Chuyên san Công ngh» thông tin và Truy•n thông ...fit.mta.edu.vn/chuyensan/Pdf_Files/042014/14.08.pdf · impedance of the open stub. ... by Ansoft HFSS 10:0, ... When the

Chuyên san Công nghệ thông tin và Truyền thông - Số 04 (4-2014)

DESIGN OF ULTRA-WIDEBAND BANDPASSFILTERS WITH NARROW NOTCHED BANDS

USING A RING RESONATOR

Nguyen Tran Quang1, Doan Minh Tan1, Ta Chi Hieu1

Abstract

In this paper, compact ultra-wideband (UWB) bandpass filters with a/two notched bands areproposed based on a ring resonator. Broad passband and narrow notched bands with controlledeven/odd-mode resonator frequencies can be adjusted conveniently by changing the characteristicimpedance of the ring resonator. Some transmission zeros are introduced to improve the selectivityand harmonic suppression of the proposed ultra-wideband bandpass filters with notched bands. Toverify the presented concept, three prototypes with 3-dB fractional bandwidth greater than 120 %are designed and fabricated. The results show that simple structure, good in/out-of-band performancecan be achieved in the proposed ultra-wideband bandpass filters.

Bài báo trình bày về mạch lọc dải thông băng siêu rộng với một và hai khe chặn can nhiễu trêncấu trúc đơn vòng cộng hưởng. Tần số dải thông và khe chặn can nhiễu có thể điều chỉnh thuậntiện bởi việc thay đổi đặc tính trở kháng của vòng cộng hưởng, một số điểm truyền “0” được tạora nhằm tăng tính chọn lọc của mạch lọc cũng như triệt tiêu các hài bậc cao. Để chứng minh tínhđúng đắn của các kết quả nghiên cứu, một vài bộ lọc băng thông siêu rộng với các đặc tính tốt đãđược thiết kế.

1. INTRODUCTION

Recently, more and more attentions have been paid to the development of ultra-wideband(UWB) systems since Federal Communications Commission (FCC)’s decision to permit theunlicensed operation band from 3.1 to 10.6 GHz in February 2002 [1]. As one of themost important microwave components, ultra-wideband bandpass filters (UWB-BPF) withhigh performance and low cost are imperatively needed in UWB communication systems.Various UWB filters employing multiple-mode resonators, complementary split-ring resonator(CSRR) and multilayer aperture-coupled patches are designed and analyzed [2]-[8]. In [9]-[16],cascaded low-pass/high-pass filters, electromagnetic (EM) loaded bandgap and T/Y-shapedresonators are used to extend the upper stopband bandwidth and improve the selectivityof the passband. In addition, some other UWB filters and differential filters are realizedbased on the transversal signal-interference concept in [17]-[24], by introducing intentionallya passband constructive interference and out-of-band signal energy cancellations to producepower transmission zeros, high-selectivity filtering responses and harmonic suppression can beachieved in these filter structures. In addition, due to the frequency band of the UWB indoor

(1)Le Quy Don Technical University

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Tạp chí Khoa học và Kỹ thuật - Học viện KTQS - Số 160 (4-2014)

limit (3.1-10.6 GHz) covers existing wireless communication systems, such as the 3.5-GHzband WiMax, 5.2-5.8 GHz wireless local-area network (WLAN) and some 8.0-GHz bandsatellite-communication systems, it is desirable to introduce single or multiple notch bands toavoid the interferences from existing wireless communication systems in the design of UWB-BPF. A number of UWB filters with a/multi notched bands are investigated and discussed in[25]-[32]. However, little research has described the application of the ring resonator in theUWB filters with single/multi-notched bands based on transversal signal-interference concept.Lately, the authors have presented an effective and simple method for wideband filter design[36]-[38]. The UWB-BPF with notched band using stubs loaded multi-mode ring resonatorwas presented [39]. However, for those structure the attenuation in the upper stopband stillremains high and the selectivity needs to be further improved. In this paper, compact UWB-BPF with a/two notched bands are proposed based on a ring resonator and open/shorted stubs.A broad passband with 3-dB fractional bandwidth greater than 120% can be realized with athree-quarter wavelength ring resonator and a shorted stub. Two transmission zeros are usedto improve the selectivity and harmonic suppression based on transversal signal-interferenceconcept. In addition, another two UWB-BPF with a/two notched bands for WiMax band,WLAN band and satellite-communication systems 8.0-GHz band are proposed using the three-quarter wavelength ring resonator and an open stub. The bandwidth and center frequencies ofthe notched bands can be easily adjusted by changing the electrical length and characteristicimpedance of the open stub. All the structures are designed and fabricated on the dielectricsubstrate with er = 2.65, h = 0.5 mm, and tanδ = 0.002. The design parameters obtainedby Ansoft HFSS 10.0, and the measurement is accomplished using Agilent 8753 ES networkanalyzer. Good agreement can be found between the theoretical and measured results.

2. DESIGN OF PROPOSED UWB BPF WITH SINGLE CENTRALLYLOADED OPEN STUB

Figs. 1 (a) and (b) show the top view and the circuit of the ultra-wideband bandpass filterwith a three quarter wavelength ring resonator, two different transmission paths with electricallength 2θ1 and θ1, characteristic impedance Z1 are connected ports 1, 2 symmetrically. Acentrally loaded shorted stub with electrical length θ2 and characteristic impedance Z2 isattached to center of the path 1. Due to the symmetry of the ring resonator, the even/odd-modeequivalent circuits of the ring resonator are shown in Figs. 2(a) and (b) [33] for theoreticalanalysis, respectively.

2.1. Even/Odd-Mode Analysis

When the even/odd-mode signals are excited form port 1 to port 2, a virtual open/short stubappears at the center of the three quarter wavelength ring resonator, and the even/odd-modeinput admittance Yine/Yino of the structures in Figs. 2(a)-(b) is given by (1) and (2).

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(a) (b)

Figure 1. Proposed ultra-wideband bandpass filter with the three quarter wavelength ring resonator and ashorted stub (a) Top view (b) equivalent circuit.

(b)(a)

Figure 2. (a) Even-mode equivalent circuit of the ring resonator (b) odd-mode equivalent circuit of the ringresonator.

Yine = jtan θ1/2

Z1

+ jtan θ1 − (Z1 cot θ2)/2Z2

Z1 + (Z21 tan θ1 cot θ2)/2Z2

(1)

Yino = −j cot θ1Z1

− j cot θ1/2

Z1

(2)

The resonance frequencies for the even/odd modes can be calculated when Yine/Yino = 0 orZine/Zino = 0, and the even/odd-mode resonance frequencies for the ring resonator under weakcoupling in Fig. 3(a) are shown in Fig. 3(b). The bandwidth for the ring resonator is mainlydetermined by the even-mode resonance frequencies feven1 and feven2, and the transmissionzero located at the even-mode frequency feven3 near the upper end of the passband helpsto improve the resonator selectivity. Moreover, another two odd-mode resonance frequenciesfodd1(θ1 = 1200), fodd2(θ1 = 1800) can be realized, the odd-mode resonance frequency fodd2is mainly caused by the 1800 phase difference of the two transmission paths at 2f0 [20]-[22].

In addition, the even/odd-mode resonator frequencies versus θ2 and different characteristicimpedance Z1 and Z2 are shown in Figs. 3(c)-(e). And the odd-mode resonator frequencies(fodd1, fodd2 do not change with θ2, Z1 and Z2. The even-mode resonator frequency feven1

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moves towards f0 as Z1 increases, while away from f0 as θ2, Z2 increase. Moreover, the even-mode resonator frequencies feven2 and feven3 move towards f0 as θ2, Z2 increase, while awayfrom f0 as Z1 increases. In this way, the bandwidth for the passband of the UWB filter withthe ring resonator can be controlled conveniently by changing the characteristic impedanceZ1, Z2 and θ2 when Z0 and θ1 are fixed

(e)

(a) (b)

(c) (d)

Figure 3. (a) Equivalent circuit of the ring resonator under weak coupling (b) |S21| under weak coupling,Z1 = 60Ω, Z2 = 70Ω, θ2 = 200 (c) even/odd-mode resonator frequencies versus θ2 at

f0, Z1 = 60Ω, Z2 = 70Ω(d) even/odd-mode resonator frequencies versus Z1, Z2 = 70Ω, θ2 = 200 (e)even/odd-mode resonator frequencies versus Z2, Z1 = 60Ω, θ2 = 200(C0 = 0.02pF, f0 = 7.7GHz).

