# Study and Investigation on 5G Technology: A Systematic Review
## 4 5G Technologies
### 5G Non-Orthogonal Multiple Access (NOMA)  
NOMA is a very important radio access technology used in next generation wireless communication. Compared to previous orthogonal multiple access techniques, NOMA offers lots of benefits like high spectrum efficiency, low latency with high reliability and high speed massive connectivity. NOMA mainly works on a baseline to serve multiple users with the same resources in terms of time, space and frequency. NOMA is mainly divided \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Approach** & **Throughput** & **Latency** & **Energy Efficiency** & **Spectral Efficiency** \\ \hline [PERSON] et al. [47] & Good & Low & Good & Average \\ [PERSON] et al. [48] & Average & Low & Average & - \\ [PERSON] et al. [8] & Good & - & Good & Average \\ [PERSON] et al. [19] & Good & Low & Average & Average \\ [PERSON] et al. [13] & Good & Average & Good & Good \\ [PERSON] et al. [51] & Average & - & Good & Average \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of massive MIMO-based approaches in 5G technology. into two main categories one is code domain NOMA and another is power domain NOMA. Code-domain NOMA can improve the spectral efficiency of mMIMO, which improves the connectivity in 5G wireless communication. Code-domain NOMA was divided into some more multiple access techniques like sparse code multiple access, lattice-partition multiple access, multi-user shared access and pattern-division multiple access [52]. Power-domain NOMA is widely used in 5G wireless networks as it performs well with various wireless communication techniques such as MIMO, beamforming, space-time coding, network coding, full-duplex and cooperative communication etc. [53]. The conventional orthogonal frequency-division multiple access (OFDMA) used by 3 GPP in 4G LTE network provides very low spectral efficiency when bandwidth resources are allocated to users with low channel state information (CSI). NOMA resolved this issue as it enables users to access all the subcarrier channels so bandwidth resources allocated to the users with low CSI can still be accessed by the users with strong CSI which increases the spectral efficiency. The 5G network will support heterogeneous architecture in which small cell and macro base stations work for spectrum sharing. NOMA is a key technology of the 5G wireless system which is very helpful for heterogeneous networks as multiple users can share their data in a small cell using the NOMA principle.The NOMA is helpful in various applications like ultra-dense networks (UDN), machine to machine (M2M) communication and massive machine type communication (mMTC). As NOMA provides lots of features it has some challenges too such as NOMA needs huge computational power for a large number of users at high data rates to run the SIC algorithms. Second, when users are moving from the networks, to manage power allocation optimization is a challenging task for NOMA [54]. Hybrid NOMA (HNOMA) is a combination of power-domain and code-domain NOMA. HNOMA uses both power differences and orthogonal resources for transmission among multiple users. As HNOMA is using both power-domain NOMA and code-domain NOMA it can achieve higher spectral efficiency than Power-domain NOMA and code-domain NOMA. In HNOMA multiple groups can simultaneously transmit signals at the same time. It uses a message passing algorithm (MPA) and successive interference cancellation (SIC)-based detection at the base station for these groups [55]. Highlights of 5G NOMA technology as follows:  
* NOMA is different than all the previous orthogonal access techniques such as TDMA, FDMA and CDMA. In NOMA, multiple users work simultaneously in the same band with different power levels. As shown in Figure 3.
* NOMA provides higher data rates and resolves all the loop holes of OMA that makes 5G mobile network more scalable and reliable. * As multiple users use same frequency band simultaneously it increases the performance of whole network.