Cognitive radios are the new paradigm
to improve the current spectrum utilization.
In this work, we discuss
 ``Duplex-observe-learn-and act'' (DOLA) a primary channel
access control
algorithm
that constitutes the first alternative to the
traditional opportunistic spectrum access for cognitive radios
``Listen-Before-Talk'' (LBT).
DOLA seeks to exploit
the bi-directional nature of many primary communication
systems to enable opportunistic secondary access while
achieving primary receiver protection.
Here, we focus our attention on ACK/NAK feedback
notifications overhearing
from the primary receiver (PU-Rx) and we show that
primary protection
can be achieved in terms of probability of
collision, that can be kept below $6\%$ fine tuning
the secondary users' reward function.