Algorithm 1: Walking speed enhancement.

Input: hallway width L            acoustic speed c  Output: User walking speed $v_{user}$ 1 Emit $f_e$ kHz sub-ultrasonic; 2 Receive $f_r$ kHz sub-ultrasonic; 3 $\mathsf{\Delta }v=c\times \frac{f_r-f_e}{f_e}$; 4 $D\leftarrow$ sub-ultrasonic TDOA method; 5 $sin\theta =\frac{L}{D}$; 6 The velocity component of $v_{encounter}$ on DopE direction; 7 $v_{encounter}^{\prime }=v_{encounter}\times \sqrt{1-{sin\theta }^2}$; 8 The velocity component of $v_{user}$ on DopE direction; 9 $v_{user}^{\prime }=\mathsf{\Delta }v-v_{encounter}^{\prime }$; 10 return $v_{user}$;