Compact Metamaterial Antenna Array for Long Term Evolution (LTE) Handset Application

Norberto Lopez, Cheng-Jung Lee, Ajay Gummalla and Maha Achour*

Rayspan Corporation, 11975 El Camino Real, Suite 301, San Diego, CA 92130

(*) Presenter Email: maha@rayspan.com

ABSTRACT: In the digital world, Moore’s Law states the number of transistors on Integrated Circuits (ICs) has been doubling every two years since 1965. While these ICs occupy only 10% of the wireless communication device area, the remaining 90%, referred to as RF Front-End Model (FEM), consists of a collection of discrete passive and active components such as antennas, filters, diplexer, duplexers, couplers, power combiner/splitter, and power amplifiers. While these powerful ICs enable effective implementation of Multiple Input Multiple Output (MIMO) digital processing, the main issue of MIMO RF FEM implementation in small devices such as handsets still remains unsolved. Without such a full solution, network providers cannot deploy next generation wireless broadband networks, such as 3GPP Long Term Evolution (LTE) networks, that can sustain tens of Mbps throughput with mobility. This paper addresses this specific problem by presenting an LTE MIMO air interface solution for handsets using metamaterial designs, which offer small, low-cost, and low-profile antennas printed directly on PCB for easy integration and simple manufacturing – all critical factors for rapid deployment and commercial success. The proposed metamaterial MIMO array consists of dual resonance antennas occupying λ0/10 × λ0/41 × λ0/387 volume at center frequency 771MHz of the LTE band 746-796MHz. The performances of two antenna array configurations with spacing of λ0/13 and λ0/5 are studied while quantifying its near-filed and far-field channel correlation. INTRODUCTION Next generation wireless devices will require multiple antennas to coexist in a small area while maintaining their low coupling to support multipath channel de-correlation. Multiple Input and Multiple Output (MIMO) technology is the most promising, if not the last frontier, in the evolution of wireless broadband access networks [1]. It allows signals from multiple antennas to exploit multipath environments to support higher channel capacity, better network coverage, and increased link reliability. 3GPP LTE is at the forefront of next generation of wide area cellular specification [2]. It uses wideband channels and advanced modulation techniques to support 100Mbps downlink and 50Mbps uplink peaks with less than 10 msec low latency using up to 4x4 MIMO systems. LTE frequency bands cover 746-796MHz, 880-960MHz, 1710-2155MHz, and 2300-2390MHz. In this paper we consider the lowest 746-796MHz band. At these low frequencies, implementing multiple antennas in a handheld device poses significant challenge in term of high antenna radiation efficiency on small ground plane, high isolation, and low far-field envelope correlation between antennas. Metamaterial is a novel man-made structure where its electromagnetic properties possess some unique behaviors [3]. Many researchers have demonstrated that metamaterial structure can significantly reduce the circuit size while maintaining the same or better performances [3-6]. Metamaterial structures have the ability to concentrate electromagnetic fields and currents near antenna structures instead of spreading them along antenna ground causing higher coupling between antennas. This allows compact antenna arrays to be realized with minimal mutual coupling to be able to de-correlate multipath channels in MIMO implementation. In this paper, a dual resonance metamaterial antenna which occupies a volume of λ0/10 x λ0/41 x λ0/387 is proposed. The simulation and measurement results of a two element metamaterial antenna array with different spacing are presented, including quantified values of antennas far-field envelope correlation measurements. ANTENNA DESIGN RULES In order to achieve high MIMO spectral efficiency numbers, the radiation efficiency for each antenna element in the array has to be maximized while minimizing their far-field envelope correlation numbers [7]. These two key challenges along with fitting two low frequency 746-796MHz LTE MIMO antennas in handset form factor are summarized in the following three factors: - First – Antenna efficiency: The metamaterial antenna element should be designed to comply with 50Ω impedance matching and high radiation efficiency over the bandwidth from 746MHz to 796MHz. The physical size of the metamaterial antenna elements is selected to fit on the small board size of a smart phone device.

