continouse OFDM with filtering

I'm trying to do the following code data (bits) QAM-MApping using (comm.RectangularQAMModulator) IFFT( to generate the OFDM symbol) then I do pulse shaping (using conv) then I add a constant value(+c) Afterwards to detect the signal I remove the constant value (-c) conv FFt QAM demapping (using comm.RectangularQAMDemodulator)

The problem is that after the QAM demapping I receive an error. If i didn't add the constant value the problem is solved .. can any one please tell me how can I solve the problem. Thanks