In-Kyeong Choi

A constrained MMSE receiver for DS/CDMA systems in fading channels

The convergence problem of minimum mean square-error (MMSE) receivers is discussed, and to overcome the problem, a constrained MMSE receiver is proposed. In addition, we propose the orthogonal decomposition-based least mean square algorithm to implement the constrained MMSE receiver adaptively. Through computer simulations, it is shown that the proposed receiver provides significant performance improvement in the bit-error rate over the conventional matched filter receiver and currently available MMSE receivers.

Statistical Eigen-Beamforming With Selection Diversity for Spatially Correlated OFDM Downlink

Statistical eigen-beamforming is shown to be effective over a spatially correlated fading channel. In the paper, we investigate the use of the statistical eigen-beamforming with short-term selection for an orthogonal frequency-division multiplexing (OFDM) downlink. The contributions of the paper are as follows: (1) an efficient statistical eigen-beamforming method with limited feedback for OFDM has been proposed; and (2) exact and asymptotic expressions of the average bit error rate for (transmitter side) spatially correlated Rayleigh fading channels are derived. In particular, the asymptotic expression allows us to gain insight to statistical eigen-beamforming with selection diversity in terms of array gain and diversity order.

An Opportunistic Beamforming Technique Using a Quantized Codebook

Opportunistic beamforming is a powerful technique increasing system throughput in a static channel environment. Since the opportunistic beamforming technique requires quite large user population to exploit sufficient multiuser diversity, a codebook-based opportunistic beamforming (COBF) technique is proposed. The COBF technique uses a unitary matrix, which changes with time, to induce larger and faster channel fluctuations in the static channel. Also, the COBF technique uses a codebook to provide further selection diversity to the conventional opportunistic beamforming technique to overcome the problem of limited multiuser diversity in a small population. Compared to the recently proposed multiple-pilot based opportunistic beamforming technique in I.M. Kim et al. (2005), the COBF technique reduces required number of pilots. Moreover, since the size of codebook, not the number of pilots, determines the amount of supplementary selection diversity, the system throughput can be increased without limitation from the number of pilots. Some computer simulation results show that system throughput of the COBF technique is higher than that of other conventional techniques and required number of unitary matrices, which needs to be kept as a memory at mobile stations, is only ten in a simulation environment.

Incorporation of adaptive interference cancellation into parallel interference cancellation

Subtractive interference cancellation receivers estimate and subtract out the multiaccess interference (MAI) from the received signals. Thus, the receivers have potential to provide significant performance improvement over the conventional matched filter (MF) receiver. However, the interference from out-of-cells limits the performance improvement of subtractive interference cancellation receivers in multicell environments, since the base station contains the information of users only in its own cell. On the other hand, adaptive MMSE receivers can suppress all interference from out-of-cells as well as in-cell, nonetheless they show performance degradation in fading environments. This paper proposes an integrated scheme of a subtractive interference cancellation receiver and an adaptive MMSE receiver, and shows through computer simulation that the proposed scheme provides outstanding performance improvement over both receivers in multicell fading environments.

Multi-mode subtractive interference cancellation for asynchronous multi-path channels

Much work on multi-user detection has been done to increase the capacity and/or to enhance the performance of DS-CDMA systems. In this paper, we propose a subtractive interference cancellation (IC) called multi-mode interference cancellation (MIC). The proposed scheme overcomes the limitations of the successive (or serial) IC (SIC) and the multistage parallel IC (PIC) schemes, and can change the IC modes according to the processing speed of the hardware, the number of users, and the power spread of received signals. Especially, the structure is designed to work efficiently in the asynchronous multi-path channels. In addition, based on computer simulations over the asynchronous multi-path Rayleigh fading channels, we determine the system parameters and evaluate the performance of subtractive IC schemes.

SIR-based group-wise parallel interference cancellation

We propose group-wise parallel interference cancellation (GPIC) which groups, for cancellation, the received signals based on the signal-to-interference-plus-noise ratio (SINR) measurement. The receiver measures the SINR of each signal, and selects the signal by comparing the measured SINR with the given threshold. The threshold is determined by the required E/sub b//N/sub 0/ of the target system, so that the scheme guarantees the system performance to fit the required E/sub b//N/sub 0/. Performance evaluation is carried out via floating-point simulations for the 3GPP IMT-2000 system.

Implementation issues on the interference cancellation over asynchronous multipath channels

Even though there are several interference cancellers that have been implemented, some issues should be considered so that they might be critical parameters in a certain environment. In this paper, we discuss a parallel interference cancellation (PIC) type technique in an asynchronous channel. Subtraction methods are also studied in detail from the viewpoint of bit-by-bit sequential processing for minimization of processing delay. Also we present the pipelined PIC structure (PLPIC) for an efficient implementation of PIC.

A symbolic reliability of multiple path required system with three states

Abstract The system reliability of a “j or more paths required system” can be obtained from the probability that there is no minimal path suffering a short-mode failure and there is at least one set of j separate paths not suffering an open-mode failure. In order to evaluate the system reliability, we use the method of generating the sum of disjoint products, which is widely used for two-state systems. Some rules and a procedure for the minimized reliability formula are presented.