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(b)(a) (b)

Figure 4. Simulated QL for the passband versus θ2, Z1 and Z2. (a) Versus Z1, Z2 = 70Ω (b) versusZ2, Z1 = 60Ω(Z0 = 50Ω, f0 = 7.7GHz, θ1 = 900).

The dependence of the loaded quality factor QL of the passband on θ2, Z1 and Z2 is shownin Fig. 4. It can be seen that the QL of the passband decreases as Z1 increases, and increasesas θ2, Z2 increase. In addition, the loaded quality factor QL and the 3-dB bandwidth ∆f3dBfor the passband can be related by [34]:

QL = f0/∆f3dB (3)

And the loaded quality factor QL can be expressed as QL = f3dB(θ2, Z1, Z2). Once Z0 andθ1 are determined, we can adjust θ2, Z1 and Z2 to satisfy the demand of QL. Obviously, therequired 3-dB bandwidth ∆f3dB and the transmission characteristic for the passband can besimultaneously obtained and further optimized based on the above discussion.

2.2. Proposed Ultra-wideband Bandpass Filter with Single Ring Resonator

Based on the above theoretical analysis, the final parameters for the filter circuit of Fig.1(b) are given as follows: Z0 = 50Ω, Z1 = 60Ω, Z2 = 70Ω, f0 = 7.7 GHz,θ1 = 900, θ2 =

230. In addition, the structure parameters for the ultra-wideband bandpass filter (15mm ×12mm, 0.56λg0× 0.45λg0) in Fig. 1(a) are listed below: L1 = 10.35mm, L2 = 2.1mm,W0 =

1.37mm,W1 = 1mm,W2 = 0.8mm, d = 0.7mm. The photograph, measured results andsimulated results are shown in Fig. 5, good agreement can be found between the measuredand simulated results, the measured insertion loss is less than 0.95 dB while the return loss isover 14 dB from 3.7 GHz to 11.6 GHz (3-dB fractional bandwidth is approximately 123.4%,2.4− 11.9 GHz). Three transmission zeros (located at 13, 14.6 and 15.7 GHz) are realized toimprove the selectivity and harmonic suppression. Furthermore, over 15-dB upper stopbandis achieved from 12.85 GHz to 18.5 GHz (2.4f0). The measured group delay is less than 0.20

ns from 2.7 GHz to 11.5 GHz.

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Figure 5. The photograph measured and simulated results of the UWB bandpass filter.

Table 1. Comparisons of Measured Results for Several Ultra-wideband Bandpass Filter Structures.

Table 1 illustrates the comparisons of measured results for several ultra-wideband bandpassfilter structures. Compared with other UWB filters [3]-[17], the effective circuit size of theproposed UWB bandpass filter is only 0.19λg0 × 0.19λg0, and the bandwidth for the UWBbandpass filter based on the ring resonator is 123.4% compared with the other UWB filterstructures based on the transversal signal-interaction concept in [18], [20]. Moreover, to extendthe upper stopband, some lowpass/bandstop networkss can be cascaded to further improvethe upper performance of the UWB filter with the single ring resonator [9]-[13].

3. DESIGN OF PROPOSED ULTRA-WIDEBAND BANDPASS FILTERSWITH A/TWO NOTCHED BANDS

3.1. Proposed Ultra-wideband Bandpass Filter with A Notched Band

To cancel the interference for WLAN signals (5.2−5.8 GHz), the UWB bandpass filter witha notched band is further proposed as Figs. 6(a)-(b), an shunt open stub with characteristic

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(a) (b)

(c) (d)

