# NOMA for VLC Downlink Transmission with Random Receiver Orientation

## TL;DR

This paper explores NOMA in VLC downlink networks with users randomly changing position and orientation, proposing feedback schemes and user ordering techniques to improve spectral efficiency and reduce complexity.

## Contribution

It introduces practical feedback schemes using mean vertical angle and two-bit feedback for group-based user scheduling in NOMA VLC systems.

## Key findings

- Mean vertical angle feedback achieves near-optimal sum-rate.
- Two-bit feedback outperforms one-bit feedback.
- Analytical expressions match simulation results.

## Abstract

Visible light communications (VLC) is an emerging technology with a promise of viable solution to spectrum crunch problem in conventional radio frequency (RF) bands. In this work, we consider a downlink multiuser VLC network where users randomly change their location and vertical orientation. In order to increase the spectral efficiency, we consider the non-orthogonal multiple access (NOMA) transmission to serve multiple users simultaneously. In particular, we propose individual and group-based user ordering techniques for NOMA with various user feedback schemes. In order to reduce the computational complexity and link overhead, feedback on the channel quality is proposed to be computed using mean value of the vertical angle (instead of the exact instantaneous value), as well as the distance information. In addition, a two-bit feedback scheme is proposed for the group-based user scheduling, which relies on both the distance and vertical angle, and differs from the conventional one-bit feedback of the distance only. The outage probability and sum-rate expressions are derived analytically, which show a very good match with the simulation data. Numerical results verify that the practical feedback scheme with the mean vertical angle achieves a near-optimal sum-rate performance, and the two-bit feedback significantly outperforms the one-bit feedback.