Energy-efficient communication is an important requirement for mobile relay
networks due to the limited battery power of user terminals. This paper
considers energy-efficient relaying schemes through selection of mobile relays
in cooperative cellular systems with asymmetric traffic. The total energy
consumption per information bit of the battery-powered terminals, i.e., the
mobile station (MS) and the relay, is derived in theory. In the Joint Uplink
and Downlink Relay Selection (JUDRS) scheme we proposed, the relay which
minimizes the total energy consumption is selected. Additionally, the
energy-efficient cooperation regions are investigated, and the optimal relay
location is found for cooperative cellular systems with asymmetric traffic. The
results reveal that the MS-relay and the relay-base station (BS) channels have
different influence over relay selection decisions for optimal
energy-efficiency. Information theoretic analysis of the diversity-multiplexing
tradeoff (DMT) demonstrates that the proposed scheme achieves full spatial
diversity in the quantity of cooperating terminals in this network. Finally,
numerical results further confirm a significant energy efficiency gain of the
proposed algorithm comparing to the previous best worse channel selection and
best harmonic mean selection algorithms.