LED non-linearity mitigation techniques for optical OFDM-based visible light communications
The visible light communication (VLC) utilising the light emitting diodes (LEDs) as transmitters has gained much interest for the indoor wireless access due to its abundance of unregulated spectrum. However, the LED non-linearities cause a detrimental non-linear distortion especially for the adoption of optical orthogonal frequency-division multiplexing (O-OFDM), which is known to suffer high peak-to-average power ratio (PAPR). Specifically, the authors consider the direct current biased O-OFDM (DCO-OFDM) to realise the O-OFDM signals. This study aims to develop the mitigation techniques for LED non-linearities at transmitter through the combined advantages of PAPR reduction and the technique of linearising the non-linear LED characteristics. The orthogonal cover code sequence is exploited to design a transform precoding to reduce the PAPR. An uncomplicated pre-distorter at transmitter is adopted to linearise the LED non-linearity. In addition, the authors propose a dynamic DC biasing method to avoid the clipping noise in DCO-OFDM. As a whole, the proposed techniques at VLC transmitter can improve the LED non-linearities even for large input signals.