# A Cost-Effective Wideband Switched Beam Antenna System for a Small Cell Base Station

TL;DR: The proposed system of the Butler matrix in conjunction with the constructed array can be utilized as a common RF front end in a wideband air interface for a small cell 5G application and beyond as it is capable to simultaneously cover all the commercial bands from 2 to 5 GHz.

Abstract: A wideband switched beam antenna array system operating from 2 to 5 GHz is presented. It is comprised of a $4\times 1$ Vivaldi antenna elements and a $4\times 4$ Butler matrix beamformer driven by a digitally controlled double-pole four-throw RF switch. The Butler matrix is implemented on a multilayer structure, using 90° hybrid couplers and 45° phase shifters. For the design of the coupler and phase shifter, we propose a unified methodology applied, but not limited, to elliptically shaped geometries. The multilayer realization enables us to avoid microstrip crossing and supports wideband operation of the beamforming network. To realize the Butler matrix, we introduce a step-by-step and stage-by-stage design methodology that enables accurate balance of the output weights at the antenna ports to achieve a stable beamforming performance. In this paper, we use a Vivaldi antenna element in a linear four-element array, since such element supports wideband and wide-scan angle operation. A soft condition in the form of corrugations is implemented around the periphery of the array, in order to reduce the edge effects. This technique improved the gain, the sidelobes, and helped to obtain back radiation suppression. Finally, impedance loading was also utilized in the two edge elements of the array to improve the active impedance. The proposed system of the Butler matrix in conjunction with the constructed array can be utilized as a common RF front end in a wideband air interface for a small cell 5G application and beyond as it is capable to simultaneously cover all the commercial bands from 2 to 5 GHz.

## Summary (2 min read)

### Introduction

- The system is operating from 2 to 5 GHz providing more than one octave of usable bandwidth.
- A hexagonal shaped approach was adopted in [18] and a wideband Butler matrix was designed, implemented and measured.
- Finally, the conclusions of this work are summarized in section V.

### II. DESIGN METHODOLOGY

- A. 4× 4 Butler matrix design Butler matrix is one of the most widespread analogue beamforming networks.
- Personal use is permitted, but republication/redistribution requires IEEE permission.
- An indicative layout geometry of the technology is illustrated in Fig. 3(b) where the continuous and dashed lines represent different layers.
- The first step is to design the individual subnetworks, which are the hybrid coupler and the phase shifter of the Butler matrix.

### C. Butler Matrix Network Implementation and Measurements

- After the implementation and verification of the two key components of the Butler matrix, the procedure continues with the implemented network verification.
- Furthermore, the distance between the output ports is set to be same as for the designed linear Vivaldi array that will be described in section III.
- Personal use is permitted, but republication/redistribution requires IEEE permission.
- To fully characterize the performance of the Butler matrix the phase difference between the output ports needs to be verified.
- Similar results for the magnitude and phase are verified for the remaining ports.

### III. LINEAR VIVALDI ARRAY DESIGN

- As an array element, an exponential tapered slot antenna (TSA) or Vivaldi [24] has been selected for its wideband and wide-scan performance.
- In this application only E-plane scan is utilized hence the motivation of the Vivaldi as a radiating element in this work.
- Personal use is permitted, but republication/redistribution requires IEEE permission.
- The edge elements were in turn matched to the new acquired impedance and tested to the Butler matrix output vectors.
- The simulated and measured active reflection coefficient of the array is depicted in Fig. 15.

### IV. SYSTEM ASSEMBLY AND PERFORMANCE

- At this point the two components have been successfully designed.
- In effect these cascaded S-parameters contain the active array impedance and the performance of the Butler matrix.
- Personal use is permitted, but republication/redistribution requires IEEE permission.
- The simulated and post processed measured results are in very good agreement.
- It is worth to note that the system is symmetric as was indicated from the measured and simulated beam patterns, see Fig. 17, and similar behavior is shown for the beams 1L and 2L.

### V. CONCLUSIONS

- The system is able to achieve four directional beams at all frequencies.
- The linear Vivaldi array offers improved wideband performance up to 6 GHz and can be utilized with a feed network in parallel with the proposed Butler matrix.
- Personal use is permitted, but republication/redistribution requires IEEE permission.
- This avoids any undesired microstrip cross sections and also improves the iterations of prototyping respectively.
- The authors have introduced a unified design methodology for such multilayer structures that can be expanded to any geometrical shape.

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##### Citations

15 citations

6 citations

### Cites background from "A Cost-Effective Wideband Switched ..."

...See https://www.ieee.org/publications/rights/index.html for more information. techniques include the use of Rotman Lens [27], Parallel plate lens [28], Butler Matrix [29], Luneburg lens [30], Phased array antenna [31], [32], and Parasitic controllable elements [33], [34]....

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...plate lens [28], Butler Matrix [29], Luneburg lens [30], Phased array antenna [31], [32], and Parasitic controllable...

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3 citations

3 citations

### Cites background from "A Cost-Effective Wideband Switched ..."

...Traditional beam switching techniques such as beamforming networks [19], [20], lens structures [21], [22] and phased arrays [23], [24], are effective....

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3 citations

##### References

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^{1}, University of Cassino

^{2}, Macquarie University

^{3}, Pompeu Fabra University

^{4}, Huawei

^{5}, Samsung

^{6}

6,462 citations

### "A Cost-Effective Wideband Switched ..." refers background in this paper

...Network densification [1] conjointly with heterogeneous networks will be in the heart of future wireless networks offering increased network capacity and spectral aggregation [2]....

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^{1}

1,270 citations

### "A Cost-Effective Wideband Switched ..." refers background in this paper

...Network densification [1] conjointly with heterogeneous networks will be in the heart of future wireless networks offering increased network capacity and spectral aggregation [2]....

[...]

714 citations

### "A Cost-Effective Wideband Switched ..." refers background in this paper

...Recent works [4], [5] have shown the potential of switched beam systems based on a Butler matrix [6]....

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...The number of couplers is equal to (N/2) log2(N) and the number of phase shifters equal to (N/2)[log2(N)−1] [6]....

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636 citations

### "A Cost-Effective Wideband Switched ..." refers methods in this paper

...characteristics, we employ a soft condition in the surrounding area of the array [25]....

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##### Frequently Asked Questions (2)

###### Q2. What are the future works in "A cost-effective wideband switched beam antenna system for a small cell base station" ?

A novel topology is proposed for an RF front end to be applied in small cell applications for the future generation wireless networks. The linear Vivaldi array offers improved wideband performance up to 6 GHz and can be utilized with a feed network in parallel with the proposed Butler matrix. The authors have introduced a unified design methodology for such multilayer structures that can be expanded to any geometrical shape.