Electronic steerable MEMS antennas - IEEE Xplore

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reflectarrays, as well as the algorithms allowing for beam shaping, beam steering, null placing etc., as well as the typical RF-MEMS circuitry for both amplitude.
ELECTRONIC STEERABLE MEMS ANTENNAS Roberto Sorrentino, Roberto Vincenti Gatti, Luca Marcaccioli, Benedetta Mencagli University of Perugia, Department of Electronic and Information Engineering, Via G. Duranti, 93, 06125 Perugia (Italy), Email: [email protected] ABSTRACT Recent advances in the technology of radiofrequency (RF) circuits, such as RF-MicroElectroMechanical Systems (MEMS) make it possible to practically implement electronically reconfigurable antenna arrays paving the way to innovative communication systems including applications such as next generation mobile communications, radar applications, remote sensing and imaging. This paper reviews the basic implementations of such antenna systems, namely phased arrays and reflectarrays, as well as the algorithms allowing for beam shaping, beam steering, null placing etc., as well as the typical RF-MEMS circuitry for both amplitude and phase control. 1.

INTRODUCTION

The increasing demand of bandwidth and service quality in modern communication systems is spurring the development of innovative and high-performance solutions subject to more and more stringent constraints. In this context, the reconfigurability of the radiated beam and the possibility of increasing the channel capacity are of paramount importance for the development of future radio systems. Reconfigurability: in both terrestrial and satellite communications, the antenna should be capable of modifying the coverage depending on the traffic demand and environmental conditions, compensating for possible element failures and minimizing the effects of interferences. In addition, antennas with fast beamscanning capabilities are required in applications such as remote sensing, radar and microwave imaging. Channel capacity: Techniques such as SDMA (Space Division Multiple Access) and MIMO (Multiple In Multiple Out) have recently been developed that can potentially boost the bit-rate of a channel without requiring any additional bandwidth nor transmitted power. Arrays of radiating elements lend themselves to both providing full reconfigurability as well as exploiting SDMA and MIMO techniques, provided that a multibeam architecture is employed. By a proper phase and/or amplitude control of the radiating elements, in fact, antenna arrays not simply provide high-directivity of the radiation pattern, but also the possibility to steer the beam, shape the radiation pattern and place nulls in prescribed directions, thus providing the ideal solution

for the implementation of SDMA techniques. On the other hand, MIMO techniques are based on the use of multiple radiators so that they can intrinsically be implemented on array antennas. Such kind of antennas with interference nulling and adaptive reconfiguration capabilities are also well known as smart antennas. Although digital techniques offer a very powerful and attractive solution to achieve beam forming capabilities, the complexity of the practical realization in the case of medium-large arrays is still preventing their actual implementation. This paper will thus be focused on analog beam forming of antenna arrays, where reconfigurability can be obtained by either modifying the excitations of the radiating elements or by integrating tuning devices in the radiating elements themselves. The latter solution leads to changing the properties of the individual radiators: although very attractive this solution is very challenging and cannot be treated in a systematic way. We will therefore concentrate on the former solution, which is based on the implementation of reconfigurable circuit devices, such as phase-shifters and power dividers, to control the phases and/or the amplitudes of the excitations. Antenna reconfigurability is thus reduced to the design of suitable circuit architectures and tuning devices. Device tunability can be obtained by employing either analog or digital devices: in both cases, RF-MEMS are very promising candidates thanks to their high RF performance and potential low cost compared to the conventional semiconductor counterpart (PIN diodes or FETs), particularly at mm-wave frequencies. Although reliability, power handling and device packaging have so far prevented an extensive use of MEMS for RF applications, it is believed that the considerable efforts being spent by both academy and industry will in the very next future pave the way to a widespread use of such devices in many practical applications, particularly for reconfigurable antennas. Two classes of reconfigurable antenna arrays are considered in this paper, namely phased arrays and reflectarrays, the difference between them being essentially the way the RF signal feeds the radiating elements: at circuit or radiation level, respectively. Furthermore, while phased arrays can either be flat or conformed to any shape, reflectarrays have tighter geometrical constraints, yet the feeding network is much simpler and with lower loss, as it simply consists of one or more radiating apertures.

The algorithms for antenna reconfigurability, based on amplitude and/or phase control are first briefly discussed in Section 2 and some of the main features that can be achieved are illustrated. After considering phased arrays in Section 3, the use of RF-MEMS in the design and realization of tuning circuits to be employed in phased arrays is then discussed and illustrated with various examples in Section 4. Reconfigurable reflectarrays are considered next, and two specific examples being developed within the AMICOM Network of Excellence, partly in collaboration with ACE NoE, are presented and illustrated. Both antennas are made of patches slot-coupled to MEMS tuneable loads, using microstrip or coplanar waveguide technology. Some conclusions are drawn at the end of the paper, showing confidence in a very promising development of this area. 2.

