Energy efficient cooperative strategies for relay-assisted downlink cellular systems

The impact of cognitive radio techniques on the energy efficiency of a downlink cellular system in which multiple relays assist the transmission of the base station toward multiple receivers is studied. In particular, the fundamental tradeoff between the power consumption at the base station and the level of cooperation at the relay nodes is investigated. By increasing its transmit power, the base station can distribute the same message to multiple relays. In turn, the common knowledge at the relays enables cooperation, which results in a reduction in the power consumption due to interference management and coherent combining gains. This implies that the overall power efficiency can potentially be improved by an increase in the power consumption at the base station. We employ an information-theoretical analysis of the attainable power efficiency based on the chain graph representation of achievable schemes. This novel theoretical tool uses a graphical Markov model to represent coding operations and allows for the automatic derivation of achievable rate regions for general networks. This approach provides an effective tool to analyze the relationship between the energy consumption at the base station and power savings provided by relay cooperation through the use of transmission strategies such as superposition coding, interference decoding and rate-splitting. We present numerical evaluations for the scenario in which two relay nodes aid the communication between the base station and three receivers. These evaluations show that cooperative strategies at the relays provide clear advantages as compared to the non-cooperative scenario for varying channel conditions and target rates.