T. Takagi

Tohoku University, Miyagi, Japan

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Publications (94)15.08 Total impact

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    ABSTRACT: In WLAN systems, distributed coordination function (DCF) is used to avoid transmission collision. When an error occurs, the transmitter extends its contention window for the next retransmission to avoid more errors. But this may cause an unfairness problem; in noisy conditions MTs may find it difficult to get transmission opportunities, especially in a heterogeneous network with a large number of WLAN access points (AP). In this paper, a new control scheme with separate control channel for WLAN is proposed. When a receiver detects a transmission error, it replies special signal via the control channel to abort transmission of corrupted data. This way the transmission of corrupted data can be reduced. Thus the it is no longer necessary to change contention window size and all MTs have the same probability to get transmission opportunities. The simulation results show that by using this technique, the average throughput of MTs whose packet error rates (PER) are larger than 0.3 can be at least 2 times compared with conventional scheme.
    No preview · Article · Jun 2015
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    ABSTRACT: Heterogeneous wireless systems such as a combination of mobile broadband wireless access (MBWA) and wireless local area network (WLAN) are candidates to attain the large capacity in the next generation mobile communication systems. For capacity expansion, small cells of multiple wireless network are deployed densely in these systems. Because a number of the cells overlap, users must discover a lot of cells at once for cell selection. As a result, radio resources are much used for cell discovery in dense small cell environment. We have proposed the efficient network selection scheme using the positioning information and the map information. Using the map information, users can select the cell which has a better channel quality without cell discovery. In this paper, for further enhancement of system performance, new selection scheme considering the traffic load is evaluated. Using this load information, user select a network based on the expected throughput and their location. Consequently, users can avoid the congested network and select one which has a low traffic demand. We discuss the user throughput of the proposed scheme by comparing with the conventional one where users select by measured instantaneous signal strength. The simulation results show that the proposed scheme can improve the average user throughput by 52.8% in comparison with the conventional one. These performances can be achieved by feasible positioning accuracy.
    No preview · Article · Jun 2015
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    ABSTRACT: We proposed an RF under sampling technique that allows the use of half the sampling frequency than is conventionally required for a single channel system. In this paper, a fs design methodology for multi-channel system is described and the application of this technique to the reception of signals in different channels is demonstrated. For the conventional technique, fs is constant even if the channel of reception changes, but in our proposed technique, fs is changed according to the channel's location in the system bandwidth. To validate the proposed fs design methodology, measurement of a front-end using the conventional and the proposed techniques was performed. The measured SNR for the proposed technique for channel one, two and three were 30.4 dB, 28.0 dB and 27.2 dB respective. Using QPSK modulation the EVM was measured to be -23.5 dB, -23.4 dB and -23.6 dB for Channel one, two and three respectively.
    No preview · Article · Mar 2015
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    ABSTRACT: A linearity enhancement technique to a CMOS triple cascode push-pull power amplifier (PA) by second harmonic feedback has been proposed. The Second harmonic feedback is optimized to improve linearity based on the concept that the linearity is depend on the second harmonic feedback amplitude and phase. In order to feed second harmonics back to the push-pull PAs, we adopt a balun with a branching filter fabricated by using multi-layered organic substrates. The PA was fabricated in 0.18-μm CMOS process and was implemented on the balun by flip-chip connection. Because of the low breakdown voltage of CMOS power transistors, triple cascode configuration was adapted. It has been demonstrated that using second harmonic feedback, PAE and output power with a wideband code division multiple access (WCDMA) modulated signal are improved by 5.3% and 2.5dB respectively at adjacent channel leakage power ratio (ACLR) of -39dBc, and it has been demonstrated PAE of 28.8% and output power of 24.5dBm, respectively.
    No preview · Article · Mar 2015
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    ABSTRACT: A series/shunt switching type sample and hold (S/H) CMOS IC for direct RF under sampling reception is presented. The series/shunt configuration enable lower loss of S/H circuit and lower leakage of RF signal into the S/H output. The optimum transistor size of switches is selected by using circuit simulation. The timing of series/shunt switching is discussed and delay circuit using 1-stage inverter is added to the shunt switch control circuit to satisfy the required delay time. The proposed and designed S/H circuit is fabricated in 90nm CMOS process. The measured results using 1 GHz-band RX signal show good agreements with simulation.
