The cell-free (CF) massive multiple-input multiple-output (mMIMO) system is recognized as a promising paradigm for future wireless networks, including sixth-generation (6G) and beyond, for providing ultrareliable and low-latency support to the high-throughput applications. The user-centric (UC) approach enhances its interference management by serving each equipments (UE) through a specific set of access points (APs), instead of all the APs. This thesis focuses on developing several computationally efficient schemes for improving the spectral efficiency (SE) performance of the UC CF mMIMO system during the uplink (UL) phase. First, a new AP-UE association scheme is proposed which leverages both the AP and UE-preferences for association assignment. It is shown that the proposed association scheme provides superior SE performance compared to the state of the art AP-preference based association scheme. Second, a pilot assignment scheme based on the auction algorithm is proposed to maximize SE and minimize interference. Numerical results demonstrate that the proposed pilot assignment scheme provides up to 30% higher average SE and up to 50% lower interference than several recent pilot assignment schemes. Third, a pilot power allocation scheme is proposed that reduces both channel estimation error and per-user pilot transmission power. Numerical results verify the effectiveness of the proposed scheme in improving the 95%-likely spectral efficiency performance by up to 16% and reducing the average pilot transmission power by up to 78% compared to existing schemes. Fourth, two centralized power allocation schemes for data transmission are proposed based on fractional programming method. Numerical results demonstrate that, compared to the no power control scheme, the proposed schemes improve the average SE performance by up to 47% while reducing the average transmission power by up to 95%. Finally, the integration of rate splitting multiple access (RSMA) technique is proposed to further enhance the SE performance for the UC CF MIMO system. Simulation results underscore the significant SE performance enhancement up to 61% when the RSMA technique is integrated. The results in this thesis prove that the proposed schemes can be viable choices for improving the SE performance of a UC CF mMIMO system during the UL phase.