Figure 6. Proposed ultra-wideband bandpass filter with single notched band (a) Top view (b) equivalentcircuit (c) even-mode equivalent circuit of the ring resonator (d) odd-mode equivalent circuit of the ring

resonator (L1 = 9.95mm,L2 = 2.84mm,L3 = 3.11mm,L4 = 2.5mm,L5 = 2.4mm,W0 = 1.37mm,W1 =

1.25mm,W2 = 1.1mm,W3 = 0.6mm, d = 0.7mm, 20mm× 12mm, 0.74λg0 × 0.45λg0).

impedance Z3 electrical length θ3 is connected in the center of the path 1. And the even/odd-mode equivalent circuits for the ring resonator with the open/shorted stubs are shown in Figs.6(c)-(d), the even-mode input admittance Yine of Fig. 6(c) can be illustrated as

Yine = jtan θ1/2

Z1


Z1 + (Z21 tan θ1 cot θ2)/2Z2

(4)

+(Z1 tan θ3)/2Z3

−(Z21 tan θ1 tan θ3)/2Z3

And the odd-mode input admittance Yino of Fig. 6(d) is the same as (2), similarly, whenthe Yine/Zine = 0, the even-mode resonator frequencies of Fig. 6(c) can be calculated. Fig.7(a) shows the even/odd-mode resonator frequencies for the ring resonator with shorted/openstubs under weak coupling, and a transmission zero (ftz1) located at 5.6 GHz, and the thirdharmonic (3ftz1) of the transmission zero can be used to further improve the upper stopband.In addition, the even/odd-mode resonator frequencies for the ring resonator of Fig. 1(a) donot change compared with Fig. 3(b).

The simulated frequency responses of the notched band versus θ3 and Z3 are shown in Figs.

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(a) (b)

(c) (d)

Figure 7. |S21| under weak coupling, C0 = 0.02pF,Z1 = 55Ω, Z2 = 60Ω, Z3 = 75Ω, θ2 = 300, θ3 = 1120 (b)notched band versus θ3, Z1 = 55Ω, Z2 = 60Ω, Z3 = 75Ω, θ2 = 300 (c) notched band versus

Z3, Z1 = 55Ω, Z2 = 60Ω, θ2 = 300, θ3 = 1120(c) notched band versus θ3 andZ3, Z1 = 55Ω, Z2 = 60Ω, θ2 = 300(C0 = 0.02pF,Z0 = 50Ω, f0 = 7.7GHz).

7(b)-(d). One can see that the center frequency of the notched band decreases as θ3 increases,and increases as Z3 increases, thus the center frequency and the bandwidth of the notchedband for WLAN band can be easily adjusted by changing θ3 and Z3 while Z0, Z1, Z2, θ1 andθ2 are fixed.

3.2. Proposed Ultra-wideband Bandpass Filter with Two Notched Bands

The UWB bandpass filter with two notched bands to cancel the interferences for 3.5-GHz band WiMax and some 8.0-GHz band satellite-communication systems are shown inFigs. 8(a)-(b), a stepped impedance resonator [35] with characteristic impedance Z3, Z4 andelectrical length θ3, θ4 is introduced to realize the two notched bands, and the even -mode

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(a)

(b) (c) (d)

Figure 8. Proposed ultra-wideband bandpass filter with dual notched bands (a) Top view (b) equivalent circuit(c) even-mode equivalent circuit of the ring resonator (d) odd-mode equivalent circuit of the ring resonator

(L1 = 9.95mm,L2 = 2.84mm,L3 = 3.16mm,L4 = 3.6mm,L5 = 2.4mm,L6 = 3.5mm,L7 =

1.5mm,L8 = 2.5mm,W0 = 1.37mm,W1 = 1.25mm,W2 = 1.1mm,W3 = 1.4mm,W4 = 0.5mm, d =

0.7mm, 20mm× 12mm, 0.74λg0 × 0.45λg0)

input admittance Yine of Fig. 8(c) can be illustrated as

Yine = jtan θ1/2

Z1


Z1 + (Z21 tan θ1 cot θ2)/2Z2

(5)

+(Z1Z3 tan θ4 + Z1Z4 tan θ3)/(2Z3Z4 − 2Z23 tan θ3 tan θ4)

−(Z21Z3 tan θ4 + Z2

1Z4 tan θ3)/(2Z3Z4 − 2Z23 tan θ3 tan θ4)

