Increasing Physical Layer Security through Scrambled Codes and ARQ

We elaborate on the proposal of non-systematic channel codes on the AWGN wire-tap channel. Such coding technique, based on scrambling, achieves secure transmission with a small security gap between the authorized receiver (Bob) and the unauthorized one (Eve). In this paper, we show that, by implementing scrambling and descrambling on blocks of concatenated frames, rather than on single frames, the security gap between Bob and Eve can be further reduced. The usage of concatenated scrambling also allows to achieve security when Bob and Eve experience the same channel quality. In this case, the introduction of an ARQ protocol with authentication is needed.