  We apply the concept of artificial and controlled interference in a
triangular relay network with an untrusted relay aiming at enhancing the
wireless communication secrecy between the source and the destination node. In
order to shield the square quadrature amplitude modulated (QAM) signals
transmitted from the source node to the relay, the destination node designs and
transmits artificial noise (AN) symbols to jam the relay reception. The
objective of our considered AN design is to degrade the error probability
performance at the untrusted relay, for different types of channel state
information (CSI) at the destination. By considering perfect knowledge of the
instantaneous CSI of the source-to-relay and relay-to-destination links, we
first present an analytical expression for the symbol error rate (SER)
performance at the relay. Based on the assumption of an average power
constraint at the destination node, we then derive the optimal phase and power
distribution of the AN that maximizes the SER at the relay. Furthermore, we
obtain the optimal AN design for the case where only statistical CSI is
available at the destination node. For both cases, our study reveals that the
Gaussian distribution is generally not optimal to generate AN symbols. In
particular, the conducted numerical results show that the Gaussian distributed
AN is far from optimal. The presented AN design takes into account practical
parameters for the communication links, such as QAM signaling and maximum
likelihood decoding.
