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The impact of cognition on the energy efficiency of a downlink cellularsystem in which multiple relays assist the transmission of the base station isconsidered. The problem is motivated by the practical importance ofrelay-assisted solutions in mobile networks, such as LTE-A, in whichcooperation among relays holds the promise of greatly improving the energyefficiency of the system. We study the fundamental tradeoff between the powerconsumption at the base station and the level of cooperation and cognition atthe relay nodes. By distributing the same message to multiple relays, the basestation consumes more power but it enables cooperation among the relays, thusmaking the transmission between relays to destination a multiuser cognitivechannel. Cooperation among the relays allows for a reduction of the power usedto transmit from the relays to the end users due to interference management andthe coherent combining gains. These gain are present even in the case ofpartial or unidirectional transmitter cooperation, which is the case incognitive channels such as the cognitive interference channel and theinterference channel with a cognitive relay. We therefore address the problemof determining the optimal level of cooperation at the relays which results inthe smallest total power consumption when accounting for the power reductiondue to cognition. A practical design examples and numerical simulation arepresented in a companion paper (part II).

Energy Efficient Cooperative Strategies for Relay-Assisted Downlink Cellular Systems, Part I: Theoretical Framework