# Clustering Approach for Reliable Wireless Communication
## 5 Conclusions  
Code-domain multiple access solutions provide simplicity with increased performance and interference control solutions. In LTE-compliant systems, MUSA codewords for users' signal separation allow for efficient design and low mathematical complexity. Therefore, this study's topic is aimed at investigating the benefits of using clusters in the MUSA code allocation process. A certain number of spreading codewords are grouped into clusters to facilitate the access of multiple users to non-interference communications (highly correlated sequences). In this research, it was also outlined that the number of clusters must be selected depending on a correlation threshold. Furthermore, a method of determining the correlation threshold considering the number of K users in the system was investigated. An extensive analysis demonstrated that for moderate overloaded systems, with limited resources, the current approach can still lead to an increase in performance. Moreover, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \begin{tabular}{c} **Scenario** \\ **No.** \\ \end{tabular} & \begin{tabular}{c} **Method Type** \\ \end{tabular} & \begin{tabular}{c} **MUSA** \\ **Code Length** \\ \end{tabular} & \begin{tabular}{c} **SNR** \\ **(dB)** \\ \end{tabular} & \
\begin{tabular}{c} **Variation** \\ \end{tabular} &
\begin{tabular}{c} **Discussions** \\ \end{tabular} \\ \hline \multirow{3}{*}{1} & No & \multirow{3}{*}{2} & \multirow{3}{*}{30} & MMSE & The combination of the clustering method and MMSE-SIC leads to better results in BER terms than the classic approach for low-length MUSA codes. \\ \cline{3-6}  & Clustering & & & MMSE-SIC & Since performance is similar, it can be stated that the clustering approach cannot be efficient for an overloaded system with users having limited resources which transmit large data. When comparing the outcomes of Rayleigh and Rice fading, Rice performs better because it emulates LoS communication. Furthermore, by analyzing BPSK and QPSK performance in this system, it was shown that a BPSK modulation scheme produces better BER values. \\ \cline{3-6}  & Clustering & & & \(K=40\) vs. \(K=62\) and random vs. PN codes & Overlapping high-length MUSA codewords (random/PN) and an increased number of users does not lead to improvement. The clustering approach performs well for a reduced number of users. \\ \hline \multirow{3}{*}{5} & 20 clusters vs. & \multirow{3}{*}{-} & \multirow{3}{*}{40} & \multirow{3}{*}{-} & Without using a sequence allocation mechanism that evaluates the correlation between clusters, it is efficient to group the code sequences into a higher number of clusters. Creating more clusters with fewer, but highly correlated, codewords leads to marginally better BER values. \\ \cline{3-6}  & & & & & better BER values. \\ \hline \multirow{3}{*}{6} & \(k\)-means & \multirow{3}{*}{4} & \multirow{3}{*}{-} & \multirow{3}{*}{-} & Both clustering algorithms (k-means and \(k\)-medoids) lead to acceptable and comparable results only for a small number of users. \\ \cline{3-6}  & vs. & & & & \\ \cline{3-6}  & & & & & \\ \cline{3-6}  & & & & & \\ \cline{3-6}  & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summative results for all scenarios. Figure 17: BER vs. SNR for Scenario 6: (**a**) BS with 2 antennas, (**b**) BS with 4 antennas. if MUSA codewords are designed starting from the classical PN code used in multiple-access code division, the quality of the system is improved because more users benefit from codes with low correlations. It was shown in all cases that no clustering technique is effective for transmission \"drawn\" into noise (negative SNR values), and performance cannot be enhanced. However, when the effects of long MUSA codewords (random/PN) are superimposed with a larger number of users, it becomes evident that no improvement is possible. For a smaller user base, the clustering method produces better BER values. The clustering approach leads to better BER values for a reduced number of users.