BEAM SHAPING ALGORITHMS

By properly weighting the amplitude and/or the phase of each radiating element of a generic linear, planar or conformal array, it is possible to reconfigure the radiation pattern in various manners. Beam steering, beam shaping, and null placing are typical applications. Beam steering is obtained in linear or planar arrays by feeding with a proper delay each radiating element (phase-only weighting). Beam steering in conformal arrays can also be achieved without phase shifters, exploiting the intrinsic directivity of different portions of the array (amplitude-only weighting). Whatever the architecture employed, when both phase and amplitude of each element can be controlled, the radiation pattern can be reconfigured with the maximum degrees of freedom. The complexity of the system however may become excessive especially when considering very large arrays. In this section, we present some examples of beam shaping corresponding to two cases: a) Both amplitude and phase are controlled b) Only the phase is controlled. The further case of amplitude-only weighting is only relevant for conformal arrays, as will be shown in the following section.

The method is based on an iterative algorithm that, by means of properly defined projections operators, converges to a solution belonging to the intersection between the set of the array factors satisfying the constraints on excitations and those fulfilling the requirements on radiation pattern. Fig. 1 shows an example obtained in the case of a planar array.

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Figure 1. Example of beam shaping obtained with a 17×17 planar array. From [11], courtesy of EuMA.

b) Phase-only weighting In [11-13] an algorithm has been introduced for the phase-only weighting of linear and planar arrays. The method is based on the optimization of proper basis functions describing the phase-distribution of the aperture.

Figure 2. Beam shaping (main lobe enlargement) for a 64 elements linear array (blue). Comparison with a uniform phase distribution (dashed black). From [11], courtesy of EuMA.

a) Amplitude and phase weighting Various approaches to perform the pattern synthesis can be found in the literature. Direct synthesis methods [1-2] allow fast and accurate shaped pattern design. Constraints on the excitations, however, arise in practical cases that cannot be handled with such methods. Alternative techniques are essentially based on optimization procedures [3-8], among which the projection method [9-10] is notable for its high efficiency and flexibility.

Figure 3. Linear array of 64 elements with Chebyshev excitation. Interfering signals at 76° and 95° in elevation. From [11], courtesy of EuMA.

In Fig. 2 a typical problem of main lobe widening is shown. In Fig. 3 the method is used to impose two nulls in the direction of arrival of two strong interfering signals. The same approach can be used in planar and conformal arrays. 3.

PHASED ARRAYS

Conventional phased arrays where both amplitude and phase weighting are implemented make use of T/R modules, where the amplitude is controlled by means of digital attenuators and the phase is varied by digital phase shifters. Those are costly devices usually employed in military applications where high performance is required and cost is not a limiting factor. The recent advances in MEMS technology makes it possible to design new classes of high-performance and low-cost devices that can efficiently replace T/R modules and implement the beam forming algorithms described in the previous section. Such devices (reconfigurable power dividers, couplers and digital or analog phase-shifters) are illustrated in the next section. Beam reconfigurability in planar arrays can be achieved with parallel or series feeding networks, as shown in Figs. 4 [14] and 5, respectively, or by combinations of the two architectures. In case of amplitude weighting, reconfigurable power dividers or couplers are used to control the amplitudes of the excitations, whereas in the case of phase-only weighting the power divisions are fixed.

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In the case of conformal arrays, beam reconfigurability is usually achieved without phase shifters, different degrees of freedom and system capabilities being achieved depending on the complexity of the feeding network. In the simplest case, for instance, a SPNT switch can be used to obtain a switched-beam antenna whilst a wide range of beam shapes can be obtained using reconfigurable power dividers that allow a full control of the amplitude distribution (see Fig. 6a) [15]. Depending on the complexity of the control algorithm, multiple interference nulling may also be implemented with this architecture. Alternatively, phase-only weighing may also be implemented as shown in Fig. 6b. The two architectures can be merged to control both amplitude and phase of the excitations.

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Figure 6. Reconfigurable beam forming network for conformal arrays: a) amplitude weighting with reconfigurable power div/comb; b) phase-only weighting with phase shifters.

