Feasibility of Steered Shaped Beam Dielectric Lens Antenna for Mobile Communications at mm-waves

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1 Feasibilit of Steered Shaped Beam Dielectric Lens Antenna for Mobile Communications at mm-waves João Eira, Pedro António, Carlos A. Fernandes Instituto de Telecomunicações, IST, Av. Rovisco Pais Lisboa Abstract Shaped dielectric lens antennas have proven to be a viable solution for wireless broadband sstems. Lens design fleibilit, good radiation characteristics and low manufacturing costs make it attractive when compared to other alternatives for mm-wave applications. This paper studies the feasibilit of lenses with steerable beam to favour reduction of multipath effects and increase antenna gain. Two lens feed configurations are analsed, one based on rectangular waveguide and the other on printed antenna technolog. Feed integration in the lens is studied for both cases. I. INTRODUCTION The growing user need for high qualit multimedia services and fast data echange requirements determines that the coming advanced commercial wireless communication services will require large transmission rates. Due to the limited bandwidth available at lower frequencies, broadband mobile communications sstems are being studied to operate in the mm-wave region. Antenna role is especiall important at these frequencies, where multipath effects can impose a severe limitation on the achievable transmission rates. Antennas ma help sstem performance enhancement and equaliser compleit reduction b providing an adequate signal level over the cell together with multipath discrimination. Shaped dielectric lens antennas have proven to be a viable solution for wireless broadband sstems [1, 2]. Lens design fleibilit, good radiation characteristics and low manufacturing cost make it attractive when compared to other alternatives. For the past few ears several dielectric lens antenna prototpes were designed and successfull tested showing good performance in mobile communications environments at mm-waves [3]. The first lens models assumed a fied shaped beam [2]. Although its performance was alread remarkable, there was still margin for further improvement. Then a prototpe was developed where the shaped beam could be electronicall switched [1]. The net step, introduced in this paper, is to design a lens with steerable shaped beam. This will enable the use of a narrower beam, allowing further multipath discrimination and an increase in antenna gain. Two feed configurations are considered in this stud, for beam steering. The first is based on rectangular waveguides, which are structurall simple and have been etensivel used in dielectric lens feeding. The second configuration uses printed antenna technolog. Problems originated b the integration of the feed into the lens are also analsed. Not onl the effect of the lens dielectric around the feed must be taken into account, but also interdependent design of feed and lens must be considered. II. WAVEGUIDE CONFIGURATION The specific lens design strateg reported in [3] dictates that the phase centres of the arra elements are placed over a circumference inside the lens, each element being contained in one lens constant ϕ-plane. In this wa the beam can be steered in aimuth, maintaining its shape in the elevation plane. Fig. 1 shows the rectangular waveguide assembl. R a) Top View b) Side View Fig. 1 Dielectric lens with multiple waveguide feeds. A. Radiation Characteristics Amplitude and phase ecitations of the ring arra must be obtained in order to produce the desired beam over a given aimuth direction. Since the ring arra is a curved structure, traditional linear arra theor cannot be applied. To obtain the phase ecitations for each waveguide the phase deviations introduced b the additional distances BD and CE in Fig. 2 must be derived. Straightforward algebra gives the phase ecitations of each element as a function of R, α and β as defined in Fig. 2 (λ is the free space wavelength of the radiated wave):

