## 2 Materials and Methods
### WSA Firmware  
1. **Operating Modes:** The firmware configuration defines the device's operating mode, addresses, radio channel, and other parameters. WSA devices communicate in master-slave mode, i.e., two main modes of operation are supported. The device (Master Unit) connected to the Processing Unit operates in Master mode. The main task of the Master Unit is to control the remote devices, acquire and forward user data from the remote devices to the Processing Unit. The remote device IMU operates in slave mode and responds to the master's instructions. Its main task is to acquire the data from the built-in inertial sensor and send it to the master. 2. **WSA Schedule:** Figure 4 shows the main operations of the WSA setup (master and five remote IMU devices) in a single 10-ms sampling cycle (5). Before the acquisition is triggered, the remote devices are in \"listening mode\", while the master device switches the radio from idle to transmitting the synchronization packet (SYNC TX) (3). All the remote units receive the synchronization packet (SYNC RX) simultaneously, start the preconfigured delay timer, and acquire data from the inertial sensor (5). After the delay timer of the first remote device expires (delay 5 ms), the payload is transmitted (IMU 1 data TX). The master device receives the first packet (IMU1 data RX) and waits for the next one. The process repeats for the remaining remote devices. At the end of the 10-ms time frame, the Master sends the acquired data via a USB for processing. The delay timers are configured to transmit the data after the second half of the 10-ms time frame. 3. **Sampling protocol--time-slot division, master side:** The protocol for sampling sensors and communication between the WSA devices uses the simple principle of time-slot division. Each remote device was assigned its own time slot when responding with the sensor payload. To begin sampling, the master device sends \"wake-up\" packets via the USB port, whereupon the master's loop timer is started. Figure 4: WSA tasks sequences in a single 10-ms sample cycle. The loop timer in the master device sends synchronization packets at intervals that are multiples of 10 ms. The synchronization interval is user configurable and was set to 30 ms.
4. **Sampling Protocol--Time-Slot Splitting, IMU side:** upon successful reception of the \"wake up\" instruction by the remote devices, the device is woken up and placed in \"listening mode\" to receive further remote instructions. In \"listening mode\" the radio receiver is on and the unit consumes most of the battery power. If no command is received within 10 s, the unit will enter power-save mode. 5. **Timer:** Receipt of the synchronization packet by the remote devices synchronizes two timers and reads the data from the sensor IMU. One timer is used for loop tasks and the other is a delay timer. The loop timer repeats the sensor sampling every 10 ms until the next synchronization packet is expected, at which point the device enters \"listening mode\". If communication with the master device is interrupted for any reason and the remote device does not receive a sync packet for 60 s, it will enter power-saving mode. The delay timer has two functions. One is to ensure that the devices send the sensor payloads one after another with a delay to avoid collisions between the radio signals, since they are all on the same radio channel. Each remote device has a different delay set before transmission. The other timer is responsible for the energy efficiency of the device. 6. **Before transmission:** Before transmission, the receiver is put into sleep mode by receiving the synchronization packet to reduce the power consumption. The sensor data is acquired from IMU and the user data is prepared for transmission. 7.