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TABLE 4. Structural analysis of proposed MIMO antenna with existing state-of-art. Fig. 26 illustrates the antenna over varied bit rates. It indicates that at 20 Gbps data rate is sup However, for a reduced ink margin for the pro ported only up to 5 mete data rate of 2 Gbps, tl posed MIMO LM of 20 dB, rs of distance. he distance is increased to 10 m. Further, a 1 Gbps data rate is supported to a maximum of 15 m. On the other side, with a reduced data rate to 100 Mbps, the proposed antenna su distance of 40 m. pports up to a Fig. 26 illustrates the link margin for the proposed MIMO

Table 4 Structural analysis of proposed MIMO antenna with existing state-of-art. Fig. 26 illustrates the antenna over varied bit rates. It indicates that at 20 Gbps data rate is sup However, for a reduced ink margin for the pro ported only up to 5 mete data rate of 2 Gbps, tl posed MIMO LM of 20 dB, rs of distance. he distance is increased to 10 m. Further, a 1 Gbps data rate is supported to a maximum of 15 m. On the other side, with a reduced data rate to 100 Mbps, the proposed antenna su distance of 40 m. pports up to a Fig. 26 illustrates the link margin for the proposed MIMO

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FIGURE 1. Development stages of antenna. (a) Stage 1, (b) stage 2, (c) stage 3, and (d) stage 4.
FIGURE 1. Development stages of antenna. (a) Stage 1, (b) stage 2, (c) stage 3, and (d) stage 4.
FIGURE 2. Modal Significance (MS) graph at f 9 for (a) stage 1, (b) stage 2, (c) stage 3, and (d) stage 4 antenna. through which it estimates the performance and radia- tion pattern [28], [29], [30]. Therefore, the above stages in this article progressively evolved through the characteris- tics mode theory (CMA) analysis. The CMA analyzes the structure surface current (J), and represent in terms of the eigenvalue (A,,) and eigen-current (J,,), as in [27] and [31]. The solution to the eigenvalue leads to the modal signif- icance (MS) and characteristic angle (CA). The MS = 1 defines the significant modes contributing to the generation of resonance in the structure. However, few modes of the structures often do not completely radiate the energy coupled. These modes store energy in electric or magnetic fields, com- promising antenna performance. Such modes can be clearly studied by eigenvalue (A,,) and CA. When the A, = 0 and the phase of the mode CA = 180°, it is an efficient mode [32]. It means it radiates most of the energy coupled to it. When dn < 0 and 1809 < CA < 270°, the mode stores electric field, which acts as a capacitive mode. When i, > O and 90° < CA < 180°, the mode stores the magnetic field, and behaves as an inductive mode [32]. The inductive or capac- itive modes must be suppressed or improved by perturbing the structures current flow. It is achieved by modifying the physical structure of the antenna. The surface current of respective modes generates unique radiation patterns. There- fore, the CMA aids antenna development and obtains the desired radiation pattern.
FIGURE 2. Modal Significance (MS) graph at f 9 for (a) stage 1, (b) stage 2, (c) stage 3, and (d) stage 4 antenna. through which it estimates the performance and radia- tion pattern [28], [29], [30]. Therefore, the above stages in this article progressively evolved through the characteris- tics mode theory (CMA) analysis. The CMA analyzes the structure surface current (J), and represent in terms of the eigenvalue (A,,) and eigen-current (J,,), as in [27] and [31]. The solution to the eigenvalue leads to the modal signif- icance (MS) and characteristic angle (CA). The MS = 1 defines the significant modes contributing to the generation of resonance in the structure. However, few modes of the structures often do not completely radiate the energy coupled. These modes store energy in electric or magnetic fields, com- promising antenna performance. Such modes can be clearly studied by eigenvalue (A,,) and CA. When the A, = 0 and the phase of the mode CA = 180°, it is an efficient mode [32]. It means it radiates most of the energy coupled to it. When dn < 0 and 1809 < CA < 270°, the mode stores electric field, which acts as a capacitive mode. When i, > O and 90° < CA < 180°, the mode stores the magnetic field, and behaves as an inductive mode [32]. The inductive or capac- itive modes must be suppressed or improved by perturbing the structures current flow. It is achieved by modifying the physical structure of the antenna. The surface current of respective modes generates unique radiation patterns. There- fore, the CMA aids antenna development and obtains the desired radiation pattern.
frequencies, these modes become capacitive and inductive. Modes 3 to 5 are slightly capacitive over 25-29.5 GHz. It is to be noted that the capacitive modes also contribute to the resonance [27].
frequencies, these modes become capacitive and inductive. Modes 3 to 5 are slightly capacitive over 25-29.5 GHz. It is to be noted that the capacitive modes also contribute to the resonance [27].
