Octagonal DGS based Dual Polarized Ring shaped Antenna for MIMO Communications
Henridass Arun a, M Gulam Nabi Alsath b
aDepartment ECE, Sri Sairam College of Engineering, W Tambaram, Chennai, India.
& Research Scholar, Anna University, Chennai, India.
E-mail:henridass.ece@sairam.edu.in
bDepartment ECE, SSN College of Engineering, kalavakkam, Chennai, India.
Octagonal DGS based Dual Polarized Ring shaped Antenna for MIMO Communications
In this article, a dual polarized Microstrip patch antenna is proposed for MIMO communications. Proposed antenna is suitable for GSM/ DCS-1800 and LTE-1900 bands as a diversity and multiple-input multiple-output (MIMO) antenna. Different from conventional MIMO antennas, the radiating aperture is shared among the radiators, which greatly reduces the overall size of the MIMO antenna system. An isolation enhancement of 30dB between the input ports is achieved by integrating cross connected octagonal shaped defected ground structure (DGS) to the ground plane. Furthermore, the Multi-antenna system performance metrics such as Envelope Correlation Coefficient (ECC), diversity gain (DG) and Mean Effective Gain (MEG) are also computed. The Proposed antenna shows a gain of 3.63dBi at 1950MHz. The simulated and measured results demonstrate that the proposed antenna has good impedance matching, isolation and dual polarization characteristics.
Keywords: Multiple-Input Multiple-Output (MIMO), Dual Polarization, Polarization diversity antenna, GSM, LTE, ECC.
Introduction
The need for higher data rates to support high definition video and online gaming, as well as multi-application support is a driving technology demand that is always on the rise. MIMO systems are deployed widely because they significantly improve wireless link performance through increased capacity (Rakesh Singh 2017). The emergence of fourth generation (4G) and LTE wireless technologies in terms of higher data rates was not possible without relying on multiple-input-multiple-output (MIMO) technology (Kildal et al 2016).
The MIMO has become an essential element for many wireless communication standards including IEEE 802.11 n (Wireless Fidelity Wi-Fi), IEEE 802.11 ac (Wi-Fi), High Speed Packet radio Access, (HSPA+ 3G Cellular), Worldwide interoperability for Microwave Access (WiMax 4G Cellular and MAN), and Long Term Evolution (LTE 4G Cellular).
MIMO systems form integral part of LTE. In the first LTE standard (3GPP Release 8), 2*2 MIMO systems was used in both uplink and downlink. Much improvements have taken place since then, and higher order MIMO antenna configurations up to 8 X8 MIMO downlink and 4X 4 uplink are supported in the latest 3GPP Release 11. The performance of multiantenna elements for MIMO systems are determined by the correlation of the signals received by different antenna elements (Sarma et al 2015). It is more challenging for the engineers to design a MIMO antenna system for portable and handheld applications. This is mainly due to space limitations as the gadgets are becoming more compact in size, thin and light in weight. And placing multiple antennas within the available compact small area, may degrade the performance benefits that one can expect from MIMO system. In addition, the transceiver design for MIMO systems and design of multiple antennas for MIMO systems are also serious challenges in multi antenna systems. (Mohammadi et al 2012). The placement of antennas within a compact restricted volume pose several issues such as, close proximity, mutual coupling and correlation of signals, which can significantly degrade the performance of the MIMO system. (Mohammadi et al 2012), (Wu et al 2012).
MIMO antennas can also be used for space and polarization diversity techniques (Hienonen et al 2012), (Chacko et al 2013). Antennas having dual polarization can be used to obtain polarization diversity. Dual polarization antennas find numerous applications in MIMO technology and in most of the wireless communications systems including air-borne synthetic aperture radars (SAR) technology.
