Table 1. List of instantaneous features extracted of interest to system.

(* indicates feature used.)

Features		Description
1.	Frontal acceleration (af)*	Dorsoventral axis acceleration values in g (i.e. range −1:1)
2.	Vertical acceleration (av)*	Anteroposterior axis acceleration values in g
3.	Lateral acceleration (al)	Left-right axis acceleration values in g
4.	Sine of body tilting angle (pitch)*	Sine of body tilting angle (θ) towards the front or back with respect to vertical in the midsagittal plane [26], approximated as: $sin(arctan\left(\frac{a_f}{a_v}\right))$
5.	Received signal power (RSSI)*	Received signal power from the sensor as received at reading antenna. RSSI is calculated as: $P_t{G}_t^2{G}_{path}^{2}K$ where Pt is the output power of the RFID reader, Gt is the RFID antenna gain, K is the W2ISP backscatter gain and Gpath is the one-way path gain of the deterministic multipath channel [44] which is inversely proportional to the square of the direct path distance (i.e. $d_0^2$). Hence, RSSI is inversely proportional to $d_0^4$.
6.	Resultant acceleration*	Magnitude of acceleration vector given by $a_t=\sqrt{a_f^2+a_v^2+a_l^2}$.
7.	Time difference Δt	Time difference with previous sensor observation (regardless of receiving antenna).
8.	Participant’s gender*
9.	Trunk yaw angle*	Rotational angle from dorsoventral axis, approximated as: $\beta \approx arctan\left(\frac{a_l}{a_f}\right)$
10.	Trunk roll angle*	Tilting angle in the coronal plane, approximated as: $\alpha \approx arctan\left(\frac{a_l}{a_v}\right)$
11.	Antenna ID (aID)	RFID antenna receiving current tag reading.