Collocated MIMO travelling wave SIW slot array antennas for millimetre waves

King Fahd University of Petroleum and Minerals, Grant/Award Number: FT161006; King Abdullah University of Science and Technology, Grant/Award Number: ST‐2016‐KKI‐2899 Abstract A novel four‐element collocated travelling wave substrate integrated waveguide (SIW) multiple‐input multiple‐output (MIMO) antenna covering millimetre wave (mm‐wave) bands (28–32 GHz) is presented. The antenna exhibits a matching bandwidth of more than 4 GHz and a measured gain of 15 dBi. The MIMO antenna elements are collocated, thus significantly reducing the size of the proposed design. To spatially isolate the beams (obtain lower correlation coefficient) of the MIMO antenna elements, the beams are oriented in different directions. The slots within each SIW array are designed to provide tilted beams, thus eliminating the need for a beam switching network. Four distinct beams are formed towards ±10° and ±30°. The dimension of the four‐element MIMO SIW design is 68 � 30.68 � 0.5 mm. The proposed antenna has a high gain, compact size, simpler feeding and enhanced MIMO capability compared to other SIW antennas proposed in the literature.


| INTRODUCTION
Multiple-input multiple-output (MIMO) technology is currently used in 4G enabled terminals because of its ability to provide enhanced data rates within limited bandwidth (BW) and power limits [1]. MIMO technology is also employed in 5G systems, with antennas operating at higher frequencies to provide wider BWs (needed to cover the data requirements), higher gains (to counter the atmospheric attenuation) and the ability to integrate with planar structures [2][3][4][5]. Antenna design at mm-waves is a challenging task and incorporating MIMO configurations makes it even more challenging.
Microstrip antennas suffer from high losses at mm-waves, and consequently, their gain is limited. A microstrip antenna as a single element normally has a relatively broad beam-width and high back lobe levels. Owing to these limitations, such antennas are normally used in array configuration to provide enhanced gain. The feed network of the array helps to form and control the beam. The use of the array and its feed not only adds complexity to the design process, but also increases the size and associated losses. A three-and six-element MIMO planar inverted-F (PIFA) antennas operating at 28 GHz were presented in [6,7] respectively, both with gains around dBi. A four-element MIMO antenna, where each element consisted of a connected array of eight PIFA sub-elements that are fed by a power divider, was presented in [8]. The connected array had a BW of 1 GHz and operated at 28 GHz. In [9], two multi-layer arrays were presented, where each array consisted of eight dipole elements operating at 28 GHz. The same configuration was also used with Bow-tie elements in [10]. A four-element Tshaped patch antenna with defected ground structures for enhanced BW and gain, operating at 29 GHz, was presented in [11]. 4G/5G integrated MIMO antenna systems for mobile handset devices were presented in [12][13][14][15][16].
Slot arrays based on substrate integrated waveguides (SIWs) offer an alternative to microstrip based antenna arrays, since they have high gain, wide BW, are compact, and can be conveniently integrated with planar structures [17][18][19].
Furthermore, when arrays are used in MIMO configurations, the most important figure of merit is the correlation coefficient (CC). The CC describes the correlation or isolation of the communication channels when all the antennas are operated simultaneously. CC depends on the radiation patterns of the MIMO antenna elements and the propagation channel. The ideal desirable value of CC for best MIMO performance is zero. In literature, several ways were proposed to improve the CC, such as introducing physical distance between antenna elements [12][13][14][15][16], orienting the beams of antenna arrays in different directions or using different antenna polarisations. The physical distance isolates the beams but increases the overall size of the MIMO antenna system. On the other hand, beams can be oriented in different directions by the use of complex feeding networks [18,[20][21][22][23]. These feeding networks tilt the beam but increase the complexity, losses and overall size and cost of the antenna arrays and when used in MIMO configurations will enlarge the size significantly.
