I am designing a VLC system with DMT. For the channel response, I have chosen BPSK on all subcarriers and calculated the SNR of each subcarrier from BER-SNR relation.

For subcarriers with SNR greater than required SNR for BER of 10^-3, I have increased the number of bits over that subcarrier. (for example if subcarriers  SNR is greater than 9.8, I have changed the bits assigned from 1 to 2).For this new bit-loading, I have found that SNR of both QPSK and BPSK loaded subcarriers getting degraded. How can I arrive at a decent bit-loading to maximize the transmission?

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