# Clustering Approach for Reliable Wireless Communication
###### Abstract
Our present strategy will apply the MMSE-SIC algorithm for bit error rate (BER) optimization in a multi-user communication system, since the reliability of MUSA is demonstrated when using the minimum mean squared error (MMSE) with soft interference cancellation (SIC) in ideal and Rayleigh fading channels [43]. Complex MUSA codes with different lengths (2, 4, 8, and 16), QPSK digital modulation, and an MMSE-SIC receiver allow for a quick decrease in the BER values of up to \(10^{-5}\) in both channel scenarios [43]. An SIC detector is also used at the receiver with the zero-forcing (ZF) technique to mitigate interference in a downlink power domain multi-user MIMO-NOMA with (2 \(\times\) 2) and (8 \(\times\) 8) configurations and single-input single-output (SISO) NOMA systems with two users. In a Rayleigh fading-affected channel, the multiple-input multiple-output (MIMO) NOMA approach overcomes the SISO-NOMA's performance by demanding a lower transmission power to achieve the same BER [44]. Moreover, [45] outlines the benefits of applying NOMA concepts in the Rayleigh fading environment. To resume the assets of MUSA, a study from 2021 [46] pointed out that this is a technique that allows grant-free transmission, that the users must not be aware of their corresponding spreading codes, and that the probability of collisions is low. In addition, because of the high-frequency diversity, massive overload (more than 700%) is easily handled concurrently. Furthermore, categorizing the spreading sequences into groups is only stated in this research. Consequently, we used clustering approaches with specific features to execute this strategy. Three CD-NOMA multiple access methods (MUSA, SCMA, and PDMA) were compared in [47], where QPSK data are broadcasted in a communication environment affected by Rayleigh fading. By using SIC to maximize the performance, the authors demonstrated that MUSA and PDMA lead to comparable results, while SCMA leads to slightly better results. Complete scenarios referring to the system's capacity were evaluated under the same circumstances in [48]. Ordered successive interference cancellation (OSIC)-based multi-user detection (MUD) and MPA-based MUD have been used at the receiver side. In [49], another three CD-NOMA multiple access techniques were compared by evaluatingthe BER behavior when the signal-to-noise ratio (SNR) varies--using MUSA, SCMA, and IDMA. An uplink SISO connection is considered when data from up to 24 users are QPSK-modulated, orthogonal frequency-division multiplexed (OFDM), and when the data are altered by Rayleigh fading on the communication channel. On the receiving end, an MMSE detector was employed. The MMSE linear detector in MUSA seeks to assist in lowering the desired power of the signal and achieving interference cancellation. It was possible to achieve an acceptable block error rate (BLER) with low/average overload multipliers even with a low diversity gain of MUSA. As a result, it is demonstrated that low code rate modulations are used in a high overload system by the MUSA technique. In addition, MUSA proved to perform better in the low SNR range compared to IDMA and SCMA. Having demonstrated the advantages of the MUSA technique with the MMSE-SIC detector above, the current study focuses on optimizing the transmission in a CD-NOMA MIMO system by introducing a novel manner of grouping and selecting the available MUSA codes from a cluster. The rest of this paper has the following structure: Section 2 advances the innovative approach of grouping and selecting MUSA codes for certain users. In Section 3, the global system model and the suitable design are described in detail. Section 4 integrates the outcomes and pertinent discussions obtained from MATLAB simulations. In Section 5, the future work perspective is included alongside the conclusions.