 at the receiver side, successive interference cancella- tion (SIC) is used to retrieve each user's signal sequentially from the SS. Specifically, a user can decode the strongest signal by treating other signals as interference. If the decoded sig- nal is its own data, SIC stops. Otherwise, the receiver subtracts the decoded signal from the SS and continues to decode the next strongest signal. Notice that SS with SIC is not new; in information theory, this duo is a capacity-achieving technique in the uplink communi- cation. However, the difference is in NOMA, the weak user has a stronger power, which 1 Introduction to 5G and Beyond Network Transmitted symbol Received symbol NOMA transmission NOMA decoding Figure 1.2 NOMA principles: transmission and decoding stage. is not information-theoretic optimal. Since its design philosophy may be combined with diverse transceivers, it has drawn tremendous attention in multiple-antenna systems and in downlink and uplink multi-cell networks. In contrast to classic orthogonal multiple access (OMA), NOMA provides simultaneous access to multiple users at the same time and on the same frequency band, for example by using power-domain multiplexing. It has been shown that NOMA is capable of achieving a higher SE and energy efficiency (EE) than OMA, such as OFDMA, time division multiple access (TDMA), and frequency domain multiple access (FDMA). Figure 1.2 shows the basic principle of NOMA with data encoding and decod- ing. S1 and S2 are the symbols for users 1 and 2, respectively. We also assume user 1 has a better channel than user 2. At the transmitter side, binary phase shift keying (BPSK) and quadratic phase shift keying (QPSK) modulation are applied, respectively, for the two users. Clearly, the average symbol power of S2 is larger to compensate for the unfavorable channel. Actual transmitted symbol is simply the addition of these two. At the receiver side, symbols with the highest power will be decoded first, in this example, S2. Besides, since the received symbol is on the 