- Second – Near-Field Coupling: The isolation between two adjacent metamaterial antenna elements is optimized to be less than -10 dB since the strong coupling between them significantly reduces radiation efficiency and MIMO channel de-correlation. This can be accomplished by carefully choosing the position and orientation of each antenna element, controlling the current distribution on the ground plane, or by using an external decoupling network. The -10dB isolation level is considered as the threshold below which any further improvement will only increase radiation

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efficiency in negligible numbers which cannot justify adding this extra de-coupling complexity. The better antenna matching is met with -10dB isolation because of the minimized effect of mutual impedance Z12 and Z21 values between antenna ports.

- Third - Far-Field Coupling: In order to de-correlate the different multipath routes that MIMO signals take between transmitted and received antenna arrays, it is essential that antennas are either spaced far apart (λ/2 or more) or have orthogonal radiation patterns to be able to see different MIMO signals. The first approach is referred to by “Spatial Diversity (SD)” and the second one by “Pattern Diversity (PD)”. While SD is suitable for base-station MIMO implementation, it cannot be used in handsets for obvious space limitation reasons. PD is met when the radiation patterns of the antennas in MIMO array are nearly orthogonal. The far-field envelope correlation [7], ρ, provides a measure of the level of overlap between two antenna far-field radiation patterns. The smaller ρ is, the lower is the overlap, and hence the higher is the MIMO gain. In this paper, ρ is calculated using equation (1) listed below:

φθθφθφθφθφθθφθφθφθ

φθθφθφθφθρ

ππππ

ππ

dpEEdpEE

dpEE

d sin),(),(),(d sin),(),(),(

d sin),(),(),(

0 2*

2

2

00 1*

1

2

0

2

0 2*

1

2

0

∫∫∫∫

∫∫

⋅⋅⋅⋅⋅

⋅⋅= rrrr

rr

(1)

where E1(θ,φ) and E2(θ,φ) are the far-field complex electric fields for the first and second metamaterial antenna element, respectively. Most diversity gain can be obtained as long as the far-field envelope correlation is less than 0.5 [7]. Pushing ρ to values much lower than 0.5 will only provide incremental improvement of MIMO gain that have no significant impact on the overall communication spectral efficiency. Given these tradeoffs, the highest priority in designing these metamaterial antennas is to maximize the radiation efficiency and optimize impedance matching over desired frequency band. Once these goals are reached, we will then optimize the designs to achieve acceptable near-field isolation and far-field envelope correlation.

Fig. 1 Top and bottom view of the dual resonance metamaterial antenna structure.

Fig. 2 Measured return loss and radiation efficiency of antenna structure shown in Fig. 1.

LTE Metamaterial Antenna Design Although the metamaterial antenna shown in [4] is designed to be small enough to fit into the handheld device, the bandwidth of the left-hand resonance is limited to due to the its high Q property and the size constraint at 771MHz. Therefore, a second resonance is created by using the meandered stub at the launch pad. This second resonance is employed to compensate the efficiency roll-off at the band edges.

Fig. 3: Antenna array in configuration 1.

Fig. 4: Antenna array in configuration 2.

Top view Bottom view

GND GND

CPW

meandered stub

via line

top patch launch

pad vias

22

11

|E|ˆ|E|

|E|ˆ|E| ˆ

222

111

θφ

θφ

θφ

θφ

θφ

θφEjEj

EjEj

eeE

eeE∠−∠−

∠−∠−

+=

+=

Fig. 1 shows the proposed LTE metamaterial antenna that is used for the MIMO application. This antenna is fabricated on a 1mm thick FR4 substrate with dielectric constant of 4.4. The PCB size is 50mm x 120mm which represents the main PCB for mobile devices such as smart phones. The MTM cell consists of a rectangular patch located on the top layer at the edge of the PCB, a metallic via that connects the top patch to the bottom layer and a via line trace which connects the via to the ground. The MTM cell is excited by a launch pad through a small capacitive gap in between them. Stemming from this launch pad is a meandered stub that is laid on both layers, having its different arms connected by several metallic vias. The antenna is fed by a coaxial cable launched on a conductor backed coplanar waveguide with 50Ω characteristic impedance. The proposed antenna is simulated by Ansoft HFSS, fabricated and measured. Fig. 2 shows the measured return loss for the antenna in Fig. 1. The antenna covers the LTE band of 746MHz to 796MHz comfortably. The measured efficiency of this single antenna was also plotted in Fig. 2, indicating over 40% across the band can be achieved. The antenna size only occupies a footprint area of 38mm x 9.5mm, making it small enough to fit two of these antennas on a 50mm x 120mm board for the MIMO application. MIMO LTE Antenna Structure Design For the MIMO application in the LTE standard, the antenna described above is placed in two different configurations. The two configurations are chosen based on physical proximity and current distribution on the ground plane to study the tradeoffs. Fig. 3 shows the antenna array configuration 1 where both antennas are located at two corners on the short side of the PCB. The physical separation between the antennas is 31mm (λ0/13). Fig. 4 shows the antenna array configuration 2 where the antennas are placed at the two corners on the long side of the PCB and oriented at 900 relative to each other. The physical separation between the two antennas is 79mm (λ0/5).

