Tetraband Small-Size Printed Strip MIMOAntenna for Mobile Handset Application

A compact printed multiple-input multiple-output (MIMO) antenna for tetraband (GSM900/1800/1900/UMTS) mobile handset application is presented. The proposed MIMO antenna, which consists of two coupled-fed loop antennas with symmetrical configuration, was printed on a 120 ∗ 60 ∗ 0.8 mm3 Fr-4 substrate of relative permittivity of 4.4, loss tangent 0.02. Each element antenna requires only a small area of 22.5 ∗ 25 mm2 on the circuit board. The edge-to-edge spacing between the two elements is only 0.03λ0 of 920 MHz. A slot and a dual-inverted-L-shaped ground branch were added in the ground plane to decrease the mature coupling between the antenna elements. The measured isolation of the proposed antenna is better than 15 dB among the four operating frequency bands. The simulated 3D radiation patterns at 900 MHz and 1900 MHz of both antenna elements show that two loop antennas in general cover complementary space regions with good diversity performance. Detailed antenna impedance matching performance comparisons were done to evaluate the benefit of using different decoupling technology. The envelop correlation coefficient is calculated to represent the diversity performance of the MIMO antenna.


Introduction
Multiple-input and multiple-output (MIMO) technology which seemed as a key technology for long-term evolution (LTE) has attracted significant attention [1][2][3].Theoretical and experimental investigations have revealed substantial improvements in channel capacity and reliability in rich scattering environments when multiple transmitter and receiver antennas are deployed [4,5].There are more limitations for engineers to design a qualified MIMO antenna for handset applications than a MIMO antenna for base station applications such as the extremely small size and the mutual coupling between the element antennas [6,7].The correlation coefficient is directly related to the mutual coupling between the element antennas.The higher the isolation was, the higher the data transmission speed could be obtained [8].
Many studies have been carried out to reduce the mutual coupling between the multiple antenna elements.A corrugated ground plane with λ/4 slot was used to reduce the interference of a current flowing in the common ground plane [9].In [10], the protruded T-shaped stub and Lshaped stub at ground plane are used to reduce the mutual coupling between two element antennas.The polarization diversity technique [11] and defected ground structure [12] are adopted to improve the separation between the element antennas.Various MIMO antennas for laptop or mobile handset applications were provided during these years.The majority of them are designed for laptop operating at WLAN (2.4-2.48GHz, 5.2 GHz) and WiMAX band [13][14][15].MIMO antennas for mobile terminal have drawn more and more research interest nowadays, and more and more designs are presented.Most of MIMO antennas for handset applications are designed to resonate at high frequencies such as GSM1800/1900/UMTS [9][10][11][12].However, fewer researches were carried out to design MIMO antennas resonating at lower frequencies such as GSM850/900/LTE 700 or to design MIMO antennas resonating at both lower frequencies and higher frequencies.In [16,17], two articles of MIMO antennas for only LTE 700 application were provided.
In this paper, we present a promising small-size on-board printed multiple-input and multiple-output antenna for tetraband wireless communication applications (GSM900/1800/1900/UMTS).The proposed antenna, which consists of two coupled-fed loop antennas with symmetric configuration, was printed on a 120 * 60 * 0.8 mm 3 FR-4 substrate of relative permittivity of 4.4, loss tangent 0.02.Two coupled-fed strip antennas were etched on the top layer of the substrate.The grounds with a 4 mm-width slot and dual-inverted-L-shaped ground branches were etched on the bottom layer of the substrate.The loop antenna is formed by a loop strip with end terminal short-circuited to the ground plane and its front section capacitively coupled to a feeding strip which is also an efficient radiator to contribute a resonant mode for the antenna's upper band to cover 1710-2170 MHz.Through the coupling excitation, the antenna can also generate a qua-wavelength loop resonant mode to form the antenna's lower band to cover the 880-960 MHz [18].The isolation between the two antenna elements highly improved when a slot and a dual-inverted-L were added on the ground.The slot on the ground also generates a resonance at about 900 MHz that broaden the antenna's lower operating band.

Antenna Design
The geometry of the proposed MIMO antenna is shown in Figure 1.Two element antennas and ground were etched on the top layer and bottom layer, respectively.The element antenna is formed by a 0.5 mm-width loop strip and a feeding strip.The end terminal of the loop strip is short-circuited to the ground via pin A1 and A2.A 2 * 4 mm 2 rectangular matching piece was added to improve the impedance matching performance.The front section of the loop strip is capacitively coupled to the feeding strip which starts from a shot section of 50 Ω microstrip.The feeding strip also generates a resonant mode for GSM1800/1900/UMTS.The ground of the proposed antenna is shown in Figure 1(a2).The substrate extends its length by L 3 = 7.5 mm.A slot with length L 1 = 55 mm and width W 1 = 4 mm was laid in the middle of the ground plane.A dualinverted-L-shaped ground branch with strip width W 2 = 3 mm and x-direction end section length L 2 = 15 mm was laid on the left and right sides of the slot.

