Interference Management in IEEE 802.11 Frequency Assignment A. Gondran, O. Baala, A. Caminada, H. Mabed SET Laboratory β UTBM β France {alexandre.gondran, oumaya.baala, alexandre.caminada, hakim.mabed}@utbm.fr AbstractβIn this article we address the frequency management during WLAN planning. Frequency management refers to channels interference and SINR computation. We propose a new approach where location selection and frequency assignment are tackled together during WLAN planning process. Two steps characterize this approach. Firstly we use all the available channels for frequency assignment. Secondly multiple signals are taken into account to compute the SINR. Several experimental results show the benefits of this new approach. Keywords- WLAN planning; access point placement; frequency channel assignment; optimisation I. INTRODUCTION Wireless Local Area Network (WLAN) planning consists in selecting a location for each transmitter and setting the parameters of all sites in order to provide users a wireless access to their local network. The objective is to respect financial requirements and to guaranty a given Quality of Service (QoS). There are two relevant stages in WLAN planning. First we have to select a set of installation sites from a list of candidates that have been identified as potential location. For each site we must choose the antenna pattern, as well as its azimuth that indicates the main propagation direction, and the emitted power of the antenna. The 4-uplet (site, antenna pattern, azimuth, emitted power) is called Access Point (AP) configuration. Selecting a set of AP configurations from a list of candidate AP configurations is a location problem usually called ACP problem for Automatic Cell Planning in cellular system. In GSM or UMTS networks the coverage area relative to a transmitter is called a cell instead this is called a base station service (BSS) in WLAN. The second important stage is to allocate one of the available frequencies to each AP configuration in order to minimize interferences. The frequency set depends on the standard (IEEE 802.11 a, b or g) and also on specific restriction on spectrum usage in each country and environment. This problem is called AFP problem for Automatic Frequency Planning and becomes very famous for designing GSM/GPRS/EDGE cellular network [19] [20]. In this paper, we evaluate the difference of QoS between networks that have been design using ACP and AFP stages successively as in current strategies, and networks designed using ACP and AFP as a joint optimisation problem to optimise [17]. The main issue of this unified approach is the on-line computation of Signal-to-Interference-plus-Noise-Ratio (SINR) during the selection of site for installation of transmitters without additional constraints linked to frequency channel assignment. The direct estimation of SINR might drive the process to a better network design offering a larger throughput to network users. The paper is organised in three main sections. The second section focuses on AFP problem and gives some methods to solve it. The third section presents four optimisation strategies tackling ACP problem and AFP problem successively or together. In the fourth section, experimentations are presented to compare different strategies and those results are analysed. Finally, we draw the conclusions and give some future work. II. AUTOMATIC FREQUENCY PLANNING Usually the design process begins choosing antenna sites then allocating the available frequencies to the selected sites. The first studies on ACP problem were defined as a covering problem [1] [2] without link with AFP. Later, various constraints were added to the ACP problem in order to ease the AFP problem. The ACP problem became over constrained. A large variety of constraints are described in the literature. The most current constraint consists to add some cell overlapping to the covering problem. For example prohibiting the selection of two close sites [3] [4] or minimizing the overlapping area between cells [5] [6] [7] or selecting BSS according to its geometrical shape [16] as in cellular [18]. More sophisticated approach is to evaluate the deviation between interfering transmitter [8] [9]. Another approach is to estimate the capacity of channel frequency reuse [10]. Now we present two different approaches to tackle AFP problem: one global view focuses on interferences at cell level, another local view focuses on interferences at user level. Those methods are general for different wireless network contexts: GSM, UMTS, IEEE 802.11β¦ However this article focuses on IEEE 802.11g wireless networks in order to put in practice our approach. A. Global interference approaches The simplest approach of frequency planning is to consider each BSS like an indivisible entity. This approach considers the average of interferences inside the BSS. This global view has the great advantage of reducing BSS to single point. The network can then be represented as an undirected graph where vertices are BSS and edges connect pairs of BSS if they are neighbours that is their coverage areas are overlapping. In this case, the AFP problem becomes a constraint satisfaction graph colouring problem: frequencies are colours to assign to graph vertices or BSS. In this graph context there are several different approaches to use frequency channels in WLAN design: