  Molecular communication often involves coexisting links where certain links
may have priority over others. In this work, we consider a system in
three-dimensional (3-D) space with two coexisting communication links, each
between a point transmitter and fully-absorbing spherical receiver (FAR), where
the one link (termed primary) has priority over the second link (termed
secondary). The system implements the underlay cognitive-communication strategy
for the co-existence of both links, which use the same type of molecules for
information transfer. Mutual influence of FARs existing in the same
communication medium results in competition for capturing the
information-carrying molecules. In this work, first, we derive an approximate
hitting probability equation for a diffusion-limited molecular communication
system with two spherical FARs of different sizes considering the effect of
molecular degradation. The derived equation is then used for the performance
analysis of primary and secondary links in a cognitive molecular communication
scenario. We show that the simple transmit control strategy at the secondary
transmitter can improve the performance of the overall system. We study the
influence of molecular degradation and decision threshold on the system
performance. We also show that the systems parameters need to be carefully set
to improve the performance of the system.
