Introduction
Wireless local area network (WLAN) is an important technology to provide an inherent property such as high-speed transmission rate, high security, low cost, low power consumption, and high signal to noise ratio. The performance of the antenna is achieved by multiple input and multiple output (MIMO) antenna [Reference Jensen and Wallace1]. In a MIMO system, more than one antenna is used as transmitter and receiver. It is used to improve the communication quality and increase the channel capacity.
Typically, the MIMO antenna faces two major problems namely the size of the antenna and mutual coupling between the antenna elements. One is to minimize the size of MIMO antenna. In order to reduce the size of the antenna, they are placed close to each other. Owing to this arrangement, it produces mutual coupling. The mutual coupling is reduced by various types of isolation techniques, namely, antenna placement and orientation [Reference Sharma, Sarkar, Saurav and Srivastava2], decouple network [Reference Goldsmith, Jafar, Jindal and Vishwanath3], inclusion of parasitic element [Reference Khan, Shafique, Naqvi, Capobianco, Ijaz and Braaten4], defected ground structure (DGS) [Reference Anitha, Sarin, Mohanan and Vasudevan5], neutralization line [Reference Chou, Cheng, Hsu and Kuo6], metamaterial structure [Reference Zhu and Eleftheriades7, Reference Ziolkowski and Erentok8] etc.
Abdulla et al. proposed two-element metamaterial MIMO antennas. It consists of two zeroth-order resonator to reduce the size of an antenna and DGS filter is introduced for avoiding mutual coupling [Reference Abdulla and Ibrahim9]. Thummalaru et al. have projected a two-monopole antenna with negative metamaterial as a decoupling network [Reference Thummaluru and Chaudhary10]. A metamaterial array is used as a substrate coated with polymer industrial adhesive [Reference Mookiah and Dandekar11]. Jusoh et al. designed two-elements MIMO antennas where each element consists of seven circles surrounded by a center circle. A partial ground plane is employed to improve the isolation [Reference Jusoh, Jamlos, Kamarudin and Malek12]. Merihpalankonden et al. developed two dipole MIMO antennas. Each dipole antenna is connected with a six-unit cell. A truncated slot is introduced on a truncated ground plane to improve the isolation [Reference Palandoken, Grede and Henke13]. Nordin et al. designed a three left-handed metamaterial unit cell. Each unit cell consists of split-ring slot with capacitive loaded. A trident feed and slotted ground are employed to match the antenna and input port [Reference Nordin, Islam and Misran14]. The two monopole antenna is designed and a quarter wavelength slot and open-ended slot are used to improve the isolation [Reference Nandi and Mohan15].
Chou et al. devised a three-loop antenna with different antenna arrangement. The middle antenna was placed at a horizontal position. It is orthogonal to other antenna elements for better isolation. A T-shaped slot and ground strip are used to enhance antenna performance [Reference Chou, Cheng, Hsu and Kuo6]. Mallahzadesh et al. reported E shape two and four-element MIMO antenna where the isolation is enhanced by invasive weed optimization algorithm [Reference Mallahazedh, Es'haghi and Alipour16]. Yu et al. proposed matching network derived using power allocation rule. The genetic algorithm was utilized to optimize the performance [Reference Yu, Wang, Wang, Wu and Shang17]. The T shaped parasitic element is implemented for the isolation [Reference Toktas and Akdagli18]. Kim et al. introduced the radiating elements and one-dimensional electromagnetic bandgap reflectors to reduce the mutual coupling [Reference Kim, Lee, Nguyen and Jang19]. Krairiksh et al. designed a two-probe circular ring. The isolation between the two probes is enhanced by the insertion of an inductor coil between the probes [Reference Krairiksh, Keowsawat, Phongcharoenpanich and Kosulvit20]. In addition, there are several four-element MIMO antennae designed and developed for WLAN, worldwide interoperability for microwave access (WiMax) applications, ultra wide band (UWB) applications, industrial scientific and medical (ISM) band, universal mobile telecommunications system (UMTS), long-term evolution (LTE) and 4G/5 G smartphone applications. It is designed using Slot, dielectric resonator antenna (DRA), complementary split ring resonator (CSRR), DGS, and inverted F/L antenna [Reference Parchin, Al-Yasir, Ali, Elfergani, Noras, Rodriguez and Abd-Alhameed21–Reference Zhai, Chen and Qing36].
