Energy- and Spectral- Efficiency Tradeoff in Nonlinear OFDM System of Visible Light Communications

light-emitting diode (LED) based visible light communication (VLC) provides a promising solution to enhance the speed and data rate for 5G and beyond, however, the nonlinear characteristic of LED that VLC greatly suffers from can lower the output power, hence causing degradation in the transmission rate. In this paper, the direct current (DC) biased optical orthogonal frequency division multiplexing (DCO-OFDM) is employed to investigate the nonlinear effect of LED on the tradeoff between energy efficiency (EE) and spectral efficiency (SE). The Rapps model is adopted to describe the nonlinear characteristic of commercial LED (OSRAM, LD-W5AM) and predistortion compensation algorithms are introduced to evaluate bit error rate (BER) affected by parameters of the LED. Based on which, the general closed-form expressions of EE-SE (ESE) tradeoff related to different system parameters under indoor VLC geometric scenario are derived. Next, effects of parameters in VLC system on the ESE tradeoff, such as the half power angle (HPA) of the LED and the effective area (EA) of the photodetector (PD), are discussed. Simulation results of the ESE tradeoff trend with different system parameters are provided, which are in well agreement with the theoretical derivation. This provides a good guideline on determining ESE-efficient parameters of the nonlinear DCO-OFDM VLC system.