To tackle these problems, here, we propose to use SIWbased slot arrays in the place of microstrip antennas at mmwaves within a MIMO configuration of four elements. To have better CC, we will not introduce physical distancing between antenna elements or use complex feeding networks. However, we will design the SIW-based slot array so that the beams are oriented towards different directions and maintain these directions at all frequencies covered. So, although the antennas will be collocated (and share the same sidewall), acceptable CC is obtained, and as a result we can have a compact size MIMO antenna systems at mm-waves. This is because of the use of the simultaneous tilted beams. It should be noted that each array is designed such that it can be fed from either end to provide two beams of +/− of its beam tilts defined. As shown in the following sections (Table 3), the proposed design has a better performance as compared to the designs available in the literature. It is compact in size, has simple feeding, provides an operating BW exceeding 4 GHz, and a maximum gain of 15 dBi. Hence, the proposed four-element MIMO configuration is well suited for 5G applications.
The study is organized as follows. Section 2 describes the theoretical background for the design of the travelling wave SIW slot array with the beam oriented along different directions. Section 3 presents the collocated MIMO antenna design details. The fabrication of the antenna and its measurement results are discussed in Section 4, and conclusions are presented in Section 5.   [17] or travelling wave [18,19] types. Standing wave SIW arrays have radiation beams directed towards broadside, while travelling wave ones can direct the radiation beam in oblique directions. Numerous techniques have been proposed for designing such antennas in the literature, based on the design principles described in [24,25], complex feed networks [17,26], multi-layer [27] and planar arrays [28,29]. All these methods are complex and have large sizes. Extending these designs methodologies to design MIMO antenna systems for mobile terminals at mm-waves will be difficult. Recently, a new design methodology for travelling wave rectangular waveguide arrays at mm-waves was presented in [30][31][32][33][34]. This methodology considered a simple feeding and can orient the beam in oblique directions. We will use the same methodology and apply it to design SIW-based slot arrays at mm-waves because SIW-based slot arrays can be easily integrated with other electronics.  The methodology to design an SIW-based slot array, with beams oriented towards the desired direction, is briefly discussed here. It is well known that by invoking the equivalence principle we can replace a slot with a magnetic dipole, which can in turn be replaced by an array of magnetic infinitesimal dipoles (IDs). An array of IDs are located at (x d , y d , z d ) and they are oriented along the angle (θ d , ϕ d ).
At an observation point (x, y, z), the $\vec{ E }$ and $ \vec{ H }$ fields radiated by an array of N magnetic IDs can be expressed as [30][31][32][33][34], ðn n � p n Þ: where c o is the speed of light in vacuum, k o is the free space wave-number and n n represents the unit vector from the nth ID location to an observation point, and it can be expressed as and the nth dipole moment (p n ) is where A d (n) is the complex excitation of the nth dipole and r n represents the distance between the observation point and the centre of the nth dipole. We employ a genetic algorithm (GA) by using the number of the slots, the desired radiation pattern, that is, direction and the side lobe level (SLL) as inputs, to determine the optimal locations of the slots on the top wall of the SIW. The cost function is the mean square error between the radiated and desired radiation patterns. The algorithm is run until either the desired radiation pattern is realised, or the preset maximum number of iterations has been reached. Our objective is to design a four-element collocated MIMO antenna, whose beams point towards 10°, −10°, 30°a nd −30°. Please note that the objective behind tilting the beams was to achieve better values of CC and not to design a tilted array. Based on our observation, beam tilts of 30°, −10°, 10°and −30°provide good values of CC and we do not need to tilt the beams further. For the first antenna element, we choose our objective to realize a tilted beam pointing at 10°in elevation. We consider a 10-element SIW slot array to achieve this goal. The GA determines the locations of the slots on the surface of the SIW as desired with an SLL of 11 dB. The geometry of the SIW antenna is shown in Figure 1(a). The width and length of the SIW slot antenna array were 7.3 and 53 mm, respectively. It was modelled on a Rohacell 51 IG-F foam substrate, whose thickness was 0.5 mm, and the distance between the first and last slot, and the ends of the SIW, were optimized using ANSYS-HFSS to improve the performance. Next, a 14element slot array with a tilt angle of 30°was designed, as shown in Figure 1(b). The width and length of the 14element slot array were also 7.3 and 53 mm, respectively. It was built on the same Rohacell substrate as above. Table 1 shows the locations of the slots on the surface of the SIW for the two tilt angles. The reflection coefficient of the antenna is plotted in Figure 2. We can observe that the antenna has a relatively wide BW (27.5-32.5 GHz) based on the voltage standing wave ratio (VSWR) < 2.0 criterion. Figure 3 clearly shows a beam tilt of 10°from broadside at different frequencies. The 2D normalised radiation pattern at ϕ = 90°cut is shown in Figure 4a. The tilt in the main beam changes with frequency because of the travelling wave behaviour of the antenna. Since, this affects all the elements, the relative tilts are still maintained as we change the frequency.
