Review on Substrate Integrated Waveguide Cavity Backed Slot Antennas

Substrate Integrated Waveguide Cavity Backed Slot Antennas (SIW CBSA) are emerging candidates for planar Integrated Circuit (IC) technology in Millimetre-wave (MMW) bands. This peculiar antenna holds the advantage of conventional metallic cavity-backed antennas, with high gain, high front-to-back ratio, and low cross-polarization level. SIW CBSA structures are the best option for planar integration with a low profile and low fabrication cost in the design of MMW systems. This paper discusses various forms of SIW CBSA structures, their operating mechanism, and methods of improving their performance in terms of gain, bandwidth, size reduction, beamforming capability, and multiple bands of operation. From this discussion, we can select the antenna with the required characteristics of gain, dual-band, and dual-polarization suited for present and future-generation applications.

elements are embedded in a chipset at a low cost, but antennas, filters, and power amplifiers can't be easily embedded with active components at MMW frequencies. So, the development of a planar platform with reliable technology for such circuits is advantageous in mm-wave wireless systems.
Substrate Integrated Waveguide technology (SIW) is a good choice for designing such a planar platform. For two decades, SIW technology has gained more attention in high-frequency bands to design microwave and MMW planar circuits due to their planar integration capability. The first planar waveguide line was proposed by Shigeki, F. in 1994 [6]. The dielectric substrate is embedded with arraying metal vias on two side walls to build waveguides in planar form is called a post-wall waveguide (laminated waveguide) structure, as shown in Fig.2. These waveguide characteristics are analyzed using a post-wall waveguide structure [7]. The leakage of electromagnetic waves in post-wall waveguides is suppressed in a laminated waveguide structure. Such laminated waveguide structures are restored with the inherent merits over conventional nonplanar waveguide structures, called Substrate Integrated Waveguide structures [8]. The SIW structures enable the planar integration of the three-dimensional structures at MMW frequencies. These SIW devices are manufactured either with the Printed Circuit Board (PCB) technology or Lowtemperature co-fired ceramic (LTCC) technology [6]. They are low-cost, lightweight, and simple to build compared to conventional metallic waveguides [9]. These SIW devices have challenges like dielectric losses and low power handling capability compared to a conventional waveguide. The dielectric substrate embedded in SIW increases the dielectric losses. Deslandes et al. proposed that by increasing the substrate thickness, conductor loss and dielectric losses are minimized, and the corresponding attenuation constant is inversely proportional to substrate thickness. The other geometrical dimensions of SIW have a negligible effect on conductor losses. Depending upon operating frequency, the substrate thickness is chosen to make the attenuation constant a low value. For example, at terahertz frequency, substrate size is limited to 0.254 mm, and at the MMW band, the substrate thickness is chosen maximum up to 0.787 mm [10]. Different types of SIW cavity-backed slot antenna construction details and operating principles proposed at the MMW band and microwave bands were the subject of this review.
This review manuscript is categorized as follows, and section II discusses the fundamental theory and construction details of SIW structures. Section III contains a technical review of several published papers on SIW cavity-backed slot antennas and arrays, large frequency ratio integrated antennas, SIW dual-slot antennas, and reconfigurable antennas. Section IV summarizes and concludes with future scope.

II. DESIGN RULES FOR SIW STRUCTURES
The SIW structure is orchestrated by two rows of vias implanted in a dielectric substrate with top and broad bottom walls that works as conducting plates, as shown in  . Two rows of vias act as narrow walls of the SIW structure. Most of the advantages of the conventional Rectangular Wave Guide (RWG) structure are preserved in the SIW structures, and full-scale integration is an attractive feature of SIW structures at MMW bands [9]. Design Rules of the SIW structures are discussed in [9] and [10]. Boundary Integral Resonant Mode Expansion (BI-RME) method and the Floquet theorem are combined to investigate the dispersion properties of the SIW structures and conclude that the SIW exhibits similar dispersion characteristics as the conventional RWG by assuming that the effective width of SIW is equal to the conventional RWG.
The effective width of SIW can be derived from Eq. (1) as follows [11] w eff = w − d 2 0.95 * s (1) where d is the via diameter, w is the width of the SIW structure, and s is the adjacent pitch distance between metal vias. Referring to (1), it does not consider the effect of d/w, so modified empirical Eq. (2) is defined by the multimode numerical calibration procedure in [12].
