Kyeongyeon Kim

Mechanical effects in peel adhesion test

—Mechanical effects in the peel strength of a thin film have been studied both experimentally and theoretically. It has been found that the adhesion strength measured by the peel test is a practical adhesion (an engineering strength per unit width) and does not represent the true interface adhesion strength. The measured value may represent a multiplication of the true interface adhesion and other work expended in the plastic deformation of the thin film. The contribution of the latter to the peel strength is found to be, sometimes, of the order of 100 times higher than the former. It is found that the major controlling factors in the peel strength are the thickness, Youngs modulus, the yield strength, the strain hardening coefficient of the film, and the compliance of the substrate as well as the interface adhesion strength. Even though the true interface adhesion strength is the same, a higher peel strength is obtained if the film is thinner or more ductile under the test conditions reported in this pa...

Adaptive Linear Turbo Equalization Over Doubly Selective Channels

Over the last decade, tremendous gains, leading to near-capacity achieving performance, have been shown for a variety of communication systems through the application of the turbo principle, i.e., the exchange of extrinsic information between constituent algorithms for tasks such as channel decoding, equalization, and multiple-input-multiple-output (MIMO) detection. In this paper, we study the practical application of such an iterative detection and decoding (IDD) framework to underwater acoustic communications. We explore complexity and performance tradeoffs of a variety of turbo equalization (TEQ)-based receiver architectures. First, we elaborate on two popular but suboptimal turbo equalization techniques: a channel-estimate-based minimum mean-square error TEQ (CE-based MMSE-TEQ) and a direct-adaptive TEQ (DA-TEQ). We study the behavior of both TEQ approaches in the presence of channel estimation errors and adaptive filter adjustment errors. We confirm that after a sufficient number of iterations, the performance gap between these two TEQ algorithms becomes small. Next, we demonstrate that an underwater receiver architecture built upon the least mean squares (LMS) DA-TEQ technique can leverage and dramatically improve the performance of the conventional implementation based on the decision-feedback equalizer at a feasible complexity. To maintain performance gains over time-varying channels, the slow convergence speed of the LMS algorithm has been improved via two methods: 1) repeating the weight update for the same set of data with decreasing step size and 2) reducing the dimensionality of the equalizer by capturing sparse channel structure. This receiver architecture was used to process collected data from the SPACE 08 experiment (Marthas Vineyard, MA). Receiver performance for different modulation orders, channel codes, and hydrophone configurations is examined at a variety of distance, up to 1 km from the transmitters. Experimental results show great promise for this approach, as data rates in excess of 15 kb/s could readily be achieved without error.

An efficient antenna shuffling scheme for a DSTTD system

A new antenna shuffling scheme for DSTTD (double space time transmit diversity) is proposed. The proposed method obtains the shuffling pattern directly from the estimated channel by maximizing minimum post-processing SNR (signal to noise ratio), while the conventional method minimizes channel correlation. Since the minimum post-processing SNR is directly related with error performance, the proposed method shows better bit error rate performance than the conventional method.

A new waveform enabling enhanced QAM-FBMC systems

Multiple filter-banks enable QAM transmission and reception in filter-bank multi-carrier (FBMC) systems. In this paper, we propose a new method to design a set of simultaneously used base filters for QAM-FBMC by optimizing the trade-off between spectrum confinement and self-interference. The increased freedom in multiple base filter designs brings merits of a tailor-made spectrum confinement and desirable bit error performance simultaneously. Simulations results show the superiority of the proposed QAM-FBMC systems to other post-OFDM waveform candidates.

QAM-FBMC: A New Multi-Carrier System for Post-OFDM Wireless Communications

Recently, as asynchronous heterogeneous network scenario becoming one of the key features for next generation wireless communications, superior spectrum confinement as well as higher spectral efficiency compared to cyclic prefixed orthogonal frequency division multiplexing (CP-OFDM) has been taken into consideration for future radio access technologies. In this paper, we propose a new quadrature amplitude modulation based filter-bank multi-carrier (QAM-FBMC) system that provides with an inherent spectral efficiency gain against CP-OFDM, which comes from reduction of the redundancies such as cyclic prefix (CP) and guard-band, while keeping the symbol rate same as in CP-less OFDM. We propose a new transceiver structure consisting of at least two different filter-bank bases at both transmitter and receiver sides. Practical algorithms including channel estimation and equalization to mitigate multi-path fading channel without CP are proposed. Various evaluation results show that the proposed system performs comparable to the CP-OFDM system even without CP and guard-band reduction is also available from the well-confined spectrum.

