A Hidden OSTBC Scheme to Enhance Physical Layer Security by Employing a Pseudorandom Precoder

The capability of exploiting Orthogonal Space-Time Block Codes (OSTBC) for increasing physical layer security of wireless systems is studied. A technique named “hidden OSTBC” is introduced, in which, a pseudorandom sequence is utilized by both transmitter and legitimate receiver to provide required security. Traditionally, employing pseudorandom sequences with methods such as spread spectrum or cooperative jamming involves huge amount of bandwidth or transmission power constraints, which are major challenges for wireless systems. Without requiring additional power or bandwidth, this study is designed to address exploitation of a pseudorandom antipodal sequence as a precoder. Elements of this sequence are multiplied to each antenna's transmitting symbol, and legitimate receiver employs the same sequence upon its combining rule. Mathematical analysis and simulations prove that an eavesdropper who does not know the pseudorandom sequence suffers from a degraded equivalent channel. Security enhancement is studied by investigating eavesdropper's higher error rate compared with that of legitimate receiver. Also, by employing lattice-based codebooks, a lower-bound is drawn for secrecy capacity, implying the achievability of nearly perfect secrecy regarding information theoretic analysis.