Hyoung-Nam Kim

Channel Estimation Based on a Time-Domain Threshold for OFDM Systems

Channel estimation for OFDM systems is usually carried out in frequency domain by the least-squares (LS) method using known pilot symbols. The LS estimator has a merit of low complexity but may suffer from noise because it does not consider any noise effect in obtaining its solution. To enhance the noise immunity of the LS estimator, we consider the estimation noise in time domain named discrete Fourier transform (DFT)-based channel estimation. Residual noise existing at the estimated channel coefficients in time domain could be reduced by reasonable selection of a threshold value. To achieve this, we propose a channel-estimation method based on a time-domain threshold which is a standard deviation of noise obtained by wavelet decomposition. Computer simulation shows that the estimation performance of the proposed method approaches to that of the known-channel case in terms of bit-error rates after the Viterbi decoder in overall SNRs.

Performance analysis of error propagation effects in the DFE for ATSC DTV receivers

The paper analyzes the error propagation phenomenon in the decision feedback equalizer (DFE) for the receivers of Advanced Television Systems Committee (ATSC) digital television (DTV) and presents the performance upper-limits of the DFE by comparing various error propagation cases and the no-error propagation case. As one approach to the performance limit, we consider a blind DFE, adopting a trellis decoder with a trace-back depth of 1 as a decision device. Through simulation, we show how much the DFE performance in ATSC DTV receivers is affected by error propagation. We found that while blind equalization is preferable to decision-directed (DD) equalization at signal-to-noise ratio (SNR) values less than 18 dB, DD equalization is superior to blind equalization at SNR values greater than 18 dB. In addition, symbol error rate curves quantitatively show that the performance difference in the DFE caused by error propagation becomes clearer at the trellis decoder following the DFE. The analysis results presented are very informative for developing equalization algorithms for ATSC DTV receivers.

Unbiased equation-error adaptive IIR filtering based on monic normalization

We present a novel may to remove the bias in equation-error based adaptive infinite impulse response (IIR) filtering by conceiving a scheme called monic normalization. It is found that normalizing all the coefficients of the denominator filter by the first coefficient after each adaptation removes the bias and leads to unbiased estimates. The analysis of stationary points is presented to show that the proposed method can indeed produce unbiased parameter estimates in the presence of noise. The computer simulation results also demonstrate that the proposed method performs better than or comparable to existing algorithms, while requiring much lower computational complexity.

Blind decision feedback equalization for VSB-based DTV receivers

We present a novel blind decision feedback equalization algorithm for VSB-based DTV receivers by modifying a stop-and-go dual-mode algorithm to improve the performance of convergence speed and residual MSE. Considering VSB modulation and RF-band effects with the baseband-equivalent VSB channel model raises the reliability of our results for practical applications.

Feedback Cancellation for T-DMB Repeaters Based on Frequency-Domain Channel Estimation

This paper presents a novel feedback canceller for a terrestrial digital multimedia broadcasting (T-DMB) on-channel repeater (OCR). There is a crucial requirement for an OCR that the isolation between the transmitter and the receiver antennas should be sufficiently secured to prevent unwanted oscillation. The required isolation, however, may not be satisfied by some physical or technical limitations. In order to overcome these obstacles, undesired feedback signals have been usually reduced by feedback cancellation techniques based on pilot-aided frequency-domain channel estimation (FDCE). In case of the T-DMB system, however, there are no pilot symbols except for the phase reference symbols (PRSs), which are transmitted only once every 76 OFDM symbols. Consequently, the convergence speed and the tracking capability of the FDCE-type feedback cancellers may not meet the required robustness in coping with dynamic feedback channels. To improve the immunity to time-varying channels, we increase the usable number of updates in our feedback canceller, where the PRS is divided into several sub-PRSs during the PRS period and pseudo pilots are generated by decision-directed estimation in the data-symbol duration. Simulation results show that the proposed methods do improve the convergence speed and the tracking capability of the conventional method in both static and dynamic feedback channels.

