Enhanced Low-Angle GPS Coverage Using Solid and ... - IEEE Xplore

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 11, NOVEMBER 2009

degrades at larger antenna scan angles. Fig. 3(a) clearly indicates that power transmission efficiency for the hy-VTR design is superior to that of both the extreme hy-CTR design cases. The depolarization characteristics of the hy-VTR and hy-CTR designs are shown in Fig. 3(b). In the case of hy-VTR design, the X-pol power transmission is well below 030 dB throughout the antenna scan range and it is superior to the hy-CTR designs. For the hy-CTR1 design, X-pol power transmission is much higher, especially around the nose-cone region, which is undesirable for airborne radar systems. Furthermore, the hy-VTR design proposed here has far superior blending characteristics for X-pol than those reported in [1]. The elevation BSE characteristics are shown in Fig. 3(c). It is readily inferred that the BSE for the hy-VTR design is again, better than that of hy-CTR designs. The BSE for the hy-CTR1 design shows a sharp variation around the nose-cone sector of the radome, which is not desirable. Further, the hy-VTR design, though not optimized for the BSE, has resulted in acceptable BSE performance as well. In order to analyze the frequency response of the hy-VTR, the EM performance parameters are computed at the end-frequencies of the given radar antenna [Fig. 4(a)–(c)]. It is observed that the deterioration of power transmission is more at 9.8 GHz than that at 9.0 GHz in the critical nose-cone sector [Fig. 4(a)]. Regarding the cross-pol transmission characteristics, it is high at 9.0 GHz as compared to that at 9.8 GHz in the nose-cone sector [Fig. 4(b)]. Even though the boresight error at 9.8 GHz shows a sharp increase in the nose-cone sector as compared to that at 9.0 GHz [Fig. 4(c)], it is well within the acceptable limits.

Enhanced Low-Angle GPS Coverage Using Solid and Annular Microstrip Antennas on Folded and Drooped Ground Planes Hussain M. Al-Rizzo, Ken G. Clark, Jim M. Tranquilla, Rami A. Adada, Taha A. Elwi, and Daniel Rucker

Abstract—Folded and drooped microstrip antennas are investigated in this communication for their potential applications in GPS marine navigation. Numerical and experimental results are reported to identify the effects of the percentage of the patch extending around to the folded side, position, and angle of the bend on the performance of the proposed antennas in comparison to the conventional flat counterparts. The folded antennas provide marginally improved 3-dB beam width and excellent phase center stability without degrading the bore-sight gain. A novel drooped square annular element operating in the TM mode is proposed and validated both numerically and experimentally. The drooped annular antenna is shown to have substantially improved above-horizon coverage to suit applications requiring acquisition of satellites from horizon to horizon with a pattern ripple less than 2 dB over the upper hemisphere and with an impedance bandwidth of 2%. The polarization rejection is marginally degraded at bore-sight. At the horizon, the cross component becomes dominant by 1.5 dB. Index Terms—Drooped microstrip antenna, GPS, low-elevation pattern coverage, marine navigation, pyramidal ground plane, TM square annular patch.

I. INTRODUCTION V. CONCLUSION The EM performance parameters are evaluated for a novel hy-VTR design based on 3-D ray-tracing with aperture integration method. The hy-VTR design based on optimized power reflection offers superior EM characteristics due to the minimization of internal reflections. The EM analysis for the present work is more accurate than the conventional approach due to the incorporation of antenna and radome as a system, and the finite-dimensional nature of the antenna. A comparative study of radome performance parameters establishes the superior electromagnetic performance of the hy-VTR design over the conventional constant thickness designs.

REFERENCES [1] R. U. Nair and R. M. Jha, “Novel design for a monolithic hybrid variable thickness radome (hy-VTR),” in Proc. IEEE Antennas Propag. Soc. Int. Symp., 2004, pp. 878–881. [2] R. U. Nair and R. M. Jha, “Novel A-sandwich radome design for airborne applications,” Elect. Lett., vol. 43, pp. 787–789, Jul. 2007. [3] R. H. J. Cary, “Radomes,” in The Handbook of Antenna Design. London, U.K.: Peter Peregrinus, 1983. [4] D. J. Kozakoff, Analysis of Radome-Enclosed Antennas. Norwood, MA: Artech House, 1997. [5] K. Siwiak, T. B. Dowling, and L. R. Lewis, “Boresight errors induced by missile radomes,” IEEE Trans. Antennas Propag., vol. AP-27, pp. 832–841, Nov. 1979.

