Energy Efficient Artificial Fast Fading for MISO-OFDM Systems

For secure wireless communications, physical layer security techniques, which degrade the received signal quality of the eavesdropper, are useful. Among the physical layer security techniques, an artificial fast fading (AFF) technique degrades the received signal quality of eavesdroppers by causing pseudo fast fading to the transmitting signals. This is realized by multiplying the signals to be transmitted by random weights every symbol interval. However, the AFF technique often increases the power of the weighted signals. In such cases, the weighted signals must be normalized before transmission. This causes energy loss in the legitimate receiver. Therefore, we consider minimizing the norm of the weight vector to prevent the power of the weighted signals from being increased. In this paper, we propose three energy efficient AFF generation schemes. In the first scheme, the phases of random weights are adjusted to minimize the norm of the non-random weight which is required to cancel AFF out at the legitimate receiver. In the second scheme, two or more non-random weights are used for minimizing the norm of the weight vector. The third proposed scheme is the combination of these two schemes. Simulation results show that the proposed schemes achieve higher ergodic mutual information as compared with the conventional AFF generation scheme.