Seok Hwan Park

Linear precoder designs for K-user interference channels

This paper studies linear precoding and decoding schemes for K-user interference channel systems. It was shown by Cadambe and Jafar that the interference alignment (IA) algorithm achieves a theoretical bound on degrees of freedom (DOF) for interference channel systems. Based on this, we first introduce a non-iterative solution for the precoding and decoding scheme. To this end, we determine the orthonormal basis vectors of each users precoding matrix to achieve the maximum DOF, then we optimize precoding matrices in the IA method according to two different decoding schemes with respect to individual rate. Second, an iterative processing algorithm is proposed which maximizes the weighted sum rate. Deriving the gradient of the weighted sum rate and applying the gradient descent method, the proposed scheme identifies a local-optimal solution iteratively. Simulation results show that the proposed iterative algorithm outperforms other existing methods in terms of sum rate. Also, we exhibit that the proposed non-iterative method approaches a local optimal solution at high signal-to-noise ratio with reduced complexity.

Distributed Beamforming Techniques for Weighted Sum-Rate Maximization in MISO Interference Channels

This letter proposes a beamforming technique for weighted sum-rate (WSR) maximization in multiple-input single-output (MISO) interference channels. In order to solve the WSR maximization problem in a distributed manner, we obtain a decoupled problem by applying high signal-to-interference-plus-noise-ratio (SINR) approximation. When there are more than two users, further approximation is employed to fully decouple the problem. Then, the decoupled problems are solved by using a zero-gradient (ZG) based algorithm which converges to a local optimal point with only a few iterations. Unlike the conventional distributed schemes where additional information should be exchanged at each iteration, each transmitter of the proposed scheme utilizes only local channel information to compute its beamformer.

Regularized Interference Alignment Based on Weighted Sum-MSE Criterion for MIMO Interference Channels

The original interference alignment (IA) scheme provides poor sum-rate performance compared to simple orthogonal access schemes such as time-division multiple access (TDMA) in low-to-medium SNR under total power constraint. In this paper, we address this problem by proposing a method of regularizing the IA scheme with a criterion of minimizing the weighted sum of the mean square error (WMSE) function. To perform the regularization process efficiently, the weight terms in the WMSE metric should be computed from the optimal zero-forcing (ZF) schemes. Thus, we first prove the optimality of the enhanced IA algorithm introduced in our previous work in the ZF sense. From simulation results, it is shown that the proposed scheme outperforms the TDMA in overall SNR regime. We can further improve the performance by repeating the proposed regularization process iteratively. Moreover, we propose a modified design that provides robustness in the presence of channel uncertainty.

New Beamforming Techniques Based on Virtual SINR Maximization for Coordinated Multi-Cell Transmission

In this paper, we propose new beamforming techniques based on virtual signal-to-interference-plus-noise ratio (VSINR) for weighted sum-rate (WSR) maximization in coordinated multi-cell transmission. In earlier works based on the VSINR maximization, the parameters which control the interference power and the noise variance were set to fixed values regardless of channel realizations and the signal-to-noise ratio level. In order to obtain an improved WSR performance, we propose a method which adaptively adjusts the parameters after establishing a connection between the WSR and VSINR. Our proposed method can be applied to the cases of various coordination levels among base stations. To address practical implementation issues, a decentralized implementation of the beamforming techniques is also proposed based on local channel state information. Numerical results confirm that the proposed centralized schemes provide near-optimal WSR performance and the proposed decentralized methods show a negligible performance loss compared to the centralized algorithms with reduced system complexity.

Sum Rate Analysis of Two-Cell MIMO Broadcast Channels: Spatial Multiplexing Gain

In this paper, we provide a precise expression of the spatial multiplexing gain (SMG) for two mutually interfering multiple-input multiple-output (MIMO) broadcast channels using linear transceiver, referred to as MIMO-IBC. The MIMO-IBC has two base stations and

Modulo Loss Reduction for Vector Perturbation Systems

K_1

A Decoupling Approach for Low-Complexity Vector Perturbation in Multiuser Downlink Systems

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Weighted Sum MSE Minimization under Per-BS Power Constraint for Network MIMO Systems

K_2

Degrees of Freedom of Multiple Broadcast Channels in the Presence of Inter-Cell Interference

users, each equipped with multiple antennas, where independent messages are transmitted over fixed channels. We observe the variation of the SMG with respect to user antenna distribution, and compare the derived result to the SMG of the interference channel with full cooperation among users. Additionally, we propose a linear precoding and decoding scheme for the MIMO-IBC in terms of maximizing the total sum rate, by extending one designed for single-cell multiple-input single-output broadcast channels. Simulation results confirm the accuracy of our theoretical SMG analysis for the MIMO-IBC.

Degrees of Freedom on MIMO Multi-Link Two-Way Relay Channels

In this letter, we present an improved precoding technique which reduces a modulo loss in vector perturbation with low complexity. Instead of searching perturbation vectors in the infinite lattice, the proposed scheme restricts the search range by utilizing the distribution of the perturbation vector depending on transmitted data. As a result, we can achieve significant complexity savings at the transmitter while providing better performance compared to the original vector perturbation.