This article presents the design of dual polarized MIMO antenna applicable for both GSM/ DCS-1800 and LTE-1900 bands. A simple annular shaped Microstrip patch antenna is taken up and two orthogonal feed lines are used to induce dual polarized radiation from the single antenna. Usually the impedance bandwidth offered by the simple slot antennas is much smaller. So a square slot is introduced at the leftmost corner of inner ring is introduced to improve the electrical length of the antenna thus it enhances the impedance bandwidth. The close proximity of orthogonal ports leads to surface current flow between the ports which in turn induces strong mutual coupling between orthogonal ports. In antennas with elements that can support more than one orthogonal sense of polarization, it is possible for the coupling to excite the polarization sense which is not directly excited by the generators, thus causing depolarization of the signal. Mutual coupling between multiple antenna elements deteriorates the potential benefits of multi-antenna system. So some isolation techniques to reduce the mutual coupling have to be inducted in the design of dual polarized antenna. DGS based mutual coupling reduction technique has been introduced to the ground plane of the proposed dual polarized antenna.
The design and characterization of dual polarized antenna is described in section II. Section III demonstrates the MIMO configuration and validation of the prototype is discussed in section IV. A comparison has been made in terms of physical dimension, impedance bandwidth, ECC, and Diversity gain with recent articles is given in section V.
Dual polarized antenna design
Conventional circular Microstrip patch antenna has been designed for the operating frequency of 1800MHz using 1.6mm thick FR-4 substrate. The radius of circular patch is 17mm. A circular slot with a radius of 9 mm is introduced in the radiating patch to radiate 1900MHz. Two orthogonal 50Ω feed lines of 12 mm length and 3mm width have been connected to the radiating patch to produce dual polarization. The orthogonal ports are used to produce horizontal and vertical polarization. The two orthogonal ports exploit both, the polarization diversity and pattern diversity. The impedance bandwidth offered by the simple ring shaped circular patch antenna is 134MHz which is narrow compared with the LTE/1900 bandwidth 0f 190MHz. To enhance the impedance bandwidth further, a square slot has been introduced at the leftmost top corner of the circular slot as shown in Figure. 1. The square slot has a dimension of 10 mm x 10 mm. The impedance bandwidth improves from 134MHz to 280MHz with the aid of square slot in the radiating patch.
Figure. 1 illustrates the geometry of proposed dual polarized antenna. The feed lines are arranged orthogonally to induce dual polarization. Proposed dual polarization antenna is fabricated and S parameters are measured using Vector Network Analyser (VNA). Figure.2 shows the fabricated prototype using FR-4 material with a thickness of 1.6mm. The S parameter characteristic of proposed dual polarization antenna is shown in Figure.3. From the measured S11 value, the impedance bandwidth offered by the dual polarized antenna is about 280MHz which is from 1.77GHz to 2.05GHz.
To achieve higher data rate/ reliability wireless link, the elements of the multi-antenna systems must be placed as wide as possible to provide statistically independent multipath components to the antenna elements. But due to space constraints in the wireless equipment’s, the antennas are usually placed close to each other which results in strong mutual coupling between antenna elements. So the mutual coupling between the elements are need to be mitigated to reap the fruits of multi-antenna systems. The mutual coupling of the proposed dual polarization antenna without DGS structure is shown in Figure.4. Figure shows mutual coupling between the input ports which leads to complete loss of advantages offered by the MIMO systems. To achieve the performance benefits of MIMO, the mutual coupling must be reduced.
The performance of MIMO antennas are greatly affected by mutual coupling between the elements. The mutual coupling greatly reduces the diversity gain and ECC of the MIMO antennas. Therefore, to get improved performance in a multipath propagation environment the mutual coupling has to be suppressed. Defected ground structures are used to modify/ improve the impedance performance of Microwave components. Authors started with octagonal shaped DGS to improve the isolation between the ports of the proposed antenna. The dimensions of inner and outer octagon 15mm and 21mm are respectively. The transition from simple ground plane to DGS based ground plane is shown in Figure. 5(a). With octagonal DGS only 10dB isolation is achieved. An octagonal ring slot with cross lines has been introduced. So to improve the isolation to optimal level cross connection is introduced in the ground plane along with existing octagonal DGS. With the introduction of cross connected octagonal DGS an isolation of 30dB is achieved.
Figure. 5(b) gives the dimensions of the modified ground plane of proposed DGS based Dual polarized antenna and the fabricated prototype is shown in Figure. 6. The introduction of DGS modifies the ground impedance of the antenna and enhances the isolation.
Figure.7 gives the measured S21 Characteristics of the proposed antenna which gives the mutual coupling level. From the graph it is evident that the DGS greatly enhances the isolation. The isolation is enhanced from -2.73dB to -35.08dB at 1.9GHz. Considering the 15dB bandwidth it provides better isolation over 570MHz ranging from1.48GHz to 2.05GHz.
