Tatsunori Obara

Tohoku University, Sendai-shi, Miyagi, Japan

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Publications (43)3 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Recently, we proposed an interference-aware channel segregation based dynamic channel assignment (IACS-DCA). In IACS-DCA, each base station (BS) measures the instantaneous co-channel interference (CCI) power on each available channel, computes the moving average CCI power using past CCI measurement results, and selects the channel having the lowest moving average CCI power. In this way, the CCI-minimized channel reuse pattern can be formed. In this paper, we introduce the autocorrelation function of channel reuse pattern, the fairness of channel reuse, and the minimum co-channel BS distance to quantitatively examine the channel reuse pattern formed by the IACS-DCA. It is shown that the IACS-DCA can form a CCI-minimized channel reuse pattern in a distributed manner and that it improves the signal-to-interference ratio (SIR) compared to the other channel assignment schemes.
    No preview · Article · May 2015 · IEICE Transactions on Communications
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    ABSTRACT: Square-root Nyquist transmit filtering is typically used in single-carrier (SC) transmission. By changing the filter roll-off factor, the bit-error rate (BER), peak-to-average power ratio (PAPR), and spectrum efficiency (SE) changes, resulting in a tradeoff among these performance indicators. In this paper, assuming SC with frequency-domain equalization (SC-FDE), we design a new transmit filtering based on the minimum variance of instantaneous transmit power (VIP) criterion in order to reduce the PAPR of the transmit signal of SC-FDE. Performance evaluation of SC-FDE using the proposed transmit filtering is done by computer simulation, and shows that the proposed transmit filtering contributes lower transmit PAPR, while there exists only a small degradation in BER performance compared to SC-FDE using square-root Nyquist filtering.
    No preview · Article · Apr 2015 · IEICE Transactions on Communications
  • Thanh Hai Vo · Shinya Kumagai · Tatsunori Obara · Fumiyuki Adachi
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    ABSTRACT: In this paper, a new analog signal transmission technique called analog single-carrier transmission with frequency-domain equalization (analog SC-FDE) is proposed. The proposed analog SC-FDE applies discrete Fourier transform (DFT), frequency-domain spectrum shaping and mapping, inverse DFT (IDFT), and cyclic prefix insertion before transmission. At the receiver, one-tap frequency-domain equalization (FDE) is applied to take advantage of the channel frequency-selectivity. As an example, in this paper, the analog voice transmission is considered. By computer simulation, we evaluate the normalized mean square error (NMSE) performance to show that analog SC-FDE achieves better NMSE performance than conventional analog signal transmission scheme.
    No preview · Article · Sep 2014 · IEICE Transactions on Communications
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    ABSTRACT: A throughput performance significantly degrades near the cell edge due to the path loss, shadowing loss, and multipath-fading. A cooperative relay is a well-known technique to increase the cell-edge throughput. In the cooperative relay, it is expected that assigning different levels of data modulation for mobile terminal (MT) and relay station (RS) may improve the throughput performance. In this paper, we propose a 2-time slot single-carrier (SC) cooperative decode-and-forward (DF) relay using adaptive modulation and evaluate, by computer simulation, its throughput performance. It is shown that the cooperative DF relay using adaptive modulation can reduce the normalized total transmit signal-to-noise power ratio (SNR) by about 1.5 dB compared to the cooperative DF relay using fixed modulation for a required throughput of 0.5 bps/Hz and can reduce the normalized total transmit SNR by about 9 dB compared to the direct transmission for a required throughput of 0.8 bps/Hz. Keywords; cooperative decode-and-forward (DF) relay, single-carrier transmission, adaptive modulation I. INTRODUCTION In the next generation mobile communication systems, broadband data services of around 1 Gbps are demanded. However, the throughput performance significantly degrades near the cell edge due to the propagation path loss, shadowing loss, and multipath-fading [1]. Therefore, the transmit power must be significantly increased to achieve the required throughput. The cooperative relay [2-5] is a well-known technique to increase the cell-edge throughput. The throughput achievable by the cooperative relay is determined by the worth link between the mobile terminal (MT) -relay station (RS) link and the RS -base station (BS) link [4]. Using different levels of data modulation at MT and RS may improve the throughput performance; lower level of modulation is used for a link with worse condition.
