The emergence of various mobile broadband networks and services is causing a substan-tial increment in mobile data traffic. Many efforts and revolutionary ideas have been proposed and explored worldwide to handle this significant mobile data traffic. Cell-free massive multiple-input multiple-output (MIMO) systems are one of the major techno-logical breakthroughs that may bring a renaissance in wireless communication networks. In cell-free massive MIMO systems, the base station (BS) antennas, which are known as access points (APs), are geographically distributed throughout the area of coverage. Hence, cell-free networks have the potential to significantly improve the key performance bottlenecks of cellular networks by cooperating between APs. To meet the system re-quirements of beyond 5G networks, designing efficient networks and architectures that achieve the required capacity with low cost and reduced resources is a critical challenge. In this thesis, we studied the optimization of spectral efficiency resources in downlink cell-free massive MIMO systems with linear precoding schemes, assuming perfect and imperfect channel state information (CSI) at the APs and time-division duplex (TDD) operation. We formulate a comprehensive optimization problem that maximizes the spec-tral efficiency (SE) of the system. We implement the power allocation strategies for upper bound, perfect and imperfect CSI scenarios. Further, we investigate different pilot as-signment schemes. Simulation results are provided, as expected minimum mean square error (MMSE) is superior to Zero-Forcing (ZF) in terms of sum-rate. For power allocation strategies, a heuristic uniform power allocation outperforms water-filling power allocation by providing over a 15% gain. To mitigate pilot contamination, we analyze different pilot assignment schemes; therefore, orthogonal pilot assignment provides better performance. Furthermore, we simulate the comparison of cellular and cell-free massive MIMO systems. The cell-free system achieved a 200% performance gain over the cellular system. Key Words: 5G, Cell-free massive MIMO, Pilot assignment, Power control, Resource allocation, Spectral efficiency