Radial Line Slot Antenna Design for Femtocell Applications in 5G

M.J. López Morales1, A. López San Millán1, M. Sierra-Castañer1 1 Grupo de Radiación. Dpto. Señales, Sistemas y Radiocomunicaciones. ETSI Telecomunicación. Universidad Politécnica de

Madrid. 28040 Madrid, Spain.

Abstract—This paper shows the design of a radial line slot antenna for applications using the future 5G standard at millimeter waves. The antenna will be designed for multimedia high speed transmission using 20 GHz band in femtocells scenarios. The paper shows the design method for this application, the design of Radial Line Slot Antennas using this design method and the fabrication and measurement of a prototype at 20 GHz band to validate the proposed design. A design method for higher frequency bands is also proposed.

Index Terms—radial line slot antenna, 5G communication, antenna synthesis, radar, array antenna.

I. INTRODUCTION & OBJECTIVES The future communication standards 5G will use new frequency bands in order to enhance capacity and velocity in the wireless communications. New applications, as multimedia distribution, automotive, machine-to-machine or gaming, among others will appear in an intensive way [1]. This paper shows design procedures for antennas for multimedia transmission in near field communications, in order to distribute multimedia in indoor environments as the cabin of a vehicle, an access point in a library and so on. The paper shows a design procedure for the use of Radial Line Slot Antennas (RLSA) in this application, and the validation of the method with the fabrication of a prototype at 20 GHz frequency band. This kind of RLSA has been selected due to the low cost and easy fabrication. Also, these antennas can be easily placed on the walls or ceiling of any room. RLSA have been applied since 1980 for satellite applications, RADAR systems or other applications [3-5]. There are also applications in millimeter frequency bands showing very cheap fabrication costs [6]. RLSA is composed of two metal parallel plates with a dielectric between them, fed in the lower one by a coaxial and with slots in the upper one. M. Ettorre et al. presented in [7] a method for designing RLSA antennas for near field focusing. The paper is divided in the following sections: section II shows the antenna specifications, section III the design process, section IV the results of a prototype at 20 GHz band, section V the design concerns at higher frequencies and section VI the conclusions and future lines.

The authors want to acknowledge the Madrid Region Government project for financing the project Space Debris Radar (S2013/ICE-3000 SPADERADAR-CM) and the Spanish Government, for the support of the project ENABLING5G “Enabling Innovative Radio Technologies for 5G networks” (TEC2014-55735-C3-1-R)

II. ANTENNA ARCHITECTURE AND AMPLITUDE EXCITATION

The antenna architecture is based on a RLSA due to its simplicity. The polarization of the antenna is circular. The dimensions should be small enough in order to be integrated in the ceiling of a room with low cost. The work region will be in near field and it will work as a femtocell for 5G.

The antenna consists of a set of pair of slots, placed in an Archimedes spiral, and excited in the center by a coaxial feed. The slots are printed on a low loss substrate. The slots are separated a quarter of wavelength in order to create the circular polarization (Fig. 1).

Fig. 1: RLSA structure and radial slot pairs. As long as we are working on the near field range, we have

to perform a study of the necessary currents distribution in the aperture of the RLSA to obtain the desired NF propagation behavior. A study using circular continuous sources and the PWS with phase changes on it for simulating field propagations is performed. For this analysis, uniform phase in the aperture is analyzed. Both z-axis and E-field intensity in the zy-plane are represented to see which amplitude distribution is the most desirable. As well as this, the amplitude of the currents in the aperture, normalized to a certain input power, is shown.

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2Horizontal position [m]

0

0.5

1

1.5

2

2.5

Am

plitu

de o

f the

mag

netic

cur

rent

UniformTriangularWelchCosine -10dBCircular

Fig. 2: Analyzed amplitude distributions of the currents on the aperture.

10-3 10-2 10-1 100 101

Propagation distance [m]

-50

-40

-30

-20

-10

0

10

20

Elec

tric

Fie

ld In

tens

ity [d

BV/

m]

Cosine -10dBCircularTriangularUniformWelchwin

Fig. 3: E-field intensity in the z-axis for different current distributions.

Fig. 4: E-field intensity in zy-plane for different current distributions.

With this study it was concluded that the amplitude distribution that obtain a higher and more uniform field in the volume is the cosine window with pedestal of -10 dB.

