AbstractâThe design and simulation of a wideband four-port steerable beamforming networks using a 4x4 Butler matrix for the 2.4 GHz applications is ...
Broadband 4x4 Butler Matrix Using Wideband 90∘ hybrid Couplers and Crossovers for Beamforming Networks Hamza Nachouane*, Abdellah Najid, Abdelwahed Tribak, Fatima Riouch
National Institute of Posts and Telecommunications Rabat, Morocco. {nachouane,najid,tribak,riouch}@inpt.ac.ma
Abstract—The design and simulation of a wideband four-port steerable beamforming networks using a 4x4 Butler matrix for the 2.4 GHz applications is presented. The aim of this work is to develop an antenna array feeding network based on Butler matrix with a large bandwidth in order to cover the IEEE 802.11b/g/n band. In order to achieve wideband features, the matrix uses two-section branch-line couplers and four-section branch-line crossovers instead of the standard ones. The proposed circuit exhibits a high isolation and wideband 4x4 Butler matrix designed and simulated by using a single-layer FR4 substrate, which covers the frequency band ranging from 2.1 GHz to 2.75 GHz. The return losses and the isolation are better than 28 dB and 30 dB, respectively, with its good performance is more suitable for IEEE 802.11b/g/n and ISM applications. Keywords-Beamforming Network ; Broadband Butler matrix ; Crossover ; Phased Array
I.INTRODUCTION Increasing the number of wireless communication systems users, in some areas very close or presence of obstacle, combined with the limited number of radio-frequency channels allocated by the Federal Communications Commission (FCC) represents a source of interference. Which reduces the transmission quality in wireless systems and limits their performances such as the cellular capacity or the frequency efficiency. To improve the quality of communication even in unfavourable conditions several techniques have been recently used, among them smart antenna arrays. The smart antenna systems can be divided into two categories: adaptive arrays, and switched beam system [1]. The first type is a device enabling to effectively reject interference since it adapts to the environment in real time to direct its beam in the desired directions using adaptive algorithms. However, it is complicated and requires a lot of signal processing than the later category. The second type is not as effective as the first because it does not use controllers; therefore it is simple and less expensive. A switched beam system produces multiple beams and selects from them the appropriate beam that gives the strangest signal level, to ensure mobile tracking in dynamic environment. This system switches from one beam to another as the mobile moves. Butler matrix is one of the well-known beamforming networks for use in switched-beam antenna systems. An NxN Butler matrix consists of N input ports and N output ports feeding N antennas where all paths between input and output
ports are equal. As there is an increased demand on wireless communications to provide high data throughput [2], it is necessary that the Butler matrix have to be operated over a wideband. The Butler matrix may be made using planar or waveguide technology. Using planar structure, the Butler matrix is composed by 3 main elements; 3-dB/90∘ quadrature couplers, crossovers and phase shifters. In order to achieve wideband features both wideband 3-dB couplers and crossovers are required. To achieve these characteristics, microstrip multi-section branch-line structures [3] have been employed. Several configurations of wideband Butler matrix have been recently reported in literature, using Conductor-Backed Coplanar Waveguide (CB-CPW) technology [4,5,6], Substrate Integrated Waveguide (SIW) technology [7,8], and SingleLayer planar technology [9,10]. In [9], the authors have introduced a Butler matrix using the standard 3-dB quadrature couplers and the four-section branch-line crossovers to obtain the wideband features. The reported result shows the proposed Butler matrix with a frequency bandwidth of 250 MHz, having a fraction bandwidth on the order of 13% around the central frequency of 1.9 GHz, the isolation is greater than 22.07 dB along with return losses less than 22.05 dB. The results presented in [10] indicate that the bandwidth can be achieved from 1.92 GHz to 2.17 GHz, the return losses and isolation were better than 23 dB and 26 dB, respectively. This paper presents the simulation results of the 4x4 Butler matrix operates over the frequency rang from 2.1 GHz to 2.75 GHz, which is 27.08% around the central frequency of 2.4 GHz, providing a good return losses and isolation which are better than 30 dB at the operating frequency. II.DESIGN CONFIGURATION The whole structure of 4x4 Butler matrix consists of four wideband hybrid couplers [11], two wideband crossovers [3], and two phase shifters, as shown in the layout below. First order microstrip line calculations were done manually [12] and then optimized using Agilent ADS line calculator [13]. All of the components assume FR-4 substrate with dielectric constant of 4.4 and thickness of 1.58 mm. The design simulation was performed using Momentum software of Agilent integrated into ADS software [13]. The choice of substrate is based on the ease of availability, low cost and low radiation losses. As the performance of the Butler matrix depends critically on the features of coupler and crossover,
they are simulated and optimized independently and then they are put together to form the entire architect of the Butler matrix using Electromagnetic Simulator Momentum, part of Agilent package.
wide bandwidth as best as possible. Its geometry is shown in Fig. 4.
