Young-Jo Ko

Effects of atomistic defects on coherent electron transmission in Si nanowires: Full band calculations

The effects of atomistic imperfections on coherent electron transmission in Si[100] quantum wires a few nanometers wide are investigated using a tight-binding Green function approach. We find a significant suppression in the electron transmission by atomistic imperfections in these extremely narrow wires. Multiple conductance peaks or oscillations can be easily developed by the presence of only several vacancy defects, which can lead to a finite zero-conductance region around the subband edge. Several substitutional defects and surface dangling bonds generally result in decreased, oscillatory conductances with more significant effects found in narrower wires.

User Grouping of Two-Stage MU-MIMO Precoding for Clustered User Geometry

This letter investigates how user grouping, more specifically group partitioning, affects the sum-rate performance of a two-stage multiuser MIMO precoding scheme referred to as joint spatial division and multiplexing (JSDM). Taking a clustered user geometry model into account, we propose a new criterion for user grouping to noticeably improve the sum-rate performance of JSDM. Numerical results confirm that the user grouping based on the proposed criterion yields substantial performance enhancement.

Error-correcting codes on scale-free networks

We investigate the potential of scale-free networks as error-correcting codes. We find that irregular low-density parity-check codes with the highest performance known to date have degree distributions well fitted by a power-law function p (k) approximately k(-gamma) with gamma close to 2, which suggests that codes built on scale-free networks with appropriate power exponents can be good error-correcting codes, with a performance possibly approaching the Shannon limit. We demonstrate for an erasure channel that codes with a power-law degree distribution of the form p (k) = C (k+alpha)(-gamma), with k> or =2 and suitable selection of the parameters alpha and gamma, indeed have very good error-correction capabilities.

Serial concatenation schemes for space-time and recursive convolutional codes

Serial concatenation scheme with a space-time trellis code as the outer code and recursive convolutional codes as the inner code are compared. A new encoding/decoding structure characterized by the inner encoding by a single recursive convolutional code and the combined decoding of simultaneous symbols from multiple antennas is proposed, which gives a large performance gain over the original multiple inner encoding/separate decoding scheme and enables a decoding algorithm much simpler than the multiple inner encoding/joint decoding scheme. Simulation results for a communication system with two transmit and a single receive antennas in quasi-static, flat Rayleigh fading channels are presented.

Doubly opportunistic beamforming for downlink multiuser MIMO

We develop a novel opportunistic beamforming scheme referred to as doubly opportunistic beamforming (DOBF), which is doubly selective in both users and beams based on a new type of channel state information (CSI) feedback to improve the limited feedback downlink system performance. DOBF benefits from very flexible scheduling at the transmitter, compared to the conventional opportunistic beamforming. We provide new findings as to how much the inaccurate signal to interference and noise ratio (SINR) estimation due to lack of scheduling information at the receivers affects the performance of zero-forcing beamforming (ZFBF) with limited feedback. On the contrary, DOBF does not suffer from such a SINR inaccuracy problem. Rather surprisingly, numerical results show that our scheme can noticeably outperform even ZFBF under a comparable amount of CSI feedback load when spatially correlated fading and performance degradation due to limited scheduling information are taken into account.

How large portion of K/2 DoF can we achieve at finite SNR in the Gaussian interference channel?

The K/2 degrees of freedom (DoF) of the K-user Gaussian interference channel (GIC) are well known to be achievable for almost all constant real channel coefficients. In this paper, we investigate the sum-rate behavior of the K-user GIC at finite SNR. To this end, we derive closed-form upper bounds on the sum capacity for the symmetric real and semi-symmetric complex cases. We further show how large portion of K/2 DoF is achievable at realistic SNR.

Noise thresholds of parallel concatenated LDPC codes

Density evolution can determine the noise threshold of an ensemble of LDPC codes in the limit of large block lengths. In this work, the capacity of code ensembles for the parallel concatenation of LDPC codes on binary-input AWGN channels is analyzed using the density evolution technique. The noise thresholds for the optimized code ensembles show that at low rates <1/3, some good parallel concatenated LDPC codes can outperform conventional ones with a similar complexity.