Coherent Detection for T-DMB Receivers in Hierarchical Modulation Mode

Hierarchical modulation has been considered for achieving higher data rates in Terrestrial-DMB (T-DMB) systems, which yields man-made noise caused by the shortened symbol distance. Such noise may produce a serious problem on a fast-fading channel for the proper operation of conventional T-DMB receivers using noncoherent differential detection because it can increase the required SNR larger than the securable SNR of the receivers. To overcome the weakness of being susceptible to fast fading of the noncoherent detection, we propose a novel decision-directed channel estimation method for the coherent detection of phase correction. In the proposed method, least-squares channel estimation is carried out with hard-decision symbols due to the absence of known pilots and then the estimated coefficients are exponentially averaged using a forgetting factor to compensate for the probable inaccuracy of the instantaneous estimation. Simulation results show that the proposed method secures higher data-rate transmission of advanced T-DMB systems by reducing the degradation caused by hierarchical modulation and fast fading.

Noise reduction for channel estimation based on pilot-block averaging in DVB-T receivers

This paper proposes a noise-reduction method to improve the channel-estimation performance in digital video broadcasting-terrestrial (DVB-T) receivers. In DVB-T, the channel estimation is carried out through two processes of pilot signal estimation and channel coefficient interpolation and thus the performance of the channel estimation is seriously affected by the accuracy of the interpolation. Since the accuracy of the interpolation depends on signal-to-noise ratio, it is important to reduce noise existing at the reference coefficients for the interpolation. To achieve the reduction of noise, we use pilot-insertion type approximation in four consecutive OFDM symbols, which construct one block with the same channel coefficients at the same pilot positions. Considering the fade rate of channels, we average the estimated reference, channel coefficients corresponding to the same subcarriers in two or more blocks. Computer simulation shows that the proposed method improves the estimation performance by more than 1 dB in terms of the symbol error rate obtained after equalization.

A Higher Data-Rate T-DMB System Based on a Hierarchical A-DPSK Modulation

Hierarchical modulation can be effectively used to enhance terrestrial digital multimedia broadcasting (T-DMB) or digital audio broadcasting (DAB) systems in response to both the demand for higher data-rate and the need to be backward compatible with legacy receivers. QAM-type modulations are well-liked for hierarchical transmission but require coherent detection based on pilot symbol aided channel estimation. In the T-DMB or the DAB system using DQPSK modulation, however, any available pilot symbols except for the phase reference symbol do not exist. Differential amplitude phase shift keying (DAPSK) modulation is easily applied to the T-DMB system for a hierarchical modulation but may be susceptible to fast fading. As a good candidate for a hierarchical modulation of T-DMB to solve the above problems, we propose an amplitude differential phase shift keying (A-DPSK) modulation which is robust to fast fading by estimating only amplitude coefficients of the channel transfer function with the use of amplitude pilots. To raise the accuracy of channel estimation, we arrange the amplitude pilots in a come-type and introduce a noise-reduction scheme of averaging estimated channel coefficients. Simulation results show that the proposed A-DPSK provides a good choice for achieving a higher data-rate over other possible modulation schemes for advanced T-DMB or DAB systems.

Effects of Synchronizing Errors on Cutting Performance in the Ultra-High-Speed Tapping

Abstract Synchronizing errors between the spindle motor and the z-axis motor directly influences the cutting characteristics in tapping, because the tapping process is accomplished by synchronizing the movement of the z-axis with the revolutionary spindle motion. The excessive synchronizing error can cause tap breakage due to the abrupt increase of cutting torque or damage the thread accuracy by overcutting the already cut threads. This paper describes the effects of the synchronizing errors on the cutting performance in the ultra high-speed tapping and presents a minimum level of synchronizing errors necessary to maintain the quality of the cut thread.

Reduced-Complexity Maximum Likelihood Direction-of-Arrival Estimation Based on Spatial Aliasing

Spatial aliasing is an undesirable phenomenon that prevents the unique determination of the direction of arrival (DOA) of impinging signals in array signal processing. However, the characteristics of spatial aliasing that generates ambiguous DOAs can be also used to reduce the computational complexity in maximum-likelihood (ML) DOA estimation. This paper proposes a structural method to dramatically reduce the computational complexity of the ML DOA estimation using the spatial aliasing generated by a nested array structure with a doubly scaled aperture. An ML full grid search is computationally simplified by the highly compressed searching range and the small number of candidate values to be searched which are derived based on spatial aliasing. Performance analyses based on the theoretical bounds and computational complexity with computer simulations show that the proposed method requires an extremely reduced computational load compared to the conventional ML DOA estimation with a uniform linear array (ULA) while achieving the performance enhancement in the estimation accuracy due to the enlarged array aperture.