MIMO Antenna System for 4G Terminals. Mohammad S. Sharawi, Yanal S. Faouri, and Sheikh S. Iqbal. Electrical Engineering Department. King Fahd University ...
Proceedings of the 6th German Microwave Conference
Design and Fabrication of a Dual Electrically Small MIMO Antenna System for 4G Terminals Mohammad S. Sharawi, Yanal S. Faouri, and Sheikh S. Iqbal Electrical Engineering Department King Fahd University for Petroleum and Minerals Dhahran, 31261 Saudi Arabia msharawi,sheikhsi @kfupm.edu.sa
Abstract—Multiple-input-multiple-output (MIMO) technology will be used by fourth generation mobile networks (also called Long Term Evolution -LTE) to achieve very high data rates in both the uplink and downlink channels. MIMO is based on the use of multiple antenna systems within the mobile terminal as well as the base station. Such antenna systems are required to fit within the hand-held (mobile) terminal which occupies a small size (typically not more than 60 100 mm2 ). Antenna integration and miniaturization are two major challenges. We propose an electrically small antenna (ESA) that is based on the meander antenna structure that operates in the 800 MHz band of LTE and 3G cellular standards. The antenna has a measured center frequency of 897 MHz, bandwidth of 185 MHz and total size of 23.5 43 mm2 . In addition, we present the design of a dual element MIMO antenna system based on the ESA antenna designed. The dual element MIMO antenna system covers the bands from 760 - 886 MHz and occupies an area of 40 50 mm2 . Both simulation and experiment results from the fabricated antennas are presented. Index Terms - Electrically Small Antennas, Meander line, LTE.
The term that is given to fourth generation mobile communication networks is long term evolution (LTE). LTE will be internet protocol (IP) based and will provide high throughput, broader bandwidth and better handoff capabilities than current third generation networks. Theoretical peak data rates of 300 Mbps and 75 Mbps for the downlink and uplink channels are expected with the use of the enabling technologies [1], [2]. LTE will support two modes of operation, time division duplexing (TDD) and frequency division duplexing (FDD). TDD is not as attractive to mobile operators as FDD mode. In FDD mode, several high and low frequency bands are covered. The 700 MHz bands represent the low frequency bands while the 2600 MHz bands represent the high frequency ones. LTE supports channel bandwidths up to 20 MHz per channel (i.e. uplink or downlink) [3]. MIMO antenna design for the lower frequency bands poses some design challenges in the antenna portion of the mobile terminal due to size limitations. The integration of more than one antenna in each mobile terminal is a challenging task. Electrically small antennas (ESA) are antennas that can be inclosed within a radian sphere, meaning that the relationship ka 1, where k 2λπ and a is the largest diameter of the circle
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inclosing the complete antenna, has to be satisfied [4]. ESAs have high input reactance and low input resistance. Therefore, they have high Quality factor (Q) and low frequency bandwidth. In [5], an expression for the Q was derived and is given by, 1 1 3 (1) Q ka ka While in [6] an expression for the maximum expected gain from an ESA based on its dimensions was determined. This maximum gain is given by, Gainmax
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It is worth mentioning that both equations (1) and (2) do not consider the presence of a ground plane near the antenna. In [7], empirical equations were used to come up with a meander line antenna (MLA) that has a resonant frequency of 1 GHz. The equations will give a prohibitively large antenna if used for the 800 MHz band. While in [8] a spiral-like printed antenna that was electrically small was proposed to operate in the 700 MHz range with a size of 40 40 mm2 . The antenna had an extremely narrow bandwidth that will not be suitable for the application at hand that requires at least 40 MHz of bandwidth to cover the downlink and uplink in LTE channels. A multi-band printed bow-tie antenna was proposed in [9] to cover the 800 MHz and 1.9 GHz bands. The size of such an antenna will occupy the whole mobile terminal size since it covers a board area of 130 77 mm2 . While in [10] a dual band ESA was proposed that covered 800MHz and 2GHz bands with a size of 25 43 mm2 . The antenna suffers from narrow bandwidth in both bands. Thus, previous work shows that the proposed antennas will not be suitable for LTE mobile terminals due to their large size or narrow operating bandwidth. In this work, we design and fabricate a single MLA with a center frequency around 850 MHz, bandwidth of at least 100 MHz and total size of 23.5 43 mm2 . In addition, we integrate two of the single antennas on a single substrate of the size of 40 50 mm2 with a ground strip separator to form a dual element MIMO antenna system. The system covers the frequencies from 760 - 886 MHz. The simulation and
14–16 March 2011, Darmstadt, Germany
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Geometry of the single Meander Line ESA. Fig. 2.