A comparison for S11 parameter has been made to analyse the effect of DGS on reflection coefficient and the same is shown Figure.8. As the DGS alters the ground impedance, it slightly affects the S11 characteristics and there is an improvement of 11dB in the S11 magnitude.
Results and Discussion
The current distribution of the designed antenna with DGS and without DGS is shown in Figure.9. From the Figure, it is evident that the DGS reduces the mutual current which is coupled from one port to another.
The radiation characteristic of designed antenna is shown in Figure.10. The Figure. 10 (a) shows radiation pattern in XZ plane and 10 (b) shows the same in YZ plane. Designed antenna offers a gain of 3.63dBi at 1950MHz. For polarization diversity antenna Co- polarization and cross-polarization must be analysed. Figure.11 shows the co-polarization and cross-polarization of both antennas. From the figure the proposed antenna’s x-pol level is atleast 30dB below when compared with co-pol.
Performance metric validation
The channel capacity of the MIMO system increases as the number of elements increases in the Multi-antenna systems. To enhance the compactness of the wireless systems the antennas are generally placed in close proximity to each other. The reduced spacing between the antennas leads to strong mutual coupling between them. The mutual coupling between the elements reduces the capacity and diversity gain obtained from MIMO configuration. (Kildal et al 2002). Therefore, cross connected octagonal shaped DGS is introduced in the proposed antenna system to mitigate the mutual coupling. To analyse the performance of the proposed antenna in the Multi-antenna environment, the performance metrics such as Envelope Correlation Coefficient (ECC), the achievable diversity gain (DG) and Mean Effective Gain (MEG) to be evaluated and validated.
A. Envelope Correlation Co-efficient (ECC or Ïe)
In small sized hand held Equipments, the antennas are closely spaced to each other. This proximity introduces unwanted coupling effects and in this way correlation results in a performance degradation. correlation coefficient is the relationship between the incoming signals at the antenna ports in an array. Envelope Correlation Coefficient quantifies how independent two antennas radiation pattern are. Thus the correlation coefficient is an important figure of merit to quantify the coupling effects in MIMO systems. Minimising the mutual coupling simply implies the reduction of correlation coefficient between pair of ports.
The square of the correlation coefficient is known as the envelop correlation coefficient. Antenna designers generally have three methods to calculate Ïe. They are the far-field Radiation pattern method, Clarke’s method and S-parameters method.
The ECC can be calculated from S parameters using the following formula,
Ï_e=|S_11^* S_12+S_21^* S_22 |^2/((1-|S_11 |^2-|S_21 |^2 )(1-|S_22 |^2-|S_12 |^2)) (1)
The Ïe value for the proposed is antenna is shown in Figure. 11. The Ïe at center frequency is 0.000432 and it is below 0.005 for the entire operating bandwidth. The benefits of MIMO/ Diversity are ensured when the value of ECC, “Ïe†is less than 0.3 as per the LTE standard released by 3GPP. The ECC analysis confirms that the designed antenna has lowest mutual coupling at the operating band.
B. Diversity Gain (ADG)
The effectiveness of diversity system is usually analysed in terms of diversity gain. The diversity Gain (DG) or Apparent Diversity Gain (ADG) is defined as the improvement in signal-to-noise ratio (SNR) from combined signals of a diversity antenna system, relative to the SNR from one single antenna in the system. It is a relative measure, and the antenna efficiency is not included in the calculations. (Zhinong Ying et al 2005).
To validate the prototype further, diversity gain has to be calculated. The Diversity gain can be obtained from the ECC values (Clarke 1998). The Apparent diversity gain of the proposed antenna can be calculated using the following formula
ADG=10*log√(1-Ï_e ) (2)
As the ECC approaches zero the diversity gain approaches maximum value. Figure. 12 shows frequency vs. diversity gain graph. For the measured operating bandwidth of over 570MHz ranging from 1.48GHz to 2.05GHz, the gain ranges from 9.98dB to 9.99dB.
C. Mean Effective Gain (MEG)
The standalone gain (i.e., the gain calculated for single antenna elements) is not a good measure of antenna performance, as the antenna is not used in an anechoic chamber, in practical applications.