In the literature, several MIMO antennas, with various decoupling structures are reported for WLAN applications where the metamaterial and DGS are mostly used for MIMO applications to minimize the mutual coupling. Further, it is also investigated that isolation is needed to be enhanced and the size of the patch is large. In order to mitigate the above-mentioned issues, the square patch with a square-shaped split-ring slot is introduced to develop 4 × 4 MIMO antenna.
In this paper, a MIMO antenna (single MIMO antenna, 2 × 2 MIMO antenna without DGS and 4 × 4 MIMO antenna without and with DGS) is proposed and designed and its functional parameters namely, return loss, VSWR, resonant frequency, isolation, radiation pattern, and envelope correlation coefficient (ECC) are investigated. DGS is introduced in 4 × 4 MIMO antenna in order to enhance the performance of the antenna. Finally, the simulated return loss is compared with that of a fabricated antenna.
This paper is arranged in the following way: Antenna geometry and design illustrates the design of single MIMO antenna, 2 × 2 MIMO antenna without DGS and 4 × 4 MIMO antenna without and with DGS and its return loss performance of simulated and measured. MIMO antenna results and discussions illustrates the simulation results of 4 × 4 MIMO antenna namely, isolation, radiation pattern and ECC. Conclusion concludes the paper.
Antenna geometry and design
The proposed antenna is designed as a low-cost FR4 substrate with relative permittivity of 4.4. The resonant frequency is calculated as per Ref. [Reference Mookiah and Dandekar11]. The width and length of the patch are 5.4 mm and 5.4 mm, respectively. The thickness of the substrate is 0.8 mm and loss tangent is 0.02. The patch height is 1.1 mm from the ground plane. According to the calculation, the single, double element, and four-element MIMO antenna are designed. The substrate has a dimension of 34.485 mm width and 42.837 mm length. The patch size is 9 mm × 9 mm.
Single element antenna
The unit element of the proposed antenna is shown in Fig. 1. The unit element is used to design a proposed MIMO antenna. It is operated at 5.8 GHz. The antenna is fed by 50 Ω microstrip line. The structural parameters and the values of the proposed antenna are listed in Table 1.
Schematic structure of unit element square patch with split-ring slot antenna.
Structural parameters of a single element (mm) antenna.
The return loss |S 11| of the proposed antenna is shown in Fig. 2. The square-shaped split-ring slot is introduced on the patch to reduce the electrical length of the antenna. The slot width is 0.45 mm. The size of the antenna is less than the operating wavelength, using the square-shaped split-ring resonator. The split-ring resonator is designed according to the radiation on the patch. The return loss of the proposed antenna is −27.2366 dB at 5.8 GHz.
Return loss response of the unit element of square patch with split-ring slot antenna.
The return loss comparison of simulated and measured single patch antenna is shown in Fig. 2. The return loss is −25 dB at 5.9 GHz. The change in return loss and the resonant frequency is due to the incorporation of SubMiniature version A (SMA) connector in the fabricated antenna.
Double element MIMO antenna
The design of a double element MIMO antenna is shown in Fig. 3. The antenna is designed as two-unit element antennas which are placed opposite to each other. However, the other structural parameters are similar to a single element antenna.
Schematic structure of double elements MIMO antenna.
The return loss of two elements MIMO is shown in Fig. 4. When port 1 is excited, port 2 is matched with the strong electromagnetic energy coupling between the two elements. As a result, the return loss of port 1 is degraded. As there is no isolation technique used between the antenna elements, it introduces mutual coupling. Owing to the mutual coupling between the ports, the return loss is reduced at port 1 which is −17.50 dB at 5.8 GHz and port 2 resonates −6 dB at 6 GHz. However, the return loss of the single element antenna is −27.2366 dB at 5.8 GHz.
Simulated return loss response of a double element MIMO antenna.