FIGURE 4. Characteristic Angle (CA) graph at f9 for (a) stage 1, (b) stage 2, (c) stage 3, and (d) stage 4 antenna. FIGURE 3. Eigenvalue (A,,) graph at fg for (a) stage 1, (b) stage 2, (c) stage 3, and (d) stage 4 antenna. The modal surface current (J;,) can envisage the radiation pattern of modes at respective stages. In this stage, the modal surface current (J;,) in Fig.5 at Mode 1 has an in-phase current with equal magnitude along the vertical edges of the ground plane (black arrow). The horizontal edges have an anti-phase current. Therefore, it resulted in bidirectional radiation at ¢ = 0 and has a quad-beam at g = 90° [34]. Likewise, Mode 2 has in-phase and anti-phase currents along vertical and horizontal edges of the ground, resulting in quad-beam and bidirectional radiation at g = 0 and g = 90°.
FIGURE 4. Characteristic Angle (CA) graph at f9 for (a) stage 1, (b) stage 2, (c) stage 3, and (d) stage 4 antenna. FIGURE 3. Eigenvalue (A,,) graph at fg for (a) stage 1, (b) stage 2, (c) stage 3, and (d) stage 4 antenna. The modal surface current (J;,) can envisage the radiation pattern of modes at respective stages. In this stage, the modal surface current (J;,) in Fig.5 at Mode 1 has an in-phase current with equal magnitude along the vertical edges of the ground plane (black arrow). The horizontal edges have an anti-phase current. Therefore, it resulted in bidirectional radiation at ¢ = 0 and has a quad-beam at g = 90° [34]. Likewise, Mode 2 has in-phase and anti-phase currents along vertical and horizontal edges of the ground, resulting in quad-beam and bidirectional radiation at g = 0 and g = 90°.
FIGURE 5. Modal significance current ((J,,) and its resultant radiation pattern at respective modes for stage 1 antenna. However, Modes 4 and 5 have in-phase current in the radiator (red arrow), resulting in bidirectional radiation at g = 0 and y = 90°. Therefore, it can be concluded that the combination of in-phase and anti-phase currents generates bidirectional radiation with quad-beam, whereas only in-phase current generates bidirectional radiation.
FIGURE 5. Modal significance current ((J,,) and its resultant radiation pattern at respective modes for stage 1 antenna. However, Modes 4 and 5 have in-phase current in the radiator (red arrow), resulting in bidirectional radiation at g = 0 and y = 90°. Therefore, it can be concluded that the combination of in-phase and anti-phase currents generates bidirectional radiation with quad-beam, whereas only in-phase current generates bidirectional radiation.
FIGURE 6. Modal significance current ((J,,) and its resultant radiation pattern at respective modes for stage 2 antenna. An addition of a circular ring inside the outer ring in stage 2 perturbed the surface current J; at all modes, as indicated in Fig. 6. In this case, all modes appear to be significant in Fig. 2(b) at fo. Also, the convergence of the eigenvalue to zero in Fig. 3(b) indicates the same. However, the CA in Fig. 4(b) reveals Modes 1 to 3 have a bandwidth from 26-29 GHz, whereas Modes 4 and 5 have a wide bandwidth from 24-36 GHz, but these have minor capacitive effects. The antenna in stage 3 (Fig. 1(c)) incorporates the plus-shape stub on the radiator circle, which increases the current oscillation (red arrow), as shown in Fig. 7. At this stage, as well, all modes continue to be significant, as depicted in Fig. 2(c). The eigenvalue of these modes in Fig. 3(c) converges to zero at fo but with a narrow bandwidth. The CA indicates Mode 1 and 2 transform from efficient resonant mode to inductive mode beyond fo, in Fig. 4(c). An addition of a circular ring inside the outer ring in stage 2
