On the Adaptive Modulation for User-Centric Cell-Free Massive MIMO Systems

The cell-free (CF) systems are a key technology for future wireless networks, which exploit the scenario spatial macro-diversity to offer high channel capacity and link reliability, uniform user quality of service (QoS), and better coverage. A technique for access point selection (APS) must be applied to improve the use of the macro-diversity. This paper proposes a low-complexity APS technique based on the derived signal-to-interference-plus-noise ratio (SINR) theoretical expression. Additionally, to increase the system's spectral efficiency (SE), an expression for the bit error probability (BEP) of the generic square M-m-ary quadrature amplitude modulation (M-m-QAM) scheme for each user is proposed and applied to an adaptive modulation (AM) algorithm. The results revealed that the proposed APS technique outperforms techniques with similar simplicity and that the AM algorithm provides higher system SE maintaining the error rate below a given threshold.