It is well known that a conventional microstrip antenna, typically mounted on a flat ground plane, suffers a remarkable decrease in gain toward low-elevation angles (10 to 30 above the horizon). Therefore, interest exists for improving this inherent coverage performance to allow for early signal acquisition from rising satellites, to avoid loss of contact and cycle slips, and to maintain signal continuity and proper system’s dilution of precision. The research reported in this communication focuses on real-time dynamic GPS marine applications. The design goal is to compensate for the motion (pitch and roll) by extending coverage over a wide range of elevation angles including 20 below the antenna’s horizon plane [1]. Some work has been reported on improving the hemispherical coverage of a crossed dipole antenna using a cylindrical ground plane with a flat elevated center surrounded by sloping sides [2]–[4]. The structure was found to be successful in improving the radiation pattern at low-elevation angles. Additional elements were also examined such as monofilar and quadrifilar helices [5], although none achieved the same degree of pattern modification as the crossed dipoles. This is attributed Manuscript received July 28, 2008; revised March 19, 2009. First published September 11, 2009; current version published November 04, 2009. This work was supported in part by the National Science Foundation (NSF) EPSCoR under Grant EPS-0701890. H. M. Al-Rizzo is with the Systems Engineering Department, Donaghey College of Engineering and Information Technology, University of Arkansas at Little Rock, Little Rock, AR 72204-1099 USA (e-mail: [email protected]). R. A. Adada, T. A. Elwi, and D. Rucker are with the Applied Science Department, Donaghey College of Engineering and Information Technology, University of Arkansas at Little Rock, Little Rock, AR 72204-1099 USA (e-mail: [email protected]). K. G. Clark and J. M. Tranquilla are with EMR Microwave Technology Corporation, Fredericton NB E3X 1N2, Canada. Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2009.2032104

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in part to the degree of ground plane illumination produced by the different sources and serves to highlight the importance of the ground plane as a secondary source with which to produce pattern changes. Success with the crossed dipoles inspired our interest in exploring the prospects of applying a similar concept to the more appealing microstrip antenna [6]. A fundamental distinction, however, exists in the relationship between a patch and the ground plane when compared with helical or dipole elements. The ground plane of a microstrip antenna forms an integral part of the radiating structure and may not best be defined as a “secondary” source. The potential benefits of the drooped microstrip antenna are such that one patent has been issued [7] proposing only downward drooped structures, although neither the dimensions nor the performances were disclosed. Later, a few attempts were reported for improving the performance of patch antennas placed on ground planes with shaped profiles to increase the gain, adjust nulls and resonant frequency [8], meet specific requirements of WLAN base station antennas [9], or increase the impedance bandwidth [10]. A corner truncated square patch, partially enclosed within a flatly folded conducting wall as described in [11], was reported in [12] with an axial ratio of 130 when mounted on a pyramidal ground plane. It should be emphasized that research reported in [7]–[12] focused only on the desired performance enhancement. There seems to be no information available on the comprehensive characterization of folded and drooped microstrip antennas intended to broaden the beamwidth coverage. This is particularly difficult to achieve without degrading other important performance metrics such as cross-polarization discrimination and phase center stability. This communication is the first to our knowledge that pursues numerical simulations, parametric studies, and measurements on prototypes of folded and drooped microstrip elements operating in the fundamental and TM30 modes, respectively, in an effort to investigate their suitability for GPS marine navigation. Unique elements involving an extreme 180 bend and an annular drooped antenna, operating in the TM30 mode, are designed, analyzed, and tested. The drooped annular element produced significant pattern flexibility as a result of interference between the widely-spaced radiating edges. The rest of the communication is organized as follows. Sections II and III present the design concept and approach followed for the folded solid patches operating in the fundamental mode and the annular elements operating in the TM30 modes, respectively. The conclusions are given in Section IV. II. FOLDED MICROSTRIP ANTENNA STRUCTURES We have developed a versatile conformal path finite-difference time domain (CPFDTD) algorithm, which incorporates the coaxial feed and detailed geometrical features, to characterize the radiation properties of the folded and drooped microstrip antennas. Our CPFDTD model has been verified against experimental measurements and then used to conduct a systematic parametric study to identify the critical parameters that affect the performance of the proposed antennas. The CPFDTD formulation for solving Maxwell’s equations is explained in detail in [13] and thus will not be presented here. In this study, probe-fed square and annular reference antennas, designed to resonate at a center frequency of 1.575 GHz, were constructed and tested. Since the resonant frequency varies slightly with the folding and bending, we adopted the following procedure to compare the performance of the proposed antenna structures. As far as the simulations are concerned, the length of the patch and feed position are adjusted through several iterative simulation steps in order to restore resonance around 1.575 GHz with good impedance matching for all the folded

