Improved physical-layer security for OFDM using data-based subcarrier scrambling

This paper presents a novel physical-layer security approach to protect the information exchanged in a wireless communication system based on OFDM. In this method, QAM symbols that are fed into the IFFT block are split into two subsets. The first subset of symbols is placed on non-scrambled (indexing) subcarriers, whereas the remaining symbols are transmitted on scrambled (data) subcarriers. Based on the bits placed on the indexing subcarriers, a permutation matrix that defines the (data-based) scrambling sequence of the data subcarriers is determined using an algorithm that is known a priori between the transmitter and receiver. The mapping between indexing bits and scrambling sequences is designed to minimize error propagation when there are erroneous received indexing bits (i.e. Gray-mapped sequences). Closed form formulas that approximate the Bit Error Probability (BEP) of the baseline (non-scrambled) and proposed (scrambled) OFDM transmissions are determined for different link configurations. The impact of the proposed physical-layer security scheme on the BEP is minimal, while increasing notably the number of combinations that an eavesdropper must check in order to execute a brute-force search attack.