## 1 Introduction  
Fifth generation (5G) wireless networks are expected to support higher spectral efficiency, lower end-to-end latency, and more connection nodes [1]. In accordance with this trend, many promising techniques are being considered [2]. Among them, a non-orthogonal multiple access (NOMA) technique has been proposed to improve the spectral efficiency of the 5G network [3]. The basic idea of the NOMA technique is to serve multiple users at the same time-frequency-space resource block via power domain or code domain multiplexing [4]. It has been known that the NOMA technique increases the spectrum efficiency and efficiently accommodates a massive number of nodes in cellular networks [5; 6]. As noted before, the NOMA techniques are, in general, classified into two categories: code-domain NOMA and power-domain NOMA [7]. With the code-domain NOMA techniques, a codeword is allocated to each user and a near optimal multi-user detection algorithm, such as a message passing algorithm, is used at the receiver. The code-domain NOMA techniques include trellis-coded multiple access (TCMA), interleave-division multiple access (IDMA), low-density signature (LDS) sequence-based code division multiple access (CDMA), sparse-code multiple access (SCMA), pattern-division multiple access (PDMA), multi-user shared access (MUSA), etc. [8]. On the other hand, with the power domain NOMA techniques, multiple users are served within a given time-frequency-space resource block by using superposition coding (SC) at the transmitter and successive interference cancellation (SIC) at the receivers, respectively, which has recently been proposed in 3 GPP LTE [3]. Multi-user superposition transmission (MUST) is a special case of the NOMA techniques, which has been studied in 3 GPP LTE standards [9]. Various NOMA techniques have been proposed and studied, focusing on multi-user non-orthogonal transmission schemes, receiver designs, and related signaling strategies in [9]. The MUST techniques are divided into three categories according to adaptive power control and bit-labeling at the transmitter side as shown in Table 1. The adaptive power ratio on each component constellation is utilized in MUST categories 1 and 2, and Gray-mapped composite constellation is adopted in MUST categories 2 and 3. The MUST techniques typically assume asymmetric downlink scenarios consisting of active cell-edge and cell-center users. At the receiver side, the cell-center user with higher received signal power decodes a super-imposed signal with SIC, while the cell-edge user with lower received signal power decodes the super-imposed signal by treating the interference signal as noise. To implement SIC at the cell-center user, codeword-level (channel coding block) SIC results in better performance than symbol-level SIC, but the signaling overhead and implementation complexity of the codeword-level SIC is much higher that of the symbol-level SIC [10; 11]. Thus, the codeword-level SIC may not be suitable in the cellular downlink, especially when the low-cost and low-power user terminals/devices are considered. Recently, several receiver designs of the NOMA techniques, including codeword-level SIC and symbol-level SIC, have been compared with each other. In particular, a log-likelihood ratio (LLR)-based low-complexity receiver design was proposed [12]. Although the LLR-based receiver design does not include the codeword-level SIC operation which accompanies high implementation complexity, it can still achieve similar performance to the codeword-level SIC receiver as well as the ideal SIC receiver. However, the performance of the proposed technique was evaluated only through link-level simulations. In addition, another low-complexity MUST technique was proposed in [13], where a NOMA transmitter sends signals to a single cell-center user over multiple (frequency) resource blocks, each of which is also occupied by a cell-edge user. Thus, the cell-center user can obtain the frequency diversity, and we call this scheme _diversity-controlled MUST_ technique in this paper.