Fig.5 shows the return loss for each of the antennas in configuration 1 and the isolation/coupling between them. Clearly the array provides 100MHz bandwidth at return loss of -6dB. The return loss shows the two radiating modes, the left hand resonance centered at 740MHz and the stub resonance centered at 800MHz. The S12 coupling between the two antennas is -10dB or better across the band. Further it should be noticed that the isolation is better around the left hand resonance at 740 MHz compared to the stub resonance at 800 MHz. This is mainly due to the current distribution of the

Fig. 5 Measured return loss and isolation of antenna array in configuration 1.

Fig. 6 Measured return loss and isolation of antenna array in configuration 2.

Fig. 7 Measured radiation efficiency of antenna array in configuration 1.

Fig. 8: Measured radiation efficiency of antenna array in configuration 2.

left hand resonance which is confined to a small region while the stub resonance depends on the ground for radiation. Fig. 7 shows the radiation efficiency measured in anechoic Chamber. Fig. 9 shows the radiation patterns in the three standard cuts XY, YZ and XZ at 800MHz. The antenna PCB was mounted vertically in XZ plane. Fig. 6 and 8 show the measured return loss, isolation and radiation efficiency for array configuration 2. In terms of radiation efficiency both array configurations are comparable. The small differences in radiation efficiency can be attributed to the impedance matching differences. The comparison of radiation efficiency, isolation and envelope correlation between the two arrays is summarized on Table 1.

Figure 9: Radiation patterns for antenna configuration 1. Port P1 in (a), (b), and (c); and P2 (d), (e), and (f).

Table 1: Comparison of the two MIMO array configurations

Frequency: 800MHz Array 1 Array 2 Radiation Efficiency 50% 45%

Isolation -10.2dB -10.5dB Envelope Correlation 0.3 0.25

CONCLUSION We have successfully designed, fabricated, and fully characterized a 2x2 MIMO handset antenna array for the LTE band based on metamaterial technology. The antennas not only have good radiation efficiency but also show low isolation and low far-filed envelop correlation numbers. These essential measured results all demonstrate the successful integration of handsets in LTE networks to reach required high data throughput. REFERENCES [1] IEEE 802.11n, Joint Proposal: High throughput extension to the 802.11 Standard: PHY, doc.:IEEE 802.11-05/1102r4. [2] Long Term Evolution of the 3GPP radio technology: http://www.3gpp.org/Highlights/LTE/lte.htm [3] C. Caloz, and T. Itoh, Electromagnetic Metamaterials: Transmission Line Theory and Microwave Applications, John Wiley

& Sons, New York, 2006. [4] G. Ajay; C. J. Lee, M. Achour, “Compact metamaterial quad-band antenna for mobile application,” Antennas and

Propagation Society International Symposium, 2008. IEEE AP-S 2008, 5-11 July 2008 Page(s):1–4. [5] G. Ajay, M. Achour, G. Poilasne, V. Pathak, Vaneet, “Compact dual-band planar metamaterial antenna arrays for wireless

LAN,” Antennas and Propagation Society International Symposium, 2008. IEEE AP-S 2008, 5-11 July 2008 Page(s):1 – 4. [6] C. J. Lee, K. M. H. Leong, and T. Itoh, “Broadband Small Antenna for Portable Wireless Application,” International

Workshop on Antenna Technology: Small Antennas and Novel Metamaterials, iWAT 2008, 4-6 March 2008 Page(s):10-13. [7] R. G. Vaughan, and J. B. Andersen, “Antenna diversity in mobile communications,” IEEE Transaction on Vehicular

Technology, vol. VT-36, no. 4, pp. 149-172, Nov. 1987.