The beam shaping and interference nulling capabilities of the proposed architectures are shown at the specific examples of Figs. 7-9. Multi-beam feeding networks can typically be realized by means of Butler matrices, Blass matrices or Rotman lenses. Reconfigurability may be obtained to these architectures by employing MEMS-reconfigurable devices mentioned in the next Section.

R15 I/O Parallel feeding network with Figure 4. Linear/planar arrays. amplitude and phase weighting (reconfigurable power dividers/combiners and phase shifters. Rn = power ratio of the n-th power divider.

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Figure 5. Linear/planar arrays. Series feeding network with amplitude and phase weighting (reconfigurable couplers and phase shifters). Cn = coupling ratio of the n-th coupler.

Figure 7. Beam shaping obtained with a 16-element linear array (architecture of Fig. 4). From [10], courtesy of EuMA.

Figure 8. Beam shaping obtained with amplitude-only weighting with a 16-element conformal array (architecture of Fig. 6a). From [15], courtesy of EuMA.

Figure 11. MEMS-based reconfigurable coupler, fabricated at ITC-irst, Italy

b) Reconfigurable couplers A reconfigurable coupler consisting of three coupled lines, two of them being connected by a pair of switches is sketched in Fig. 12 [16]. The device toggles its state between two configurations: when both switches are on, the central line is shunted to one of the two side lines, thus obtaining a strong coupling. When the switches are off, the central line is floating and the coupling is weaker. The device can be designed so as to obtain good matching and high isolation in both states. Switch

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MEMS RECONFIGURABLE DEVICES

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The beam forming architectures described in the previous Section make use of some key components such as reconfigurable power dividers/combiners, reconfigurable couplers and phase shifters. Examples of MEMS-based configurations of such components are described next.

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Figure 9. Interference nulling obtained with phase-only weighting with an 8-element conformal array (architecture of Fig. 6b).

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Figure 12. Three line asymmetrical coupler: proposed architecture.

A reconfigurable power divider is shown in Fig. 10. The circuit consists of two 3-dB branch line couplers and a phase shifter [14]. The power ratio can be changed through phase shifter. The latter can be realized using RF-MEMS technology. A prototype employing a reflection-type phase shifter is shown in Fig. 11. RF-MEMS switches

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Figure 10. Reconfigurable coupler: proposed architecture.

Figure 13. Layout (Agilent ADS) of a microstrip coupler employing RF MEMS switches and operating at 20 GHz. The device is under construction at ITC-Irst (Trento, Italy).

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Phase shifters

Different topologies of MEMS phase shifters have been investigated in recent years, most of them being based on established designs where the solid-state switch is replaced by a MEMS switch. The most common topologies are the switched-line and reflection type phase shifters [17-18]. Especially for high frequency applications, another very common topology is the distributed transmission line phase shifter [19-23]. It consists of a transmission line periodically loaded with MEMS shunt capacitances. By controlling the MEMS state, the distributed capacitance, the phase velocity and the phase shift of the line section can be varied. As an example Fig. 14 shows a W-band distributed MEMS phase shifter developed within a collaboration between our laboratory and VTT-Millilab, Finland. The fabrication on Glass substrate is in progress at ITC-irst (Trento, Italy).

Reconfigurability is typically achieved by varying the phases of the waves reflected by the elements [24-28] either by suitably varying their shape or by loading them by a variable reactive load [29-32]. Both approaches can employ RF-MEMS. The former approach, which implies the use of reconfigurable elements presents some technological difficulties particularly in the realization of the control network when employed in large reflectarrays. Moreover, the EM interaction between the elements and the MEMS reconfigurable circuit can strongly affect the radiation properties of the antenna. Such problems are strongly alleviated using the second approach, since the reactive loads, in the form of MEMS-tuned phase shifter, can be fabricated in a separate layer. Two such examples are illustrated in the following, where the radiating element is a simple patch slot-coupled to a either a coplanar or a microstrip MEMS-tuneable reactive load. Both structures have been developed within the so-called RARPA North Star Project of AMICOM. a) Patch coupled to coplanar waveguide reactive load

Figure 14. W-band distributed MEM transmission line phase shifter, fabricated at ITC-irst, Italy.

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Fig. 16 depicts the layout of the elementary cell. The patch on the top side is slot-coupled to a coplanar line on the bottom side, connected to a short-ended 7-state phase-shifter [30]. The reflectarray is designed to operate at 26 GHz. The structure is sketched in Fig. 17: the antenna is made of 10×10 elements (58 mm × 58 mm) and 600 RF MEMS switches. The entire antenna can be fabricated on a single silicon wafer, processed on both sides.

