# Virtual Sensing Directional Hub MAC (VSDH-MAC) Protocol with Power Control
## 2 Medium Access Control Protocol
#### VSDH-MAC Channel Access Algorithm
In addition, an increased number of antennas would require a narrower beam width per antenna, which implies increased gain in most cases; this would reduce the required transmission power for both the nodes and the hub in a given scenario. ## 4 Results Analysis

The offered load is distributed evenly across all of the sensor nodes. The network throughput is the channel capacity successfully used by all sensor nodes with the maximum of \(M\) Erlangs, in which \(M\) is the number of directional antennas equipped at the hub. For the purpose of understanding the link performance of the protocol, the results will be expressed as the total number of data bits successfully received per unit time. Figure 5 shows the throughput of directional hub Aloha (DH-Aloha) protocol [5] averaged over 10 randomly generated networks for each of the three directional antenna types. As predicted in our previous work [5], the antenna pattern has a significant effect on throughput. The idealized antenna pattern with no overlap between sectors, shows a substantially larger throughput than can be achieved with the real antennas with patterns that have some overlap. As depicted in Figure 4, due to the shape of the antenna pattern, the reason that Ant 1 has a higher throughput than Ant 2 is due to the fact than although Ant 2 has a narrower beam-width, it has a larger back lobe. Using the analysis in [5], the back lobe increases the overlapping ratio (\(r\)), as \(\theta_{A}\) increases. This results in more packet collisions caused by interference from antenna patterns overlapping. In Figure 6, the throughput of the VSDH-MAC protocol for Ant 1 and Ant 2 is presented. The difference between the throughputs of the two antennas are significantly smaller than in Figure 5. This is because the power control mechanism reduces the effect of antenna pattern overlap by adjusting the node transmission power. The adjusted transmission power reduces the interference caused by the back lobe. Figure 5: Throughput comparison of different antenna patterns with the DH-Aloha protocol with \(M=4\). It is useful to compare the throughput performance of the VSDH-MAC and DIFS-VSDH-MAC protocols with other directional MAC protocols. Two other directional MAC protocols have been replicated for the purpose of performance comparison. Both protocols have been replicated with the parameters described in Section 3. In the DMAC (directional virtual carrier sensing MAC) protocol proposed in [24], the simulation results showed a saturation throughput of 0.225 Mbps with the same simulation setup as in Section 3. However, it is assumed that all nodes are equipped with beamforming directional antennas and global positioning system (GPS). The DMAC protocol is based on the IEEE 802.11 standard and nodes are required to perform channel sensing prior to transmission. The use of GPS and channel sensing has significantly increased the node energy consumption. With idealized antenna patterns, the VSDH-MAC and DIFS-VSDH-MAC protocols can achieve a saturated throughput of 0.336 Mbps and 0.426 Mbps, which is approximately a factor of 1.5 or 1.9 higher than the saturated throughput of DMAC. The CMDMAC (cooperative multichannel directional MAC) is a similar protocol that requires multiple channels [25]. It requires one radio channel for control packets such as RTS, CTS and ACK, and a second radio channel for data transmission. Our simulation results indicate a saturated throughput of 0.45 Mbps from the CMDMAC protocol with the same simulation setup as in Section 3 and ideal directional antennas. All sensor nodes must be equipped with an omni-directional antenna for overhead communication and an idealized steerable directional antenna for data transmission. The additional channel provides continuous channel sensing which provides advantages in terms of throughout performance.