A Kalman Filter Approach for Distinguishing Channel and Collision Errors in IEEE 802.11 Networks I. Tinnirello * , A. Sgora + ∗ Universit` a di Palermo, Dip. di Ing. Elettrica, Elettronica e delle Telecomunicazioni, Italy (ilenia.tinnirello@tti.unipa.it) + University of Aegean, Department of Information and Communication Systems Engineering, Greece (asgora@aegean.gr) Abstract— In the last years, several strategies for maximizing the throughput performance of IEEE 802.11 networks have been proposed in literature. Specifically, it has been shown that opti- mizations are possible both at the Medium Access Control (MAC) layer, and at the Physical (PHY) Layer. In fact, at the MAC layer, it is possible to minimize the channel waste due to collisions and backoff expiration times, by tuning the minimum contention window as a function of the network congestion level. At the PHY layer, it is possible to improve the transmission robustness, by selecting a suitable modulation/coding scheme as a function of the channel quality perceived by the stations. However, the feasibility of these optimizations rely on the availability of MAC/PHY measurements, which are often impracticable or very rough. In this paper, we propose a joint MAC/PHY estimator based on a bi-dimensonal extended kalman filter, devised to separately track the collision probability and the channel error probability suffered by each station. To this purpose, we derive a relationship between the unobservable system state and measurements which are perfomed in a distributed way by all the competing stations. I. I NTRODUCTION One of the key factor for the wide success of IEEE 802.11 Wireless Local Area Networks (WLANs) is the simplicity and robustness of the Medium Access Control (MAC) protocol em- ploying the Distributed Coordination Function (DCF). Based on the well-known carrier sense paradigm, with an exponential backoff mechanisms devised to minimize the probability of simultaneous transmission attempts by multiple stations, DCF is able to work in presence of interference, which is very critical for networks operating in unlicensed spectrum. Sources of interference affecting a given station may include not only other stations sharing the channel on the same network, but also external noise, for example, from microwave ovens and overlapping networks. The former endogenous interference affects the MAC layer; the latter exogenous interference affects the physical (PHY) layer. In the last years, several strategies for maximizing the throughput performance of DCF in presence of interference have been proposed in literature. Specifically, it has been shown that optimizations are feasible both at the MAC layer and at the PHY layer. On one side, it is possible to minimize the channel waste due to collisions and backoff expiration times, by tuning the minimum contention window as a function of the number of interfering stations [2], [3], [4]. While, in the standard IEEE 802.11 protocol [1], the backoff parameters were hard-wired in the PHY layer, the idea of adaptively setting the backoff window has been recently taken into consideration in the new 802.11e standard amendment [5]. By exploiting this new possibility, adaptive tunings of the contention have been proposed in [6], [7], [8]. On the other side, it is possible to improve the transmission robustness, by selecting a suitable modulation/coding scheme as a function of the channel quality perceived by the stations. Different rate selection algoritms, known as link adaptation algorithms, may be implemented in commercial cards, according to the hardware latency for switching from a rate to another and to the capability of buffering per-packet rate descriptors [9]. However, the feasibility of these optimizations rely on the availability of MAC/PHY measurements, which are often impracticable or very inaccurate. Regarding the estimation of the MAC interference, we have to consider that the protocol operations do not allow to directly retrieve the network con- gestion level. In fact, DCF does not rely on a privileged station to control the access to the channel. Even considering the existence of an Access Point (AP), as in infrastructure mode, the information available at the AP is limited to the number of associated stations, a number which may be very different from the number of stations actually in contention. Moreover, in presence of generic traffic sources, the number of active stations is not directly related to the network congestion status. Regarding the estimation of the PHY interference, we have to consider that the detection of frame errors due to poor Signal to Noise Ratio (SNR) is not immediate, since the collision detection is not available for wireless transmissions. Different solutions, based on special control frames, acknowledgement monitoring or tracking of consecutive failures [10], [11], [12], have been designed for indirectly distinguishing between collision-induced and channel-induced errors. Most of them have some drawbacks in terms of overhead or accuracy of the channel error rate estimator. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings. 978-1-4244-2324-8/08/$25.00 © 2008 IEEE.