Hae-Sock Oh

Frame selection algorithm with adaptive FFT input for OFDM systems

Orthogonal frequency division multiplexing (OFDM) systems transmit a cyclic prefix, called a guard-interval, which is inserted in front of the effective symbol period along with signals copied from some of the last parts in the effective symbol period. This guard-interval is then used to reduce distortions which are due to multipath conditions. However, conventional OFDM systems produce intersymbol interference (ISI) under severe multipath or pre-ghost channel conditions even though they accurately estimate the effective symbol period. Accordingly, the current paper proposes a frame selection algorithm for a fast Fourier transform (FFT) operation in the OFDM receiver to remove the ISI produced in conventional OFDM systems under multipath conditions. The proposed algorithm replaces some of the rear parts of the effective symbol period with some of the guard-interval as an FFT input frame. The replaced period for the FFT input frame is changed according to the multipath environment. Four estimation methods for determining the replaced period are also presented. The proposed algorithm can increase the receiver performance by more than 3 dB under multipath conditions in a terrestrial digital video broadcasting (DVB-T) system without changing the equalization method.

An adaptive double-dwell PN code acquisition system in DS-CDMA communications

An adaptive double-dwell acquisition system for pseudo-noise (PN) sequences is presented for direct-sequence spread-spectrum (DS-SS) systems. Since existing acquisition systems have a fixed threshold value, they are unable to adapt to varying mobile communication environments resulting in a high false alarm rate or low detection probability. Accordingly, this study presents an adaptively varying threshold scheme that uses a constant false alarm rate (CFAR) algorithm. The performance of the proposed system is compared to the conventional double-dwell system based on deriving formulas for the detection probability, false alarm rate, and mean acquisition time of the proposed system. The results confirm that the proposed system can produce a highly improved performance over the conventional system.

Sparse equalizer using adaptive weight activation for fast start-up in ATSC DTV systems

A sparse equalizer using a data-recycling algorithm is proposed for 8-vestigial sideband (VSB) digital television (DTV) equalizers. The proposed sparse equalizer with the data-recycling algorithm selects effective equalizer taps for channel equalization, thereby improving the convergence speed and output signal-to-noise ratio (SNR). The output SNR, convergence speed, and number of required equalizer taps are compared between the proposed and conventional algorithms in Brazilian channels and a single ghost dynamic channel.

Adaptive hybrid beamformer for mobile reception of ATSC DTV

An adaptive hybrid beamformer is proposed to improve the reception performance of the advanced television system committee (ATSC) digital television (DTV) in a mobile environment. Dynamic multipaths and Doppler shifts severely degrade the reception performance of the ATSC DTV receiver. Accordingly, a hybrid beamformer, called a Capon and least mean square (CLMS) beamformer, is presented that uses direction of arrival (DOA) information and the least mean square (LMS) beamforming algorithm. The proposed CLMS algorithm efficiently removes dynamic multipaths and compensates for the phase distortion caused by Doppler shifts in mobile receivers. When the CLMS beamformer has an insufficient degree of freedom (DOF), the subsequent use of an equalizer removes any residual multipath effects, thereby significantly improving the performance of DTV receivers. The performances of the proposed CLMS, Capon, and LMS beamformers are compared based on computer simulations. In addition, the overall performance of the CLMS beamformer followed by an equalizer is also considered.

Equalizer with data recycling algorithm for fast convergence in 8-VSB DTV systems

The 8-vestigial sideband modulation (VSB) for digital television systems transmits a training sequence called field sync every 313 segments with a period of 24.2 ms. However, the field sync sequence is not long enough to guarantee the operation of an equalizer. So it takes many field sync sequences for the convergence of the equalizer especially in severely distorted channel conditions. In this paper, an algorithm called data recycling is proposed for fast convergence of the equalizer in 8-VSB systems. The data recycling algorithm utilizes only one field sync sequence for several times thereby reducing the convergence time for the equalizer. The control algorithm for the operation of the equalizer is also proposed with comparison of conventional equalizer.

Monopulse angle estimation with constrained adaptive beamforming using simple mainlobe maintenance technique

A new monopulse radar system is proposed to overcome the difficulties with conventional monopulse techniques under jamming conditions. The proposed system uses a simple mainlobe maintenance technique based on the estimated direction of the mainlobe jammer. Thereafter, an adaptive array is used to create pattern nulls in the incoming directions of jammers, while maintaining the shape of the mainlobe. As a result, the proposed system can track the target angle without any correction of the adaptive sum and difference beam outputs.

Adaptive sparse equalizer robust to fast fading and long delay spread for ATSC DTV

An equalization algorithm is proposed to guarantee a stable performance in fast fading channels for the advanced television system committee (ATSC) digital television (DTV) systems, in channels with high Doppler shifts, the conventional equalizer shows severe performance degradation. The conventional equalizer with long filter taps to overcome long delay profiles is not suitable for fast fading channels. The proposed sparse equalization algorithm is robust to the multipaths with long delay profiles as well as fast fading by utilizing channel estimation and an equalizer initialization. Fast fading channels with high Doppler shifts can be compensated with an adaptive tap selection technique as well as variable step-sizes. Under the ATSC test channels, the proposed algorithm is analyzed and compared with the conventional equalizer.

Adaptive double-dwell PN code acquisition in direct-sequence spread-spectrum systems

An adaptive double-dwell acquisition system of pseudo-noise (PN) sequences is presented for direct-sequence spread-spectrum (DS-SS) systems. Since existing acquisition systems have a fixed threshold value, they are unable to adapt to various mobile communications environments. Accordingly, this results in a high false alarm rate or low detection probability. Therefore, an adaptively varying threshold scheme is proposed through the use of a constant false alarm rate (CFAR) algorithm. By deriving formulas for the detection probability and false alarm rate of the proposed adaptive system, its performance was analyzed and compared to an existing one. The results showed that the proposed adaptive double-dwell acquisition system. Provides a better performance than conventional systems.

Adaptive hybrid acquisition with antenna diversity in CDMA systems

Since existing acquisition systems have a fixed threshold value, they are unable to adapt to varying mobile communication environments. This results in a high false-alarm rate or low detection probability. In this paper, an adaptive hybrid acquisition system that adopts an antenna diversity technique and the cell-averaging constant false alarm rate (CA-CFAR) algorithm is presented to acquire a pseudo-noise (PN) signal for CDMA systems. The CA-CFAR algorithm and an antenna diversity technique are adopted to adjust the decision threshold according to environments and to improve detection performance, respectively. The detection probability, false alarm, rate, miss detection probability, and mean acquisition time in the Rayleigh fading channel are derived and compared to those of the conventional hybrid acquisition system with fixed threshold.

Bidirectional DFE Arbitrated by Viterbi Decoder Path Metrics

A bidirectional decision feedback equalizer (DFE) is proposed with the arbitration of the Viterbi decoder. The proposed DFE uses the minimum path metric between Viterbi decoders for the forward and reverse DFEs in the arbitration process. It has over 2 dB SNR gain against the conventional bidirectional DFE and the single DFE.