II. DESIGN STRUCTURE

A) Geometry of the MIMO antenna system

The design process of the single element of the proposed antenna is presented in [Reference MoradiKordalivand and Rahman18–Reference MoradiKordalivand, Rahman, Ebrahimi and Hakimi19]. Figure 1 shows the geometric details of the proposed MIMO antenna system and the photographs of the prototype. The antenna is etched on a FR-4 dielectric substrate with relative permittivity ε _r = 4.3, thickness h = 1.6 mm, loss tangent = 0.025, length L _S = 105 mm, and width W _S = 125 mm. The radiated element and the L-corner ground plane are on the top and bottom of the dielectric substrate, respectively, and both are made of copper material with thickness t = 0.035 mm and conductivity σ = 5.96 × 10⁷ s/m. The L-corner ground plane length and width are L _G = 230 mm and W _G = 18.5 mm, respectively. To achieve a 50 Ω output impedance matching with the SubMiniature version A connector (SMA), a transmission line feed with width W _F = 3 mm and length L _F = 20 mm is used.

Fig. 1.

(a) Geometry of top view of the MIMO antenna system. (b) MIMO antenna system integrated with a WAP. (c) Top view of the fabricated prototype. (d) Bottom view of the fabricated prototype.

B) Chain shape parasitic element (CSPE) structure

As mentioned in Section I, there are various methods to increase the isolation between MIMO antenna elements. One of the methods is insertion of a PE to create a secondary current path to minimize the current distribution effects between the antenna elements. A novel PE structure is proposed in order to improve the isolation. In this new structure, two groups of rings are placed on the top and bottom of the substrate, as shown in Figs 2(a) and 2(b), respectively. Each group consists of four rings is connected to each other through formation of a chain. Thus, the proposed PE structure is called CSPE. Since both antenna ports are connected to the common radiator and the L-corner ground plane, the current is distributed along them. The CSPE is added between the common stepped line and L-corner ground plane to achieve high isolation. Referring to Fig. 2(a), two of the four rings of the top CSPE, numbered 1 and 2 are placed at the edge of the L-corner ground plane and the other two, numbered 3 and 4 are connected to them. On the other hand, rings numbered 5 and 6 of the bottom CSPE rings are placed exactly at the edge of stepped line and other two rings, numbered 7 and 8 are connected to them, as illustrated in Fig. 2(b). The positions of rings 3 and 4 of the top CPSE are aligned with the position of rings 7 and 8 of the bottom CPSE, respectively. The dimensions of the rings are the same where the inner radius R _in = 5 mm and the outer radius R _out = 7 mm.

Fig. 2.

(a) Configuration of the CSPE on the top. (b) Configuration of the CSPE on the bottom.

Based on the design structure and the locations of the CSPEs, rings 1 and 2 are used to trap the coupling current via L-corner ground plane; while rings 5 and 6 are applied to trap the current coupling via the stepped line. The rings 3–4 and rings 7–8 are connected to rings 1–2 and rings 5–6, respectively. The position of rings 3–4 and rings 5–6 are physically aligned with each other in order to allow current coupling between them and subsequently create a second current path to flow through between the radiator and the ground plane, via the thin substrate.

The isolation characteristics of the proposed MIMO antenna system can be varied by changing the location of the rings around the stepped line and L-corner ground plane. In order to study the effect of ring locations on the isolation between ports, the parametric studies on the gap (g _p) between ring 1 and L-corner ground plane is shown in Fig. 3. It can be seen that by changing the g _p from 0 to 2 mm, the S ₂₁ parameter is lowered from −12 to −18.5 dB at 2.4 GHz.

Fig. 3.

Parametric studies on the gap between ring 1 and L-corner ground plane.

The isolation characteristics will also be affected by the position of the rings along the L-corner ground plane or the stepped line. Figure 4 illustrates the effect of changing the location of ring 1 along the L-corner ground plane, Rs (see Fig. 1(a)). It is shown that, the S ₂₁ in the range of −14 to −21 dB at 2.4 GHz with different location relative to line 0 (see Fig 1(a)) has been obtained. Rings 2, 5, and 6 experienced the same effect on the isolation performance as ring 1.

Fig. 4.

Parametric studies on location of ring 1 along the L-corner ground plane.