The propagation characteristics of SIW were analyzed by the Method of Lines (MoL) as provided in Eq. (3) [13].
where, λ g is the guided wavelength of propagating wave in SIW [14].
where, λ o operating wavelength and λ c The cutoff wavelength of the wave.   integration has been presented for integrability purposes in [9] and [10]. A new construction method is used to design the waveguide with the help of SIW structures, as shown in Fig. 5(a). Here microstrip line and rectangular waveguide are connected via a simple tapering module on the same substrate. This new transition helps design a complex waveguide component in planar form. As shown in Fig. 5 (b) & (c), a coplanar waveguide and a rectangular waveguide are fully integrated on the same substrate to make planar circuits without any additional mechanical tuning to operate at the MMW band as proposed in [16]. Power dividers are also designed in a planar form using the standard PCB process. These planar SIW power dividers have been basic building blocks for designing complex microwave circuits. For the First time, Li-yan et al. experimented with SIW slot array antennas by loading the longitudinal slots on the top conducting layer of SIW [17]. The proposed design is advantageous compared to conventional waveguide slot array antennas in terms of size, weight, and cost, with a slight decrement in gain and efficiency.

III. TECHNICAL REVIEW
Primarily, the slot and cavity dimensions have been calculated for the same resonant frequency with the excitation of TE 101 mode. The cavity dimensions are calculated in Eq. (6) [13].
133506 VOLUME 10, 2022 where ε is permittivity, µ is permeability, W eff and L eff are effective width and length of the SIW cavity, respectively. The SIW cavity width and length (W siw and L siw ) can be approximated as follows in Eq. (7) & Eq. (8) [11]: where d and s parameters are similar to the parameters discussed in SIW design. To minimize the guided power leakage from the sidewalls of the cavity, the geometry of the linear array of vias should follow Eq. (4) and (5), which are discussed in the earlier section. Based on the utilization of SIW structures, several types of antennas are implemented for high-frequency applications. SIW CBSA array classification is listed in Fig. 6. Some of the proposed research is discussed in detail to understand SIW CBSA configurations. Miniaturized SIW slot antennas based on negative order resonance are proposed in [18] and [19]. SIW CBSA's are loaded with metamaterials to achieve a good amount of miniaturization. As shown in Fig.7(a), an interdigital slot is loaded on the top plane of the SIW cavity to operate the resonator as Composite Right Left-Handed (CRLH) SIW resonator [18]. A ramp-shaped slot is loaded on the SIW cavity to perform the resonator presented in Fig.7(b) in the first negative order mode resonance [19]. The proposed antenna in [19] achieves 87% miniaturization with a gain of 3.6 dBi at 2.1 GHz. From [18] and [19], we can conclude that even the antenna resonates in lower frequencies instead of increasing the size, slots are loaded with capacitive loading to operate in the negative order mode with the same size. Due to the loading effect, antenna size remains unchanged, and miniaturization is achieved.
Earlier, a fragile cavity-backed microstrip-fed slot antenna is investigated in [20]. This work provides a path to design cavity-backed slot antennas in SIW structures. A thin cavitybacked crossed slots antenna is proposed to operate at 2.34 GHz for dual broadcast systems such as satellite radio and terrestrial communication [21]. A novel design method of low-profile SIW CBSA is described in [22]. A grounded  [18], (b) An ultra-Miniature SIW CBSA [19], (c) a very thin cavity SIW CBSA structure [22]. coplanar waveguide (GCPW) feed line excites the TE 120 mode in the cavity resonator.
The slot length is chosen in such a way that the E-field across the sides of the slot has a phase reversal, and energy can radiate into space. Here 19.8×23.8×0.5mm 3 size, CBSA is presented as shown in Fig. 7(c). It has achieved a gain VOLUME 10, 2022 of 5.4 dBi, Front to Back Ratio (FTBR) of 16.1 dB, and 1.7% bandwidth at 10 GHz frequency. From this article [22], we can observe that the impedance bandwidth is affected by substrate thickness and slot width. A half-wavelength dimension slot is considered for better radiation efficiency at the resonant frequency. The radiation patterns of the crossed slot antenna vary from Left Hand Circular Polarization (LHCP) in the elevational direction to Vertical Polarization (VP) at the horizon. Here crossed-slot aperture utilizes the dual resonance technique for circular polarization. By utilizing the crossed slots, in the far-field region, radiation from the two orthogonal slots with 90 0 phase differences produces Circular Polarization (CP) towards the elevational direction. The slots and cavity are tuned together to get the radiating modes. The cavity thickness and slot width dimensions are optimized to improve the operational bandwidth. Here crossed slots loaded cavity antenna supports four modes. The two modes are related to crossed slots, and two additional modes are related to interference. SIW CBSA with different slot configuration characteristics is compared in Table 1. We discussed low-profile CBSA and miniaturized antennas earlier. Apart from that, bandwidth enhancement of low-profile CBSA is proposed in [23]. In the proposed design combination of TE 110 and TE 120 modes is excited in the SIW-backed cavity, as shown in Fig. 8. Wide bandwidth is achieved by utilizing the two-hybrid modes. Impedance bandwidth is enhanced from 1.4% to 6.3%, and its gain is slightly improved. There are several techniques, like increasing the substrate thickness and utilizing dual modes in the SIW cavity to improve the bandwidth, But these antennas usually suffer from a lower value of gain. The cavity-backed patch antenna grabbed particular interest to enhance the bandwidth [24], [25].
