MIMO antenna design and simulation

The geometry of the proposed slot-based four-port MIMO antenna is shown in Fig. 1. The antenna is constructed using a low loss of 0.5 mm thin Taconic TLX9 substrate with dielectric constant 2.5 and loss tangent 0.0015. The array consists of two interlocked printed circuit boards (PCBs) where each PCB consists of two slot radiators as shown in Fig. 1(a). The top view and the perspective view of the interlocked MIMO scheme are shown in Figs 1(b) and 1(c), respectively. The basic antenna element in the PCB consists of one-quarter sector corresponding to the annular slot ring antenna.

Fig. 1. The different views of the proposed four-port interlocked MIMO antenna (all the units are in mm).

The evolution of the basic antenna element is described in Fig. 2. The reference antenna is an annular ring antenna which exhibits structural symmetry along the x-axis. The antenna is excited by a microstrip line terminated by a sector-shaped stub line of sector length defined by two arc angles 23° and 40°, respectively. The annular ring is defined by an inner and outer radius of 7 and 11 mm, respectively. This proposed slot antenna is constructed from one bottom quadrant of the annular ring antenna in Fig. 2(a). The bisected slot antenna is shown in Fig. 2(b). The antenna in Fig. 2(b) is replicated through mirror imaging to create the two-port MIMO antenna as illustrated in Fig. 2(c). The antennas are separated by a distance of 2 mm. To have a connected ground configuration, the top metal of both the slot antennas is connected by a simple microstrip line of length 2 mm and width 1 mm. The footprint of the proposed two-element antenna in Fig. 2(c) is 32 × 15 mm². The two-port MIMO antenna is extended to four-port by creating a slit in the substrate and adopting the interlocking scheme as shown in Fig. 1(c). The interlocking scheme results in the generation of vertically polarized signals making the antenna suitable for roof top-loading under the shark fin housing. The optimized size o the proposed antenna is 32 × 15 × 32 mm³. The antenna design and simulations are carried out using CST Microwave Studio. The reflection coefficient characteristics and the mutual coupling characteristics of the proposed four-port MIMO antenna are shown in Fig. 3. During the design, port 1 and port 3 are connected to the antennas sharing the same PCB while port 2 and port 4 are connected to the antennas printed on the orthogonal PCB. Thus, the characteristics calculated from port 1 are identical to port 4 and the characteristics calculated from port 2 are identical to port 3. From the figure, it is evident that the proposed MIMO antenna offers UWB characteristics covering frequencies from 2.8 to 9.5 GHz with a reference voltage standing wave ratio (VSWR) of 2 corresponding to |S ₁₁| ≤ −10 dB. The antenna ports are separated by a large distance and hence the proposed antenna offers inherently high isolation. The calculated isolation is greater than 15 dB throughout the operational bandwidth. Thus, the proposed configuration does not require a separate decoupling arrangement to enhance the isolation characteristics of the MIMO antenna. The arrangement of the antennas is done in such a way that, the Port 1(2) and Port 3(4) offer polarization diversity while all the ports offer different radiation patterns with pattern maxima at different directions resulting in pattern diversity. Thus, the proposed configuration contributes to both polarization and pattern diversity.

Fig. 2. Evolution of the proposed antenna element (i) Reference antenna, (ii) Proposed slot antenna, (iii) Two-port MIMO antenna developed on a single microwave laminate, (a) Front view, (b) Rear view (black: top metal; gray: bottom metal).

Fig. 3. Simulated S-parameter characteristics of the proposed four-port MIMO antenna.

Fabrication & measurement

S-parameter characteristics

The prototype MIMO antenna is fabricated using photolithography and the fabricated prototype is shown in Fig. 4. A total of 50 Ω sub-miniaturized A (SMA) connectors are used to connect the feedlines of the proposed MIMO antenna. The antenna S-parameter measurements are carried out using Keysight's microwave analyzer N9917A. The measured S-parameter characteristics are shown in Fig. 5. The measured S-parameters are in close correlation with the simulation results. The measured reflection coefficient bandwidth starts from 2.95 GHz and extends over 10 GHz. However, the radiation pattern is not stable for frequencies over 10 GHz. Thus, the fundamental operating limit is considered to be 9.5 GHz as calculated in simulation. The measured port-to-port isolation with reference to port 1 to other ports is greater than 15 dB. The difference between the simulated and measured reflection coefficient is attributed to the fabrication tolerance and loss due to SMA soldering. The real-time interlocking has resulted in small air gaps which are also attributed to the deviation.

Fig. 4. Photograph of the fabricated prototype.

Fig. 5. Measured S-parameter characteristics of the proposed four-port MIMO antenna.

