On the equivocation region of relay channels with orthogonal components

We consider the secrecy rate of a relay network where an eavesdropper is co-located with the relay node. This exemplifies a scenario where the relay node is not malicious by nature, but is located in an "untrusted region", and hence is potentially compromised. Given that the aim now is to keep the relay node completely oblivious to the information sent from the source to the destination, an interesting question is whether the relay node should be deployed at all. We investigate this question for two types of relay networks with orthogonal components. For the first model, we find the equivocation capacity region and prove the relay node should not be deployed. For the second model, we present an achievable secrecy rate based on compress-and-forward, and conclude that the relay node is potentially useful as it can, without being able to decode the source data, facilitate secret communication between the source and the destination that would not be possible without the relay.