A Sectored Phased Array for DBF Applications

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IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 54, NO. 6, NOVEMBER 2005

A Sectored Phased Array for DBF Applications Antonis Kalis, Member, IEEE, and Michael J. Carras, Member, IEEE

Abstract—In this paper, we present the idea of using switching techniques in phased arrays in order to reduce the size and cost of the RF/IF circuitry on a smart array system with a given number of array elements. The principle is illustrated using a sectored phased array antenna working on the 5.2 GHz industrial, scientific and medical (ISM) band. The antenna is a four-element dipole array, with two active and two parasitic elements. The array uses a reflective 2P4T switch to route the RF signals to consecutive elements of the array and four pin diode SPST switches for short or open circuiting the antenna elements. The performance of the proposed antenna system surpasses the performance of traditional two-port phased array systems and is comparable to that of four-element planar arrays. Simulated results of the antenna performance and beam-forming capabilities are presented.

Fig. 1.

Proposed four-element sectored phased array.

Index Terms—Adaptive arrays, array signal processing, sectored antennas, switched antennas.

I. INTRODUCTION EXT-GENERATION mobile communications are very demanding in terms of channel throughput and bit error rate. However, the wireless channel imposes several restrictions to these goals, due to high levels of multipath fading and cochannel interference. The use of smart antennas and space-time coding techniques [1] has demonstrated great effectiveness in overcoming these restrictions. Although advancements in the fields of processing power, power consumption and cost of digital signal processing (DSP) technology has enabled space-time coding algorithms to be integrated in mobile terminals, traditional smart array architectures are still too costly for integration in user terminals. Reported smart arrays [2], [3] use a number of active radiating elements and a distinct signal path for each of these elements. The deployment of multiple uncorrelated signal paths (multiple RF front ends), however, results in costly implementations, unsuitable for mobile terminal use. Therefore, traditional smart antenna architectures are only suitable for integration on access points and base stations rather than user terminals, since the wireless terminal size and cost requirements impose several restrictions on that goal. Recently, smart arrays using only one front end have been reported [4]. These electronically steerable passive array radiator (ESPAR) antennas consist of one active and a number of passive radiators, and they are suitable for beam steering and null steering applications. However, the reported ESPAR antennas suffer from bandwidth degradation with respect to the number of antenna elements used [5]. Moreover, the reflection coefficient and consequently the frequency response of these structures vary with respect to the steering angle of the array [5].

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Manuscript received May 13, 2004; revised Jan 18, 2005. The review of this paper was coordinated by Prof. T. Lok. The authors are with Athens Information Technology, Peania, Attiki 19002, Greece (e-mail: [email protected]; [email protected]). Digital Object Identifier 10.1109/TVT.2005.858168

Fig. 2. Radiation pattern of the array of 1, when elements 1 and 2 are driven with equal phase and power, while elements 3 and 4 are short circuited.

Fig. 3. Frequency response of the proposed array, when two consecutive elements are driven with equal amplitudes and phases, while the other two are short circuited.

In this paper, the idea of using switching techniques in phased arrays is presented, in order to be able to build arrays with only two distinct signal paths, regardless of the total number of antenna elements. The advantage of the architecture that is presented in this work is that the frequency response and the

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KALIS AND CARRAS: A SECTORED PHASED ARRAY FOR DBF APPLICATIONS

Fig. 4.

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Radiation patterns of the proposed array when elements 1 and 2 have the same power level and (a) 60 ◦ phase difference. (b) −60 ◦ phase difference.

II. SECTORED PHASED ARRAY

Fig. 5.

Proposed array feeding network block diagram.

return loss coefficients of the array remain constant, regardless of the beam or null direction of the radiation pattern. The beamsteering capabilities of the presented sectored phased array surpass those of traditional two-port adaptive arrays of comparable dimensions, and are similar to those of traditional four-element adaptive arrays [6]. In Section II, the proposed sectored phased array architecture is presented using an example four-element array working in the 5.2-GHz band. Section III shows how the proposed array performs with respect to two-element linear and four-element planar phased arrays in beam-steering and null-steering applications. Finally, we conclude our work and present proposals for further work on this area in Section IV.

The requirements of mobile terminal applications result in several restrictions on the use of smart antenna systems. These restrictions include low cost, small size, high efficiency, and on-the-horizon coverage. This section illustrates the basic idea of the sectored phased array structure and functionality that meets the requirements of mobile terminal applications, using a four-element example, targeting the 5.2-GHz band. The proposed array functionality and performance was simulated using the IE3D electromagnetic simulation tool, based on the method of moments. Fig. 1 shows the proposed sectored array architecture. It consists of four dipole antennas that are placed on the periphery of a circle with radius d = 10.5 mm, or 0.182λ, and the center of the circle is the symmetrical center of the array geometry. The dipoles are of length L = 25.4 mm, or 0.44 λ, and are simulated as cylinders of diameter a = 0.9 mm. Only two consecutive elements of the array are driven, while the other two are short circuited. Due to the four-way symmetry of the array topology, it is adequate to show the performance of the array for driving elements 1 and 2, while short circuiting elements 3 and 4. The array performance will be identical for all cardinal directions. Fig. 2 shows the radiation pattern of the array when we drive elements 1 and 2 with equal phase and power level, while short circuiting elements 3 and 4. The S11 parameter for this structure is less than −15 dB in the frequency range of 5.15–5.5 GHz, as shown in Fig. 3. The directivity is 8.1 dBi, and the power efficiency is 98%. It is evident that by driving the two active elements of the array with equal phase and power level, the structure acts as a switched beam array,

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Fig. 6. Combinations of beam maximum and null direction satisfying two conditions simultaneously, for the array of 1, when elements 1 and 2 are driven and elements 3 and 4 are short circuited. (a) With grating lobes. (b) Without grating lobes.

