Secure index-modulation based hybrid free space optical and millimeter wave links

An enhanced security algorithm for wireless communication system deploying hybrid free space optical (FSO)-millimeter wave (mmWave) links is proposed in this clause. The activation of either links, FSO or mmWave, is determined through index modulation (IM) technique. In particular, the first bit to be transmitted determines the active link; while remaining bits modulate a conventional symbol to be transmitted over the active link. Yet, before transmitting the symbol, it is precoded by another signal that is extracted from the channel coefficient of the active link. The channel coefficients for both links are assumed to be available at the transmitter and the receiver through typical channel estimation algorithms. It is revealed that the security of the proposed system is significantly enhanced, where IM concept degrades the performance of an eavesdropper who is attacking any of the communication links. Also, the use of precoding worsens the functioning of an eavesdropper who is attacking both links. The impact of channel estimation errors along with the spectral efficiency, turbulence conditions, channel fading, and eavesdropper capability on the overall performance is investigated and thoroughly discussed.