Energy Efficient Routing Protocols for Multi-Hop Cellular Networks Baldomero Coll-Perales and Javier Gozalvez Ubiquitous Wireless Communications Research Laboratory, Uwicore, http://www.uwicore.umh.es University Miguel Hernández of Elche, Avda de la Universidad, s/n, 03202 Elche, Spain Mail: j.gozalvez@umh.es , bcoll@umh.es , tlf: +34 966658955 Abstract—Multi-hop Cellular Networks using mobile relays require efficient multi-hop networking protocols that provide a high end-to-end performance and reduce the terminal’s energy consumption. In this context, this paper proposes a set of novel multi-hop routing protocols that exploit the location of the destination node to reduce their network signalling and overall energy consumption. These two aspects are crucial factors to progress towards the future implementation of Multi-hop Cellular Networks based on mobile relays. Keywords—Multi-hop Cellular Networks, routing protocols, cost function, energy efficiency. I. INTRODUCTION Future 4G cellular systems are required to provide high and homogeneous bit rates over the complete cell coverage area. Traditional cellular architectures, where each Mobile Station (MS) directly communicates with the Base Station (BS), are not capable to provide such homogeneous high bit rates due to the signal attenuation with the increasing distance. Achieving the defined 4G objectives requires installing a higher number of base stations, or integrating cellular and ad-hoc networking technologies. Increasing the number of base stations has an economical and social cost due to the growing social reaction against base stations. The integration of cellular and ad-hoc technologies, also referred as Multi-hop Cellular Network (MCN) [1], has gained significant research attention given its capacity to achieve the 4G objectives by substituting a direct MS-BS link by multi-hop links using intermediate nodes to retransmit the information from source to destination. In this case, MCN networks offer the potential to increase the overall multi-hop transmission rate by reducing the communications distance and signal loss in each hop. MCN networks can either use fixed (MCN-Fixed Relay, MCN-FR) or mobile relays (MCN-Mobile Relay, MCN-MR). MCN-MR networks are characterised by a lower implementation cost than MCN- FR networks, but a higher management complexity due to the participation of mobile terminals. However, exploiting the mobile terminals communications capabilities in a decentralized and distributed manner, also increases the potential and future perspectives of MCN-MR networks. To reach such potential, it is necessary to overcome important technological challenges, such as the design and optimization of robust, adaptive, context-aware and energy-efficient multi- hop routing protocols. Different multi-hop routing protocols and cost functions have been proposed in the literature for wireless mesh networks. The first proposals were mainly based on the number of hops between the source and destination nodes [2]- [3]. This parameter has a direct impact on the multi-hop latency and stability, and has since then been generally taken into account in multi-hop routing protocols and cost functions. Other cost functions have proposed to consider diverse parameters that are relevant to the network and system performance and operation. For example, parameters such as the energy [4], network congestion [5], Packet Error Rate (PER) [6] and throughput [7] have been shown to impact the multi-hop wireless mesh networking performance. An interesting multi-hop routing protocol combining some of these parameters, and referred to as Multiple-Metric (MM), has been reported in [8]. In particular, the MM cost function combines the energy, channel congestion and number of hops from source to destination for its multi-hop routing decisions. Despite these important contributions, novel reliable multi- hop networking protocols minimising energy consumption or signalling load are still needed to improve the feasibility of mesh networking systems, and in particular of MCN-MR networks that heavily rely on the collaboration from mobile nodes. In MCN-MR networks, the communications between a mobile station and a base station is established through mobile relaying nodes. Considering the perspective future implementation of MCN-MR systems, it would be reasonable to assume that, in the uplink direction, the source node (MS) knows the location of the destination node (BS). In fact, many countries currently release online the location of all their base stations nationwide. Knowing the location of the destination node for downlink communications would require the use of location technologies, which is not unrealistic given the continuous increase of devices with embedded GPS capabilities. The long range signalling capabilities offered by cellular architectures would then make possible to feedback this information to the source node (BS). This work focuses on uplink communications and proposes several location- based routing protocols that exploit the location of the destination and intermediate nodes to efficiently reach the addressed node. Most location-based routing protocols (e.g. LAR [9]), extensively studied in the literature, need to assume that the destination node’s locations is known and accordingly limit the search area in the discovery route process. Contrary to existing routing techniques, this work proposes to exploit the knowledge of the BS’s location to design a set of efficient multi-hop routing protocol capable to reduce the energy