  In this paper, a buffered decode and forward (DF) relay based three-node
underlay cooperative cognitive relay network (CRN) is considered with a direct
path to the destination. The source and the relay use multiple rates, and joint
rate and link selection are performed to maximize throughput. Optimum link and
rate selection rules are evolved that ensure buffer stability, and expressions
are derived for the throughput assuming peak power and peak interference
constraints on the transmit power of the secondary nodes. The expressions are
written in a manner that yields useful insights on buffer stability and role of
the direct link on performance. A scheme in which the direct link signal is
combined with the relayed signal is also considered, and it is demonstrated
that it offers additional improvement in performance only in some scenarios.
Computer simulations have been presented to verify the accuracy of derived
expressions.