    No preview · Article · Mar 2015

  • No preview · Conference Paper · Oct 2014
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    ABSTRACT: A 60GHz band 2×4 planar dipole phased array antenna using MMIC mixers is proposed for wide beamforming coverage area. 5 substrates are stacked vertically by using 3-D system-in-package (SiP) technology, and the element antenna is installed on the substrates. 8 passive double balanced MMIC diode mixers are used. By controlling the phase of each IF signal, proposed phased array antenna can operate as both transmitter and receiver. In this work, the beamforming coverage area where gain exceeds 10 dBi is measured by operating the fabricated array antenna as a receiver. The reflection at mixer's input port causes re-radiation and coupling to other element antennas. To reduce the fluctuation of phase and amplitude of received signal by the re-radiation and coupling, flip chip mounting are selected. Measured 3-D radiation patterns are in good agreement with simulated radiation patterns and show that the designed array antenna has a wide coverage area, covering 70° and 80° in the θ and φ directions, respectively, with gain exceeding 10 dBi. The proposed planar dipole array structure using 3-D SiP technology is feasible as a 60 GHz band 2-D beamforming antenna in small wireless terminals.
    No preview · Conference Paper · Jan 2013
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    ABSTRACT: A 5 GHz-band direct digital radio frequency (RF) modulator using current-mode digital-to-analog converter (DAC) with idle current is proposed and fabricated in 90-nm complementary metal oxide semiconductor (CMOS) process. Since the proposed modulator directly converts digital base band (BB) parallel input signal into RF signal, small die size and low dc operation can be achieved. To reduce the higher order distortion components at the RF output, idle current is provided to BB current output from the current-mode DAC. By providing idle current of 10% of full scale DAC output current, 2nd and 3rd order distortion components near 5 GHz-band carrier are reduced and the improvement of effective number of bits (ENOB) is achieved. Measured ENOB of 6.36 bit (spurious-free dynamic range (SNDR) of 40.0 dB) with RF modulated output power of -28.4 dBm is obtained at 5 GHz. The core size of the proposed modulator integrated circuit (IC) is 200μm × 200μm. The dc power consumption of core circuit excluding inverter section for digital BB signal converter is 2.5 mW (2.1 mA/1.2 V).
    No preview · Conference Paper · Jan 2013
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    ABSTRACT: This paper presents an flip-chip assembled 60GHz CMOS receiver front-end. The receiver front-end consists of a cascode LNA, a balun, a harmonic mixer, and an IF amplifier fabricated by 90nm CMOS. The flip-chip assembled cascode LNA performs 31.1dB gain as well as on-wafer probing with NF of 5.8dB and output P1dB of -3.9dBm at 60GHz while consuming 26.9mW. The flip-chip assembled receiver front-end performs 21.3dB of conversion gain.
    No preview · Conference Paper · Sep 2012
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    ABSTRACT: Wireless systems using license-free 60 GHz band have been focused as Gbps ultra-high speed data rate short range communication. Beamforming techniques based on array antennas have been employed to obtain high directional antenna gain. In order to realize an array antenna in millimeter-wave frequency range, the accuracy of positioning of the antenna elements becomes considerably important. Conventional 60 GHz band beamforming antennas have used a patch array antenna configuration. Since the patch antenna is patterned on a substrate, high accuracy of the positioning is obtained. These patch array antennas realize a main beam in the vertical direction of the substrate. In order to realize a main beam in the horizontal direction of the substrate, 1×4 dipole array antenna was developed and beam scanning in the azimuth plane was possible. For 2-dimensional (azimuth/elevation) beam scanning, 3×2 vivaldi array antenna using multi-layered substrate was reported. It showed the 2-dimensional beam scanning function, but a high antenna gain could not be obtained. Since the antenna elements were fabricated in a multi-layered substrate, the antenna elements were placed very close (nearly 1/25 λ0) in elevation direction and it caused relatively low antenna gain.