    Preview · Article · Feb 2014 · IEICE Transactions on Communications
  • Hiroyuki Miyazaki · Tatsunori Obara · Fumiyuki Adachi
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    ABSTRACT: In this paper, joint transmit/receive frequency-domain equalization (FDE) is proposed for analog network coded (ANC) singlecarrier (SC) bi-directional multi-antenna relay. In the proposed scheme, diversity transmission using transmit FDE is performed at relay station (RS) equipped with multiple antennas while receive FDE is carried out at base station (BS) and mobile terminal (MT) both equipped with single antenna. The transmit and receive FDE weights are jointly optimized so as to minimize the end-to-end mean square error (MSE). We evaluate, by computer simulation, the throughput performance and show that the joint transmit/receive FDE obtains the spatial and frequency diversity gains and accordingly achieve better throughput performance compared to either the transmit FDE only or the receive FDE only. It is also shown that ANC SC bi-directional multi-antenna relay can extend the communication coverage area for the given required throughput compared to conventional direct transmission. Copyright © 2013 The Institute of Electronics, Information and Communication Engineers.
    No preview · Article · Dec 2013 · IEICE Transactions on Communications
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    ABSTRACT: In this paper, a DFT-based frequency-domain spread spectrum (FDSS) technique for SC transmission is introduced. The transmitted data is initially transformed to frequency-domain signal by using DFT. Frequency-domain spreading can achieve large frequency diversity gain through joint frequency-domain equalization (FDE) and spectrum combining. Performance of the proposed FDSS is evaluated by computer simulation in aspects of bit-error rate (BER), peak-to-average power ratio (PAPR), and average throughput, and then compared to the conventional TDSS scheme with spreading sequence.
    No preview · Conference Paper · Dec 2013
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    ABSTRACT: The well-known scheduling algorithms are Max-map, Max-Min, and Proportional fairness (PF)-map. Using the above scheduling algorithms, the capacity and the fairness among users vary according to the channel variation. Flexible system design is possible if the sum capacity and the fairness can be controllable. In this paper, assuming single-carrier frequency division multiple access (SC-FDMA) uplink, to control the trade-off between the capacity and the fairness among users, we present modified Max-map, Max-Min, and PF-map. We evaluate, by computer simulation, the sum capacity, the user capacity, and the fairness among users when using the modified scheduling algorithms. It is shown that the modified scheduling algorithms can control the trade-off relationship between the capacity and the fairness among users by changing the number of simultaneously accessing users.
    No preview · Conference Paper · Sep 2013
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    ABSTRACT: Broadband data services are demanded even under high mobility environment, such as high-speed railway. In this paper, we consider single-carrier (SC) linearly distributed antenna network (DAN). The distributed antenna-mobile terminal (MT) link will be a line-of-sight link and therefore, fading can be characterized as the frequency-selective Nakagami-Rice fading. Distributed antenna diversity using single frequency network (SFN) can mitigate the impacts of path and shadowing losses as well as the frequency-selective fading. In this paper, frequency-domain space-time block coded-joint transmit/receive diversity (FD-STBC-JTRD) is employed as SFN linearly distributed antenna diversity. It is confirmed by computer simulation that FD-STBC-JTRD is a powerful diversity to improve the transmission performance under a high mobility environment.
    No preview · Conference Paper · Aug 2013
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    ABSTRACT: In this paper, a direct/cooperative relay switched single carrier-frequency division multiple access (SC-FDMA) using amplify-and-forward (AF) protocol and spectrum division/adaptive subcarrier allocation (SDASA) is proposed. Using SDASA, the transmit SC signal spectrum is divided into sub-blocks, to each of which a different set of subcarriers (resource block) is adaptively allocated according to the channel conditions of mobile terminal (MT)-relay station (RS) link, RS-base station (BS) link, and MT-BS link. Cooperative relay does not always provide higher capacity than the direct communication. Switching between direct communication and cooperative relay is done depending on the channel conditions of MT-RS, RS-BS, and MT-BS links. We evaluate the achievable channel capacity by the Monte-Carlo numerical computation method. It is shown that the proposed scheme can reduce the transmit power by about 6.0 (2.0) dB compared to the direct communication (the cooperative AF relay) for a 1%-outage capacity of 3.0 bps/Hz.
    No preview · Article · Apr 2013 · IEICE Transactions on Communications
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    ABSTRACT: In this paper, we propose a new broadband single-carrier (SC) multiple-input multiple-output (MIMO) transmission method to reduce the residual inter-antenna interference (IAI) and inter-symbol interference (ISI) after the minimum mean square error based receive filtering (Rx MMSE filtering). The proposed method jointly performs transmit and receive MMSE filtering (joint Tx/Rx MMSE filtering) to transform the MIMO channel to the orthogonal eigenmodes and allocate the transmit power based on MMSE criterion. Also, as the received signal-to-interference plus noise power ratio (SINR) of each eigenmode is significantly different, rank adaptation and adaptive modulation are jointly introduced to narrow the received SINR gap between each eigenmode. The superiority of the proposed method (adaptive SC-MIMO transmission using joint Tx/Rx MMSE filtering) is confirmed by computer simulation.
