Abstract:
Light fidelity (LiFi) is a means of high speed wireless data transmission along with room illumination using the existing light emitting diode (LED) infrastructure. As a data encoder for LiFi, different variants of orthogonal frequency division multiplexing (OFDM), such as direct current biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM) and asymmetrically, and symmetrically clipped optical OFDM (ASCO-OFDM) have been considered. However, the choice of appropriate modulation format should consider both the data communication performance and the brightness control/dimming flexibility, since a low intensity/brightness of light signal is likely to hamper reliable communication. Recently, pulse-width modulation (PWM) is shown to be a suitable method for obtaining dimming. This work evaluates the bit error rate (BER) performance and dimming flexibility of LiFi scheme having PWM based dimming flexibility with ASCO-OFDM data encoding. For this, a framework is developed to incorporate the PWM scheme for ASCO-OFDM transmitters and receivers. In this framework, the generated ASCO-OFDM signal in the electrical domain is multiplied with the PWM signal and the resultant signal is converted to the optical signal by optical modulators. The pulse width of the PWM based ASCO-OFDM signal is varied in accordance with the dimming or brightness level. Next, simulations are performed using MATLAB tool to evaluate the effectiveness of PWM based dimming for ASCO-OFDM. In the simulations, the dimming level is varied from a very low value to the standard value required for room illumination. The BER performances of ASCO-OFDM, DCO-OFDM and ACO-OFDM are investigated for both electrical and optical power limited channels. The performance evaluation is done for a number of OFDM subcarriers and for different constellation sizes. Finally, simulations are performed to compare ACO-OFDM, DCO-OFDM and ASCO-OFDM in terms of BER performance and dimming capacity. Results show that for a given data rate, ASCO-OFDM is 3 dB better than ACO-OFDM and 4 dB better than DCO-OFDM. Results also show that when 50% dimming is applied, an ASCO-OFDM system required 6 dB more power compared to a system with full brightness.
