Optimal System Performance in Multihop Energy Harvesting WSNs Using Cooperative NOMA and Friendly Jammers

In this paper, we investigate the system performance of multihop energy harvesting (EH) wireless sensor networks (WSNs) with imperfect channel state information (CSI) using cooperative non-orthogonal multiple access (NOMA) and friendly jammers in the presence of multiple passive eavesdroppers (EAVs). Specifically, we propose a two-phase communication protocol consisting of EH and information transmission (IT). In the first phase, relays in all clusters harvest energy from power transfer station (PTS) signals. In the first time slot of the second phase, the gateway simultaneously broadcasts information and interference signals. In the subsequent time slots, a relay acting as a friendly jammer in each cluster uses the harvested energy to send an interference signal. Simultaneously, another EH relay applies the NOMA technique to transmit the information signal according to an optimal scheduling scheme based on the maximum signal-to-interference-plus-noise ratio (SINR) of a far user (MSm) and a near user (MSn). To ensure security performance, we propose an algorithm for determining the EH time constraint for a friendly jammer. Additionally, closed-form expressions for the outage probability and throughput of the considered system are derived. Accordingly, an optimal power allocation coefficient algorithm is proposed to achieve throughput fairness for pairs of users. The results of the mathematical analysis are verified by Monte Carlo simulations. Finally, the numerical results demonstrate that the MSn scheme is recommended for guaranteeing throughput fairness for pairs of users.