A wideband SIW cavity-backed E-shaped Patch Antenna is shown in Fig. 9(a) [24]. This structure consists of three metallization layers and two substrates. The top metal layer is loaded with E-shaped patches, a feedline line with vias and tunable loads working as a feeding structure in the middle layer, and the bottom layer behaves as a ground plane. Here E shaped patches are excited with feeding vias connected to the strip line, as shown in Fig. 9 (a). From Fig. 9 (b), we can observe that a lower resonant frequency (f L ) can't be created with E shaped patch antenna because the probe inductance is very low and the patch and ground exhibit a large capacitance value.
To enhance the inductance value at a lower resonant frequency, tunable stubs are connected across feeding via in middle layer. By properly selecting the length and width of stub parameters lower resonant mode is excited. Without loaded stubs, E shaped patch antenna resonates at f R and which is slightly lower than the higher resonant frequency (f H ). After loading the stub parameters f L and f H are created. The proposed E-shaped patch single elements design achieves a bandwidth of 10.9%, and a gain of 7.7 dBi, while a 2×2 array achieves a bandwidth of 10.5%, and a gain of 13.2 dBi for X band applications. For modern wireless communication, dual-.polarized, high gain, and broadband antennas are desirable to improve the link quality, and communication capacity [5]. Different types of SIW-fed array configurations are presented in [26], [27], [28], and [29] to improve bandwidth and gain. However, Substrate loss is dominant at the MMW band when designing a broadband, dual-polarised high-gain antenna. In [26], a SIW fed patch antenna array in a multilayer structure with high gain and bandwidth for the 60 GHz band. The proposed antenna is suitable for future mm-wave wireless communication. A 4×4 antenna array is designed to achieve a 27.5% and 22.6% bandwidth and 19.6 dBi gain. In [27], they have adopted a dual-polarized Magneto Electric (ME) dipole array with air-filled cavities and metallic pillars to minimize substrate loss. By adopting the ME dipoles in the SIW feed array, broadband and high gain in a unidirectional pattern with a lower cross-polarisation value are achieved.
The proposed 8 × 8 dual-polarized antenna array 3-D view is shown in Fig.10 (a). It consists of four layers, ME dipole array in the top layer, 1 to 4 power divider in the second layer, port 1 in the third layer, and port 2 in the fourth layer. Measurement and simulated results are compared in Fig.10 (b). Semicircle cut edges are introduced on the conventional cross-shaped slots to improve the bandwidth of the antenna array [28]. In this article, a dual-layer SIW feed CBSA array (8 × 8) is constructed to obtain a gain of 22.3 dB for 60 GHz applications.
An aperture-coupled vertical network couples the power to the second metal layer. A slot is etched on the bottom metal layer for H-polarization (H-pol). A cross-shaped slot is inscribed simultaneously on the middle layer for Vertical/Horizontal-Polarized. Semicircle cut on the slot edges introduces new resonances in the higher band. By increasing the diameter of the semicircles, these multiple resonances at high frequencies are merged, and board bandwidth is achieved. The fabricated prototype of the 8×8 semicircular antenna array is presented in Fig.11(a), and the return loss characteristics of H-pol and V-pol antennas are presented in Fig. 11(b). An array of antenna elements 2 × 2, 4×4, and 8 × 8 configurations are analyzed in [29] for covering the 50-74 GHz band applications. The proposed antenna element consists of slots surrounded by two dipoles  in parallel for better performance. The measurement values of fabricated antenna prototypes discussed in [26], [27], [28], and [29] are presented in Table 2.
Discussion: We have discussed low-profile SIW CBSA, and miniaturized antennas. This low-profile antenna BW and gain are improved by increasing the substrate layers by utilizing the dual resonant modes in the cavity. These planar antennas with high integration capability can be used practically. But practically high gain dual polarized dualband with a large frequency ratio or small frequency ratio is required. In the future for 5G and beyond applications System In Package (SIP) chipsets are required.

B. DUAL-BAND ANTENNAS
If we look into satellite communication applications, an antenna operating at a dual-band with a small frequency ratio is required. In [30], based on the advantage of aperture VOLUME 10, 2022 FIGURE 11. (a) An 8×8 dual-polarized semicircular cross-shaped slot antenna array, (b) S parameters of the proposed antenna array [28].
sharing a dual-band shared aperture antenna array for MMW bands(K-/Ka-bands) is proposed for low earth orbit (LEO) satellite communication. In another method, SIW CBSA is reported as operating in the dual-band by exciting two different modes in design [31], [32], [33], [34]. A dual-band operating frequency is obtained by simultaneously exciting the two modes in the triangular ring slot (TRS) [31]. This resonant operation can be analyzed with the help of field distributions in the TRS as shown in Fig.12(a). At lower frequencies TRS radiates and at higher frequencies patch inside the TRS is radiated. The fabricated SIW cavity-backed TRS antenna is presented in Fig.12(a) and return loss characteristics are presented in Fig.12(b).
