Highly Compact MIMO Antenna System for LTE / ISM Applications

Planar monopole antenna is proposed as the antenna element to form a compact dual-element multiple-input-multiple-output (MIMO) antenna system for LTE2300 (used in Asia and Africa) and ISM band operation. The system can cover a 310MHz (2.20– 2.51 GHz) operating bandwidth, with the total size of 15.5mm × 18mm × 1.6mm. Measured isolation higher than 16 dB is obtained without any specially designed decoupling structures, while the edge-to-edge element spacing is only 7.8mm (0.08λ at 2.20GHz). Radiation characteristics, correlation coefficient, and the performance of the whole system with a metal sheet and a plastic housing show this system is competitive for practical MIMO applications. The antenna element is further used to build an eight-element MIMO antenna system; also good results are achieved.


Introduction
MIMO antenna systems are widely used in modern wireless terminals [1,2].The use of multiple antennas can increase the data rate without extra need of bandwidth and power levels.However, in practical mobile terminal design, the space for antenna system is highly limited.When elements are close to each other, strong electromagnetic coupling will occur.How to obtain a compact MIMO antenna system with a wide bandwidth while maintaining adequate isolation between the elements becomes a challenging task [3][4][5].
Electromagnetic band-gap (EBG) structures [6,7] are used to suppress the surface wave and enhance the isolation.But for this method, the size of the EBG elements is always larger than 0.1 0 and the structure is complicated in practical design and can affect the radiation characteristics of the whole system.Defected ground plane structures (DGS) are used in [8]; the performance will be deteriorated when placed closely to conductive objects.Elements are placed orthogonally in [9]; the arrangement makes the antenna elements have orthogonal polarization which can reduce the mutual coupling between the antennas.Besides the arrangement of antenna elements, placing metal strips between elements [10,11], changing the structure of the ground [12][13][14], and adding lumped element on the ground [15] are used to further improve the isolation of the whole system.In [16,17], filtering structures like slotted meander-line resonators and modified serpentine structure are used to enhance the isolation between two microstrip patch antennas.The structures are complicated in design and only effective at single resonant frequency, and the total systems are large in size and not wide in bandwidth (60 MHz for [17]).
In this paper, two symmetric planar monopole antennas are used to form the MIMO antenna system.The total dimension of the MIMO system is only 15.5 × 18 mm 2 , less than 12.8% of the size reported in [17].A measured operating band of 310 MHz (2.20-2.51GHz) is obtained to cover the LTE 2300/ISM2.45GHz operation.Isolation higher than 16 dB can be achieved with a small edge-to-edge separation of 2.6 mm (0.02).The correlation coefficients between the two elements are well less than 0.3 and total efficiencies are larger than 78%, which made this MIMO antenna system competitive for mobile MIMO terminal designs.

2
International Journal of Antennas and Propagation  In this paper, the radiation patch and ground of the antenna are on the same side of the substrate.In order to design the antenna to resonate at the desired frequency, we use the following two equations in parameters optimization:

MIMO Antenna Design
with In Figure 2, both elements are excited and when   = 0.2 mm, the resonant frequency is 2.45 GHz and the length  of the surface current path (the dot line in Figure 2) at this frequency is 18.6 mm, which is one-quarter of the wavelength calculated by (1).When   = 0.4 mm, the resonant frequency shifts to 2.53 GHz, and the length of the surface current path changed to 18.0 mm, which is just one-fourth of the wavelength at this frequency.The changes of   and   have similar effect on the antenna resonance.Figure 3 shows the relationship between the width of   ,   , and   and the reflection coefficients.Figure 4 is the surface current when only element number 1 is excited at 2.45 GHz.Because of the high isolation between antenna elements, little coupling current is on the other element.Compared to the current rotation in Figure 2, the direction of the coupling current on element number 2 in Figure 4 is opposite with the excited current on element number 2 in Figure 2.
Two types of dual-element antenna arrays are demonstrated in Figures 5(a) and 6(a).The two elements are no longer symmetrical but are in the same direction.For Type I, the edge-to-edge separation of the ground is the same with the proposed one; considering the shape of the radiation patch, the distance between the two elements is decreased and the isolation deteriorated, shown in Figure 5(b).
For Type II, the space between the two radiation patches is kept the same with the proposed one; the actual distance between the antenna elements is increased, but the isolation is still lower than the proposed MIMO antenna system.So the isolation is affected not only by the distance between elements, but also by the rotation of the currents on the radiation patch.Due to the well known skin effect, the two elements can be seen acting like two loop antennas at the operating band.So when only one antenna is excited, the coupled current on the other element has the same rotation or polarization with the exited one (Figure 4); while two elements are excited simultaneously, the rotation or the polarization is just opposite (Figure 2); this to some extent reduced the coupling between the elements.
The two antenna elements can also be orthogonally placed as shown in Figure 7(a).Compared to the proposed MIMO antenna system, the isolation is 1-2 dB higher due to the arrangement of the antenna element.However, the size of the total system is larger than the proposed one.So the final choice is a compromise between space and system performance.The simulated and measured  parameters of the MIMO antenna system are shown in Figure 9.The simulated results were achieved by Ansoft HFSS version 15, while the measured results were obtained by using Agilent 85058E vector network analyzer.The simulated bandwidth was 320 MHz (2.26-2.58GHz), the simulated  11 curve coincides with the simulated  22 curve, and the measured bandwidth was 310 MHz (2.20-2.51GHz).The difference between them was probably due to the fabricated imperfections and the loss of the feeding cable and connectors.The measured −10 dB impedance bandwidth can cover the desired LTE 2300 and 2.45 GHz ISM band.   Figure 10 shows the isolation between antenna elements.Through the whole operating band, the isolation is higher than 16 dB.

