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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 |
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