In [32], a novel dumbbell-shaped slot is introduced to create additional hybrid modes without disturbing the conventional TE 120 mode. The frequency ratio of two bands can be tuned to certain values by changing the dimension of the slot. Design details and return loss characteristics are presented in Fig.13.
In [33], A Dual-band Cavity-backed SIW array antenna with a small size is presented. This dual-band is achieved by exciting higher-order hybrid mode (TM 310 +TM 130 ) and second higher-order mode (TM 320 ) into the SIW cavity by an inductive window. The proposed 2 × 2 elements design, as shown in Fig.14 (a) and (b), achieves a gain of 16 and 17.4 dBi, respectively, at two operating bands (21 and 26 GHz).
In [34], A Dual-band circularly polarized SIW cavitybacked slot antenna is presented. Two annular exponential slots are carved on the surface of the SIW cavity to excite the cavity mode and surface mode. A two-order matching circuit using an inductive window is presented with the help of the differential evolution method to achieve dual-band operation. The proposed antenna operates at 37 and 48 GHz bands, as shown in Fig.15.
This dual-band SIW CBSA is rarely involved due to the improper frequency ratio of bands [31], [34]. Proximitycoupled slot apertures are used to excite the two consecutive  resonant modes (TM 020 and TM 120 ). Meanwhile, degenerate polarization modes are depressed [35]. In [35], a SIW circular cavity is constructed with two rectangular slots as radiators on the top plane, and a rectangular SIW with a power divider  is formed in the middle layer as a feeding structure. Furthermore, the proximity-coupled slot is inscribed on the bottom layer to improve the impedance matching, and a matching post is utilized. The construction details of the three layers are indicated in Fig.16(a).
Proximity coupled slot aperture couples the energy to a circular cavity single element by exciting the TM 020 and TM 120 modes. The 2 × 2 circular cavity-backed slot array prototype achieved a gain of 13.2dBi at 28GHz and 14.6dBi at 38 GHz for 5G application, as clearly indicated in Fig.16(b). if we look into the geometry of the circular cavity, the radius of the SIW circular cavity is calculated from the following Eq. (9). [13] where f nm0 is the resonating frequency of the TM nm0 mode, c is the free space light velocity, P nm and is the m th root of the n th order Bessel function. The relation between the radius R c and R eff is calculated with the following Eq. (10) [11]  R c is the SIW circular cavity radius, D eff is the conventional circular cavity diameter, d c is the diameter of the metallic vias, and s is the distance between two adjacent metallic vias. s ≤ 2×d c , d c /λ • < 1 to ensure that no electromagnetic energy leakage from the cavity as stated above [30], [34], several arrays are implemented through the multilayer substrate, which increases the cost and complexity of the structure, on the other hand, the single layer of substrate SIW cavity slotted array is proposed in [36]. The cavity is loaded with a rhombic slot and a split ring slot resonating on TE 101 and TE 102 modes to operate at MMW bands (28 and 38 GHz). The first single element is analyzed later 1 × 4 linear array with a broadband power divider is used to excite four cavity elements. The miniaturized SIW slot antenna array (28.7 × 30.8mm 2 ) exhibits impedance bandwidth of 1.05 % and 5.5 % with a realized gain of 10.9 dBi and 12.1 dBi at 28 and 38 GHz, respectively. The measured half-power beamwidth is 20.70 o . Discussion: In this section, dual-frequency operating antennas are discussed. But these two frequency bands' ratios are small and utilized for specific band applications. Designing a large frequency ratio antenna has wide applications compared to small frequency ratio antennas.

C. CIRCULARLY POLARIZED SIW ANTENNAS
Along with the bandwidth and gain enhancement factors, circular polarization (CP) is an attractive factor for 5G wireless communication. There are several approaches used to achieve CP, for example, dielectric resonator antennas, helical antennas, and crossed dipole utilization, but the complexity and multilayers phenomenon suffer from the issues like nonplanar form and feeding network configuration. A high-gain lowprofile CP antenna structure is presented with a multilayer structure [37]. The bottom layer consists of a SIW cavity with 4 × 4 slots inscribed on the top layer of the cavity. The SIW cavity structure is fed with a coaxial cable to excite TE 440 mode at 28 GHz. The slots in the upper layer of the cavity are arranged on each standing wave peak as shown in Fig. 17 (a) To achieve circular polarization, linear to circular converters are constructed on another substrate layer with dipoles above the slotted cavity as shown in Fig. 17(b). The proposed structure achieves a 16 dBi gain and radiation efficiency of 96% at the Ka-band.
