted and parasitic structures is proposed in [11].Defected ground and notching structures in feed line is presented in [12][13][14].However, band notch characteristics proposed in all these designs are permanent and for the sake of interference free communication, utili-zation of the whole UWB spectrum may not be possible even if there is no conflicting narrow band system working in the close proximity.Therefore, for the improvement in performance of an UWB system, antennas with reconfigurable structures exhibiting switchable band notch capability are highly desired [16][17][18][19][20][21][22][23][24].In [21][22][23][24] the reconfigurability is not electronically controllable and for UWB and band notch operations one has to design two separate designs.Electronically reconfigurable antennas are preferred due to their multi-operability.They can utilize the whole UWB spectrum, and for interference mitigation, the antenna can behave like a band notch antenna within the same system when desired.Different types of microwave switches such as PIN diodes and RF MEMS, can be used for switching purpose in reconfigurable antenna structures [18].In [19], optically controlled switches are used in antenna system to provide switchable band-notch functionality, which is used in cognitive radio systems.However, in [20], [25], PIN diodes connected on a microstrip slot antenna are used to achieve the purpose of reconfigurable band notching.
In this paper, a miniaturized reconfigurable band notch dual port MIMO antenna is proposed for UWB applications.Compact flower shaped symmetrical antenna elements are designed.The antenna exhibits a rectangular defected ground plane on the rear side of a substrate.The antenna system operates at a wide range, covering frequency band of 3 to 12 GHz.A slotted rectangular shaped structure is designed on the ground plane to enhance decoupling between antenna elements.This decoupling structure provides isolation of more than 25 dB over the entire UWB band, while keeping other MIMO/diversity parameters (ECC, TARC and CCL) in an acceptable range.Moreover, electronically reconfigurable band notch characteristic in the proposed design is achieved by inserting PIN diodes in inverted Ω shaped slots present in the main radiating elements.The antenna system provides switchable band notch capability for WiMAX (3.2-3.8GHz) band.For design and optimization of the proposed antenna geometry, full wave electromagnetic simulations have been carried out in Ansys High Frequency Structural Simulator (HFSS) using Finite Element Method (FEM).

Design and Configuration of MIMO Antenna 2.1 MIMO Antenna Element Structure
The structure of the proposed antenna system is presented in Fig. 1.The proposed antenna system is simulated and fabricated on FR4 substrate with relative permittivity, loss tangent and thickness of 4.4, 0.018 and 0.8 mm, respectively.The proposed MIMO antenna has a compact overall size of 14 mm × 30 mm.The upper layer comprises of two center fed flower shaped radiating elements, whereas, the rear side of the substrate contains the ground plane.The proposed radiating elements are symmetrical and

MIMO Antenna Decoupling Structure
The effect of decoupling structure on the mutual coupling (S 21 ) is analyzed.To achieve high isolation between antenna elements, decoupling structure is added on the rear side of the substrate.The design of the decoupling structure for UWB-MIMO antenna is shown in Fig. 1(b).The decoupling structure consists of a defected rectangular stub line.The decoupling structure is placed between ground planes to isolate antenna elements that are placed in sideby-side arrangement.The decoupling structure effectively improves the isolation above 25 dB.Table 1 shows the detailed parameters of the decoupling structure.

Design of Reconfigurable Band Notch Structure
The reconfigurable band notching in UWB-MIMO antenna is achieved by using PIN diodes.The effective length of the notching structure is required to filter out the desired frequency band.Notching at the desired frequency bands can be obtained using (1).At the notching frequency the structure works as a quarter wave resonator.Ω shaped slot is created in the main radiator of each antenna element.
where L Total = 2S L + 2πR 2 and C is the constant of electromagnetic wave velocity.The slot provides band notch characteristics at WiMAX 3.5 GHz.The current around the edges of slots reverses its direction causing anti-resonance at the desired notched frequency band.Moreover, to achieve the reconfigurable band notch functionality in UWB-MIMO antenna, PIN diode is added to the slot of each radiator, as shown in Fig. 1

Impedance Matching
The proposed design is fabricated on low cost FR4 laminate, the prototype is shown in Figs.5(a resulting effect on the reflection coefficient is analyzed and the results are shown in Fig. 7 for Case II.
The DC block inline SMA module has been installed with VNA so no onboard DC block capacitor is installed.However, the RF choke is added to both elements.Since the ground plane is electrically small so ferrite beads have been added to the measurement cables to reject the reverse current propagating on the outer side of the test cables.

