A comparative performance study of different low power wireless protocols for event-driven sEMG acquisition system

Nowadays, wireless systems are more and more important in biomedical world, both to eliminate cables encumbrance as well as integration of embedded systems inside Internet of Things (IoT) world. In days where electronic health and fitness monitoring, as well as telemedicine are well-studied topics, the need for a fast, reliable and at low power consumption wireless protocol is of extreme importance. This master thesis project aims to analyze and characterize performances of different wireless communication protocols, both already existing and not, to send sEMG (surface Electromyography) data acquired with custom PCB boards using classical front-end electronics together with Average Threshold Technique (ATC). Every time muscular signal overcomes a fixed pre-defined threshold, a digital events is made available: in this way both power consumption, as well as circuitry complexity is reduced, due to lower data size acquired. Protocols studied, implemented, and tested are Bluetooth Low Energy, IEEE 802.15.4, and a custom protocol, tailor-made for this application. The proposed system is composed of two nRF-52840 Development Kit(s), which simulate transmitter and receiver side: in particular, the former has the task to collect events inside a time window, assigning each of them with a relative timestamps, and at the end send the resulting packet to the receiver side, in which further processing (as gesture recognition) or ATC reconstruction could be performed. After implementing them, protocols have been tested using lab instrumentation, to measure both power consumption, throughput and delay between transmission and reception: all protocols behave well in terms of transmission reliability using 1.5 m distance between transmitter and receiver. Features mentioned above, are used to compare protocols and choose the best one. Considering throughput, all protocols show promising results, suitable for this application: BLE shows 215 kbits−1 versus 116 kbits−1 for IEEE 802.15.4 and 322 kbits−1 for custom protocol. The delay between transmission and reception is a key concept in order to allow for a real-time system: over all performed trials BLE shows a latency of 2.5 ms but with a high variability due to stack used, versus 4.5 ± 0.02 ms for IEEE 802.15.4 and 800 ± 10µs for custom protocol. Results from energy consumption, important for battery life, list here refer to one single transmission event: 38µC for BLE versus 125µC for IEEE 802.15.4 and 32 µC for custom protocol. Considering the above results, if we exclude the need for standardization, in which case BLE shows better performance than IEEE 802.15.4, it is clear that the new proposed protocol is the best among all.