Performance Analysis of a Decode-and-Forward Based Hybrid-ARQ Protocol Ilmu Byun, Duho Rhee, Young Jin Sang, Mingyu Kang, and Kwang Soon Kim † Department of Electrical and Electronic Engineering, Yonsei University 134, Shinchong Dong, Seodaemun Gu, Seoul 120-749, Korea E-mail: {dlfan, dh.rhee, yjmich, kangys8, ks.kim}@yonsei.ac.kr † Corresponding Author ABSTRACT Cooperative hybrid-ARQ protocols have been widely studied because the efficiency of time resources of them is higher than that of cooperative protocols. In this paper, a cooperative hybrid-ARQ protocol based on incremental re- dundancy hybrid-ARQ technique is proposed and analyzed with respect to the relay location. Based on the analysis, the optimum initial rate is searched and the maximum throughput is obtained with respect to the relay location. INTRODUCTION Recently, it is shown that cooperative protocols can improve the system performance using automatic repeat request (ARQ) techniques [1]-[4]. In [1], an incremental relaying protocol which can be viewed as a extension of hybrid-ARQ to the cooperative context was proposed and the performance of the incremental relaying protocol is better than that of a fixed relaying protocol. A dynamic decode and forward (DDF) protocol, in which the source transmits until a relay successfully decodes the information bits, was proposed in [2]. The DDF protocol can not achieve the optimal D-M tradeoff when the multiplexing gain r e is in the region 0.5 <r e ≤ 1. In [3], it is shown that the DDF based ARQ protocol can achieve the optimal D-M tradeoff when the maximum allowable number of transmission is greater than 2. Also, in [4], it is shown that a cooperative ARQ protocol using a convolutional code outperforms a non-cooperative ARQ protocol. Additionally, there have been many researches to de- velop and analyze cooperative ARQ protocols [5]-[7]. In [5], three cooperative ARQ protocols were proposed and their outage behavior was analyzed based on simple ARQ systems in which a destination terminal does not combine received packets. In [6], [7], cooperative hybrid- ARQ protocols for multiple relays were analyzed and the upper bound of a cooperative incremental redundancy (IR) hybrid-ARQ protocol was developed for a scenario where relays are close to a source. However, in wireless systems, the relay can be selected from various locations. Thus it is necessary to analyze a cooperative protocol with respect to the relay location. Furthermore, it is necessary to find the optimum initial rate in cooperative hybrid- ARQ protocols to obtain the maximum throughput. In this paper, the performance of a cooperative protocol is analyzed using a state diagram. Based on the analysis, the optimum initial rate is searched stepwisely and the maximum throughput is shown with respect to the relay location and SNRs. The analyzed protocol is a decode- and-forward based cooperative hybrid-ARQ (DF-HARQ) protocol. In [10], it is shown that coded cooperation protocol, in which the relay terminal transmits incremental redundancy to the destination terminal instead of repeating received bits, outperforms the repeated DF protocol. Thus the IR hybrid-ARQ technique is used for the DF-HARQ protocol. The rest of this paper is organized as follows. In Section ‘SYSTEM MODEL AND PROTOCOL DESCRIPTION’, the signal model and the DF-HARQ protocol are given. In Section ‘PERFORMANCE ANALYSIS’, the DF-HARQ protocol is analyzed using a state diagram. Simulation is performed and the maximum throughput is given in Section ‘NUMERICAL RESULTS’.Concluding remark is given in Section ‘CONCLUSION’. SYSTEM MODEL AND PROTOCOL DESCRIPTION A. Protocol Description An orthogonal cooperative ARQ communication model with three half-duplex terminals (a source, a destination, and a relay) is assumed as in [5]. The source terminal transmits data to the destination terminal with the aid of the relay terminal and the destination and the relay terminals transmit ACK/NACK signals for retransmission. It is assumed that ACK/NACK messages are decoded without error at each terminal and all ACK/NACKs are known to all terminals. The relay terminal transmits data only when it has successfully decoded data from the source terminal and the destination terminal fails to decode. The initial transmission rate is selected based on the long-term average signal to noise ratio (SNR) at the receiver. 978-1-4244-2677-5/08/$25.00 c 2008 IEEE