Four-element MIMO antenna
The proposed four-element MIMO antenna is shown in Fig. 5. The antenna is designed as a four-unit element antenna which is placed at the center of each side. The length and width of the patch remain the same as a single element MIMO antenna.
Schematic structure of four-element MIMO antenna without DGS.
The simulated return loss response of four-element MIMO antenna is shown in Fig. 6. There is no isolation technique used between the antenna elements to minimize the mutual coupling. Hence, the return loss of port 1 resonates −11.69 dB at 5.8 GHz, port 2 resonates −8.8 dB at 6.1 GHz, port 3 resonates −5.8 dB at 5.8 GHz and port 4 resonates −9.12 dB at 6.1 GHz. There are several types of isolation techniques reported for antenna placement and orientation [Reference Jensen and Wallace1], algorithms [Reference Anitha, Sarin, Mohanan and Vasudevan5], parasitic elements [Reference Sharma, Sarkar, Saurav and Srivastava2], DGSs, neutralization lines [Reference Chou, Cheng, Hsu and Kuo6], and metamaterial structures [Reference Zhu and Eleftheriades7]. Here, the DGS is introduced to enhance antenna performance.
Simulated return loss response of four-element MIMO antenna without DGS.
In between the antenna elements, mutual coupling is introduced which reduces the performance of the antenna. For avoiding the mutual coupling, the two meandered line-shaped slots like DGS is introduced on the ground plane. The two meandered line-shaped slot is placed on the ground plane in between the first antenna element because the radiation of this antenna element affects the nearby antenna element. The thickness of the slot is 0.4 mm. The return loss is improved at port 1 by this structure. The four-element MIMO antenna with the size of the DGS is shown in Fig. 7.
Schematic structure of four-element MIMO antenna with DGS.
The return loss of four-element MIMO with DGS is shown in Fig. 8. Two meandered line slots introduced on the ground plane are used to enhance the return loss performance. The return loss of port 1 resonates −28.75 dB at 5.8 GHz, port 2 resonates −5.77 dB at 5.9 GHz, port 3 resonates −7 dB at 6.1 GHz, port 4 resonates −4 dB at 5.9 GHz. The return loss is improved by incorporating the DGS in between the antenna elements. DGS suppresses the unwanted wave in the ground plane. As a result, the return loss is enhanced from −11.69 dB to −28.75 dB.
Simulated return loss response of four-element MIMO antenna with DGS.
The impedance bandwidth is just the normal bandwidth of the antenna. Eventually, it is defined as the range of frequencies over which the return loss is acceptable. The 6 dB impedance bandwidth of the proposed antenna without and with DGS is about 0.10 GHz and 0.35 GHz, respectively. The impedance bandwidth of the proposed antenna without DGS is 0.10 GHz which spans from 5.8 GHz to 5.9 GHz. The 0.35 GHz impedance bandwidth arrives with DGS over the ranging from 5.6 GHz to 5.95 GHz. The impedance bandwidth of the proposed antenna meets the requirements for 802.11a/n/ac.
The antenna efficiency is calculated. The antenna efficiency of the proposed 4 × 4 MIMO antenna with/without DGS is reported in the revised paper. The antenna efficiency of an antenna with/without DGS is 87.5% and 83%, respectively. The fabricated four-element patch antenna's front and back view are shown in Fig. 9. The overall size of an antenna is very small (about 42.837 mm × 34.485 mm × 0.8 mm.).
Fabricated four-element MIMO antenna with DGS (a) front view and (b) back view.
The return loss comparison of simulated and measured four-element MIMO antenna is shown in Fig. 10. The measured results are slightly varied from the simulated result. During the simulation, the SMA connector is not included. Owing to the insertion loss the resonant frequency is shifted to 6.1 GHz with the return loss of −22 dB.
Return loss response of dual-element MIMO antenna with DGS.
MIMO antenna results and discussions
The 4 × 4 MIMO antennas is designed and the isolation, radiation pattern, and ECC are simulated and presented here.