For circularly polarized antennas, AR bandwidth enhancement is a significant factor [38]. A SIW fed antenna exhibit a low AR bandwidth. A simple rotated elliptical cavity fed by slots on the broad sidewall of the SIW is utilized to improve Axial Ratio (AR) bandwidth. By using SIW power dividers, Axial Ratio (AR) bandwidth is improved. A 45 0 rotated elliptical cavity slot is constructed as radiating element fed through a slot on the SIW in the bottom layer.
Earlier, a single elliptical cavity antenna element is analyzed. It exhibits an Axial Ratio (AR) bandwidth of 5.6% and a gain of 7.8 dBi within the operating bandwidth (23)(24)(25)(26)(27). Later 4 × 4 antenna array with dimensions 50 × 50 × 5.3 mm 3 was designed, simulated Axial Ratio (AR) bandwidth of 21.4 to 29.8 GHz (33%) and operating bandwidth is 22.2 -27.6 GHz (21.7%) with a peak gain of 18.7 dBi at 27 GHz is achieved. Gain of an antenna can be improved in several methods, i.e., array configuration, utilizing Dielectric Resonator Antenna (DRA), a single circular patch loaded by a conical horn, and Fabry-Perot Cavity antenna, but all these structures are limited to fabrication at MMW bands. In [39], By utilizing the higher-order modes in the cavity an economical wideband, high-gain slotted cavity antenna is constructed for MMW applications. Here a longitudinal slot etched on the top broad wall of the square SIW cavity is fed with a coaxial cable. LP and CP of antennas are analyzed. CP of an antenna is obtained by placing a linear to circular converter above the LP antenna. LP antenna obtains again up to 14 dBi, impedance bandwidth 22.4% by using the proposed structure. Broad bandwidth is achieved by the proximity of exciting modes TE 310 , TE 130 , and TE 330 modes. The proposed CP antenna obtains 11% AR bandwidth and impedance bandwidth in Ka-band.
Discussion: In this section, types of circularly polarized SIW antenna designs are discussed, which are widely used in wireless communications. Practically designing a target-based antenna with this feature is useful for future communications.

D. LARGE FREQUENCY RATIO INTEGRATED ANTENNA
Integration of microwave and MMW antennas has become an important issue in wireless communication. Several designs are reported to operate with a large frequency ratio [40], [41], and [42] but all of them provide a fixed beam direction at MMW bands. But beam steerable with an array of elements is a good choice for mm-wave propagation [4], [43]. A common aperture 5G antenna with dimensions 75 × 25 × 0.254 mm 3 is proposed in [44]. A dipole and tapered slots structures are jointly used to operate at multi bands, i.e., 3.6 GHz and 28 GHz. For the mm-wave tapered slot arrays, the dipole acts as a footprint, and this dipole operates at microwave frequency. Meanwhile, tapered slots are designed to excite a dipole and work as a high-gain antenna at MMW frequency. This design approach achieved a multiband operation and large space coverage of 120 • by compact footprint with a realized gain of 8 dBi. The prototype of the large frequency ratio antenna and corresponding results are presented in Fig. 18 (a), (b), and (c).
An aperture-shared antenna with a frequency-switching function is designed for 5G applications [45]. Here a thick patch antenna containing a slotted cavity array (TPCSCA) is presented with two different feeding structures for S-band and Ka-band, as shown in Fig.19(a). For an S-band of operation, TPCSCA operates at TM010 mode. For the Ka-band, TE110 mode is generated through a coaxial probe in the coupling layer. This dual-frequency antenna covers ±40 • beam steering operation in Ka-band, as shown in Fig.19 (d).In [46], a 28 GHz beam steerable four-element array is integrated into a 3.5 GHz dipole antenna. The SIW technique is used to integrate both antennas and maintain the radiation characteristics without mutual interference. These two antenna elements are fed with two different feeding structures, which facilitate the beam steering conditions at 28 GHz. With beam steering and dual-frequency operation capability, the proposed technique is suitable for Satcom terminals applications. Instead of two different ports for large frequency ratio antennas, a single port aperture-fed dual-band antenna is proposed in [47]. As shown in Fig.20(a), an annular ring patch antenna element operates at the microwave band, and an inscribed SIW with a longitudinal slot element operates at the MMW band. A 50 microstrip line is employed underneath the bottom substrate to feed the SIW slot element and annular ring elements. The structure results in misalignment in-field distribution. It can be resolved by placing an annular ring with a small radius to combine the fields. As shown in Fig. 20(b) proposed structure achieves a gain of 10.4 dBi at the C band and 8.51 dBi at the Ka-band, and a good amount of frequency ratio is achieved. The CP is obtained by replacing the annular ring patch antenna element with a circular patch antenna containing shorting vias and stubs, as shown in Fig.20(d). The structure radiates in circular polarization at the microwave band and linear polarization at the MMW band, providing a gain of 4.5 dBi/9.37 dBic as shown in Fig.20(d). A thick patch antenna working at 3.5 GHz and a 12 × 12 SIW slot array antenna operating at 60 GHz is proposed as a dual-band large frequency ratio antenna [48]. The main advantages of this work are high aperture reuse efficiency and high channel isolation between different antennas. This design achieves large Radiating Aperture Reuse (RAR) efficiency of 77%.
