## 1 Introduction  
To satisfy the multiplied demands in system capacity and throughput performance, non-orthogonal multiple access (NOMA) has been widely considered a novel and promising candidate cellular multiple access scheme for the fifth generation (5G) mobile communication systems [1; 2; 3; 4; 5; 6]. Many theoretical analyses and experimentation have proved that NOMA can achieve a higher sum rate than orthogonal multiple access (OMA) adopted in the fourth generation (4G) wireless networks [7; 8; 9; 10; 11; 12]. In NOMA, multiple users with different channel gains can be multiplexed together in the same subchannel and decoded at the receivers through successive interference cancellation (SIC) techniques [13; 14; 15; 16; 17; 18]. In this mechanism, the power domain is exploited to simultaneously serve multiple users at different power levels, whereby spectrum efficiency can be significantly improved. There is a lot of research literature in the previous work focusing on further enhancement to achieve a significant spectrum efficiency gain. For example, by fully exploiting the feature that users with better channel conditions have prior knowledge about the message of other users, [PERSON] et al. proposed a cooperative NOMA scheme which significantly improves the system performance [19]. In [20], [PERSON] et al. studied the ergodic capacity maximization problem for the Rayleigh fading multiple-input multiple-output (MIMO) NOMA systems with statistical channel state information at the transmitter. They proposed optimal and low complexity suboptimal power allocation schemes to maximize MIMO NOMA systems' ergodic capacity with total transmission power constraint and minimum rate constraint of weak users. In addition, the design of minimum Euclidean distance (MED) precoders for MIMO-NOMA downlink is developed in [21], where MED precoders are extended to general multi-user scenarios. In addition, [PERSON] et al. proposed an efficient user pairing, channel assignment, and power optimization technique for the secondary users while primary users' performance is guaranteed through interference temperature limits [22]. Their solution maximizes the achievable rate of a multi-user multi-channel NOMA based cognitive radio system. Despite its great benefit for spectrum efficiency, there are still some tricky problems with the NOMA techniques. A notable issue targets energy consumption [23; 24; 25; 26; 27]. To support simultaneous transmission in NOMA systems, much more transmission and circuit power consumption are inevitable. For many potential applications of NOMA (e.g., the well-known Internet-of-Things networks [28; 29; 30; 31]), the lifetime of networks is critical in practice due to the high price of battery replacement. As a result, the energy-efficient transmission is one of the most important research directions in these areas [32; 33; 34]. Without proper energy management, NOMA's performance gain may have to be discarded due to energy scarcity. On the other hand, it is also essential for NOMA systems to efficiently utilize low energy from economic and environmental concerns. Nowadays, information and communication technologies are responsible for about 5% of the total world energy consumption. This percentage is rapidly growing due to the exponential increase of connected devices and infrastructure nodes [35; 36; 37]. The massive energy consumption aggravates the fiscal spending for mobile operators and increases the carbon dioxide (CO\({}_{2}\)) emission, which will exacerbate the greenhouse effect and lead to serious environmental problems. All told, energy efficiency is a crucial issue of NOMA systems that should be valued as spectrum efficiency. Recently, the green communication in wireless networks, especially in the upcoming 5G networks, has become an urgent need [38; 39; 40; 41; 42]. Unfortunately, despite its complete spectrum efficiency studies, the energy-efficient architectures in NOMA systems have not yet been well developed.