# Analysis of Power Allocation for NOMA-Based D2D Communications Using GADIA
## 7 Conclusions and Future work  
Several innovative techniques have been introduced for the next mobile generation. The next part is to control the URLLC, which pursues super reliable and low latency requirements. Precisely, D2D fits within URLLC, especially in mission-critical communications. This article discussed the critical concept and advantages of NOMA techniques, which constitute promising future 5G systems. The principal NOMA schemes have been presented to compare their operating principles, key features, and receiver complexity. We also highlighted a range of critical challenges, opportunities and future research trends related to NOMA's design with eMBB, mMTC and URLLC. It is expected that NOMA will play an essential role in future 5G wireless communication systems sustaining massive connectivity and low latency. In this paper, we investigated opportunistic spectrum sharing for a D2D-based URLLC system. A novel performance examined a NOMA-based D2D communication system coexisting with a cellular network, where the power resource requires to be carefully allocated based on the shortage in practice. We formulated a max-min fairness optimization problem with energy budget constraints to provide a reasonable boundary rate for the downlink to all devices and cellular users in the network while still limiting the whole system's total transmit power. Moreover, we consider two scenarios where a D2D system coexists with a mobile cellular system within the same cell. Two frequency bands are available, D2D users only use the first one, and D2D and cellular network users share the second one. D2D users use GADIA to allocate them to one of the two frequencies. GADIA is a fully distributed algorithm for finding a sub-optimal frequency band allocation to the network users. Each D2D group includes several paired devices so that the NOMA technique can be used to mitigate co-subchannel interference. Then, simulations in theoretical and practical scenarios are examined, comparing the maximum achievable rate and the EE at a given SE when using NOMA and OFDMA. The simulations were shown in MATLAB, and the results confirmed that NOMA obtains better performance than OFDMA in the categories compared. Furthermore, we study the mean interference received by D2D users by applying three different approaches, i.e., GADIA controlled frequency allocation, random frequency allocation, and without using any frequency sharing. However, there are still diverse, challenging problems to be solved. Further challenges should also be determined in the context of NOMA systems, including the allied reference signal design and channel estimation, the loss of the PAPR in multi-carrier NOMA systems, such as LDS-OFDM, sustaining system scalability, the effects of channel-quality feedback design, BS cooperation, etc. Moreover, the existing multiple access design routinely allows  
Figure 17: Mean D2D user interference level for different environments after 200 iterations. a single scheme for all applications, regardless of their distinct requirements. Therefore, different system design factors have to be estimated to accommodate the worst-case situation, which leads to inefficient multiple access design in various applications. Therefore, the software-defined multiple access technology is assumed to maintain the flexible configuration of multiple access schemes, and thus multiple services, as well as applications, can be advanced in 5G. It is anticipated that NOMA solutions will achieve further performance enhancements by addressing these challenges. [PERSON] as the principal investigator takes the primary responsibility for this research and analyzed the results; [PERSON] and [PERSON] conceived of the study, and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.