• Journal of electromagnetic engineering and science
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• v.11 no.2
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• pp.113-121
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• 2011

Multiple-input multiple-output with orthogonal frequency division multiplexing technology (MIMO-OFDM) is considered to be the ultimate solution for increasing system throughput and for enhancing communication reliability. In this paper, we propose to increase the uplink (UL) throughput by assigning the same UL resources to multiple single-antenna mobile stations. This leads to the loss of orthogonality among sub-carriers. Thus, at the base station (BS), MIMO-OFDM detection techniques are used to separate the streams of different users assigned the same UL resources. To obtain a realistic performance evaluation, different channel scenarios are applied with different correlation values among the antennas of the users. Simulation results show that the proposed MIMO-OFDM system linearly increases the uplink capacity of the OFDM system while maintaining a mobile station transmitter as simple as that used in a conventional OFDM system. For instance, when 4 users are assigned the same UL resources, the throughput of the proposed system is 3.07 times that achieved by a conventional single input single output OFDM system.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.42 no.2
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• pp.291-298
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• 2017

In this paper, we derive the achievable degrees of freedom (DoF) for multiple-input multiple-output (MIMO) Z-interference channels (Z-IC) with reconfigurable antennas at the receivers, assuming that channel state information is not available at the transmitters. We propose a new linear scheme to align interfering signals and to decode desired signals through the designed preset mode switching pattern of reconfigurable antennas at the receivers. The key idea of our scheme is to use interfering signals as a side information at the interfered receiver by being silent at the corresponding transmitter during some time slots. Consequently, it is shown that the reconfigurable antennas at the receivers can bring a DoF gain if the number of preset modes is greater than the number of RF chains at the receivers.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.13 no.4
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• pp.1-7
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• 2013

In the next generation of mobile communication systems, high data rates and high capacity will be possible if multiple antennas are used at new frequencies. This paper presents the correlations between channel parameter path loss (PL), delay spread (DS), angular spread (AS) and K-factor established based on channel measurements. To avoid interference from Korean DTV broadcasting, we measured the channel characteristics in urban/rural/suburban areas on Jeju Island using a channel sounder and $4{\times}4$ antennas. The correlations between channel parameter show that the wireless channel characteristics can be determined and effective communication system design can be produced for use in similar environments.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.41 no.8
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• pp.896-902
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• 2016

In this paper, we investigate a minimum mean-squared error based nonlinear successive precoding method as a practical solution of dirty paper coding for multiuser downlink channels where each user has more than one antenna in the presence of other cell interference (OCI). Unlike conventional zero-forcing (ZF) based methods, the proposed scheme takes the OCI plus noise into account when suppressing the inter-cell multiuser interference, which results in improvement of the received signal-to-interference-plus-noise ratio. Simulation results show that the proposed scheme outperforms conventional methods in terms of sum rate for various OCI configurations.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.8A
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• pp.768-776
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• 2010

In this paper, we consider a linear precoding for the effective spectrum sharing in multiple-input multiple-output (MIMO) cognitive radio system where a secondary user coexists with primary users. The secondary user employs the orthogonal space time block coding (OSTBC) at the transmitter. Assuming a flat fading channel and a maximum-likelihood receiver, the optimum precoder forces transmission referred to as eigen-beamforming. In this paper, to eliminate the interference, ZF criterion based eigen-beamforming is not only used but also the precoding weight is chosen to cancel the remaining interference. This weight is computed by vector's likelihood. Simulation results show stronger interference suppression capability, better SER performance, and higher capacity than the algorithm in [4].

• Journal of Satellite, Information and Communications
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• v.11 no.2
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• pp.60-64
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• 2016

In this paper, we propose a scheme for MIMO beamforming for the backscatter communication using a multi-tag to improve the efficiency of energy harvesting and the BER of received signals. We obtain a normal channel information through a communication between the H-AP and multi-tag. The H-AP sets parameters for the transmission scenario of the spatial channel model (SCM) using the obtained channel information and generates a SCM channel information. Then, the H-AP transmits signals that have optimal transmission power to increase the signal-to-interference-plus-noise ratio (SINR) to each of tags. Tags perform a backscatter communication with signals. The receiver performs a time switching technique of energy harvesting using backscatter signals from the multi-tag. Simulation results demonstrate effectiveness of the proposed scheme, and the harvesting efficiency and BER at the receiver is greatly improved.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.44 no.7 s.361
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• pp.59-69
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• 2007

