Een-Kee Hong

MIMO cooperative diversity with scalar-gain amplify-and-forward relaying

We analyze diversity performance of scalar fixed- gain amplify-and-forward (AF) cooperation in multiple-input multiple-output (MIMO) relay channels with ns source antennas, nR relay antennas, and nD destination antennas. We first derive the exact symbol error probability (SEP) for maximum likelihood decoding of orthogonal space-time block codes with M-ary phase-shift keying modulation over such channels and then characterize the effect of MIMO cooperative diversity on SEP behavior in a high signal-to-noise ratio regime. We show that the simple scalar-gain AF cooperation can create the diversity order d_AF les n_Sn_D + n_Sn_Rn_D/max{n_S, n_R, n_D} with the equality if 2max{n_S, n_R,n_D} ges n_S + n_R + n_D - 1. This finding reveals that the number of relay antennas greater than or equal to n_S + n_D - 1 is required to achieve the additional diversity order n_Sn_D by scalar-gain AF relaying. We also present the asymptotic SEP as the number of relay antennas tends to infinity.

On Transmission Efficiency for Wireless Broadcast Using Network Coding and Fountain Codes

This paper investigates the benefits of applying fountain codes (FCs) in improving the transmission efficiency in broadcasting systems. Particularly, an exact expression of the transmission efficiency for wireless broadcast using FCs is derived. This derivation allows us to compare the transmission efficiency of the fountain code approach (FCA) and network coding approach (NCA) in wireless broadcast. The numerical results demonstrate that FCA achieves better performance than NCA when the number of users is large and vice versa when the number of users is small.

Synchronous transmission technique for the reverse link in DS-CDMA terrestrial mobile systems

In this letter, a timing control algorithm is described, which realizes timing synchronization on the direct sequence code-division multiple-access reverse link. In addition, the uncoded bit-error probability is analytically derived. Numerical results, confirmed by simulation, show that our synchronous transmission technique can offer better performance than conventional asynchronous transmission, especially for an exponentially decaying multipath intensity profile with a large decay factor.

Energy-Efficient Bandwidth and Power Allocation for Multi-Homing Networks

This paper investigates resource allocation for multi-homing networks where users can simultaneously transmit data to multiple radio access networks (RANs) using multiple air interfaces. We aim at optimally assigning the bandwidth and power to each user-RAN connection so as to maximize energy-efficiency of the entire network subject to user specific QoS requirements as well as the available resource budgets. First, we study the problem of resource allocation for scenarios where the connections between the users and the RANs are predefined, which naturally leads to a fractional program. To obtain the optimal solution efficiently and facilitate distributed implementations, we further equivalently transform the design problem into a convex program using a parameter-free approach and develop a decentralized algorithm based on the alternating direction method of multipliers. Second, we investigate the problem of joint link selection and resource allocation for energy-efficiency maximization. The problem is cast as a mixed integer nonlinear convex program for which we particularize the branch and bound method to find an optimal solution. Then, two suboptimal low-complexity designs are proposed: the first one is based on greedy algorithm, which gradually maximizes the virtual link energy-efficiency; the other one is based on the concept of sparsity-inducing norm. Simulation results are presented to demonstrate the potential gains in terms of energy-efficiency of the proposed methods.

Multiuser Diversity for Successive Zero-Forcing Dirty Paper Coding: Greedy Scheduling Algorithms and Asymptotic Performance Analysis

Dirty paper coding (DPC) scheme is the capacity achieving transmission technique in multiuser MIMO downlink channels. As a suboptimal solution to DPC, successive zero-forcing DPC (SZF-DPC) has been proposed recently. The zero-interference constraint in designing the precoding matrices limits the number of supportable users. In this correspondence, we propose three low-complexity suboptimal user scheduling algorithms to exploit the multiuser diversity gain in SZF-DPC as the number of users grows. The first algorithm greedily maximizes the true sum rate. The second algorithm is based on eigenvalues. The third algorithm relies on the diagonal elements of the effective channel matrix since eigenvalues and diagonal entries of a Hermitian matrix have a strong relationship. Simulation results show that the proposed scheduling algorithms can obtain a significant fraction of sum rate of the optimal solution. Furthermore, the performance analysis is provided to prove that the proposed user selection algorithms can achieve the same asymptotic sum rate as that of DPC.