. The mean effective gain (MEG) of the antenna, Ge, is then defined as the ratio between the power which the mobile actually receives and the total which is available. (Simon R. Sanders 2007)
The mathematical expressions for the mean effective gain calculation using Cross Polarization Ratio (XPD) is given in Equation (4).
MEG=∫_0^2π▒∫_0^π▒[XPD/(1+XPD) G_θ (θ,φ) P_θ (θ,φ)+ 1/(1+XPD) G_φ (θ,φ) P_φ (θ,φ)]sinθdθdφ (4)
Where Gθ (θ, φ), and Gφ (θ, φ), are antenna gain components, and Pθ (θ, φ), and Pφ (θ, φ), represent the statistical distribution of the incoming waves in the environment, assuming that the two are not correlated. The XPD stands for Cross Polarization ratio.
The MEG values for the Dual Polarized antenna system with cross-polarization discrimination (XPD-Γ) levels of 0 dB and 6 dB are calculated and presented in Table I. for 1850MHz and 1950MHz. From the table it is evident that the ratio of MEG1/MEG2 < 3 dB at both bands of operation with both XPD values which is acceptable MEG values for Multi-antenna systems.
Comparison with existing literatures
The table II relates the performance of dual polarized antenna with the existing works. All the presented work in table I tend to achieve mutual coupling reduction using decoupling unit cells. The authors have considered improvement in isolation along with bandwidth and dimensions as the prime importance. From the table it is evident that the proposed dual polarization antenna performs well compared with existing works in terms isolation enhancement and ECC.
Conclusion
Dual polarized ring shaped MIMO patch antenna for GSM/DCS 1800 and LTE 1900 bands has been presented. An annual shaped aperture is shared among vertical and horizontal ports which greatly reduces the size of the antenna. Further a square slot is introduced in the radiating patch to enhance the impedance bandwidth of the proposed antenna from 134 MHz to 280MHz. To achieve the benefits of multi-antenna systems, the mutual coupling between the elements of multi-antenna systems has to me mitigated. So, a cross connected octagonal shaped DGS slot has been introduced to the ground plane which greatly reduces the mutual coupling. An isolation enhancement of 30dB is achieved with the cross connected octagonal shaped DGS. The proposed design is fabricated and validated using measurements. ECC, Diversity gain and MEG analysis demonstrate that the designed antenna has good isolation, polarization diversity, and Diversity gain.
References
Rakhesh Singh Kshetrimayum, “Fundamentals of MIMO Wireless Communicationsâ€, Cambridge University press, 2017. ISBN-10: 1108415695.
P. S. Kildal and K. Rosengren, "Correlation and capacity of MIMO systems and mutual coupling, radiation efficiency, and diversity gain of their antennas: simulations and measurements in a reverberation chamber," in IEEE Communications Magazine, vol. 42, no. 12, pp. 104-112, Dec. 2004. doi: 10.1109/MCOM.2004.1367562.
A. K. Sarma, H. Arun, M. Kanagasabai, S. Velan, C. Raviteja and M. G. N. Alsath, "Polarisation diverse multiple input–multiple output antenna with enhanced isolation," in IET Microwaves, Antennas & Propagation, vol. 9, no. 12, pp. 1267-1273, 9 17 2015. doi: 10.1049/iet-map.2015.0040.
Mohammadi, A., & Ghannouchi, F. M., (2012) “RF Transceiver Design for MIMO Wireless Communicationsâ€, Springer-Verlag. DOI:10.1007/978-3-642-27635-4.
B. Wu and K. M. Luk, "A 4-Port Diversity Antenna With High Isolation for Mobile Communications," in IEEE Transactions on Antennas and Propagation, vol. 59, no. 5, pp. 1660-1667, May 2011. doi: 10.1109/TAP.2011.2123060.
S. Hienonen, A. Lehto and A. V. Raisanen, "Simple broadband dual-polarized aperture-coupled microstrip antenna," IEEE Antennas and Propagation Society International Symposium. 1999 Digest. Held in conjunction with: USNC/URSI National Radio Science Meeting (Cat. No.99CH37010), Orlando, FL, USA, 1999, pp. 1228-1231 vol.2. doi: 10.1109/APS.1999.789535.
