Matched-Decision AP Selection for User-Centric Cell-Free Massive MIMO Networks

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

“…A two-layer APS algorithm with different strategies for fine-tuning was proposed in [19], either decreasing the number of UEs per AP, without decreasing the system SE or maintaining the energy efficiency (EE). In the first layer, each UE connects to an intermediate AP based on the LSF coefficient, preventing the signal from dropping, and in the second layer, each UE expands its cluster by connecting to more APs to increase the SE.…”

“…The expression is applied to perform slow APS using a simple methodology that aims to increase each UE average SINR, which differs from of the fast APS proposed in [8], [10]- [12], to secure low backhaul capacity requirement. Furthermore, differently from the works that used the LSF to perform slow APS [13]- [19], the usage of the SINR for AP to UE assignment incorporates information about three different characteristics simultaneously: the interference, the noise, and the signal powers. Thus, the proposed APS may present an improved behavior for real scenarios.…”

“…Additionally, if close enough UEs are served by the same AP, high interference levels may be experienced by each other. Thus, a proper technique to assign APs to UEs must be applied to improve the system performance by decreasing the multiuser interference [8], [10]- [19].…”

“…The slow AP to UE assignment performs APS within the time in which the largescale fading (LSF) remains constant, which is assumed to vary slowly compared to the small-scale fading (SSF). In this strategy, the APS focuses on maximizing, for example, the average system SE, but may fail to attain it if the channel VOLUME 11, 2023 varies suddenly [13]- [19]. In the fast APS, the AP to UE assignment is performed on the SSF coherence time, so, whenever it varies, the system must be able to adjust the selected APs for each UE [8], [10]- [12].…”

“…As in slow APS, the assignment is performed on the LSF coherence time, in this approach, the computational cost decreases, sometimes at the expense of rate gains. In this context, several works analyzed different ways to perform AP selection to increase the system performance [8], [13], [14], [16], [18], [19].…”