A High Isolation MIMO Antenna without Decoupling Structure for LTE 700 MHz

This paper presents a long-term evolution (LTE) 700MHz band multiple-input-multiple-output (MIMO) antenna, and high isolation between the two symmetrical antenna elements is obtained without introducing extra decoupling structure. Each antenna element is a combination antenna of PIFA and a meander monopole antenna. The end of the PIFA and the meander monopole antenna are, respectively, overlapped with the 50Ωmicrostrip feed line, the two overlapping areas produce additional capacitance which can be considered decoupling structures to enhance the isolation for theMIMO antenna, as well as the impedance matching of the antenna elements. TheMIMO antenna is etched on FR4 PCB board with dimensions of 71 × 40 × 1.6mm; the edge-to-edge separation of the two antenna elements is only nearly 0.037 λ at 700MHz. Both simulation and measurement results are used to confirm the MIMO antenna performance; the operating bandwidth is 698–750MHz with |S 11 | ≤ −6 dB and |S 21 | ≤ −23 dB.


Introduction
With the development of wireless communication, mobile terminals require much higher data-rate transmission than the past.As a key technology of wireless communication, long-term evolution (LTE) standard allows employing multiple-input-multiple-output (MIMO) technique, which has been considered one of the most promising technologies to provide increased channel capacity and enhanced spectrum efficiency, without the need for additional transmit power.But it is a challenge to design MIMO antenna in slim mobile terminal applications, especially for lower band LTE 700 MHz.
Many works have been carried out to reduce mutual coupling between MIMO antenna elements.In [1], an idea was to use field cancellation to enhance isolation by creating an additional coupling path between antenna elements.A tree-like decoupling structure was used to achieve high isolation, as mentioned in [2].A T-shaped ground plane was placed between two orthogonal linear monopole antennas [3].According to research in [4], an LC-based branch-line hybrid coupler had been integrated with the antenna to decouple the ports.Utilizing EBG structures is an effective method to reduce the mutual coupling between antenna elements [5][6][7].Two bent slits had been etched into the ground plane to improve isolation at lower frequency and widen impedance bandwidth at higher frequency [8].Defected ground structure (DGS) has also been proved to provide a significant effect to improve isolation by changing the ground current flowing [9].A suspended neutralization strip was used in [10] to deliver one antenna element signal to the other antenna element in order to cancel out the existing mutual coupling.
As mentioned above, there are several methods that have been reported to improve the isolation between antenna elements, but decoupling structure is expensive and occupies a large area, especially for lower band.That is disadvantageous to portable devices because of their small volumes and low costs.
In order to study the decoupling of LTE MIMO antenna at lower frequencies, MIMO antenna for LTE 700 MHz with small size of 71 × 40 × 1.6 mm 3 is presented in this paper.The proposed antenna has a high isolation without extra decoupling structure and performs over the lowest LTE band.It is found that the design can achieve more than 23 dB isolation over the LTE 700 band and even better than 35 dB over frequency range of 690-725 MHz.Details of our antenna design and experiment results are presented and discussed.

Antenna Design
2.1.Configuration of the Proposed Antenna. Figure 1 shows the geometry of the proposed LTE MIMO antenna, which is etched on FR4 PCB board with length  sub =   + 0.8 mm + 7.2 mm + 13 mm = 71 mm, width   = 40 mm, height ℎ = 1.6 mm, relative permittivity   = 4.4, and loss tangent tan  = 0.02.The MIMO antenna consists of two symmetric antenna elements; each element includes a mender monopole antenna and PIFA.The mender monopole antennas are printed on both sides of the substrate and connected by the metal strips which are located at the flank of PCB board.The ground plane is located on the front side of the PCB board, and the PIFAs are grounded by the short pins.Each 50 Ω microstrip feed line has a protruded part; it has an overlapping area with the end of its corresponding PIFA.Moreover, the strip of each monopole printed on the front side of the PCB board also has an overlapping part with its corresponding 50 Ω microstrip feed line.