Fig. 7. Combinations of beam maximum and null direction satisfying two conditions simultaneously, for all the sectors of the array of (a) With grating lobes; (b) without grating lobes.

producing beams of 91 ◦ . However, it is possible to further steer the radiation pattern of the array by adjusting the power and phase difference between the two active elements. Fig. 4 illustrates an example of the beam-steering capabilities of the array. Fig. 4(a) shows the radiation pattern that is produced when the power level of elements 1 and 2 is equal, and the phase difference between elements 1 and 2 is 60 ◦ . Fig. 4(b) shows the radiation pattern when the phase difference is equal to −60 ◦ . In both cases, the front-to-back ratio of the beams is less than −25 dB. An alternative of the proposed configuration is the ESPAR antenna architecture as described in [7] and [8] which has

only one active element and relies on using variable loads on parasitic elements to modify the gain pattern. However, since ESPAR antennas have only one active element any processing to find the directions of arrival (DoA) of incoming signals or interference must be done by taking multiple samples with different (known) gain patterns. The proposed configuration, on the other hand, permits the use of conventional DoA estimation techniques at the DSP level, for example, subspace techniques like the MuSiC algorithm. Although a detailed analysis of the properties of the electronics supporting the functionality of the proposed array is not in

KALIS AND CARRAS: A SECTORED PHASED ARRAY FOR DBF APPLICATIONS

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Fig. 9. Combinations of beam maximum and null direction satisfying two conditions simultaneously, for the array of 1, when all elements are driven.

Fig. 8. Combinations of beam maximum and null direction satisfying two conditions simultaneously, for a two-element linear array with d = 0.364 λ.

the scope of this paper, the feeding network of this structure is simple and based on prior work in the field of switched beam arrays [9]. A block diagram showing the concept of the feeding network is displayed in Fig. 5 and consists of a single reflective 2 P4 T RF switch, connecting the two front ends to the four elements of the array. Open and short circuiting of the antenna elements can be implemented using fast RF (pin diode) switches, using common switching techniques, as these are described, for example, in [9]. III. PROPOSED ARRAY PERFORMANCE In this section, the performance of different adaptive arrays and the proposed sectored phased array is compared with respect to their directivity, beam-steering, and null steering capabilities. A performance analysis was made by importing the impedance matrix of the proposed array into MATLAB, where the response of the array to random excitations was evaluated. In order to compare the beam steering and null steering capabilities of the different array structures, we present the combination of the beam maximum with respect to the nulls of this pattern. The corresponding diagram showing the beamsteering and null-steering capabilities of a single sector of the proposed array after 100 000 iterations are plotted in Fig. 6(a). In this diagram, each grey area shows the azimuth angles where a beam maximum greater than 5 dBi and a null that is less than −30 dBi are satisfied simultaneously. It is evident that for some excitation combinations, the array produces large grating lobes outside the sector of interest. If these lobes are undesirable, and the corresponding excitation combinations are rejected, then the beam- and null steering capabilities of the array are reduced to the angle combinations represented in Fig. 6(b). Switching between different sectors extends the steering capabilities of the

sectored phased array. In Fig. 7, grey areas show the azimuth angles where a beam maximum greater than 5 dBi and a null that is less than −30 dBi are satisfied simultaneously, when all sectors are used. Fig. 7(a) shows the performance of the array when grating lobes are accepted, and Fig. 7(b) shows the performance of the array when large grating lobes are rejected. Traditional adaptive arrays utilizing two RF signals consist of two radiating elements. The simplest two-element adaptive array, described in [6], is the two-element linear array. In order to compare this structure to the proposed sectored phased array, a two-element linear array is considered with interelement spacing equal to the maximum dimension of the proposed array, equal to 2d = 21 mm or 0.364 λ. The two-element structure has maximum directivity equal to 4.31 dBi, while the sectored phased array’s directivity is higher and equal to 8.1 dBi. In order to compare the beam steering and null steering capabilities of the different array structures, we present the combination of the beam maximum with respect to the nulls of this pattern. The results of this simulation for the two-element array after 100 000 iterations are plotted in Fig. 8. In this diagram, each grey area shows the azimuth angles where a beam maximum greater than −1 dB than the antenna directivity and a null that is less than −20 dB than the antenna directivity are satisfied simultaneously. It is evident that both the directivity characteristics and the beam-steering and null-steering capabilities of the proposed array surpass the performance of two-element adaptive arrays. Finally, the sectored phased array capabilities are compared to those of a four-element adaptive array of the same dimensions. The directivity of the adaptive array is equal to 8.2 dBi, which is naturally very close to the directivity of the proposed structure, since the dimensions in both cases are identical. The beam- and null steering capabilities of the adaptive array are shown in Fig. 9, where grey areas show the azimuth angles for which a beam maximum greater than 5 dBi and a null that is less than −30 dBi are satisfied simultaneously. It is evident that the steering capabilities of the two arrays are comparable. The adaptive array is capable of covering the 84% of the total ϕG max /ϕG min plane, while the sectored phased array is capable of covering the

