Orthogonal frequency division multiplexing (OFDM) is one of the promising
technologies to improve the spectral efficiency, enhance system capacity and mitigate
inter-symbol interference in the wireless communication system. Adaptive
modulation (AM) with OFDM has been suggested as a bandwidth efficient
transmission technique in wireless fading environments. In this paper, adaptive
modulation techniques is used with OFDM system to improve the throughput
performance of the system using four modulation schemes BPSK, QPSK, 8QAM,
and 16QAM. Two algorithms to estimate SNR are suggested. The first algorithm is to
estimate the SNR for each OFDM symbol in the frame. The second algorithm is to
estimate the channel SNR for each subcarrier in the OFDM symbol. The comparison
between the results of the two algorithms on the performances of BER and
throughput for OFDM system is shown. Furthermore, the effect of various types of
channel equalization on performances of system will be determined. The obtained
results show that a significant improvements in terms of bit error rate (BER) and
throughput can be achieved demonstrating the superiority of the adaptive modulation
schemes compared to fixed transmission schemes.