SIR-based interference-maps for TDMA-based outdoor mesh networks

A TDMA-based MAC protocol is essential to achieve high throughput and low delay/jitter values, in WiFi mesh networks. An important component of such a network is the interference map, which quantifies the level of interference between pairs of links in the network. Past work in this domain has been relatively sparse, although many protocols and algorithms have been proposed which assume an interference map. In a TDMA network where carrier-sense is disabled, all interference shows up as hidden-terminal cases. Prior work on building interference maps has not focused on the hidden terminal case. Further, most prior evaluation has been in indoor settings. In this work, we present an SIR based interference measurement technique. We start by measuring the average RSS for each link in the network. This is subsequently used to get an estimated average SIR. The average SIR is then used in conjunction with an SIR-versus-PER plot, to estimate the link error rate under interference. We show through measurements on an outdoor 802.11b/g testbed, that the above estimate is accurate for (1) non-interfering links, as well as (2) fully interfering links, but is less accurate for (3) intermediate link pairs. The resultant 3-way classification of inter-link interference will be useful input for subsequent TDMA scheduling algorithms.