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Article  <  Archive  <  Home
Dual-Polarized Small Base Station Antenna Integrated RF Module Applicable to Various Cell Environments for Next-Generation Mobile Communication Service
Jung-Nam Lee, Yuro Lee, Bong-Hyuk Park, and Tag-Joong Kim
vol. 39, no. 3, June. 2017, pp. 383-389.
http://dx.doi.org/10.4218/etrij.17.0116.0382
Keywords : Next generation mobile communication service, dual-polarization, base station antenna, isolation, simple isolator, correlation coefficient

This is an Open Access article distributed under the term of Korea Open Government License (KOGL) Type 4: Source Indication + Commercial Use Prohibition + Change Prohibition (http://www.kogl.or.kr/news/dataView.do?dataIdx=97).
Manuscript received  June. 09, 2016;   revised  Jan. 09, 2017;   accepted  Mar. 08, 2017.  
  • Abstract
    • Abstract

      A small dual-polarized base station antenna with a simple isolation patch is presented. A high isolation is achieved when using a shorted metallic isolation patch. The experimental results indicate that the measured impedance bandwidth of the proposed antenna is 1.72 GHz to 1.89 GHz for small cell systems and that the isolation is more than 30 dB. The proposed antenna exhibits good radiation patterns with a peak gain of 8 dBi.
  • Authors
    • Authors

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_a001.jpg

      Corresponding Author jnlee77@etri.re.kr

      Jung-Nam Lee received his BS and MS degrees from the Department of Information and Communication Engineering, Hanbat National University, Daejeon, Rep. of Korea, in 2004 and 2006, respectively. He received his PhD degree in radio wave engineering from Hanbat National University in 2010. He then joined the Radio Core Technology Research Section of the ETRI, Daejeon, Rep. of Korea, where he is currently a senior member of the engineering staff. His research interests are small antennas, RFID antennas, UWB antennas, ESPAR antennas, beamforming antennas, and small base station antenna design.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_a002.jpg

      yurolee@etri.re.kr

      Yuro Lee received his BS and MS degrees in electrical engineering from Seoul City University, Rep. of Korea, in 1997 and 1999, respectively. Since 2001, he has been with ETRI, Daejeon, Rep. of Korea, where he is currently a principal research engineer. His current research interests are broadband wireless transmission technologies and 5G mobile communications.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_a003.jpg

      bhpark@etri.re.kr

      Bong-Hyuk Park received his BS degree in electrical engineering from Kyungpook National University, Daegu, Rep. of Korea, in 1996, his MS degree in mechatronics from the Institute Science and Technology, Gwangju, Rep. of Korea, in 1998, and his PhD degree in electrical engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 2010. From 1998 to 1999, he was an RF application engineer with Ansoft Corporation. Since 1999, he has been with the ETRI, Daejeon, Rep. of Korea. His main research interests are ultrawideband (UWB) RF transceiver front-end circuit design, fractional-N phase-locked loop (PLL) design, system-level integration of transceivers, and 5G RF technology.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_a004.jpg

      aisma@etri.re.kr

      Tae-Joong Kim received his BS, MS, and PhD degrees from Yonsei University, Seoul, Rep. of Korea in 1991, 1993, and 1998, respectively. From 1998 to 2000, he was a senior researcher at ETRI, Daejeon, Rep. of Korea. He joined Eonex Ltd. in 2001, where he worked first on chipset system design and then on software protocol stacks and field tests until 2006. He returned to ETRI in 2006 and developed 4G-LTE and 5G mobile communication systems. He is currently the managing director of the Mobile Transmission Research Department at ETRI, managing 5G projects sponsored by the government.

  • Full Text
    • I. Introduction

      Several methods have been studied to increase the capacity of communication services, expand frequency resources, develop advanced communication services such as the 4G and Beyond 4G and 5G techniques [1][4], and shorten the distance between the antenna and user to reduce the path loss. Although various advanced technologies such as active array antennas [5][9], MIMO [10][14], and beam-forming techniques [15][17] have been developed, they have not been able to keep up with the increasing speed of mobile traffic capacity.

