Anti‐eavesdropping FrFT‐OFDM system exploiting multipath channel characteristics

This study exploits wireless channel response and fractional Fourier transform (FrFT) to achieve anti‐eavesdropping downlink transmission over a multipath channel. The proposed system works in time‐division duplexing mode, and only the transmitter knows the main channel response through uplink services. With the use of the main channel information, a time‐varying spread‐spectrum orthogonal frequency division multiplexing (TS‐OFDM) subsystem is proposed to realise confidential transform order (TO) transmission, and a security‐enhanced FrFT‐OFDM (SE‐FrFT‐OFDM) subsystem is presented for high‐rate data transmission. The optimal power allocation algorithms that optimise the average secrecy outage capacity of the TS‐OFDM subsystem and the bit error rate performance of the SE‐FrFT‐OFDM subsystem are also provided. Given channel reciprocity and spatial decorrelation, the legitimate user can achieve optimal anti‐multipath fading performance, but the intercepted data symbols will be randomised by both channel fading and TO deviations. To avoid randomisation, the eavesdropper has to test all possible TO combinations of multiple SE‐FrFT‐OFDM symbols, which is usually an impractical task. Compared to other typical methods, the transmitting power of the proposed system can be used more efficiently because the data symbol is not transmitted along with additional interference. Computer simulations are carried out to validate this scheme.