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

Abstract

Frame aggregation is a MAC-layer technology proposed in 802.11n 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 $F^{\prime }/F_{\text{max}}$. 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.

Introduction

IEEE 802.11n [1] is proposed as an amendment of the previous IEEE 802.11 wireless networking standard to significantly improve network throughput. It aims at providing a data transmission rate of up to 600 Mbps. The version 2.0 draft specification for the next generation IEEE 802.11n WLANs has been approved in March 2007. It has introduced substantial enhancements at both the PHY (physical) and MAC (medium access control) layers for high throughput, efficiency and robustness [2], [3] for the wireless system. In the PHY layer, based on the MIMO-OFDM (Multiple Input Multiple Output – Orthogonal Frequency Division Multiplexing) technology, 802.11n can use spatial multiplexing to transmit two or more data streams simultaneously. It also provides transmitter spatial diversity to improve reception by spreading the spatial streams across multiple antennas [4]. Beamforming, specified as an optional feature, can further improve packet transmission efficiency. The 802.11n defines a new set of the modulation and coding schemes (MCS), and the MCS is an index value that determines the modulation, coding and number of spatial streams in MIMO-OFDM systems. The actual transmission scheme is composed of both the MIMO mode and the MCS. The efficiency improvements at the MAC layer are frame aggregation, block acknowledgment (block ACK, also backward compatible with 802.11e [5]), etc. Frame aggregation [6], [7], [8] can improve MAC-layer efficiency by reducing the transmission time for preamble and frame headers, and the random backoff period for successive frame transmissions. They are particularly applicable to voice traffic where the voice frame is short and continuous traffic such as video or large file transfers.

We tackle the wireless scheduling problem from a cross layer optimization angle. We have done much research on the link adaptation algorithms for opportunistic scheduling in [9]. However, while the link adaptation improves transmission on a physical link, the aggregate system performance is very much dependent on multi-user scheduling and cross layer optimization mechanisms, which are also heavily coupled with underlying link adaptation. This cross layer optimization becomes more imperative in the 802.11n wireless systems since the standard has introduced many significant options in the MAC layer. In this paper, we will take advantage of the frame aggregation in the 802.11n for designing a multi-user scheduling algorithm. The scheduler tries to improve the system performance, in particular, in terms of the quality of service (QoS) efficiency.

Different from the well studied opportunistic scheduling that monitors the channel continuously and decides the locally optimized strategy to send packets to one user, scheduler with frame aggregation can send packets to several users simultaneously. How to select this set of users to be serviced is a challenging problem, which can be easily modeled into a knapsack problem that is well known to be NP hard [10]. We argue that, however, in practice we do not have to find the optimal solution to the knapsack problem to achieve a good performance. A simple greedy algorithm that has less complexity works sufficiently well with practical implementation of the frame aggregation.

The rest of this paper is organized as follows. In Section 2, we present the system model and conventions used throughout the paper. We then model the scheduling problem in the wireless system with frame aggregations into a knapsack problem in Section 3. In Section 4, we propose a simple greedy algorithm to do the scheduling and analyze its performance. In Section 5, we bring forward some practical considerations and argue that the greedy algorithm performs well in practice. In Section 6, some simulations and performance evaluation are present comparing our algorithm with the Round-Robin scheduling. Then, we conclude the paper in Section 7.