SIGW Based MIMO Antenna for Satellite Down-Link Applications

In this paper, a 4-port multi-input multi-output (MIMO) antenna based on substrate integrated gap wave guide (SIGW) is presented for down-link satellite applications. The proposed MIMO antenna consists of four rectangular-shaped radiating slots, which are fed via SIGW to minimize dispersion and insertion loss. Firstly, single antenna element (reference antenna) is designed to have impedance matching and stable radiation pattern over the bandwidth 13.4–13.65 GHz. Afterward, different antenna configurations are studied to combine four identical elements of the reference one for the sake of minimizing mutual coupling between them. In this work, the edge-to-edge separation between the antenna elements for orthogonal and opposite orientations are 9.3mm and 17.3mm, respectively. The mutual coupling between antenna elements is suppressed by etching the upper ground in the region between the antenna elements. Moreover, further optimization is carried out by changing the dimensions of the etched slot to reach higher isolation between radiating slots for better MIMO diversity performance. With this approach, the realized gain is increased by 3-dBi and the maximum mutual coupling achieved at 13.5 GHz between port (1,2) and port (1,3) are −63 dB and −78 dB, respectively with isolation enhancement of 38 dB between port (1,2) and 48 dB between port (1,3). Both simulated and measured results indicate that the proposed antenna has a bandwidth of 13.3–13.8 GHz, with a high isolation greater than 40 dB. Furthermore, the realized antenna gain is around 9-dBi with radiation efficiency of 86%. In addition, the designed MIMO antenna offers excellent radiation characteristics and stable gain over the whole operating band. To explore the diversity of the MIMO system the envelope correlation coefficient (ECC), multiplexing efficiency (ME), diversity gain (DG), channel capacity loss (CCL) and total active reflection coefficient (TARC) are studied. Finally, good consistency between simulation and measured outcomes is obtained confirming the validity of the MIMO antenna for real life satellite application systems.