Isolation
The decoupling structure comprising of a vertical strip along with rectangular slots in the middle has been added to the ground plane of the proposed antenna system in order to achieve the desired improved isolation.Although the coupling between the two antenna elements at low frequencies is below -15 dB without the decoupling structure, it is acceptable for MIMO operations [2,6,15,16,23].However, the incorporation of the decoupling structure further reduces the coupling between antenna elements by 10 dB for most of the operating band and is well below -20 dB for lower frequencies in our desired band [1], [17].This enhanced isolation ensures minimal correlation among the signals coming from different paths, thus improving the diversity gain further when a suitable combining scheme is employed.The effect of decoupling structure on mutual coupling of antenna elements, for both Case I and II, can be observed clearly from Fig. 8.The overall simulated as well as measured isolation with decoupling structure is better than 25 dB.Separate ground planes for MIMO antennas is not a new concept as reported in [7], [17].The main reason behind good isolation performance of the proposed antennas is non-conventional shape of antenna elements and the incorporated decoupling structure.These antennas with separate ground Simulated Measured Without Decoupling Structure  planes also offer better impedance matching over operating frequency band.The effect of the common ground plan for the proposed antenna can be analyzed in Fig. 9.The figure shows that the connected ground plane mainly affects the impedance matching of the antenna.The isolation with the connected ground plane is also less than 20 dB for most of the UWB band, which indicates that the isolation enhancement in the proposed MIMO antenna is mainly because of the decoupling structure between the antenna elements as shown in Fig. 8.

Radiation Characteristics
The simulated and measured radiation patterns in E and H plane of the proposed MIMO antenna (Case I and Case II), observed at 3.5, 7, 9.5 and 12 GHz are plotted in Fig. 10.It can be noted that the simulated and measured radiation patterns are well within the acceptable range.

Gain and Efficiency
The proposed MIMO antenna system shows band notch characteristics.Therefore, in order to have a good analysis of the notched WiMAX band, gain and efficiency characteristics of the proposed prototype have been compared for both cases.In general, it is desirable to have lower gain and efficiency at the notching frequencies.The simulated gain and efficiency results are shown in Fig. 11.It clearly depicts that the proposed antenna offers a good gain and efficiency response over the whole UWB band except for the notched WiMAX band.

Diversity Performance of Antenna
The diversity performance of the proposed MIMO antenna system is investigated by analyzing different parameters like Envelop Correlation Coefficient (ECC), Total Active Reflection Coefficient (TARC) and Channel Capacity Loss (CCL).ECC is an important diversity parameter of MIMO system.For an isotropic rich scattering environment ECC can be estimated from S-parameters using (2), the expression is derived in [28].By using this approach, we can analyze the explicit influence of mutual coupling and input match, without considering the radiation pattern of the antenna system [3,7,16,17,26,29].ECC value below -3 dB is desired for optimum MIMO operations [26].As shown in Fig. 12(a), (b), the results are well within an acceptable range for both cases (I, II).
TARC and CCL are other important diversity parameters.TARC represents the ratio between square roots of the total reflected power and the total incident power, as shown in (3), whereas CCL elaborates the loss of capacity induced due to the correlation between MIMO channels.CCL is calculated from S parameters using (4) where ψ R is a 2 × 2 correlation matrix.
For optimum MIMO operation it is desired to have TARC < 0 dB and CCL < 0.5 bits/sec/Hz in the operating band [22].Figure 12(c) and (d) show that the values of TARC for Case I and Case II are well within the allowable limits.In Figs.12(e) and (f), CCL parameter is calculated for antenna elements placed in side-by-side arrangement.Results suggest that CCL value is well below 0.5 bits/sec/Hz for Case I.In Case II the CCL value at WiMAX (3.2-3.8GHz) band is above 0.5 bits/sec/Hz, because of the anti-resonant effect of band notching structure.Group delay for UWB MIMO antenna elements for transmit and receive antenna is also analyzed and shown in Figs. 12 (g) and (h).The proposed MIMO antenna elements are compact as compared to various antennas reported in literature previously [4][5][6][7].The comparison of the proposed antenna characteristics with other MIMO antenna designs [1,15,17,27] is listed in Tab. 2.

