Beam-Domain-Based Pilot Assignment for Energy Efficient Cell-Free Massive MIMO

In this letter, an efficient beam-domain-based pilot assignment scheme is proposed to mitigate the pilot contamination caused by pilot reuse in cell-free massive multiple-input multiple-output systems. By exploiting the information provided through beam management during the initial access, we construct a graph that captures the potential interference in the beam domain and apply graph coloring for pilot assignment. In addition, we consider a radio frequency chain switch on/off strategy to improve energy efficiency. Numerical results verify that the proposed pilot assignment scheme can significantly improve both spectral efficiency and energy efficiency compared to existing schemes.