## 2 Directional and Omnidirectional Antennas--A Brief Comparison  
Traditionally, communication inside WSNs is done using omnidirectional antennas which broadcast radio signal almost uniformly in all directions. Omnidirectional antennas are small, inexpensive and simply to deploy, but they suffer from poor spatial reuse, high collisions, reduced energy efficiency and are susceptible to security attacks [4; 5]. A relevant example of omnidirectional antennas is a simple dipole, having the radiation pattern depicted in Figure 1.  
If the mentioned drawbacks dramatically affect the normal WSN operation and security, wireless nodes can be equipped with directional antennas either alone or in conjunction with existing omnidirectional antennas [6]. A directional antenna, also known as beam antenna, is the type of antenna which emits or receives greater power in a particular direction (Figure 2) [7; 8]. By focusing its radiation pattern in a specific direction, they reduce the interferences and collisions, increase the gain and enhance the security against eavesdropping, jamming or other malicious attacks.  
A brief comparison between the two types of antennas [9] is provided in Table 1, highlighting three aspects of practical interest for directional antennas usage in future WSN technologies:  
Figure 1: Omnidirectional radiation pattern (dipole antenna).  
Figure 2: Directional radiation pattern for the binomial array antenna.  
* Improved energy consumption; the wireless data transmission is proved to be the most energy-intensive operation of a sensor node [10; 11]. By focusing their transmitted power in the needed direction, directional antennas have the potential to reduce the energy usage [12] and therefore to extend sensor nodes' lifetime.
* Longer transmission range; reporting information inside WSNs using fewer hops [13; 14] or reducing the risk for nodes or groups of nodes to become isolated (due to malfunctions, battery depletion or malicious attacks) [15] can significantly improve the WSN performance, when using directional antennas.
* Higher security; derived either from their immunity to eavesdroppers [16] or jammers [17] placed outside their narrow radiation region or from the feature of determining the exact position of a sender node using the signal's angle of arrival [18], the directional antennas can mitigate the risk of security attacks.  
Directional antennas are generally constructed by combining simple antenna elements (e.g., dipoles) into antenna arrays. Their overall radiation patterns are influenced by the type, the number and the geometrical configuration of the elements and also by the characteristics of the signal applied to each element. There are basically two types of directional antennas: traditional directed antennas and smart antennas. Traditional directed antennas [19] (e.g., Yagi-Uda, helix, aperture horn, reflector, patch antennas, _etc._) have a fixed beam that can be oriented in the desired direction by mechanical rotation. Smart antenna [20] is a generic name that describes an antenna array endowed with digital signal processing techniques, which automatically optimize its radiation/reception pattern. Smart antennas can be classified [21; 22] as either switched beam or adaptive array systems. A switched beam antenna [23] can generate multiple fixed beams, automatically switching from one beam to another every time when needed. The second type of smart antennas--adaptive array systems [24]--possess the ability to actively locate and track desired signal in order to dynamically mitigate interferences, optimizing the signal reception.