Study of LTE MIMO Antenna.
As mentioned above, the LTE MIMO antenna element is composed of a mender monopole antenna and a PIF antenna.Monopole antennas are used in mobile devices popularly for their inherent advantages.Here, we designed a mender monopole antenna firstly and adjust its length to  monopole = 11.5 + 20.2 + 12 + 18 + 7.5 + 3 = 72.2mm, a little shorter than quarterwavelength at 800 MHz.A parasitical capacitance is produced by the overlapping area between the feed line and end of monopole antenna, which can help the antenna excite a resonant mode at about 800 MHz as shown in Figure 2. To cover the whole bandwidth of LTE 700 MHz, a PIF antenna is produced by adding a short pin, and another resonance mode is introduced at about 710 MHz.The production of this resonant mode is benefited from the overlapping area between the ground branch and the feed line, which can introduce additional capacitance for the inductive PIF antenna.Thus, an operating bandwidth covering 700-750 MHz is obtained.Figure 4  the antenna structure, which can be considered a combined antenna of a monopole antenna and a PIF antenna, and it is chosen as antenna element for the MIMO antenna shown in Figure 1.
As we know, isolation is an important parameter of MIMO antenna.Performance of PIF antenna is stable, so the isolation for the frequencies at about 700 MHz is good enough without need to introduce additional decoupling structure.Figure 5 shows the simulated | 12 | of the MIMO antenna with varying  last , which is the length of the last segment of the monopole antenna, as shown in Figure 1.We can find from Figure 5 that the effect of varying  last is slight for the frequencies around 700 MHz, because this resonance International Journal of Antennas and Propagation

Results and Discussions
The MIMO antenna is designed and simulated using High Frequency Structure Simulator V13.0 and is fabricated and measured too.The photographs of the proposed MIMO antenna are shown in Figure 6.
With one port excited and the other terminated with 50 Ω impedance, the simulated surface current distributions of the proposed MIMO antenna are shown in Figure 7.At 700 MHz, the currents mainly distribute on the PIFA but mainly distribute on the monopole antenna at 750 MHz.That further explains that the resonant modes at 700 MHz and 750 MHz are produced by the PIFA and the meander monopole antenna, respectively.Observing from Figure 7, we also can know why the coupling between two elements is more serious at the higher frequency.As shown in Figure 7(b), there are more currents distributing on the ground plane, and that is because of the good stability of PIFA.
The measured and simulated | 11 | of the proposed antenna can be seen in Figure 8.There are two resonant modes at about 700 MHz and 750 MHz, and they are produced by the PIFA and meander monopole, respectively.These two resonant modes are interrelated and interact with each other, so a good operation band is achieved.With | 11 | ≤ −6 dB, the operating band is from 698 to 750 MHz, and LTE 12 and LTE 17 bands are covered.
Figure 9 shows the measured and simulated | 12 | of the proposed LTE MIMO antenna.The deviation of measured and simulated results is due to the ground currents, which has strong effect on the mutual coupling between the two antenna elements and are susceptible to the test instruments.Without any decoupling structure, we can see that the simulated result shows that the isolation between two ports is better than 23 dB.
Figure 10 shows the measured envelope correlation coefficient results; the calculated correlation is obtained from the radiation efficiency and -parameters [11] The values are less than 0.06 over the operating bandwidth, and that means the MIMO antenna has a good diversity performance.
When measuring 2D patterns of the proposed MIMO antenna, port 1 is excited and port 2 is terminated to a load with 50 Ω impedance.The simulated and measured results are shown in Figure 11.Because the two antenna elements are symmetric, the radiation patterns of the other antenna element are not shown.

Conclusions
In this paper, LTE 700 MHz MIMO antenna without decoupling structure has been presented.The MIMO antenna has a small size and a good performance.With edge-to-edge separation of the two antenna elements nearly 0.037 at 700 MHz, and without decoupling structure, high isolation is obtained.Both simulation and measurement results had shown that isolation of the proposed antenna is better than 23 dB, and the impedance bandwidth is 698-750 MHz with | 11 | ≤ −6 dB.

Figure 6 :
Figure 6: Prototype photograph of the proposed antenna: (a) top layer; (b) bottom layer.

Figure 10 :
Figure 10: Measured ECC of the proposed antenna.