imaginary straight line between AP1 and AP2, such that
y1 and y2 both have their RSS values from AP1 larger than
those from AP2. To enable the discriminativeness, we try
to involve more APs in the comparison. It is possible to
show that we can have a high-order comparison feature that
is robust and discriminative, but due to space limitation we
skip the details here. Finally, because we cannot trivially
enumerate all the possible combinations of multiple APs
(which is exponential to the number of APs), we propose
to automatically learn