Tatsunori Obara

Tohoku University, Sendai-shi, Miyagi-ken, Japan

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Publications (30)0.54 Total impact

  • [show abstract] [hide abstract]
    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.
    Personal Indoor and Mobile Radio Communications (PIMRC), 2013 IEEE 24th International Symposium on; 01/2013
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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.
    Vehicular Technology Conference (VTC Fall), 2013 IEEE 78th; 01/2013
  • 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.
    Wireless Communications & Signal Processing (WCSP), 2013 International Conference on; 01/2013
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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.
    Intelligent Signal Processing and Communications Systems (ISPACS), 2013 International Symposium on; 01/2013
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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.
    Vehicular Technology Conference (VTC Fall), 2013 IEEE 78th; 01/2013
  • 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.
    Vehicular Technology Conference (VTC Spring), 2012 IEEE 75th; 01/2012
  • 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.
    Communications (APCC), 2012 18th Asia-Pacific Conference on; 01/2012
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    ABSTRACT: Single-carrier transmission is a promising transmission technique due to its low peak-to-average power ratio (PAPR) property compared to multicarrier transmission, and using frequency-domain equalization (FDE) at the receiving side also mitigate the adverse effect of channel frequency-selectivity. An introduction of transmit filter can improve the system performance in terms of either PAPR or error probability. In this paper, we focus on the transmit filter aiming to reduce PAPR by employing the minimization of variance of instantaneous transmit power. Filter roll-off factor is also considered in order to provide excess-bandwidth transmission. With a combination of an FDE and spectrum combining at the receiver, excess-bandwidth transmission inherits additional frequency diversity gain, and therefore improves error probability. Performance evaluation of the proposed filtering algorithm is done by computer simulation, while the PAPR and bit-error rate (BER) performance of proposed algorithm are compared with conventional square-root Nyquist filter.
    Communications (APCC), 2012 18th Asia-Pacific Conference on; 01/2012
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    ABSTRACT: In this paper, we propose a joint transmit/receive frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion for multi-input multi-output (MIMO) analog network coding (ANC) in single-carrier (SC) bi-directional relay communications. In the proposed scheme, the relay station (RS) equipped with multiple antennas carries out antenna diversity and one-tap transmit FDE, and the base station (BS) and mobile terminal (MT) receivers, both equipped with single antenna, carry out one-tap receive FDE only. The FDE weights at RS, BS and MT are jointly optimized so as to minimize the end-to-end mean square error (MSE). We evaluate, by computer simulations, the bit error rate (BER) performance when using the proposed scheme, and discuss how the transmit FDE at RS operates and affects the BER performance.
    Vehicular Technology Conference (VTC Fall), 2012 IEEE; 01/2012
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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.
    Network Infrastructure and Digital Content (IC-NIDC), 2012 3rd IEEE International Conference on; 01/2012
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    ABSTRACT: Recently, we proposed a joint transmit/receive frequency-domain minimum mean square error based filtering (joint Tx/Rx MMSE-FD filtering) to improve the transmission performance of broadband single-carrier (SC) multiple-input multiple-output (MIMO) spatial multiplexing. For wireless packet communications, hybrid automatic repeat-request (HARQ) plays an important role as the error control technique to achieve a higher throughput performance. In this paper, we modify and optimize the joint Tx/Rx MMSE-FD filtering for its application to SC-MIMO spatial multiplexing and HARQ packet combining. At each retransmission of the same packet, joint Tx/Rx MMSE-FD filtering is carried out so as to minimize the MSE after packet combining. Computer simulation results show that joint Tx/Rx MMSE-FD filtering offers an improved throughput performance compared to the conventional Rx MMSE-FD filtering.
    Communication Systems (ICCS), 2012 IEEE International Conference on; 01/2012
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    ABSTRACT: In a frequency reusing wireless local area network (WLAN), the co-channel interference (CCI) environment will change when new access points (APs) join the network. Therefore, the user throughput reduces if the available channels are not properly re-allocated. A newly joined AP is designed to measure the CCI levels on the available channels to select the best channel which experiences the lowest CCI (this is referred to as the conventional channel assignment). However, old APs' channels are not anymore the best since the CCI environment has changed after the new one joined. If each AP periodically measures the CCI environment and reselects the channel experiencing the lowest CCI, the throughput may improve. This is called the dynamic channel assignment (DCA). In this paper, we apply the channel segregation based DCA (CS-DCA) to WLAN. In order to examine the channel distribution pattern obtained by CS-DCA, we introduce three indicators: the autocorrelation of channel distribution pattern indicating the degree of stability, the deviation of channel reusing, and the minimum co-channel AP distance. We confirm by computer simulation that the CS-DCA can autonomously form a channel distribution pattern which minimizes the CCI at each AP and that the CS-DCA improves the signal-to-interference-ratio (SIR) compared to the conventional scheme.