    No preview · Conference Paper · Jan 2013
  • H. Miyazaki · T. Obara · F. Adachi
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    ABSTRACT: In analog network coded (ANC) relay with conventional channel estimation, the feedback of the estimated channel state information (CSI) is required, and hence, the bandwidth efficiency decreases. A cyclic-shifted pilot aided channel estimation (CSPACE) is used to simultaneously estimate two equivalent channels which are required for joint transmit/receive frequency-domain equalization (FDE) and own transmitted signal removal and hence, it requires no CSI feedback. In CSPACE, the delay time-domain windowing is used to separate two equivalent channels and suppress the impact of noise. However, in single-carrier (SC) ANC multi-antenna bi-directional relay (SC-ANC-MBDR) with the joint transmit/receive FDE, the equivalent channel is a concatenation of the propagation channel and the transmit FDE, and hence, its impulse response spreads over the entire delay time-domain. Therefore, the optimal delay time-domain window varies according to changing instantaneous received signal-to-noise power ratio (SNR), and as a consequence, the channel estimation accuracy degrades if the window width for the delay time-domain windowing is not adapted to the instantaneous received SNR. In this paper, we propose an adaptive window width control (AWWC) for CSPACE. The proposed AWWC adaptively changes the window size so as to minimize the mean square error (MSE) between the channel estimate and the actual channel. It is confirmed by the computer simulation that CSPACE using our proposed AWWC can always achieve BER performance superior to when using fixed window width.
    No preview · Conference Paper · Jan 2013
  • Ryusuke Matsukawa · Tatsunori Obara · Fumiyuki Adachi

    No preview · Article · Jan 2013
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    ABSTRACT: Joint transmit/receive frequency-domain equalization based on minimum mean-square error criterion (joint Tx/Rx MMSE-FDE) is a promising frequency-domain-based technique suppressing inter-symbol interference (ISI) in broadband single-carrier (SC) transmission. However, it changes the frequency-domain spectrum shape and hence increases the peak-to-average power ratio (PAPR) of transmit signal. In this paper, we introduce the selective mapping (SLM), which is typically used in orthogonal frequency division multiplexing (OFDM), to reduce PAPR of SC transmission using joint Tx/Rx MMSE-FDE. The SLM is applied to the frequency-domain SC signal after applying the transmit FDE (Tx-FDE) and prior to inverse fast Fourier transform (IFFT). It is shown, by computer simulation, that the SLM can reduce PAPR without significant effect on bit-error rate (BER). Performance of SLM is also evaluated in various phase-rotation sequence types, in both deterministic and random points of views, and number of candidates. Computational complexity of the proposed transmission scheme employing SLM algorithm is also discussed.
    No preview · Conference Paper · Jan 2013
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    ABSTRACT: In this paper, we propose a joint transmit and receive filtering based on minimum mean square error criterion (joint Tx/Rx MMSE filtering) and joint rank adaptation/adaptive modulation for single-carrier (SC) multiple-input multiple-output (MIMO) packet transmission. Joint Tx/Rx MMSE filtering transforms the MIMO channel to the orthogonal eigenmodes to avoid the inter-antenna interference (IAI) and allocates the transmit power based on MMSE criterion to sufficiently suppress the inter-symbol interference (ISI). Joint rank adaptation/adaptive modulation narrows the received signal-to-interference plus noise power ratio (SINR) gap between eigenmodes. The superiority of the SC-MIMO packet transmission using joint Tx/Rx MMSE filtering and joint rank adaptation/adaptive modulation is confirmed by computer simulation.
    No preview · Conference Paper · Jan 2013
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    ABSTRACT: In this paper, we propose a power allocation method for the direct/cooperative amplify-and-forward (AF) relay switched SC-FDMA using spectrum division/adaptive subcarrier allocation (SDASA). The cooperative relaying is used only when it can achieve higher channel capacity than the direct communication. In the proposed power allocation method, transmit power is adaptively allocated to mobile terminal (MT) and relay station (RS) according to the channel conditions of MT-RS link and RS-base station (BS) link when the use of cooperative relay is selected. We evaluate the achievable channel capacity of the proposed power allocation method by Monte-Carlo numerical computation method. It is shown that the proposed power allocation method can achieve almost the same channel capacity as the grid search method.
    No preview · Conference Paper · May 2012
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    Tetsuya Uchiumi · Tatsunori Obara · Kazuki Takeda · Fumiyuki Adachi
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    ABSTRACT: In the conventional iterative superimposed pilot-assisted channel estimation (SI-PACE), simple averaging of the instantaneous channel estimates obtained by using the pilot over several single-carrier (SC) blocks (called the frame in this paper) is taken in order to reduce the interference from data symbols. Therefore, the conventional SI-PACE has low tracking ability against fading time variations. To solve the tracking problem, Wiener filtering (WF)-based averaging can be used instead of simple averaging. However, WF incurs high computational complexity. Furthermore, the estimation error of the fading autocorrelation function significantly degrades the channel estimation accuracy. In order to improve the channel estimation accuracy while keeping the computational complexity low, a new iterative SI-PACE using sliding WF (called iterative SWFSI-PACE) is proposed. The channel estimation is done by sliding a WF having a shorter filter size than the measurement interval. The bit error rate (BER) and throughput performances of SC-FDE using iterative SWFSI-PACE are investigated by computer simulation to show that the proposed scheme achieves good BER and throughput performances while keeping the computational complexity low irrespective of the fading rate (or maximum Doppler frequency).