(a)
(b)
Figure 2. Layouts of hybrids: (a) narrow band 3-dB hybrid; (b) wideband 3dB hybrid.
Figure 1. Layout of the 4x4 Butler matrix
(a)
A. 3-dB hybrid coupler The 3-dB hybrid coupler is the most significant part within the Butler matrix, since it is the most element exist in the structure, and also the crossover is obtained by cascading two couplers. Fig. 2 shows the standard and the broadband 3-dB hybrids. Using the two-section branch-line coupler we will be able to increase the bandwidth. The theoretical part is shown in [11]. The return losses, coupling and isolation of the designed hybrid coupler are shown in Fig. 3(a). The return losses S11 and isolation S41 are about 32.87 dB and 37.43 dB, respectively, around the resonant frequency. Therefore, it can be concluded that we got a good matching and a very good isolation around the operating frequency 2.4 GHz. The coupling is about 3 dB throughout the operating frequency band, which shows that the power is halved on both output ports. In terms of phase, the output signals on ports 2 and 3 are almost in phase quadrature. The phase difference is shown in Fig. 3(b), which is around 89.5∘ , throughout the bandwidth. B. Crossover As it is mentioned above, the combination of two broadband coupler results a broadband crossover [3]. The crossover is placed at the point where lines intersect to prevent the combination of signals. In a planar single-layer implementation of Butler matrix, crossovers must assure isolation between signals at the crossing lines. Therefore, the main role of our crossover is to guarantee isolation and reach a
(b) Figure 3. Momentum simulation of the proposed 3-dB hybrid coupler. (a) The coupler return loss S11, insertion losses (S31 and S21), and isolation S41 ; (b) phase difference between the output ports.
Figure 4. The geometry of the wideband crossover
The design and simulation are finished using ADS [13]. Fig. 5 shows the momentum simulated S-parameters of the proposed crossover. The isolation between the two input ports is greater than 27 dB, the return losses less than 25 dB, while
the isolation between Port 1 and Port 2 is better than 23 dB. It can be noted that the proposed crossover exhibits good features such as the wide bandwidth, the isolation between the input ports and between input/output ports, and an excellent matching around the resonant frequency. The coupling ratio is almost constant and around 0 dB over the bandwidth, which indicates that the power in the input Port 1 is totally transmitted to Port 3. The differences from theoretical values are acceptable.
(b) Figure 6. The layout and simulation of the proposed phase shifter. (a) the Layout ; (b) phase simulation.
S22,S33
S11,S44 Figure 5. Momentum simulation of the proposed crossover. S11: return loss, S41 : isolation, and S31 : insertion loss.
C. Phase Shifter The last part of our 4x4 Butler matrix design is to find how to implement phase shifters. The phase shifter is implemented using microstrip line. The transmission line introduces a phase !" shift θ = L, where L is the length of the line, λ is the !
wavelength on the substrate defined as: λ =
!! !!""
(a)
, where λ! is
the free space wavelength and ε!"" is the effective dielectric constant of the microstrip line [12]. Fig. 6 represents the layout and ADS simulation result of the phase shifter. Since the phase shift is implemented using simple transmission line therefore it is linearly frequency dependent, at the resonant frequency the phase is about 91.65∘ . D. 4x4 Butler Matrix Performances The described 3-dB hybrid coupler, crossover, and phase shifter are combined together so as to achieve a wideband 4x4 Butler matrix using single-layer microstrip line, as shown in Fig. 1, with a total area of 17.37×17.3cm. This structure was implemented and simulated using Momentum software of Agilent integrated into ADS software [13] based on Method of Moments (MoM). The momentum simulated S-parameters are shown in Fig. 7-8.