Furthermore, by introducing a compact microstrip resonant cell [CMRC] structure of better than 100 dB, isolation is achieved between two antennas at 60 GHz. The frequency ratio of the proposed work is 17, which is utilized for space communication. Comparisons of some of the discussed large frequency ratio integrated antennas are listed in Table 4.
Discussion: In this section, Large frequency ratio antenna designs and characteristics are discussed. These antennas cover microwave and MMW band applications. In future design, an adaptive beam steerable antenna is a good choice for communication.

E. SIW CAVITY-BACKED DUAL SLOT ANTENNAS
For MMW communication, the SIW CBSA array is based on double slots presented in [49]. Here double slot structures are used as radiating elements instead of previously discussed slot structures. The introduction of dual-slot controls the radiation over a wide bandwidth. Single-slot and double-slot radiators are evaluated and compared to know the doubleslot performance characteristics. An array of double slots is excited with TE 320 mode by a corporate feeding network consisting of rectangular coupling apertures fed with coaxial cable. The detailed structure and field excitation is presented in Fig.21 (a) and (b). After parametric analysis of the single slot and double slot array in CST studio, it is concluded that the double slot array provides more freedom to choose desired frequency bands. A compact wideband two-port SIW antenna with a central double-slotted metallic plate bounded by two paired corrugations for radar application was proposed in [50] to operate in the X band. Wideband characteristics are achieved by combining transversal slot resonance and higher-order mode resonance of the SIW cavity. After investigating modal analysis without the SMA connector, with the SMA connector of the central slot in the cavity, it concluded that TE 103 and TE 302 modes are excited in the SIW antenna with the central slot.
As in [50], double slots are used as radiators to obtain good radiation over a large bandwidth in the X-band and slot resonant length depends on its offset position regarding the SIW centre axis. The second slot is placed across the centre VOLUME 10, 2022  axis of SIW and acts as a passive element. The second slot, placed slightly away from the axis, acts as an active element. A metallic post placed near the double slot acts as a reflector that enhances radiation. The detailed antenna structure is shown in Fig. 22.
In [51] an H-shaped subarray with four slot pair system is used to improve the impedance matching over a large operating bandwidth as shown in Fig. 23. As discussed, these double slot antennas improve the performance characteristics but they occupy a large area.
A simple technique for the bandwidth enhancement of a CBSA is achieved by placing a via hole above the slot instead of double slots, as shown in Fig. 24 [52]. Here an H -shaped  slot is used to decrease the resonant slot length, and via hole, position is optimized above the slot for dual resonance first. The slot via hole creates the second resonance without using double slots and enhances the bandwidth in the limited area.
Discussion: In this section, dual slot-based SIW antenna designs are discussed. This new method improves bandwidth performance, and all other radiation characteristics make it suitable for future communications.

F. RECONFIGURABLE SIW ANTENNA
Reconfigurable Radio Frequency Microelectromechanical system (RF-MEMS) antennas were first introduced by E.Brown in 1998 [53]. However, conventional Reconfigurable pattern antennas require expensive solid-state phase shifters and create higher insertion loss at high frequencies. Moreover, the microstrip antennas suffer from losses at a higher frequency. Hence Reconfigurable MEMS switches used in SIW technology for pattern reconfiguration are proposed in [54]. Sekar et al. proposed a new technique to tune the SIW filters by loading the SIW cavity with tunable posts [55]. These posts are controlled by PIN diodes. The field distribution inside the cavity is manipulated. Instead of directly loading the slots with a fixed length, slot length is varied by adding switches to the slot, and the field variation parameter is considered to tune in the SIW CBSA antenna [56]. Four shorting post behaviour depends on the switching mechanism of the PIN diode. Connecting the shortening post alters the field inside the cavity, and the switching configuration of four posts exhibits different tuning stages that achieve octave band tuning. Via post positions have been chosen by observing the resonance contours inside the SIW CBSA. Here, single via post effect and four via post effects are analyzed for frequency reconfigurability in the octave band (1.1-2.2 GHz). So to achieve wide bandwidth, PIN diodes are used in SIW CBSA. We can reconfigure the antennas in terms of polarization, pattern and frequency for different applications. Here simple beam scanning SIW CBSA using a post-loaded varactor is presented [57], where the pattern reconfigurable has shown a good advantage over phased arrays in terms of size and flexibility to model antenna arrays. The beam steering with reconfiguration is achieved by electronically controlling the state of switches on the SIW aperture [58]. Four wide dipoles are used to generate the beams, and the corresponding aperture is switched on to excite the dipole. In [59], RF-micro electro-mechanical system switches are used in single-arm spiral antennas to reconfigure the radiation pattern. In [60], SIW CBSA with frequency, polarization, and radiation pattern agility was realized. In [57], a SIW CBSA with three transversal radiating slots and placing 16 varactors in a paired form in the vicinity of the longitudinal centerline is proposed to scan the main beam in a different direction, which is presented in Fig.25(a). the junction capacitance of the varactor controls the amplitude and phase of the fields across the slot. As shown in Fig. 25(b), the main beam directions change from −30 • to 41 • after applying four sets of bias voltages to varactors at 2.6 GHz. Here beam steering can be achieved by employing varactor diodes in SIW CBSA.
