At an energy harvesting relay, securing residual harvested energy, net remaining energy after each receiving and forwarding cycle, is of importance for sustainable operation. However, there exists a tradeoff between the achievable rate and residual harvested energy, whereby understanding this tradeoff concretely is crucial for energy harvesting relay system design. This paper analyzes the rate-energy (R-E) region for achievable rate and residual harvested energy in two-way decode-and-forward (DF) relay systems with a power splitting based energy harvesting relay. In particular, we characterize R-E regions for multiple access broadcast (MABC) and time division broadcast protocols. Moreover, we propose a new energy harvesting relaying protocol, namely, information and energy signals multiple access broadcasts (IEBC), to improve the achievable R-E region. The boundary of the R-E regions is obtained by optimizing a power splitting factor in each protocol. Moreover, to have better analytic comparisons and useful insights on performance, we derive approximated R-E regions of all protocols for high and low signal-to-noise ratio cases. Based on the approximated R-E region, it is shown that if the required residual energy is large, the IEBC outperforms the others, but if the required residual energy is small, either the IEBC or MABC is preferred.

We propose the source and relay precoder designs to maximize sum rate under individual power constraints in two-way relay system with one relay and multiple sources. However, it is difficult to directly solve the sum rate maximization problem. For this reason, we establish the relationship between sum rate and weighted mean square error(WMSE), and show that the solution of the sum rate maximization problem can be also the solution of the WMSE minimization problem if the weight matrices are properly chosen. Then, based on the relationship between sum rate and WMSE, the sum rate maximization problem is equivalently formulated as the WMSE minimization problem. We finally propose an iterative algorithm for WMSE minimization problem. Simulation results show that the proposed scheme has better sum rate performance than conventional scheme at all SNR regimes.

This paper considers the problem of detecting the pilot contamination attack for multi-antenna secrecy systems. The detection problem is formulated as a binary hypothesis problem and the likelihood ratio or the generalized likelihood ratio is employed as a decision statistic for the detection of the pilot contamination. Using the Neyman-Pearson criterion, we then develop several detection methods under the different assumptions of the channel and noise statistics. To be specific, the cases of interest are as follows: (i) exact knowledge of all covariance matrices, (ii) exact knowledge of the legitimate channel and noise covariance matrices, but no knowledge of the jamming channel covariance matrix and (iii) no knowledge of all covariance matrices. The performance comparison of the proposed schemes with the conventional scheme is performed by simulations.