• ETRI Journal
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• v.35 no.1
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• pp.51-57
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• 2013

In this paper, a small internal antenna for a mobile handset is presented using multiband, wideband, and high-isolation multiple-input multiple-output techniques. The proposed antenna consists of three planar inverted-F antennas (PIFAs) that operate in the global system for mobile communication (GSM900), the digital communication system (DCS), the personal communication system (PCS), the universal mobile telecommunication system (UMTS), and wireless local area network (WLAN) bands with a physical size of $40mm{\times}10mm{\times}10mm$. A resonator attached to the folded PIFA creates dual resonances, achieving a wide bandwidth of approximately 460 MHz, covering the DCS, PCS, and UMTS bands; a meander shorting line is used to improve impedance matching. Additionally, a modified neutralization link is embedded between diversity antennas to enhance isolation, which results in a 6-dB improvement in the isolation and less than 0.1 in the envelope correlation coefficient evaluated from the far-field radiation patterns. Simulation and measurements demonstrate very similar results for S-parameters and radiation patterns. Peak gains show 3.73 dBi, 3.77 dBi, 3.28 dBi, 2.15 dBi, and 5.86 dBi, and antenna efficiencies show 56.15%, 72.15%, 68.59%, 52.92%, and 82.93% for GSM900, DCS, PCS, UMTS, and WLAN bands, respectively.

• Journal of Communications and Networks
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• v.18 no.5
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• pp.688-698
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• 2016

In this paper, we investigate the achievable sum rate and energy efficiency of downlink massive multiple-input multiple-output antenna systems with zero-forcing precoding, by taking into account the randomness of user locations. Specifically, we propose two types of non-uniform user distributions, namely, center-intensive user distribution and edge-intensive user distribution. Based on these user distributions, we derive novel tight lower and upper bounds on the average sum rate. In addition, the impact of user distributions on the optimal number of users maximizing the sum rate is characterized. Moreover, by adopting a realistic power consumption model which accounts for the transmit power, circuit power and signal processing power, the energy efficiency of the system is studied. In particular, closed-form solutions for the key system parameters, such as the number of antennas and the optimal transmit signal-to-noise ratio maximizing the energy efficiency, are obtained. The findings of the paper suggest that user distribution has a significant impact on the system performance: for instance, the highest average sum rate is achieved with the center-intensive user distribution, while the lowest average sum rate is obtained with the edge-intensive user distribution. Also, more users can be served with the center-intensive user distribution.

• 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].

• ETRI Journal
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• v.34 no.1
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• pp.9-16
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• 2012

Precoding in the multiple-input multiple-output orthogonal frequency division multiplexing system is investigated. In conventional wideband precoding (WBP), only one precoder, obtained from the decomposition of the subcarrier independent channel matrix, is used for all subcarriers. With an investigation of the relationship between the subcarrier independent channel matrix and the temporal/frequency channels, an improved WBP scheme is proposed for practical scenarios in which a part of subcarriers are allocated to a user. The improved WBP scheme is a generalized scheme of which narrow-band precoding and conventional WBP schemes are special modes. Simulation results demonstrate that the improved WBP scheme almost achieves the optimum performance of a single precoder and outperforms the conventional WBP scheme in terms of the bit error ratio and ergodic capacity with slight complexity increase. The largest advantage of the improved WBP scheme on signal-to-noise ratio in simulation results is over 2.1 dB.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.43 no.10 s.352
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• pp.1-7
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• 2006

Lattice decoding concept has been proposed for the implementation of the Maximum-Likelihood detection which is the optimal receiver from the viewpoint of the BER (Bit Error Rate) performance for MIMO (Multiple Input Multiple Output) systems. Sphere decoding algorithm and K-best decoding algorithm are based on the lattice decoding concept. A K-best decoding algorithm shows a good BER performance with relatively low complexity. However, with small K value, the error propagation effect severely degrades the performance. In this paper, we propose an adaptive K-best decoding algorithm which has lower average complexity and better BER performance than conventional K-best decoding algorithm.

• Journal of Communications and Networks
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• v.6 no.3
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• pp.258-268
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• 2004

We demonstrate rotationally invariant space-time (ST) trellis codes with a 4-D rectangular signal constellation for data transmission over fading channels using two transmit antennas. The rotational invariance is a good property to have that may alleviate the task of the carrier phase tracking circuit in the receiver. The transmitted data stream is segmented into eight bit blocks and quadrature amplitude modulated using a 256 point 4-D signal constellation whose 2-D constituent constellation is a 16 point square constellation doubly partitioned. The 4-D signal constellation is simply the Cartesian product of the 2-D signal constellation with it-self and has 32 subsets. The partition is performed on one side into four subsets A, B, C, and D with increased minimum-squared Euclidian distance, and on the other side into four rings, where each ring includes four points of equal energy. We propose both linear and nonlinear ST trellis codes and perform simulations using an appropriate multiple-input multiple-output (MIMO) channel model. The 4-D ST codes constructed here demonstrate about the same frame error rate (FER) performance as their 2-D counterparts, having however the added value of rotational invariance.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.40 no.7
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• pp.1257-1265
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• 2015

In this paper, we describe a greedy user selection scheme for multiuser multiple-input multiple-output (MIMO) systems. We propose a new metric which has significantly improved performance compared to the Frobenius norm metric. The approximation of projection matrix is applied to increase the accuracy of Frobenius norm of effective channel matrix. We analyze the computational complexity of two metrics by using flop counts, and also verify the achievable sum rate through numerical simulation. Our simulation result shows that the proposed metric can achieve the improved sum rate as the number of user antenna increases.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.48 no.6
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• pp.14-21
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• 2011

In this paper, we propose an efficient symbol mapping method based on LLR (log-likelihood ratio) statistic for HARQ employing CoRe (constellation rearrangement) with 64QAM. The signal constellations for 64QAM with limited number of retransmissions are selected by the proposed mapping rule. Then we determine the transmission order of selected constellations. The proposed scheme reduces the performance deviation between transmit symbols so as to improve the system performance. Through simulation results, the proposed and existing CoRe mapping methods are compared with respect to error rate and throughput in MIMO-OFDM system over fading channel and we confirm the applicability of the proposed scheme in practical wireless communications environment.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.48 no.5
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• pp.8-15
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• 2011

LTE-Advanced aims at peak data rates of 1Gbits/s for the downlink and 500 Mbits/s for the uplink, which can be accomplished only by using wide spectrum allocation of 100MHz as well as advanced multiple input multiple output antenna techniques to the uplink. This paper analyzes the uplink precoding techniques which include LTE codebook of downlink, singular value decomposition codebook, and equal gain transmission codebook over LTE defined single carrier frequency division multiplexing systems. Finally considering nonlinear transmit power amplifier model, it is shown that link-level performance of EGT is superior to those of any other precoding schemes.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.15 no.5
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• pp.1025-1030
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• 2011

To improve the performance of mobile communication systems, beamforming technology is being considered. However, the number of antenna should be more than four to implement beamforming technology. Therefore, the existing 2-Tx RU (radio unit) for $2{\times}2$ MIMO (multiple input multiple output) should be replaced by 4-Tx RU for beamforming function. In this paper, we propose a new method to implement 4-Tx beamforming RU reusing the existing 2-Tx RU. We think that our method is very useful from the viewpoint of cost and environment because the existing 2-Tx RUs can be reused.