# Joint Transmit Power Allocation and Splitting for SWIPT Aided OFDM-IDMA in Wireless Sensor Networks
## 1 Introduction
[14] adopted a similar system to that in [13] but focused on the joint optimization of the transmit power of SN along with the tradeoff between energy harvest and information transmission durations. [15] considered the cooperative multi-user networks based on the SWIPT-NOMA scheme. And the main focus was on the relay-user selection strategies, with the purpose of achieving a lower outage probability or better throughput. Inspired by the previous work mentioned above, in this paper we propose the SWIPT aided OFDM-IDMA scheme. Our purpose is to provide enough power to RNs through wireless energy harvesting, so that the iterative MUD is capable to work with sufficient number of iterations. Such purpose along with the unique properties of OFDM-IDMA, make the proposed system to be different from the previous work in two aspects:  
* From the aspect of EH, in the proposed system the harvested energy is fully devoted to recharging the circuits of MUD. As a consequence, the energy harvesting requirement is related to the iteration number of MUD, which should be treated as variables rather than a fixed constant as that in traditional detectors. * From the aspect of Information Decoding (ID), the performance of the proposed system is determined by the final Signal to Interference plus Noise Ratio (SINR) after iterative decoding. Hence it relies on the iteration number of MUD, the number of RNs and also the initial power assigned to them. Such iterative detection process is difficult to track, which makes the system design to be more complicated. Bearing these unique features in mind, in this paper we focus on the transmit power minimization problem in the proposed system with two constraints, which are (1) providing sufficient energy for MUD to work; and (2) satisfying the desired Bit Error Rate (BER) requirements of the RNs. Against such objective, we first setup the constraint for EH, which is a non-linear function with respect to the transmit power and is also sensitive to the iteration number of MUD. Then we investigate the explicit relationship between the final SINR and BER performance of SN, where the iterative MUD is taken into account with the help of SNR evolution function [16]. The power minimization problem is therefore formulated, which subjects to not only the transmit power assignment and power splitting ratio, but also the iterations number of MUD for each RN. As the optimal solution to the original problem is hardly obtained, we decompose the original problem into two independent sub-problems, where one solely considers the power allocation with the assumption of given PS ratio and number of iterations, and the other one deals with the optimization of PS ratio and iteration number with fixed power profile. We then propose a sub-optimal algorithm by solving these two sub-problems in an iterative manner. And the solution to the original solution can be finally approached through such algorithm. Specifically, the main contributions of this work are summarized as follows:  
* We introduce the concept of Simultaneous Wireless Information and Power Transfer (SWIPT) into OFDM-IDMA, resulting in the proposed SWIPT-aided OFDM-IDMA system. Not only inherits the advantages of OFDM-IDMA, the proposed system also allows the SN to wireless recharge the RNs along with information transmission. The energy limitation of RNs is therefore alleviated and the iterative MUD can be carried out with sufficient iteration numbers. Hence the proposed system is suitable for the power-constraint scenarios of 5G, such as Internet of Things and Wireless Sensor Networks. * With the proposed system, we also investigate the impacts of iteration number of MUD, BER requirements, as well as PS ratio on the transmit power of SN. Aiming at minimizing the transmit power of SN, we propose a sub-optimal algorithm to jointly coordinate the power allocation and power splitting in an iterative approach. The energy efficiency of the whole system is significantly improved through our algorithm. The rest of this paper is organized as follows.