OFDM is one of the promising modulation candidates for a fourth generation broadband mobile communication system because of its robustness against intersymbol interference (ISI). The adaptive modulation scheme is also an efficient scheme to increase the transmission rate by changing the channel modulation scheme according to the estimated channel state information. Since its implementation depends on the channel environment of the system and control period by using feedback information, this paper presents an evaluation for the effects of various modulation scheme combinations, target BER, Doppler frequency, and various adaptation intervals as control period on the performance of adaptive OFDM. We also propose a predicted feedback information scheme which increases the adaptation interval using the predicted power estimation in order to reduce the transmission time of feedback information from receiver to transmitter. Computer simulation results show that the case with BPSK, QPSK and 16QAM modulation combination at target BER 10/sup -2/ achieves 2Mbit/s improvement over other combination cases in high Doppler frequency. On the other hand, at target BER 10/sup -3/, the case with BPSK, QPSK, 8PSK and 16QAM modulation combination achieves 3Mbit/s improvement compared to the case of target BER 10/sup -2/. It is also shown that the predicted feedback information scheme effectively reduces the transmission time of feedback information from the receiver to transmitter.
['Chang Jun Ahn', 'Iwao Sasase']
The effects of modulation combination, target BER, Doppler frequency, and adaptation interval on the performance of adaptive OFDM in broadband mobile channel