Two-Dimensional DOA Zero-Forcing Precoding for Enhanced Spectral Efficiency in Cell-Free Massive MIMO

Energy and spectral efficiency in mobile networks are enhanced by Cell-free Massive-MIMO systems utilizing macro-diversity to optimize overall performance. However, multi-user interference, especially due to pilot reuse within the same cell, continues to pose significant challenges. This research introduces a Two-Dimensional Direction of Arrival (2D-DOA) Zero-Forcing Precoding method. It addresses both intra-cell and inter-cell interference, removing the requirement for Channel State Information exchange between Access Points (APs). A new expression for downlink spectral efficiency is developed, considering the impacts of imperfect CSI and pilot contamination. Utilizing UESPRIT and MUSIC -based 2D-DOA estimation techniques, the proposed method surpasses pilot-contaminated systems and deterministic Zero-Forcing precoding, demonstrating enhanced spectral efficiency even under constrained pilot resources.