Conclusion
A low profile dual port reconfigurable MIMO antenna system is proposed in this research work for ultrawideband applications.The antenna system consists of two symmetrical flower shaped elements.Slotted rectangular shaped decoupling structure is introduced on the ground to enhance isolation.The proposed MIMO antenna operates over a frequency band of 3-12 GHz, while exhibiting isolation (S 21 ) of more than 25 dB.Moreover, the reconfigurable band notch characteristics for WiMAX (3.2 GHz to 3.8 GHz) band have also been achieved by installing PIN diodes along Ω shaped filtering structure in the main radiating elements.Diversity parameters like ECC, CCL and TARC are also within the allowed limits.The simulated and measured results are in good agreement which confirms its appropriateness for reconfigurable UWB-MIMO operation.
Pakistan, in 2006 and PhD from the University of Manchester, United Kingdom in 2011.He worked on antennas, channel modeling and interference aspects of Ultra Wideband systems during his PhD and was also member of a team designing and testing arrays for the Square Kilometer Array project.Currently, he is working as an Assistant Professor at the University of Engineering and Technology (UET), Taxila, Pakistan where he is supervising several postgraduate students and heading the MAP (Microwave Antennas and Propagation) research group.His research interests include antennas, angle-of-arrival based channel modeling, microwave periodic structures and metamaterials.

Fig. 1 .Tab. 1 .
Fig. 1.Geometry of dual port antenna system: (a) Front view.(b) Rear view.Parameter L W L 1 L 3 L 4 L 5 L G L C Value(mm) 14 30 0.7 3.8 0.75 1.2 10.2 1.5 Parameter P 1 P 2 P 3 P W1 W 1 W 2 W G W G1 Value(mm) 0.7 2.6 0.6 2.2 0.5 0.7 4.5 3.2 Parameter G L G L1 D A R 1 R 2 R 3 F L1 S L Value(mm) 3.3 0.8 14.4 1.5 0.8 0.4 4.7 3.4 Parameter P W P D W G2 W G3 G L2 W G4 --Value(mm) 4 2.1 1.6 1 1.8 4 --Tab.1. Optimized values of proposed antenna.are placed in side-by-side configuration in y-dimension.Partial grounding technique is used for ultra-wideband matching.Each antenna element is fed independently by a rectangular transmission line of 50 Ω.The feed line has width of 1.6 mm while the length is F L1 .The feed lines are 14.4 mm apart and placed 7.2 mm from the edges of the substrate.The partial ground plane is a notched rectangular strip of width W G2 .The antenna system operates at a wide frequency band of 3 to 12 GHz except at the desired WiMAX notch frequency.The detailed parameters of antenna structure are listed in Tab. 1.

Fig. 12 .
Fig. 12. Measured MIMO performance parameters: (a) ECC for Case I. (b) ECC for Case II.(c) TARC for Case I. (d) TARC for Case II.(e) CCL for Case I. (f) CCL for Case II.(g) Group delay for Case I. (h) Group delay for Case II.
Farhan SHAFIQUE received the B.Eng. degree from Hamdard University, Karachi, Pakistan, in 2003, M.S. degree from the University of Paris East Marne-La-Vallée, Paris, France, in 2005 and PhD in Electronic and Communications Engineering from the University of Leeds, Leeds, UK in 2010.His research interests involve multilayer microwave device fabrication on LTCC technology, electromagnetic modeling of microwave structures, RF antenna, filters and MEMS packaging.He is also involved in dielectric characterization of materials using microwave techniques and fabrication of ceramic microfluidic devices.He is working as an Associate Director at the Center for Advanced Studies in Telecommunications (CAST) where he has established the MCAD (Microwave Components and Devices) research group.He has also setup a wide range of research facilities in the area of RF engineering which involves 4 state-of-the art laboratories.He is a reviewer of various journals and also a Senior Member of IEEE.Sabih ur REHMAN is a Lecturer in Computing with the School of Computing and Mathematics at Charles Sturt University, Australia.Sabih has completed his PhD in the area of vehicular ad-hoc networks for which he was the recipient of competitive scholarship from Charles Sturt University.Sabih obtained his Bachelor degree from the University of South Australia, Adelaide in Electronics & Telecommunication Engineering with Honours.Sabih's research expertise lies in the areas of Quality of Service (QoS), Cross-layer Protocol Architecture Designing, Wireless Propagation and Antenna Modeling using mathematical/stochastic models and conducting performance analysis.Sabih's current research is focused on the emerging area of Internet of Things (IoT) especially in context of Intelligent Transport Systems and Precision Agriculture.