Access Point-User Association and Auction Algorithm-Based Pilot Assignment Schemes for Cell-Free Massive MIMO Systems

In this article, two effective schemes are proposed for association and pilot assignment to tackle the problem of spectral efficiency (SE) maximization and interference management in a user-centric cell-free (CF) massive multiple-input–multiple-output (mMIMO) system. First, a novel access point (AP)–user equipment (UE) association scheme is proposed, which ensures both scalability and association for all the UEs in a CF mMIMO system. After that, a pilot assignment scheme is also proposed which has an iterative structure that utilizes the auction algorithm to solve symmetrical assignment problems for pilot allocation. Numerical results demonstrate the superior performance of the proposed AP–UE association scheme compared to the existing AP preference-based association scheme. Moreover, the proposed pilot assignment scheme provides up to 30% average SE performance and up to 50% reduction in the average interference to signal power ratio (ISR) over that of several competing alternatives. Numerical simulated results further reveal that the proposed pilot assignment scheme improves the average SE and ISR reduction performance regardless of the number of UEs, APs, and pilot sequences in the system. Finally, the proposed pilot assignment scheme manifests the best balance in the tradeoff among the considered performance criteria relative to its comparatives.