Pilot Assignment with Graph Coloring and Power Control Algorithms in Cell-Free Massive MIMO Systems

‍ ‍A large number of wireless access points （AP） in cell-free massive multiple input multiple output （CF-mMIMO） system is an advanced technology in next-generation wireless communications， which serves all users in the same time-frequency resource with high network coverage probability and spectrum efficiency. However， the limitation in the number of orthogonal pilot and the difference in the channel quality of the users lead to more severe inter-user interference. To optimize the interference problems， the pilot assignment with graph coloring and power control algorithms are proposed in cell free massive MIMO systems. In the scheme， the pilot assignment algorithm with graph coloring is proposed to reduce the pilot contamination and improve the accuracy of channel estimation. Furthermore， the max-min fairness power control algorithm is presented to inhibit the multi-users co-frequency interference and thus satisfying the various quality-of-service requirements of users. Numerical simulation results show the proposed algorithms can significantly improve 95% user spectrum efficiency and the minimum user rate of the system， the inter-user interference is also well suppressed.