CoRe-MAC: A MAC-Protocol for Cooperative Relaying in Wireless Networks Helmut Adam 1 , Wilfried Elmenreich 1 , Christian Bettstetter 1,2 , and Sidi Mohammed Senouci 3 1 Mobile Systems Group, University of Klagenfurt, Austria [firstname.lastname]@uni-klu.ac.at 2 Lakeside Labs GmbH, Austria 3 Orange Labs, 22307 Lannion Cedex, France, sidimohammed.senouci@orange-ftgroup.com Abstract—Cooperative relaying methods can improve wireless links, but introduce overhead due to relay selection and resource reservation compared to non-cooperative transmission. In order to be competitive, a cooperative relaying protocol must avoid or compensate for this overhead. In this paper, we present a MAC protocol for relay selection and cooperative communication as an extension to CSMA/CA which addresses resource reservation, relay selection, and cooperative transmission while keeping the overhead in terms of time and energy low. We discuss the efficiency of this protocol for packet error rate, throughput, and message delay in a multi-hop network. Simulation results show that the protocol performs similar and without noticeable overhead compared to standard CSMA/CA for good SNR while it is able to significantly improve throughput and reliability at larger distances. Keywords: cooperative relaying, cooperative diversity, MAC, relay selection, CSMA/CA I. I NTRODUCTION Mobile radio communications suffer from large-scale and small-scale fading effects that attenuate the communication signal. While large-scale fading is caused by a distance- dependent path loss and shadowing effects, small-scale fading is caused by multipath propagation. For mobile receivers or transmitters, small-scale fading can cause rapid fluctuations of the received signal-to-noise ratio (SNR); if a mobile device moves only a small distance, it may experience deep fading, even if it had perfect signal reception just an instant before. Cooperative relaying [1] is a concept, where a relay node assists the communication between two nodes when the direct link is affected by small scale fading. The information is relayed via a spatially different path which is likely not affected by the same fading effects as the direct link at the same time. Thus, using such a relay communication channel can improve the network capacity by implementing spatial diversity for the communication paths [2]. With the growing number of networked wireless devices in everyday appliances, there are more potential relay nodes within transmission range of a sender and receiver. Henceforth, cooperative relaying will gain additional importance in the near future. Cooperative diversity is expected to be more beneficial, if the cooperative relaying protocol is designed according to the following: First, it should have a low overhead. A large number of communication attempts are expected to succeed without the need for alternative communication paths. Thus, in the case of a successful transmission, a cooperative relaying protocol should have minimal overhead in comparison to non- cooperative transmission schemes. Second, the protocol should exploit cooperative diversity to an extent that makes the effort for the more complex interaction between wireless nodes worth it. Finally, the protocol should be implementable with state-of-the-art hardware. In this paper, we propose a novel Cooperative Relaying Medium Access protocol (CoRe-MAC) for wireless networks which extends the standard Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA) protocol. The objective is to increase reliability and throughput of the communi- cation if the Signal-to-Noise-Ratio (SNR) over the direct link between source and destination is below an acceptable level, while avoiding overhead to the communication if the direct transmission is successful. To this end, in the proposed protocol a reactive (on demand) relay selection is invoked only in case of transmission failures. The remaining part of this paper is structured as follows: Section II presents related work and summarizes design options on cooperative diversity and relaying. Section III introduces the proposed CoRe-MAC protocol. Section IV pro- vides an evaluation of CoRe-MAC and a standard CSMA/CA approach for comparison reasons in a realistic scenario. Fi- nally, we conclude and give an outlook to future research in Section V. II. RELATED WORK AND DESIGN OPTIONS Cooperative relaying can be divided into three main phases: direct transmission, relay selection, and cooperative transmis- sion. In the direct transmission phase the source transmits its data, whereas destination and relay (or potential relays) try to receive it. In the relay selection phase a neighboring node of source and destination is selected. The cooperative transmission phase, where the relay forwards the data to the destination, occurs only if the destination has failed to retrieve the data from the source during the direct transmission. The relay selection phase has a great impact on the per- formance of the whole cooperative relaying process [3]. The major selection criteria is the link quality of the communi- cation participants which is typically measured by probing packets [4]–[6]. The selection can be further refined by us- ing additional factors like residual power [7]. Note that the selection process also depends on the actual environment, i.e., it is important to know how frequently a relay needs to be selected for a given source destination pair, since a node may © IEEE, 2009. This is the author's version of the work. Personal use of this material is permitted. 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