  Relay nodes-enhanced architectures are deemed a viable solution to enhance
coverage and capacity of nowadays cellular networks. Besides a number of
desirable features, these architectures reduce the average distance between
users and network nodes, thus allowing for battery savings for users
transmitting on the uplink. In this paper, we investigate the extent of these
savings, by optimizing relay nodes deployment in terms of uplink energy
consumption per transmitted bit, while taking into account a minimum uplink
average user delay that has to be guaranteed. A novel performance evaluation
framework for uplink relay networks is first proposed to study this
energy-delay trade-off. A simulated annealing is then run to find an optimized
relay placement solution under a delay constraint; exterior penalty functions
are used in order to deal with a difficult energy landscape, in particular when
the constraint is tight. Finally, results show that relay nodes deployment
consistently improve users uplink energy efficiency, under a wide range of
traffic conditions and that relays are particularly efficient in non-uniform
traffic scenarios.
