# Is Satellite Ahead of Terrestrial in Deploying NOMA for Massive Machine-Type Communications?
## 6 Possible Commonalities with 5G mMTC  
At the beginning of the first 5G activities within the 3 GPP standardization group (i.e., Release 15), advanced NOMA techniques were proposed and investigated for massive machine-type communication (mMTC) services. A very good summary and categorization of the proposed NOMA schemes are reported in [35; 36], while a comparative performance analysis based on selected 5G system-level assumptions has been summarized in [37]. Even though none of those NOMA candidates have been selected and standardized thus far for 5G mMTC services, the different proposals from the terrestrial wireless industry players can be grouped into three main multiple access (MA) categories: (a) codebook-based; (b) sequence-based; (c) interleaver/scrambler-based. Hereafter, a very brief summary of the key features is presented in order to understand the possible commonalities with the S-MIM/F-SIM technology.  
* Codebook-based MA maps the user data packet stream in a multi-dimensional codeword belonging to a codebook. The mapping is conducted in a way to achieve signal spreading and introduce zero elements to mitigate inter-user interference. The decoding process is obtained through a relatively complex iterative message passing algorithm.
* Sequence-based MA exploits non-orthogonal complex number sequences (short or long sequences) to separate users sharing the same spectrum, thus easing the multi-user detection process. Affordable complexity linear MMSE plus SIC or parallel interference cancellation (PIC) is proposed for the packet detection.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**IURA Cr2205f16 Ds38**} \\ \hline  & **0 SIC** & **1 SIC** & **2 SIC** & **3 SIC** \\ \hline Throughput (bit/chip) & 0.06 & 0.25 & 0.43 & 0.465 \\ PLR & 0.001 & 0.001 & 0.001 & 0.001 \\ \hline Throughput (bit/chip) & 0.12 & 0.42 & 0.5 & 0.55 \\ PLR & 0.01 & 0.01 & 0.01 & 0.01 \\ \hline \hline \end{tabular}
\end{table}
Table 4: UC#3: IURA results.  
Figure 23: UC#3 IURA PLR performance per SIC iterations.  
Finally, interleaver/scrambler-based MA utilizes different interleavers to separate users sharing the same bandwidth. Some repetition/scrambling is also adopted to spread the signals and achieve some interference-averaging effect. Depending on the size of the interleaved bit stream, simpler MMSE-SIC or more complex soft SIC decoding techniques will be used.  
From the proposed NOMA categorization in 3 GPP, it shall be easier to associate the E-SSA random access protocol at the sequence-based MA techniques, and, in particular, specific commonalities can be found in the MUSA [37] and in the RSMA [38] proposals. As already said, the current 5G standard (from Release 15 to Release 17) has not included NOMA techniques for mMTC services. Nevertheless, the advanced RA techniques (such as E-SSA or others) may benefit in the future to be part of 5G and beyond terrestrial standards by exploiting the current promising trend to work for a full and seamless integration between satellite and terrestrial networks.  
The rapid evolution of NGSO systems, resulting today in the deployment of thousands of LEO satellites, and the renewed interest in 3 GPP in the integration between satellite-based and terrestrial-based 5G systems are good precursors to the integration of NOMA-based multiple access systems, starting from the 3 GPP Release 18. A proposal inspired by the long development and operational experience cumulated in the last ten years, in the actual implementation of E-SSA-based systems, may pave the way to its consideration for future mMTC satellite and terrestrial applications. NOMA-based technologies, such as E-SSA, have demonstrated the ability to operate in mobility and in GSO and non-GSO-based orbits and provide a massive scalability, high efficiency and low user terminal cost.