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Fig. 1. Geometry of the two downward bend antennas.

and drooped antennas considered to facilitate comparison against their respective reference benchmarks. The measured far-field radiation patterns, however, are recorded at the measured resonant frequency which is slightly different from the simulated 1.575 GHz resonant frequency due to the inevitable fabrication tolerances and simulation inaccuracies. This approach is justified since the radiation patterns were found not to significantly change with respect to the observed shift in the measured resonant frequency. Fig. 1(a) and (b) show the two prototypes that were constructed and tested to verify the performance of the CPFDTD model. The first consists of a 45 mm 2 45 mm patch, 40 mm square elevated section, printed on a 1.5 mm thick substrate with a relative permittivity of "r = 4:2. The antenna is constructed from a 100 mm square flat ground plane with a 60 droop angle, starting from the edges of the flat section. The second is a 61 mm 2 61 mm patch, 50 mm square elevated section, printed on a 3 mm substrate, "r = 2:2, 100 mm square flat ground plane, and a 30 droop angle. A vector network analyzer was used to measure the reflection coefficient (S11 ) in an anechoic chamber. Fig. 2 presents the numerically and experimentally determined frequency spectrum of S11 for the antennas shown in Fig. 1. The measured and simulated E and H plane far-field radiation patterns measured inside an anechoic chamber are shown in Fig. 3 at the resonant frequency of the dominant mode. Evidently, the accuracy of the computed S11 spectrum shown in Fig. 2 deteriorates for frequencies well above the dominant mode. This is caused by the inevitable numerical dispersion since the mesh was optimized at the desired L1 resonant frequency for the purpose fast convergence and reasonable simulation times. The resonant frequencies of the dominant mode computed by the CPFDTD model match their corresponding measured values within less than 0.5%. The resonant frequencies evaluated from the CPFDTD model for the antennas shown in Fig. 1(a) and (b) are 1.5755 GHz, 3.1809 GHz, 3.5452 GHz; and 1.5751 GHz, 3.2298 GHz, 3.5953 GHz, respectively. The corresponding measured values are 1.5832 GHz, 3.1912 GHz, 3.5986 GHz; and 1.5685 GHz, 3.2971 GHz, 3.5482 GHz. The maximum difference is 1.48%. The measured impedance bandwidths of the GPS L1 band for the antennas shown in Fig. 1(a) and Fig. 1(b) are 16.5 MHz and 26.7 MHz, respectively, which are in good agreement with the corresponding values of 13.2 MHz and 21 MHz as predicted form the CPFDTD model. The success of the CPFDTD model, as evidenced by the excellent agreement observed between simulated and measured resonant frequencies of the dominant mode (L1 GPS band) and far-field radiation patterns as demonstrated in Figs. 2 and 3, respectively, prompted further investigation to explore the feasibility of improving the performance by further manipulating the ground plane. The folded antenna is derived by extending the bend to 180 . Specifically, the edges are folded completely under and flat against the back, creating a section of enclosed ground plane with the element corners projecting towards the center of the back. This resulted in a flat antenna, where the backside resembles a pair of crossed slots as shown in Fig. 4. A possibility exists that resonant modes with little radiation losses could replace the fundamental mode for the extreme bend case.

