# Analysis of Power Allocation for NOMA-Based D2D Communications Using GADIA
## 4 Principle of NOMA System  
In this section, we briefly explain the fundamental principles of orthogonal multiple access (OMA) and NOMA. Traditional OMA schemes are: frequency-division multiple access (FDMA), time-division multiple access (TDMA), code-division multiple access (CDMA) and orthogonal frequency-division multiple access (OFDMA) used for 1G, 2G, 3G, and 4G, respectively. Multiple users are allocated to orthogonal radio resources in frequency, time, code-domain, or combinations. More particularly, every user sends a unique user-specific signal through its unique frequency resource in FDMA. Therefore the receiver can quickly identify all users' data in their identical frequency bands, respectively. Furthermore, in TDMA, a particular time slot is allocated to each user; accordingly, it is straightforward to notice the distinct users' signals at the receivers in the time domain. In CDMA, various users can share the same time-frequency resources, while the transferred symbols of distinctive users may be correlated to orthogonal spreading sequences, e.g., Walsh-Hadamard codes. Hence, a low-complexity decorrelation receiver enables the use of multi-user detection (MUD). OFDMA can be considered an intelligent combination of FDMA and TDMA where the radio resources are orthogonality divided in the time-frequency grid [47]. Theoretically, as an advantage of orthogonal resource allocation, there is no interference between OMA systems users. Consequently, low-complexity indicators with linear complexity can assign different users' signals. The maximum number of sustainable users is tightly limited by the available number of orthogonal resources in conventional OMA schemes which results in reducing the extensive connectivity in 5G. Moreover, it has been proven that OMA cannot regularly obtain the maximum achievable sum-rate of multi-user wireless systems. Unlike OMA techniques, NOMA can accomplish \begin{table}
\begin{tabular}{p{85.4 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**NOMA Use Case** & **Opportunities** & **Challenges and Research Directions** \\ \hline \multirow{6}{*}{**eMBB**} & Downlink NOMA, D2D NOMA, edge caching, Better spectral efficiency and user’s fairness, Increased number of several users satisfying their QoS & Design a comprehensive RRA, IM, caching placement and strategy that includes down-link NOMA and D2D NOMA \\ \hline \multirow{6}{*}{**mMTC**} & Grant-free NOMA in the uplink transmissions, Reduced energy consumed in signaling, Higher transmission success probabilities compared to OMA & Optimize the number of multiplexed users through NOMA \\ \hline \multirow{6}{*}{**URLLC**} & Downlink NOMA by increasing the networking availability, Up link NOMA by reduce the collision, Data storage and computation at the network edge & Analyze the joint provision of high reliability and low latency \\ \cline{1-1} \cline{2-4}  & & Analyze the joint provision of high reliability and low latency \\ \cline{1-1} \cline{2-4}  & & \\ \cline{1-1} \cline{2-4}  & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Integration of NOMA in different radio access network technologies. the multi-user capacity with the aid of time-sharing or rate-splitting if required [48]. To address the above constraint of OMA, NOMA has been recently introduced as a potential solution. The key characteristic of NOMA scheme is to maintain reliable communication for a massive number of users higher than the number of orthogonal resource slots supported by non-orthogonal resource allocation [49]. The complex inter-user interference cancellation can perform at the cost of an enhanced receiver complexity, e.g., computational complexity with polynomial or exponential order. An effective way to eliminate the interference is to exploit powerful multiuser detectors. NOMA schemes can be introduced in two domains: power-domain NOMA and code-domain NOMA. NOMA in the power-domain, for distinct users, indicates different power levels based on their channel quality, as long as the same time-frequency-code resources are shared between multiple users.