Joint Power Control in Wiretap Interference Channels

Interference in wireless networks degrades the signal quality at the terminals. However, it can potentially enhance the secrecy rate. This paper investigates the secrecy rate in a two-user interference network where one of the users, namely user 1, requires to establish a confidential connection. User 1 wants to prevent an unintended user of the network to decode its transmission. User 1 has to transmit such that its secrecy rate is maximized while the quality of service at the destination of the other user, user 2, is satisfied, and both user's power limits are taken into account. We consider two scenarios: 1) user 2 changes its power in favor of user 1, an altruistic scenario, 2) user 2 is selfish and only aims to maintain the minimum quality of service at its destination, an egoistic scenario. It is shown that there is a threshold for user 2's transmission power that only below or above which, depending on the channel qualities, user 1 can achieve a positive secrecy rate. Closed-form solutions are obtained in order to perform joint optimal power control. Further, a new metric called secrecy energy efficiency is introduced. We show that in general, the secrecy energy efficiency of user 1 in an interference channel scenario is higher than that of an interference-free channel

Beamforming optimization for two-way relay channel

In this thesis, we focus on the optimization of the two-way relay channel (TWRC), which can double the data rate of communications comparing to the traditional one-way relay channel (OWRC). Because of the broadcasting nature of wireless transmissions, secure transmission is an appealing research topic. We take secrecy rate consideration into the optimization of the TWRC. Overall we provide near-optimal solutions for the secrecy rate maximization problems of the TWRC with imperfect channel state information (ICSI). A much lower complexity optimal SOCP solution is provided for SNR balancing of the TWRC without secrecy consideration. We first look at a flat fading TWRC network model with a multiple-input multiple-output (MIMO) relay where perfect channel state information (CSI) is assumed available. We then formulate an optimization problem, with the objective to minimize the relay’s power usage under the constraints that the signal-to-noise ratio (SNR) of the two transceivers should exceed a preset threshold. A low-complexity optimal beamforming solution is provided to this optimization problem by reformulating it in the form of second-order cone programming (SOCP). Later in the thesis, we consider the presence of an eavesdropper and address the beamforming optimization for minimizing the relay’s power with the constraints of the secrecy rates of the two transceivers. A semi-definite programming (SDP) based searching algorithm is proposed to find a near-optimal solution. For each search of the proposed approach, the previous non-convex optimization problem is transferred into an SDP problem, which can guarantee the optimality of the beamforming matrix. Afterwards, more realistic imperfect CSI (ICSI) situations are considered for the TWRC network models. As ICSI completely changes the structure and the property of the optimization problems, we reformulate the optimization problems into two scenarios. For the first case, we consider that the relay is an untrusted eavesdropper and in this case an SDP solution is provided to maximize the joint-decoding sum-secrecy rate. For the second case, we investigate the robust beamforming problems where the relay is trusted but there is an external eavesdropper, another SDP solution is provided to maximize the sum-secrecy rate