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