Secure communication in frequency selective channels with fade-avoiding subchannel usage

The random nature of fading channels is an enabling factor for achieving secrecy against eavesdropping. By considering the channel frequency selectivity, we propose an adaptive transmission scheme in which the faded subchannels of the legitimate channel are not used for conveying information. Thus, capacity reduction in legitimate channel is minimized while causing a reduction of the eavesdropper channel capacity proportional to the unused subchannels. Besides improving communication secrecy with the intelligent subchannel usage, populating the unused subchannels with artificially-generated noise that further disturbs the eavesdropper's reception is proposed. Since each receiver has its own channel state information (CSI) but not other's, eavesdroppers cannot discard the distortion which is integrated into the transmitted signal as a function of the legitimate pair's CSI. Positive secrecy capacity and outage probabilities are provided as well as error performance example in a communication scenario. It is showed that while careful usage of the subchannels in frequency selective channels improves the secrecy, introducing the fade-filling noise creates an error floor for the malicious nodes.