Energy-Efficient Zero-Forcing Precoding Design for Small-Cell Networks

We consider small-cell networks with multiple-antenna transceivers and base stations (BSs) cooperating to jointly design linear precoders to maximize the network energy efficiency, subject to a sum power and per-antenna power constraints at individual BSs, as well as user-specific quality of service (QoS) requirements. Assuming zero-forcing precoding, we formulate the problem of interest as a concave-convex fractional program to which we proposed a centralized optimal solution based on the prevailing Dinkelbach algorithm. To facilitate distributed implementations, we transform the design problem into an equivalent convex program using Charnes-Coopers transformation. Then, based on the framework of alternative direction method of multipliers (ADMM), we develop a decentralized algorithm, which is numerically shown to achieve fast convergence. Since BSs are generally power-hungry, it may be more energy-efficient if some BSs can be shut down, while still satisfying the QoS constraints. Toward this end, we investigate the problem of joint precoder design and BS selection, which is a mixed Boolean nonlinear program, and then provide an optimal solution by customizing the branch-and-bound method. For real-time applications, we propose a greedy algorithm which achieves near-optimal performance in polynomial time. Numerical results are provided to demonstrate the effectiveness of the proposed algorithms.

Effect of Line-of-Sight on Dual-Hop Nonregenerative Relay Wireless Communications

In this paper, we analyze the effect of the line-of-sight (LOS) on the symbol error probability (SEP) of nonregenerative cooperation in multiple-input multiple-output (MIMO) dual-hop relay channels with nS source antennas, nR relay antennas, and no destination antennas-referred to as a (nS,nR,nD)-MIMO dual-hop nonregenerative relay channel. In particular, we consider the channels of the source-to-relay (S-to-R) and the relay-to-destination (R-to-D) links are Rayleigh and Rician fading distribution, respectively. To be specific, we derive the exact SEP for maximum likelihood (ML) decoding of orthogonal space-time block codes (OSTBCs) over such channels and validate the analytical results by comparing with Monte-Carlo simulation. It is shown that for a fixed channel gain a strong LOS component degrades the error performance, e.g. SEP, of MIMO cooperative communications due to the lack of scattering.

Error probability of binary and M-ary signals with spatial diversity in Nakagami-q (Hoyt) fading channels

We analyze the exact average symbol error probability (SEP) of binary and-ary signals with spatial diversity in Nakagami- (Hoyt) fading channels. The maximal-ratio combining and orthogonal space-time block coding are considered as diversity techniques for single-input multiple-output and multiple-input multiple-output systems, respectively. We obtain the average SEP in terms of the Lauricella multivariate hypergeometric function. The analysis is verified by comparing with Monte Carlo simulations and we further show that our general SEP expressions particularize to the previously known results for Rayleigh ( = 1) and single-input single-output (SISO) Nakagami- cases.

An Efficiency Maximization Design for SWIPT

A joint power splitting and beamforming design for multiuser multiple-input single-output (MISO) systems where receivers have capability of decoding information and harvesting energy simultaneously from received signals is considered. The objective is to maximize the ratio of the achieved utility to the total power consumption subject to harvested power requirements and power budget at a base station (BS). The utility function of interest combines the sum rate and the total harvested power. The design problem is nonconvex, and thus, global optimality is difficult to achieve. To solve this problem locally we first convert the problem into a more tractable form, and then propose an iterative algorithm which is guaranteed to achieve a Karush-Kuhn-Tucker solution. Numerical results are provided to demonstrate the superior performance of the proposed method.

Multiuser Cooperative Relay Communication Employing Hierarchical Modulation

We develop a scheme to improve data throughput for multiuser cooperative relay communications operating in half-duplex mode. Cooperative relay, while effective in mitigating fading effects, might reduce the overall throughput of the network since it requires additionally resources for relay. In order to overcome this limitation, we develop a multiuser cooperative communication scheme that employs hierarchical modulation. This scheme exploits the different fading conditions for each user and allows the system to transmit two independent data streams simultaneously. Since the bit-error rate of the proposed system is a function of the distance parameters of hierarchical modulation, transmission reliability can be controlled flexibly by varying the distance parameter. We derive the error-rate expressions of the proposed scheme and compare its symbol-error rate performance for specific distance parameters with that of conventional relay schemes.