Physical-Layer Secrecy for OFDM Transmissions Over Fading Channels

This paper considers the information theoretic secrecy rates that are achievable by an orthogonal frequency-division multiplexing (OFDM) transmitter/receiver pair in the presence of an eavesdropper that might either use an OFDM structure or choose a more complex receiver architecture. The analysis is made possible by modeling the system as a particular instance of a high dimensional multiple-input multiple-output wiretap channel. The secrecy capacity is formulated as a maximization problem under a trace constraint, and simple expressions are given for its high signal-to-noise (SNR) limit. The low rate limit of the secrecy outage probability is also evaluated under a fading channel model. As for the finite SNR case, the secrecy rates that can be achieved with particular inputs are considered. Numerical results are provided under a Rayleigh fading channel model and under dependence of the main and eavesdropper channels. The secrecy loss due to the OFDM structure constraints, and the information gain for an eavesdropper that uses a more complex receiver, are also considered.