      Therefore, the next-generation mobile communication base station (BS) system separates the radio frequency unit (RFU) from the base band unit and integrates the BS antenna and RFU to minimize the cable loss between the BS antenna and the system, thus minimizing the power loss. It is important to develop a functionally distributed base station that is applicable to a variety of cell environments (indoor cells, small cells, macro cells, and so on) using a module-type (BS module) BS antenna and RFU. A BS module has the advantages of easy installation and repair, small power loss, energy savings, and simple implementation of the next-generation mobile communication BS system.

      The current mobile communication BS system has the disadvantage of high power consumption because it uses a passive element array antenna structure [18][21] with 10% to 20% efficiency. Existing array antennas use a 3-dB power divider to distribute the input power to each antenna element. As each power divider has a constant loss, the antenna efficiency decreases. Higher power is used to compensate for these reduced efficiencies. In addition, the maintenance costs increase when replacing the entire base station if a failure occurs in the BS antenna. However, a next-generation mobile communication BS using a BS module can be repaired by replacing just the failed BS module instead of replacing the entire system.

      The core technology of a BS module is the antenna. A cube antenna [22][27] is considered for application to a variety of cell environments and is easy to replace. However, a cube antenna has certain problems that have to be overcome. First, when the BS antenna is enveloped in a metal-type cube, the antenna boundary conditions are changed, and the antenna characteristics (bandwidth and antenna gain) deteriorate. Second, a small-sized base station is necessary for application to a variety of cells. The BS antenna size will determine the size of the entire base station. It is very difficult to design a small-sized dual-polarized antenna for a limited metal cube space.

      In this paper, we present the design of a small dual-polarized BS antenna with a simple isolator applicable to various cell environments for next-generation mobile communication services. It is not possible to place a conventional dual-polarized BS antenna in a metal cube owing to its large size and because of its complex structure that ensures high isolation [28][32]. High isolation is achieved using a simple isolation patch. The proposed BS antenna has a small size, dual polarization, wide bandwidth, high isolation, and high antenna gain despite being enveloped in a metal cube. We improved the impedance matching and bandwidth using a coupled feed method.

      The BS antenna is operated at 1.72 GHz to 1.89 GHz, and the measured isolation is greater than 30 dB at the operating frequency band. The measured antenna peak gain is 8 dBi. We propose improved methods for the isolation between the two BS modules for the MIMO base station system. The simulated isolation is over 50 dB between the two BS antennas.

      II. Design of a Small Dual-Polarized Base Station Antenna

      1. Antenna Design

      We designed and implemented a small dual-polarized BS antenna, as shown in Fig. 1. The size of the metal cube is 100 mm × 100 mm × 30 mm, and the antenna element size is 80 mm × 80 mm × 15 mm. The microstrip (MS) coupled feed lines and simple isolation patch are placed below the MS radiation patch. The small BS antenna consists of a bottom PCB (Taconic TLY-5A, εr: 2.17), MS coupled feed lines, two metallic feeding posts (4.5 mm × 4.5 mm × 15 mm), a shorted metallic isolation post, a top PCB (FR-4, εr: 4.4), and a metal cube. The MS rectangular radiator is placed on the top plane, and the MS coupled feed lines and simple isolation patch are placed on the bottom plane of the top PCB. The MS coupled feed lines and metallic feeding posts are combined with the microstrip line printed on the bottom of the PCB. MS feeding lines were designed to have a 50 Ω impedance. The simple isolation patch is positioned at the bottom plane of the top PCB and is connected to the substrate ground plane using a metallic isolation post.

      Fig. 1.

