Wireless Network-Level Partial Relay Cooperation Nikolaos Pappas † , Jeongho Jeon ‡ , Anthony Ephremides ‡ , Apostolos Traganitis † † Computer Science Department, University of Crete, Greece Institute of Computer Science, Foundation for Research and Technology - Hellas (FORTH) ‡ Department of Electrical and Computer Engineering and Institute for Systems Research University of Maryland, College Park, MD 20742 Email: npapas@ics.forth.gr, {jeongho, etony}@umd.edu, tragani@ics.forth.gr Abstract—In this paper, we evaluate the benefits of using one user of a two-user random access system to relay traffic of the other user. I. I NTRODUCTION Cooperative communication helps overcome fading and at- tenuation in wireless networks. Its main purpose is to increase the communication rates across the network and to increase reliability of time-varying links. It is known that wireless communication from a source to a destination can benefit from the cooperation of nodes that overhear the transmission. The classical single relay channel [1] exemplifies this situation. Further work on the relay channel in [2] and [3] has enabled substantial performance improvement. However, there is evidence that additional gains can be achieved with “network-layer” cooperation (or packet-level cooperation), that is plain relaying without any physical layer considerations [4] and [5]. In this work, we focus on this type of cooperation. The work in [6] investigated the network-level cooperation in a network consisting of a source and a relay by considering the cases of full or no cooperation at the relay. A key difference between physical-layer and network-layer cooperation ideas is that the objective rate function that is maximized is the so-called stable throughput region which captures the bursty nature of traffic from the source. In [6], it was shown that the stability region of full cooperation under random-access does not always strictly contain the non- cooperative stability region. The main contribution in this paper is to introduce the notion of partial network-level cooperation by adding a flow controller for the traffic coming to the relay from the source. We prove that the system is always better than or at least equal to the system without the flow controller. Specifically, we provide an exact characterization of the stability region of a network consisting of a source, a relay and a destination node as shown in Fig. 1. We consider the collision channel with erasures and random access of the medium. The source and N. Pappas was supported by ”Heracleitus II - University of Crete”, NSRF (ESPA) (2007-2013) and is co-funded by the European Union and national resources. J. Jeon was partially supported by NIST-ARRA Fellowship Program. This work was supported in part by MURI grant W911NF-08-1- 0238, NSF grant CCF-0728966, and ONR grant N000141110127.This work was partially funded by the Marie Curie IAPP ”AVID-MODE” grant within the 7th European Commission Framework Programme. S  2 Regulator R  1 D  1 2 Queue 1 Queue 2 p 13 p 23 p 12 p a Fig. 1. Network model with regulator at the relay the relay node have external arrivals; furthermore, the relay is forwarding part of the source node’s traffic to the destination. Unlike the work in [6], the relay node is equipped with a flow controller that regulates the internal arrivals from the source based on the conditions in the network to ensure the stability of the queues. We characterize the stable throughput region under conditions of no cooperation at all, full cooperation, and probabilistic (opportunistic) cooperation. By probabilistic cooperation we mean that under certain conditions in the network, the relay may accept a packet from the source. The characterization of the stability regions is known to be challenging because the queues of the users are coupled (i.e., the service process of a queue depends on the status of the other queues). A tool that bypasses this difficulty is the stochastic dominance technique [7]. II. SYSTEM MODEL We consider a time-slotted system in which the nodes are randomly accessing a common receiver as shown in Fig. 1. We denote with S, R, and D the source, the relay and the destination, respectively. Packet traffic originates from S and R. Because of the wireless broadcast nature, R may receive some of the packets transmitted from S and then relay those packets to D. The packets from S which failed to be received by D but were successfully received by R are relayed by R. As we impose half-duplex constraint, R can overhear S only when it is idle. Each node has an infinite size buffer for storing incoming packets, and the transmission of each packet occupies one time slot. Node R has separate queues for the exogenous arrivals and the endogenous arrivals that are relayed through R. But, we can let R to maintain a single queue and merge all the arrivals into a single queue as the achievable stable throughput region is not affected [6]. This is because 2012 IEEE International Symposium on Information Theory Proceedings 978-1-4673-2579-0/12/$31.00 ©2012 IEEE 1127