This paper investigates the use of instantly decodable network coding (IDNC) for minimizing the mean decoding delay in multicast cooperative data exchange systems, where the clients cooperate with each other to obtain their missing packets. Here, IDNC is used to reduce the decoding delay of each transmission across all clients. We first introduce a new framework to find the optimum client and coded packet that result in the minimum mean decoding delay. However, since finding the optimum solution of the proposed framework is NP-hard, we further propose a heuristic algorithm that aims to minimize the lower bound on the expected decoding delay in each transmission. The effectiveness of the proposed algorithm is assessed through simulations. I. INTRODUCTIONImagine that a group of geographically close wireless clients are interested in downloading the same set of packets from the base station. We assume that these clients have collectively received the set of the packets broadcast from the base station but individually missed some packets. In such scenarios, instead of solely relying on the base station to complete their download, an increasingly attractive strategy is for the clients to cooperate with each other by exchanging network coded packets until all of them have received their requested packets [1]-[6]. The main advantage of such cooperative scheme is reducing the load of the base station. In addition, short-range transmission links among the clients can potentially result in a faster, more reliable and cheaper delivery of the information to them.In this paper, we consider the problem of cooperative multicast exchange of packets to a set of clients. We assume that a set of clients are interested in receiving a subset of packets broadcast from the base station. Here, we consider the applications in which the received packets should bring new information to the clients, regardless of their reception order. Examples of such applications are multiple description coded systems and sensor/emergency networks [7]. In such applications, if a received packet at a client does not bring new information or cannot be instantly decoded, that client will experience decoding delay.To minimize the decoding delay of the clients, a possible strategy is to employ instantly decodable network coding (IDNC) [7]-[11] schemes and encode the transmitted packets such that the decoding delay across all clients is minimized in each transmission. IDNC is an important subclass of network coding in which the sender exploits the diversity of the received and lost packets at different clients for making network coding decisions that provide instant packet decodability at a subset or all clients upon successful packet reception. In addition to being

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This paper studies the complicated interplay of the completion time (as a measure of throughput) and the decoding delay performance in instantly decodable network coded (IDNC) systems over wireless broadcast erasure channels with memory, and proposes two new algorithms that improve the balance between the completion time and decoding delay of broadcasting a block of packets. We first formulate the IDNC packet selection problem that provides joint control of the completion time and decoding delay as a statistical shortest path (SSP) problem. However, since finding the optimal packet selection policy using the SSP technique is computationally complex, we employ its geometric structure to find some guidelines and use them to propose two heuristic packet selection algorithms that can efficiently improve the balance between the completion time and decoding delay for broadcast erasure channels with a wide range of memory conditions. It is shown that each one of the two proposed algorithms is superior for a specific range of memory conditions. Furthermore, we show that the proposed algorithms achieve an improved fairness in terms of the decoding delay across all receivers.

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