I. INTRODUCTION
With the rapid evolution in the modern wireless communication systems, the great demand for high data rate, high channel capacity and reliability is dramatically increasing for satellite technology, earth monitoring, mobile communication and radar. Single-band antennas with different adaptable polarizations has attracted attention for such applications as it can improve the data resolution, transmission rate and dramatically reduce signal interference [1]. To maximize dominated by a strong line of sight (LOS) path which may lead to lack in the spatial MIMO channel matrix. Subsequently, the MIMO may not appear as a candidate for SAT-COM to provide spatial diversity. However, SATCOM can benefit from MIMO using orthogonal polarizations, multiple satellites, multiple ground terminals and multiple user concepts even in absence of scattering in SATCOM propagation path. In addition, high MIMO gains in strong LOS channels can be achieved with particular antenna geometries scenarios [5], [6].
Satellite applications in Ku-band such as telecommunication broadcasting, direct-to-home services, tracking data relay and global position system (GPS) can benefit from the outstanding features of MIMO antenna systems [7], [8].
To enhance the performance of MIMO based system, low correlation between antenna elements is needed. This can be achieved by increasing the isolation between the antenna ports. Lower isolation may lead to degradation in the radiation pattern of each element of the multi-element system. As a result, it may lower antenna efficiencies and increase correlation coefficients [9]. Thus, the primary aim of MIMO antenna design is to reduce correlation between the received signals.
Various techniques have been used to increase isolation between MIMO antenna elements including neutralization technique [10], orthogonal feeding or elements, which results in considerable polarization and pattern diversity [11], [12], simultaneous matching [13] and etching slits in the middle of the ground-plane [14]. In addition, electromagnetic band gap (EBG) substrates, defected ground structure (DGS), metamaterials, metasurfaces, split ring resonator and parasitic resonators have been presented to reduce coupling between antenna elements [15]- [20]. These techniques require considerable circuit board space. In another approach the isolating slot is etched between the antenna elements [21]. Various slot configurations have been explored including a vertical slot [22], a T-shaped slot [23], F-shaped stubs [24] and L-shaped slot [25]. Although these isolating slots minimize the mutual coupling between the radiating antennas, they do not improve the system gain.
Numerous antenna design methods have been proposed in the Ku-band to reduce mutual coupling between MIMO antenna elements as well as gain enhancement and reduction in the overall size of the antenna [26]- [32]. In [26], meta-surface isolator was used as a decoupling structure for 2-element patch MIMO antenna to offer an average of 10 dB of mutual coupling suppression with average gain enhancement of 2 dBi in the Ku-band. Also, linear slots were placed between radiating oval shaped slots of 4-element MIMO antenna to reduce mutual coupling of about 20 dB in Ku-band with gain improvement of 2 dB in [27]. Moreover, two port textile MIMO antenna with two half ring shaped antenna elements was presented in [28]. The ground plane is defected to achieve a minimum isolation of 15.5 dB in the Ku-band. In [29], a four-element compact wideband MIMO antenna with four T-shaped radiating elements was introduced. The minimum isolation of 15 dB and maximum gain of 5 dBi were achieved by using reduced ground plane and stubs at the bottom ground. Also, in [30], inverted pair of L-shaped stubs placed in the ground of dual patched wideband MIMO antenna are used to obtain better isolation of about 20 dB with peak gain of 5.32 dBi. The main disadvantage of such system is the complexity of design and low realized gain. Moreover, some of the MIMO systems available in the literature have good miniaturization with good isolation characteristics but they have a drawback in terms of poor gain. Thus, this makes them less suitable for wireless applications. To improve system gain, a dumbbell shaped slot is located between two inverted E-shaped monopole elements to decrease mutual coupling in [31]. A high isolation of more than 20 dB is presented with average gain of 8 dBi in the Ku-band. Also, in [32], a 4 × 6 array MIMO antenna is used to achieve high antenna gain but the design was bulky. Although a significant effort in the recent years has been directed to introduce featured MIMO antennas with reliable diversity parameters, there is a still a room of improvement to achieve higher isolation along with a high stable gain along the operating band which remains a challenging task.
In this paper, a four-port MIMO antenna system is designed, fabricated and measured for satellite down link application covering frequency from 13.4-13.65 GHz. It exhibits an isolation over 40 dB with gain around 9 dBi. The design is based on orthogonal orientation of the antenna elements in conjunction with slots etched in the ground plane between the antenna elements. A configuration without any isolation technique is used as a reference of evaluation for the isolation and gain enhancement. The feeding network is substrate integrated gap waveguide (SIGW) based technology. It is used to inhibit the dispersion relative to either microstrip or coplanar traditional feeding networks. The antenna is considered a compact one taking into consideration the topology of MIMO system based on GW technology.

II. FEEDING NETWORK DESIGN AND TRANSITION
In this section, the design of proposed feeding network is presented. It is SIGW of mushroom unit cell which is based on GW technology. The main feature of the GW technology is guiding the signal over ridge surrounded by high impedance textured surface called Artificial Magnetic Conductor (AMC). It creates a stop band forcing all parallel-plate modes to be prohibited [33]. Three rows of AMC structure can be enough to inhibit the leakage and prevent radiation [34]. One of the key elements in the GW configuration is the design of the unit cell that constructs the AMC surface.
As shown in Fig. 1, the unit cell consists of four layers; bottom and top ground layers which are made of lossy copper annealed and two substrate layers which are sandwiched between them. The substrate material for the lower layer is Rogers RO4350B with relative dielectric constant of 3.66 and for the upper layer is Rogers RO4003C of 3.55. The array of periodic metallic vias implanted on the bottom layer and ended with a layer of periodic patches representing the mushroom structure. This unit cell is placed periodically around the ridge forming SIGW transmission line. The dispersion diagram of the mushroom unit cell and the transmission line are carried out using Eigen mode solver (CST microwave studio) and are illustrated in Fig. 1 and Fig. 2, respectively. As it can be seen from Fig. 2, there is a band-gap between 6 GHz to 20 GHz. In this band-gap, the fields will be confined over the ridge with a quasi-TEM field distribution resulting in a dominant mode with minimal dispersion [35]. The dimensions of the unit cell and ridge are demonstrated in table 1.  Transition between SIGW and microstrip is required to excite and measure GW structure. For the sake of feeding the proposed antenna, a microstrip to SIGW transition is designed to operate in the Ku-band. It is a microstrip line structure with characteristics impedance 50 ohm as shown in Fig. 3(a). It is implemented on ROGERS RO4003C with relative dielectric constant 3.55, thickness 0.203mm and loss tangent 0.0027. The width of the microstrip line (ws) is 0.45mm. To connect the transition with ridge, the transition is turned upside down such that the microstrip line touches the upper surface of the ridge. Quarter wavelength transformer with optimization is used to match the transition with feeding network. The s-parameter of the microstrip-SIGW transition is illustrated in Fig. 3(b). The reflection coefficient shows a behavior better than -10 dB over the desired Ku-band. Also, it is clear that the transmission coefficient is close to -1 dB, which is due to the insertion losses from the microstrip transitions input/output lines in the whole operational bandwidth.