Radiation characteristics

The radiation characteristics of the proposed four-port MIMO antenna is shown in Fig. 6 for the two reference frequencies, viz. 6 and 8 GHz. The simulated radiation characteristics provide an average peak gain of the antenna is 5.41 dBi at 8 GHz with an efficiency greater than 73%. The radiation characteristics are measured in an anechoic chamber. A broadband double ridged waveguide horn antenna operating between 1 and 12 GHz is used as a reference antenna to estimate the gain properties of the proposed antenna. The pattern diversity characteristics of the four-port MIMO antenna is evident through the radiation pattern plots showing pattern maxima in different directions. Thus, in the event of deep fading, at least one of the antennas will receive a better-quality signal leading to improved signal-to-noise ratio (SNR) performance. Thus, the proposed antennas increase the link reliability and also enhances the channel capacity. The realized gain of the antenna and the total efficiency of the four-port MIMO antenna is illustrated in Fig. 7. The measured peak gain of the antenna is greater than 5 dBi in all the ports and this peak occurs at 8 GHz. The mean of the gain offered by all the four ports is calculated and presented in Fig. 7. The authors could not measure the efficiency of the antenna due to resource limitation. The simulated efficiency of the antenna ranges between 73 and 86%.

Fig. 6. Measured radiation pattern at 6 GHz (a and b) and 8 GHz (c and d).

Fig. 7. Gain and efficiency characteristics of the proposed four-port MIMO antenna.

Performance evaluation of MIMO metrics

The proposed four-port MIMO antenna is subjected to the evaluation of MIMO performance using some of the performance metrics such as ECC, apparent diversity gain (ADG), effective diversity gain (EDG) and mean effective gain (MEG).

ECC using S-parameters and far-field data

The ECC is a measure of the correlation between the radiation pattern of the individual antenna elements in the MIMO scheme. This parameter signifies the independent operation of the basic antenna elements. The ideal value of ECC is 0 and the acceptable level is 0.5. The more accurate form of ECC calculation is by using the far-field data. However, in places of resource limitations, the S-parameter-based equations can be used to estimate the ECC. The ECC using S-parameters and far-field data is given by equations (1) and (2), respectively. The ECC using measured S-parameters is shown in Fig. 8. From the figure, it is evident that the proposed antenna has a correlation coefficient <0.1 throughout the operating bandwidth. The simulated ECC using far-field data is found to be <0.4. During the calculation of far-field-based ECC, the cross-polarization ratio (XPR) is set be 1 dB relevant to the outdoor scenario. The power distribution is assumed to be Gaussian with mean and standard deviation set to 10° and 20°, respectively.

(1)$${\rm ECC\ } = \displaystyle{{{\vert {S_{ii}^\ast S_{ij} + S_{\,ji}^\ast S_{\,jj}} \vert }^2} \over {( {1-{\vert {S_{ii}} \vert }^2-{\vert {S_{\,ji}} \vert }^2} ) ( {1-{\vert {S_{\,jj}} \vert }^2-{\vert {S_{ij}} \vert }^2} ) }}$$

(2)$$\rho _e = \displaystyle{{\vert \iint {\left({\mathop {\mathop F\nolimits_i }\limits^\to ( \theta , \;\phi ) \bullet \mathop {\mathop F\nolimits_j }\limits^\to ( \theta , \;\phi ) } \right){\rm d}\Omega } \vert ^2} \over {\iint {\vert \mathop {\mathop F\nolimits_i }\limits^\to ( \theta , \;\phi ) \vert ^2{\rm d}\Omega \iint {\vert \mathop {\mathop F\nolimits_j }\limits^\to ( \theta , \;\phi ) \vert ^2{\rm d}\Omega } } }}$$

Fig. 8. ECC using measured S-parameter data.

Diversity gain using S-parameters and far-field data

The diversity gain (DG) is another useful metric in evaluating the MIMO performance of the proposed antenna. The ADG is the measure of the effective increase in the SNR after adopting the MIMO scheme. The theoretical limit for ADG is 10 dB. Another useful form of DG which takes into account the radiation losses from the antenna is the EDG. The ADG and EDG are calculated using (3) and (4) from the ECC values computed above. Under both circumstances, the ADG and EDG should be greater than 7.5 dB for an effective MIMO scheme. Figure 9 illustrates the ADG and EDG of the proposed four-port MIMO antenna using measured S-parameter data. From the figure, it is evident that the ADG is close to 10 dB and the EDG is greater than 7.5 dB throughout the operating bandwidth. The ADG and EDG using far-field data also satisfy the minimum requirement of the MIMO scheme. The ADG and EDG using far-field data are greater than 9 and 7 dB, respectively, under the settings described in the section “ECC using S-parameters and far-field data.”

(3)$$ADG = 10 \times \sqrt {1-\vert ECC\vert ^2} $$

(4)$$EDG = \eta _{total} \times ADG = \eta _{total} \times 10 \times \sqrt {1-\vert ECC\vert ^2} $$

Fig. 9. ADG and EDG using measured S-parameter data.