FIGURE 6. Modal significance current ((J,,) and its resultant radiation pattern at respective modes for stage 2 antenna. An addition of a circular ring inside the outer ring in stage 2 perturbed the surface current J; at all modes, as indicated in Fig. 6. In this case, all modes appear to be significant in Fig. 2(b) at fo. Also, the convergence of the eigenvalue to zero in Fig. 3(b) indicates the same. However, the CA in Fig. 4(b) reveals Modes 1 to 3 have a bandwidth from 26-29 GHz, whereas Modes 4 and 5 have a wide bandwidth from 24-36 GHz, but these have minor capacitive effects. The antenna in stage 3 (Fig. 1(c)) incorporates the plus-shape stub on the radiator circle, which increases the current oscillation (red arrow), as shown in Fig. 7. At this stage, as well, all modes continue to be significant, as depicted in Fig. 2(c). The eigenvalue of these modes in Fig. 3(c) converges to zero at fo but with a narrow bandwidth. The CA indicates Mode 1 and 2 transform from efficient resonant mode to inductive mode beyond fo, in Fig. 4(c). An addition of a circular ring inside the outer ring in stage 2
FIGURE 7. Modal significance current ((J,,) and its resultant radiation pattern at respective modes for stage 3 antenna. indicates that Modes 3 and 5 are capacitive and converge close to the 180° phase angle. The surface current (Fig. 8) at Mode | has an in-phase current in the ground plane (black arrow) and an anti-phase current in the radiator (red arrow), resulting in bidirectional radiation at ¢ = O and hexa-beam atg = 90°. In the case of Modes 2, 3, and 5, the cur- rent in the radiator has an in-phase current. However, in the ground plane, the current is in the horizontal edges with in- phase, but in the opposite direction. Therefore, both result in bidirectional radiation but nulls at different gy. Therefore, the proposed single-element antenna has a single dominant resonant mode (Mode 2) and two minorly contributing modes (Modes 3 and 5) operating over 27-30 GHz.
FIGURE 7. Modal significance current ((J,,) and its resultant radiation pattern at respective modes for stage 3 antenna. indicates that Modes 3 and 5 are capacitive and converge close to the 180° phase angle. The surface current (Fig. 8) at Mode | has an in-phase current in the ground plane (black arrow) and an anti-phase current in the radiator (red arrow), resulting in bidirectional radiation at ¢ = O and hexa-beam atg = 90°. In the case of Modes 2, 3, and 5, the cur- rent in the radiator has an in-phase current. However, in the ground plane, the current is in the horizontal edges with in- phase, but in the opposite direction. Therefore, both result in bidirectional radiation but nulls at different gy. Therefore, the proposed single-element antenna has a single dominant resonant mode (Mode 2) and two minorly contributing modes (Modes 3 and 5) operating over 27-30 GHz.
FIGURE 8. Modal significance current ((J,,) and its resultant radiation pattern at respective modes for stage 4 antenna. Figs. 9(a & b) shows the final dimensions of the proposed single-element antenna. The dimension variables are indi- cated in mm, which are as follows: STw = 0.5, Ry = 4.5, Ro = 3.5, R3 = 2.75, R4 = 2, SBw = 20, SB, = 15, Fy = 5.55, Fw = 1.5, GW, = 2, GL; = 3, GW2 = 19, GLn = 3, GW3 = 1, and GL3 = 1.5. The above CMA analysis is performed through surface excitation. However, when the antenna port is excited, the reflection coefficient |S11| for different stages is displayed in Fig. 9(c). Therefore, the stage 4 proposed antenna resonates at fo = 28 GHz, with a wide bandwidth from 24.3-31.7 GHz.
FIGURE 8. Modal significance current ((J,,) and its resultant radiation pattern at respective modes for stage 4 antenna. Figs. 9(a & b) shows the final dimensions of the proposed single-element antenna. The dimension variables are indi- cated in mm, which are as follows: STw = 0.5, Ry = 4.5, Ro = 3.5, R3 = 2.75, R4 = 2, SBw = 20, SB, = 15, Fy = 5.55, Fw = 1.5, GW, = 2, GL; = 3, GW2 = 19, GLn = 3, GW3 = 1, and GL3 = 1.5. The above CMA analysis is performed through surface excitation. However, when the antenna port is excited, the reflection coefficient |S11| for different stages is displayed in Fig. 9(c). Therefore, the stage 4 proposed antenna resonates at fo = 28 GHz, with a wide bandwidth from 24.3-31.7 GHz.
FIGURE 9. Proposed single-element antenna. (a) Top view, (b) bottom view, and (c) reflection coefficient |S11| curves after excitation from respective stages.