III. MIMO ANTENNA DESIGN A. SINGLE-ELEMENT STRUCTURE
The design of MIMO antenna systems usually starts with a single element. The geometrical configuration of the proposed single element antenna structure is illustrated in Fig. 4. It is designed to operate in 13.4-13.65 GHz frequency range for downlink satellite applications. It consists of SIGW structure to feed rectangular shaped slot antenna which is etched in the top ground plane to drive the electromagnetic waves toward the z-direction. The slot antenna has the dimensions of 5.85 mm x 7.37 mm to resonate at 13.5 GHz based on microstrip patch antenna design equations [36]. Two quarter    wavelength transformer sections are used to provide matching between the guiding structure and the antenna radiating slot. An optimization process is performed to obtain a better matching level with reflection coefficient depicted in Fig. 5 where

B. TWO-ELEMENT MIMO STRUCTURE
The two-element structure of the proposed MIMO antenna is discussed in this section. First, the orientation of antenna elements is studied. Fig. 6 illustrates the two configurations which are used in the antenna design. In the first configuration shown in Fig. 6(a), the two elements have 90 • between them (orthogonal orientation) with spacing of 9.3 mm between the edges of two antennas. While elements in the second configuration are placed in opposite orientation (180 • ) with spacing of 17.3 mm between the edges, as illustrated in Fig. 6(b). The simulated scattering parameters of the two antenna configurations are demonstrated in Fig. 7. It is apparently obvious that the antenna with opposite orientation exhibits lower mutual coupling of -29 dB between elements due to the spacing. On the other hand, antenna with orthogonal orientation has mutual coupling lower than -25 dB within the entire  frequency band. Finally, it can be concluded that the two orientations (90 • and 180 • ) have acceptable low mutual coupling which will lead to good MIMO antenna performance.

C. FOUR-ELEMENT MIMO STRUCTURE 1) ANTENNA WITHOUT ISOLATION ENHANCEMENT TECHNIQUE
Based on the aforementioned discussion, the four-element MIMO antenna is arranged as depicted in Fig. 8(a). The two orientations of previous section are combined to form four-element MIMO system. The antenna element at port 1 is oriented orthogonally on antennas at port 2 & 4 and in opposite direction with the antenna at port 3. The proposed four-element MIMO antenna is fabricated as shown 35968 VOLUME 10, 2022 in Fig. 8. Fig. 9 illustrates the simulated and the measured scattering parameters of the proposed antenna which shows good trend between both results. In Fig. 9(a), it is obvious that the antenna operates at frequency band from 13.28 GHz to 13.73 GHz with good reflection coefficients performance lower than -10 dB at all ports which meets the intended satellite application. Furthermore, Fig. 9