In [61], a reconfigurable SIW antenna with H-shaped slots for beam scanning is proposed. Here H shaped slot is designed as a central longitudinal slot that does not cut the surface current, so to radiate the slot, four transversal slots are associated with exciting the currents. These four transversal slots are loaded with PIN diodes. The shape of the slot (L, U, S) depends on the state of PIN diodes (ON/OFF). The radiation characteristics are changed by altering the current flow on the radiation aperture by changing the shape of the slot. The proposed antenna exhibits eight different phase shifts by controlling the length of the shape of the slot and beam scanning between ±45 • in the same plane. A novel frequency reconfigurable SIW antenna is proposed in [62] and [63]. In [62], as discussed earlier, miniaturization is achieved by loading the Inter-Digital Capacitor (IDC) slot on SIW, which resonates in Zeroth Order Resonance (ZOR). Apart from miniaturization, frequency tuning is achieved by loading the IDC slot with a varactor diode and proper bias network. The detailed structure is shown in Fig.26 (a). From Fig.26(b), results provided that 9% frequency tuning (4.13 GHz-4.50 GHz) by changing the bias voltage from 0 V to 36 V.  SIW frequency reconfigurable antenna switching from S to C band is proposed [63]. A rectangular slot and longitudinal slot are inscribed on the SIW cavity top layer. Twelve PIN diodes (SMP1345) are wired together in a specific order along the rectangular ring slot. The detailed construction is shown in Fig. 27(a). The coaxial bias tee provides biasing to the diodes, and the size of the SIW cavity and rectangular patch facilitates dual-band frequency. Moreover, via post is inserted to adjust the resonant frequency. Xr is the distance of the coaxial feed position from the centre of the C-band patch antenna. Xp and Yp are the distances between the centre of the S-band patch antenna and to C-band patch antenna. Fig. 27 (b) and (c) indicate the return loss characteristics when the Xr, Xp, and Yp are varied.
Reconfiguration of the antenna can be achieved in several ways, as discussed earlier, and PIN diodes are utilized to reconfigure resonant frequency [64], [65] and [66]. In [65], a Reconfigurable bow-tie antenna is proposed here, and a two-sided bow-tie antenna is fed by parallel strips fed with a microstrip line integrated with pair of PIN diodes and varactors to achieve wideband frequency. The proposed reconfigurable bow-tie antenna is shown in Fig. 28(a). As shown in Fig. 28(b), the impedance bandwidth of 15.3-39.7% is achieved at 3-5 GHz by applying a different combination of bias voltages.
As shown in Fig. 29(a), Polarization reconfigurability is presented by controlling the state of Single Pole Double Throw (SPDT) switches connected to the antenna array [67]. The antenna array's polarisation can be adjusted between the four polarization modes V-LP, H-LP, LHCP, and RHCP, as shown in Fig. 29(b). These polarization states are achieved by considering a two-layer structure composed of a SIW cavity and four parasitic patches that are excited by ring slots etched onto the surface.
Instead of MEMS and diodes, ferrite slabs are used in the SIW cavity for frequency tuning, as shown in Fig. 30(a) [68]. The position of ferrite slabs alters the field distribution in the cavity, which alters the antenna's resonant frequency. As depicted in Fig. 30(b), the resonant frequency varies as the magnetic field biasing changes.
Discussion: We have discussed frequency, pattern, and polarization reconfigurable antenna designs using switching techniques. These designs may be utilized in multiple applications within a limited area. In the future, these reconfigurable antennas may replace many existing antennas for better performance.

G. TEXTILE SIW ANTENNA
Recent advances in wireless communication make that wearable low-profile textile antennas are presently utilized in various applications, including military, medical monitoring, services, and physical training. All surfaces of the SIW structures, except those with excitation ports, have conducting plates, making them resistant to the impacts of the environment. This characteristic encourages the adoption of SIW antennas in textile antenna design. It is challenging to develop an effective wearable antenna insensitive to the environment. A few textile SIW antenna designs are discussed in this section.
