# Strategy for Non-Orthogonal Multiple Access and Performance in 5G and 6G Networks
## 3 NOMA Code-Separated Techniques
### Low-Density Spreading by CDMA Technology  
LDS sequences, with CDMA and OFDMA were used to create the first code division NOMA schemes [14]. LDS-CDMA, an enhancement of CDMA employing low-density codes, was introduced over ten years ago [15]. The LDS-CDMA technology uses LDS sequences to apply a multi-user reception method, which is close to the principal-component-criterion optimal algorithm [16]. Due to LDS sequences, LDS-CDMA technology can use a multi-user reception algorithm that is comparable to the algorithm that maximizes probability of users [17]. As users multiply in a typical CDMA system, reciprocal interference increases. To reduce noise, orthogonal codes with a changeable base were proposed, but such a system is not meant to work in overload mode, i.e., when the number of subscribers greatly exceeds the number of viable orthogonal code sequences. LDS-CDMA uses low-density coding sequences with many zero elements. Such LDS sequences reduce subscriber signal interference and allow mass connection and overload mode operation. LDS-LDS CDMA's code format simplifies user-signal processing compared to CDMA. Consider an LDS-CDMA system with \(K=6\) users using sequences of length \(N=4\). Information symbols reflect into code sequences using the following formula:  
\[S=\begin{bmatrix}1&1&1&0&0&0\\ 1&0&0&1&1&0\\ 0&1&0&1&0&1\\ 0&0&1&0&1&1\end{bmatrix} \tag{1}\]  
\[M^{k}M^{w}\]  
In this case, the signal base is represented by the number of rows in the matrix \(S\), and the number of subscribers who access these sequences is represented by the number of columns. With a loading ratio of 150%, the system can support far more users than standard CDMA networks. A huge number of zero elements characterize the structure of the matrix \(S\). A graphical representation of this system is provided in Figure 2; \(x_{i}\) represents subscriber information symbols and \(y_{i}\) represents elements in received code sequences.  
The MPA algorithm (message passing algorithm) can repair mistakes on the receiving side of the NOMA multi-user demodulation system. The MPA algorithm simplifies a demodulator using sequential interference suppression and employs a graph format to show informational symbols as a resource for demodulation. Errors can be fixed with a turbo decoder. These iterative processing approaches can improve demodulation quality (but increase processing complexity). NOMA systems with LDS code sequences make multi-user demodulators, especially sequential interference suppression-based ones, easier to build. Low density spreading CDMA sequences reduce user-signal interference even more than regular CDMA systems. CDMA and NOMA systems with code separation have the following reception processing computational complexity. The receiver complexity in a typical CDMA system (optimal by maximum likelihood criterion) is \(M^{k}\), where \(K\) is the number of active users and \(M\) is the modulation multiplicity. This can be used to evaluate the computational difficulty of processing at the receiver for CDMA and NOMA systems with code division. The receiver's complexity is \(M^{\varpi}\), \(W\) < \(K\) if the NOMA system uses \(W\) code sequences, where \(K\) is the number of active users and \(M\) is the modulation multiplicity. If the NOMA system uses \(W\) code sequences, complexity of an LDS-computational OFDM's receiver system is larger than of OFDMA due to LDS sequences.