2 Line 3 Line 2 Line 1 Table I Results for α = 15º, R = 1 mm, aperture dimensions 2.4 mm 1.2 mm and f = 62.5 GH (structure immersed in an infinite mean with ε r = 2.53). A R α α D β β B E C Simul. β. [ º] D [dbi] SLL [db] Targetβ [ º] BW -3 db (Pl H) [ º] Fig. 2 XY projection of the waveguide ring arra. δ º (1.a) 1 = 36º 36º δ = BD = R (1.b) [ cos( α + β ) β ] 2 cos λ λ 36º 36º δ 3 = CE = 2 R sinα sin β (1.c) λ λ It is kept in mind that the main target of the work is to obtain beam scanning without radiation pattern (RP) degradation. It seems obvious that element 1 in Fig. 2 contributes to deflect the beam awa from direction β, so onl elements 2 and 3 should be ecited in this situation. Etending the above principle to all elements in an arra introduces a significant simplification of the beam scanning because onl two elements will be ecited at a time (the two that are more close to the desired beam direction), independentl of the total number of elements in the arra. A simple law was adopted in which the amplitude ecitation (a i ) is proportional to the proimit of the element to the desired beam direction: α β a i = (2) α The above ring arra was simulated using the numerical electromagnetic solver WIPL D [4]. The arra was considered immersed in unbounded dielectric media with the same permittivit of the lens. Considering R = 1 mm, α = 15º and the frequenc f = 62.5GH, results were obtained for the beam direction, directivit (D), Side Lobe Level (SLL) and 3 db beam width (BW -3 db ), which are presented in Table 1. The values for β coincide with those obtained via theoretical analsis. The SLL is alwas below 1 db, which is acceptable, although further improvement is desirable. The results for 7.5º < β < 15º are smmetric with respect to plane β = 7.5º. Fig. 3 shows the simulated RP for the situations considered in Table I. a) H - Plane (aimuth) b) E-Plane (elevation) Fig. 3 Radiation Pattern for α = 15º, R = 1mm, aperture dimensions mm and f = 62.5 GH Up to this point, onl the aimuth RP was analsed. However the lens design is based on the elevation RP. If the lens is to be a bod of revolution, a unique profile must be designed and the feed elevation RP must be the same for all ϕ-planes. Simulation shows Fig. 3 b) that this is the case. B. Phase Centre β = 5º β = 5º β = 7.5º β = º β = º β = 7.5º The phase centre (PC) of an antenna is the apparent point in space where the spherical wave originates. This point must be placed in the lens focus so that the beam will not be distorted b the lens presence. For such an arra, it is necessar to verif that a PC eists, where it is located and how its position varies with beam scanning. However, eact determination of PC position is comple. Fig. 4 presents the simulated phase of the waveguide arra electric field in the - plane for several beam directions. It

3 clearl shows the eistence of constant phase surfaces, at least in the main lobe region. Although it is not possible to conclude from these figures about the accurate PC location, it is safe to sa that the PC eists in the main lobe region. ε r primar feed r η θ a) b) Fig. 5 a) Prototpe of microstrip patch antenna used as lens primar feed, for 4 GH band operation. b) Lens configuration with embedded feed. a) θ = º b) θ = 2.5º The measured patch RP inside the dielectric is shown in Fig. 6. It was obtained using the long proven procedure of embedding the radiator into a hemispherical lens [1, 2]. E- and H-plane are almost coincident, ecept for a peak near 9º that seems to be due to surface-wave ecitation. This topic requires future consideration. H - Plane c) θ = 5º d) θ = 7.5º -18º arg (E ) 18º Fig. 4 E Phase (-3cm < < 3cm; cm < < 6cm). III. PRINTED ANTENNA The drawback of waveguide arras is its bulk, which limits the minimum separation between arra elements. Results have shown that the minimum phsicall possible separation between waveguide apertures still causes important grating lobes. It is desirable to replace the waveguide elements b printed elements, which are smaller and moreover enable the integration with Monolithic Millimetre Wave Circuits (MMIC) technolog. Promising results were previousl obtained for a shaped lens fed b a printed modified V- antenna [5], but the proposed V-antenna is not the best choice for the ring arra that is required in this application. Microstrip patch antennas are considered in the following. Using the numerical electromagnetic code ENSEMBLE [6], a ver simple square patch was designed to function at f = 43 GH. A prototpe is shown in Fig. 5, together with the transition from microstrip to rectangular waveguide. E - Plane Fig. 6 Radiation pattern of the patch inside the dielectric, at f = 43 GH. Onl one patch is eperimentall tested: due to lens circular smmetr, and compatible smmetr of the feeding ring arra, the elevation pattern is practicall independent of the number of arra elements. IV. LENS SINTHESYS The basic lens configuration is presented in Fig. 5b). A three dimensional shaped dielectric lens is fed directl b the arra of waveguides or microstrip patch elements. The design formulation is based on Geometrical Optics (GO), considering large lens dimensions in terms of wavelength. The profile of the lens output surface is calculated so that it transforms the source pattern into the desired shaped output beam. The design formulation is the same that was used with previous lenses [1]. Phsical Optics (PO) formulation is used to predict accuratel the lens output pattern and improve lens design [1,2]. The lens in this case is intended for the Base Station of a wireless or mobile sstem. The desired output pattern is of

