  Two-hop security communication with an eavesdropper in wireless environment
is a hot research direction. The basic idea is that the destination,
simultaneously with the source, sends a jamming signal to interfere the
eavesdropper near to or co-located with the relay. Similar as physical layer
network coding, the friendly jamming signal will prevent the eavesdropper from
detecting the useful information originated from the source and will not affect
the destination on detecting the source information with the presence of the
known jamming signal. However, existing investigations are confined to Gaussian
distributed signals, which are seldom used in real systems. When finite
constellation signals are applied, the behavior of the secrecy rate becomes
very different. For example, the secrecy rate depends on phase difference
between the input signals with finite constellations, which is not observed
with Gaussian signals. In this paper, we investigate the secrecy capacity and
derive its upper bound for the two-hop relay model, by assuming an eavesdropper
near the relay and the widely used M-PSK modulation. With our upper bound, the
best and worst phase differences in high SNR region are then given. Numerical
studies verify our analysis and show that the derived upper bound is relatively
tight.