measurement results of the resonance frequencies, bandwidths and radiation patterns are presented. I. M ODELING OF THE A NTENNAS A meander line antenna shrinks the electrical length of a regular monopole or dipole antenna by folding its length back and forth to create a structure with multiple turns. This method has advantages when antennas with low frequency of operation are of interest, since this will reduce the size of the antenna significantly. The size of the antenna will even get smaller because of the use of a dielectric substrate. Printed meander antennas usually have good radiation efficiency and close to omni-directional radiation patterns. The designed single Meander antenna structure is shown in Figure 1. Antenna dimensions were optimized using HFSST M . The dimensions of the antenna are in mm and given by, L 43, W 23 5, Lg 16 2, W 1 15 5, W 2 1 65, W 3 1, W 4 1, W 5 1, L1 12 27 and L2 5 93. The antenna was etched on an FR-4 substrate with 1.56 mm thickness. 1 oz. copper was used. A right angle PCB mount SMA connector was used as the feeding port for the antenna. The antenna size provided consumes 1/4 of that of a regular mobile terminal. This is of great importance since LTE handsets are to provide MIMO capability, and thus multiple antennas are to be squeezed in a very small area. The dual element MIMO system designed followed the geometry in Figure 2. Antenna dimensions were optimized using HFSST M and the separating GND strips were used for improving the isolation later on. The dimensions of the antennas in mm are given by, L 40, W 50, Lg 17, W 1 16 5, W 2 1 5, W 3 1 05, W 4 1 05, W 5 1 05, W6 1, W 7 1, L1 19 and L2 5. The edge of the meander from the GND arms was 4 mm while its distance from the sides of the substrate was 2 mm. Same substrate material and thickness were used as the single meander design.
Geometry of the dual Meander Line MIMO system.
antenna is 2.23 cm. The factor ka 0 79 which shows that this is an ESA as per the definition in [4]. Figure 4 shows the measured and simulated reflection coefficients. An HP 8514B Network Analyzer was used to conduct this measurement. The correlation between the two is very well observed. The simulated fc was 850 MHz, while the measured one was 897 MHz. The simulated -10 dB bandwidth was 234 MHz while the measured one was 185 MHz. Using equation (1) gives Q 3 29 which in turns gives a bandwidth of 182 MHz. This shows a good match between the two, although some discrepancy is expected due to the presence of the GND plane. The MLA total size is 23.5 43 mm2 . Figure 5 shows the fabricated dual element MIMO antenna along with its reflection coefficient performance. A 50 Ω termination was used on the other port during the Sii measurement, where i 1 2 . Antenna 1 resonated with a center frequency of 840 MHz and a bandwidth of 155 MHz. While antenna 2 had a center frequency of 820 MHz and a bandwidth of 146 MHz. The 2.4% difference between the center frequencies of the two antennas and the 5.5% difference of the measured bandwidths might be attributed to the tolerances of the SMA connectors used, the termination resistance as well as the hand soldering that was performed. The measured radiation patterns for the single element antenna are shown in Figures 6-9. The patterns were acquired in a laboratory environment using a yagi antenna as the
II. R ESULTS AND D ISCUSSION Figure 3 shows the top and bottom views of the fabricated single MLA antenna. The radius of the sphere inclosing this
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Fabricated MLA antenna.
Single MLA vs. Reference Dipole, AZ plane, θ=90o
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Fig. 6. Single element measured radiation pattern in dB (MLA vs. Dipole). Azimuth plane (θ 90o ), x-y plane.
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Fig. 5. Reflection coefficient measurements of the fabricated the 2 1 MIMO antenna. (a) Top side of fabricated antenna, (b) bottom side.
Fig. 7. Single element measured radiation pattern in dB. Elevation plane (φ 0o ), y-z plane..
transmitter, and the reference antenna (as well as the designed antenna) as the receiver. The antenna separation was 1.75 m, 2 which is more than 2D λ at 800 MHz. The antenna heights were about 1.7 m above the ground. A reference dipole antenna was used to validate the measurement setup configuration. Then the fabricated antenna was tested. The Azimuth (AZ) plane and Elevation (EL) plane radiation patterns were recorded. Figure 6 shows the AZ (x-y plane in Figure 1) plane cut for the MLA as well as the reference dipole antenna in dB at θ 90o . It is evident that the MLA will have a higher gain than the reference dipole, but the metal post that was used to hold the MLA was different than that holding the dipole, as the dipole had a pre-configured holding post. The MLA holding post had a L-shaped metal edge that is believed to have added to the GND plane of the antenna, and thus would add to the front directivity observed in Figure 5, which explains the large gain observed (about 6 dB, while the reference dipole antenna was about 1.9 dB). Also, comparing the measurement values with that of equation (2), the measurements also show higher gain values that the maximum theoretical limit. This is due to the fact the that theoretical equation does not take the GND
plane effect into account. The radiation pattern in AZ shows that this antenna will provide good coverage in the x-y plane (see Figure 1). Figure 7, shows the EL (x-z plane) plane of the MLA. The radiation pattern also shows that the coverage of the antenna when held in the talking position within a mobile phone, will have a peak between 30o and 90o in EL, which is where we expect the base-station incoming wave is coming from. This is also the y-z plane pattern in Figure 1. The maximum gain level is a little less than the x-y plane radiation because the L-shaped edge is below the antenna in this testing position, and thus had a minimal effect on the measurement. A maximum gain value of about 4 dB is expected in the elevation plane cut around 65o . The gain was always calculated using the gain substitution method with the dipole antenna used as the reference. The azimuth (x-y plane, i.e. θ 90o ) co-polarized radiation pattern of MIMO antenna system is shown in Figure 8. Also, in blue color, a reference dipole antenna of known maximum gain was used to find the gain of the MIMO antenna system. The dipole had a maximum gain of about 1.9 dBi. The maximum gain of the MIMO antenna system was approximately 2.2 dBi
Dual MLA, AZ plane, Co−Pol, θ=90o
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The antenna performance parameters are very well suited for operation in the 800-900 MHz band of the 3G and 4G cellular handsets. The radiation patterns, and antenna size satisfy the requirements of both standards, and the compact size of the proposed MLA antenna allows its use as a MIMO antenna system since a single element only occupies 1/4 of the mobile phone size (assuming a 4 10 cm2 standard phone size). It is worth mentioning that the proposed antenna is 40% less in area size than that in [8].
