Kyungchul Kim

Spatial-Correlation-Based Antenna Grouping for MIMO Systems

We investigate antenna-grouping algorithms, which are hybrids of beamforming and spatial multiplexing. We partition transmit antennas into several groups and use beamforming in a group and spatial multiplexing between groups. With antenna grouping, we can achieve diversity gain through beamforming and spectral efficiency through spatial multiplexing. In this paper, we review existing criteria and present several new criteria in multiple-input-multiple-output (MIMO) antenna-grouping systems where the number of transmit antennas is larger than that of receive antennas. We then propose a novel low-complexity antenna-grouping algorithm, which reduces the computational complexity with little degradation of the bit error rate (BER). Comparing the BER and complexity with existing criteria, we demonstrate the benefits of the proposed algorithm. We also derive the BER lower bound of the proposed algorithm in independent and identically distributed (i.i.d.) channels by using the probability density function of the largest eigenvalue of a Wishart matrix.

Mode selection between antenna grouping and beamforming for MIMO communication systems

Antenna grouping algorithms are hybrids of beamforming and spatial multiplexing. With antenna grouping, we can achieve diversity gain through beamforming, and high spectral efficiency through spatial multiplexing simultaneously. In an IID channel, at a fixed transmission rate, we can get better BER performance by antenna grouping compared with beamforming. But if the channel is ill-conditioned or there is some correlation between transmit or receive antennas, the BER performance of antenna grouping is significantly degraded. In that case, beamforming is better than antenna grouping algorithm. We introduced existing antenna grouping algorithms and proposed our new simple antenna grouping algorithms in our previous paper. In this paper, we investigate mode selection algorithms which select antenna grouping or beamforming mode. By selecting a suitable mode for instantaneous channel, we can prevent fluctuation of BER performance caused by channel variation and antenna correlation. We introduce some existing mode selection criteria and also propose a new criterion which is derived from the simple antenna grouping algorithm in the previous paper. This proposed mode selection criterion requires little additional calculations. By comparing with existing mode selection criteria, we verify the performance of our criterion.

Interference compensation technique for multilevel flash memory

Multilevel cell flash memory devices are gaining popularity because it can increase the memory capacity by storing two or more bits to a single cell. However, when the number of levels of a cell increases, the inter-cell interference which shifts cell (threshold) voltage becomes more critical. Thereare two approaches to alleviate the errors caused by the voltage shift. One is the error correcting codes, and the other is the signal processing methods. We focus on signal processing methods to reduce the cell to cell interference which causes the voltage shift, and propose algorithms which reduce the voltage shift effects by classfying and compensating erratic cells. The simulation results show that the proposed algorithms are effective for interference reduction.

Adaptive selection of antenna grouping and beamforming for MIMO systems

Antenna grouping algorithms are hybrids of transmit beamforming and spatial multiplexing. With antenna grouping, we can achieve combining gain through transmit beamforming, and high spectral efficiency through spatial multiplexing. In an independent identically distributed channel, the antenna grouping method has better bit error rate (BER) performance than the beamforming method. However, if the channel is correlated, then the BER performance of antenna grouping degrades. In that case, it is better to use beamforming instead of antenna grouping. In this article, we investigate the mode selection algorithms which select between the antenna grouping and the beamforming modes. By selecting a suitable mode for a given channel, we can achieve more robustness of the system performance. We introduce several mode selection criteria as well as a low complexity criterion which is derived from a low complexity antenna grouping algorithm. Simulation results show that the proposed mode selection algorithm performs better than the antenna grouping and the beamforming modes in various channel conditions.

BER Based Multiuser MIMO User Selection with Block Diagonalization

It has been shown that multiuser MIMO systems have better capacity performance than single-user MIMO systems. For a large number of users, the transmitter selects a subset of users with good channels to maximize the system performance. Shannon capacity has been used most of the time as the performance measure for multiuser MIMO systems. In this paper, we consider BER as an alternative performance metric, and propose two new user selection algorithms with block diagonalization (BD), which eliminates inter-user interference in a multiuser MIMO system. The first approach is BER minimization with fixed rate, and the second approach is rate maximization with fixed target BER. Simulation results show that the proposed algorithms produce lower BER and/or higher rate when they are compared to the conventional capacity based multiuser MIMO user selection algorithm with comparable computational complexity.

Antenna grouping techniques for MIMO beamforming systems

In this paper, we investigate antenna grouping algorithms which are hybrids of beamforming and spatial multiplexing. With antenna grouping, we can achieve diversity gain through beamforming, and high spectral efficiency through spatial multiplexing. We introduce existing criteria and present several new criteria in multiple-input multiple-output antenna grouping system where the number of transmit antennas is larger than that of receive antennas. We also propose a low complexity antenna grouping algorithm. Comparing bit error rate (BER) and complexity, we demonstrate the benefits of our algorithm. We also derive the BER approximation of our algorithm.

Interference Reduction Modulation Based on Chirp Spread Spectrum for Capsule Endoscopy

In this paper, chirp spread spectrum (CSS) based modulation schemes for capsule endoscopy applications are proposed. The On-Off-Keying (OOK) technique has been conventionally used for the capsule endoscopy applications, but it is known to be susceptible to narrow-band interference such as TV signals. In order to enhance robustness to interferences, we propose two CSS based modulation schemes which are chirp spread spectrum differential binary phase shift keying (CSSDBPSK) and chirp spread spectrum on-off keying (CSS-OOK). The CSS-DBPSK scheme performs better than the CSS-OOK scheme in terms of BER, but it has high implementation complexity due to the strict synchronization requirement. On the other hand, the CSS-OOK scheme has both low receiver complexity and comparable BER performance. A key contribution of this paper is to use a low complexity chirp spread spectrum technique for interference mitigation in capsule endoscopy. A human body channel model is used in the simulations. Simulation results show that the BER performance of the CSS-OOK modulation is in between the CSS-DBPSK and the OOK schemes. We will show that the CSS-OOK modulation is a practical solution that achieves good compromise between the interference robustness and the receiver complexity.