Subwavelength Radio Repeater System Utilizing ... - IEEE Xplore

More documents

Recommendations

Info

2684 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 59, NO. 7, JULY 2011Fig. 1. Schematic of radio link using radio repeater.Fig. 3. Modified MMA: (a) topology of modified MMA for small radio repeater;(b) design parameters.TABLE IDESIGNED PARAMETERS OF MODIFIED MMAFig. 2. Schematic of proposed small radio repeater.series of small radio repeaters can enhance radio connectivityby establishing a LOS with the repeater nodes.B. Benefits of the Proposed Small Radio RepeaterFig. 2 shows the proposed Small Radio Repeater. It consistsof two miniaturized planar antennas capable of supporting anomnidirectional pattern and vertical polarization. Additionally,a metamaterial based isolator structure is used, and active RFamplification circuitry as well as a battery is integrated intothe radio repeater platform. Since RX/TX of the proposed repeatershows omnidirectionality over the H plane, the uplink anddownlink signal paths can be established through a single circuitpath. This can reduce complexity and power consumption ofRF circuitry. In addition, pure vertical polarization allows for asimple antenna structure for the base station and end-node/unitsas well as a decreased path-loss along the channel. It is wellunderstood that for near-earth wave propagation scenarios verticallypolarized waves experience much less path-loss comparedto horizontally polarized waves as presented in [9]. Toachieve the compact dimension of the radio repeater, a metamaterialbased channel isolator is utilized. By generating thenormal magnetic field along the signal path between RX andTX, artificial magnetic walls are generated that serve to suppressthe electromagnetic wave propagation from RX to TX antennas.The proposed radio repeater occupies a very small areawith a very short height without its activecircuitry. The passive components such as two miniaturizedantennas and metamaterial based isolator are presented andverified in this paper. The prototype is fabricated using acommercially available dielectric substrate. In addition, acommercially available RF amplifier and battery are used toverify the operational feasibility of the proposed repeater. Thisconfiguration is shown to boost the power level of the receivedsignal by 32 dB.II. DESIGN SPECIFICATIONSA. Miniaturized Repeater AntennaAccording to antenna theory, the intensity and polarization ofthe radiated field are proportional to the level and direction of thecurrent distribution over the antenna. In many practical miniaturizedantennas, the level of the excited current is limited bythe impedance mismatch between the feeding network and theantenna itself. To achieve a low-profile miniaturized antenna,a quarter-wave microstrip resonator fed near the short-circuitedend is used. To achieve miniaturization and impedance matchinga four-arm spiral shape quarter-wave resonator structure wasutilized and presented in [10]. Although a good input reflectioncoefficient at the frequency of operation can be obtained,this antenna requires two layers; an upper layer consisting ofthe open-ended spiral shape line and a lower layer consisting ofthe short-circuited transmission line for the matching network toradiate the power effectively. This physical structure increasescomplexity and cost for fabrication. And any misalignment betweenthe two layers can shift the operation frequency.The proposed new approach is to place the matching networkat the same layer of the miniaturized antenna as shownin Fig. 3. Although the main radiation is from the shorted pins,some radiation is emanated from the spiral arms. The polarizationof the radiated field from the spiral arms is horizontal.Therefore, each of the spiral arms should be placed in a symmetricalmanner in order to minimize horizontally polarized radiatedfield. Symmetry of these arms is essential to cancel suchradiated fields and eventually achieve an omnidirectional verticallypolarized radiation pattern. In addition, symmetry andclose spacingof the shorting pins enablethe excited currents through these pins to be in-phase. The shortcircuited currents that pass through the four vertical pins are thedominant radiating elements and responsible for the verticallypolarized radiated field. The optimized miniaturized multi-elementsmonopole antenna (MMA) is designed using a commercialfinite element method solver (Ansoft’s HFSS ver. 11.1).To be incorporated into the small radio repeater, the optimizedMMA must be further modified to achieve smaller dimensions.Because of the sub-wavelength dimension of the proposed radio
SARABANDI AND SONG: SUBWAVELENGTH RADIO REPEATER SYSTEM UTILIZING MINIATURIZED ANTENNAS 2685Fig. 4. Simulated responses of modified MMA: (a) S response; (b) radiation pattern in E(zx) plane; (c) radiation pattern in E(yz) plane; (d) radiation pattern inH(xy) plane.repeater, a pair of the optimized MMA with four arms can producea high level of mutual coupling within the small groundplane. Thus, the objective is to modify the MMA geometry sothat mutual coupling can be reduced while maintaining the polarizationpurity and the desired radiation pattern. By utilizingonly two arms of the MMA with a symmetric topology as shownin Fig. 3, the horizontal current cancellation and reduction ofmutual coupling can be achieved at the expense of an asymmetricradiation pattern in the E plane. The modified MMAis designed and verified using Ansoft’s HFSS. The designedtopology and parameters are shown in Fig. 3. The physical dimensionsare optimized for operation around 2.5 GHz and summarizedin Table I. And all line length and spacing (betweentraces) values are set to 0.4 mm, except for , and whichare set to 0.6 mm. Fig. 4(a) presents the simulated input reflectioncoefficient of the modified MMA. As can be seen, the modifiedMMA shows a 18.5 dB of input reflection coefficientat the design frequency. The radiation pattern of the modifiedMMA is shown in Fig. 4(b), (c), and (d). The vertical polarizationin the H-plane shows an omnidirectional pattern, similarto that of a monopole. In addition, the level of the horizontalpolarization in the H-plane is negligible compared to the verticalpolarization, which implies that the cancellation betweenhorizontal electric current is achieved effectively. Furthermore,the transmission line based antenna can be fabricated using thestandard printed circuit technology. This serves to reduce thealignment error observed in the multi-layer design, and hencethe fabrication process and the discrepancy between the simulationand measurement are reduced significantly.B. Metamaterial Based Channel IsolatorIn practical antenna systems, the mutual coupling betweenadjacent antennas restricts compact integration of multiple antennasin a small area for applications such as multiple inputand multiple output (MIMO) communication systems. To suppressthe mutual coupling, various approaches have been studiedand presented in [11]–[13]. In general, these studies can be categorizedinto two approaches. The first method is to engineerthe electric and magnetic properties of the material, such as thepermittivity and permeability, by introducing an artificial structure.For example, a mushroom-like structure can suppress themutual coupling by introducing a negative refractive index, asshown in [11] and [12]. Another method utilizes metamaterialinsulators to block the EM energy from being transmitted acrossthe insulation boundary, as shown in [13]. This metamaterial insulatorconsists of magneto-dielectric embedded circuits, whichcan be modeled as parallel LC resonant circuits. However, bothapproaches have intrinsic limitations when attempting to suppressthe mutual coupling of antennas at the commercial ISM
Page 1: IEEE TRANSACTIONS ON ANTENNAS AND P
Page 5 and 6: SARABANDI AND SONG: SUBWAVELENGTH R
Page 7 and 8: SARABANDI AND SONG: SUBWAVELENGTH R

Subwavelength Radio Repeater System Utilizing ... - IEEE Xplore

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?