This work presents a revolutionary compact wideband multiple-input-multiple-output (MIMO) antenna. On the basis of the S-parameters and the surface current distributions, the working mechanism of the dual-antenna is investigated. The measured − 10dB impedance bandwidth is 6.8 GHz, and the measured mutual coupling is less than 15 dB throughout the whole band, according to a prototype. It is possible to attain a clearance area of 30 mm x 20 mm. To ensure the diversity performance of the proposed MIMO antenna, MIMO parameters such as ECC, DG, CCL, and multiplexing efficiency are explored.
Research Article
Wideband Mimo Antenna with Reduced Mutual Coupling
https://doi.org/10.21203/rs.3.rs-1605228/v1
This work is licensed under a CC BY 4.0 License
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MIMO
mutual coupling
coplanar wave guide (CPW)
MIMO (multiple-input, multiple-output) technology has been widely used in various systems to greatly increase channel capacity. The use of wideband MIMO antennas in portable devices has recently gotten a lot of interest. In a portable device, however, the wideband mutual coupling between MIMO elements will influence the antenna performance. Furthermore, improving wideband isolation is a difficult task.
Several approaches have recently been investigated and used to minimise MIMO antennas' wideband mutual coupling. High isolation was achieved using the band notched feature on an ACS supplied UWB MIMO antenna [1]. Improved isolation is accomplished by placing an I-shaped slot strip between the two slot antennas in a CPW fed UWB MIMO antenna [2]. A unique planar decoupling structure that is put between the two antennas achieves the wide isolation [3]. In order to provide wideband isolation over the operating frequency band, a dual L-shaped and a rectangular defect was produced in the ground plane [4]. To reduce mutual coupling, a flag-shaped stub is inserted in the ground plane in [5]. To eliminate mutual coupling between the elements, [6] employs a wideband neutralisation technique. Pattern diversity is used to achieve isolation in [7]. By inserting slots in the ground plane, mutual coupling in a quad element MIMO antenna is reduced in [8]. In [9], polarisation diversity is used to reduce reciprocal coupling between the elements.
Figure 1 depicts the proposed wideband antenna's geometry. The suggested antenna is constructed from a low-cost FR-4 substrate. The substrate has a thickness of 1.4 mm, a permittivity of 4.3, and a loss tangent of 0.025, respectively. Take a circle with a radius of c = 7mm and a cylinder with an outer radius of 6mm and an inner radius of 5.5mm. An arc is formed from this cylinder. On either side of the circular patch, the identical arc is replicated and symmetrically organised.Table 1 shows the optimal characteristics of the proposed antenna.
|
Parameter |
Ws |
Ls |
a |
b |
c |
Wf |
|---|---|---|---|---|---|---|
|
Unit (mm) |
15 |
20 |
4 |
5.5 |
7 |
3 |
Return loss
Figure 2 depicts the proposed antenna's return loss curve. The proposed antenna covers the frequency range of 5.4 to 12.2 GHz. The current distribution on the antenna is examined in order to provide a physical understanding. Figure 3 depicts the surface current distributions at four sample frequencies.
The gain and efficiency of the antenna at four sample frequencies shown in Table 2.
|
Frequency/Parameter |
6 GHz |
8 GHz |
10 GHz |
12 GHz |
|---|---|---|---|---|
|
Gain (dB) |
2.7 |
3.5 |
4.3 |
3.5 |
|
Efficiency |
89 |
92 |
94 |
91 |
As illustrated in Fig. 4, the same construction was expanded to a total of 2×2 MIMO antenna. The suggested MIMO antenna's size are 32×20 mm2. d = 2mm (0.038 λ0) is the end-to-end distance between two elements. Table 3 shows the optimum dimensions of the proposed MIMO antenna.
|
Parameter |
Ws |
d |
W |
L |
T |
||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
|
Value (mm) |
32 |
2 |
5 |
16 |
1 |
||||||
Figures 5 and 6 show the simulated and measured S-parameters (S11, S21). Figure 5 shows that the operating frequency range of MIMO antenna is similar to that of single antennas (5.4–12.2 GHz). It's also worth noting that the simulated and measured S11 are nearly identical.
Mutual coupling S21 is the most significant metric to consider when evaluating the performance of a MIMO antenna. Mutual coupling happens when two or more antennas are put in close proximity to each other. Mutual coupling has a negative impact on impedance, radiation pattern, and received voltages, among other things, hence it's critical to reduce it. Mutual coupling throughout the full operational frequency band is below − 15dB, as shown in Fig. 5.