FIGURE 9. Proposed single-element antenna. (a) Top view, (b) bottom view, and (c) reflection coefficient |S11| curves after excitation from respective stages.
TABLE 1. RLC values of a single-element equivalent circuit.
TABLE 1. RLC values of a single-element equivalent circuit.
FIGURE 10. RLC equivalent circuit model of single-element antenna. transform to deliver the entire input power from port | to the load resistance RL.
FIGURE 10. RLC equivalent circuit model of single-element antenna. transform to deliver the entire input power from port | to the load resistance RL.
FIGURE 11. |S11| response of simulated antenna and its equivalent RLC circuit. Note: Resistance R is in ohms, Inductance L is in nH, and capacitance C is in pF.
FIGURE 11. |S11| response of simulated antenna and its equivalent RLC circuit. Note: Resistance R is in ohms, Inductance L is in nH, and capacitance C is in pF.
TABLE 2. Ground plane dimensions of four-element MIMO antenna (in mm). C4 represents the gap in the middle. L18 and L17, and the gap by C7 represents the lines N5 and N6. L16, L15, and L12 imitate the line N2, the notches by capacitance C6 and C5. L13 and L14 represent the open circuit on N2. For the analysis purpose, L1, L2, L3, L4, L5, Cl, and C2 are tuned to match the |S21| simulation curve, as depicted in Fig. 15(b). Likewise, to obtain the |S31| curve, L1, L3, L4, L6, L7, L8, L9, L18, L16, L13, L12, Cl, C3, C4, C7 and C6 need to be tuned. The final tuned values of LC are mentioned in Table 3.
TABLE 2. Ground plane dimensions of four-element MIMO antenna (in mm). C4 represents the gap in the middle. L18 and L17, and the gap by C7 represents the lines N5 and N6. L16, L15, and L12 imitate the line N2, the notches by capacitance C6 and C5. L13 and L14 represent the open circuit on N2. For the analysis purpose, L1, L2, L3, L4, L5, Cl, and C2 are tuned to match the |S21| simulation curve, as depicted in Fig. 15(b). Likewise, to obtain the |S31| curve, L1, L3, L4, L6, L7, L8, L9, L18, L16, L13, L12, Cl, C3, C4, C7 and C6 need to be tuned. The final tuned values of LC are mentioned in Table 3.
FIGURE 13. Influence of surface wave current on adjacent and opposite elements. (a) For initial DCS structure. (b) For proposed DCS structure.
FIGURE 13. Influence of surface wave current on adjacent and opposite elements. (a) For initial DCS structure. (b) For proposed DCS structure.
Note: Inductance L is in nH, and capacitance C is in pF. TABLE 3. LC values of DCS equivalent circuit. a triple beam. The reason for such a pattern is justified in the CMA analysis. The proposed design has Mode 2 as an efficient resonant mode supported partially by Modes 3 and 5. The surface current in these modes has an in-phase current in
Note: Inductance L is in nH, and capacitance C is in pF. TABLE 3. LC values of DCS equivalent circuit. a triple beam. The reason for such a pattern is justified in the CMA analysis. The proposed design has Mode 2 as an efficient resonant mode supported partially by Modes 3 and 5. The surface current in these modes has an in-phase current in
FIGURE 14. Influence of surface wave current on adjacent and opposite elements. (a) For initial DCS structure. (b) For proposed DCS structure.
FIGURE 14. Influence of surface wave current on adjacent and opposite elements. (a) For initial DCS structure. (b) For proposed DCS structure.
FIGURE 15. (a) LC equivalent circuit of proposed DCS structure. (b) |S21| curve generated from equivalent circuit comparable with a simulated response.
FIGURE 15. (a) LC equivalent circuit of proposed DCS structure. (b) |S21| curve generated from equivalent circuit comparable with a simulated response.
FIGURE 16. Prototype fabricated MIMO antenna and its measurement setup. (a) Top view of fabricated antenna. (b) Bottom view of fabricated antenna. (c) |Sii| and |Sij| measurement setup.
FIGURE 16. Prototype fabricated MIMO antenna and its measurement setup. (a) Top view of fabricated antenna. (b) Bottom view of fabricated antenna. (c) |Sii| and |Sij| measurement setup.
FIGURE 17. Simulated and measured results of proposed MIMO antenna. (a) |S-parameter|, (b) gain plot.
FIGURE 17. Simulated and measured results of proposed MIMO antenna. (a) |S-parameter|, (b) gain plot.
