Fixed Cluster-Based Collision Resolution Random Access for Cell-Free Massive MIMO

In this paper, we investigate the grant-based massive random access in cell-free (CF) massive multiple-input multiple-output (mMIMO) networks. A fixed cluster-based collision resolution random access (FCCRRA) protocol is proposed to mitigate severe pilot collisions for massive Internet of Things (IoT) by leveraging the difference in received power distribution across distributed access points (APs). Particularly, the cooperation among clustered APs and the enhanced normalized conjugate beamforming (ECB) are jointly exploited to achieve a low-complexity distributed contention resolution. Moreover, we derive the theoretical received power distribution of a user equipment (UE) across different APs and coverage areas and then formulate an optimization problem to determine a suitable parameter for fixed clustering. The results verify the superior performance of the proposed FCCRRA to benchmarks in terms of throughput and access success probability.