  In this paper, the minimization of time-averaged age-of-information (AoI) in
an energy harvesting (EH) source-equipped remote sensing setting is considered.
The EH source opportunistically samples one or multiple processes over discrete
time instants and sends the status updates to a sink node over a time-varying
wireless link. At any discrete-time instant, the EH node decides whether to
probe the link quality using its stored energy and further decides whether to
sample a process and communicate the data based on the channel probe outcome.
The trade-off is between the freshness of information available at the sink
node and the available energy at the energy buffer of the source node. To this
end, an infinite horizon Markov decision process theory is used to formulate
the problem of minimization of time-averaged expected AoI for a single energy
harvesting source node. The following two scenarios are considered: (i) single
process with channel state information at the transmitter (CSIT), (ii) multiple
processes with CSIT. In each scenario, for probed channel state, the optimal
source node sampling policy is shown to be a threshold policy involving the
instantaneous age of the process(es), the available energy in the buffer, and
the instantaneous channel quality as the decision variables. Finally, numerical
results are provided to demonstrate the policy structures and trade-offs.
