# GPU Accelerated PIC and SIC for OFDM-NOMA
## 1 Introduction  
Non-orthogonal multiple access (NOMA) in the power domain has been proposed as a promising multiple access technique for the upcoming fifth-generation cellular networks. In theory, NOMA fulfills the spectral efficiency requirements of 5G by serving multiple users simultaneously in the same frequency band [1]. For example, in the reverse link of a NOMA system, each user transmits at the same time and in the frequency band so that the base station receives the superimposed version of the transmitted signals by each user. This is in contrast to the multiple access methods used in conventional cellular systems where one user is allocated per time slot or frequency unit [2]. Early works on NOMA demonstrated that NOMA can significantly improve the sum capacity and cell-edge user throughput [3, 4].  
In NOMA systems, since each user is seen as an interference to others, advanced interference cancellation is required for successful decoding [5]. In the literature, successive interference cancellation (SIC) and parallel interference cancellation (PIC) techniques are considered for NOMA. However, both techniques require massive computation power. For example, in [6], the authors reported that their NOMA testbed can support only nine users due to the limitations in the computation power of their personal computers.  
5G networks are expected to boost the data rates while significantly reducing the latency, which requires powerful and efficient baseband processing at the receiver. In this work, the two most popular interference cancellation schemes, PIC and SIC, are implemented and compared in a graphics processing unit (GPU) using CUDA to speed up the most time-consuming process of a NOMA receiver. Numerical results show that PIC, due to its parallel architecture, is more suitable for GPU implementation and outperforms the SIC with a speedup of over 70-times. Moreover, an interference cancellation processing times as low as 7 ms, even for large number of users, is observed with PIC in GPU. The results also reveal the feasibility of interference cancellation techniques for NOMA and their competitiveness in commercial solutions particularly when accelerated by GPU using CUDA.  
In addition to their computation performances, the reliability of the transmission with SIC and PIC receivers may differ [7]. In SIC receivers, multi-user interference is removed in a successive manner so that the reliability of the system heavily depends on the correct decoding of the first user. Moreover, in SIC receivers, the received powers from each user should be different so that the receiver can distinguish the users in the power domain and perform successful decoding. In PIC receivers, on the other hand, all the users are decoded at once in parallel, and its performance does not depend on the difference between the received powers as in SIC receivers. However, in order to support a large number of users, PIC-based NOMA systems may require signature signals for each user as in code division multiple access (CDMA) and are more suitable for code domain NOMA [8; 9]. In this work, we further present the relevant signal models for both SIC and PIC receivers and discuss their bit-error-rate (BER) performances.  
This work is organized as follows. In the next section, we present the system models where the SIC and PIC receivers for OFDM are described. In the Section 3, the CUDA implementation of the receivers is described. Numerical results are presented and discussed in Section 4. We summarize the study in Section 5.