• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.6C
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• pp.572-579
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• 2010

Filtered multi-tone (FMT) is a form of multicarrier modulation utilizing frequency-domain equalization efficient in multi-path fading channels. Doppler frequency information can be employed for channel estimation and link adaptation to improve the performance. However, most previous studies have concentrated on the orthogonal frequency division multiplexing (OFDM) instead of FMT. Moreover, they have not considered the synchronization error that can commonly occur in practical systems. In this paper, we propose Doppler frequency estimation scheme that is effective in FMT systems with residual synchronization error and high noise levels.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.10A
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• pp.974-983
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• 2007

In this paper, a low complexity subcarrier allocation scheme is proposed for multiuser MIMO-OFDM systems with zero-forcing beamformer (ZFBF) so that the total transmit power can be minimized satisfying given target data rate. Since the optimal method requires very high computational complexity, we propose a low complextiy suboptimal method. Using the fact that the effective channel gain is proportional to the orthogonallity of channels of multiplexed users, a user set with the highest orthogonality of channel among users is assigned to each subcarrier in order to minimize required transmit power. The numerical results show that the proposed suboptimal method can reduce computational complexity with little performance loss.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.2A
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• pp.110-118
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• 2006

In part-I of the paper, two sub-channel structures, either contiguous or distributed, were considered. Modeling the SNR distribution over a sub-channel as Ricean in general, the statistical chracteristics were investigated. In this part of the paper, we develop a generalized two step channel/resource allocation algorithm, which incorporates the two statistical measurements, and analyze the spectral efficiency of OFDMA in terms of average frequency utilization for the two sub-channel structures. In OFDMA with distributed structure, the key design parameter would be the sub-channel bandwidth. To give an insight into the impact on this parameter, we show in the numerical results the frequency utilization as a function of sub-channel bandwidth normalized to coherence bandwidth. As confirmed by numerical results, for contiguous sub-channel structure, we obtain the nominal multiuser diversity gain when the sub-channel bandwidth is smaller than the coherence bandwidth and lose the gain as it is getting larger.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.12C
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• pp.1186-1192
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• 2005

In this paper, we propose a novel channel estimation method based on pilot channel in a frequency interleaved multicarrier code division multiple access (MC-CDMA). Using the chip interleaving (CI) technique in the frequency domain make it possible to achieve higher frequency diversity gain than the system with conventional symbol interleaving. However, in CI-MC-CDMA systems, a pilot channel-based channel estimation (PCCE) cannot be applied because the orthogonality between pilot symbols and the data symbol is not maintained. The proposed method alters the system structure in order to maintain orthogonality between data and pilot channels over two consecutive subcarriers. Therefore, it can obtain accurate channel state information (CSI) in CI-MC-CDMA.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.43 no.11 s.353
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• pp.8-15
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• 2006

In this paper, we propose the iterative receiver for LDPC-coded OFDMA systems in uplink environments. Applying the Wiener filtering to pilot symbols, an initial channel estimation can be performed effectively. To reduce the complexity of the Wiener filtering, we approximate Wiener filtering coefficients to pre-determined coefficients according to estimated correlation of channel. After an LDPC decoding process, soft symbol derived by extrinsic information of decoder outputs is used to estimate channel. we also derive the error variance of channel estimation and maximum ratio combined results. Using combined results, the channel correlation is re-estimated. Then the proper Wiener filtering coefficients are chosen according to the re-estimated result of the channel correction. Using a computer simulation, we show that the proposed receiver structure has the better performance than the receiver using only pilot symbols.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.46 no.6
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• pp.78-85
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• 2009

In time-varying multipath fading channels, orthogonal frequency division multiple access (OFDMA) uplink systems suffer severe performance degradation caused by inter-channel interference (ICI). In this paper, we propose a hybrid interference cancellation (HIC) for suppressing the degradation effect of ICI. The proposed HIC can achieve both exact interference cancellation and low detection complexity through efficient combination of parallel detection and serial cancellation. Simulation results show that, as the effect of Doppler increases, the proposed HIC achieves bit error rate (BER) performance enhancement in compared with severe performance degradation of conventional OFDMA receivers. In addition, both the computational complexity and total detection time are reduced.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.11C
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• pp.1087-1094
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• 2007

In this paper, we propose a new peak-to-average power ratio (PAPR) reduction scheme of orthogonal frequency division multiplexing (OFDM) system, called a modified selected mapping (SLM) scheme, which considerably reduces the computational complexity with keeping the similar PAPR reduction performance compared with the conventional SLM scheme. The proposed scheme is analytically and numerically evaluated for the OFDM system specified in the IEEE 802.16 standard. For the OFDM system with 2048 subcarriers, the proposed scheme with 4 binary phase sequences can reduce the complex multiplications by 63.5% with the similar PAPR reduction compared with the SLM scheme with 16 binary phase sequences.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.6C
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• pp.444-449
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• 2008

Selected mapping is well-known scheme for peak to average power ratio(PAPR) reduction in orthogonal frequency multiplexing(OFDM) systems. In this paper, we proposed a new scheme for generating a set of candidate transmission sequences. In conventional SLM, candidate sequences were generated by rotating the phases of the frequency domain OFDM M-ary symbols but in proposed scheme, candidate sequences are generated by multiplying the m-sequences with data in bit format of OFDM signals. Computer simulation results show that, as compared to the conventional SLM scheme, the proposed approach has better PAPR reduction performance and especially in case of OFDM systems which has small number of subcarriers has better performance than large ones.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.38A no.2
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• pp.201-208
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• 2013

In this paper, the PTS scheme generate alternative orthogonal frequency frequency division multiplexing (OFDM) signal sequences by multiplying all the time domain samples with phase rotating vectors to find an OFDM signal having the minimum peak-to-average power ratio (PAPR). However, it needs an exhaustive search which causes large computational complexity. In order to solve this problem, we propose two efficient methods based on the crest factor. The first proposed scheme is to select time domain sample with large magnitude to calculate PAPR, and the next scheme is to calculate the absolute value of real and imaginary part of the sample at each subblock. The simulation results show that the proposed schemes achieve better PAPR reduction performance than existing PTS schemes.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.43 no.3 s.345
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• pp.39-48
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• 2006

In this paper, we analyze the spectral efficiency of multicarrier-code division multiple access (MC-CDMA) scheme. First, we derive a generalized formula for the spectral efficiency according to the number of subcarriers involved in, code division multiplexing and the number of codes used (i.e., loading factor), under a given set of channel coefficients. Also, we derive a generalized formula for spectral efficiency of various reduced-complexity systems that divide the full sets of subcarriers into several groups of subcarriers for code division multiplexing. Then, through these derivations, we establish an inter-relationship between the frequency selectivity and diversity order according to the number of multipaths. From the results, we choose the smallest code length while maximizing the diversity effect, provide an optimum subcarrier allocation strategy, and finally suggest a system structure for capacity-maximizing under the smallest code length. Through numerical analyses under simulated environments, we analyze the properties of spectral efficiency of various systems with reduced complexity and choose a major contributing factors to system design and a better system design methodology. Finally, we compare the spectral efficiency of the MC-CDMA scheme and orthogonal frequency division multiplexing (OFDM) scheme to make a relationship between both schemes.