Figure 7 depicts the surface current distributions between elements with and without T-shaped stubs. When a stub is inserted between the elements, surface currents are prevented from propagating from one element to the next.
The performance of the MIMO antenna is evaluated in terms of gain, ECC, diversity gain and channel capacity loss. The fabricated MIMO antenna is shown in Fig. 8. The gain of the antenna is 2.6, 3.5, 4.3 and 3.4 at 6GHz, 8 GHz, 10 GHz and 12 GHz respectively as shown in Fig. 9.
Diversity Gain
The diversity gain of the MIMO antenna is 9.98, 9.98, 10 and 9.99 at 6 GHz, 8 GHz, 10 GHz and 12 GHz respectively.
DG = 10 \(\sqrt{1-{\left|ECC\right|}^{2}}\) (1)
Multiplexing Efficiency
Multiplexing efficiency and total efficiency for two element antenna is
$${\left|{\rho }_{e}\right|}^{2}=1-\frac{{\eta }_{mux}}{{\eta }_{1}{\eta }_{2}}$$
2
Effective Diversity Gain (EDG)
The EDG is 8.6, 9, 9.2 and 9.1 at 6 GHz, 8 GHz, 10 GHz and 12 GHz respectively. The antenna efficiency, diversity gain and effective diversity gain are related as
EDG = DG × 𝞰ant (3)
Envelope Correlation Coefficient (ECC)
The ECC for the proposed MIMO antenna can derived from S-parameters. The ECC is 0.003, 0.003, 0.001 and 0.002 at 6 GHz, 8 GHz, 10 GHz and 12 GHz respectively.
|
Frequency/Parameter |
6 GHz |
8 GHz |
10 GHz |
12 GHz |
|---|---|---|---|---|
|
Mutual coupling (dB) |
-16 |
-17 |
-20 |
-17 |
|
Efficiency |
86 |
91 |
92 |
91 |
|
Diversity Gain |
9.98 |
9.98 |
10 |
9.99 |
|
Gain |
2.6 |
3.5 |
4.3 |
3.4 |
|
Multiplexing Efficiency |
-0.11 |
-0.08 |
-0.05 |
-0.06 |
|
Effective Diversity Gain |
8.6 |
9 |
9.2 |
9.1 |
|
CCL |
0.25 |
0.23 |
0.25 |
0.24 |
|
ECC |
0.003 |
0.003 |
0.001 |
0.002 |
Channel Capacity Loss (CCL)
Channel capacity is one of the important performance index for the MIMO antenna. The channel capacity loss plot is shown in Fig. 9. It is observed that CCL is 0.15, 0.24, 0.23 and 0.18 at 6 GHz, 8 GHz, 10 GHz and 12 GHz respectively.
Closs = -\({\text{log}}_{2}det\left({\varPsi }^{R}\right)\) (5)
𝝭R =\(\left[\begin{array}{cc}{\rho }_{11}& {\rho }_{12}\\ {\rho }_{21}& {\rho }_{22}\end{array}\right]\)
|
Antenna |
Size (mm2) |
Bandwidth (GHz) |
Mutual coupling (dB) |
ECC |
Clearance area (mm2) |
|---|---|---|---|---|---|
|
[10] |
35 × 33 |
3.1-5 |
-22 |
< 0.1 |
35 × 16 |
|
[11] |
37 × 45 |
3.1–5.15 |
-20 |
NA |
30 × 25 |
|
[12] |
60 × 50 |
3–6 |
-20 |
NA |
60 × 40 |
|
[13] |
35 × 40 |
3.1–10.6 |
-16 |
NA |
35 × 27.25 |
|
[14] |
26 × 40 |
3.1–10.6 |
-20 |
NA |
32 × 26 |
|
Proposed |
32 × 20 |
5.4–12.2 |
-25 |
0.001 |
27 × 20 |
The proposed MIMO antenna has a 6.8 GHz bandwidth. 0.038 λ0 is the edge-to-edge distance between the elements. The mutual coupling over the full band is kept below − 15dB by introducing a stub between the elements. The MIMO antenna's performance is further assessed in terms of gain, EDG, ECC, DG, and CCL. The findings of the simulation and the measurements are in good agreement.
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No competing interests reported.