Need assistance with Digital Filter for Power Line Channel

Hi,

I am working on power line communication channel. I am getting some problem through simulation the model. When I used a sample 256 FFT and multiple modulation scheme then there is no error(when only considering Gaussian noise, if I consider more noise like Rician noise then get 49 percent error).

When I have used 64 IFFT for simulation then I also get the 49 percent error. I need some help.

Please find the attested link of the block diagram.

64 IFFT with 49% error https://docs.google.com/file/d/0B6M8AMfF-BvhTmlzVGNwaXdGSDQ/edit?usp=sharing

256 IFFT with No error - working model https://docs.google.com/file/d/0B6M8AMfF-Bvhd29rUXZDU0hJSmc/edit?usp=sharing

Please advise.

Sagar