We proposed an improved hybrid optical orthogonal frequency division multiplexing (O-OFDM) and pulse-width modulation (PWM) scheme for visible light communications. In this scheme, a bipolar O-OFDM signal is converted into a PWM format where the leading and trailing edges convey the frame synchronization and modulated information, respectively. The proposed OFDM-PWM scheme is mainly used to address the high PAPR issues of O-OFDM and also enhance the BER performance.

Due to the PWM can be employed to linearly represent the bipolar signal, we can take the advantage of the anti-symmetry property of ACO-OFDM signal. We just convert the first half bipolar O-OFDM frame and use the last half frame time slots to extend the PWM signal to overcome the high bandwidth requirement of the PWM signal. And the generated OFDM-PWM signal has the same throughput with the original ACO-OFDM.

Since the average duty cycle of OFDM-PWM is greater than the sample of ACO-OFDM, the proposed scheme provides a higher illumination level if we set the high level of OFDM-PWM is equal to the maximum amplitude of ACO-OFDM sample. In addition, we can decrease the high level of the OFDM-PWM signal to have a better resilience to nonlinearity of optical source.

Because the OFDM-PWM scheme makes LEDs only switch ‘on’ and ‘off’ between two points. Therefore, the tight requirement on high peak-to-average-power-ratio (PAPR) in OFDM is no longer a major issue. The simulation and experimental results also demonstrate that the proposed scheme offers an improved bit error rate performance compared to the traditional ACO-OFDM.