# Evaluation of BLE Star Network for Wireless Wearable Prosthesis/Orthosis Controller +
## 5 Discussion
In general, the greatest power consumption is due to the operation of the BLE antenna, which is only active during a connection event. For low- or battery-powered applications, using a longer connection interval--if latency requirements allow--can minimize the system power consumption, as the BLE radio is less frequently activated. Prosthesis control applications, however, demand low transmission latency. Our measurements of the instantaneous maximum power supply current (again measured from the entire Adafruit Feather board) reached almost 50 mA, which was an order of magnitude higher than the average currents. In a system that is field ready, some of these functions may be modified or eliminated, which may result in a lower maximum power consumption. The normalized latency measurements showed that the time delay between when the input signal was applied and detected on the peripheral follows a uniform-shaped distribution, with a mean of 0.54 connection intervals and standard deviation of 0.27 connection intervals. This finding is consistent with the step input being manually triggered; thus, its time of arrival within the peripheral node ADC data block was equally likely at each time within the connection interval. Once queued for transmission in the peripheral node, the transmit time is unsynchronized with ADC data block completion but occurs within a connection interval. Thus, again, this delay is equally likely from the 0 to 1 connection interval. Hence, the measurements of overall time delay from when the step occurred on the peripheral to when it was detected in the received BLE data on the central node showed a triangle-like distribution with a mean \(\pm\) std. dev. of 1.14 \(\pm\) 0.46 connection intervals, consistent with the sum of two independent and uniform delay processes [34]. Some small additional delays were observed between each stage in the data flow, as expected, to account for intermediate processing of the data, data queuing, the BLE interframe space, etc. Additional measurements could help refine the shape of the latency distributions. When looking at the normalized latencies, on average, a little over one connection interval (maximum of just over two connection intervals) elapses between when the step was applied on the peripheral and when it was detected on the central node. In real-time applications, lower latencies are best, which are facilitated by shorter connection intervals. For prosthesis controllers, connection intervals well below 100 ms--which leave as much time as possible for EMG smoothing--should be used to avoid long delays for the user. This choice of connection interval may not result in the lowest power consumption and may limit the number of simultaneous connections that can be sustained. Hence, a trade-off must be made. Additional work can be completed to investigate methods that can be used to improve the performance of the biosensor system in terms of power consumption, maximum network size, and latency. With respect to power consumption, reductions may be achieved using power-saving techniques such as the use of low power modes, disabling of functions on the Adafruit feather board, power cycling schemes, or BLE version 5.3's connection subrating feature [35]. Power cycling schemes minimize the average power consumption during times of inactivity, which results in a lower average current consumption. Introduced in BLE version 5.3, the connection subrating feature allows for quick and dynamic modification from long to short BLE connection intervals. The connection subrating feature can be used to quickly reduce the data throughput during times of inactivity to reduce power consumption. To increase the number of connections that can be sustained over distance, future work should consider increasing the transmission strength using the BLE Power Control feature. Introduced in BLE version 5.2, the Power Control feature modifies the transmit power based on the received signal strength, which helps to maintain quality communication between connected devices [36].