# Virtual Sensing Directional Hub MAC (VSDH-MAC) Protocol with Power Control
## 1 Introduction  
Wireless sensor networks (WSNs) have been employed in a wide range of health care, industrial and environmental monitoring applications [1; 2; 3]. For many applications, the use of batteries in sensor nodes places constraints on the energy budget, so it is important to maximize the performance of the network whilst minimizing the sensor node energy consumption. Interference, lack of fairness and energy consumption are the key constraints in WSNs, which poses challenges to the design of medium access control (MAC) protocols. Directional antennas provide the potential to increase transmission range and/or reduce transmission power, to reduce interference along with the prospect of allowing spatial reuse. In order to make the best use of directional antennas, suitable MAC protocols must be designed. On selection of suitable MAC protocols for WSNs, one could consider either contention-based or contention-free protocols. Contention-based protocols can be less efficient than those without contention in terms of throughput performance for large star topologies due to the large number of collisions when the data traffic offered load is high. However, they are simpler and typically provide lower delay in smaller WSNs [4]. Contention-based protocols are a promising approach for directional MAC protocols, as they enable multiple nodes to simultaneously access a channel without the need for synchronization. Scheduling and synchronization are the main challenges for contention-free protocols, especially for WSNs with mobile nodes and/or a varying number of nodes. In [5], we proposed a simple directional hub MAC protocol for star topology WSNs based on the Pure Aloha protocol, in which the performance differences between a realistic directional antenna pattern and idealized directional antenna pattern were demonstrated. It is shown that although directional antennas can provide high throughput performance, the antenna pattern may still have a significant effect on spatial reuse and network performance. In this proposed protocol, node complexity and power consumption are minimized by having only a single omnidirectional antenna on the basic sensor nodes. The hub carries multiple directional antennas and can be continually powered as its complexity and power consumption are not considered critical compared with the basic sensor nodes. Energy consumption and fairness were considered in [6], in which a MAC protocol with transmit power control on nodes was analyzed. The performance enhancement that can be achieved by the use of power control and directional hub antennas was demonstrated, in terms of network throughput, node power consumption, and fairness. Most previous works on the use of directional antennas have assumed the use of idealized antenna patterns where each antenna beam is distinct, with no overlap with adjacent beams and having a constant antenna gain across the beam [7; 8; 9; 10; 11; 12; 13; 14; 15]. Some work has assumed that the nodes are capable of knowing each other's position [7; 9; 10; 11] or that nodes have complex, steerable antennas [7; 12]. In addition, multiple antennas are often required at the nodes as well as the hub [7; 8; 9; 10; 11; 12; 13; 16] which increases both the complexity and energy consumption of the nodes. Some of the protocols proposed also require multiple channels to successfully operate [8; 13; 16]. Only a few papers within the literature [14; 17; 18; 19] have considered the energy consumption of the protocol, which is an important factor for low power nodes [20]. In this paper, a modified directional CSMA/CA (carrier sense multiple access with collision avoidance) protocol is proposed, which is similar to the IEEE 802.11 WI-FI standard and the IEEE 802.15.4 standard for WSNs. A version without the traditional physical carrier sensing is used (to reduce energy consumption). Here virtual carrier sensing is performed via handshaking packets. A version with physical carrier sensing similar to the CSMA/CA protocol is also considered for comparison.