REFLECTARRAYS

A reflectarray is a flat array illuminated by an external feed. The reflected radiation pattern depends on the reflections from the individual elements. A schematic of the operating principle is shown in Fig. 15. FEED

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Figure 15. Schematic of a generic reflectarray.

Reflectarrays have the potential of satisfying stringent requirements in scanning, multi-beam and reconfiguration capabilities while maintaining low-mass and low-cost, exhibiting a complexity intermediate between reflector antennas and phased arrays.

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Figure 16. Reflectarray element: (a) top layer: directly coupled patch (designed at the University of Perugia); (b) bottom layer: CPW open or short-ended phase-shifter (designed by ISIT-ULM).

The length d of the line connecting the phase-shifter to the slot (Fig. 16b) introduces a delay in each element in such a way as to control the direction of the reflected plane wave. To reduce the complexity of the biasing network the antenna is controlled at subarray-level: all phase-shifters of the same row are biased simultaneously (Fig. 17). The complete reflectarray is being fabricated at the Fraunhofer ISiT (Itzehoe, Germany).

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Figure 18. Layout of the radiating elements (CST Microwave Studio) (a), and the phase-shifter layer (b).

Figure 17. Layout of the bottom layer and zoom of a particular (Agilent ADS). The lines biasing the rows of the reflectarray are evident, as long with the short-ended phase-shifters.

b) Patch coupled to a microstrip reactive load The patch is slot-coupled to a MEMS-tuned microstrip reactive load [31] fabricated on a different substrate, as shown in Fig. 18. The reflective patch is made on a Rogers RO4003C laminate (εr = 3.38), while the tuneable load is printed on Silicon substrate. The tuneable load consists of an open-ended 3-bit phase shifter that employs seven series RF MEMS switches. The reflectarray is designed to operate at 35 GHz: this choice is justified by possible implementations in an outdoor mm-wave imaging system. The antenna is made of 12×12 elements (52 mm × 52 mm) and 1008 switches. It is controlled at subarray level, like that previously described in Sec. 3.1, thus there are 84 biasing signals. The complete reflectarray is to be realized at the ITCIRST (Trento, Italy). To show the feasibility of this solution, a prototypal antenna at 25 GHz has been designed and fabricated [32]. The reflectarray is made of 20×20 elements spaced by half a wavelength in both directions, to avoid grating lobes. A hardwired version of the phase shifting layer has been realized steering the main beam 45 degrees in the Hplane.

The feed, a low gain horn designed to have a 3dB beamwidth equal to 60°, is placed at 20 λ0 from the centre of the reflectarray. The fabricated reflectarray antenna is shown in Fig. 19a, while Fig. 19b depicts the phase shifting layer. It can be observed that together with the phase shift necessary to steer the beam, a correction had to be provided to account for the spherical-wave phase distribution produced by the feed. This is also evident from Fig. 19c, where the adopted phase distribution is depicted.

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Figure 19. Prototypal 20×20 reflectarray antenna @ 25 GHz: (a) reflectarray and feed horn; (b) phase shifting layer; (c) reflectarray phase distribution. From [32], courtesy of EuMA.

The measured radiation patterns at 24.5, 25.0 and 25.5 GHz are shown in Fig. 20, while the comparison between the simulations and measurements at 25 GHz are shown in Fig. 21 showing a good agreement. Reflectarray Directivity (dB) : Measurements 0 24.5 GHZ 25 GHz 25.5 GHz

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ACKNOWLEDGMENT Original contributions provided in this paper are the result of the work of many members of research group of the Microwave. The contributions of Paola Farinelli, Alessandro Ocera are gratefully acknowledged. Most of the activities have been carried out in the framework of the NoE AMICOM. REFERENCES

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Figure 20. Reflectarray radiation pattern at three frequencies: (a) simulations; (b) measurements. Directivity: 23.9, 26.4, 25.5 dBi. From [32], courtesy of EuMA. 25 GHz : Simulation and Measurement

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Figure 21. Reflectarray simulated and measured radiation pattern at 25GHz. From [32], courtesy of EuMA.

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CONCLUSIONS

A review of the basic implementations of reconfigurable antenna arrays, namely phased arrays and reflectarrays, as well as the algorithms allowing for beam shaping, beam steering, null placing etc., has been presented along with the typical RF-MEMS circuitry for both amplitude and phase control. The practical development of such antenna systems is expected to be soon made possible by the advances in the RF-MEMS technology, paving the way to the practical exploitation of highly innovative RF systems, such as smart antennas for next generation wireless networks, remote sensing and millimetre-wave radar and imaging.

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