  Timely information updates are critical to time-sensitive applications in
networked monitoring and control systems. In this paper, the problem of
real-time status update is considered for a cognitive radio network (CRN), in
which the secondary user (SU) can relay the status packets from the primary
user (PU) to the destination. In the considered CRN, the SU has opportunities
to access the spectrum owned by the PU to send its own status packets to the
destination. The freshness of information is measured by the age of information
(AoI) metric. The problem of minimizing the average AoI and energy consumption
by developing new optimal status update and packet relaying schemes for the SU
is addressed under an average AoI constraint for the PU. This problem is
formulated as a constrained Markov decision process (CMDP). The monotonic and
decomposable properties of the value function are characterized and then used
to show that the optimal update and relaying policy is threshold-based with
respect to the AoI of the SU. These structures reveal a tradeoff between the
AoI of the SU and the energy consumption as well as between the AoI of the SU
and the AoI of the PU. An asymptotically optimal algorithm is proposed.
Numerical results are then used to show the effectiveness of the proposed
policy.
