## 2 Materials and Methods
### Architecture  
The overall architecture of the smart lifejacket is presented in Figure 1. A Wi-Fi-and Bluetooth-capable microcontroller unit lies at the center of the system. Various peripherals are connected to it, providing localization, NFC, the actuators of the haptic navigation system and an IMU. The schematics are included in the Figures 1 and 2.  
A microcontroller unit was required to host the main program and communicate with the required peripherals. It was important to keep the cost low by having multiple interfaces  
Figure 1: Architecture of the smart lifejacket.  
for on-board communication with the various components. With that in mind, a device [27] was selected which had both Wi-Fi and Bluetooth wireless interface integrated, both for cost reduction and to simplify and facilitate the design process. With these two interfaces, it became possible to connect to the ship's central system, responsible for recording the location data and providing the route on request, as well as also any peripheral devices the passenger could have in order to gather additional information.  
An ultra-wideband transceiver module [28] was selected to carry out the indoor localization measurement. It contains firmware which abstracts the basic calculations and provides a final three-axis position to the MCU via UART. This technology was selected due to its properties which make it more suited for indoor localization than any alternatives, such as Bluetooth. The module has an integrated antenna and relies on preinstalled transceivers or anchors, which each have a known location for calculating to allow for position estimation in a coordinate system.  
An advanced 3-axial 14-bit inertial motor unit was selected [29] and connected to the MCU via I\({}^{2}\)C. It integrates multiple sensors and performs the fusion on-chip, thus reducing the load of the MCU and simplifying the development procedure. This module provides additional information to the smart lifejacket to help offset any measurement errors during the navigation procedure.  
A low-power magnetic switch is responsible for activating the smart lifejacket. While the lifejackets are stored, the switch detects the magnetic field of a magnet, also located on the smart lifejacket, thus keeping the battery circuit open. When the lifejacket is removed from storage and unfolded, then the circuit is closed and the device is powered on. This is the simplest way to power on the device from an end-user perspective, instead of a traditional mechanical switch that the user has to remember to turn on.  
A Near-Field Communication (NFC) reader module based on the PN532 controller [30] was integrated on the lifejacket to allow for pairing with peripheral devices. The NFC controller communicates with the MCU via the I\({}^{2}\)C bus and utilizes an external antenna. The antenna is connected to the rest of the PCB via a cable to allow for placement independent of the PCB. In Figure 2, the test fixture used to tune the antenna along with the simulation and measurement results is depicted. The antenna is tuned at a slightly higher frequency than the 13.56 MHz of NFC to compensate for the capacitance that will be added by the human body.  
Figure 2: (**a**) Measured return loss of the antenna with the matching circuit. (**b**) NFC antenna and matching circuit test fixture. (**c**) Simulated impedance seen at the input of the matching circuit and return loss. (**d**) Equivalent circuit of the matching network and the antenna.  
Additional components included a low-profile metal push-button to be used for activating the navigation functionality, as well as two vibration motors, left and right, to help navigate the passenger through the route by means of haptic indications.