III. OPTIMIZATION PROCESS OF SLOTS In order to design the desired antennas, an optimization algorithm based on two algorithms is used: firstly a global algorithm based on simulated annealing is used and secondly a local one based on a conjugate gradient is used [8]. With this, the number of necessary iterations for the first algorithm is reduced. Since the antenna is large enough, the RLSA does not have to be finished in short circuit or absorber. On the other hand, the substrate will just be extended.

In the design of these antennas it is important to assure that the antenna radiates most of the power. Therefore, in the situation treated in this paper the optimization function will have two components, the first one related to the spillover efficiency (to maximize it) and the second one related to the currents distribution in the aperture of the RLSA (to make it the most similar possible to the desired one). If the spillover efficiency were not included in the optimization function, the algorithm would give very small slots as a result. The error function to minimize in this work considers the amplitude difference respect the ideal currents, the phase difference and the spillover efficiency. Basically, the amplitude depends on the length of the slots, while the phase error depends on the separation between pairs of slots. The spillover error is

decreased when the slots are closest to the resonance length. However, there it is more difficult to adjust the other two parameters. The optimization function uses the analysis method presented in [9]. This algorithm is a simplified method of moments with only one base function per slot and a lossless substrate. However, for this application gives enough accuracy and it allow the optimization of medium size antennas in a reasonable time. As, it is presented in [10], if not high losses are included, the effect of those can be neglected. The output of the algorithm is the length of the slots and the position of each slot. In fact, the algorithm performs the optimization of two ideal axis (horizontal and vertical) and the position and lengths of the slots are interpolated according to the Archimedes spiral configuration.

IV. RESULTS OF A FIRST PROTOTYPE In order to validate the optimization process, a first

prototype has been designed. For this case, a working frequency of 20GHz has been selected, left hand circular polarization and a maximum diameter of 20cm diameter. The slots are printed on the upper side of a PTFE substrate, following the cosine distribution explained in Section II. The optimization process presented in Section III has been applied. Then, the near field radiated by the array of slots is calculated. Fig.6 shows the result of the optimization process: position and length of the slots. Fig. 5 shows the amplitude and phase on each slot. It is observed that the phase is uniform, while the amplitude has a relatively high ripple. However, it is really difficult to reduce this ripple in this RLSA. Fig. 6 shows the position and length of the control points of the algorithm (ideal positions in vertical and horizontal axes). The amplitude is the one necessary to get the cosine distribution, while the position is close to 0.95 wavelengths. The difference with respect one wavelength is due to the effects of the slots on the radiated and transmitted field in the RLSA. Once the RLSA has been optimized a prototype has been fabricated and measured. Fig. 7 shows the final prototype and Fig. 8 the radiation pattern has been measured in the Spherical Near Field System of the LEHA-UPM.

Fig. 5: Amplitude and Phase distribution on the slots.

Amplitude(linear) Phase(deg)

Slot Slot

0º

180º

-180º

0.6

1

0.2

Fig. 6: Position and length of the slots.

Fig. 7: Fabricated prototype at 20 GHz band. The measured losses are around 1dB (they depend slightly on the frequency), and the peak gain 26.4 dBi (at 19.3 GHz). The central frequency has been shifted slightly due to the fabrication process. However, in order to check the design process, the planar near field has been measured on the Planar Near Field System of the LEHA-UPM (correcting the effect of the probe). It is observed that a close uniform response is obtained at least in the size of the antenna (20 cm), as it was expected. Other cuts at different distances were done with similar results. These measurements validate the design process and confirm the validity of this RLSA for the near field application.

V. DESIGN OF THE ANTENNA AT HIGHER FREQUENCIES When the frequency design is very high, in order to keep the same aperture, the number of slots increases. The optimization process becomes very slow, even with the simplified analysis method, and the optimization time becomes unpractical.

Fig. 8: Far field radiation pattern.

Fig. 9: Near field pattern at 1.5 m. Therefore, it is necessary to use another method for designing RLSA with larger radius in terms of the signal wavelength. Fig. 6 has shown the results for the length and position of the slots. These results have been confirmed with other sizes of antennas. In the case of the position (phase variations), it is observed that the separation between laps/rings is almost constant and equal to 0.95λg. For the amplitude design, the process is based on the theorem of energy conservation in a pair of slots, where a physical approximation is carried out [9]. If the theorem of energy conservation in a pair of slots is used, the input energy has to be equal to the sum of the energy radiated by the pair of slots, the energy dissipated in the dielectric material and the energy transmitted to the next pair of slots. This concept is shown in Fig. 10.

