• ETRI Journal
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• v.29 no.3
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• pp.381-389
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• 2007

In this paper we propose a graph-theoretic method based on linear congruence for constructing low-density parity check (LDPC) codes. In this method, we design a connection graph with three kinds of special paths to ensure that the Tanner graph of the parity check matrix mapped from the connection graph is without short cycles. The new construction method results in a class of (3, ${\rho}$)-regular quasi-cyclic LDPC codes with a girth of 12. Based on the structure of the parity check matrix, the lower bound on the minimum distance of the codes is found. The simulation studies of several proposed LDPC codes demonstrate powerful bit-error-rate performance with iterative decoding in additive white Gaussian noise channels.

• ETRI Journal
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• v.40 no.2
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• pp.218-226
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• 2018

Multiuser detection (MUD) and channel estimation techniques in space-division multiple-access aided orthogonal frequency-division multiplexing systems recently has received intensive interest in receiver design technologies. The maximum likelihood (ML) MUD that provides optimal performance has the cost of a dramatically increased computational complexity. The minimum mean-squared error (MMSE) MUD exhibits poor performance, although it achieves lower computational complexity. With almost the same complexity, an MMSE with successive interference cancellation (SIC) scheme achieves a better bit error rate performance than a linear MMSE multiuser detector. In this paper, hybrid ML-MMSE with SIC adaptive multiuser detection based on the joint channel estimation method is suggested for signal detection. The simulation results show that the proposed method achieves good performance close to the optimal ML performance at low SNR values and a low computational complexity at high SNR values.

• ETRI Journal
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• v.42 no.1
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• pp.26-35
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• 2020

Herein, we consider uplink multiuser massive multiple-input multiple-output systems when multiple users transmit information symbols to a base station (BS) by applying simple space-time block coding (STBC). At the BS receiver, two detection filters for each user are used to detect the STBC information symbols. One of these filters is for odd-indexed symbols and the other for even-indexed symbols. Using constrained output variance metric minimization, we first derive a special relation between the closed-form optimal solutions for the two detection filters. Then, using the derived special relation, we propose a new blind adaptive algorithm for implementing the minimum output variance-based optimal filters. In the proposed adaptive algorithm, filter weight vectors are updated only in the region satisfying the special relation. Through a theoretical analysis of the convergence speed and a computer simulation, we demonstrate that the proposed scheme exhibits faster convergence speed and lower steady-state bit error rate than the conventional scheme.

• Proceedings of the IEEK Conference
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• 2000.09a
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• pp.229-232
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• 2000

Orthogonal frequency division multiplexing(OFDM) has meanwhile become part of several telecommunicati ons standards, such as satellite and terrestrial digital audio broadcasting(DAB), digital terrestrial TV broad casting(DVB), asymmetric digital subscriber line(ADSL) for high-bit-rate digital subscriber services on twisted-pair channels, and broadband indoor wireless systems. In his paper, we show that OFDM signals contain sufficient structure to accomplish blind channel estimation using second order statistics only. This method doesn't require redundancy as cp in transmitter. And the result is compared with PSAM channel estimation as least square, linear minimum mean square, singular value decomposition.

• Journal of the Korean Institute of Telematics and Electronics D
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• v.35D no.8
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• pp.69-75
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• 1998

We demonstrate wavelength conversion of 2.5Gb/s optical signals by four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). We investigate the effect of input pump and signal powers on the coversion efficiency, optical signal-to-noise ratio (OSNR) and extinction ratio to be a measure of performance in a wavelength converter. As a result, we show that the maximum bit error rate (BER) performance can be obtained by co promising among high-vonversion efficiency (minimum Pprobe), high-OSNR (maximum Pprobe) and low-cross-gain saturation effects (Pprobe kept at least 6dB weaker than Ppump). In our experiment, we obtain optimum performance at +3 dBm pump power and -6dBm signal power. The power penalty incurred in the wavelength conversion can be minimized by careful selection of the input pump and signal powers. We show that about 0.5dB power penalty for 3.2nm wavelength coversion at 10-10 BER is achievable.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2011.07a
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• pp.294-295
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• 2011

