Hello NAsir
According to your question you have implemented a VLC system with different formats and couldnt get a Acceptable BER.For the case of OFDm have you used BER analyzer or BER DUT set,Because it is recommended to use BER DUT set instead of BER analyzer for OFDM due to large sequence length.As far as i know that you have implemented system for 4-QAM ,16-QAm and 256 QAM.Then in general with the increase of the bits per symbol,noise increase and effects of non-linarities for changing the phase.Use of non linearity compensate is good for tht problem.