In this paper, a link adaptation technique based on constellation error power per sub-carrier for an OFDM communication system is described. The algorithm is based on the estimation of signal to noise ratio for each sub-channel, from which estimates for the overall symbol error rate for different modulation modes are obtained. Next, taking the operation of the Viterbi decoder into account, an estimate of the packet error rate (PER) is found for each case. The actual throughput rate for each transmission mode can then be computed for a given length of the packet. The estimates for PER as well as throughput rate can be used to select a suitable transmission mode as well as the length of fragments.