Abstract
In this paper, we consider the sum secrecy rate maximization problem in multiuser multiple-input single-output (MU-MISO) systems in the presence of multiple energy harvesters (EHs) which also have potential to wire-tap the information users (IUs). To facilitate delivering secure information to the IUs and increase the total harvested energy by the EHs simultaneously, we optimize the transmit beamforming vectors to direct the information signals toward the IUs and artificial noise (AN) toward the EHs. We assume that each EH relies on itself to decode the information signal intended for an individual IU. Therefore, the corresponding problem is to maximize the worst-case sum secrecy rate under transmit power and energy harvesting constraints. The problem is optimally solved by transforming it into a convex iterative program using a change of variables, semi-definite relaxation (SDR), and linearization of quadratic terms. We prove that rank-one optimal solutions for the IUs beamforming covariance matrices can be obtained from the optimal relaxed unconstrained solution. Also, we provide three suboptimal solutions based on null space projection and power control of the beamforming vectors for the low and high harvested energy constrained regions. A special case of cooperative EHs in which the EHs can collaboratively cancel the signal of all IUs except the one they intend to eavesdrop is also investigated, and the optimal solution is derived in a comparable way as in noncooperative EHs case. Our simulation results reveal an understanding of how the tradeoff between the AN and information signal can jointly improve both the sum secrecy rate and the total harvested energy. We also show that, within the low total harvested energy region, the suboptimal solution in which the AN is projected in the null space of the IUs channels outperforms the suboptimal solution which ignores AN alignment at the IUs, and vice versa over the high total harvested energy region; and the suboptimal solution that combines both of them achieves close to optimal performance.
Highlights
The technologies for extending the lifetime of energy constrained wireless networks such as wireless sensor networks have acquired much interest
We considered secure downlink transmission in simultaneous wireless information and power transfer (SWIPT) multiuser multi-input single-output (MU-MISO) systems comprising multiple information users (IUs) and multiple energy harvesting (EHs) which have the potential to wire-tap the IU’s signal
We proposed joint optimization of the information and artificial noise (AN) beamforming vectors for worst-case sum secrecy rate maximization with a constraint on the total harvested energy by the EHs
Summary
The technologies for extending the lifetime of energy constrained wireless networks such as wireless sensor networks have acquired much interest. The work in [9] assumed that each IU employs a power splitting approach to decode information and harvest energy simultaneously and each user is cooperatively wire-taped by the remaining IUs. The considered problem was to minimize the total transmit power under constraints on individual harvested energy and secrecy rate. We employ transmit beamforming for the information signal and the AN to maximize the worst-case sum secrecy rate of the IUs and the sum secrecy rate for the case of cooperative EHs with a lower limit on the total harvested energy by the EHs (the problem with individual harvested energy constraints has been considered).
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