Seong-Woo Ahn

Effect of Channel-Estimation Error on BER Performance in Cooperative Transmission

In this paper, a framework for evaluating the bit-error-rate (BER) performance of amplify-and-forward (AF) relay-assisted cooperative transmission is provided in the presence of imperfect channel estimation. First, the AF relaying is classified into two cases - a variable gain and a fixed gain. For each case, the closed-form average BER is derived by using the moment-generating function (MGF) of the effective output SNR, which quantifies the SNR penalty due to the noisy channel estimate. Moreover, the asymptotic BER bound is also provided at a high SNR regime for the insight of our approach. In particular, the effect of the feedforward delay on informing the source-relay channel estimate as to the destination is presented in the variable-gain relay. Numerical investigation shows that the analytic result makes an exact match with the simulation, and the comparative superiority between the fixed- and variable-gain relay is discussed.

A Simple Technique to Enhance Diversity Order in Wireless Fading Relay Channels

Relaying has proven to be an effective method of providing additional spatial diversity on wireless fading channels. This paper proposes a system of XORing at the relay with iterative maximum a posteriori (MAP) decoding at the destination. An XOR-relay offers an increased number of diversity paths, enabling the destination to construct a cycle- free decoding structure, and resulting in an enhanced diversity order. Simulation results clearly show that the proposed scheme approaches 3rd-order diversity in the topology of second diversity paths, with no need for additional resources.

Throughput–Delay Tradeoff of Proportional Fair Scheduling in OFDMA Systems

This work presents an analytical model of the throughput-delay tradeoff found in proportional fair scheduling used in orthogonal frequency-division multiple access (OFDMA) systems in the context of user multiplexing. For this purpose, we propose a simple and efficient resource allocation scheme based on 2-D resource partitioning in the frequency and time domains. Basically, frequency-domain partitioning is determined by the degree of frequency diversity, and time-domain partitioning determines the degree of multiplexing (DoM), which is the number of users to be served in a subchannel of a frame. We investigate the effect of the DoM and frequency diversity on the delay quality-of-service (QoS) performance of OFDMA systems by analyzing the effective capacity. In addition, an energy efficiency analysis in the wideband regime shows that insufficient frequency diversity increases the impact of the delay QoS constraint on energy efficiency, and in this case, greater energy efficiency can be achieved by using an appropriate DoM.

The Effect of Multiplexing Users in QoS Provisioning Scheduling

In this paper, we consider the problem of quality of service (QoS) provisioning for K users sharing a downlink wireless channel. The channel to each user is assumed to be a block Rayleigh fading channel. We develop simple and efficient scheduling schemes which can yield substantial capacity gain. Our approach is to maximize the throughput in subject to a given delay QoS constraint by choosing the appropriate number of users served in a frame (or a coherent duration). We call this number the degree-of-multiplexing (DoM). By analysis of the effective capacity, we investigate the effect of DoM on QoS performance in wireless multiuser scheduling. Finally, numerical investigation shows that the analytic results make asymptotically an match with the simulations, and the comparative superiority of our approach to the previous ones is discussed.

Subband interference suppression in channel shortening for OFDMA downlink systems

In orthogonal frequency-division multiple access (OFDMA) downlink systems, user equipments over large delay spread channels reduce the cell spectral efficiency due to intersymbol interference (ISI). Channel shortening is an attractive filtering technique to mitigate ISI over such large delay spread scenarios, but the filter should be designed carefully to prevent subsequent spectrum null phenomenon. In this paper, we propose subband interference suppression (SIS) in channel shortening for OFDMA downlink systems. In our framework, we first develop the minimum subband interference (MSI) filter which reduces the interference and noise enhancement within the subcarriers of interest. To reduce its complexity, we present the delay preselection method, which predetermines significant delay candidates on the basis of estimation results. Simulation results show that the proposed SIS framework is effective over cellular scenarios, where conventional schemes do not work properly.

Analysis of proportional fair scheduling in OFDMA systems with delay QoS constraints

We present an analytical model of throughput-delay tradeoff of the proportional fair scheduling of OFDMA Systems in context of user multiplexing. For this purpose, we propose simple and efficient resource allocation schemes which are based on 2-dimensional resource partitioning in frequency and time domain. Basically, frequency-domain partitioning is determined by degree of frequency diversity and time-domain partitioning is determined by degree of multiplexing which is number of users to be served in a sub-channel of a frame. Our approach is aimed at maximizing the delay constrained throughput by choosing the appropriate degree of multiplexing for a given degree of frequency diversity. We investigate the effect of degree-of-multiplexing and frequency diversity on delay QoS performance of OFDMA system by analyzing the effective capacity, and then develop an approach to finding the optimal degree of multiplexing by the asymptotic approximation.