    Preview · Article · Mar 2012 · IEICE Transactions on Communications
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    Iulia Prodan · Tatsunori Obara · Fumiyuki Adachi · Haris Gacanin
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    ABSTRACT: Recently, broadband analog network coding (ANC) has intensively been studied due to its potential to increase the network capacity by exploiting the broadcasting nature of the wireless channel. However, channel state information (CSI) knowledge is required for self-information removal and signal detection. A low-complexity pilot-assisted channel estimation (PACE) scheme has been presented for broadband ANC, where feedback of the CSI estimates from the relay to the users is required. In this study, we propose a PACE scheme without CSI feedback from the relay for broadband ANC using orthogonal frequency-division multiplexing. In the first time slot the users transmit their respective pilots to the relay and in the second time slot the relay simply amplifies and forwards the received pilot signals to both users. Each user can then estimate all the CSI it needs for self-information removal and coherent signal detection, without requiring any feedback of the CSI estimates from the relay. We theoretically analyze the channel estimator’s mean square error (MSE) performance and evaluate the bit error rate (BER) and throughput performance of broadband ANC using the proposed PACE by computer simulation. The results show that the increase in the MSE of the proposed CE scheme causes only a slight BER performance degradation compared to the conventional PACE scheme with ideal feedback. However, the benefit of eliminating the CSI feedback can be seen in the throughput performance.
    Preview · Article · Jan 2012 · EURASIP Journal on Wireless Communications and Networking
  • R. Matsukawa · T. Obara · F. Adachi
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    ABSTRACT: In this paper, we propose a dynamic channel assignment (DCA) scheme for distributed antenna networks (DANs). DANs, in which many antennas are distributed in each cell, significantly reduce the transmit power compared to conventional cellular networks (CNs). In DAN, a different group of channels should be assigned for each distributed antenna to avoid the interference. Since DAN can also reduce the interference power due to its low transmit power property, the same channel groups can be reused even within the same cell. Proposed DCA scheme dynamically assigns the channels based on the co-channel interference measurement. Computer simulation results demonstrate that the DAN using proposed DCA achieves higher spectrum efficiency than the conventional CN.
    No preview · Conference Paper · Jan 2012
  • Y. Sato · M. Ryusuke · T. Obara · F. Adachi
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    ABSTRACT: The system capacity of single-carrier frequency division multiple access (SC-FDMA) can be increased by allocating subcarriers to users who are in a good channel condition. However, there is a tradeoff between system capacity and fairness among users. Nash bargaining solution (NBS) in cooperative game theory can be used to solve this tradeoff problem. However, very high computational complexity is required to find the NBS. In this paper, we propose a reduced complexity suboptimal NBS based subcarrier allocation, which requires lower computational complexity while achieving the system capacity and fairness among users similar to the NBS. The proposed suboptimal NBS subcarrier allocation is applied to distributed antenna network (DAN) using frequency-domain space-time transmit diversity (FD-STTD). Numerical computation results show that the proposed NBS based subcarrier allocation achieves the system capacity comparable to Proportionally Fair (PF) map while achieving a higher fairness than PF map.
    No preview · Conference Paper · Jan 2012
  • K. Kimura · T. Obara · F. Adachi
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    ABSTRACT: An introduction of the adaptive modulation improves the throughput of a 2-time slot cooperative relay. The throughput depends on conditions of mobile terminal (MT)-base station (BS) link, MT-relay station (RS) link, and RS-BS link. However, the cooperative relay is effective when the MT-BS direct link quality is worse than the others. Therefore, the best modulation combination can be chosen using instantaneous CSIs of MT-RS link and RS-BS link In this paper, by neglecting the MT-BS direct link contribution, we introduce an adaptive modulation using instantaneous channel state information (CSI) of MT-RS link and RS-BS link. We evaluate, by computer simulation, the throughput performance of single-carrier (SC) 2-time slot cooperative decode-and-forward (DF) relay using adaptive modulation. It is shown that the introduction of adaptive modulation can reduce by about 1.8 dB the required transmit power for achieving a 1%-outage throughput of 1.4 bps/Hz compared to the conventional method.
    No preview · Conference Paper · Jan 2012