  This letter attempts to design a surveillance scheme by adopting an active
reconfigurable intelligent surface (RIS). Different from the conventional
passive RIS, the active RIS could not only adjust the phase shift but also
amplify the amplitude of the reflected signal. With such reflecting, the
reflected signal of active RIS could jointly adjust the signal to interference
plus noise ratio (SINR) of the suspicious receiver and the legitimate monitor,
hence the proactive eavesdropping at the physical layer could be effectively
realized. We formulate the optimization problem with the target of maximizing
the eavesdropping rate to obtain the optimal reflecting coefficient matrix of
the active RIS. The formulated optimization problem is nonconvex fractional
programming and challenging to deal with. We then solve the problem by
approximating it as a series of convex constraints. Simulation results validate
the effectiveness of our designed surveillance scheme and show that the
proposed active RIS aided surveillance scheme has good performance in terms of
eavesdropping rate compared with the scheme with passive RIS.
