Multi-timescale scheme for cooperative user association and hybrid beamforming in mmWave MIMO systems

Hybrid beamforming has received significant attention as a solution to the thermal issues, costs, and implementation complexities associated with fully digital mmWave extremely large MIMO (XL-MIMO) systems. The hybrid approach offers a balance between system flexibility and performance. However, in scenarios involving a large number of user equipments (UEs), even optimal beamforming techniques cannot provide satisfactory network delay and throughput if the data rate is not effectively shared among them.. Considering multiple forms of dynamism of an indoor wireless channel, we propose a multi-layer scheme to optimize resource allocation to the UEs in a cooperative multi-base station (BS) setup. We evaluated various resource allocation techniques to effectively share the bandwidth among UEs, rather than simply maximizing total downlink throughput by concentrating on well-off ones. Our findings indicate that queue length-based allocation strategies yield the lowest delay under different downlink traffics and UE/BS deployment densities. Moreover, selectively serving a subset of UEs during each channel block, rather than serving them all simultaneously, enhances network delay and throughput. To further improve resource utilization and overall performance, we introduce an extension called sub-coherence time allocation. This technique considers early downlink queue exhaustion and speculatively calculates multiple digital precoders to be activated sequentially within a channel block. Simulation results demonstrate that this approach improves delay and jitter with minimal computational overhead, achieving a 25% and 15% decrease in digital and hybrid modes, respectively.