Volume 2, Spl. Issue 2 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
A Review Paper on Triangular Patch MIMO Antennas for Wireless Communication Sheifali Gupta1 , Amanpreet Kaur2 Department of Electronics and Communication Engineering , Thapar University, Punjab, India 1
sheifali4pisces@gmail.com, 2amanpreet.kaur@thapar.edu
Abstract – This paper presents a review article on the design of MIMO systems with three equilateral triangular patches. The impact of mutual coupling on system performance is analysed . The wide beam characteristics and the high gain of each antenna element , improvement in SNR and interference rejection of MIMO system are also studied. Both correlation effects and variation of antenna gain due to mutual coupling mechanism are considered . Due to mutual coupling and correlation , it is shown that the antenna achieves good polarisation and diversity gain. Index terms – MIMO systems; mutual coupling; correlation; microstrip antennas; equilateral triangular patch antenna ; diversity gain. 1.
INTRODUCTION
Due to the development in multimedia technology, there is need to raise the capacity and reliability for wireless communication systems. It is becoming difficult to achieve all the above requirements using SISO systems, due to limited channel capacity[1 , 2]. By using various diversity schemes , i.e transmitting diversity , reception diversity and channel coding techniques, we can implement MIMO (Multiple Input Multiple Output) systems which are able to transmit multiple decorrelated signals of same power level, through spatial multiplexing . These signals are then received at receiver and combined using diversity techniques . Main advantages of MIMO are increased capacity & reliability over SISO , with same power and no bandwidth expansion. Major requirements of MIMO systems are maximum channel capacity and minimum BER . To achieve these requirements , we need to use antennas in a configuration called arrays in the MIMO systems . Antennas used for making array are preferably the microstrip antennas as these are most successful and revolutionary antenna technology ever , with several advantages such as;
Arrays that are used in MIMO systems are required to have high gain , wide lobe pattern and high isolation between the antenna elements in order to achieve an optimised performance. This paper presents the review work done on realization of MIMO antenna array with equilateral triangular patch antennas . The arrangement of triangular patch antennas is done in special manner , so that good radiation pattern and polarisation diversity can be achieved.[6]
2. TRIANGULAR PATCH ANTENNA ARRAY DESIGN MIMO antenna array are realized with equilateral triangular patches which are arranged in a configuration that provides good polarisation and radiation properties to be used in mobile applications. We prefer triangular patches over rectangular patches for array elements because as the no. of array elements increases the gain and directivity increases both for rectangular and triangular patch antennas but the side lobe levels in rectangular patch is worse than that in triangular patch antennas . Also , the antenna height required for triangular patch antennas are low , so this solves the issue of compactness for the mobile applications[4]. Here the various design configurations for triangular patch antenna arrays are given by different authors which are: 2.1 DESIGN OF 4*1 TRIANGULAR PATCH ANTENNA ARRAY Author Yahya S. H. Khraisat & Melad M. Olaimat designed 4x1 array of triangular patches , quarter wave transformer is used to feed the elements[4].The radiation pattern for this array is in broadside direction.The radiation pattern improves as no. of array increases therefore impact of mutual coupling reduces for the array.
light weight , low profile , easy and low cost fabrication , conformable to planar and non-planar surfaces , mechanical robustness , easy association in arrays. These antennas are used in many and varied military & commercial applications such as radar , satellite communication , WLANS, Ultra wideband & radio frequency identification (RFID).[9]
251
BUEST, Baddi
RIEECE -2015
Volume 2, Spl. Issue 2 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
Figure 1. The configuration of 4*1 antenna array 2.2DESIGN OF THREE EQUILATERAL TRIANGULAR PATCHES The geometry of a typical MIMO antenna array that are used for MIMO applications is shown in Figure 2, and the corresponding prototype of the antenna array is shown in Figure 3. These antennas are position such that their inward pointing vertices all point towards a point. In each antenna element , there is a feed point located away from the vertices. To increase the isolation between each antenna element , the ground plane on the back side is divided into three sections. The antenna array is fabricated on an FR4 substrate. Each antenna is fed by coaxial cable with an SMA connector. Figures 2 and 3 show the geometery and the fabricated antenna of the proposed structure.