2) ANTENNA WITH ISOLATION ENHANCEMENT TECHNIQUE
To get higher isolation between antenna elements, the upper ground plane of the presented antenna is subjected to different modifications. First, the conductor of the upper ground is etched in the middle position between the antenna elements (Antenna A) to suppress the surface currents, as shown in Fig. 10. An optimization process on width 'Wc' is conducted to achieve more isolation with selected values of 'Wc', as demonstrated in Fig. 11. It is clear that 'Wc' of 0.8 mm reduces the mutual coupling (S21) and (S31) by 2 dB and 5 dB compared to antenna with no enhancement technique, respectively. Further modification is accomplished on the etched slots (Antenna B), as shown in Fig. 12. Fig. 13 shows a parametric sweep on length 'Lc' to achieve higher isolation. It is obvious that 'Lc' of 20.7 mm enhances the isolation between port (1,2) and (1,3) by 5 dB and 20 dB, respectively. It is clear that the orthogonal orientation has dominant effect on the isolation. To achieve higher isolation enhancement and gain especially between port (1,2), the slot is segmented into three cascaded linear slots with spacing 'S', as depicted in Fig. 14 (Antenna C). As shown in Fig. 15,  further optimization is held on the lengths of slots 'Xc' and 'Yc' to achieve better isolation. It is clearly obvious that antenna with 'Xc' of length 12.5 mm and 'Yc' of length 3.9 mm shows the best isolation enhancement. The isolation between port (1,2) is better that 40 dB in the whole bandwidth, while the magnitude of S31 is lower than -45 dB. Perhaps this enhancement is due to the suppression of higher order modes conducted from segmented slots. The simulated and measured reflection coefficient of the optimized design is depicted in Fig. 15(c), which shows good agreement. On the other hand, one can notice that the reflection coefficient is nearly the same before and after the enhancement process. Current density distribution over the proposed MIMO antenna with and without isolation enhancement technique are illustrated in Fig. 16. It is observed that surface current is suppressed at the adjacent radiating antennas by introducing the three cascaded linear slots. Thus, this decoupling technique plays a vital role for preventing the current flow to the other antenna which leads to higher isolation. Regarding the antenna gain, Fig. 17 shows that the proposed decoupling technique increases the average realized gain by 3 dBi to reach a gain of 8.8 dBi. Due to effective impedance matching, radiation efficiency reaches around 86% as seen in Fig. 18. Table 3 shows a comparison between s-parameters and gain of MIMO antenna with and without the proposed isolation technique. The radiation properties of the MIMO antenna with and without the proposed isolation technique are plotted in Fig. 19. It is clear that no significant degradation in the radiation pattern has been observed.

IV. DIVERSITY PERFORMANCE OF THE DESIGNED MIMO ANTENNA
A. THEORETICAL BACKGROUND Several important parameters are analyzed to evaluate the diversity performance of the proposed MIMO antenna. The parameters used to serve this purpose are the envelope corre-   lation coefficient (ECC), diversity gain (DG), channel capacity loss (CCL), multiplexing efficiency (ME) and total active reflection coefficient (TARC). The first parameter is the ECC which measures the degree of correlation between different antenna elements of MIMO structure [37]. Lower ECC means lower the degree of dependence between the antenna elements. Thus, higher MIMO diversity performance. The ECC can be calculated from both the far field radiation pattern and scattering parameters as in equations (6) and (7), respectively [29], [37]. where i,j = 1, 2, 3, 4, ρ eij is the ECC between ith and jth antenna elements, while F i (θ , φ) is the 3D radiation pattern field with excitation at port ''i'' and ''•'' denotes the hermitian product and '' '' is the solid angle.
where '' * '' is the complex conjugate of the s-parameter. However, it is important to mention that computing the ECC values from s-parameters is not accurate and underestimates its values when evaluating any lossy antenna. Subsequently, the values obtained from far-field radiation patterns are more practical as they have higher accuracy [38]. On the other hand, this process requires complex and advanced calculations. For good MIMO antenna diversity, ECC should be lower than the acceptable limit 0.5 [2]. Diversity gain (DG) is another important parameter, obtained from ECC, to evaluate the performance of the MIMO antenna system, defined as [2]: It is observed that the relationship between the correlation and diversity gain are inversely proportional, as seen in equation (8). Thus, a larger DG value indicates a better MIMO VOLUME 10, 2022  antenna performance. Since the DG is related to ECC, it can be calculated from both the far field radiation pattern and scattering parameters. The third one is CCL which defines the maximum reachable limit of the information transmission rate [39]. In other words, it measures the optimum information transmission rate [39]. Smaller CCL means the easier signal to transmit with lower losses. To have good diversity performance for MIMO antenna, CCL value should to be smaller than the acceptable value 0.4 bps/Hz [38].  For a four-port MIMO antenna system, it is mathematically expressed in equation (9) [39]. where To have more evaluation on the MIMO antenna system, the ME has been computed. It is defined as the power of the real antenna over the power of the ideal one. It is the power ratio between the real antenna and the ideal one [37]. The maximum multiplexing efficiency has been calculated as [38]: where η i is the total efficiency of the i th antenna elements and ρ eij is the magnitude of complex correlation coefficients between ith and jth antenna elements. The last parameter is TARC which is defined as the square root of the total reflected power divided by the square root of the total incident power [3], [29]. It represents the effective operating bandwidth of MIMO antenna system as it can be calculated from the S-parameters [12]. It is considered an important parameter because it accounts the inter-port coupling, port matching, and the effect of the random phases of incoming signals into each antenna element, therefore describing the performance in a more realistic situation. Under certain conditions, the random phases of incoming signals have an important impact on MIMO array behavior, and TARC is the only MIMO parameter that considers this information. For four-port MIMO antenna, the TARC expression can be calculated as the following equation [39]: where is the excitation phase difference between the two ports. For a MIMO communication system, the TARC value should be <0 dB [3]. Fig. 20 illustrates the ECC metric. It depicts the simulated ECC based on far field equation (6) using (FDTD) (CST Microwave studio), the calculated and the measured ECC based on equation (7). The value of the ECC between port (1,2) at the frequency band is below 10 × 10 −5 , 0.25 × 10 −5 and 10.6 × 10 −5 extracted from simulated, the measured, and calculated results, respectively. While the ECC between port (1,3) at the operating band is below 8.7 × 10 −5 , 0.12 × 10 −5  and 0.17 × 10 −6 , respectively. It is observed that calculated and measured values are well matched. Moreover, the corresponding DG values are illustrated in Fig. 21. It is obvious that the obtained DG is around 10 dB within the operating bandwidth. The calculated and measured CCL are shown in Fig. 22. It shows that the CCL value at the entire operating band is below 0.4 bps/Hz. The simulated, measured and calculated ME are around -1 dB within the band from 13.4 GHz to 13.65 GHz, as illustrated in Fig. 23. Finally, Fig. 24 clearly shows that the value of TARC for the proposed antenna is < −10 dB for the entire operating band. Nevertheless, the measured results may show a little deviations with the simulated ones and this can be due to the fabrication tolerance, soldering process, test connectors, connecting cables and the surrounding test environment.

