# Multi-Objective Optimization of a Wireless Body Area Network for Varying Body Positions
## 2 Wireless Body Area Sensor Network  
The WBAN analyzed in this research, which we designed originally for monitoring the vital signs of firefighters, consists of miniature nodes which use transceivers operating in the 2.4 GHz ISM (Industrial, Scientific, and Medical) band. The studied network consists of three nodes which measure certain physiological parameters of the human body and transmit the data both to each other (on-body channels) and to a remote node located away from the body (off-body channel). In our computer simulations, all nodes use half-wave dipole antennas tuned to 2.4 GHz. We designed dipoles with shortened geometry to match the antenna port impedance to 50 \(\Omega\), which resulted in a total length of 54 mm, a conductor diameter of 1 mm, and a feeding point gap of 1 mm. One node (connected to antenna 1; see Figure 1) contains a pulse-oximeter to measure blood oxidation and heart rate. The sensor is attached to the ear-lobe and the node, therefore, has to be located close to the ear. It was assumed that this node has a fixed position on the left side of the head. Node 1 has a low-power transceiver and a miniature battery, which enables data transmission only to the two other nodes located on the user's trunk, constituting an on-body network. The trunk node, located on the user's chest, measures the respiratory rate as well as the temperature and humidity of the firefighter's clothes (antenna 2 in Figure 1). Another node performs the function of a pedometer and is placed on the user's calf (antenna 3 in Figure 1). Both the trunk and leg nodes can receive data from the pulse-oximeter and from each other (on-body transmission). Nodes 2 and 3 are also capable of transmitting measurement data to the remote receiver (off-body transmission). Figure 2 presents the off-body transmission scenario. The received power in this link depends strongly on the wearable antenna gain, especially in an outdoor environment, in which the multipath effect is very small [6]. Due to the influence of the human body on the wearable antenna radiation pattern for a wide range of radiation angles, the antenna gain can be very low. Depending on the position of the antenna on the human body, the antenna gain can be lower than \(-10\) dBi for a sector wider than \(100^{\circ}\) in the horizontal plane [20]. When the propagation path between the human body and the remote receiver falls into the low gain zone of the antenna radiation pattern, the power of the received signal can drop below the receiver's sensitivity. Figure 2 also presents the differences in the radiation patterns of the wearable antennas. Antenna 2 is located on the right arm and antenna 3 is placed on the left side of the calf. In this case, the radiation of antenna 2 (with gain G\({}_{2}\)) presented in Figure 2a covers the right side of the body with very low gain on the left side, while the radiation of antenna 3 presented in Figure 2b (with gain G\({}_{3}\)) is directed to the left side and does not cover the right side of the body completely. In the proposed system, both transceivers 2 and 3 operate at the same frequency, but different time slots are utilized for transmitting the measurement data from the body to the remote receiver in the off-body link. The remote receiver can select the stronger signal of the two that come from nodes 2 and 3, as presented in Figure 2c. This two-antenna transmission scenario improves the off-body communication performance because the minima of the radiation pattern of one antenna can be filled in by the radiation from the other antenna, depending on the positions of the antennas on the human body. In Figure 2c, the gain of the wearable antennas of transceivers 2 and 3 operating in different time slots is defined as G\({}_{23}(\phi)=sup(\)G\({}_{2}(\phi)\), G\({}_{3}(\phi))\). The resultant gain G\({}_{23}\) is equal to the greater of the  
Figure 1: The wireless body area sensor network on the simplified cylindrical human body model.