## 3 Design Process
### DAid(r) Smart Socks-Based Biofeedback System Parameter Selection
#### 3.3.5 Technical Innovations and System Architecture for Multi-User  
Biomechanical Feedback  
The DAid(r) smart socks-based biofeedback system tackles technical challenges through innovative design and engineering solutions. The study concept is to manage the training process for three soccer players at the same time, wearing the HMDs which means managing the training process of all the players from one PC. The system must handle high-frequency sensor data transmission from six smart socks simultaneously, ensuring that data are processed, analyzed, and displayed on a head-mounted display (HMD) within a latency of less than 100 ms. This is tackled both by data processing logic and a system architecture approach. This requires robust data processing and hardware capable of supporting such high throughput. The use of a layered system architecture enables modular and efficient handling of tasks. Therefore, in order to avoid the processing capacity issues, which would arise if the data processing and data transfer were performed in the HMD, all the data processing and data transfer logic were conducted on the PC (client and server sides of the program) and only a minimum of commands and data were sent to the HMD. The data processing module employs a real-time parser to normalize raw data, compute center of pressure (COP) values, and pass results into the MR application. Additionally, the adoption of Bluetooth for sensor data and Wi-Fi/TCP for HMD communication strikes a balance between bandwidth and latency. Simultaneously connecting and managing data from multiple Bluetooth channels for six socks while ensuring stable and interference-free connections in environments prone to congestion is complex. The system implements a dedicated BluetoothService to maintain robust connections, using reconnection logic to address potential disconnections. By optimizing the communication protocol and assigning specific Bluetooth addresses to each sensor, the system ensures seamless, concurrent data streams. Simultaneouslymanaging multiple Bluetooth connections for six smart socks in environments prone to interference and congestion requires innovative solutions. The system implements a multi-threaded communication management framework that assigns a dedicated thread to each Bluetooth connection. This framework incorporates a priority-based scheduling algorithm to prevent collisions, packet loss, or delays in data transmission. Additionally, an adaptive reconnection logic ensures that disconnections are handled seamlessly without impacting overall performance. For example, during exercises such as \"vertical jumps\", where the user has to jump as quickly as possible in different directions thus complicating the real-time feedback, the feedback uses adaptive reconnection logic, creating feedback out of data received in between the repetitions of the exercise. By leveraging these innovations, the system ensures reliable and interference-free communication across all six socks. Creating an immersive MR environment on the Meta Quest 3 HMD (Meta Platforms, Inc., Menlo Park, CA, USA) that visualizes real-time feedback while maintaining high-quality rendering and low latency is technically demanding. The HMD must seamlessly integrate with the application to provide an engaging user experience. The Meta Quest 3 headset was selected for its rendering capabilities, low latency, and high-resolution displays as well as pass-through to reduce motion sickness and controller-free functionality for freedom of movement which is crucial for soccer players. The MRCCommunicationService transforms sensor data into a format compatible with the HMD, enabling dynamic visualizations. This innovation allows real-time adjustments to be displayed in an interactive and visually engaging manner. The HMD application was developed to provide minimum processing but receive most of the needed calculations and feedback from the PC side. Therefore, it only receives commands, data, and shows the visualizations.