Hyun Seok Ryu

The performance of relay-enhanced cellular OFDMA-TDD network for mobile broadband wireless services

A multihop relay (MR) and repeater are useful means for improving system throughput and coverage in a cellular mobile packet access system, as the carrier-to-interference ratio can be improved when deploying them in a heavily shadowed region. In this paper, we report on our investigation of bandwidth efficiency and the associated service outage performance for different relay scenarios, using system level simulation for a cellular Orthogonal Frequency Division Multiple Access-Time Division Duplexing (OFDMA-TDD) system. We have demonstrated that network throughput gain by typical optical repeaters, which have a simple amplify-and-forwarding capability in a full-duplexing mode, could be minimal in open space subject to cochannel interference from all repeaters in the neighboring cells. This is true, even though they are generally useful for warranting the outage performance with a multiple order of combining gain, especially in the destructive area, for example, basements or indoors with heavy wall attenuation, that naturally shields interference. Meanwhile, we show that multihop relays increase the average system capacity (almost doubling the system throughput) by fully reusing the frequency in every relay station, while improving the per-user data rate in the cell edges or improving the outage performance in the heavily shadowed areas.

Relay Selection Scheme for Orthogonal Amplify-And-Forward Relay-Enhanced Cellular System in a Multi-Cell Environment

In this paper, we consider the relay station (RS) selection schemes for an orthogonal amplify-and-forward relaying system with half-duplexing in a multi-cell environment. The selection schemes are to to maximize the instantaneous received SINR either when the channel gains for the other cells are fully known or not. We propose a sub-optimal RS selection scheme, which is given by the harmonic mean of the ratios of the SNRs (signal-to-noise ratios) and INRs (interference-to-noise ratios) among the adjacent cells. Furthermore, we analyze its outage performance. Our analysis presents the performance comparison between the optimal and sub-optimal RS selection policies, including the existing selection schemes. The proposed sub-optimal scheme provides a simple yet useful performance metric to improve the outage performance without resorting to the full channel knowledge in the multi-cell environment.

Performance Analysis of Relay Systems in an Interference-Limited Environment

In this paper, we analyze the outage performance of relay-enhanced cellular systems in an interference-limited environment, where the co-channel interference (CCI) is a crucial factor that governs the overall system performance as compared to an AWGN (additive white Gaussian noise). Furthermore, the relay station and destination are subject to the different but arbitrary number of CCIs on each node. Assuming that the interference channels are statistically independent and non-identically distributed (i.ni.d), we derive the outage probabilities for amplify-and-forward (AF) and decode-and-forward (DF) relay systems. We show that the statistics obtained from our analyses, e.g., probability density function (PDF) and cumulative density function (CDF), including the outage probability, closely coincide with simulation results.

BER analysis of dual-carrier modulation (DCM) over Rayleigh fading channel

In this paper, we consider a dual carrier modulation (DCM) which is employed in multi-band orthogonal frequency multiplexing (OFDM) ultra wide-band (UWB) system. DCM transforms two QPSK symbols to two 16-QAM symbols with different bit-to-symbol mappings. We find that the basic idea of DCM is the same as that of the rotation code. More specifically, both the rotation code and DCM can provide not only diversity gain by symbol repetition in the time or frequency domain, respectively but also coding gain by constellation rearrangement. We find that these features allow to derive the tight upper bound on the bit error rate (BER) of DCM over the Rayleigh fading channel. We show that our analysis is virtually identical to simulation results over the acceptable range of Eb/No, e.g., higher than 4dB.

Transmission Protocol for Cooperative MIMO with Full Rate: Design and Analysis

In this paper, we propose two different types of cooperative transmission protocols, referred to as spatial multiplexing with receive diversity (SMRD), that can support a full rate, i.e., one symbol transmission per each time slot. We show that the BER performance can be significantly improved with a proper design of SMRD under the AF (amplify-and-forward) mode of relaying, when there is no interference among all symbols transmitted in the same time slot. Furthermore, we consider a more practical implementation of SMRD with successive interference cancellation (SIC) to deal with co-channel interference. Our simulation result shows that the proposed transmission protocol achieves 6 dB gain over the case of noncooperation (direct transmission) without any bandwidth expansion.

