Yeong-Hyeon Kwon

A new LDPC decoding algorithm aided by segmented cyclic redundancy checks for magnetic recording channels

We introduce a new low-density parity-check (LDPC) decoding algorithm that exploits the cyclic redundancy check (CRC) information of data segments. By using the error detection property of the CRC, we can successively decode data segments of a codeword corrupted by random errors and erasures. The key idea is that the messages from the variable nodes with correct checksum are fixed to deterministic log likelihood ratio values during LDPC iterative decoding. This approach improves the decoding speed and codeword error rate without significant modification of the LDPC decoding structure. Moreover, the CRC is also used for an early stopping criterion of LDPC decoding. Simulation results verify our claims.

Determination of fragmentation size in wireless system

In noisy channel, wireless communication packets become erroneous, and retransmission reduce throughput. If an error rate (ER) is known, some parameters can be adjusted such that maximum throughput is obtained. In this paper, a new ER estimation method is proposed, which is asymptotically unbiased, and transmission time cost based on the estimated ER is declared. With cost function, minimum transmission time is identified, which is corresponding to the maximum throughput and the optimal fragmentation size. The proposition is verified with IEEE 802.11b, DSSS PHY system.

Design and performance analysis of UWB TH-MA scheme using multi-code based PPM

In this paper, we propose an UWB time-hopping multiple-access (TH-MA) scheme using multi-code based PPM. The multi-code signal, which is generated by spreading the desired users M transmitting bits with M orthogonal codes, is pulse-position modulated. At the receiver, the received signal is demodulated through two stages. In the first stage, the multi-code based PPM signal is demodulated through the Bayes decision rule. Then, in the second stage, the demodulated multi-code signal is de-spreaded by using the M orthogonal spreading codes that are used in the transmitter. The BER performance and bit transmission rate (R/sub b/) of the proposed scheme is analyzed. Numerical results show that the proposed scheme has a much higher bit transmission rate than the conventional PPM scheme as M increases and also has better BER performance than the conventional PPM scheme in the same bit transmission case because it can achieve an additional spreading gain.

A new LDPC decoding algorithm aided by segmented CRCs for erasure channels

This letter introduces a new low-density parity-check (LDPC) decoding algorithm that exploits the cyclic redundancy check (CRC) information of data segments. By using the error detection property of CRC, we can successively decode data segments in the codeword corrupted by magnetic recording channels. The key idea is that variable nodes corresponding to data segments with correct checksum are fixed to deterministic log likelihood ratio (LLR) values (or probabilities) during the LDPC iterative decoding. In this way, the performance regarding the decoding speed and codeword error rate can be improved without significant modification of the LDPC decoding structure. Moreover, the CRC information is also used for an early stopping criterion of LDPC decoding. Simulation results verify our claims.

Efficient packet transmission scheme using minipackets in wireless networks

An efficient packet transmission scheme is proposed to avoid unnecessary retransmission of correctly received symbols. In the proposed scheme, we implement small packets (referred to as minipackets) in a transmission payload. The use of minipackets makes it possible to retransmit only erroneous received symbols, which leads to high throughput and low packet delay through harsh wireless channels. Moreover, as the performance is dependent on the length of the minipackets, we present a method of searching for the optimal minipacket length. Numerical simulations show that the performance of the proposed scheme is significantly improved compared with the conventional single payload scheme, and even better than that of the combined hybrid-ARQ scheme.

Blind iterative channel estimation and LDPC decoding for OFDM systems

This paper aims at designing a novel blind iterative receiver for LDPC coded OFDM systems, where pilot symbols for estimating the channel are not inserted in the transmitter. Incorporating the differential QPSK scheme and LDPC decoder, we show that the decision-directed channel estimator can produce the channel information to improve performance of the LDPC decoder. The resulting blind iterative receiver can resolve the drawbacks of differential detection, an improved channel estimate and BER performance in the frequency selective channel, while keeping bandwidth efficiency. Simulation results verify performance of our scheme.

A novel blind adaptive equalization in time-varying channels with ISI

This paper addresses novel adaptive blind equalization for time-varying channels with inter-symbol interference (ISI). In this paper, a combined receiver structure using the CMA and Kalman filters investigated to utilize the advantages of both algorithms: a simple implementation for a blind algorithm and tracking capability. Based on this receiver structure, we present two techniques to improve the performance and reduce the complexity of channel equalization. One is a new cost function, which achieves faster convergence to adapt time variation of the channels. The other is termed a prior ISI reduction scheme, which reduces ISI before equalization and in turn leads to low-complexity equalization. The performance of our algorithms is confirmed by the mean square error (MSE) analysis and computer simulations.

Optimal erasure selection of M-ary PAM signaling for errors and erasures decoding algorithms

Erasure selection for errors-and-erasures decoding algorithms is investigated and analyzed, where declaring erasures provides additional gain for hard decision decoding. To derive an analytical criterion for optimal erasure selection, we first investigate erasure effects on decoding performance, by which a metric function for erasure selection is derived. From the derived metric, a suboptimal threshold is defined, which can be used for errors and erasures decoding of large code-distance channel codes. Moreover, application to M-ary (M>2) PAM constellation is also described. For verification of the proposed erasure threshold, simulation results with popular channel codes are included.

Novel Blind Adaptive Equalization over Doubly-Selective Fading Channels

A new receiver structure that combines the constant modulus algorithm (CMA) and the Kalman filter (KF) is investigated to exploit the advantages of both algorithms; simple implementation of blind algorithms, and excellent tracking ability, respectively. The proposed scheme achieves faster convergence and adaptability to the channel variation, which is verified through comparative simulations in doubly-selective (time- and frequency-selective) fading channels.

Reduced modulation modes for bit and power allocation in multi-carrier transmissions

A new scheme is proposed to optimally reduce the modulation modes for bit and power allocation algorithm in multi-carrier transmissions. By reducing the modulation modes, computational load and hardware complexity can be reduced without any performance degradation. When modulation modes for rectangular QAM are available at a transmitter, it is shown that the modulation modes for rectangular QAM can be reduced to those for square QAM which is the subset of rectangular QAM. After identifying a power allocation property of rectangular QAM, it is proved that the optimal allocation with square QAM has exactly the same performance as that with rectangular QAM. Numerical results show the reduction of computational load and verify our theoretic presentations.