    Communication Systems (ICCS), 2012 IEEE International Conference on; 01/2012
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    ABSTRACT: Multiple-input multiple-output (MIMO) spatial multiplexing is known to increase the transmission rate without bandwidth expansion. However, in cellular networks (CNs), the transmission rate of a user close to the cell edge significantly degrades because the received signal-to-interference plus noise power ratio (SINR) degrades due to the presence of strong co-channel interference (CCI) from neighboring cells. Distributed antenna network (DAN), in which many antennas are spatially distributed over the cell, is suitable for MIMO spatial multiplexing because the received SINR improves over the entire cell. In this paper, assuming block transmission with cyclic prefix (CP) insertion, we theoretically derive an expression for the downlink spectral efficiency of DAN-MIMO spatial multiplexing in a multi-cell environment. Then, we propose the optimal and suboptimal transmit power allocation schemes for DAN-MIMO spatial multiplexing. We evaluate the spectral efficiency distribution by Monte Carlo numerical computation method to show that DAN allows single frequency reuse and achieves higher spectral efficiency compared to CN.
    Vehicular Technology Conference (VTC Fall), 2012 IEEE; 01/2012
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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.
    Vehicular Technology Conference (VTC Spring), 2012 IEEE 75th; 01/2012
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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).
    IEICE Transactions on Communications. 01/2012; E95.B(3):924-932.
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    ABSTRACT: For gigabit wireless data services, there are three important technical issues to be addressed: limited bandwidth, severe frequency-selective fading, and limited transmit power. A distributed antenna network (DAN) is a promising solution to the above three technical issues. In this paper, recent advance in gigabit DAN is presented. In DAN, each mobile user is served by using multiple distributed antennas close to it. Particular attention is paid to frequency-domain multi-input/multi-output (MIMO) diversity, relay, beamforming, and multiplexing jointly used with frequency-domain equalization (FDE) to significantly improve the signal transmission quality over a service area.
    Circuits and Systems (MWSCAS), 2011 IEEE 54th International Midwest Symposium on; 09/2011
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    ABSTRACT: Relay using analog network coding (ANC) can achieve the same maximum throughput as the direct transmission while reducing the required transmit power. In this paper, we propose an adaptive power allocation scheme for bidirectional single-carrier (SC) ANC. The proposed scheme adaptively allocates the transmit power so as to make the instantaneous signal-to-interference plus noise power ratios (SINRs) of both up and down links identical and maximized. Since it is quite difficult to obtain the optimal solution, we derive an approximate solution. We evaluate, by computer simulation, the distributions of bit error rate (BER) and throughput when using the proposed power allocation. It is shown that the proposed scheme can improve the BER and throughput performances compared to equal power allocation.
    01/2011;
  • Fumiyuki Adachi, Tatsunori Obara, Tetsuya Yamamoto
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    ABSTRACT: Single-carrier (SC) waveform has a lower peak-to-average power ratio than multi-carrier waveform. Furthermore, it can exploit the channel frequency-selectivity through frequency-domain equalization (FDE) to improve the transmission performance. SC-FDE is a block transmission. The cyclic prefix (CP) is inserted in front of each data block. Instead of CP insertion, the known training sequence (TS) insertion and zero padding (ZP) can be used. In this paper, performance comparison is made among CP-, TS-, and ZP-SC in terms of the channel capacity and average bit error rate (BER) performance in a frequency-selective Rayleigh fading channel.
    01/2011;
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    ABSTRACT: Broadband analog network coding (ANC) has been recently introduced 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. Recently, a pilot-assisted channel estimation (PACE) scheme has been presented for broadband ANC, where feedback of the channel estimates from the relay to the users is required. In this work, we introduce a PACE scheme without feedback from the relay for broadband ANC using orthogonal frequency-division multiplexing (OFDM). 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 from the relay. The bit error rate (BER) performance of broadband ANC using the proposed PACE is evaluated by computer simulation. It was shown that the proposed PACE scheme causes only a slight BER performance degradation compared to the conventional PACE scheme while eliminating the feedback channel requirement.
    01/2011;
  • Tatsunori Obara, Kazuki Takeda, Fumiyuki Adachi
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    ABSTRACT: Joint MMSE-FDE & Spectrum Combining for a Broadband Single-Carrier Transmission in the Presence of Timing Offset
    IEICE Transactions. 01/2011; 94-B:1366-1375.