The fundamental problem of selective protection scheme for encoded bitstreams is to find an efficient algorithm to select the set of frames required to be encrypted that can maximize protection effect with the minimum amount of protected data is required. In this paper, we propose an optimal selective protection scheme for SVC bitstreams by protecting the best combination of frames for selective protection in the sense that the amount of data required for protection is minimized and the resulting visual quality degradation is maximized. The selection of the frames to be encrypted is done by first expressing R-Q (protection rate - visual quality) relationship with Lagragian cost model. The experimental results show that, compared to protecting SVC bitstreams layer by layer, the proposed scheme gives superior performance in terms of protection effectiveness due to its better selection of frames for protection given protection bit budgets.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.5A
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• pp.331-340
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• 2009

In this paper, we propose a new power loading algorithm for orthogonalized spatial multiplexing(OSM) systems over flat-fading multiple-input multiple-output (MIMO) channels. Compared to SVD-based transmission scheme, the OSM scheme exhibits a good system performance with lower complexity and feedback overhead. To further improve the performance in OSM systems with power loading, we introduce a geometric approach on the Euclidean distance between the constellation points in the effective channel. Using this approach, we show that the optimal power loading parameters in terms of the minimum distance can be obtained. Simulation results demonstrate that our algorithm provides a 5dB gain at a bit error rate (BER) of $10^{-4}$ over that of no power loading case with both QPSK and 16-QAM. Consequently, our power loading algorithm allows us to significantly improve the system performance with one additional feedback value.

• Proceedings of the IEEK Conference
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• 2003.07a
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• pp.218-221
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• 2003

본 논문에서는 사전등화를 이용하는 상향링크 MC-CDMA(multicarrier-code division multiple access)/TDD(time division duplexing) 시스템에서 사전등화를 위한 상향링크 채널을 추정하는 방법들을 제안하고 시스템의 성능분석을 수행한다. 제안된 방법들에서는 하향링크 슬롯구간에서의 채널변화를 적절한 차수의 다항식으로 모델링하고, 이 다항식을 상향링크 슬롯구간으로 확장함으로써 상향링크 슬롯구간의 채널을 추정한다. 하향링크 슬롯구간에서의 채널변화는 MMSE(minimum mean squared error)curve fitting 방법이나 Lagrange 보간법 등이 사용되며 1차, 2차, 3차 다항식으로 근사화 된다. 성능지표로 정확도보다 시스템 성능이 중요 하므로 BER (bit error rate)을 사용한다. 다양한 시스템 및 채널환경에서의 모의실험 결과로부터 Lagrange 보간법은 하향링크 채널정보가 정확한 경우에는 MMSE 방법보다 성능이 다소 우수하지만 하향링크 채널추정 오류에 매우 민감하며, 2 차 다항식을 사용한 MMSE curve fitting 방법은 다양한 환경에서 우수한 성능을 가질 뿐만 아니라 채널추정 오류에도 매우 강인함을 알 수 있다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.2 no.4
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• pp.209-221
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• 2008

In this paper, we propose a subcarrier allocation method and a composite linear minimum mean square error (LMMSE) channel estimator to increase spectrum efficiency in orthogonal frequency division multiplexing (OFDM) transmit diversity. The pilot symbols for OFDM transmit (Alamouti) diversity are exclusively allocated in two OFDM symbols in different antennas, which causes serious degradation of spectrum efficiency. To reduce the number of pilot symbols, our subcarrier allocation method uses repetition-coded data symbols, and the proposed channel estimator maintains good bit error rate (BER) performance.

• Journal of Communications and Networks
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• v.9 no.1
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• pp.34-42
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• 2007

Orthogonal frequency division multiplexing (OFDM) combined with time division multiplexing (TDM), in this paper called OFDM/TDM, can overcome the high peak-to-average-power ratio (PAPR) problem of the conventional OFDM and improve the robustness against long time delays. In this paper, the bit error rate (BER) performance of OFDM/FDM in a frequency-selective Rayleigh fading. channel is evaluated by computer simulation. It is shown that the use of frequency-domain equalization based on minimum mean square error criterion (MMSE-FDE) can significantly improve the BER performance, compared to the conventional OFDM, by exploiting the channel frequency-selectivity while reducing the PAPR or improving the robustness against long time delays. It is also shown that the performance of OFDM/FDM designed to reduce the PAPR can bridge the conventional OFDM and single-carrier (SC) transmission by changing the design parameter.