Isolation
Isolation is used to find out how much power is coupled between the adjacent antenna elements by using the structure of antenna elements. Figure 11 shows the isolation curve of proposed four-element MIMO antenna. The isolation is calculated in between port 1 and 2, port 1 and 3, port 1 and 4, port 2 and 3, port 2 and 4, port 3 and 4.
Simulated isolation response of four-element MIMO antenna with DGS.
Radiation characteristics
The simulated radiation patterns in H- and E-planes are presented in Fig. 12. The simulated radiation pattern of the proposed antenna is at the frequency of 5.8 GHz. The representation of the 2D radiation pattern of the simulated result is unidirectional in E plane and bidirectional in H plane.
Radiation patterns of four-element MIMO antenna with DGS antenna.
Envelope correlation coefficient
The capacity of the proposed MIMO antenna is analyzed by ECC. The MIMO antenna is necessary to obtain a low ECC. ECC computes the correlation between the branch signals received by the different elements, and lower ECC signifies higher pattern diversity [Reference Thummaluru and Chaudhary10]. By assuming the proposed antenna as a lossless antenna in an isotropic scattering environment, the ECC between i and j, is calculated using the following equation. The ECC of two elements with DGS is shown in Fig. 13:
Simulated ECC response of four-element MIMO antenna with DGS.
The ECC is calculated in between port 1 and 2, port 1 and 3, port 1 and 4, port 2 and 3, port 2 and 4, port 3 and 4. The ECC of four elements with DGS structure is <0.0001. The ECC is improved by the DGS. The values are well below the acceptable limit of 0.5 [Reference Chou, Cheng, Hsu and Kuo6]. From the above analysis, it can be seen that the designed antenna has the smallest size and has low mutual coupling between the antennas. Hence, the proposed antenna is highly desirable for WLAN applications.
Conclusion
In this paper, four-element MIMO antennas, with DGS are proposed and designed. The functional characteristics of the proposed antenna, namely return loss, isolation, radiation pattern, and ECC are investigated. The return loss of the four-element MIMO antenna is −28.75 dB at 5.8 GHz. However, the return loss is reduced to −22 dB after the fabrication due to SMA connector. The overall size of the antenna is 42.837 mm × 34.485 mm × 0.8 mm. The radiating patch-size is 9 mm × 9 mm. The mutual couplings between the antennas elements are minimized through DGS. The ECC is <0.0001. Hence, the proposed antenna meets the requirements for WLAN-applications.
K. Malaisamy M.E. was born in Tamilnadu, India. He is currently working as Assistant Professor, Department of Electronics and Communication Engineering, Saranathan College of Engineering, Tiruchirapalli, Tamilnadu, India. He has published eight papers in International Journals and has presented 10 papers in International and National Conferences in India. He is a life member of ISTE, ISSE. His current research interest areas are Antennas, RF and microwave devices.
M. Santhi M.E., Ph.D., was born in Tamilnadu, India in 1969. She is currently working as Professor & Head, Department of Electronics and Communication Engineering, Saranathan College of Engineering, Tiruchirapalli, Tamilnadu, India. She has 23 years of teaching experience. She is the reviewer in reputed journals like IEEE Transactions on VLSI, IEEE Access, Springer, etc. She has published 18 reputed international journals and 20 reputed international conferences. Her research areas are asynchronous techniques, MB-OFDM UWB, RF and microwave devices
S. Robinson M.E., Ph.D., was born in Tamilnadu, India in 1982. He is currently working as Associate Professor, Department of Electronics and Communication Engineering, Mount Zion College of Engineering and Technology, Pudukkottai, Tamilnadu, India. He has published more than 100 papers in International Journals and presented more than 100 papers in International and National Conferences in India and foreign countries. He has published one book and five books chapters. He is the reviewer for around 25 refeeed journals like IEEE, SPIE, Elsevier, Springer, PIER, and HJEEE Journals. He is a life member of ISTE, IETE, CSI, IE (I), and member of IEEE. His current research interests are in the areas of Optical Communication, Photonic Crystals, Photonic Integrated Circuits, Optical Devices, Microwave communication, Microwave Devices, and Optical Networks.