    No preview · Conference Paper · Jan 2012
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    ABSTRACT: For millimeter wave beam forming system, instead of using phase shifter at very high frequency of radio frequency (RF) or local oscillator (LO) frequency, low frequency baseband (BB) phase shifter has more potential for realization. Conventional BB phase shifters use variable gain amplifiers (VGA), which have low linearity, and calibration is required for over 3-bit BB phase shifters. In this paper, we propose a novel BB phase shifter using fixed gain amplifiers (FGA) matrix with in/out-phase switches. Since FGA normally has higher linearity than VGA, it is possible to reduce overall power consumption to achieve same linearity. FGA has fixed reflection and transfer characteristic, so mismatch between phase shift states are reduced. By dividing phase shifter into smaller stages, the effects of process mismatches on very nearby transistors are small. So proposed structure has robustness against process mismatch. Fabricated 5-bit BB phase shifter using proposed method has a very low power consumption to achieve almost same linearity as other works and has a 3-dB bandwidth of 1.2 GHz, 1.5° of rms phase error, lower than 0.2 dB of rms gain error without calibration. For ambient temperature from -20 °C to 80°C, simulated rms phase error varied by only 0.2°, and did not vary with process mismatch.
    No preview · Conference Paper · Jan 2012
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    ABSTRACT: We have proposed a hybrid single-carrier (SC) and multi-carrier (MC) system for next-generation mobile communication systems, where both high data rate and wide coverage system are attained by appropriately selecting SC or MC signal. We have confirmed effectiveness of the proposed hybrid system under single-cell environment. In this paper, the uplink throughput performance under multi-cell environment in the presence of inter-cell interference was calculated using computer simulation. The simulation results show that the hybrid system attains more than 10 times higher throughput than orthogonal frequency division multiple access (OFDMA) at a cumulative probability of 10% and 1.2 times higher throughput than SC-frequency division multiple access (FDMA) at a cumulative probability of 90%. The above result show that the proposed hybrid systems is useful in multi-cell environment as well as single-cell environment.
    No preview · Conference Paper · Jan 2012
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    ABSTRACT: A 5 GHz-band direct digital radio frequency (RF) modulator is proposed and fabricated in 90-nm complementary metal oxide semiconductor (CMOS) process. Since this modulator directly converts digital base-band (BB) parallel input signal into RF signal, small die size and low d.c. operation are achieved. It consists of an inverter section to obtain digital differential BB signal, a differential current-mode digital-to-analog converter (DAC) section and a local oscillator (LO) switch section. The differential DAC configuration enables low glitch performance at the BB current output and low spurious emission at the RF output. The fabricated direct digital RF modulator performs RF output power of -38.6 dBm with LO leakage of -88.1 dBm at 5GHz LO and 1-MHz BB signals. The core size of fabricated integrated circuit (IC) is 200μm × 170μm and d.c. power consumption is 2.5 mW (2.1 mA/1.2 V).
    No preview · Conference Paper · Jan 2012
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    ABSTRACT: Using 60-GHz band, multi-GHz bandwidth can be achieved for wireless personal area network (WPAN). However, WPAN requires high gain wide coverage area phased array antenna (PAA) for practical uses. In measurement of PAA, 3-D radiation pattern measurement is preferred. However, 2-D measurement is usually used because of limitations of measurement system. In this work, we propose a measurement method that can reduce measurement time for PAA while providing all radiation patterns; and a fixture to measure the 3-D radiation pattern which is compact and provides low interference on antenna's radiation pattern.
    No preview · Conference Paper · Jan 2012
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    ABSTRACT: A low cost, small size antenna integrated transceiver module is one of the key technologies for the diffusion of short range millimeter-wave wireless communication. In this paper, 60GHz antenna integrated module technology is introduced, which based on 3-D system in package (SiP) technology. This module has a multi-level stacked organic substrates configuration. The use of low cost / low dielectric constant organic multi-layer substrates, stud bump bonding (SBB) for RFIC/MMIC flip-chip mounting and Cu ball interconnection for multi-level organic substrate stacking enable to realize low production cost and small size transceiver modules. Thank to the lower dielectric constant of organic substrate than that of conventional ceramic substrate, integration of antenna into the RF module becomes easy. The realization of phased array anttena module using same structure is also described.
