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Pilot allocation in downlink for cell-free communication systems

Abstract : In Cell-Free massive Multiple-Input-Multiple-Output (Cell-Free massive MIMO) systems, we distribute in a coverage area a massive number of access points, mastered by central processing units (CPUs), to simultaneously serve much smaller number of user equipments (UEs) over the same time/frequency resources. In contrast to the centralized massive MIMO, cell-free massive MIMO is characterized by a channel hardening not sufficiently accentuated, thus, it will be appropriate to include downlink (DL) pilots to estimate the DL channel. This thesis considers a DL pilot assignment for the cell-free massive MIMO systems by defining a metric, involving the inter-user interference (IUI). This metric gives insights about DL pilot contamination. A threshold is then defined to optimize the number of DL pilots, which maximizes the minimum per-user DL throughput. This approach gives a conflict graph, where each UE is regarded as a vertex of the graph. It consists in a combinatorial optimization problem that can be approximated using a graph coloring algorithm. It is a greedy algorithm whose steps are described as follows. By fixing the adequate threshold, maximizing the minimum per-user DL throughput, a conflict or interference graph is constructed. It models the potential interference among interfering UEs, the UEs between which there is an edge are in conflict, i.e., present a high IUI. Then, the proposed scheme mitigates the potential IUI by appointing different DL pilots to connected UEs with high IUI and same DL pilots to UEs with low IUI in the conflict graph in accordance with some coloring rules. The simulation results validate that the minimum per-user DL throughput based on the proposed approach outperforms the conventional methods, i.e., statistical channel state information, the orthogonal and the random pilot assignment in the DL training. Our analysis underlines also the reduction of the DL pilot overhead ratio using the DL pilot assignment based on our proposed scheme, compared to the conventional methods aforementioned.
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Submitted on : Monday, June 21, 2021 - 10:08:13 AM
Last modification on : Tuesday, June 22, 2021 - 3:34:16 AM


Version validated by the jury (STAR)


  • HAL Id : tel-03265624, version 1



Wafa Haj Hmida. Pilot allocation in downlink for cell-free communication systems. Electronics. Université de Limoges; Systèmes de Communications (Tunis), 2020. English. ⟨NNT : 2020LIMO0079⟩. ⟨tel-03265624⟩



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