BER Analysis of Constellation Rearrangement for Cooperative Relaying Networks over Nakagami-m Fading Channel

In this paper, we consider the BER analysis of the constellation rearrangement (CoRe), which is not subject to the Gray-encoding constraint for the first transmission. This type of CoRe can be applied in a parallel transmission system such as cooperative relaying networks. Furthermore, it allow us to construct the accumulate constellation, which obtained by separating and rearranging the received symbols over two different frequency bands. Based on this accumulated constellation, we derive the tight bound on the BER of CoRe over Nakagami-m fading channel. We show that the proposed bound is much tighter than the existing loose bound, e.g., Chernoff bound. Especially, for the Rayleigh fading channel, i.e., m=1, we show that our bound is virtually identical to the simulation result over the acceptable range of Eb/No.

Dynamic Topology Control for Multi-Hop Relaying in a Cellular TDD-OFDMA System

This paper introduces a notion of divide-by-K reuse strategy, which allows for K orthogonal chunks (subzones) of OFDMA data regions to be fully reused for all relay links throughout each cell in a N-hop relay-enhanced cellular TDD-OFDMA system with K ≤ N. Our objective is to configure a topology of relay stations subject to the divide-by-K reuse strategy, such that the end-to-end routes for relay stations are determined while maximizing the bandwidth efficiency of resource available for relay links. We propose a sub-optimal algorithm for a topology control that reduces co-channel interference while balancing the time-varying traffic load in a dynamical manner. Our system-level simulation results show that the average system throughput can be significantly improved by the proposed algorithm for the divide-by-K reuse strategy, especially with K = 3.

Performance analysis of interference-limited relay systems over large-scale fading channels

In this paper, we consider an interference-limited dual-hop relay system, in which the overall system performance is governed not by an additive white Gaussian noise (AWGN), but by co-channel interference (CCI), due to aggressive frequency reuse for high spectrum utilization. We consider that the received signal-to-interference power ratio (SIR) at each hop is affected by large-scale fading (path-loss and shadowing) as well as small-scale Rayleigh fading. Assuming that the multiple CCIs in each hop are distributed as a Poisson point process on the infinite plane, we investigate the outage performance of the AF and DF relay systems. In order to analyze the performance of AF relaying, we exploit the upper and lower bounds, which can be directly obtained from the outage probability of the DF relay system. Even though the closed-form expression for the outage probability of the DF relaying cannot be found, the results can be obtained by simple numerical method, requiring only one integration calculus. For the various propagation parameters and interferer density, we present how the CCIs influence on the outage performance of dual-hop relay systems, and show that the performance gap between the upper and lower bounds is not significant.

Achieving maximum system throughput with cooperative relaying: A case study of IEEE 802.16j multi-hop relay

Various types of cooperative relaying (CR) schemes exhibit different levels of throughput and outage performance because of their inherent trade-off between diversity gain and opportunity cost; in other words, the overhead that is associated with cooperation. This article attempts to answer whether cooperative communication is beneficial or not from the system-level viewpoint and furthermore, if it is, how its average throughput can be maximized while maintaining the target outage rate. In order to improve throughput at the required outage performance, we propose a unified selection criterion to deal with different levels of combining gain and opportunity cost associated with each scheme, which allows for the employment of different CR schemes for various positions of the mobile station. Our system-level simulation results for an IEEE 802.16j multi-hop relay confirm the varying levels of trade-offs among different CR schemes and furthermore, show that CR will be a useful means of maximizing the average throughput for a multi-hop relay system as long as each type of the cooperating scheme is carefully selected, depending on the position of the mobile stations.

Dual constellation diversity-enhanced modulation for network-coded bidirectional relaying in an asymmetric channel

In this paper, we consider a modulation scheme for a network-coded bi-directional relaying (CBR) scheme in which two different signals from mutually communicating nodes are network-coded in the relay node. It employs dual constellations as a useful means of dealing with its performance degradation in the CBR over asymmetric channel (i.e., the different signal-to-noise ratio in each link), which provide not only a diversity gain from the symbol repetition but also a coding gain from the constellation rearrangement to protect the network-coded symbols. We study the channel utilization and bit error rate performance of the different CBR schemes to show that the proposed diversity-enhanced CBR system always outperforms other existing systems as long as traffic load is properly balanced along the bi-directional links.