QoS based scheduling in the downlink of multi-user wireless systems (extended)

Frame aggregation is a MAC-layer technology proposed in 802.11 n WLAN. The base station can serve two or more users in one frame simultaneously, which can improve MAC-layer efficiency by reducing the transmission time for preamble and frame headers, and the random backoff period for successive frame transmissions. This fact enables us to design a more QoS-aware scheduler from the MAC layer. In this paper, we first formulate the scheduling problem with frame aggregation into a knapsack problem that is shown NP hard. Then we propose a simple approximation algorithm (LUUF) based on the unit urgency concept. Our analysis shows that the complexity of LUUF is O(n log n) and it achieves an approximation ratio of F'/F-max. We then show that in practice the complexity can be further reduced to O(n) and the approximation ratio can be made very near to 1, which makes LUUF a promising candidate for wireless systems that support frame aggregation. We also conduct simulations comparing LUUF with the widely used Round-Robin scheduler and find that LUUF can significantly improve the quality of service for various numbers of users and different maximum aggregation frame sizes. (C) 2009 Elsevier B.V. All rights reserved.