And the odd-mode input admittance Yino of Fig. 6(d) is also the same as (2), similarly, whenthe Yine/Zine = 0, the even-mode resonator frequencies of Fig. 8(c) can be calculated. Theeven/odd-mode resonator frequencies for the ring resonator in Fig. 8(a) under weak couplingis shown as Fig. 9(a), two transmission zeros are located at 3.5 GHz (ftz1) and 8.0 GHz(ftz2), respectively. In addition, the two notched bands versus the characteristic impedanceZ3, Z4 and electrical length θ3, θ4 are shown in Figs. 9(b)-(e), the center frequency of the firstnotched band decreases as θ3, Z3 increase, and the center frequency of the second notchedband also decrease as θ4, Z4 increase. In addition, the center frequency of each notched bandcan be adjusted independently when the other notched band is fixed. Moreover, the bandwidthof the notched bands can also be adjusted by changing the characteristic impedance Z3, Z4

and electrical length θ3, θ4 when Z0, Z1, Z2, θ1andθ2 are fixed.

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(a)

(b) (c)

(d) (e)

Figure 9. |S21| under weak coupling,C0 = 0.02pF,Z1 = 55Ω, Z2 = 30Ω, Z3 = 43Ω, Z4 = 90Ω, θ2 = 300, θ3 = 1230, θ4 = 1000 (b) the first

notched band versus θ3 and Z3, Z1 = 55Ω, Z2 = 60Ω, Z4 = 90Ω, θ2 = 300, θ4 = 1000 (c) the first notchedband versus θ4 and Z4, Z1 = 55Ω, Z2 = 60Ω, Z3 = 43Ω, θ2 = 300, θ3 = 1230 (d) the second notched bandversus θ4 and Z4, Z1 = 55Ω, Z2 = 60Ω, Z3 = 43Ω, θ2 = 300, θ3 = 1230 (e) the second notched band versusθ3 and Z3, Z1 = 55Ω, Z2 = 60Ω, Z4 = 90Ω, θ2 = 300, θ4 = 1000(C0 = 0.02pF,Z0 = 50Ω, f0 = 7.7GHz).

3.3. Measured and Simulated Results of Proposed UWB Bandpass Filters with A/TwoNotched Bands

Based on the above theoretical analysis, the final parameters for the UWB filter circuitwith a notched band of Fig. 6(b) are: Z0 = 50Ω, Z1 = 55Ω, Z2 = 60Ω, Z3 = 75Ω, f0 =

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7.7GHz, θ1 = 900, θ2 = 300, θ3 = 1120; and the final parameters for the UWB filter circuitwith dual notched bands of Fig. 8(b) are: Z0 = 50Ω, Z1 = 55Ω, Z2 = 60Ω, Z3 = 43Ω, Z4 =

90Ω, f0 = 7.7GHz, θ1 = 900, θ2 = 300, θ3 = 1250, θ4 = 1000. The photographs, measuredresults and simulated results of the UWB filters with a/two notched bands are shown inFig. 10. For the UWB filter with a notched band (Fig. 10(a)), the measured 3-dB fractionalbandwidth is approximately 126% (2.3− 12 GHz), a notched band located at 5.7 GHz with10-dB bandwidth of 7.0% (5.5−5.9 GHz). The group delay is 2.0 ns in 5.7 GHz. Furthermore,over 15-dB upper stopband is achieved from 12.6 GHz to 19.2 GHz (2.5f0). The measuredgroup delay is less than 0.22 ns from 2.1 GHz to 12 GHz. For the UWB filter with twonotched bands (Fig. 10(b)), the measured 3-dB fractional bandwidth is approximately 127%

(2.2 − 12 GHz), two notched bands located at 3.5 and 8.1 GHz with 10-dB bandwidth of10% (3.4 − 3.75 GHz) and 4.3% (7.95 − 8.3 GHz). The group delay is 2.1 ns in 3.5 GHzand 1.6 ns in 8.1 GHz. The measured group delay is less than 0.25 ns from 2.2 GHz to11.6 GHz. The slight frequency discrepancies between the measured and simulated resultsare mainly caused by the limited fabrication precision and measurement errors. In addition,Table 2 illustrates the comparisons of measured results for several ultra-wideband bandpassfilter structures with single/dual notched bands. Compared with other UWB filters [25]-[32],the proposed UWB filters with a/two notched bands have very compact effective circuit sizes(0.35λg0 × 0.19λg0, 0.35λg0 × 0.32λg0) and simpler structures. And some transmission zeroscan be realized to improve the upper stopband based on transversal signal-interaction concept.

