Scheduler Algorithms for MU-MIMO

University essay from Lunds universitet/Institutionen för elektro- och informationsteknik

Abstract: In multi-user multiple input multiple output (MU-MIMO), the complexity of the base-station scheduler has increased further compared to single-user multiple input multiple output (SU-MIMO). The scheduler must understand if several users can be spatially multiplexed in the same time-frequency resource. One way to spatially separate users is through beamforming with sufficiently many antennas. In this thesis work, two downlink beamforming algorithms for MU-MIMO are studied: The first algorithm implements precoding without considering inter-cell interference (ICI). The second one considers it and attempts to mitigate or null transmissions in the direction of user equipments (UEs) in other cells. The two algorithms are evaluated in SU-MIMO and MU-MIMO setups operating in time division duplex (TDD) mode and serving with single and dual-antenna terminals. Full-Buffer (FB) and file transfer protocol (FTP) data traffic profiles are studied. Additionally, various UE mobility patterns, UE transmit antenna topologies, sounding reference signal (SRS) periodicity configurations, and uniform linear array (ULA) topologies are considered. Simulations have been performed using a system level simulation framework developed by Ericsson AB. Another important part of this thesis work is the functional verification of this simulation framework, which at the time of writing is still undergoing development. Our simulation results show that in SU-MIMO, the second algorithm, which considers ICI, outperforms the first one for FB traffic profile and all UE speeds, but not for FTP traffic profile and medium (30 km/h) or high (60 km/h) UE speeds. In this case, the first algorithm, which does not consider ICI, can be used with advantage. In MU-MIMO, cell downlink throughput gains are observed for the second algorithm over the first one for low and medium system loads (number of users). For both algorithms, the cell throughput is observed to decrease with increasing UE speed and sounding periodicity.

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