III. RESULTS AND DISCUSSION

In order to validate the simulation results, a prototype has been fabricated and tested by Rohde and Schwarz ZVL network analyzer. Figure 5 shows the comparison of the simulated and measured reflection coefficients and isolation of the proposed MIMO antenna system. Figure 5(a) shows that the individual antenna element has a wideband performance from the range of 1.5–2.95 GHz based on −10 dB reflection coefficients. From Fig. 5(b), the MIMO antenna system with CSPEs based on 15 dB isolation has a wideband performance from 2.3 to 2.9 GHz, which is suitable for WiFi (2.4 GHz) and LTE (2.6 GHz) applications. Voltage standing wave ratio (VSWR) of the proposed MIMO antenna system for both ports 1 and 2 is shown in Fig. 6. It can be seen that VSWR below 1.5 can be achieved in the frequency range of 2.3–2.9 GHz.

Fig. 5.

Simulated and measured S-parameters of the MIMO antenna system. (a) Reflection coefficients. (b) Isolation.

Fig. 6.

VSWR of the proposed MIMO antenna system.

In order to investigate the impact of the CSPEs on the performance of the MIMO antenna system, simulation and measurement results at 2.4 GHz is presented here as a case study. In Fig. 7, simulated surface current distributions at 2.4 GHz at the both ports with and without the CSPEs are depicted. It can be observed that the coupling current between the ports without the CSPEs is strong where a large portion of the surface current flows along the stepped line and L-corner ground plane. After the insertion of the CSPEs, the majority of the coupling current is being trapped by the rings as shown in Fig. 7. As discussed earlier, changing the location of the rings in the CSPE technique will affect the isolation results. An optimization process is performed in order to obtain optimal ring placement at the expected frequency band. By optimally inserting CSPEs between the ports, isolation is substantially decreased so that the behavior of each port is exactly the same as an independent antenna.

Fig. 7.

Current distributions of the MIMO antenna system at 2.4 GHz with and without CSPE. (a) Port 1 excited (b) Port 2 excited.

The comparison of simulated S ₁₂ of the MIMO antenna system with and without CSPE using the CST Microwave Studio is shown in Fig. 8. It can be seen that the simulated S ₁₂ with CSPE dropped between 2 and 15 dB in the expected frequency band. For example, at 2.3 GHz the S ₁₂ is equal to −10 and −25 dB without and with CSPE.

Fig. 8.

Comparison of simulated S ₁₂ with and without CSPE.

The simulated full spherical radiation pattern at 2.4 and 2.6 GHz for ports 1 and 2 are shown in Figs 9 and 10, respectively. Owing to the location of the feeding ports, radiation patterns at both ports are relatively similar in form with a 90° rotation in the x–y plane. This proves the orthogonal polarization state. The slight difference is due to the dimensions of the radiated element and the ground plane.

Fig. 9.

3D radiation patterns of the antenna at 2.4 GHz. (a) port 1. (b) Port 2.

Fig. 10.

3D radiation patterns of the antenna at 2.6 GHz. (a) port 1. (b) Port 2.

Figure 11 shows the comparison of the simulated and measured radiation patterns of the MIMO antenna system at 2.4 GHz. Due to the structure of the ports, i.e. they are in the orthogonal mode, a dual polarized MIMO antenna system is achieved. The E-plane (θ = 0°–180° and φ = 0°) radiation pattern of port 1 is shown in Fig. 11(a), which produces the vertical polarization. Figure 11(b) demonstrates the E-plane (θ = 0°–180° and φ = 90°) of radiation pattern port 2, which produces the horizontal polarization. The electrical field is orthogonal too due to the orthogonal mode of the ports. Therefore, with port 1 excited, the y–z plane is the electric field. With port 2 excited, the x–z plane is the electric field. Therefore, polarization diversity is achieved using the proposed antenna. Figures 11(c) and 11(d) show the H-plane radiation pattern of ports 1 and 2, respectively. The MIMO antenna exhibits a nearly omni-directional pattern within the expected frequency band.

Fig. 11.

Simulated and measured of normalized radiation pattern at 2.4 GHz. (a) E-Plane port 1. (b) E-plane port 2. (c) H-plane port 1. (d) H-plane port 2.

