“…The frequency of background light is different from the transmitted signal and can be ignored during demodulation. Supposing that the LEDs have a Lambertian radiation pattern, when only the line-of-sight (LOS) channel is considered, the distance between an LED and the receiver can be estimated by measuring the received signal power, which can be expressed by [12]. Pri=Ar(m+1)2πdi2cosmfalse(ϕifalse)cosfalse(φifalse)Ttransg(φi)Pti,i=1,2,3,4 where Pri(i=1,2,3,4) is the received signal power of the four LEDs, Pt is the transmitted signal power, Ar is the physical area of the detector, d is the distance between the LED and the receiver, ϕ is the angle of irradiance, φ is the angle of incidence, Ttrans is the optical transmittance, g(φ...…”