Terminal Orientation in OFDM-Based LiFi Systems

Light-fidelity (LiFi) is a wireless communication technology that employs both infrared and visible light spectra to support multiuser access and user mobility. Considering the small wavelength of light, the optical channel is affected by the random orientation of user equipment (TIE). In this paper, a random process model for changes in the TIE orientation is proposed based on the data measurements. We show that the coherence time of the random orientation is in the order of hundreds of milliseconds. Therefore, an indoor optical wireless channel can be treated as a slowly varying channel as its delay spread is typically in the order of nanoseconds. A study of the orientation model on the performance of direct-current-biased orthogonal frequency-division multiplexing (DC-OFDM) is also presented. The performance analysis of the DC-OFDM system incorporates the effect of a diffuse link due to reflection and blockage by the user. The results show that the diffuse link and the blockage have significant effects, especially if the TIE is located relatively far away from an access point (AP). It is shown that the effect is notable if the horizontal distance between the TIE and the AP is greater than 1.5 m in a typical 5 × 3.5 × 3 m3indoor room.