Feeding the antenna is a challenging task at mm-waves and the performance of the antenna depends significantly on how it is fed. In the present design, a microstrip to the SIW transition was placed at both ends of the SIW slot array. The transition was optimized for the reflection coefficient and the radiation patterns desired tilts. The geometry of the SIW antenna with its feed transitions is shown in Figure 6a, while the reflection coefficient and the radiation pattern of the antenna are presented in Figures 2 and 5a, respectively. Based on the VSWR <2.0 criterion, we found that the BW performance is not affected significantly by the microstrip-to-SIW transition, though, it does affect the SLL.
The design of a SIW slot antenna with a tilt of 30°and low SLL is even more challenging because it becomes increasingly difficult to control the SLL when the tilt angle is high. We observe that the separation distance between the slots for the 30°tilt case is less than the 10°case; consequently, we increased the number of slots while keeping in mind the length constraints that should remain unchanged from the case of Antenna 1 (10°case). A total of 14 slots are found to produce a tilt of 30°over a wide band, while meeting the length (SIW size) criterion. The geometry of the proposed 14 slot array is shown in Figure 1 -819 respectively. We can also observe a slight shift in the antenna beam maximum with respect to frequency. A microstrip to SIW transition was used to feed the antenna and was optimized based on the reflection coefficient and the radiation patterns. The geometry of the proposed antenna along with the microstrip-to-SIW transition is shown in Figure 6b. The reflection coefficient and the radiation pattern are shown in Figures 2 and 5b, respectively. The SLL was around 8.5 dB.
To realize a tilt in the negative direction, that is, −10°and −30°are relatively straight forward, since the antenna has two feed points. The antenna is typically fed from the left while the other side (right) is terminated with a matched load. To tilt the beam in the backward direction, for example, −10°, we feed the antenna from the opposite end, that is, from port 2, as shown in Figure 4c and d. This gives an added feature of beam control and dynamic change when needed.

| FOUR-ELEMENT MIMO
Following the design of single antenna elements with beam tilt angles of 10°and 30°, accompanied by a wide BW and acceptable SLL performance, we designed a collocated fourelement MIMO antenna with adjacent elements sharing the via walls. Our goal is to have the smallest possible antenna, so that it is suitable for future wireless terminals and access points.
The geometry of the proposed antenna is shown in Figure 7, where the first antenna element produces a tilt of −30°, second +10°, third −10°while the fourth has a tilt of +30°, all with respect to broadside. The collocated placement of the antennas affect their performance in terms of the BW, SLL level and radiation. The distance between the slot and the edge of the SIW, as well as the slot positions, were optimized for the reflection coefficient and the radiation pattern with a view to improve the performance of the antennas. The proposed design focused on somehow fixed beams from the arrays to guarantee low correlation between them at all covered frequencies and to maintain small size and enough operating BW. The design can be modified to have wider scanning beams, but a trade-off will be made.
The reflection coefficients and the coupling performance of the four-element MIMO antenna design are presented in Figures 8a and b, respectively. We can observe that all the MIMO antenna elements have an impedance BW ranging from 27 to 33 GHz. The antenna elements have very good isolation, better than 20 dB. The radiation patterns of all the four elements are shown in Figure 9 for the ϕ = 90°cut at the three frequencies of 30, 31 and 32 GHz. The SLLs of the antennas in the middle are affected more compared to those of the neighbouring antennas, and a possible reason for this degradation is the presence of the collocated antenna on both sides of the middle section and sharing of the via walls. Figures 9b and c show the degradation of the SLL.
