  Age-of-information is a novel performance metric in communication systems to
indicate the freshness of the latest received data, which has wide applications
in monitoring and control scenarios. Another important performance metric in
these applications is energy consumption, since monitors or sensors are usually
energy constrained. In this paper, we study the energy-age tradeoff in a status
update system where data transmission from a source to a receiver may encounter
failure due to channel error. As the status sensing process consumes energy,
when a transmission failure happens, the source may either retransmit the
existing data to save energy for sensing, or sense and transmit a new update to
minimize age-of-information. A threshold-based retransmission policy is
considered where each update is allowed to be transmitted no more than M times.
Closed-form average age-of-information and energy consumption is derived and
expressed as a function of channel failure probability and maximum number of
retransmissions M. Numerical simulations validate our analytical results, and
illustrate the tradeoff between average age-of-information and energy
consumption.