Figure 2. The geometry of the antenna array
BUEST, Baddi
RIEECE-2015
Figure 3. The actual antenna array 2.3 DESIGN OF SLOT RING TRIANGULAR PATCHES This antenna was designed byFitri Yuli Zulkifli, Daryanto, & Eko Tjipto Rahardjo depicted in Figure. 3. In this the antenna shape is triangular patch with insertion of slot ring to obtain broadband characteristic . The antenna is printed onFR4-Epoxy substrate The antenna consists of four elements perpendicular to each other to form the 2 x 2 MIMO configuration. Two antenna elements are for the transmitters (port 2 and port 4) and two elements for the receivers (port 1 and port 3). For this design antenna elements achieved mutual coupling suppression to lower than 25 dB by placing the single element perpendicular to each other [10].
Figure 4 . The proposed antenna design for slot ring triangular patches
252
Volume 2, Spl. Issue 2 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
3. MUTUAL COUPLING ANALYSIS OF ANTENNAS FOR MIMO SYSTEMS In mobile applications , the spacings between each antenna are generally smaller , smaller than λ/4. So, the impact of mutual coupling can’t be neglected. Since mutual coupling changes the current distribution in antennas which causes distortion in radiation pattern for each antenna element, this results in the degradation in system’s performance. In MIMO although antenna’s performance is degraded due to the channel correlation [3], the deformation of radiation pattern induces polarization diversity, which enhances MIMO system performance and diversity gain[3]. The mutual coupling between each element can be obtained by measuring the isolation between each element Figure 5 shows the isolation characteristics between each antenna element.
Figure 5. The isolation between two ports (Sij) At 2.65 Ghz , S31 was -32dB which was lower than S21 and S23 (these are -28dB & -29dB) [8]. This is due to the distance between antenna 1 & 3 was longer than the distance between 2 and other antennas and therefore the isolation between 1 & 3 was relatively lower. Due to the mutual coupling between antenna elements , the radiation pattern of each element is different from their isolated counterparts. The antenna elements are positioned in a way that one vertex of antenna point towards a point, so that we can achieve the polarisation in vertical direction. Antenna 1 & 3 are also affected by mutual coupling , but their radiation patterns [8] retained characteristics of equilateral triangular patch antennas. When the geometry of triangular patches changed as shown in figure [4] two antenna elements are for the transmitters (port 2 and port 4) and two elements for the receivers (port 1 and port 3). This antenna design is very compact because it consists of the elements are placed directly near to each other without any spacing. This design excites linear polarization. The polarization of port 2 is linear in the x direction , while port 1 in the y direction. Therefore, this can reduce the mutual coupling between antenna port 1 and port 2. The perpendicular position of the antenna plays an important role towards the mutual coupling reduction of the antenna. After experimenting author achieved the mutual coupling suppression of S21, S41, S23 and S43 at centre frequency 2.35 GHz is -33.1 dB, -40.1 dB, 32.9 dB and -34.9 dB, respectively[10 ] shown in Figure 6.. This mutual coupling result is suppressed well below -20 dB . This configuration well describes how to suppress the mutual coupling between the antenna elements and enhancement of gain.
Figure 6. Mutual coupling measurement
253
BUEST, Baddi
RIEECE -2015
Volume 2, Spl. Issue 2 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
4. ANTENNA DIVERSITY PERFORMANCE ANALYSIS With the antenna diversity , the problems of mobile channels are attacked directly. Antenna diversity improves the channel capacity by adding extra equipment (antenna , combiner ) to the receive end of link , there is no need of extra spectrum. While all other schemes need extra spectrum to improve channel capacity. The only disadvantage of antenna diversity is the cost and the inconvenience of extra equipment.[5] The diversity performance of the MIMO antenna array depends on cross-correlation & relative mean power level among the signals delivered from each antenna element. The cross-correlation of signals are characterised by envelope correlation coefficient ‘ ρ’. For uniform random field where antenna losses are not considered , the envelope correlation is given from S parameters as[7]
ρ=
S*11S12
+
S*21S22
(1 − |S11|2 -|S21|2)(1 - |S22|2-|S12|2) The mutual coupling causes the correlation coefficient between each antenna element to increase under some conditions but at same time mutual coupling causes the radiation pattern of each antenna element to deform , which induces diversity gain that cause correlation coefficient to decrease. For figure 2, we have vertical polarisation , this induce a mutual coupling effect that reduces the correlation between antenna elements . To achieve diversity characteristics ρ < 0.7 at base station or ρ < 0.5 at mobile. The correlation coefficient between 1 & 3 = 0.4 and between 2 and other two elements (1 and 3) = 0.5 . These values are sufficient to fulfil the requirements for MIMO systems.