A simplified adaptive modulation scheme for D-STTD systems with linear receivers

In this letter, we consider D-STTD (Double Space Time Transmit Diversity) with an adaptive modulation (AM) scheme. The simple criteria for AM are proposed according to linear receivers and the corresponding AM schemes are derived to minimize bit error rate under the constraint of the total data rate and total transmit power. The D-STTD system using the proposed AM scheme requires low computational complexity and small feedback information. The performance of D-STTD system using the proposed scheme is improved by 7 dB or more in signal to noise ratio corresponding to 10/sup -4/ bit error rate.

Introduction to QAM-FBMC: From Waveform Optimization to System Design

The asynchronous heterogeneous network scenario is becoming one of the key features for 5G mobile communications. Compared to CP-OFDM, which is 4G waveform, superior spectrum confinement as well as higher spectral efficiency has been taken into consideration for 5G waveform design. In this article, we introduce a new waveform called QAM-FBMC that provides superior spectrum confinement and higher spectral efficiency simultaneously against CP-OFDM. The article explains fundamental trade-offs in waveform design and shows how they can be optimized for QAM-FBMC. We also introduce the transmission and reception procedures for QAM-FBMC including its similarities and differences compared to well-known waveforms such as CP-OFDM and OQAM-FBMC. Performance evaluation results show where the spectral efficiency gain of QAM-FBMC over CP-OFDM comes from.

One-Dimensional Soft-Demapping Algorithms for Rotated QAM and Software Implementation on DSP

To improve detection performance of quadrature amplitude modulation (QAM), signal space diversity (SSD) has been exploited and adopted for the second generation of digital video broadcasting (DVB-T2) system. Maximum-likelihood detection (MLD) to get full SSD is avoided because of enormous computational complexity. Its max-log approximated detection (full search algorithm) and subregion based soft-demappers are also too complex to be implemented due to their two-dimensional (2D) Euclidean distance calculation. In particular, the complexity becomes the main burden for the software implementation, which is attractive for multistandard broadcasting receivers. To tackle the main bottleneck, we propose one-dimensional (1D) soft-demappers. By reformulating a rotated QAM signal as two layered pulse amplitude modulation (PAM) signals, the full search algorithm is simplified to an MMSE decorrelation followed by 1D soft-demapping, where Gaussian approximation is used for the interferences. Additional interference cancellation is considered to further suppress its residual interference. For 256-QAM with 4/5 code rate in memoryless Rayleigh channels with/without erasures, the performance gap to the full search is within 0.15 dB at 10-3 bit error rate (BER), while the complexity is less than 8%. Due to the significant complexity reduction of the proposed algorithms, the software implementation of a DVB-T2 receiver on DSP is feasible with 73% less computations than the one with the full-search-based soft-demapper.

Software-defined DVT-T2 demodulator using scalable DSP processors

This paper describes the feasibility of software-defined demodulator of DVB-T2 standard using the scalable DSP processor. This paper focuses mainly on the DVB-T2 receiver design and implementation of four major software blocks of the demodulator: FFTs, channel estimator, multi-level de-interleavers and rotated QAM soft-demapper. In particular, 2K-point FFT function is analyzed and mapped on the scalable architecture of coarse-grained reconfigurable array (CGRA) processors resulting 51dB signal to quantization noise ratio (SQNR) performance. The computational burden of frequency interpolator and frequency/cell deinterleavers are greatly reduced with specially designed intrinsics, 30% and 85%, respectively, from the original implementation. The softdemapper for rotated QAM constellation becomes feasible with the latest 1D-MMSE decorrelation method though it is still the most expensive function covering 40% of DTG106 mode. By implementing full chain demodulation software including abovementioned four major functions, it is demonstrated that the software-defined DVB-T2 demodulator is realizable with software on two scalable CGRA processors running at 300MHz.

Link-Adaptive MIMO Systems With Ordered SIC Receiver Using Stream-Ordering Algorithms in Multiuser Environments

This paper proposes new algorithms to improve the bit error rate (BER) and data rate of link-adaptive multiple-input-multiple-output (MIMO) systems with ordered successive interference cancellation (OSIC) receivers in a multiuser environment. The proposed algorithms are as follows: (1) the power adaptation (power allocation-equal rate) scheme; (2) the rate adaptation (equal power-adaptive modulation) scheme; and (3) the power and rate adaptation (power allocation-adaptive modulation) scheme. Additionally, as the OSIC receiver is exploited in the proposed algorithms, the detection order of each users receiver is considered since the performance of the system using it is seriously affected by the detection order. Finally, the proposed algorithms are executed to minimize the BER or to maximize the data rate of the system by simultaneously considering link adaptation techniques and the detection order of the OSIC receiver.