Heung-No Lee

On the estimation of target spectrum for filter-array based spectrometers

Miniature spectrometers have been drawn researchers much attention due to its wide variety of possible applications. In this paper, we show the achievability of a fine spectrometer on-a-chip based on a low-performance, low-cost filter-array. A low quality filter-array is augmented with digital signal processing techniques. A series of estimators for recovering target spectrum is introduced. By exploiting non-negative nature of spectral content, a non-negative least-square algorithm is found particularly useful for spectrum recovery. The concept is verified in a hardware implementation.

Extraction of Sources of Tremor in Hand Movements of Patients With Movement Disorders

This paper proposes an efficient method to acquire sources of tremor in patients with movement disorders based on blind source separation of convolutive mixtures. The extracted sources indicated neural activities that might be generated in the central nervous system. Four patients with essential tremor were tested in a set of movement tasks. Subjects wore a data glove that measured finger movements of the hand. The experimental data were then fed to a convolutive-mixture model, which revealed sources that imbibed in them the tremor frequency components of 2--8 Hz. Time--frequency analysis of these sources might be of potential help to clinicians to devise tasks that can manifest visible tremor from patients.

Performance Analysis for LDPC-Coded Modulation in MIMO Multiple-Access Systems

We consider a low-density parity-check (LDPC)-coded modulation scheme in multi-input multi-output (MIMO) multiple-access systems. The receiver can be regarded as a serially concatenated iterative detection and decoding scheme, where the LDPC decoders perform the role of outer decoder and the multiuser demapper does that of the inner decoder. In this paper, we investigate the performance of the scheme with simulation results and bounds. Union upper bounds are derived, which can be used as additional means to evaluate the performance of the MIMO multiple-access system.

Best mapping for LDPC coded modulation on SISO, MIMO and MAC channels

Mapping has a significant impact on multilevel modulation schemes. These schemes for conventional codes with iterative demapping and decoding on static channel and Rayleigh fading channel have been studied previously. In this paper, we first investigate the mapping influence on LDPC-code based multilevel modulations on static and Rayleigh fading channels. Then we extend our work to multiple-input multiple-output (MIMO) fading channels and MIMO multiple access channels (MAC). A conclusion can be made as that the Gray map is the best with both iterations and super-iterations for all these channels, which is different from the results of conventional codes.

LDPC coded modulation MIMO OFDM transceiver: performance comparison with MAP equalization

We study the performance of coded modulation schemes where the Gallager code is used as the underlying frequency-space code over MIMO fading ISI channels. Two systems are compared: one is the OFDM based system; the other is the single-carrier system, which employs the turbo-iterative decoding, and equalization at the receiver. With simulations over a number of ISI channels, we confirm that in all channel settings detection performance-perhaps already well known. In this paper, a particular attention is given to answer the question: how close is the performance of OFDM system to the performance of the optimal time-domain equalization when the LDPC code is used as the frequency-space code. It is shown that over the ensemble of MIMO fading channels, the performance degradation of OFDM is only about 2 dB, when two transmit and two receive antennas are used.

Underwater Channel Analysis and Transmission Method Research via Coded OFDM

The underwater channel is known to offer poor communications channel. The channel medium is highly absorptive and the transmission bandwidth is limited. In addition, the channel is highly frequency selective; the degree of selectiveness depends on a detailed geometry of the channel. Furthermore, the response changes over time as the channel conditions affecting the response such as water temperature, sea surface wind and salinity are time-varying. The transceiver design to deal with the frequency and time selective channel, therefore, becomes very challenging. It has been known that deep fading at certain specific sub-carriers are detrimental to OFDM systems. To mitigate this negative effect, the proposed coded OFDM system employs an LDPC code based modulation. In this paper, we aim 1) to provide a detailed underwater channel model; 2) to design a robust LDPC coded OFDM system; 3) to test the proposed system under a variety of channel conditions enabled by the channel model.

Performance analysis on LDPC-Coded systems over quasi-static (MIMO) fading channels

In this paper, we derive closed form upper bounds on the error probability of low-density parity-check (LDPC) coded modulation schemes operating on quasi-static fading channels. The bounds are obtained from the so-called Fano-Gallagers tight bounding techniques, and can be readily calculated when the distance spectrum of the code is available. In deriving the bounds for multiple-input multiple-output (MIMO) systems, we assume the LDPC code is concatenated with the orthogonal space-time block code as an inner code. We obtain an equivalent single-input single-output (SISO) channel model for this concatenated coded-modulation system. The upper bounds derived here indicate good matches with simulation results of a complete transceiver system over Rayleigh and Rician MIMO fading channels in which the iterative detection and decoding algorithm is employed at the receiver.

Union bounds on coded modulation systems over fast fading MIMO channels

Novel closed-form upper bounds on the error performance of coded modulation systems over fast-fading multi-input multi-output (MIMO) channels are obtained for two different space-time schemes: direct transmission and orthogonal space-time block codes. The concept of the distance spectrum for associated space-time codes is developed and used to derive the bounds which can be calculated readily through polynomial expansion. Comparison of the bounds for the two systems indicates the performance of direct transmission is superior. Comparison with simulation results shows that the bounds are tight and useful for benchmarking the practical iterative decoding process.

Iterative MMSE-sphere list detection and graph decoding MIMO OFDM transceiver

An enabling transceiver system which uses a larger digital signal constellation for higher spectral efficiency is proposed in this paper in the context of multi-input and multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. We approach this problem by using a low density parity check (LDPC) code combined with a simple constellation mapping scheme at the transmitter, and a turbo-iterative demapping and decoding scheme at the receiver. The receiver utilizes a reduced complexity soft-input and soft-output detection scheme - a regularized minimum mean-square sphere list detection. In simulation we show that the proposed transceiver achieves a near capacity limit while the computational complexity of the receiver is kept at a certain manageable size. Simulation results indicate that the results are less than a few dB away from the outage capacity at the spectral efficiency of 8 bits/sec/Hz using 16-QAM symbols on all four antennas and a rate 1/2 LDPCC of length 16 K.

A Power-Efficient Communication System between Brain-Implantable Devices and External Computers

In this paper, we propose a power efficient communication system for linking a brain-implantable device to an external system. For battery powered implantable devices, the processor and the transmitter power should be reduced in order to both conserve battery power and reduce the health risks associated with transmission. To accomplish this, a joint source-channel coding/decoding system is devised. Low-density generator matrix (LDGM) codes are used in our system due to their low encoding complexity. The power cost for signal processing within the implantable device is greatly reduced by avoiding explicit source encoding. Raw data which is highly correlated is transmitted. At the receiver, a Markov chain source correlation model is utilized to approximate and capture the correlation of raw data. A turbo iterative receiver algorithm is designed which connects the Markov chain source model to the LDGM decoder in a turbo-iterative way. Simulation results show that the proposed system can save up to 1 to 2.5 dB on transmission power.