Simulation Analysis
The  All the dimensions that may affect the return Loss and isolation performances of the antenna are examined.Figure 1(a2) shows the dimensions that influence the antenna's return loss and isolation performances.L 1 is the length of the slot, and W 1 is the width of the slot.L 2 is the length of the inverted-L stub's x-direction end section.L 3 represents the extended length of the PCB. Figure 3    The simulated three-dimensional (3D) total power radiation patterns at 900 MHz and 1900 MHz of the proposed small-size MIMO antenna are plotted in Figure 4.The radiation patterns are seen from four different directions (front, back, top, and bottom) considering the cases of element antennas work respectively and two element antennas work together.For the lower frequency at 900 MHz, each element antenna generates an oblique dipole-like radiation pattern with the center axes orthogonal to each other.Two bolique dipole-like radiation patterns combined to a unique dipolelike radiation patterns with it center axis directly oriented toward z-axis.The same diversity performance could also be

Measurement Result
The antenna was fabricated and tested in the school of Electronic Engineering of Beijing University of Posts and Telecommunications (BUPT).The S-parameters were measured by a Vector Network Analyzer, and the radiation pattern measurement is carried out inside an anechoic chamber.
The measured S 11 and S 21 curves of the antenna are plotted in Figure 5 The ECC (envelope correlation coefficient) is usually used to evaluate the diversity capability of a multiantenna system and should ideally be computed using the 3D radiation pattern [20].Assuming that the antennas will operate in a uniform multipath environment, it can be alternatively calculated by using the scattering parameters.The ECC of two antennas is given by ( 1) [21].The calculated envelope correlation coefficient (ECC) curve is plotted in Figure 10 to evaluate the performance of the MIMO antenna.The ECCs of the two element antennas are always below 0.05 over the whole frequency band.This leads to perfect performance in terms of diversity: (1)

Conclusion
A dual-element small-size printed strip multiple-input and multiple-output (MIMO) antenna was proposed in this paper.The edge-to-edge spacing between the two elements is only 0.03 λ 0 of 920 MHz.A ground plane with a slot and dual-inverted-L-shaped stub were used to decrease the mutual coupling between the element antennas.Antenna performances of with-and without-decoupling techniques were listed.Prototypes were fabricated and measured after parameter optimizations.

Figure 1 (
a1) depicts the layout of the two coupled-fed loop antennas; the edge-to-edge spacing is 10 mm.The detailed dimension of the element antenna is shown in Figure1(b).Each element antenna occupies a footprint of 22.5 * 25 mm 2 .

Figure 1 (
c) is the photograph of the fabricated MIMO antenna.Port no. 1 and port no. 2 were connected with two SMA female connectors for S-parameter measurements and radiation patterns measurements.

Figure 2 :
Figure 2: Simulated S 11 and S 21 curves: (a) simulated S-parameter curves of the antenna without slot on ground plane and ground branch, (b) simulated S-parameter curves of antenna with slot on the ground plane, (c) simulated S-parameter of antenna with dual-inverted-L shaped ground branch, (d) simulated S-parameter of antenna with both slot and ground branch.

5 Figure 4 :
Figure 4: Simulated total power 3D radiation pattern of antenna no. 1 and antenna no. 2 at 900 MHz and 1900 MHz.

Figure 6 :
Figure 6: Measured E-Plane radiation pattern of antenna no. 1 at 900 MHz.
plane Horizontal Ant01 H-plane Vertical

Figure 7 :
Figure 7: Measured H-Plane radiation pattern of antenna no. 1 at 900 MHz.

Figure 8 :
Figure 8: Measured E-Plane radiation pattern of antenna no. 1 at 1900 MHz.
plane Horizontal Ant01 H-plane Vertical
. The S 11 < −6 dB frequency bandwidth covers from 870 MHz to 1010 MHz at lower frequency band and from 1710 MHz to 2290 MHz at higher frequency band.The measured S 21 values of the MIMO antenna are smaller than −15 dB over the entire frequency band of 800 MHz-2300 MHz.The return loss and isolation of the antenna fully satisfies the requirement of MIMO antenna operation at GSM900/1800/1900/UMTS.The 900 MHz and 1900 MHz measured radiation patterns of antenna no. 1 are depicted in Figures 6, 7, 8, and 9.The measured E-Plane and H-Plane radiation patterns of antenna no. 2 are consistent with radiation patterns of antenna no. 1 except a 180-degree rotation because of the exactly symmetrical configuration.Each figure shows the radiation pattern at one frequency including H-plane's (xy plane)/E-plane's (x-z plane) horizontal radiation pattern and vertical radiation pattern.
The measured −6 dB bandwidth are 870 MHz to 1010 MHz and 1710 MHz to 2290 MHz.The isolations are better than 15 dB covering all the frequency band.The simulated and measured radiation patterns of the antenna show good diversity characteristic.The calculated ECCs are below 0.05 over the whole band.The features above proved that the proposed antenna is a promising product for mobile terminals.The S 11 < −6 dB frequency bandwidth covers from 870 MHz to 1010 MHz at lower frequency band and from 1710 MHz to 2290 MHz at higher frequency band.