Minki Ahn

Three-Dimensional Beamforming: A new enabling technology for 5G wireless networks

It is anticipated that the mobile data traffic will grow 1,000 times higher from 2010 to 2020 with a rate of roughly a factor of two per year. This increasing demand for data in next-generation mobile broadband networks will lead to many challenges for system engineers and service providers. To address these issues and meet the stringent demands in coming years, innovative and practical solutions should be identified that are able to provide higher spectral efficiency, better performance, and broader coverage. Next generations of wireless cellular networks, which are known as fifth generation (5G) or beyond fourth generation (B4G) wireless networks, are expected to produce higher data rates for mobile subscribers in the order of tens of gigabits per second (Gbit/s) and support a wide range of services. Despite the absence of official standards for the 5G, the data rate of 1 Gbit/s per user anywhere for 5G mobile networks is expected to be deployed beyond 2020.

Diversity Analysis of Coded Spatial Multiplexing MIMO AF Relaying Systems

In this paper, we analyze the diversity performance of source-relay joint precoding in spatial multiplexing (SM) multiple-input-multiple-output (MIMO) amplify-and-forward (AF) relaying systems with channel coding. First, we evaluate the achievable diversity order of singular-value-decomposition-based precoding schemes as a function of a code rate. Then, we show that a significant diversity improvement can be obtained by utilizing signal space diversity. Our analysis provides helpful insights on the tradeoff between the code rate and the diversity order in SM MIMO AF relaying systems. Simulation results demonstrate that our analytical expressions match well with the numerical results.

SLNR-Based User Scheduling for MISO Downlink Cellular Systems

In this paper, we consider a user scheduling problem which maximizes the weighted sum-rate (WSR) in multicell multiple input single output (MISO) downlink systems. Since an exhaustive search algorithm requires high computational complexity, we propose a low complexity algorithm which finds a user set in terms of WSR maximization. We first consider a greedy user selection algorithm which almost achieves the system performance of the exhaustive search algorithm in high SNR regime. Next, a distributed user scheduling algorithm is proposed by employing the maximized signal-to-leakage-and-noise ratio (SLNR) in the selection criteria. This leads to a reduction of the system overhead of exchanging channel state information and computational complexity. From simulation results, we show that the performance of our proposed scheme is very close to the conventional greedy user scheme with lower overhead.

A Low Complexity User Selection Algorithm for Full-Duplex MU-MISO Systems

In this paper, we propose a new user selection algorithm for full-duplex (FD) multiuser multiple-input single-output (MU-MISO) systems, where an FD base station (BS) communicates with multiple half-duplex users in both downlink and uplink channels simultaneously. Due to self-interference at the BS and co-channel interference among users, a joint downlink and uplink user selection to maximize system performance incurs high search complexity. To reduce the complexity, we introduce a two-step user selection algorithm, which successively chooses downlink users followed by uplink users based on the decomposed sum rate of the FD systems. In addition, we analyze the average sum rate performance of our proposed user selection algorithm for FD MU-MISO systems and derive a tight approximation of the performance. From the numerical results, we confirm that our analysis matches well with simulation results, and the proposed user selection algorithm for the FD MU-MISO systems exhibits a small performance loss compared with the optimal user selection algorithm with much reduced complexity.

Precoding Techniques for MIMO AF Relaying Systems With Decision Feedback Receiver

In this paper, we provide new precoding schemes which jointly optimize the source and relay precoders in multiple-input multiple-output amplify-and-forward relaying systems with minimum mean-squared error decision feedback equalizer (DFE) at the destination node. Instead of conventional schemes which resort to an iterative method, we propose simple precoding schemes based on a closed-form solution. To this end, we first extend the decomposable property of the error covariance matrix for a linear receiver to relaying systems with DFE receivers. Then, we suggest two closed-form solutions which successively identify the source and the relay precoders. To improve the performance, a mode selection which adaptively chooses one of the two closed-form precoding schemes according to the channel conditions is introduced. Then, we propose a simple eigenvalue based mode selection algorithm, and analyze its selection probability behavior for Rayleigh fading channels. Simulation results demonstrate that the performance of the proposed method is almost identical to the iterative solution with much reduced complexity.

Diversity of Coded Beamforming in MIMO-OFDM AF Relaying Systems With Direct Link

This paper investigates the diversity order of beamforming schemes in frequency-selective multiple-input multiple-output (MIMO) amplify-and-forward (AF) relaying channels with a nonnegligible direct link between the source and the destination. In this system, orthogonal frequency-division multiplexing and bit-interleaved coded modulation schemes are adopted to exploit multipath diversity. Due to a nonconvex property of the problem, the optimal performance of flat-fading MIMO relaying channels, which is imperative to derive the diversity order, is still unknown. To this end, we first identify upper and lower bounds of the signal-to-noise ratio (SNR) of the flat-fading channels, which lead to upper and lower bounds of diversity order of coded beamforming in frequency-selective channels. Then, we establish the diversity order as a closed form by showing that these bounds are tight. In addition, our analysis provides an insightful guideline for code constructions to achieve a proper diversity gain of the system. The accuracy of our analysis is validated through simulation results.

An Efficient User Selection Technique for Full-Duplex MU-MISO Systems

In this paper, we propose a new user selection algorithm for full-duplex (FD) multiuser multiple-input single-output (MU-MISO) systems where a FD base station (BS) communicates with multiple half-duplex (HD) users in both downlink and uplink channels simultaneously. Due to self-interference at the BS and co-channel interference among users, a joint downlink and uplink user selection to maximize system performance incurs high search complexity. To reduce the complexity, we introduce a two step user selection algorithm which successively chooses downlink users followed by uplink users based on the decomposed sum rate of the FD systems. From the numerical results, we confirm that the proposed user selection algorithm for the FD MU systems exhibits a small performance loss compared to the optimal user selection algorithm with much reduced complexity.

Pairwise MSE Balancing Transceiver Designs for Multipoint-to-Multipoint MIMO AF Relay Systems

In this paper, we consider multipoint-to-multipoint multi-input multi-output relay systems where multiple sourcedestination pairs simultaneously communicate through a single multiple antenna relay. In this system configuration, we minimize the maximum pairwise mean-squared-error (MSE) to guarantee fairness for all pairs. This problem is non-convex, and thus it is quite complex to find an analytical solution. In order to simplify this problem, we apply error covariance decomposition approach and broadcast channel to multiple access channel MSE duality, and then a pairwise MSE balancing scheme which utilizes global channel state information (CSI) is proposed. In order to reduce the complexity and CSI requirements, a new method is developed. Through numerical simulations, we confirm the effectiveness of our proposed schemes.

Achievable Throughput of SDMA System with Linear Receivers

This paper basically considers a space-division multiple-access (SDMA) systems where all nodes including the base station and mobile stations are equipped with equal number of antennas. In the SDMA system, for each transmit antenna, the user corresponding to the largest signal-to-interference-plus-noise (SINR) is selected to be supported. In this environment with the minimum mean squared error receivers, we analyze the average throughput using the previous works.

Vertical sectorization techniques for active antenna systems

In this paper, we study vertical sectorization techniques for multiple-input single-output (MISO) downlink active antenna systems (AAS). In the AAS, antenna beam patterns can be adjusted in each sector and multiple vertical beams can form the vertical sectorization. Since an exhaustive search based vertical sectorization algorithm requires high computational complexity to find the optimal tilt angles, we propose a low complexity vertical sectorization algorithm which is close to the performance of the exhaustive algorithm with much reduced complexity.