(a)
(b)
(c) Figure 7. Momentum simulated S-parameters of the 4x4 Butler matrix. (a) return losses ; (b) isolation of the input ports ; (c) insertion losses S51, S61, S71, and S81.
Fig. 7(a) shows the return losses of the four input ports. From these curves, it can be concluded that the bandwidth is about 650 MHz, which is 27.08% around the central frequency 2.4 GHz. The isolation of Port 1 when the other input ports are matched with a resistance of 50 Ω, is shown in Fig. 7(b). It can be noted that the isolation losses between input ports was better than 30 dB, which shows a relatively high isolation over such a wideband frequency band ranging from 2.01 GHz to 2.75 GHz. These results were more better compared to those found in [9] and [10]. Fig. 7(c) shows the insertion losses when Port 1 is fed. The simulated transmission coefficients are around -6 dB, except S51 and S61 which deviate slightly below the other coefficients from the ideal value of -6 dB, the value of 6 dB means that the power of the Port 1 has been divided equally between the four output ports. These errors are mainly due to the accumulation of errors coupling and phase errors of the four couplers and two crossovers constituting the matrix. The phases of output ports vary linearly with frequency. At resonance frequency, the phase of Port 5, Port 6, Port 7, and Port 8 are respectively 46.42∘ , 84.64∘ , 136.85∘ , and −177.46∘ , as shown in Fig. 8(a). These values differ from ideal values by 1.42∘ , 5.36∘ , 1.85∘ , and 2.54∘ , respectively. Therefore the matrix introduces an average error of 2.8∘ , that is much better than [4], [9] and [14]. Fig. 8(b) shows the simulated phase difference between two adjacent output ports when the Port 1 is fed. The theoretical value of relative phase difference is 45∘ . It is evident that in Fig. 8(b) the average errors of three phase differences are within 6.12∘ at the operating frequency compared with the desired value of 45∘ . Table 1 shows various performances of the proposed Butler matrix when the Port 1 is fed and a comparison between other Butler matrices cited in literature. The comparison exhibits clearly the supremacy of our design in terms of the bandwidth, the impedance matching, the isolation, and the phase errors which are major factors in our tender specifications. The other designs in literature were selected based primarily on the choice of the technology, i.e. the single-layer microstrip technology, and the operating frequency. The choice of a low cost substrate and the favorable results obtained prove the feasibility of such a matrix for beamforming in MIMO wireless communication systems.
(a)
-‐45° -‐45°
(b) Figure 8. Momentum simulated phase (a) and phase differences (b) between the output ports of the 4x4 Butler matrix.
III.CONCLUSION & PERSPECTIVE A novel wideband 4x4 Butler matrix in single layer microstrip technology has been studied, designed and simulated. The bandwidth achieved from 2.1 GHz to 2.75 GHz, which covers the IEEE 802.11b/g/n and ISM band. The large bandwidth of 27.08% is achieved by using broadband structures such as couplers and crossovers. Its good performance makes it suitable for use as a beamforming for multibeam antenna array for angle diversity in wireless communication systems, to reduce interference problem. The design needs to be verified experimentally. As it can be noted from Table 1, the proposed matrix exhibits good features in terms of bandwidth, isolation, phase errors and matching compared with other structures in literature. To complete this work, it is useful to include an antenna array to visualize the effect on the beams. Moreover, it is the main objective of the Butler matrix. This work will be also complete by associating a switching circuit to allow four beams with different distribution angles using PIN diodes. TABLE I.
PERFORMACES OF THE PROPOSED BUTLER MATRIX AND COMPARISON WITH OTHER DESIGNS IN LITERATURE.
Parameters
Proposed design
[9]
[10]
[14]
𝑆!! (dB)
-35
-22.05
-23
-33.7
∆𝑓 (MHz)
650
250
250
100
Isolation (dB)
-30
-22.07
-26
-48.59
Phase errors (deg)
4.41
1.75
13
8
Resonant frequency (GHz)
2.4
1.9
2
8.3
Size (cm)
17.37x17.3
27.94x11.43
28x14
5.37x4.67
ACKNOWLEDGEMENT The authors would like to thank Departamento de Ingenieria de Comunicaciones Escuela Técnica Superior de Ingenieros de Telecomunicacion, University of Cantabria (UNICAN) of their help in ADS simulations. REFERENCE [1] [2] [3] [4]
[5] [6] [7] [8]
[9]
[10] [11] [12] [13] [14]
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