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Fig. 3. Simulated (at 1.575 GHz) and measured elevation patterns for the antennas shown in Fig. 1(a) (measured at 1.583 GHz), and Fig. 1(b) (measured at 1.5685 GHz).

Fig. 2. Measured and simulated S (b).

for the antennas showed in Fig. 1(a) and

The initial folded structure was examined experimentally. A folded microstrip antenna was constructed on a Teflon substrate with a 50 mm fold width (45 mm enclosed ground plane) using a 75 mm wide patch, which created a 5 mm slot gap. A swept frequency measurement of S11 was performed to obtain the Q factor. The structure was found to have a Q of approximately 30 compared to 40 for a flat antenna of a similar size. Because of the low losses associated with the construction material, this reduction accounts for a radiation loss comparable to that of the flat microstrip. The radiation patterns were measured from which the gain was obtained from a comparison against a half wavelength dipole. The folded structure exhibits a bore-sight gain (slot side) of slightly more than 3 dB compared to a dipole gain of 2.1 dB. Having realized that the structure did perform adequately as a radiator, we next employed the CPFDTD model to determine if the antenna could achieve any significant advantage over the flat structure. Once the element size is selected, the size of the ground plane is the only remaining parameter, and this could only be adjusted within limits. If too small, the element would wrap around, and the distance separating the element tips on the back would disappear (zero slot width). If too large, the element would not reach the edge, and the structure reverts to a simple flat microstrip. For the case of the 40 mm wide element, the ground plane limits ranged from 30 to 50 mm. The phase error is evaluated following the procedure described in [14]. The measured upper hemispherical phase pattern is matched in a 5 2 5 grid to an ideal hemisphere using equal solid angle weighting. The position of the ideal hemisphere is adjusted to minimize the RMS

Fig. 4. Geometry of the 180 folded structure.

error between the measured and ideal case. The origin of this hemisphere is defined as the apparent source of radiation. The difference between the measured and ideal phase is defined as the phase residual or phase error. A summary of the performance of the folded antenna element is provided in Table I as the fraction of the element wrapped around increases from 0 to 50 %. It is observed that the beam width and the near horizon gain roll off improved slightly with respect to the flat microstrip antenna for the "r = 2:2 substrate without degrading the bore-sight gain. In addition, the RMS phase error is lower than the fat case for the two substrates considered which is advantageous for applications requiring improved phase center stability. An examination of the field distribution between the element and ground plane revealed that the folded corners are not part of the principal resonance but rather act as sub-resonant loads along the edge of an otherwise flat rectangular element. For this reason, the crossed slot assumption may not be adequate to explain the antenna operation. III. THE DROOPED ANNULAR MICROSTRIP ANTENNA The antennas examined in Section II resonate at the fundamental mode of the solid patch. Simulated and experimental results revealed a


TABLE I RESULTS FOR THE FOLDED MICROSTRIP ANTENNA, "

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= 2:2 AND 4:2

Fig. 5. Geometry of the drooped annular structure and range of parameter variations studied.

limited ability to alter the radiation pattern of the flat microstrip for the wide range of parameters examined. In this section, additional structures are considered based on the use of multiple array elements. Because the individual microstrip element radiates normal to the ground plane, this suggests that a discrete element on each drooped face could provide the desired low-angle coverage by varying the amplitude and phase of the excitation on each element. However, this requires a complex feeding network to excite the elements with the proper amplitude and phase. An alternative solution is to connect the discrete elements in a manner that would allow them to be driven as a single unit. Once this is accomplished, the structure emerges as that of a square annular patch placed on the drooped ground plane with the open section coincident with the flat area on the top of the ground plane, as seen in Fig. 5. A higher order TM30 mode is excited in order to create a discrete element resonating in the fundamental TM10 mode on each face of the drooped plane. This causes opposing arms to oscillate with two radiating edges, the inner and outer, in a manner similar to the single element in the TM10 mode. The resulting structure allows the radiating edges to produce the interference required to introduce nulls into the upper hemispherical pattern. To this end, the design objective is to manipulate the drooped edges to control these nulls and create a pattern with minimal ripples. The radiation patterns were examined as a function of the width of the open region and the bend angle. The most promising case predicted by the model was then tested experimentally. A selection of the elevation patterns is displayed in Fig. 6 for a 4 mm-thick substrate with "r = 2:2. Fig. 6 shows that the structure is capable of producing a wide range of radiation patterns in the upper hemisphere. The final design, which reduces the pattern ripple to about 2 dB above the horizon, is obtained by choosing a 6 cm open center, coupled with a 60 bend. To validate