The CC can be calculated as [35].
where →F i (θ, ϕ) represents the 3D-radiation pattern, when ith port is excited, the * is the Hermitian product and Ω represents the solid angle. The CC calculation between the MIMO antenna elements at different frequencies is presented in Table 2. The values obtained from the field correlation calculation are well below the desired limit and it is due to the beam tilt. The gain and efficiency of the antenna elements are presented in Figures 10(a) and (b), respectively. We observe that the gain varies between 14 and 16.5 dBi, across the 3 GHz BW. The novelty of our proposed design compared to the existing ones (available in the literature) is shown in Table 3. It is the most compact design as there is no separation distance between adjacent MIMO antennas (they use same sidewalls). Furthermore, it offers the highest gain as compared to all the available MIMO designs in the literature. It should be noted that the design was optimized for having at worst a 3-dB gain variation within the BW of interest and not for the widest impedance BW. In addition, it provides four-tilted beams without the use of a feeding network (i.e. Butler matrix) with the possibility of being fed from both sides. For MIMO antennas, the SLL is not an important figure of merit, that is, why the majority of previous works did not focus on it. For MIMO antennas, spatial correlation of the antenna is the most important parameter of -821 performance evaluation, but for collocated MIMO antenna elements, if the SLL is close to the level of the main lobe, then the CC value will be high and, as a result, a poor MIMO performance will be obtained. As long as the beams are uncorrelated, the MIMO antenna system will enhance the channel capacity (in an isotropic environment).

| FABRICATION AND MEASUREMENTS
The fabrication of the four-element SIW slot array with collocated MIMO design was a challenging task because it required a large number of vias in a very confined region. To handle this problem, we used a Rohacell 51 IG-F foam of 0.5 mm thickness. To introduce top and bottom metal surfaces on a Rohacell 51 IG-F foam, we used an adhesive material 3M VHB double-sided tape, whose thickness was 0.025 mm. Dupont AP 8525 is a flexible polyimide material (Kapton) that was used between the adhesive and copper with a thickness of 0.05 mm. A 0.035-mm layer of copper plated on both sides was added. The layered view of the geometry is shown in Figure 11. Furthermore, instead of using vias, we used a via wall to connect the different layers, as well as act as a barrier between the MIMO antenna elements. This is because it is not possible to create a large number of vias within the Rohacell. To address this issue, a via wall was built with a Rogers 5880 substrate with a thickness of 0.381 mm, which was plated with copper strips (0.017 mm thickness) on both sides to represent the vias.
The fabrication process consisted of several steps. Initially, we cut a flexible laminate and applied a photosensitive film on the top side of the material. After exposing and developing the film, features of the top and bottom were etched on two different pieces of the material. The off-the-shelf Rohacell foam 51 IG-F material, whose thickness was 1 mm, was machined down to 0.5 mm. Then, the internal layers of the  polyimide were coated with 3M VHB double-sided tape. The top and bottom layers were bonded to the Rohacell foam 51 IG-F. Next, the via wall was fabricated by using the Roger 5880 substrate with a copper layer etched on both sides, which was machined by using a special jig (or you can use tool) designed for this type of job. Then the strips were soldered on both sides to connect the top and bottom sides. Solder wires were placed next to the connector position through the holes. Connectors (mini-SMPs) were soldered and the entire design was cleaned in an isopropyl alcohol bath. The fabricated geometry of the four-element co-located MIMO SIW design is shown in Figure 12.  -823 The measured reflection and isolation curves of the collocated four-element MIMO antenna design are shown in Figures 13(a) and (b), respectively. We can observe a slight difference between the measured and the simulated results, this can be attributed to fabrication errors introduced by the sequential fabrication steps. The radiation patterns were measured in a mm-wave far-field chamber at the Polytechnique Montreal, and the measurement setup is shown in Figure 14. Figure 15 presents the measured radiation patterns for the same cut as that in Figure 9. In the worst scnario, SLL was around 6 dB but still acceptable CC values were achieved. Note that only the front half of the pattern is presented because of the limitation imposed by the measurement setup, which only covered the upper hemisphere. We observe that the main beam is in the desired direction, but there is a slight increase in the SLL. The measured gain levels were close to those predicted by the simulations (ranged between 14 and 15 dBi). We believe that the fabrication tolerances are responsible for the differences between the simulated and measured results, and a slight bend in the material may also have contributed to these differences.

| CONCLUSIONS
A four-element, collocated travelling wave SIW MIMO antenna operating at mm-waves has been presented in this work. Each element of the antenna system is composed of slot arrays whose radiated beams tilt along different angles in space to provide low correlation. The antenna elements have all been designed to cover the BW of 4 GHz ranging from 28 to