REFERENCES 1. M.Karaboikis,C.Soras,G.Tsachtsiris, and V.Makois , “Compact Dual – Printed Inverted- F Antenna Diversity Systems for Portable Wireless Devices , ” IEEE Antennas and Wireless Propagation Letters , 3,2004,pp.9-14. 2. Hyundong Shin and Jae Hong Lee , “ Capacity of Multiple Antenna Fading Channels : Spatial Fading Correlation , Double Scattering and Keyhole,”IEEE Transactions on Information Theory, 49,10,October 2003 ,pp.2636-2647. 3.Bruno Clerckx,Christophe Craeye , Danielle Vanhoenacker-Janvier, and Claude Oestges, “Impact of Antenna Coupling on 2*2 MIMO Communications,” IEEE Transactions on Vehicular Technology,56,3,June2007,pp.1009-1018. 4. Yahya S. H. Khraisat, Melad M. Olaimat, “Comparison Between Rectangular and Triangular Patch Antennas Array,”19th Inetrnational Conference on Telecommunications( ICT 2012) Vol 4,No.2;2012 5. Rodney G. Vaughan and J.Bach Andersen, “Antenna Diversity in Mobile Communications,” IEEE Transactions on Vehicular Technology,36,4,November1987 , pp.149-172. 6. Jeen-Sheen Row and Yen-Yu Liou , “ Broadband Short-circuited Triangular Patch Antenna ,” IEEE Transactions on Antennas and Propagation,AP-54,7,July 2006,pp.2137-2141. 7. S. Blanch, J. Romeu, and I. Corbella, “Exact representation of antenna system diversity performance from input parameter description,” Electron. Lett., vol. 39, no. 9, pp. 705–707, May 2003. 8. Haili Zhang, Zhihong Wang , Jiawei Yu and Jia Huang, “A Compact MIMO Antenna for Wireless Communication,” IEEE Antennas and Propagation Magazine , Vol.50,No.6,December 2008 . 9. C.A.Balanis. Antena Theory:Analysis and Design , 2nd ed. United States of America , John Wiley&Sons Inc.,1997. 10. Fitri Yuli Zulkifli, Daryanto, Eko Tjipto Rahardjo , “Slot Ring Triangular Patch Antenna with Stub for MIMO 2x2 Wireless Broadband Application ,” Antenna Propagation and Microwave Research Group (AMRG),Vol 02 ;October 2013,pp. 885-887.
5. CONCLUSION The paper presents an analysis of Triangular Microstrip antennas for a MIMO system. The detailed analysis including the effect of mutual coupling and antenna diversity is presented in this paper. The analysis was done considering an antenna array of three antenna elements . The equilateral triangular patch antenna elements are arranged in a configuration that provides good polarisation , radiation properties and compact enough to be used in mobile applications. The wide lobe pattern of the triangular patch antenna elements, combined with the omni directional radiation pattern of the centre antenna element , allow the antenna array to receive a wide range of multipath signals and to increase the SNR of MIMO systems. The high gain of each antenna element , combined with high isolation characteristics between antenna elements , allow the antenna array to deliver the high diversity gain required by MIMO systems. When the array is arranged to slot ring triangular patches perpendicular to each other there occurs reduction in mutual coupling due to linear polarisation between antenna elements. .
BUEST, Baddi
RIEECE-2015
254