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We evaluate the upper bound, lower bound and an approximate expression for secrecy outage probability of a dual hop Amplify-and-forward (AF) relay system when the single cooperating relay among the relays is selected without the knowledge of eavesdropper's instantaneous channel state information (CSI). This situation arises when eavesdropper is not a legitimate node, or being a trusted node, it acts maliciously and does not share its instantaneous CSI. We also obtain the tighter bounds in high signal-to-noise ratio (SNR) of the source to relay links and in high SNR of the relay to destination links. We show that when the SNR of the source to relay links tends to zero, it becomes the bottle neck for secure communication, and the secrecy outage tends to 1. We show that the approximate expression follows the simulated outage closely and the outage decreases with an increase in the number of relays and increases with an increase in the required secrecy rate.

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In this paper, we consider the problem of resource allocation in an OFDMA system with single source and M untrusted users in presence of a friendly jammer. The jammer is used to improve either the weighted sum secure rate or the overall system fairness. The formulated optimization problem in both the cases is a Mixed Integer Non-linear Programming (MINLP) problem, belonging to the class of NP-hard. In the sum secure rate maximization scenario, we decouple the problem and first obtain the subcarrier allocation at source and the decision for jammer power utilization on a per-subcarrier basis. Then we do joint source and jammer power allocation using primal decomposition and alternating optimization framework. Next we consider fair resource allocation by introducing a novel concept of subcarrier snatching with the help of jammer. We propose two schemes for jammer power utilization, called proactively fair allocation (PFA) and on-demand allocation (ODA). PFA considers equitable distribution of jammer power among the subcarriers, while ODA distributes jammer power based on the user demand. In both cases of jammer usage, we also present suboptimal solutions that solve the power allocation at a highly reduced complexity. Asymptotically optimal solutions are derived to benchmark optimality of the proposed schemes. We compare the performance of our proposed schemes with equal power allocation at source and jammer. Our simulation results demonstrate that the jammer can indeed help in improving either the sum secure rate or the overall system fairness.

We evaluate the bounds on the secrecy outage probability of a dual hop Amplify-and-forward (AF) system with an eavesdropper tapping the second hop, when all the links undergo Rayleigh fading. An approximate expression of outage is also obtained when the signal-to-noise ratio of the first hop is high. Based on the developed outage expressions we evaluate the bounds and the approximate outage for optimal relay selection when full instantaneous channel state information (FCSI) i.e. of all the links is available. We also propose a novel relay selection scheme when only statistical channel state information (SCSI) of all the links is available. The proposed scheme can be used in networks where feedback of FCSI to the decision making node is power intensive and CSI of the eavesdropper cannot be obtained at all instants. In the results section we show that the approximate expression follows the simulations exactly and the outage for relay selection in FCSI decreases with an increase in the number of relays.