  This paper investigates the average age of information (AoI) minimization in
status update systems in which the update packets are transmitted with fixed
rate. It is assumed that the source avoids queue-induced delay by generating
and sending a new status update packet in each time slot. Assuming that the
transmit power can be adapted based on the number of successive NACKs (negative
acknowledgements) received, a closed-form expression for the average AoI is
derived considering transmissions over block fading channel models. An
optimization problem for minimizing the average AoI under average power
constraints is formulated and a stochastic optimization algorithm for obtaining
a feasible solution is proposed. Numerical results on the proposed power
control policy show that more power is preferred to be allocated to packets
after a certain number of failure packets in the low-power regime and the
proposed power control reduces the average AoI compared to the constant power
policy. An alternative on-off power control policy is also proposed and shown
to achieve satisfactory performance.
