  We investigate a relay network where a multiantenna source can potentially
utilize an unauthenticated (untrusted) relay to augment its direct transmission
of a confidential message to the destination. Since the relay is untrusted, it
is desirable to protect the confidential data from it while simultaneously
making use of it to increase the reliability of the transmission. We present a
low-complexity scheme denoted as imbalanced beamforming based on linear
beamforming and constellation mapping that ensures perfect physical-layer
security even while utilizing the untrusted relay. Furthermore, the security of
the scheme holds even if the relay adopts the conventional decodeand- forward
protocol, unlike prior work. Simulation results show that the proposed
imbalanced signaling maintains a constant BER of 0.5 at the eavesdropper at any
SNR and number of source antennas, while maintaining or improving the detection
performance of the destination compared to not utilizing the relay or existing
security methods.