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73% of the degree combinations in the same plane, when grating lobes are accepted. Therefore, the use of the proposed sectored phased array is more attractive for digital beam-forming algorithms running on mobile terminals, due to the reduced number of front ends required.

[8] K. Ito, A. Akiyama, and M. Ando, “Bandwidth of electronically steerable parasitic array radiator antennas in single beam scanning,” IEICE Trans. Commun., vol. E86-B, no. 9, pp. 2844–2847, Sep. 2003. [9] N. Scott, M. Leonard-Taylor, and R. Vaughan, “Diversity gain from a single-port adaptive antenna using switched parasitic elements illustrated with a wire and monopole prototype,” IEEE Trans. Antennas Propag., vol. 47, no. 6, pp. 1066–1070, Jun. 1999.

IV. CONCLUSION This paper presented a sectored phased antenna array system that uses only two front ends but surpasses the performance of traditional two-element adaptive antenna designs. Moreover, it has been shown that the proposed antenna directivity and beam- and null steering capabilities are comparable to those of four-element planar adaptive arrays. The antenna is capable of performing adaptive beam-forming in different sectors of the azimuth plane. The main advantages of this design are that it can be easily implemented, reducing the number of IF/RF front ends required when compared to a conventional four-element planar adaptive array by a factor of two. The idea of using switched parasitic elements in circular phased arrays can be applied in future work to arrays with more elements, with the same number of front ends. REFERENCES [1] P. D. Karaminas and A. Manikas, “Super-resolution broad null beamforming for cochannel interference cancellation in mobile radio networks,” IEEE Trans. Veh. Technol., vol. 49, no. 3, pp. 689–697, May 2000. [2] S. Bellofiore, J. Foutz, R. Govindarajula, I. Bahceci, C. Balanis, A. Spanias, J. Capone, and T. Duman, “Smart antenna system analysis, integration and performance for mobile ad-hoc networks (MANETs),” IEEE Trans. Antennas Propag., vol. 50, no. 5, pp. 571–581, May 2002. [3] J. Liu, Y. Yuan, L. Xu, R. Wu, Y. Dai, Y. Li, L. Zhang, M. Shi, and Y. Du, “Research on smart antenna technology for terminals for the TD-SCDMA system,” IEEE Commun. Mag., vol. 41, no. 6, pp. 116–119, Jun. 2003. [4] K. Gyoda and T. Ohira, “Design of electronically steerable passive array radiator (ESPAR) antennas,” in Proc. IEEE Antennas and Propagation Society Int. Symp., vol. 2, Jul. 2000, pp. 922–925. [5] R. Schlub, L. Junwei, and T. Ohira, “Frequency characteristics of the ESPAR antenna,” in Proc. Asia-Pacific Microwave Conf., vol. 2, Dec. 2001, pp. 697–700. [6] C. A. Balanis, Antenna Theory: Analysis and Design, 2nd ed., New York: Wiley, 1997. [7] A. Akiyama, K. Gyoda, T. Ohira, and M. Ando, “Numerical simulations on beam/null forming performance of ESPAR antennas,” Electronics and Communications in Japan (Part 1: Communications), ECJA. New York: Wiley, vol. 86, no. 12, pp. 1–11, Dec. 2003.

Antonis Kalis (M’00) received the electrical engineering diploma degree from the Electrical Engineering Department and the Ph.D. degree with honors from the University of Patras, Patras, Greece, in 1997 and 2002, respectively. He is an Assistant Professor with Athens Information Technology and an Adjunct Assistant Professor of Carnegie Mellon University Pittsburgh, PA, and has worked for the Lab of Electromagnetics at the University of Patras, participating in various R&D projects for the Greek Government and the European Union, as research staff. In 2000, he worked as a Research Engineer and an Assistant Research Unit Manager the Computer Technology Institute. His research interests are in the areas of radio communications, antenna design, and wireless networks. He has numerous journal and conference publications, and a U.S. patent. Dr. Kalis won the 2000 Chester Sall award of the Consumer Electronics Society. He is a member of the Technical Chamber of Greece and the AFCEA.

Michael J. Carras (M’04) was born in London, U.K., in 1982. He received the M. Eng. degree in information systems engineering from Imperial College, London, in 2003, where he is pursuing the Ph.D. degree. Since November 2003, he has been with Athens Information Technology, Athens, Greece, where he is a research scientist in the Broadband Wireless & Sensor Networks (B-WiSE) group. His key research interests are in the areas of wireless communications, including antennas, array processing, spreadspectrum, and MIMO systems.