A Novel Proportional Fairness Criterion for Throughput Allocation in Multirate IEEE 802.11

arXiv:0809.1061v1 [cs.NI] 5 Sep 2008 1 A Novel Proportional Fairness Criterion for Throughput Allocation in Multirate IEEE 802.11 M. Laddomada, F. Mesiti, M. Mondin, and F. Daneshgaran Abstract This paper focuses on multirate IEEE 802.11 Wireless LAN employing the mandatory Distributed Coordination Function (DCF) option. Its aim is threefold. Upon starting from the multi-dimensional Markovian state transition model proposed by Malone et.al. for characterizing the behavior of the IEEE 802.11 protocol at the Medium Access Control layer, it presents an extension accounting for packet transmission failures due to channel errors. Second, it establishes the conditions under which a network constituted by N stations, each station transmitting with its own bit rate, R(s) d , and packet rate, λs, can be assumed loaded. Finally, it proposes a modified Proportional Fairness (PF) criterion, suitable for mitigating the rate anomaly problem of multirate loaded IEEE 802.11 Wireless LANs, employing the mandatory DCF option. Compared to the widely adopted assumption of saturated network, the proposed fairness criterion can be applied to general loaded networks. The throughput allocation resulting from the proposed algorithm is able to greatly increase the aggregate throughput of the DCF, while ensuring fairness levels among the stations of the same order as the ones guaranteed by the classical PF criterion. Simulation results are presented for some sample scenarios, confirming the effectiveness of the proposed criterion for optimized throughput allocation. Index Terms DCF, Distributed Coordination Function, fairness, IEEE 802.11, MAC, multirate, non-saturated, proportional fairness, rate adaptation, saturation, throughput, traffic, unloaded, unsaturated. I. INTRODUCTION Consider the IEEE802.11 Medium Access Control (MAC) layer [1] employing the DCF based on the Carrier Sense Multiple Access Collision Avoidance CSMA/CA access method. The scenario envisaged in this work considers N contending stations; each station generates data packets with constant rate λs by employing a bit rate, R(s) d , which depends on the channel quality experienced. In this scenario, it is known that the DCF is affected by the so-called performance anomaly problem [2]: in multirate networks the aggregate throughput is strongly influenced by that of the slowest contending station. After the landmark work by Bianchi [3], who provided an analysis of the saturation throughput of the basic 802.11 protocol assuming a two dimensional Markov model at the MAC layer, many papers have addressed almost any facet of the behaviour of DCF in a variety of traffic loads and channel transmission conditions. Contributions proposed in the literature so far can be classified in two main classes, namely DCF Modelling and DCF Throughput and Fairness Optimization. Massimiliano Laddomada is with the Electrical Engineering Dept. of Texas A&M University-Texarkana, email: mladdomada@tamut.edu. F. Mesiti and M. Mondin are with DELEN, Politecnico di Torino, Italy. F. Daneshgaran is with ECE Dept., California State University, Los Angeles, USA. 2 DCF modelling. This is the topic that received the most attention in the literature since the work by Bianchi [3]. Papers [4]-[6] model the influence of real channel conditions on the throughput of the DCF operating in saturated traffic conditions, while [7]-[9] thoroughly analyze the influence of capture on the throughput of wireless transmission systems. Paper [10] investigates the saturation throughput of IEEE 802.11 in presence of non ideal transmission channel and capture effects. The behavior of the DCF of IEEE 802.11 WLANs in unsaturated traffic conditions has been analyzed in [11]-[18]. In [19], the authors look at the impact of channel induced errors and of the received Signal-to-Noise Ratio (SNR) on the achievable throughput in a system with rate adaptation, whereby the transmission rate of the terminal is modified depending on either direct or indirect measurements of the link quality. Multirate modeling of the DCF has received some attention quite recently [20]-[24] as well. In [20] an analytical framework for analyzing the link delay of multirate networks is provided. In [21]-[22], authors provide DCF models for finite load sources with multirate capabilities, while in [23]-[24] a DCF model for networks with multirate stations is provided and the saturation throughput is derived. Remedies to performance anomalies are also discussed. In both previous works, packet errors are only due to collisions among the contending stations. DCF throughput and fairness optimization. This is perhaps the issue most closely related to the problem dealt with in this paper. The main reason for optimizing the throughput allocation of the 802.11 DCF is the behaviour of the basic DCF in heterogeneous conditions, with stations transmitting at multiple rates: the same throughput is reserved to any contending station irrespective of its bit rate, with the undes

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