In this paper, we propose a MIMO-OFDMA (Multi Input Multi Output-Orthogonal Frequency Division Multiple Access) system based on Grassmannian beamforming for performance improvement of downlink real-time traffic transmission in harsh channel conditions with low CIR (Carrier-to-Interference Ratio). In the proposed system to reduce feedback information for the beamforming, we also apply Grassmannian Beamforming. Furthermore, we propose antenna selection scheme which performs the beamforming with more useful transmit antennas. In the proposed system, the optimal combination of transmit antennas with maximum MRT (Maximum Ratio Transmission) beamforming gain, is selected. Simulation results reveal that the proposed MIMO-OFDMA system achieves significant improvement of spectral efficiency in low CIR region as compared to a typical open-loop MIMO-OFDMA system using pseudo-orthogonal space time block code.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39A no.2
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• pp.69-76
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• 2014

Multiple-input and multiple-output (MIMO) technique is used in many communication fields in order to increase the channel capacity. However, this MIMO system has difficulty of miniaturization of antenna size due to the multiple RF chains Also, multiple RF chain raises some problems which increase power consumption at RF circuit and degrade the system performance due to the interference between RF chains. Because of these reasons, beamspace MIMO (BS-MIMO) technique with only single RF chain was proposed for MIMO transmission. This BS-MIMO system basically uses electronically steerable parasitic array radiator (ESPAR) antenna. Existing ESPAR antenna has a 5-element structure. So, it is possible to do only $3{\times}3$ MIMO transmission. Therefore, in order to extend BS-MIMO dimension, extension of ESPAR antenna structure is essential. In this paper, we show that BS-MIMO dimension can be increased according to the extension of structure of the ESPAR antenna, as in the conventional MIMO techniques. For example, we show that it is possible to design the $7{\times}7$ BS-MIMO transmissions with the 13-element ESPAR antenna. Also, when the number of parasitic elements of ESPAR antenna increases by two elements, MIMO dimension is expanded by 1.

• ETRI Journal
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• v.33 no.5
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• pp.661-669
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• 2011

In this paper, we propose an efficient soft-output signal detection method for spatially multiplexed multiple-input multiple-output (MIMO) systems. The proposed method is based on the ordered successive interference cancellation (OSIC) algorithm, but it significantly improves the performance of the original OSIC algorithm by solving the error propagation problem. The proposed method combines this enhanced OSIC algorithm with a multiple-channel-ordering technique in a very efficient way. As a result, the log likelihood ratio values can be computed by using a very small set of candidate symbol vectors. The proposed method has been synthesized with a 0.13-${\mu}m$ CMOS technology for a $4{\times}4$ 16-QAM MIMO system. The simulation and implementation results show that the proposed detector provides a very good solution in terms of performance and hardware complexity.

• Journal of Communications and Networks
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• v.18 no.6
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• pp.892-901
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• 2016

In the multi-user multiple-input multiple-output (MIMO) system, orthogonal random beamforming (ORBF) scheme is proposed to serve multiple users simultaneously in order to achieve the multi-user diversity gain. The opportunistic space-division multiple access system (OSDMA-S) scheme performs multiple weighting matrices during the training phase and chooses the best weighting matrix to be used to broadcast data during the transmitting phase. The OSDMA-S scheme works better than the original ORBF by decreasing the inter-user interference during the transmitting phase. To save more time in the training phase, a partly random multiple weighting matrices selection scheme is proposed in this paper. In our proposed scheme, the Base Station does not need to use several unitary matrices to broadcast pilot symbol. Actually, only one broadcasting operation is needed. Each subscriber generates several virtual equivalent channels with a set of pre-saved unitary matrices and the channel status information gained from the broadcasting operation. The signal-to-interference and noise ratio (SINR) of each beam in each virtual equivalent channel is calculated and fed back to the base station for the weighting matrix selection and multi-user scheduling. According to the theoretical analysis, the proposed scheme relatively expands the transmitting phase and reduces the interactive complexity between the Base Station and subscribers. The asymptotic analysis and the simulation results show that the proposed scheme improves the throughput performance of the multi-user MIMO system.