A novel key-based Direct Sequence Spread Spectrum (DSSS) technique is proposed as a safety function for Communications-Based Train Control (CBTC) systems. According to well-known railway standards, namely IEEE 1474, IEC 61508, and EN 50128, CBTC systems should satisfy acceptable safety, security, and reliability levels. However, due to their inherent low-power features, prevalent CBTC systems cannot meet those levels perfectly in front of co-band interferences or attackers. Thus, a safety function is required which can be implemented without consuming much energy and restricted resources of CBTC systems. In this paper, the DSSS technique is based on a Cryptographically Secure Pseudo-Random Number Generator (CSPRNG). Comprehensive time-domain analysis shows how this safety function enhances the safety, security, and reliability of CBTC systems concerning the standards mentioned above. It is proved that any inauthentic signal turns into a Gaussian term with significantly mitigated power at the output of the CBTC receiver. It is also established how to determine appropriate communications parameters based on railway properties. The proposed scheme’s failure rate performance is compared with that of similar works. Additionally, computer simulations and experiments verify the efficiency of the proposed safety function.

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.