1. Introduction
The increase in the demands of the expected new services and data traffic, such as cloud-based architectural applications and among such, is the main challenge for the 5G wireless network. Thus, there are challenging requirements to achieve these services, such as high massive connectivity, much higher data rates (100-1000 times faster than current 4G technology), and low latency. As a result, these requirements cannot be achieved by using OMA [1 –3]. Non-orthogonal multiple access (NOMA) is proposed to address the drawbacks of OMA scheme. Serving multiple users at the same time, frequency, code and space domain, but with different power levels, at the expense of minimal inter-user interference (IUI) is the main concept of NOMA scheme [4, 5]. NOMA Scheme intends to share domains among users via superposition and consequently it requires to employ multiple user detection (MUD) strategy to separate interfered users sharing the same degree of freedom, as illustrated in Fig. 1 [6]. As result, interesting multiple access technique for the fifth generation (5G) network, can be obtained by NOMA scheme [7, 8]. Moreover, NOMA techniques are classified into two domains, i.e., the power and code domains. Power domain NOMA is more interesting technique regarding to its implementation in 4G techniques. In addition, Serving and supporting multiple users with some degree of freedom (DoF) can be satisfied by using power domain NOMA, in which it performs successive interference cancellation (SIC) at receivers with better channel condition. In practice, multiplexing technique is achieved by using superposition coding at the transmitter and SIC at the receiver [10], [11]. On the other hand, the code domain NOMA uses user-specific spreading sequence which are either non-orthogonal cross-correlated sequences of low correlation coefficient or sparse sequences. This scheme is further divided into different classes, such as low-density spreading-based OFDM(LDS-OFDM), low-density spreading CDMA (LDS-CDMA), and sparse code multiple access (SCMA) [12, 13]. Generally, there are three NOMA transmission scenarios viz. downlink NOMA transmission scenario, cooperative NOMA transmission scenario and uplink NOMA transmission scenario. In this paper, we take a consideration of the downlink NOMA transmission scenario as well as cooperative NOMA transmission scenario.