AN INTERFERENCE AWARE USER-GROUP PILOT ASSIGNMENT FOR CELL-FREE MASSIVE MIMO SYSTEMS

Abstract

A cell-free (CF) massive multiple-input multiple-output (MIMO) system is a nextgeneration technology where a large number of access points (APs) are distributed over a large geographical area to serve multiple users to fulfill the higher data rate requirements. However, due to a large number of users and the reuse of pilot sequences, the CF network suffers from severe inter-user interference, which deteriorates the achievable throughput. In this thesis, we propose an interference aware user-group (IA-UG) pilot assignment scheme that addresses the issue of inter-user interference while supporting a large number of users. In the proposed IA-UG scheme, we count the number of common APs between two user pairs and use it as a likely measure of the number of interferers. The higher the number of common APs between two users, the higher will be the number of interferers and, thereby, the inter-user interference between them. Based on this point, users having the minimum number of interferers are formed into a group by assigning them the same orthogonal pilot. Subsequently, the algorithm focuses on minimizing the number of interferers in the group having the maximum count. For the minimization, a traverse procedure will be implemented until the user picks the suitable pilot. Numerical results show that the proposed pilot assignment scheme outperforms the existing schemes and yields a superior quality of channel estimates as well as the throughput.