Small-scale fading describes the rapid fluctuation of the signals over a short period of time or distance. On the other hand, large-scale shadowing represents a random effect which occurs over a large number of measurement locations which have the same distance between the transmitter and the receiver, but have different levels of obstacles on the propagation path. It is well-known that the log-normal shadowing propagation model captures this effect. The log-normal shadowing propagation model describes the random variation of the received power around the mean (nominal) value, and the power variation in decibel (dB) follows a normal distribution [16].Figure 1 illustrates the transmission ranges when the two-ray ground reflection model (left) and the log-normal shadowing propagation model (right) are used.

Under the deterministic channel model, transmission range of a node is circular for a given transmit power, as shown in the left figure in Figure 1. Under shadowing channels as in a typical wireless network environment, the transmission range is not circular anymore. As can be observed in the right figure, although Node B is within the mean transmission range of the center node (the dotted circle), it may not receive the center node’s transmission due to shadowing. On the other hand, although Node C is out of the mean transmission range of the center node, it can receive the center node’s transmission. The free space or two-ray ground reflection channels do not model the actual radio propagation precisely, and such inaccuracy may have a considerable impact on the MAC protocol performance since the set of one-hop neighbors is not deterministic anymore.

Such randomness caused by shadowing effects should be taken into account in the MAC protocol design to avoid potential collisions and leverage spacial reuse.Figure 1.Transmission ranges for the two-ray ground reflection model (left) and the shadowing model (right).Motivated by these observations, in this paper we study the problem of how to mitigate the exposed terminal problem in the presence of log-normal shadowing channels. We propose a location-assisted extension to the IEEE 802.11 MAC protocol for opportunistically scheduling ��concurrent�� transmissions in the neighborhood of a ��free�� transmission, i.e., the transmission between the two nodes that first win the channel with RTS/CTS handshake.

We assume Drug_discovery node location information as in many prior works (e.g., the class of geographic routing protocols [17, 18]). We assume that such location information can be obtained via the global positioning system (GPS) if such service is available, or by using an effective localization scheme proposed in the literature [19]. However, our main objective is to exploit location information for improved network-wide performance, while localization is not the focus of this paper.