# Joint Transmit Power Allocation and Splitting for SWIPT Aided OFDM-IDMA in Wireless Sensor Networks
## 1 Introduction  
Allowing multiple users to communicate with each other in the same network, Multiple Access (MA) has been considered as one of the most important techniques in the area of wireless communications. It is widely recognized that the existing MA schemes can be classified into two categories, namely, Orthogonal Multiple Access (OMA), and Non-orthogonal Multiple Access (NOMA) [1]. In the former case, different users access to the network utilizing different resource blocks (i.e., frequency, time, or orthogonal codes). The Multiple Access Interference (MAI) is therefore absent, leading to the lower detection complexity. While for the latter, multiple users are served with the same resource block, where MAI exists but can be mitigated through Successive Interference Cancellation (SIC). According to information theory, NOMA is superior than OMA in terms of Spectral Efficiency (SE). As a consequence, despite of the higher complexity paid for interference cancellation, NOMA is still expected to be the most potential candidate for the future communication networks. Among numerous NOMA schemes, Interleave Division Multiple Access (IDMA) was proposed in [2] and has drawn increasing research interests. Different from the traditional NOMA operated inpower domain, in IDMA signals from different users are labelled by different interleaving patterns. The iterative Multi-User Detection (MUD) is utilized to recover the signals of each user at the receiver. It is reported that the signal-user performance can be achieved by IDMA with a substantial number of users, given MUD the sufficient number of iterations together with proper transmit power allocation [2]. In addition, IDMA can be easily integrated with other existing MA paradigms, such as Orthogonal Frequency Division Multiplexing (OFDM) [3]. The resulted OFDM-IDMA system inherits the advantages of both OFDM and IDMA, and hence is considered as one promising MA approach for the fifth generation (5G) wireless communications. Despite of the excellent performance, the implementation of OFDM-IDMA is still a difficult task due to the employment of iterative MUD. In fact, the attractive features of OFDM-IDMA, such as the high spectral efficiency and good error-rate performance, rely on the iterative MUD employed at the receiver side. However, such iterative detection is more computation costly than traditional non-iterative one, leading to higher power consumption. In some power-constrained scenarios of 5G such as Internet of Things (IoT) and Wireless Sensor Networks (WSN), the receive nodes (RNs) are usually supplied by batteries with limited power [4; 5]. Consequently it is difficult to implement OFDM-IDMA into such case since the RNs are short of enough energy to execute MUD with sufficient iteration numbers. On the other hand, Simultaneous Wireless Information and Power Transfer (SWIPT) is a novelly proposed concept who is expected to alleviate such energy-constrained problem. Based on the fact that the Radio Frequency (RF) signals carry both information and energy, in SWIPT RNs are allowed to harvest the energy from the received signals while decoding them [6]. By properly coordinating the wireless information and power propagation through Power Splitting (PS), Time Switching (TS) or Antenna Switching (AS), the wireless recharging can be realized without interrupting the data transmission [7; 8]. The early literatures about SWIPT in multi-user scenario mainly focused on the OMA schemes, i.e., OFDM in [9; 10; 11] and Time Division Multiple Access (TDMA) in [12]. While the research on the implementation of SWIPT in the power domain NOMA system is conducted in [13; 14; 15]. In [13], the authors proposed a wireless-powered NOMA system, where the RNs are wireless recharged by Source Receive (SN) during the downlink transmission and then use the harvested energy to transmit their own information to SN during uplink phase. A joint design on the energy harvest time-slot and decoding orders was also proposed to balance the maximum achievable data rate and fairness among the RNs.