Cooperation in Multi-hop Cellular Networks ∗ Naouel Ben Salem † , Levente Butty´ an ‡ , Jean-Pierre Hubaux † , Markus Jakobsson § Technical Report Nr. IC/2003/55 Abstract In multi-hop cellular networks, the existence of a communication link between the mobile station and the base station is not required: a mobile station that has no direct connection with a base station can use other mobile stations as relays. Compared with conventional (single-hop) cellular networks, this new generation can lead to a better usage of the available spectrum and to a reduction of infrastructure costs. However, these benefits would vanish if the mobile nodes do not properly cooperate and forward packets for other nodes. In this paper, we propose a charging and rewarding scheme to encourage the most fundamental operation, namely packet forwarding. We analyse the robustness of our protocols against rational and malicious attacks. We show that our protocols thwart rational attacks and detect malicious attacks. We also show that our solution makes collaboration rational for selfish nodes. Keywords: Cooperation, Multi-hop Cellular Networks, Hybrid Cellular Networks, Ad Hoc Networks, Packet Forwarding, Security, Pricing, Charging, Billing 1 Introduction The geographic area covered by a conventional cellular network is populated with base stations that are connected to each other via a backbone. A mobile node can use the network when it has a direct (single- hop) connection to a base station, but as soon as it is beyond the reach of the base stations coverage, the mobile node is disconnected from the cellular network. For the operator, the usual solution to this problem is to increase the coverage of the network by adding antennas, and for the user to move until he reaches a covered region. An alternative solution would be to allow multi-hop communications in the cellular network, which makes it possible for the isolated node to ask other nodes to relay its traffic to or from a base station. The resulting network [1, 34, 12, 2, 32], frequently called multi-hop cellular network, presents several benefits [24, 13, 25, 22]. First of all, the coverage of the network is increased while the number of fixed antennas is kept relatively small. Second, the energy consumption of the nodes can be reduced because the signal has to cover a smaller distance. And finally, as the radiated energy is reduced, the interference with other nodes diminishes as well. Given the advantages listed above, multi-hop cellular networks represent a new and promising paradigm. However, the proper operation of this new family of networks requires the mobile nodes to collaborate with ∗ The work presented in this paper was supported (in part) by the National Competence Center in Research on Mobile Infor- mation and Communication Systems (NCCR-MICS), a center supported by the Swiss National Science Foundation under grant number 5005-67322 † Laboratory of Computer Communications and Applications (LCA), Swiss Federal Institute of Technology – Lausanne (EPFL), Switzerland ({naouel.bensalem, jean-pierre.hubaux}@epfl.ch) ‡ Budapest University of Technology and Economics, Department of Telecommunications, Hungary (buttyan@hit.bme.hu) § RSA Laboratories, 174 Middlesex Turnpike, Bedford, MA 01730, USA (mjakobsson@rsasecurity.com) 1