Seokkwon Kim

A multi-user beamforming scheme in MIMO downlink channels for multi-cell networks

In this paper, a multi-user beamforming scheme is presented in multi-user multi-input multi-output (MIMO) downlink channels with other cell interference (OCI). The proposed scheme consists of maximizing signal-to-leakage-and-noise ratio (SLNR) and whitening the OCI for power allocation. The conventional scheme which eliminates perfectly intra-cell interference has several problems such as antenna constraints and noise enhancement. The proposed scheme has resolved these problems and could obtain an additional channel gain by the number of antennas. We confirm the increasing of the performance via numerical results.

Beamforming of Amplify-and-Forward Relays under Individual Power Constraints

This paper investigates a distributive beamforming scheme for the amplify-and-forward (AF) protocol when the second-order statistics, i.e., the mean and variance, of the channel state information (CSI) are available to relay networks. For maximizing the signal-to-noise ratio (SNR) of a destination node, we derive an analytical solution and the maximum achievable SNR when there is an individual power constraint for each relay. Based on the derived results, a greedy algorithm is proposed to sequentially find beamforming weights. It is shown that the complexity is linear to the number of relays per iteration, and the number of iterations is less than or equal to the number of relays. The necessary condition and sufficient condition are analyzed, respectively, for the proposed algorithm to be optimal. The derived analytical solution and the proposed algorithm generalize the previous results in the literature, which are optimal for particular cases. Simulation results show that the performance of the proposed algorithm is near to that achieved using semidefinite programming (SDP).

Optimal Beamforming with Combining for the Non-Orthogonal Amplify-and-Forward (NAF) Protocol in Cooperative Communication Systems

This paper expands optimal beamforming with combining from the amplify-and-forward (AF) protocol to the non-orthogonal AF (NAF) protocol for environments with perfect channel state information (CSI) and a sum power constraint. After formulating an optimization problem for maximizing the signal-to-noise ratio (SNR), we propose optimal beamforming and combining vectors. After exhaustive comparison of the optimal beamforming weights with those for the previous results in and the characteristics of the beamforming weights are analyzed according to the protocols and the power constraints. Moreover, it is shown that the optimal combining vector is given succinctly with reduced computational complexity, and that the optimal beamforming could be accomplished using partial CSI. Simulation results verify that the proposed schemes enhance performance.

Average approach to amplify-and-forward relay networks

In this paper, the analysis of an average received signal-to-noise ratio (SNR) for a cooperative communication network with amplify-and-forward (AF) relays is presented. In AF systems, the overall instantaneous SNR at the receiving end can be expressed as the form of a harmonic mean of perhop SNRs. This means that the received SNR can be analyzed by applying the statistical expectation of the harmonic mean of two exponential random variables, which was derived in a closed-form expression. The proposed result can help examining the performance of AF relay systems by using the average sense approach. The simulation results demonstrate the exact derivation of the expectation of harmonic means. The result also shows that the destination node can obtain larger average received SNR as the number of relay nodes is increased.

Development of a demeaning filter for small object detection in infrared images

The demeaning filter detects a small object by removing a background with a mean filter as well as the covariance of an object and backgrounds. The factors considered in the design of the demeaning filter are the method of demeaning, which involves subtracting the local mean value from all pixel values, and the acquisition of templates for both the object and the background. This study compares the sliding window method and the grid method as a demeaning method, and studies the method of acquisition of an object template. Moreover, a method involving the use of previous frames, a mean filter, and an opening operation are studied in an effort to acquire a background template. Based on the results of this study, a practical design of a demeaning filter that is able to detect a small object in an IR image in real time is proposed. Experiment results demonstrate the superiority of the proposed design in detecting a small object following a 2-D Gaussian distribution even under severe zero-mean Gaussian noise.

Robust Method for Detecting an Infrared Small Moving Target based on the Facet-based Model

In this paper, a new condition for the target is proposed to increase the robustness of the facet-based detection method for zero-mean Gaussian noise. In the proposed algorithm, the pixels detected from the maximum extremum condition are checked further to discern if they are false maximum points in the proposed scheme. The experimental results show that the proposed algorithm is much more robust for zero-mean Gaussian noise than the conventional detection method.

A CSI feedback scheme for beamforming at AF relays

In cooperative wireless communications, beamforming at relays can be employed to increase the signal-to-noise ratio (SNR). Since beamforming at relays requires that channel state information (CSI) be exchanged, an efficient CSI feedback scheme is proposed to reduce the feedback overhead incorporating an optimization of the combining and beamforming weights. With 802.16e OFDMA time-division duplexing (TDD) systems, the proposed scheme feeds common information back to relays for beamforming, which should be acquired by a destination for combining. The simulation results verify that the proposed scheme reduces the feedback overhead and improves computational efficiency.

Derivations of decoder and beamforming vector for amplify—and—forward relays based on the Cauchy-Schwartz inequality

For multiple amplify—and—forward (AF) relays, optimal decoder and beamforming vector have been presented [3], [4]. The derivations of these optimal ones have been based on the generalized eigenvalue problem. In this work, we show the derivations using the Cauchy-Schwartz inequality. Moreover, the intimate relation between decoder and beamforming vector is analyzed.

A Low Complexity Resource Allocation Algorithm with Increasing Capacity in Cooperative OFDMA Systems

We investigated a conventional adaptive resource allocation scheme for the downlink of multi-hop orthogonal frequency division multiple access (OFDMA) systems. In addition, we formulated an optimization problem for an adaptive allocation for the subcarriers, paths, and power so that the system capacity could be maximized while minimum resources are guaranteed for each user. Since the optimization problem should be performed in real-time, a heuristic algorithm was recently proposed as a means of reducing the complexity. However, the implementation in cooperative OFDMA systems with radio frequency (RF) relays is not appropriate; moreover, the quantity of feedback is excessive for real-time operation, and the relay load balancing scheme is iterative and slightly inefficient. Thus, we propose an improved heuristic resource allocation algorithm that performs better than the conventional scheme. The proposed algorithm is suitable for cooperative OFDMA systems with RF relays; furthermore, it can reduce the complexity and increase the capacity.