Abstract-This paper addresses the problem of inter-session network coding to maximize throughput for multiple communication sessions in wireless networks. We introduce virtual multicast connections which can extract packets from original sessions and code them together. Random linear network codes can be used for these virtual multicasts. The problem can be stated as a flowbased convex optimization problem with side constraints. The proposed formulation provides a rate region which is at least as large as the region without inter-session network coding. We show the benefits of our technique for several scenarios by means of simulation. The optimal solution for inter-session network coding might require nonlinear encoding functions [6] and even deciding a linear inter-session coding problem can be NP-complete [7] . Nonetheless, suboptimal linear coding schemes have been proposed both for wireline and wireless networks. Previous work on inter-session coding for wireline networks has focused on a graph-theoretic characterization of network codes [8], [9] . For example, in [10] , the authors present an approach to translate the solution for the well-known butterfly network to larger networks. In [11] inter-session network coding is combined with multiple description coding. Recent work [12] presented an interference alignment approach to perform intersession coding. We focus on wireless networks, where the most prominent technique is the COPE protocol [13] which exploits the broadcast nature of the wireless medium. As shown in Fig. 1a , the number of transmissions can be reduced compared to routing if packets from different sessions are combined. This technique is called reverse carpooling in [14] due to the fact that the combined information has to travel in opposite directions. The interplay of the COPE protocol with medium access has been analyzed in [15] and approaches to model its performance were proposed in [16] , [17] . COPE has been successfully implemented and shows gains especially when the reverse carpooling situation occurs.
The flow-based problem formulation stated in REF provides a rate region which is at least as large as the rate region that can be achieved without inter-session network coding.
Wireless Inter-Session Network Coding - An Approach Using Virtual Multicasts
2011 IEEE International Conference on Communications (ICC)
2011 IEEE International Conference on Communications (ICC)
Computer Science