For Wireless Body Area Networks (WBAN), an all-textile SIW cavity-backed circular ring slot antenna was proposed [69]. According to Fig. 31, the suggested antenna is made entirely of textile materials, including felt, conductive fabric, and conductive thread. A semisolid phantom was employed to examine how the human body impacted the results. This one operates at the operating frequency of 5.8 GHz for the ISM band, as shown in Fig. 32. The semisolid phantom effect is negligible, as observed from return loss characteristics. Based on this design, is improving the No.of bands, the operation is a good task for the present scenario. The size of the antenna VOLUME 10, 2022  needs to be reduced. In [70], a SIW textile antenna inspired by metamaterials is proposed for off-body applications to reduce the size. The antenna consists of two layers of wool felt, and conductive fabric Slot configuration exhibits negative order resonance and operates below waveguide cutoff frequency. The prototype is given in Fig. 33. It exhibits a gain of 5.35 dBi, an efficiency of 74.3% and a specific absorption rate (SAR) of 0.38 mw/g averaged over 10g of tissue. It is appropriate for the wearable application at 2.45 GHz. Improving the antenna performance in terms of a band of operation is required.  The Coupled-Mode Substrate Integrated Cavity (CMSIC) antenna with a slotted textile is implemented in [71]. As shown in Fig. 34(a), two coupled cavities comprised of two back-to-back Half Mode Substrate Integrated Cavities (HMSIC), and through the coupling aperture, the probe excites the lower HMSIC and couples it to the upper HMSIC. The antenna prototype with embroidered textile material is presented in Fig. 34(b). This CMSIC antenna offers two resonances, and a slot on the top layer adds a third resonance. The suggested antenna was tested for wearable applications in the Wireless Body Area Network (WBAN), Industrial, Scientific, and Medical (ISM) band, and Long-Term Evolution (LTE) -7 band, as illustrated in Fig. 34(c).
The planar CMSIC antenna used in this work provides a new method for bandwidth expansion on wearable SIW antennas. In one of the HMSICs, the author has recommended a rectangular slot; increasing the number of slots may  improve bands of operation. Even slot geometry has a great effect on the bandwidth achieved.
Easily fabricated wearable badge antenna with standard textile material for a military application is presented in [72]. As depicted in Fig. 35(a), a Planar SIW cavity with an aircraft-shaped slot is proposed to be used, which is suited for badge size. Aircraft with meander slots resonating at 8 GHz with 1.05 GHz band. The proposed antenna was tested with on-body performance, and results are provided in Fig. 35(b). Peak Specific Absorption Rate (SAR) value is less than 30 dBm, as illustrated in Fig. 36, making it appropriate for on-body applications.
Discussion: In this section, low-profile textile SIW antennas are discussed. Present Era, on-body and off-body antennas have been used for ISM applications. Designing a reconfigurable SIW textile antenna with beam-steering capability may be helpful with great importance.

H. A CRITIQUE OF SIW TECHNOLOGY OVER GROOVE GAP WAVEGUIDE TECHNOLOGY
In the MMW band, as the operating frequency increases, transmission losses are increased. Instead of working with transmission lines and coplanar waveguides, a hollow waveguide performs with high efficiency. However, meeting high integration is not an easy task. The gap waveguide model relies upon using boundary conditions to guide electromagnetic wave propagation inside a parallel plate in a specific direction [73]. To understand the analysis between SIW technology [74] and Groove gap waveguide (GGW) technology, summarised data or properties of both technologies is presented in Table.5. After comparison of several parameters, even though GGW technology has several advantages, it is in three-dimensional (3D) structure [75], [76]. GGW components have the capability of high-quality factors making them operate in a narrow band compared to SIW components. Several types of filters, power dividers, amplifiers, and antennas are designed using GGW technology [77].

IV. CONCLUSION
A comprehensive review of Substrate Integrated Waveguide (SIW) cavity-backed slot antenna (CBSA) with different types of classifications like miniaturized, large frequency ratio, dual-slot, and reconfigurable antennas, textile SIW antennas have been studied in this paper. The antennas reviewed in this paper exhibit different applications like dualband, dual-polarized, circular polarized, and beam steering, which are essential for 5G communication. Out of all the antenna classifications, the antennas working at a large frequency ratio with a shared aperture are advantageous for wireless applications. Additionally, polarization, frequency, and pattern reconfigurability are achieved easily with reconfigurable SIW CBSA. Achieving this reconfiguration by connecting the diodes to SIW CBSA is advantageous for future applications, and SIW antennas developed with textile materials are discussed for on-body and off-body applications. Finally, designing an aperture-shared SIW CBSA antenna with reconfigurability may be helpful in future communications.