SUGGESTED SOLUTIONS FOR THE NEAR-FAR EFFECT IN MULTIMODE WLANS

A recent paper [1] has highlighted the near-far effect in IEEE 802.11b [2-3]. It has been observed that when some mobile node use a lower bit rate than the others, the performance of all nodes is considerably degraded. For example, a node transmitting at 1 Mbps reduces the throughput of all other nodes transmitting at 11 Mbps to a low value below 1 Mbps. Thus, the near- far effect can cause unfairness and significantly reduce the capacity of a cell. This problem has been illustrated with IEEE 802.11b. However, any multimode WLAN systems based on DCF [2], i.e., 802.11a/b/g, is affected in the same manner. In this paper, we provide a detailed analysis of the problem through simulations in the case of IEEE 802.11b. In order to mitigate the near- far effect, several solutions can be proposed: (a) adapt the packet size to the chosen physical (PHY) mode in case of uplink transmissions; (b) extend the infrastructure with a relay node; (c) adopt a centralized scheduling strategy; (d) adapt the future IEEE 802.11e standard. Some of these might need the modification of the standard. This paper addresses the two first solutions.