  As a key in cognitive radio networks (CRNs), dynamic spectrum access needs to
be carefully designed to minimize the interference and delay to the
\emph{primary} (licensed) users. One of the main challenges in dynamic spectrum
access is to determine when the \emph{secondary} (unlicensed) users can use the
spectrum. In particular, when the secondary user is using the spectrum, if the
primary user becomes active to use the spectrum, it is usually hard for the
secondary user to detect the primary user instantaneously, thus causing
unexpected interference and delay to primary users. The secondary user cannot
detect the presence of primary users instantaneously because the secondary user
is unable to detect the spectrum at the same time while it is transmitting. To
solve this problem, we propose the full duplex wireless communications scheme
for CRNs. In particular, we employ the Antennas Cancellation (AC), the RF
Interference Cancellation (RIC), and the Digital Interference Cancellation
(DIC) techniques for secondary users so that the secondary user can scan for
active primary users while it is transmitting. Once detecting the presence of
primary users, the secondary user will release the spectrum instantaneously to
avoid the interference and delay to primary users. We analyze the packet loss
rate of primary users in wireless full duplex CRNs, and compare them with the
packet loss rate of primary users in wireless half duplex CRNs. Our analyses
and simulations show that using our developped wireless full duplex CRNs, the
packet loss rate of primary users can be significantly decreased as compared
with that of primary users by using the half duplex CRNs.