    No preview · Conference Paper · Jan 2012
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    ABSTRACT: In this paper, we report on a mobile broadband wireless access (MBWA) system field trial with Fast Low-latency Access with Seamless Handoff Orthogonal Frequency Division Multiplexing (FLASH-OFDM), which was carried out at Sendai city in Japan. This paper describes coverage estimates of an downlink 64 quadrature amplitude modulation (QAM) signal under the condition of constant base-station output power when the signal bandwidth is up to 20.48 MHz. The estimates are based on the field trial results of 1.28 MHz signal bandwidth. The coverage estimates based on a propagation model are also discussed. The estimation results show that wider signal bandwidth yields narrower coverage area. 64 QAM can be used within 90 %, 80 %, 65 %, and 50 % of the field trial area when the signal bandwidths are 2.56 MHz, 5.12 MHz, 10.24 MHz, and 20.48 MHz, respectively. We consider that the downlink 64 QAM signal is useful to provide a broadband wireless data communication service to users in an urban area even if the signal bandwidth is 20 MHz.
    No preview · Article · Nov 2010
  • S. Yoshida · S. Kameda · T. Takagi · K. Tsubouchi
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    ABSTRACT: 5-GHz band have been used for wireless local area network (WLAN) application such as IEEE802.11a/b/g. For next generation WLAN system, IEEE802.11n with multiple input multiple output (MIMO) option has been ratified. Since multi-antenna will be required, an antenna should be small. To satisfy these requirements, many researches on small antenna have been conducted. A tapered meander line antenna was designed at 2.55GHz[1]. A printed meander-line antenna was designed at 2.4 GHz band using FR-4 substrate[2]. A multi-band meander line antenna was designed[3]∼[5]. Radiation efficiencies were improved by applying folded configurations to a meander line antenna for UHF radio frequency identification (RFID) tags[6]. However, only a few researches of small antenna that can directly connect to 50Ω devices were conducted at 5-GHz band. In this paper, we propose a novel 3-stacked meander line antenna (MLA) with open-ended and short-ended structure using multi-layered organic substrates for 5-GHz band. Generally, electrical small antenna has low input resistance. Applying well-known folded structure, we design 3-stacked structure in the multilayered substrate to increase input resistance. Return loss, radiation pattern, and actual gain are evaluated. Then we show the 3-stacked MLA has enough potential being used as a 5-GHz band small antenna.
    No preview · Conference Paper · Aug 2010
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    ABSTRACT: In this paper, we report on a mobile broadband wireless access (MBWA) system field trial with Fast Low-latency Access with Seamless Handoff Orthogonal Frequency Division Multiplexing (FLASH-OFDM), which was carried out at Sendai city in Japan. This paper describes coverage estimates of an uplink 16 quadrature amplitude modulation (QAM) signal under the condition of constant terminal output power when the bandwidths are from 2.56 MHz to 20.48 MHz. The estimates are based on the field trial results of 1.28 MHz bandwidth. The results show that wider bandwidth yields narrower coverage area. 16 QAM can be used within 55 %, 45 %, 35 %, and 25 % of the field trial area when the signal bandwidths are 2.56 MHz, 5.12 MHz, 10.24 MHz, and 20.48 MHz, respectively. We consider that 16 QAM is useful even if the signal bandwidth is wider than that 10 MHz.
    No preview · Conference Paper · May 2010
  • S. Yoshida · S. Kameda · T. Takagi · K. Tsubouchi
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    ABSTRACT: A planar monopole antenna was designed for 60 GHz band ultra-small and low-cost front-end modules using organic substrates. The planar monopole antenna was simply patterned on the top layer substrate of the module. Simulated results showed that bandwidth and gain of the antenna were sufficient for 60 GHz wireless personal area network (WPAN) system. We fabricated and evaluated the module. Maximum actual gain of 5.1 dBi at 63 GHz was measured even though single element and very easy structure antenna was used. Although lossy substrate was used for cost reduction, the antenna patterned on organic substrates for 60 GHz band module had enough potential for being used as a low-cost and simple construction antenna for 60GHz band WPAN modules.
    No preview · Conference Paper · May 2010
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    ABSTRACT: In this paper, we report on a mobile broadband wireless access (MBWA) system field trial with fast low-latency access with seamless handoff orthogonal frequency division multiplexing (FLASH-OFDM), which was carried out at Sendai city in Japan. This paper discusses the feasibility of uplink transmission with the maximum modulation level of 64 QAM, using the measured uplink throughput performances when the maximum modulation levels are quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (QAM). The field trial results show that 64 QAM improves the uplink throughput performance in 30% of the field trial area. It is considered that the maximum modulation level of 64 QAM is feasible to provide a mobile broadband data communication service with a higher throughput in uplink as well as downlink.
    No preview · Conference Paper · Jan 2010