      Geometry of proposed small BS antenna: (a) entire antenna structure, (b) top and side planes, and (c) photograph of antenna.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f001.jpg

      2. Measured and Simulated Results

      The measured reflection coefficient and isolation of the BS antenna, with and without the metal cube, are shown in Fig. 2(a). The proposed BS antenna was tested using an Anritsu Vector Network Analyzer (MS46122A) in an anechoic chamber. The BS antenna without a metallic cube was operated at 1.9 GH2 to 2.2 GH2 and did not satisfy the operating frequency band. Figure 2(b) shows the reflection coefficient and isolation with the metal cube. The proposed BS antenna was experimented on with and without a simple isolation patch. As shown in the figure, the proposed antenna was tested with and without a simple isolation patch. The measured frequency is 1.72 MHz to 1.89 MHz (port-1 and -2) with and without a simple isolation patch, respectively. When a simple isolation patch is used, the proposed antenna has a high isolation value.

      Fig. 2.

      Measured results of proposed BS antenna: (a) without metallic cube and (b) with metallic cube.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f002.jpg

      The measured port-to-port isolation of the small BS antenna is greater than 30 dB over the operating frequency band (max = 45 dB). The simulations were implemented in perfect free space (vacuum), and no loss of measured cable or manufacturing losses were considered. However, the prototype antenna was operated in real free space, and there were production and measured cable losses. Therefore, the simulation and measurement results differed slightly. However, the simulated results are in reasonable agreement with the measured results.

      The measured and simulated radiation patterns of the BS antenna with a metal cube are shown in Fig. 3. As shown in the figure, both the cross polarization and back lobe are suppressed using a metal cube. Measured BS antenna peak gains of 8.0 dBi at 1.735 GHz, 7.9 dBi at 1.755 GHz, 7.8 dBi at 1.83 GHz, and 7.8 dBi at 1.85 GHz were obtained. The measured 3-dB beamwidths in the yz- and xz-planes were 100º at all operating frequencies.

      Fig. 3.

      Measured and simulated radiation patterns: (a) yz-plane at port-1, (b) yz-plane at port-2, (c) xz-plane at port-1, and (d) xz-plane at port-2.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f003.jpg

      We simulated the effects of the simple isolator with an excited E-field distribution at 1.83 GHz. The E-field distribution was simulated using ANSYS HFSS. As shown in Fig. 4(a), when not using the simple isolator, the E-field excited at port-1 affects port-2. When using the simple isolator, the E-field is mostly focused at the isolator. As shown in Fig. 2(b), the results confirm high isolation characteristics with the simple isolator.

      Fig. 4.

      Simulated E-field distribution at 1.83 GHz: (a) without and (b) with simple isolator.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f004.jpg

      III. Inter-module Improving Isolation Techniques for MIMO System Configurations

      A small cell base station is composed of a power supply part, an IP based module, and a BS module, as shown in Fig. 5. The BS module is composed of a dual-polarized BS antenna and an RF module. The dual-polarized BS antenna is located on the top plane, and the RF module is integrated at the bottom plane.

      The RF module consists of a front-end part, a PA, and a low-noise amplifier (LNA). The front-end part consists of a transmitter-receiver RF filter, a circulator, and an LNA. As shown in Fig. 5, the BS antennas are located in close proximity to each other. A signal excited from each BS antenna will significantly decrease the performance of the transmitter and receiver. It is important to suppress the leakage wave to improve the isolation between the BS antennas.

      Fig. 5.

      Small cell base station configuration.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f005.jpg

      In particular, a strong leakage wave is transmitted in the feeding portion. As shown in Fig. 6, the BS antennas are arranged with different feeding directions, and the best isolation is chosen. We conducted a simulation with four vertical arrangements. The isolation values were compared to select a high isolation position. The distance between the BS antennas was lamda at 1.83 GHz.

      Fig. 6.

      BS antenna position setup: (a) case-1, (b) case-2, (c) case-3, and (d) case-4.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f006.jpg

      Figure 7 shows the simulated isolations for the four cases. We conducted simulations for S31, S41, S32, and S42 to confirm the isolation between the BS antennas. The isolation values were simulated to be over 30 dB at all positions. The BS antenna position for case-2 showed a high isolation value, and thus, case-2 was chosen.

