  Energy-efficient communication is an important requirement for mobile
devices, as the battery technology has not kept up with the growing
requirements stemming from ubiquitous multimedia applications. This paper
considers energy-efficient transmission schemes in cooperative cellular systems
with unbalanced traffic between uplink and downlink. Theoretically, we derive
the optimal transmission data rate, which minimizes the total energy
consumption of battery-powered terminals per information bit. The
energy-efficient cooperation regions are then investigated to illustrate the
effects of relay locations on the energy-efficiency of the systems, and the
optimal relay location is found for maximum energy-efficiency. Finally,
numerical results are provided to demonstrate the tradeoff between
energy-efficiency and spectral efficiency.
