Chae-Hyun Lim

Robust LS channel estimation with phase rotation for single frequency network in OFDM

A channel estimation technique has been proposed with the least square (LS) based on comb-type pilots for orthogonal frequency division multiplexing (OFDM). An interpolator should be used to obtain the channel information between adjacent pilots for a comb-type pattern. However, an interpolation method causes additional errors due to the interpolation properties of not detecting long delayed paths and generating fictitious paths, OFDM has its strength in realistic possibility of constructing the single frequency network (SFN). However, synchronization and channel estimation errors are revealed by multi-path fading with long delayed paths in SFN channels. We propose a robust channel estimator overcoming the adverse effects of long delayed signals in SFNs. A new channel estimator is proposed with the interpolated LS by applying phase shifted samples in the frequency-domain. In particular, the proposed channel estimator is more robust to the single frequency network (SFN) whenever the delay spread is less than the cyclic prefix length. For a Rayleigh channel, the proposed estimator has an almost 3 dB gain comparing to conventional estimator

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.

Channel capacity enhancement using virtual array elements in smart antenna systems

A smart antenna system using virtual array elements is proposed to increase the channel capacity of mobile communication systems. The proposed smart antenna system increases the degree of freedom using virtual array elements generated by complex conjugations of real array elements signals and sign compensations using the orthogonality between the user identification codes in the I and Q channels. The performance of the proposed system is analyzed under cdma2000 specifications in urban cell environments.

Viterbi decoder aided equalization and sampling clock tracking for OFDM WLAN

IEEE 802.11a is a standard for the high-speed wireless local area network (WLAN), supporting from 6 up to 54 Mbit/s in a 5 GHz band. There are training sequences for the IEEE802.11a standard in the preamble for channel equalization and sampling timing tracking. However, the preamble is only at the initial transmission. Thereby, it is not easy to track channel variations and sampling phase errors even though there are four pilot samples in each symbol of 52 subcarriers. We propose a channel equalization algorithm and a sampling clock tracking algorithm by utilizing the Viterbi decoder output of the IEEE 802.11a WLAN standard. The proposed channel equalizer adaptively compensates channel variations. The proposed system uses re-encoded Viterbi decoder outputs as reference symbols for the adaptation of the channel equalizer. It also extracts sampling phase information with the Viterbi decoder outputs for fine adjustment of the sampling clock. The proposed sampling timing recovery and equalizer are more robust to noise and frequency selective fading environments than conventional systems using only four pilot samples.

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.

Equalization of ATSC DTV Systems Based on Space-Time Signal Processing for Mobile Reception

A joint operation of the Capon and the least mean square (LMS) beamformers is proposed to get reliable reception performance for the advanced television system committees (ATSCs) digital television (DTV) system. The proposed beamformer called a Capon and LMS (CLMS) beamformer provides an initial weight set for the operation of the LMS beamformer during the training sequence. The Capon beamformer is operated during the payload data period. By the way, the Capon beamformer could be unstable because of frequent changes of the steering beam direction in mobile conditions. To get a stable operation in mobile conditions with the CLMS beamformer, the signal-to-noise ratio (SNR) is estimated to control the operation by utilizing the Viterbi decoder output. When the SNR level is below the threshold of visibility (TOV), the Capon beamformer is operated to get a fast initialization of the LMS beamformer. When the receiver is moving with a speed of 60 km/h for 700 MHz band, the successful reception probability of the CLMS beamformer based on SNR estimation is 76% compared to 60% for that of the conventional CLMS beamformer

Mobile reception of broadband signal with joint operation of blind and LMS adaptive beamforming

A beamforming technique is proposed to guarantee a feasible reception performance in mobile communications. The proposed algorithm called CLMS is jointly operating a Capon and a least mean square (LMS) adaptive beamforming techniques. The CLMS algorithm works as the Capon and the LMS beamformers for unknown and known data periods, respectively. The CLMS algorithm is evaluated in a mobile reception channel of the geometrically based single bounce circular model (GBSBCM). Compensation abilities over multipaths and a phase distortion are analyzed and compared for the proposed, Capon, LMS beamforming algorithms. A decision feedback equalizer (DFE) is also considered to mitigate remaining multipaths by concatenating the beamformer.