Superposition Modulated Cooperative Diversity for Half-duplex Scenario Koji Ishii Department of of Reliability-based Information Systems Engineering, Kagawa University, Japan Email: kishii@eng.kagawa-u.ac.jp Abstract— In this paper, the cooperative diversity with super- position modulation, which has been proposed as an instance of “dirty paper coding”, is theoretically analyzed using the outage probability. However, the conventional system with superposition modulation cannot ideally perform dirty paper coding. Thus, we propose to apply constellation rotation technique and iterative processing to the superposition modulated system, which yields a performance close to the ideal one. Index Terms— cooperative diversity, dirty paper coding, superposition modulation, constellation rotation, iterative processing I. INTRODUCTION In sensor networks, wireless transmitters and receivers, which we shall call nodes throughout this paper, may not be able to support multiple antennas owing to size, com- plexity, power, or other constraints. As a consequence, a great deal of effort has been, recently, put on the ideal of cooperative communication. Cooperative transmission between pairs of nodes has been suggested [1]-[3], [6]- [11],[16] as a means to achieve diversity gain. In the existing cooperative transmission, several protocols have been considered in the literature, and these are largely classified into amplify-and-forward and decode-and- forward schemes [1] [2]. In this paper, we restrict our attention to decode-and- forward schemes and consider the half-duplex environ- ment that the node cannot transmit and receive simultane- ously. Specially we focus on the superposition modulated cooperative transmission which has been proposed in [3] as an embodiment of relay channel theory or even as an instance of dirty paper coding theory [4]. In the superposition modulated cooperative transmission system, a node transmits its own signal superposed to other node’s signal to the destination node. Therefore the node transmits one signal which consists of its own information and other node’s information, simultaneously. The other node’s information can be considered as an interference factor for its own information. Here, the transmit-node knows this interference factor. If the system can ideally perform dirty paper coding theory and the transmitter This paper is based on “Cooperative Transmit Diversity Utilizing Su- perposition Modulation,” by K. Ishii, which appeared in the Proceedings of the IEEE Radio and Wireless Symposium, California, USA, January 2007. c ⃝ 2007 IEEE. knows the interference factor, the capacity at the des- tination node does not decrease even if the destination node (decoding side) does not know the interference factor [4] [5]. In [3], the cooperative transmit diversity has been proposed using one-dimensional superposition modulation and evaluated by only computer simulations. This paper analyzes the superposition modulated cooper- ative transmission with outage probability and expands to two-dimensional modulation. Moreover we apply a constellation rotation technique to this system in order to achieve maximum coding gain. A similar idea from the view point that the transmitted symbol is combining its own information and other user’s information, has been proposed in [6] [7]. During inter- user communications phase, each node communicates in a different sub-band in a full-duplex scenario. During cooperative communications phase, the nodes transmit space-time coded QAM symbols combining their own information and other user’s information. The aim of this paper is to realize dirty paper coding theory. In order to realize it, we utilize a constellation ro- tation technique and iterative processing to the superposi- tion modulated cooperative transmission. The cooperative transmission using iterative processing has been proposed in [8]. This system is only for full-duplex scenario. The source node transmits coded data to relay and destination nodes, while the relay node simultaneously forwards its estimate for the previous coded block to the destination after decoding and re-encoding. The destination decodes the received signals with iterative decoding. A character- istic of this system is that the decoding scheme at the destination node jointly operates over all the transmitted blocks. We expand this cooperative transmission with iterative processing from full-duplex scenario to half- duplex scenario using superposition modulation. The rest of the paper is organized as follows. Sec- tion II presents several transmission schemes including the superposition modulated cooperative transmission and proposes to apply the iterative detection to it. In section III, we analyze each system by outage probability under the assumption that they can ideally perform dirty paper coding. In section IV, we propose to apply a constellation rotation technique to the superposition modulated system and discuss the optimum rotation angle and superposition ratio by minimum-distance analysis. In section V, we evaluate the system with computer simulations and outage 20 JOURNAL OF COMMUNICATIONS, VOL. 2, NO. 7, DECEMBER 2007 © 2007 ACADEMY PUBLISHER