V. COMPARISON AND DISCUSSION
Comparison of the proposed MIMO antenna system with the recently published MIMO work operating in 13-14 GHz band is carried out in table 4. It summarizes several MIMO antenna parameters in terms of number of ports, size, bandwidth, mutual coupling, realized gain, radiation efficiency and   different MIMO diversity performance parameters. Compared with [3], [7], [26], [28], [30], [31], [40]- [42], the proposed antenna has more ports. In terms of antenna size, the proposed antenna is more compact than [12], [39] which have same number of ports. Although antennas in [2], [29] are more compact, they achieve lower MIMO antenna performance in terms of realized gain, radiation efficiency and isolation between antenna elements. Generally, it is clear that the proposed designed MIMO antenna system performs well in terms of gain which reached 8.8 dBi, mutual coupling lower than -40 dB and good radiation efficiency of 86%. Although its size is not the smallest one, it shows significant high isolation improvement and better MIMO diversity performance compared to other literature work. This permits the proposed antenna to be utilized in real satellite communication applications. Furthermore, it is important to point out that the simplicity of proposed design and its symmetric geometry allow this design to be easily integrated to larger MIMO antennas with more radiation elements.

VI. CONCLUSION
A novel 4-element MIMO antenna system with high isolation fed by SIGW is presented. Designs using SIGW technology have the advantages of low dispersion and insertion loss. Thus, this design can be considered as a promising candidate for down-link satellite applications. The MIMO antenna element is of rectangular-shaped radiating slot to operate in band of 13.4-13.65 GHz. A prototype of the four-element MIMO antenna is fabricated. The measured and simulated results show good agreement in the entire operating band. Isolation enhancement technique of three linear cascaded slots between antenna elements is proposed to enhance isolation of lower than -40 dB. In addition, the realized antenna gain is enhanced by 3-dBi. All the simulated and measured radiation results demonstrate that the proposed antenna offers important characteristics with stable high gain, acceptable radiation efficiency and good radiation patterns over the operating band. The diversity performance of proposed MIMO antenna structure is obtained by calculating different parameters like ECC, DG,,CCL, ME and TARC, which show a good diversity performance of the proposed antenna. A comparison with other reported antenna structures has been presented to highlight the novelty and significance of the proposed work. Therefore, the SIGW based highly isolated four-element MIMO antenna can be considered as an attractive candidate for real satellite applications.