This paper proposes full-duplex transmissions from legitimate nodes to achieve channel estimation performance deterioration at an eavesdropper as compared to the legitimate receiver. The proposed discriminatory channel estimation (DCE) technique comprises of two stages where, in the first stage, the self-interference channel is estimated by the respective legitimate nodes. Followed by in-band full-duplex transmission from both legitimate nodes for channel estimation at legitimate nodes, while providing equivocation at the eavesdropper due to the superposition of two signals. The discrimination of channel estimation performance provides secrecy against the passive eavesdropper while delivering information to the legitimate receiver. We provide the mean square error (MSE) to indicate the performance achieved by linear minimum mean square error (LMMSE) estimators. We have also provided bit error rate (BER), and secrecy capacity analysis to indicate the performance of secure communication achieved by securing the channel estimates from the eavesdropper. The BER analysis shows that for proposed DCE, BER at the eavesdropper is close to 0.1 while the legitimate node is able to robustly decode the information. Finally, simulation results show that the proposed DCE outperforms existing DCE techniques for the considered scenario.

The random nature of wireless channel can be utilized to achieve secure communications. Recently, there have been some contributions towards finding practical approaches to provide physical layer security, but robust and secure channel estimates are critical in achieving secrecy. In this paper, we propose that in-band full-duplex communication can be utilized by the legitimate transceiver pair to mask the channel estimates from the eavesdropper. The proposed secure channel estimation technique comprises of two stages. In the first stage, the self- interference channel is estimated by respective nodes. In the second stage, in-band full-duplex transmission from both legitimate nodes is utilized to achieve channel estimates at legitimate nodes while providing equivocation at the eavesdropper. The performance analysis shows that the proposed scheme provides robust channel estimation at the legitimate nodes while maintaining equivocation at the eavesdropper.

We develop a practical layered detect-compress-and-forward (LDCF) coding framework for a three-node relay network. Assuming all transmissions to be mapped to Quadrature Amplitude Modulation (QAM) constellations, the LDCF framework at the relay utilizes maximum-likelihood detection, followed by a multi-level convolutional coding structure to achieve distributed joint source-channel coding of the detected indices. The layering is achieved by mapping the more important detection bit-planes to the more significant bit-planes of the QAM constellation, thus providing them a higher level of protection. As a result, the proposed strategy adapts to heterogeneous channel conditions by letting the destination extract as many CF cooperation layers as the channel conditions allow. At the destination, we propose an iterative message passing algorithm to achieve joint decoding of the relay's detection layers before recovering the original source information. Simulations results with off-the-shelf convolution codes indicate a performance gain of approximately 3 dB over the traditional unlayered CF strategy.

This paper presents a prototype implementation and experimental performance evaluation of compress- and-forward (CF) relaying using software-defined radios. Under QPSK transmissions from all nodes, the relay demodulates the received signal before applying distributed joint source-channel coding (DJSCC) implemented with the help of an off-the- shelf convolutional code. The destination employs two stages of standard BCJR decoding; a DJSCC decoding stage for the relay's demodulated data followed by another to recover the source's message. Under varying channel conditions, we propose and implement an opportunistic Decode-or- Compress-and-Forward (DoCF) relaying strategy in which the relay forwards the decoded packet if a cyclic redundancy check (CRC) is satisfied, and uses CF relaying otherwise. The prototype implementation of DoCF is carried out using National Instruments USRP 2921 devices. Over-the- air experiments in an indoor office environment indicate that the proposed DoCF strategy can achieve up to ten times smaller bit-error-rate than a standard DF implementation. To the best of our knowledge, this work is the first in the literature to carry out an experimental study of CF relaying.