  In this paper, an intelligent reflecting surface (IRS) assisted spectrum
sharing underlay cognitive radio (CR) wiretap channel (WTC) is studied, and we
aim at enhancing the secrecy rate of secondary user in this channel subject to
total power constraint at secondary transmitter (ST), interference power
constraint (IPC) at primary receiver (PR) as well as unit modulus constraint at
IRS. Due to extra IPC and eavesdropper (Eve) are considered, all the existing
solutions for enhancing secrecy rate of IRS-assisted non-CR WTC as well as
enhancing transmission rate in IRS-assisted CR channel without eavesdropper
fail in this work. Therefore, we propose new numerical solutions to optimize
the secrecy rate of this channel under full primary, secondary users' channel
state information (CSI) and three different cases of Eve's CSI: full CSI,
imperfect CSI with bounded estimation error, and no CSI. Simulation results
show that our proposed solutions for the IRS-assisted design greatly enhance
the secrecy performance compared with the existing numerical solutions with and
without IRS under full and imperfect Eve's CSI. And positive secrecy rate can
be achieved by our proposed AN aided approach given most channel realizations
under no Eve's CSI case so that secure communication also can be guaranteed.
All of the proposed AO algorithms are guaranteed to monotonic convergence.