FIGURE 18. Normalized simulated and measured radiation pattern at three different frequencies in the band of interest. (a & b) XZ & YZ plane at 26 GHz, (c & d) XZ & YZ plane at 28 GHz, (e & f) XZ & YZ plane at 30 GHz.
FIGURE 18. Normalized simulated and measured radiation pattern at three different frequencies in the band of interest. (a & b) XZ & YZ plane at 26 GHz, (c & d) XZ & YZ plane at 28 GHz, (e & f) XZ & YZ plane at 30 GHz.
FIGURE 20. Extension of four-element MIMO to six-element MIMO antenna. (a) Top view, (b) bottom view. as compared to the proposed four element MIMO antenna. The antenna element of ports 1, 3, 5, and 6 has an open circuit on one end and DCS at the other end of the ground plane.
FIGURE 20. Extension of four-element MIMO to six-element MIMO antenna. (a) Top view, (b) bottom view. as compared to the proposed four element MIMO antenna. The antenna element of ports 1, 3, 5, and 6 has an open circuit on one end and DCS at the other end of the ground plane.
FIGURE 22. Surface wave distribution of six-element MIMO antenna. (a) When port 1 is excited. (b) When port 2 is excited. FIGURE 21. |S-parameter| of six-element MIMO antenna. (a) Reflection coefficient |Sii|, (b) Isolation |Sijj.
FIGURE 22. Surface wave distribution of six-element MIMO antenna. (a) When port 1 is excited. (b) When port 2 is excited. FIGURE 21. |S-parameter| of six-element MIMO antenna. (a) Reflection coefficient |Sii|, (b) Isolation |Sijj.
FIGURE 23. Comparison of SISO and MIMO antenna channel capacity over varied SNR. and nature of the environment [35]. The channel capacity is directly related to a number of transmitting (TX) and receiving (RX) MIMO antenna, as defined in equation (2). Therefore, the channel capacity of the proposed four-element MIMO antenna is compared with Single-Input-Single-Output (SISO), MIMO with 2 TX and 2 RX, MIMO with 3 TX and 3 RX, and proposed MIMO with 4 Tx and 4 RX antenna. The performance of the MIMO antenna increases with the increase in SNR, as displayed in Fig. 23. At an SNR of 20 dB, SISO has a capacity of 5.3 Gbps. In contrast, the proposed MIMO has 10.35 Gbps, double the SISO capacity. For these calculations, the considered bandwidth is 6.8 GHz from the proposed antenna.
FIGURE 23. Comparison of SISO and MIMO antenna channel capacity over varied SNR. and nature of the environment [35]. The channel capacity is directly related to a number of transmitting (TX) and receiving (RX) MIMO antenna, as defined in equation (2). Therefore, the channel capacity of the proposed four-element MIMO antenna is compared with Single-Input-Single-Output (SISO), MIMO with 2 TX and 2 RX, MIMO with 3 TX and 3 RX, and proposed MIMO with 4 Tx and 4 RX antenna. The performance of the MIMO antenna increases with the increase in SNR, as displayed in Fig. 23. At an SNR of 20 dB, SISO has a capacity of 5.3 Gbps. In contrast, the proposed MIMO has 10.35 Gbps, double the SISO capacity. For these calculations, the considered bandwidth is 6.8 GHz from the proposed antenna.
FIGURE 24. Virtual LOS environment demonstrating the path loss and link budget estimation for a communication system.
FIGURE 24. Virtual LOS environment demonstrating the path loss and link budget estimation for a communication system.
TABLE 5. Performance of proposed MIMO antenna with existing state-of-art.
TABLE 5. Performance of proposed MIMO antenna with existing state-of-art.
VIII. CONCLUSION FIGURE 26. Calculated link margin of proposed MIMO over varied bit rates, Note: Ref — Reference, Dim — Dimension, Res-Resonance, BW - Bandwidth, Iso — Isolation, Gain — Peaks gain, GD — Ground, Con — Connected ground, Dis — Disconnected ground, NA — Not available.
VIII. CONCLUSION FIGURE 26. Calculated link margin of proposed MIMO over varied bit rates, Note: Ref — Reference, Dim — Dimension, Res-Resonance, BW - Bandwidth, Iso — Isolation, Gain — Peaks gain, GD — Ground, Con — Connected ground, Dis — Disconnected ground, NA — Not available.
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