  In this paper, we consider a light fidelity (LiFi)-enabled bidirectional
Internet of Things (IoT) communication system, where visible light and infrared
light are used in the downlink and uplink, respectively. In order to improve
the energy efficiency (EE) of the bidirectional LiFi-IoT system, non-orthogonal
multiple access (NOMA) with a quality-of-service (QoS)-guaranteed optimal power
allocation (OPA) strategy is applied to maximize the EE of the system. We
derive a closed-form OPA set based on the identification of the optimal
decoding orders in both downlink and uplink channels, which can enable
low-complexity power allocation. Moreover, we propose an adaptive channel and
QoS-based user pairing approach by jointly considering users' channel gains and
QoS requirements. We further analyze the EE of the bidirectional LiFi-IoT
system and the user outage probabilities (UOPs) of both downlink and uplink
channels of the system. Extensive analytical and simulation results demonstrate
the superiority of NOMA with OPA in comparison to orthogonal multiple access
(OMA) and NOMA with typical channel-based power allocation strategies. It is
also shown that the proposed adaptive channel and QoS-based user pairing
approach greatly outperforms individual channel/QoS-based approaches,
especially when users have diverse QoS requirements.