      Figure 8 shows the simulated and calculated results when the case-2 arrangement is used. We applied an aluminum supporting plate to the bottom plane of the BS antennas. We improved the isolation by placing a metal isolator stub between the BS antennas. Figure 8(b) shows the values of the simulated isolation when using a metal isolator stub. Without the metal isolator stub, as shown in Fig. 7(b), the average isolation value is about 38 dB at S31, S41, S32, and S42. However, with the metal isolator stub, the average isolation value is approximately 50 dB at S31, S41, S32, and S42.

      Fig. 7.

      Simulated isolation results for four cases: (a) case-1, (b) case-2, (c) case-3, and (d) case-4.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f007.jpg
      Fig. 8.

      Base station configuration using an isolator for 2 × 2 MIMO system: (a) structure, (b) simulated isolation, and (c) calculated correlation coefficient.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f008.jpg

      A correlation coefficient was used to confirm the interference between the antennas. An acceptable correlation coefficient for a typical MIMO system is less than 0.5 [33]. In this study, the correlation coefficient was calculated using the scattering parameter [34], [35] because this method provides sufficiently accurate results and a simple calculation.

      ρ e = | S 11 * S 12 + S 21 * S 22 | 2 ( 1 ( | S 11 | 2 + | S 21 | 2 ) ) ( 1 ( | S 22 | 2 + | S 12 | 2 ) ) .

      Figure 8(c) shows the calculated correlation coefficient using ANSYS HFSS and MATLAB. The correlation coefficients are less than 0.05, and the two BS antennas are highly decorrelated.

      IV. Conclusion

      A small dual-polarized BS antenna integrated RF module applicable to various cell environments for next-generation mobile communication services was designed and tested. We tested the proposed BS antenna with and without a simple isolator. When using the simple isolator, the measured isolation value was greater than 30 dB. We located a metal isolator stub between two BS antennas, and the isolation between the two BS antennas was substantially improved. A wide bandwidth, high isolation, low cross-polarization, high gain, and high decorrelation were achieved.

      Footnotes

      Jung-Nam Lee (corresponding author, jnlee77@etri.re.kr), Yuro Lee (yurolee@etri.re.kr), Bong-Hyuk Park (bhpark@etri.re.kr), and Tae-Joong Kim (aisma@etri.re.kr) are with the Mobile Transmission Research Department, ETRI, Daejeon, Rep. of Korea.

      This work was supported by an Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korean government (MSIP) (No. R2014-0-00282, Development of 5G Mobile Communication Technologies for Hyper-Connected Smart Services).

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      Fig. 1.

      Geometry of proposed small BS antenna: (a) entire antenna structure, (b) top and side planes, and (c) photograph of antenna.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f001.jpg
      Fig. 2.

      Measured results of proposed BS antenna: (a) without metallic cube and (b) with metallic cube.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f002.jpg
      Fig. 3.

      Measured and simulated radiation patterns: (a) yz-plane at port-1, (b) yz-plane at port-2, (c) xz-plane at port-1, and (d) xz-plane at port-2.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f003.jpg
      Fig. 4.

      Simulated E-field distribution at 1.83 GHz: (a) without and (b) with simple isolator.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f004.jpg
      Fig. 5.

      Small cell base station configuration.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f005.jpg
      Fig. 6.

      BS antenna position setup: (a) case-1, (b) case-2, (c) case-3, and (d) case-4.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f006.jpg
      Fig. 7.

      Simulated isolation results for four cases: (a) case-1, (b) case-2, (c) case-3, and (d) case-4.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f007.jpg
      Fig. 8.

      Base station configuration using an isolator for 2 × 2 MIMO system: (a) structure, (b) simulated isolation, and (c) calculated correlation coefficient.

      images/2017/v39n3/ETRI_J001_2017_v39n3_383_f008.jpg