Mean effective gain

MEG is another useful parameter in the estimation of MIMO performance. Since identical antennas are used to construct the MIMO antenna, the gain offered by individual antenna elements should not deviate by a large number. The ideal theoretical limit for MEG is 0 dB. However, a deviation by ±3 dB is permissible. The MEG is calculated using the gain pattern and power pattern of the antenna along both elevation plane and principal plane as given by equation (5). Table 1 describes the MEG ratio between the adjacent antennas in the MIMO antenna scheme. The calculation is made with reference to port 1. From the table, it is inferred that the deviation is within the acceptable level making the scheme more suitable for MIMO communications.

(5)$$MEG = \int\limits_0^{2\pi } {\int\limits_0^\pi {\left[\matrix{\displaystyle{{XPR} \over {1 + XPR}}G_\theta ( \theta , \;\phi ) P_\theta ( \theta , \;\phi ) \hfill \cr + \displaystyle{1 \over {1 + XPR}}G_\phi ( \theta , \;\phi ) P_\phi ( \theta , \;\phi ) \hfill} \right]} } \sin \theta d\theta d\phi $$

Table 1. MEG ratio calculated with reference to port 1.

On-car performance evaluation

The on-car performance of the proposed MIMO antenna is evaluated using CST Microwave Studio. The radiation pattern of the individual antenna elements is used as a far-field source to evaluate the on-board characteristics. CST's asymptotic solver is used to estimate the on-car performance of the proposed antenna. The asymptotic solver is used to calculate the fields radiated by an antenna in the presence of large electrical structures. The asymptotic solver efficiently computes the scattered fields using 3D ray tracing method. The 3D CAD model of Toyota's Mazda is used for the calculation of the far-field properties. Antennas for automotive applications should satisfy certain important requirements to achieve high performance. For instance, the antenna should be placed high above the ground to avoid ground reflections. The car's roof is an ideal location for loading the antenna in automobiles. In this region, the antenna sees no metallic obstructions except the car roof. Therefore, the proposed MIMO antenna is mounted at the rear end of the roof where the shark fin housing is loaded. The shark fin has a typical dimension of 179 mm × 88 mm × 55 mm which can easily accommodate the proposed four-port antenna whose footprint is 32 mm × 15 mm × 32 mm. The antenna is positioned 1 cm above the car's roof. The 1 cm gap between the vehicle's roof and the antenna base corresponds to the base of the shark-fin housing [Reference Chakam, Schürmeier, Butscher and Mierke19]. In the simulation, the effect of housing neglected and hence the surrounding environment is assumed to be free space. Figure 10 illustrates the simulated on-car performance of the proposed MIMO antenna at 6 GHz. Furthermore, Figs 10(a)–10(d) correspond to the radiation pattern of antenna elements 1–4, respectively. From the figure, it can be inferred that the radiation patterns are unique for each antenna element. The nulls present in some antenna elements are compensated by maximum gain offered by other antenna elements. To understand the overall radiation performance, the combined radiation pattern is plotted in Fig. 10(e). From Fig. 10(e), it can be seen that the proposed MIMO antenna provides a maximum gain in all directions. Furthermore, the directivity of the proposed MIMO antenna shows at least 3 dB increment since the roof is assumed to be a conductor which diffracts the backward radiation. This phenomenon has been extensively studied in [Reference Zhao, Yeo and Ong1, Reference Naga Sai, Suneel Kumar, Sunil Kumar and Kandimalla20]. The diffraction effect increases the overall realized gain of the proposed antenna. Since the antenna is loaded in the car's roof, the other parts of the vehicle body have negligible influence on the antenna's radiation characteristics.

Fig. 10. Simulated on car performance of the proposed MIMO antenna at 6 GHz calculated at (a) Port 1, (b) Port 2, (c) Port 3, (d) Port 4, (e) Combined radiation pattern.

Thus, the operation of the antenna in the presence of the scattering environment is validated making the antenna suitable for real-time application. Table 2 compares the performance of the proposed antenna with the state-of-the-art reported in the literature. The proposed antenna is a 3D solution with vertical polarization unlike many other antennas [Reference Zhao, Yeo and Ong1–Reference Wei and Feng15] reported in the literature. The antenna is constructed using common ground configuration unlike [Reference Sipal, Abegaonkar and Koul7–Reference Raheja, Kanaujia and Kumar9, Reference Premalatha and Sheela17, Reference Reddy, Kamma, Kharche, Mukherjee and Mishra18]. Most of the MIMO antennas [Reference Liu, Lu, Zhang and Gong12–Reference Wei and Feng15] are reported for smartphone applications. However, the proposed antenna is more suitable for the vehicular environment. The 3D antenna in [Reference Srivastava, Kanaujia, Dwari, Kumar and Khan16] occupies more space than the proposed antenna. Though the antenna in [Reference Bactavatchalame and Rajakani21] is smaller than the proposed antenna, the bandwidth is limited to 3.75 GHz.

Table 2. Performance comparison with other four-port MIMO antennas.