Distributed Interleave-Division Multiplexing Space-Time Codes for Coded Relay Networks Petra Weitkemper, Dirk W¨ ubben, Karl-Dirk Kammeyer Department of Communications Engineering, University of Bremen Otto-Hahn-Allee 1, 28359 Bremen, Germany E-mail: {weitkemper, wuebben, kammeyer}@ant.uni-bremen.de Abstract—In this paper the application of the recently pro- posed Interleave-Division Multiplexing Space-Time Codes (IDM- STC) in coded relay networks is investigated. IDM-STC are a very flexible scheme to exploit space diversity with the benefits of IDM such as robustness against asynchronism and efficient iterative detection. While IDM-STC have been investigated for MIMO systems, relay networks induce additional constraints like imperfect decoding at the relay and limited cooperation. Consequently, not only the choice of the diversity scheme, but also the applied relay functionality significantly influences the overall performance. In this paper different relay schemes in combination with IDM-STC are considered for coded relay networks. Amplify-Forward (AF) does not assume any signal processing at the relays and just forwards the received sig- nal. Decode-Forward (DF) fully decodes the received signal which can lead to error propagation. Another relaying scheme called Estimate-Forward (EF), which combines the advantages of AF and DF, was recently extended to coded systems and is called Decode-Estimate-Forward (DEF). A coded IDM-STC relay system with different relay schemes is investigated to show the good performance of IDM-STC combined with DEF in realistic scenarios with small overhead and robustness against asynchronism. I. I NTRODUCTION Multiple-Input Multiple-Output (MIMO) systems have been intensively investigated in the last years. Depending on the considered system, either multiplexing techniques like V-BLAST or diversity techniques as Space-Time Codes (STC) are applied to deal with severe fading in wireless scenarios. More recently, distributed relay systems have attracted much attention. Several relays assisting a source to transmit data to a destination build up a virtual MIMO (VMIMO) system where the relays are combined to a virtual antenna array (VAA). Due to this similarity to classical MIMO systems, diversity techniques known to be powerful for MIMO, can be adopted to relay networks. Many approaches to apply STC in relay networks have been proposed in the last years, however, several restrictions in relay networks have not been addressed. One important point is the imperfect cooperation between the virtual antennas. It is not possible to achieve perfect cooperation in terms of knowledge of all channel states and decoded data of the other relays with reasonable signaling overhead. Even synchronization is a hard task in a system with distributed relays. Consequently, some approaches are hard to apply to VMIMO directly as, e.g., Orthogonal Space-Time Block Codes (OSTBC). A Space-Time Code based on Interleave Division Multi- plexing (IDM) was introduced in [1] and [2] which does not need synchronization or any knowledge about other antennas. This IDM-STC is a very promising diversity technique also for VMIMO systems with small signalling overhead and robustness against asynchronism.In [3] IDM-STC was applied to a multi-user cooperative system. The focus was on high throughput at the cost of nearly full cooperation between the users requiring a large overhead. Additionally, despite of a repetition coded system, perfect decoding was assumed at all relays, which is a hard task in fading channels even with strong codes. In [4] an IDM-STC for an uncoded single-user relay system was considered, but only for Decode-Forward (DF) protocol. However, not only the choice of the diversity scheme, but also the applied relay functionality significantly influences the overall performance. In this paper different relay schemes in combination with IDM-STC are considered for coded relay networks. Amplify-Forward (AF) does not assume any signal processing at the relays and just forwards the received signal. DF fully decodes the received signal which can lead to error propagation. In this case the results presented in [3] will not be achievable. To combine the advantages of AF and DF, Estimate-Forward (EF) was proposed in [5] for uncoded sys- tems. EF forwards reliability information and therefore avoids error propagation and exploits the discrete signal alphabet. The idea of EF is the transmission of MMSE estimates conditioned on the received signal. This basic idea was recently extended to coded systems [6] and also to higher order modulation schemes [7]. This so-called Decode-Estimate-Forward (DEF) shows very good performance and is a reasonable choice in relay networks. Transmission of Log-Likelihood Ratios (LLRs) is an alter- native soft relay protocol called Decode-Amplify-Forward (DAF) and was investigated in combination with IDM-STC for an uncoded multi-user system in [8]. But as the DAF approach was already shown to be suboptimum in [6], we will not consider it here. In this paper a coded IDM-STC relay system with different relay schemes is investigated and the good performance of IDM-STC combined with DEF in realistic scenarios with small overhead and robustness against asynchronism is shown.