Coverage investigations for adaptive modulation in 5GHz WLANs

Transmission in orthogonal frequency division multiplex (OFDM) systems is realized with the help of orthogonal sub-carriers, broadband wireless radio networks, like ETSI hiper-LAN [ETSI BRAN, April 2000.] or IEEE 802.11a [IEEE Std 802.11a, 1999.] apply OFDM to realize their high data-rates. Thereby, the data to be transmitted are equally distributed onto the respective sub-carriers. However, due to multipath propagation, the radio channel shows a frequency selective behavior, which results in a different reception quality for each sub-carrier. As long as the channel properties do not change too fast over time, it makes sense to distribute the amount of transmitted data differently on each sub-carrier depending on the channel quality per sub-carrier i.e. the signal to noise ratio per sub-carrier. The aforementioned technique, called adaptive modulation, thus is helpful to achieve better performance results in terms of e.g. packet error ratio as long as the channel is not too dynamic or in other words as long as a terminal´s velocity is not too high. In this paper, investigations on enhanced coverage i.e. transmission range when applying adaptive modulation are presented and compared to ´ordinary´ OFDM transmission.