  Wireless surveillance is becoming increasingly important to protect the
public security by legitimately eavesdropping suspicious wireless
communications. This paper studies the wireless surveillance of a two-hop
suspicious communication link by a half-duplex legitimate monitor. By exploring
the suspicious link's two-hop nature, the monitor can adaptively choose among
the following three eavesdropping modes to improve the eavesdropping
performance: (I) \emph{passive eavesdropping} to intercept both hops to decode
the message collectively, (II) \emph{proactive eavesdropping} via {\emph{noise
jamming}} over the first hop, and (III) \emph{proactive eavesdropping} via
{\emph{hybrid jamming}} over the second hop. In both proactive eavesdropping
modes, the (noise/hybrid) jamming over one hop is for the purpose of reducing
the end-to-end communication rate of the suspicious link and accordingly making
the interception more easily over the other hop. Under this setup, we maximize
the eavesdropping rate at the monitor by jointly optimizing the eavesdropping
mode selection as well as the transmit power for noise and hybrid jamming.
Numerical results show that the eavesdropping mode selection significantly
improves the eavesdropping rate as compared to each individual eavesdropping
mode.