Fig. 10: Energy conservation in a pair of slots scheme. If ∆r→0 (an acceptable approximation for relatively large

aperture radius), the differential equation that depends on the amplitude distribution of the currents in the aperture can be solved [9]. The solution of this differential equation and the correct calculations will give the necessary coupling depending on the radial position in the upper plate of the RLSA.

The coupling obtained in a pair of slots (1) can be expressed as:

τ r (r) =Wrad (Δr)Win (r)

= 2 ⋅αr (r) ⋅h (1)

where h is the height of the RLSA. Once, this coupling is calculated, the length of the slots can be obtained for a certain amplitude distribution, solving the differential equation obtained from the energy conservation scheme.

ddr

P(r) ⋅ r( ) = −2P(r) ⋅ r ⋅ αr (r)+αL[ ] (2)

When the coupling factor αr is expressed in terms of the amplitude distribution, the resulting equation is (3), that can be solved to calculate the coupling factor depending on the radial

Separationbetweentworings Slotlength

Controlpoint(xandyaxis)

1

1.1

0.9

0.8 Controlpoint(xandyaxis)

0.44

0.5

0.38

ΔR/λg L/λeff

position. The dependence of the length with the coupling factor can be calculated through simulations

(3)

(4)

(5) In that expression, P(r) is the power density and g(r) the amplitude distribution on the slots and αl the dielectric losses.

αr (r) =g2 (r) ⋅ r−2 ⋅X(r)

(6)

X(r)=e2αL⋅(ro−r )⋅ Xo+e−2αLro⋅ t⋅g2 (t)

ro

r∫ ⋅e2αLtdt⎡

⎣⎢⎤⎦⎥ (7)

In the case of the antenna presented in Section IV (20 GHz), the result is shown in Fig. 11. It is observed that the result is close to the one presented in Fig. 6, excluding the first part, where there are not physically slots (close to the coaxial feed). Another design has been performed for a 60 GHz (higher frequency) antenna with a 60 cm radius to see the result obtained with this method (Fig. 12). In this case, the maximum length is truncated to the resonance length of the slots.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10.3

0.32

0.34

0.36

0.38

0.4

0.42

0.44

r en m

L/la

mbd

a eff

Longitud de las ranuras en función del radio de la antena

Fig. 10: Slots length design depending on radial position for a cosine amplitude distribution for the RLSA designed in Section IV.

Fig. 11: Slots length design depending on radial position for a cosine amplitude distribution.

VI. CONCLUSIONS AND FUTURE LINES This paper has shown a design method of RLSA for near

field communications for the future 5G standard for high-speed transmission (multimedia). A prototype at 20 GHz has been fabricated to check the validity of the design method. For higher frequency bands and electrically larger antennas, an analytical design approach has been used. The results for the length of slots agree well with the ones obtained through the optimization process.

Another approach that could be used for designing these antennas for compact-range communication systems is based on changing the phase distribution of the currents in the aperture instead of changing only the amplitude distribution. By changing the phase distribution the wave will propagate in the directions perpendicular to the phase pattern, therefore increasing the coverage obtained with a single RLSA. Depending on the application, this second approach will be useful for wider coverage areas.

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[3] M. Ando, K. Sakurai, N. Goto, K. Arimura, and Y. Ito, “A radial line slot antenna for 12 GHz satellite TV reception,” IEEE Trans. Antennas Propag., vol. AP-33, no. 12, pp. 1347–1353, Dec. 1985.

[4] T. Yamamoto, T. C. Nguyen, M. Ando, N. Goto, M. Hirayama, T. Ohmi, “Design of Radial Line slot Antenna at 8.3 GHz for Large Area Uniform Plasma Generation,” Jpn.J.Appl.Phys., Vol.38, Part 1, No.4A, pp.2082-2088, April 1999. �

[5] M. Sierra-Castañer, M. Sierra-Pérez, M. Vera-Isasa and J. L. Fernández- Jambrina, “Low cost monopulse Radial Line Slot Antenna”, IEEE Trans. Antennas Propagat., vol. 51, pp. 256-263, 2003.

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[8] T. Salmerón-Ruiz, T. Díez-Ricondo, M. Sierra-Castañer “An optimization procedure for Radial Line Slot Antennas with arbitrary pattern”. Proceedings of 8th European Conference on Antennas and Propagation, EuCAP 2014. Den Haag (Netherlands). April 2014.

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[10] T. Nguyen, J. Hirokawa, Makoto Ando, M. Sierra Castañer, “Design of mm-Wave RLSAs with Lossy Waveguides by Slot Coupling Control Techniques. IEICE Trans. Commu., Vol.E98-B, No.09,Sep. 2015.