Reinvestigating Channel Orthogonality - Adjacent Channel Interference in IEEE 802.11n Networks

In this paper we analyze the adverse effects of Adjacent Channel Interference (ACI) on 802.11 with a focus on new 802.11n standard. ACI is causing problems that are related to the carrier sensing mechanism in 802.11. On the one hand, the carrier sensing is sometimes too restrictive thus preventing concurrent transmissions which leads to a variant of the exposed terminal problem. On the other hand, the carrier sensing is sometimes too optimistic thus causing packet collisions which is a form of the hidden node problem. Both problems are especially severe in multi-radio systems, where the radios are very closely spaced. Such problems already investigated in 802.11a/b/g still remain with 802.11n. Our results show that the number of available orthogonal channels in IEEE 802.11n depends on the spatial spacing between the radios, channel width (HT20 vs. HT40), RF band (2.4 vs. 5 GHz) and traffic pattern. In a multiradio system the situation is worst, e.g. in the 2.4 GHz we were not able to find more than 1 orthogonal channel. The adverse effect of ACI can be reduced in two ways. First, by increasing the spatial separation between the radios; a spacing of less than 1 meter already improves the situation significantly, e.g. 40 cm are sufficient to get 2-3 orthogonal 20 MHz channels in the 2.4 GHz band with reduced transmission power. Furthermore, a distance of 90 cm is also sufficient so that a 40 and a 20 MHz channel can be used simultaneously without any interference. However, in a multi-radio system the spatial spacing between the radios cannot be increased due to space limitations. The only option to overcome ACI related problems is to reduce the transmit power making power control essential. Finally, our analysis revealed that 802.11 is an inappropriate protocol for multi-channel MAC/routing protocols based on multi-radio systems where an explicit MAC layer link-scheduling is more promising.