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Fig. 8. Measured azimuth (x-y plane) radiation patterns of the MIMO antenna system at 800 MHz along with a reference dipole antenna (blue), antenna 1 (solid) and antenna 2 (dashed).
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Fig. 9. Measured elevation (x-z plane) radiation patterns of the MIMO antenna system at 800 MHz, antenna 1 (solid) and antenna 2 (dashed).
and the radiation patterns of both antennas followed the same trends. The obtained radiation pattern shows good coverage around the antenna with some dips due to the presence of the feeding ports (backwards), or when at 90o when the non-radiating antenna is facing the transmit antenna. These dips in the radiation pattern are not believed to affect the coverage of the antenna system as they represent the bottom of the hand-held terminal and the far opposite side of the operating/radiating antenna. The elevation plane (z-x plane) co-polarized radiation patterns for the MIMO antenna system are shown in Figure 9. The two elements show identical behavior with very good coverage in front of the MIMO system (the case when the user is holding the phone closer to his/her ear). The maximum gain is approximately 2 dBi. Since one antenna is active at a time, the radiation pattern shows some directive behavior between 25 - 90 degrees. The back radiation is more than 5 dB lower due to the presence of the GND arms. This is an advantage, as we want to minimize the radiation towards the head of the user.
A compact electrically small antenna (ESA) design and fabrication that is based on the meander antenna is presented. The antenna is intended for the use in the 800-900 MHz of the LTE standard for mobile handsets. Simulation and measurement results are compared. A dual element MIMO antenna system is also fabricated to operate in the same band using the same compact element presented. Antenna radiation patterns are shown for both planes; the Elevation plane and Azimuth plane. The antenna size allows for its integration as a 2 2 MIMO antenna system within a 4 10 cm2 handset size for 4G mobile standards. The single antenna has a measured center frequency of 897 MHz, bandwidth of 185 MHz and total size of 23.5 43 mm2 , while the dual antennas cover the frequency band from 760 - 886 MHz and occupies an area of 40 50 mm2 . ACKNOWLEDGEMENT The authors would like to thank King Fahd University for Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia, for supporting this research through grant number SB100009 for the year 2010-2011. R EFERENCES [1] V. K. Garg, Wireless Communications and Networking, Elsevier-Morgan Kaufmann, 2007. [2] A. Larmo, et. al., “The LTE Link-Layer Design,” IEEE Commun. Mag., Vol. 47, No. 4, pp. 52-59, April 2009. [3] 3GPP TS 36.101, V8.3.0, “EUTRA User Equipment Radio Transmission and Reception,” September 2008. [4] H. A. Wheeler, “Fundamental Limits of Small Antennas,” Proceedings of The I.R.E., pp. 1479-1484, December 1947. [5] J. McLean, “A Re-Examination of the Fundamental Limits on the Radiation Q of Electrically Small Antennas,” IEEE Transactions on Antennas and Propagation, Vol. 44, No. 5, pp. 672-675, May 1996. [6] Roger F. Harrington, “Effect of Antenna Size on Gain, Bandwidth, and Efficiency,” Journal of Research of the National Bureau of StandardsD, Radio Propagation, Vol. 64D, No. 1, p. 112, January 1960. [7] Alok Singh, et. al., “Empirical Relation for Designing the Meander Line Antenna,” IEEE International conference on recent advances in Microwave Theory and Applications, pp. 695-697, Jaipur, India, Nov. 2008. [8] H. K. Kan and R. B. Waterhouse, “Shorted Spiral-like Printed Antennas,” IEEE Transactions on Antennas and Propagation, Vo. 50, No. 3, pp. 396397, March 2002. [9] G. Wang, et. al., “Coaxial-Fed Double-Sided Bow-Tie Antenna for GSM/CDMA and 3G/WLAN Communications,” IEEE Transactions on Antennas and Propagation, Vo. 56, No. 8, pp. 2739-2742, August 2008. [10] H. Urabe, et. al., “Design and Performance of an 800MHz/2GHz Dual Band Small Planar Antenna,” IEEE Asia Pacific Microwave Conference, pp. 2671-2674, Singapore, December 2009.
