Enhanced Spatial Reuse in Multi-Cell WLANs

Abstract : When IEEE 802.11 access points (APs) share the same channel in a multi-cell WLAN, their downlink transmissions can interfere. Typically, an AP whose scheduled transmission to some user is blocked by another cell will apply the CSMA/CA back-off algorithm and continue to make reattempts to the same user. Through analytical models and simulations, we demonstrate that significant capacity gains can be attained by choosing an alternative destination for the reattempt. Results demonstrate that a simple random choice of alternative destination brings almost the same gain as a more sophisticated algorithm that seeks to maximize spatial reuse. I. INTRODUCTION The downlink traffic capacity of multiple interfering IEEE 802.11 Access Points (APs) can be improved by introducing opportunism in the way APs schedule their transmissions. The envisaged multi-cell WLAN consists of a number of APs transmitting on the same frequency channel to a population of users distributed over the coverage area. We assume a stochastic traffic model where finite size flows arrive according to a Poisson process. Traffic capacity is measured by the maximum load at which the network remains stable in the sense that the number of flows in progress does not explode. Traffic capacity depends on the way the APs interfere. Under the IEEE 802.11 Distributed Control Function (DCF), channel access is governed by the CSMA/CA algorithm. Three features of this algorithm are relevant to our study: (i) the backoff mechanism implemented by each station effectively provides random access opportunities to the APs downloading flows to their users; (ii) the two small RTS/CTS frames exchanged between transmitter and receiver in an access attempt inhibit interfering stations from attempting a simultaneous transmission; (iii) when a packet retransmission is blocked, the AP will make repeated attempts to send the same packet on successive expirations of the backoff counter. Our proposal is to modify the last feature by allowing an AP to transmit to an alternative destination after observing an initial collision. The problem is more easily explained with reference to a simple example. In Figure 1 we depict a network consisting of two
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Thomas Bonald, Ali Ibrahim, James Roberts. Enhanced Spatial Reuse in Multi-Cell WLANs. Infocom, 2009, Rio de Janeiro, Brazil. ⟨10.1109/INFCOM.2009.5062200⟩. ⟨hal-01244784⟩

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