B. P. Chacko, G. Augustin and T. A. Denidni, "Uniplanar Slot Antenna for Ultrawideband Polarization-Diversity Applications," in IEEE Antennas and Wireless Propagation Letters, vol. 12, no. , pp. 88-91, 2013. doi: 10.1109/LAWP.2013.2242841.
ConstantinosVotis, George Tatsis, PanosKostarakis, (2010) “Envelope Correlation Parameter Measurements in a MIMO Antenna Array Configuration†Int. J. Communications, Network and System Sciences, pp. 350-354.
J. F. Li, Q. X. Chu and T. G. Huang, "A Compact Wideband MIMO Antenna With Two Novel Bent Slits," in IEEE Transactions on Antennas and Propagation, vol. 60, no. 2, pp. 482-489, Feb. 2012. doi: 10.1109/TAP.2011.2173452.
R. H. Clarke, "A statistical theory of mobile-radio reception," in The Bell System Technical Journal, vol. 47, no. 6, pp. 957-1000, July-Aug. 1968. doi: 10.1002/j.1538-7305.1968.tb00069.x.
S. C. K. Ko and R. D. Murch, "Compact integrated diversity antenna for wireless communications," in IEEE Transactions on Antennas and Propagation, vol. 49, no. 6, pp. 954-960, Jun 2001. doi: 10.1109/8.931154.
R. G. Vaughan and J. B. Andersen, "Antenna diversity in mobile communications," in IEEE Transactions on Vehicular Technology, vol. 36, no. 4, pp. 149-172, Nov. 1987. doi: 10.1109/T-VT.1987.24115.
Y. Gao, X. Chen, Z. Ying and C. Parini, "Design and Performance Investigation of a Dual-Element PIFA Array at 2.5 GHz for MIMO Terminal," in IEEE Transactions on Antennas and Propagation, vol. 55, no. 12, pp. 3433-3441, Dec. 2007. doi: 10.1109/TAP.2007.910353.
Kildal, P.-S., Rosengren, K., Byun, J. and Lee, J. (2002), Definition of effective diversity gain and how to measure it in a reverberation chamber. Microw. Opt. Technol. Lett., 34: 56–59. doi:10.1002/mop.10372.
J. Granholm and K. Woelders, "Dual polarization stacked microstrip patch antenna array with very low cross-polarization," in IEEE Transactions on Antennas and Propagation, vol. 49, no. 10, pp. 1393-1402, Oct 2001.doi: 10.1109/8.954928.
C. H. Ahn, S. W. Oh and K. Chang, "A Dual-Frequency Omnidirectional Antenna for Polarization Diversity of MIMO and Wireless Communication Applications," in IEEE Antennas and Wireless Propagation Letters, vol. 8, no. , pp. 966-969, 2009. doi: 10.1109/LAWP.2009.2030135.
W. Hu, X.-S. Ren, B. Li, Y.-Z. Yin, X. Yang & J.-H. Yang (2012) Dual-band dual- polarized printed antenna for wireless communication, Journal of Electromagnetic Waves and Applications, 26:17-18, 2242-2248, DOI: 10.1080/09205071.2012.732025.
Bin Wang, Cencen Huang, Honggang Hao, Bo Yin & Wei Luo (2016) Broadband Dual-Polarized Dipole Antenna with Metallic Cylinder for Base Station, Electromagnetics, 36:8, 515-523, DOI: 10.1080/02726343.2016.1236049.
Mohammad S. Sharawi., (2013). ‘Printed Multi-Band MIMO Antenna Systems and Their Performance Metrics [Wireless Corner]’, IEEE Antennas and Propagation Magazine, vol. 55, no. 5, pp. 218-232.
3GPP (2013), “The Mobile Broadband Standard LTE-Advanced†release 11. 3GPP, a Global initiative.
Valenzuela-Valdes, J.F., Manzano, M. F., Landesa, L. (2012) ‘Deepening True Polarization Diversity for MIMO System’, IEEE Antennas Wireless. Propag. Lett., vol. 11, pp. 933 -936.
R. Clarke, "A statistical theory of mobile-radio reception," Bell Syst. Tech. J, vol. 47, pp. 957-1000, 1968.