flexRLM: Flexible Radio Link Monitoring for Multi-User Downlink Millimeter-Wave Networks

Exploiting millimeter-wave (mm-wave) for high-capacity multi-user networks is predicated on jointly performing beam management for seamless connectivity and efficient resource sharing among all users. Beam management in 5G-NR actively monitors candidate beam pair links (BPLs) on the serving cell to simply select the user’s best beam, but neglects the multi-user resource sharing problem, potentially leading to severe throughput degradation on overloaded cells. We propose flexRLM, a coordinator-based flexible radio link monitoring (RLM) framework for multi-user downlink mm-wave networks. flexRLM enables flexible configuration of monitored BPLs on the serving and other candidate cells and beam selection jointly considering link quality and resource sharing. flexRLM is fully 5G-NR-compliant and uses the LTE coordinator in non-standalone mode to continuously update the monitored BPLs via measurement reports from periodic downlink control synchronization signals. We implement flexRLM in ns-3 and present full-stack simulations to demonstrate the superior performance of flexRLM over default 5G-NR RLM in multi-user networks. Our results show that flexRLM’s continuous updating of monitored BPLs improves both link quality and stability. By monitoring BPLs on candidate cells other than the serving one, flexRLM also significantly decreases handover decision delays. Importantly, flexRLM’s low-complexity coordinated load-balancing achieves a per-user throughput close to the single-user baseline.

I. Introduction

The spectrum-rich millimeter-wave (mm-wave) bands are a key candidate for solving the capacity crunch in 5G-and-beyond cellular networks [1]. Exploiting the high-capacity promise of mm-wave links for high-capacity multi-user networks is predicated on jointly performing agile beam management for seamless connectivity and efficiently sharing the resources among all users. This is a challenging task due to the directional and blockage-prone nature of mm-wave links, requiring robust control procedures for maintaining precise beam alignment between the serving cell and the user, as well as coordinated spatial link and time-frequency resource allocation among the users served on the cell [2], [3].