Non-orthogonal multiple access (NOMA) is a promising multiple access technique that significantly improves the spectral efficiency of mobile communication networks. Unlike conventional orthogonal multiple access (OMA), which uses time/frequency/code domains, NOMA uses the power domain to serve multiple users simultaneously at the same time/frequency/code with different power levels. This approach allows users with poor channel conditions to access all subcarrier channels, improving spectral efficiency and balancing system throughput with user fairness. NOMA is envisioned as a key component of 5G networks and has been studied in the context of 3GPP LTE and 5G. NOMA can be combined with multiple-input multiple-output (MIMO) technologies to achieve different trade-offs between reception reliability and data rates. It also has potential in cooperative NOMA, where users with stronger channel conditions act as relays to assist users with weaker channel conditions. The interplay between NOMA and cognitive radio (CR) is also discussed, highlighting how NOMA can be viewed as a special case of CR networks, where users with stronger channel conditions can help those with weaker conditions. Recent developments in NOMA include its application in downlink and uplink scenarios, as well as its integration with beamforming and spatial multiplexing techniques. The performance of NOMA is compared with OMA, showing that NOMA can achieve higher data rates, especially in scenarios with users having different channel conditions. Research challenges include user pairing/clustering, hybrid multiple access, MIMO-NOMA, imperfect channel state information (CSI), and cross-layer optimization. The standardization of NOMA in LTE and 5G networks has been actively pursued, with various non-orthogonal transmission schemes proposed and studied. These include superposition transmission with adaptive power ratios, label-bit assignment, and other techniques. Other promising non-orthogonal multiple access schemes, such as sparse code multiple access (SCMA), pattern division multiple access (PDMA), and multiuser shared multiple access (MUSA), are also being explored for 5G. In conclusion, NOMA offers significant improvements in spectral efficiency and user fairness, and is a key technology for future mobile networks. However, challenges remain in its implementation, including system complexity, power allocation, and the integration with other technologies. Ongoing research aims to address these challenges and further enhance the performance of NOMA systems.Non-orthogonal multiple access (NOMA) is a promising multiple access technique that significantly improves the spectral efficiency of mobile communication networks. Unlike conventional orthogonal multiple access (OMA), which uses time/frequency/code domains, NOMA uses the power domain to serve multiple users simultaneously at the same time/frequency/code with different power levels. This approach allows users with poor channel conditions to access all subcarrier channels, improving spectral efficiency and balancing system throughput with user fairness. NOMA is envisioned as a key component of 5G networks and has been studied in the context of 3GPP LTE and 5G. NOMA can be combined with multiple-input multiple-output (MIMO) technologies to achieve different trade-offs between reception reliability and data rates. It also has potential in cooperative NOMA, where users with stronger channel conditions act as relays to assist users with weaker channel conditions. The interplay between NOMA and cognitive radio (CR) is also discussed, highlighting how NOMA can be viewed as a special case of CR networks, where users with stronger channel conditions can help those with weaker conditions. Recent developments in NOMA include its application in downlink and uplink scenarios, as well as its integration with beamforming and spatial multiplexing techniques. The performance of NOMA is compared with OMA, showing that NOMA can achieve higher data rates, especially in scenarios with users having different channel conditions. Research challenges include user pairing/clustering, hybrid multiple access, MIMO-NOMA, imperfect channel state information (CSI), and cross-layer optimization. The standardization of NOMA in LTE and 5G networks has been actively pursued, with various non-orthogonal transmission schemes proposed and studied. These include superposition transmission with adaptive power ratios, label-bit assignment, and other techniques. Other promising non-orthogonal multiple access schemes, such as sparse code multiple access (SCMA), pattern division multiple access (PDMA), and multiuser shared multiple access (MUSA), are also being explored for 5G. In conclusion, NOMA offers significant improvements in spectral efficiency and user fairness, and is a key technology for future mobile networks. However, challenges remain in its implementation, including system complexity, power allocation, and the integration with other technologies. Ongoing research aims to address these challenges and further enhance the performance of NOMA systems.