Fig. 6. Elevation patterns of the drooped annular antenna with fixed bend angle (top) and fixed bend position (bottom).

our design, a prototype was constructed and tested. The prototype consists of a square annular element, 18.6 cm in side length, with a 6.6 cm square inner hole, printed on a 26 cm 2 26 cm grounded substrate. The substrate is 4 mm in thickness with a relative permittivity of "r = 2:2. The drooped antenna is then formed by bending the flat element by 60 beginning at the edge of the flat opening. The dimensions of the drooped annular element are chosen in the CPFDTD model to ensure that the TM30 mode is resonating at a central frequency of 1.575 GHz. The simulated and measured elevation patterns shown in Fig. 7 support the model prediction of above horizon coverage with minimum ripples. The bandwidth for a 2:1 standing wave ratio was found to be 2%. Further testing revealed that, along with the pattern improvement, other performance measures have been degraded with the higher mode operation. First, the polarization rejection near the horizon is reduced to the point where the cross component becomes dominant by 1.5 dB. Second, the measured phase pattern as shown in Fig. 7 displays large variations in the elevation cut. However, the position of the phase center has moved by 6.12 cm (116 at 1.5703 GHz), down from the top of the ground plane where the calculated and measured patterns are referenced. IV. CONCLUSION In this communication, folded and drooped microstrip antennas have been investigated for potential applications in GPS marine navigation,

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[10] C. L. Tang, J. Y. Chiou, and K. L. Wong, “A broadband probe fed patch antenna with a bent ground plane,” in Proc. Microw. Conf., 2000, pp. 1356–1359. [11] C. L. Tang, J. Y. Chiou, and K. L. Wang, “Beamwidth enhancement of a circularly polarized microstrip antenna mounted on a three-dimensional ground structure,” Microw. Opt. Technol. Lett., vol. 32, no. 1, pp. 149–153, 2002. [12] C. W. Su, S. K. Huang, and C. H. Lee, “CP microstrip antenna with wide beamwidth for GPS band application,” Electron. Lett., vol. 43, no. 20, Sep. 27, 2007. [13] T. G. Jurgens, A. Taflove, K. R. Umashankar, and T. G. Moore, “Finitedifference time-domain modeling of curved surfaces,” IEEE Trans. Antennas Propag, vol. AP-40, pp. 357–366, Apr. 1992. [14] J. M. Tranquilla and S. R. Best, “Phase center considerations for the monopole antenna,” IEEE Trans. Antennas Propag, vol. AP-34, pp. 741–744, May 1986. Fig. 7. Amplitude and phase of the elevation patterns for the drooped annular antenna. The measured patterns are shown at the measured resonant frequency of 1.5703 GHz and the simulated patterns at 1.575 GHz.

particularly by observing the effects of shape and orientation of the ground plane upon the amplitude and phase patterns. Results showed that the beam width has been slightly increased for the folded antenna without altering the bore-sight gain accompanied with improved phase center stability. A novel drooped antenna operating in the TM30 mode has been presented using a square annular element. The drooped annular element permits significantly greater control over the radiation pattern as a result of the interference between the four radiating edges. Variations of the angle and position of the bend revealed a certain combination, giving complete upper hemispherical coverage with the pattern ripple reduced to 2 dB. Notably, in general, there is a tradeoff in achieving coverage over the entire upper hemisphere and low cross polarization. If a broad beam width is of precedence, it may be necessary to operate the drooped antennas under less than optimal conditions in regard to cross polarization performance.

