Masked-OFDM: A physical layer encryption for future OFDM applications

Security is an important topic of research for future 4G applications employing OFDM modulation. Physical layer security is an emerging candidate, based on the concept of perfect secrecy. In this paper we present a symmetric key physical layer encryption for OFDM systems, termed Masked-OFDM (MOFDM). The OFDM signal becomes practically undetectable by a potential eavesdropper by being masked in a non-orthogonal FDM signal of approximately the same overall bandwidth. This is equivalent to the transmission of roughly twice as much data as the channel degrees of freedom. For the OFDM symbol, a positive secrecy capacity can be achieved in AWGN even in cases where the wiretap channel has a better SNR than the main channel. The system secrecy capacity due to using a structured helping interferer is higher than compared to using a noise-like mask signal. The proposed scheme does not compromise either the bandwidth efficiency, or the error performance of the OFDM system, at the cost of increased transmission power.