4 the sec 2 θ form, so that constant illumination of the whole cell is achieved. Using a smoothed version of the measured RP of Fig. 6 the lens profile was computed using GO for f = 43 GH. The correspondent RP obtained at this frequenc using PO, is shown along with the eperimental result in. Fig. 11 (sec. V) E Ema [db] Eperimental V. EXPERIMENTAL RESULTS A. Waveguide Feeding Arra Fig. 7 shows a prototpe of the waveguide ring arra lens. Used parameter values are R = 1 mm and α =3º. Simulated Fig. 9 Elevation radiation pattern for maimum plane with the ecitations of Fig. 8, at f = 62.5 GH. The densit plot of Fig. 1 corresponds to the lens far-field RP. The radial direction corresponds to elevation angle θ, and aimuth to ϕ. The figure shows that the lens elevation pattern approimates the target not onl for ϕ = 15º, but also within a surrounding 3º angular region. ϕ ϕ Fig. 7 Lens prototpe fed b ring-arra of waveguides, for operation at f = 62.5 GH The prototpe was tested with the ecitation scheme shown in Fig db Eθ / Ema db -3 db Eϕ / Ema db ai = δ = º ai = δ = º 3º a i = 1 δ = º ai = 1 δ = º ai = δ = º Fig. 8 Ecitation scheme for the measured lens prototpe. Figure 9 shows the lens RP obtained for ϕ = 15º, that is, for the maimum radiation direction (β = 15º). Simulated and target patterns are superimposed on the same figure. Measured result shows good agreement with simulations. Ripple is mainl due to edge diffraction. a) E θ b) E ϕ Fig. 1 Far Field plot for the situation shown in Fig. 8, measured at f = 62.5 GH. The high SLL is also well visible. This is a direct consequence of the waveguide arra SLL, a strong reason for the introduction of printed antennas as previousl discussed. B. Printed Antenna Configuration The final lens prototpe fed b the printed patch is shown Fig. 11. Corresponding measured results are plotted in Fig.12, along with target and simulated RPs. Measured and simulated results show ver good agreement, which confirm that reliable and predictable lens performance can be obtained also with printed patch feed elements, using the developed methods and design tools.

5 shaped dielectric lenses, to produce electronicall scanned shaped beams, or even adaptive shaped beams. Fig. 1 Lens prototpe fed b microstrip patch (assembl for RP measurements at 43 GH). Eperimental Simulated Fig. 11 Elevation radiation pattern using the patch as lens feed, f = 43 GH. REFERENCES [1] C.A. Fernandes, V. Brankovic, S. Zimmermman, M. Filipe, L. Anunciada, Dielectric Lens Antenna for Wireless Broadband Communications, Wireless Personal Communications Journal, Vol. 1, No. 1, pp , June [2] C.A. Fernandes, Shaped Dielectric Lenses for Wireless Millimeter-Wave Communications, IEEE Antennas and Propagation Magaine, Vol. 41, No. 5, pp , Oct [3] C.A. Fernandes, J. Fernandes, Performance of Lens Antennas in Wireless Indoor Millimeter-Wave Applications, IEEE Transactions on Microwave Theor and Techniques, Vol. 47, No. 6, Part I, pp , June [4] B. Kolundija, T. Sarkar, R. Harrington, J. Ognjanovic, WIPL: Electromagnectic Modelling of Composite Wire and Plate Structures, Artech House, [5] M. Rodrigo, C.A. Fernandes, Shaped Dielectric Lens fed b a Printed Element for WLANS at 4 GH, in Proc. Of Millenium Conference on Antennas and Propagation AP2, Davos, Switerland, April 2 [6] Ensemble Design, Review & Arra Snthesis, vs. 5.1 Users Guide, Ansoft Corporation, Nov. 19 As previousl stressed, onl one patch is enough to prove the concept, owing to the circular smmetr of the lens. The arra behaviour is epected to improve in this case because, unlike the waveguide case, printed elements can be placed closer to each other in the ring arra, thus contributing to improve SLL characteristic. VI. CONCLUSIONS This paper analsed the feasibilit of steerable shaped beam antennas at mm-waves based on dielectric lenses. It was shown, using a ring waveguide arra and an appropriate lens that beam-scanning could be achieved. Measured results showed good agreement with simulation, although confirming a quite high SLL resulting from the phsical impossibilit to mount waveguide apertures closer to each other. This problem can be overcome b the use of printed feed elements in the ring arra, instead of the waveguides, but first of all, it requires that the integration of microstrip antennas with shaped dielectric lens is proved feasible. A lens prototpe fed b an ebedded rectangular patch was designed to produce a sec 2 radiation pattern. The prototpe was fabricated, and correponding measured results showed ver good agreement with simulation. This stud opens the wa for the future integration of active printed antenna arras with