# Optimal Design of Switchable Wearable Antenna Array for Wireless Sensor Networks
## 1 Introduction  
Wireless sensor networks consist of small measurement devices that can transmit data using integrated wireless modules. Such arrangements can utilize wireless sensors that are located directly on the human body or are embedded in the clothes. This makes it possible to remotely monitor physiological parameters or vital signs of people as well as their motion, body posture, location, etc. Networks of such wearable wireless sensors can find numerous applications in healthcare, sports, or public safety because they unobtrusively gather data [1, 2]. The critical feature of a wearable wireless sensor network is the size of the node that should be small enough not to limit the comfort of use. This requires small batteries that suffer from limited energy resources, which, as a consequence, reduces the maximum transmit power of the wearable nodes. For this reason, the power budget of the wireless link is very tight, and the separation of signal from interference is relatively small. Wireless sensor that works in the proximity of the human body utilizes an antenna that operates in the complex environment consisting of open space, human tissues, and clothes. The antenna radiation pattern that presents the angular distribution of gain is influenced by the body and is also sensitive to the placement of antenna [3]. In general, the antenna that is located close to the body (it is a so-called wearable antenna) has the radiation pattern with a wide main lobe, and its minima are directed towardsthe body. In such a case, for a wide range of angles, the antenna has almost constant gain. Wireless sensor networks that operate in the off-body mode transmit a signal between the remote transceiver that is located far from the body and the wearable sensor. When the wearable antenna radiation pattern has a wide main lobe, the useful signal, as well as the interference, can be received from different directions with a similar antenna gain and with similar power levels. Then, the signal-to-interference ratio (SIR) is low, which is unfavorable (see Figure 1a). Several approaches to the improvement of SIR are known [4; 5; 6; 7; 8], mainly relying on optimum antenna design or simple switchable/reconfigurable antennas. However, to our best knowledge, on-body phased arrays have not been used to achieve this goal yet. A phased array antenna consists of a set of elementary antennas organized in a particular geometrical arrangement: linear, planar, circular, cylindrical, etc. The signal from the transceiver is connected through the combiner and feedlines to each elementary antenna. The radiation pattern of the array for a given number of elementary antennas and their spatial arrangement depends on the phase relationship at the antenna feed points, which follows from the phase shift in each elementary antenna feeding path. Signal-to-interference ratio (SIR) can be improved by forming the radiation pattern to direct the maximum gain of the array towards the signal and the minimum towards the interference [6; 9] (see Figure 1b)--with constraints on the radiation pattern following from the array design. Phase switching can be automatically performed during system operation to improve the quality of transmission. There have been numerous studies into the problem of circular antenna array optimization, e.g., [10; 11; 12; 13; 14; 15; 16]. In the case of uniform arrays [10; 13; 14], beamforming is controlled by the adjustment of amplitudes and phases in the feeding network of elementary antennas. Additionally, for non-uniform arrays [11; 12; 15; 16] angular positions of the antennas can be adjusted, which provides even more degrees of freedom to the optimization algorithm and yields greater flexibility of the directivity pattern shaping. Among many algorithms successfully applied for antenna array optimization, particle swarm optimization [10; 12; 13] and genetic algorithms [11; 14] are often used.