(a) (b)

Figure 10. The photographs, measured and simulated results of the UWB bandpass filters with notched bands(a) Single notched band (b) dual notched bands.

4. CONCLUSION

In this paper, compact UWB bandpass filters with a/two notched bands are proposed basedon only a ring resonator with open/shorted stubs. Several transmission zeros can be realized toimprove the upper stopband for the filter structures. Compared with former UWB structures,the proposed UWB filters have more compact effective circuit size, wider bandwidth, simpler

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Table 2. Comparisons of Measured Results for Several Ultra-wideband Bandpass Filters with Notched Bands.

design theory and low insertion loss. Good agreements between simulated and measuredresponses of the filters are demonstrated, indicating the validity of the design strategies

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[33] D. M. Pozar, Microwave Engineering, 2nd ed. New York: Wiley, 1998.[34] J.-S. Hong and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, New York: Wiley, 2001.[35] M. Makimoto, S.Yamashita, “Bandpass filters using parallel coupled stripline stepped impedance resonators,” IEEE

Trans. Microw. Theory Tech., vol. 28, no. 12, pp. 1413–1417, Nov. 1980.[36] Minh Tan. Doan, W. Q. Che and X. N. Tran, “Novel wideband bandpass filter using open stub and short stub-loaded

multiple-mode ring resonator,” In Proc. Int. Conf on Advanced Technologies for Communications., pp 341-344. Oct2012.

[37] Minh Tan. Doan, W. Q. Che and H. D. Nguyen, “Novel compact wideband based on square ring short stub-loadedresonator,” In Proc. Int. Conf on Advanced Technologies for Communications., pp 62-64. Oct 2012.

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[38] Minh Tan. Doan, W. Q. Che and P. L. Nguyen, “A novel wideband bandpass filter using open stubs multiple-modering resonator,” In Proc. Int. Conf on Advanced Technologies for Communications., pp 180-181. Oct 2012.

[39] Minh Tan. Doan, “Novel Ultra-wideband bandpass filter with notched band using stubs loaded multiple-mode ringresonator,” In Proc. Int. Conf on Advanced Technologies for Communications., pp 479-482. Oct 2013.

Manuscript received 12-02-2014; accepted 06-05-2014.

Tran Quang Nguyen graduated Faculty of Radio-Electronics Engineering, Le Qui Don TechnicalUniversity, Hanoi, Vietnam in 2000, completed the master training course specialized the technique ofelectronic at Le Quy Don Technical University in 2009. He is currently a scientific and technologicalmanagement assistant of Vietnamese Defence Ministry’s General Technical Department, and a fellowof The Scientific and Technological Military Institute.

MinhTan Doan received the B.Eng and M. Eng degrees in Radio Electronic Engineering from LeQuy Don Technical University in 2000 and 2003, respectively, Ph.D. degree in Department of Com-munication Engineering Nanjing University of Science and Technology, Nanjing, China in 2012. Heis currently a lecturer of Faculty of Radio-Electronics Engineering, Le Qui Don Technical University,Hanoi, Vietnam. His research interests include the design of microwave filters and associated RFmodules for microwave and signal processing for communications.

Chi Hieu Ta was born in Vinh Phuc in 1970. He graduated the Military Technical Academy in1994 with honor. He got his MSc degree in electronic engineering in the National Defense Academyof Japan in 2002 and his PhD degree in signal processing in the University of Strathclyde, UnitedKingdom in 2008. He is currently working at the Faculty of Radio Electronics, Military TechnicalAcademy. His research interests include precoding and equalization for MIMO systems, microwaveengineering and computational electromagnetics.

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