  This paper studies the wireless surveillance of a hybrid automatic repeat
request (HARQ) based suspicious communication link over Rayleigh fading
channels. We propose a proactive eavesdropping approach, where a half-duplex
monitor can opportunistically jam the suspicious link to exploit its potential
retransmissions for overhearing more efficiently. In particular, we consider
that the suspicious link uses at most two HARQ rounds for transmitting the same
data packet, and we focus on two cases without and with HARQ combining at the
monitor receiver. In both cases, we aim to maximize the successful
eavesdropping probability at the monitor, by adaptively allocating the jamming
power in the first HARQ round according to fading channel conditions, subject
to an average jamming power constraint. For both cases, we show that the
optimal jamming power allocation follows a threshold-based policy, and the
monitor jams with constant power when the eavesdropping channel gain is less
than the threshold. Numerical results show that the proposed proactive
eavesdropping scheme achieves higher successful eavesdropping probability than
the conventional passive eavesdropping, and HARQ combining can help further
improve the eavesdropping performance.
