Simulate BER curves for OFDM with convolutional coding
Look at one OFDM symbol at the point where it is transmitted, i.e. right before it enters the channel. Figure out its average power, $P$, or its average energy, $E = PT$, where $T$ is the symbol's duration. Also, figure out how many information bits, $I$, are carried by the symbol.

In your simulation, $E_b = E/I$.

To get a desired $E_b/N_0$, calculate $\sigma_n^2 = N_0 / 2$. Then, add AWGN noise to the symbol right before it enters the receiver; i.e. after all other channel effects such as multipath. The noise should have variance $\sigma_n^2$ in each of the I and Q branches.