Figure 12 shows the plots of the envelope correlation coefficient (ECC) through the far-field and polarization diversity gain of the MIMO antenna system. According to [Reference Colburn, Rahmat-Samii, Jensen and Pottie20], the value of ECC less than 0.5 is acceptable to achieve satisfactory diversity performance. The envelope correlation can be computed from the S-parameters using the following formula:

(1)$${\rho _e}=\displaystyle{{{{\vert {S_{11}^{\rm \ast } {S_{21}}+S_{12}^{\rm \ast } {S_{22}}} \vert}^2}} \over {\vert {\lpar 1 - \vert {S_{11}}{\vert^2} - {{\vert {{S_{21}}} \vert}^2}\rpar \lpar 1 - \vert {S_{22}}{\vert^2} - {{\vert {{S_{12}}} \vert}^2}\rpar } \vert}}.$$

Fig. 12.

ECC and simulated diversity gain.

The ECC in term of radiation pattern (ρ _e^rp) can be calculated as:

(2)$$\rho _e^{rp}=\displaystyle{{\oint {{A_{ij}}\lpar {\rm \Omega }\rpar d{\rm \Omega }} } \over {\sqrt {\oint {{A_{ii}}\lpar {\rm \Omega }\rpar d{\rm \Omega } \bullet \oint {{A_{jj}}\lpar {\rm \Omega }\rpar d{\rm \Omega }} } } }}\comma \;$$

where ${A_{ij}}\lpar {\rm \Omega }\rpar d \!=\! \Gamma \bullet {E_{\theta i}}\lpar {\rm \Omega }\rpar \bullet E_{\theta j}^\ast \lpar {\rm \Omega }\rpar \bullet {P_\theta }\lpar {\rm \Omega }\rpar +\! {E_{\varphi i}}\lpar {\rm \Omega }\rpar \bullet E_{\varphi j}^\ast \lpar {\rm \Omega }\rpar$$\bullet {P_\varphi }\lpar {\rm \Omega }\rpar$ in which E _θ and E _ϕ are θ and φ components of the complex electric field radiation pattern, respectively, and P _θ and P _ϕ are the θ and φ components of probability distribution function of income wave, respectively. The asterisk denotes complex conjugate. The parameter Γ is the cross-polarization discrimination (XPD) (ratio of vertical to horizontal power density) of the incident field.

The ρ _e^rp for a two-antenna system can be calculated as:

(3)$$\rho _e^{rp}=\displaystyle{{{{\left\vert {\vint_{4\pi } {\lsqb {{{\vec F}_1}\lpar \theta\comma \; \, \varphi \rpar \bullet \vec F_2^\ast \lpar \theta\comma \; \, \varphi \rpar } \rsqb d{\rm \Omega }} } \right\vert }^2}} \over {\vint_{4\pi } {{{\vert {{{\vec F}_1}\lpar \theta\comma \; \, \varphi \rpar } \vert}^2}d{\rm \Omega }\vint_{4\pi } {{{\vert {{{\vec F}_2}\lpar \theta\comma \; \, \varphi \rpar } \vert}^2}d{\rm \Omega}}}}}.$$

For the proposed MIMO antenna system, the ECC values lower than 0.15 and the polarization diversity gains more than 9.98 dB are achieved at the expected frequency bands. These results suggest that the proposed antenna system is suitable for MIMO applications.

Figure 13 shows the measured efficiencies of the proposed MIMO antenna system from the frequency bandwidth of 1.5–3 GHz. It can be observed that, the 60% efficiency bandwidth is 2.1–2.7 GHz. Table 1 lists the measured gains and efficiencies of the microstrip-fed monopole antennas for both ports at 2.4 and 2.6 GHz when one port is excited and the other port is terminated with 50 Ω impedance matching. It is observed that, the gains from 4.35 to 5.1 dBi and efficiencies from 65 to 68% have been achieved. Table 2 lists the comparison between the results of the proposed MIMO antenna system and related works. It can be seen that the proposed antenna provides good performance in term of isolation and gain than other related works.

Fig. 13.

Measured efficiency of the proposed MIMO antenna system.

Table 1.

Measured gains and efficiencies of the microstrip-fed monopole antennas at 2.4 and 2.6 GHz.

Table 2.

Comparison between the results of the proposed design and related works.