  Physical layer network coding (PNC) has been proposed for next generation
networks. In this contribution, we investigate PNC schemes with embedded
perfect secrecy by exploiting structured interference in relay networks with
two users and a single relay. In a practical scenario where both users employ
finite and uniform signal input distributions we propose upper bounds (UBs) on
the achievable perfect secrecy rates and make these explicit when PAM modems
are used. We then describe two simple, explicit encoders that can achieve
perfect secrecy rates close to these UBs with respect to an untrustworthy relay
in the single antenna and single relay setting. Lastly, we generalize our
system to a MIMO relay channel where the relay has more antennas than the users
and optimal precoding matrices which maintain a required secrecy constraint are
studied. Our results establish that the design of PNC transmission schemes with
enhanced throughput and guaranteed data confidentiality is feasible in next
generation systems.