Results and Discussion
The correlation coefficient () is an important parameter to evaluate the diversity performance of the multiantenna systems.The lower  results in better MIMO system performance.It is usually calculated from 3D radiation patterns.However, this procedure is rather complicate; recent research indicates that the correlation coefficient can be easily derived from the -parameters [19]    Figure 11 shows the correlation coefficient between the antenna elements of the proposed MIMO antenna system.The antenna efficiency was higher than 78% through the working band.The results are well below 0.3, which is the maximum value set for 4 G standards [20].
The gain of the proposed MIMO antenna system is shown in Figure 12.The simulated peak gain in the + direction is 0.72 dBi at 2.41 GHz and the measured peak gain is 0.62 dBi at 2.4 GHz.
The radiation pattern of the proposed MIMO antenna system is shown in Figure 13.The radiation pattern was obtained when one element was excited and the other is terminated with a 50 ohm load.The radiation pattern is just like a monopole, which has an omnidirectional radiation in the  plane at the resonant frequency.

Eight-Element MIMO Systems.
According to [21], the increase of the number of transmitter antennas and receiver antennas can improve the communication quality and increase the channel capacity without extra radiation power and spectrum bandwidth.An eight-element MIMO antenna system was also fabricated and measured.Its −10 dB bandwidth is 340 MHz (2.18-2.52GHz), as shown in Figure 14(a); good agreement is realized; for the sake of clarity, we only put the results of four antennas here.The measured isolation between antennas is lower than −15 dB at the whole operating band.The simulated radiation efficiency of the antenna element is higher than 77% over the whole band.The correlation coefficient can be calculated by using (3).As plotted in Figure 14(d), the value is far smaller than 0.3.

Practical Applications.
For practical applications, MIMO antenna system is integrated with other components such as circuit board and LCD screen, which means that a conductive plane under the system should be taken into consideration.
In Figure 15(a), the MIMO antenna system was placed above a metal sheet separated by free space.The separation between the conductive board and antenna bottom side was varied from 1 to 5 mm in 1-mm steps.Figures 15(b   It was found that, by using a metal sheet, the resonant frequency of the individual antenna element shifted.As the distance between the antenna and conductive plane increased, the effect weakens.When the board is closely placed under the system like 1 mm, the resonant frequency changed most, but the working band can still cover the LTE2300 and 2.45 GHz ISM band.This is unlike the system reported in [22]; the antenna system does not have any special designed structures on the back side of the substrate, so it is not obligatory to place the system carefully such that its bottom side maintains a minimum distance like 3 to 5 mm from any conductive plane for practical design. Figure 16(a) shows a 1 mm thick plastic housing which is used to simulate the practical case of the wireless terminals.In simulation, the relative permittivity of the plastic housing is International Journal of Antennas and Propagation   set as 3.3 and the loss tangent is 0.02, the same as [23].Figures 16(b) and 16(c) show the influence of the plastic housing on reflection coefficients and isolation.Differences between the two cases are small and acceptable and show this MIMO system is competitive for practical application.

Conclusion
Symmetric planar monopole antenna for compact multielement MIMO antenna systems is proposed.Isolation higher than 15 dB over the operating band is realized by using polarization diversity of the antenna elements.Correlation coefficient, -parameters, radiation characteristics, and acceptable performance when placed with conductive plane and plastic housing show that the proposed antenna arrays can be used for practical MIMO applications.

2. 1 .
Proposed Antenna Configuration.Figure1shows the geometry of the proposed dual-element MIMO antenna for LTE/ISM operation.In this paper, the presented antenna system is formed by two symmetric planar monopole antennas and is printed on a 1.6 mm thick FR4 substrate of relative permittivity 4.4 and loss tangent 0.02.The detailed dimensions of the proposed MIMO antenna system are as follows:  sub = 15.5 mm,  sub = 18 mm,  0 = 1.8 mm,  1 = 3.3 mm,  2 = 2 mm,  3 = 0.6 mm,  4 = 0.9 mm,   = 0.4 mm,   = 0.2 mm,   = 1 mm,   = 0.2 mm,   = 4.6 mm,  1 = 5.4 mm,  2 = 4 mm,   = 5.5 mm,   = 7.8 mm,   = 2.6 mm, and   = 3 mm.2.2.Design Procedure.The limitation of space becomes the most challenging task in designing antennas for modern wireless devices.Meanwhile, electrically small antenna always faces a narrow bandwidth problem.Planar monopole antenna has attractive characteristics such as simple structure, omnidirectional radiation pattern, low profile, and wide bandwidth, which made it a competitive candidate for MIMO antenna systems.

InternationalFigure 4 :
Figure 4: Surface current distribution when only one element is excited at 2.45 GHz.

3. 1 .
Measured Results. Figure 8 shows the photograph of the fabricated antenna.The antenna elements are fed by 50 Ω MCX connectors [18].

Figure 5 :
Figure 5: (a) MIMO antenna system Type I, (b)  parameters of Type I.

Figure 6 :
Figure 6: (a) MIMO antenna system Type II, (b)  parameters of Type II.

Figure 8 :Figure 9 :
Figure 8: Photograph of the fabricated MIMO antenna system.
) and15(c)  show the reflection coefficients and the isolation characteristics of the antenna element over a metal sheet at different distances.

Figure 14 :
Figure 14: (a) Geometry of the 8-element MIMO antenna system.(b) Reflection coefficients of the MIMO antenna system.(c) Measured isolation between MIMO antenna elements.(d) Correlation coefficient || curves for the proposed MIMO antenna system.

Figure 15 :
Figure 15: (a) Proposed MIMO antenna with a metal sheet separated by free space.(b) Reflection coefficients of the MIMO antenna system with a metal sheet.(c) Isolation of the MIMO antenna system with a metal sheet.