REFERENCES [1] G. Lachapelle, M. Casey, R. M. Eaton, A. Kleusberg, J. Tranquilla, and D. Wells, “GPS marine kinematic positioning accuracy and reliability,” Canadian Surveyor, vol. 41, no. 2, pp. 143–172, Oct. 1987. [2] J. M. Tranquilla, The Experimental Study of Global Positioning Satellite Antenna Backplane Configurations NASA Jet Propulsion Lab., Radiating Systems Research Lab., Univ. New Brunswick, Fredericton, NB, Canada, Tech. Rep., 1988, Contract 957959. [3] J. M. Tranquilla and B. G. Colpitts, “GPS antenna design characteristics for high precision applications,” presented at the ASCE Conf. GPS-88 Eng. Applicat. of GPS Satellite Surveying Technol., Nashville, TN, May 11–14, 1988. [4] J. M. Tanquilla and B. G. Colpitts, “Development of a class of antennas for space-based NAVSTAR GPS applications,” in Proc. 6th Int. Conf. on Antennas and Propag. (ICAP 89), Coventry, U.K., Apr. 4–7, 1989, pp. 65–69. [5] J. M. Tranquilla and S. R. Best, “A study of the quadrifilar helix antenna for global positioning systems (GPS) applications,” IEEE Trans. Antennas Propag., vol. 38, pp. 1545–1550, Oct. 1990. [6] K. G. Clark, “The Finite-difference time-domain technique applied to the drooped microstrip,” Ph.D. dissertation, Dept. Elect. Eng., Univ. New Brunswick, Fredericton, NB, Canada, Jul. 1996. [7] W. Feller, “Three Dimensional Microstrip Patch Antenna,” U.S. Patent 5 200 756, Apr. 1993. [8] N. Fayyaz, N. Hojjat, and S. Safavi-Naeini, “Rectangular microstrip antenna with a finite horn-shaped ground plane,” in Proc. IEEE Antennas and Propag. Society Int. Symp., Jul. 13–18, 1997, vol. 2, pp. 916–919. [9] H. Nakano, S. Shimada, J. Yamauchi, and M. Miyata, “A circularly polarized patch antenna enclosed by a folded conducting wall,” in IEEE Topical Conf. on Wireless Commun. Technol., Oct. 15–17, 2003, pp. 134–135.

High-Gain Yagi-Uda Antennas for Millimeter-Wave Switched-Beam Systems Ramadan A. Alhalabi and Gabriel M. Rebeiz

Abstract—A high-efficiency microstrip-fed Yagi-Uda antenna has been developed for millimeter-wave applications. The antenna is built on both sides of a Teflon substrate ( = 2 2) which results in an integrated Balun for the feed dipole. A 7-element design results in a measured gain of 9–11 dB at 22–26 GHz with a cross-polarization level of 16 dB. The antenna is matched to 50 (microstrip feed). A mutual coupling of 20 dB is measured between two Yagi-Uda antennas with a center-to at 24 GHz), and a two-element array recenter spacing of 8.75 mm (0 7 sults in a measured gain of 11.5–13 dB at 22–25 GHz. The planar Yagi-Uda antenna results in high radiation efficiency ( 90%) and is suitable for mm-wave radars and high data-rate communication systems. Index Terms—Automotive radars, endfire antennas, millimeter-wave antennas, millimeter-wave communication systems, planar antennas, Yagi-Uda antenna.

I. INTRODUCTION Planar Yagi-Uda antennas are very attractive for many microwave and millimeter-wave applications due to their high gain, low cost, high radiation efficiency and ease of fabrication. The Yagi-Uda antenna is one of the most popular endfire antennas which can be designed to achieve a medium gain with relatively low cross-polarization levels. Previously, Kaneda et al. presented a microstrip-fed Quasi-Yagi antenna at X-band with a gain of 3–5 dB and a cross-pol. level of < 0 15 dB [1]. Grajek et al. showed a Yagi-Uda antenna with a directivity of 9.3 dB at 24 GHz [2]. These antennas utilize planar microstrip-to-coplanar stripline (CPS) transition which is based on a

Manuscript received July 28, 2008; revised March 12, 2009. First published July 07, 2009; current version published November 04, 2009. This work was supported in part by Intel Corporation and in part by the UC-Discovery Program. The authors are with the Electrical and Computer Engineering Department, University of California, San Diego, CA 92122 USA (e-mail: ralhalabi@gmail. com; [email protected]). Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2009.2026666

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