Frequency-time domain transmit power adaptation for multicarrier system in fading channels

Multicarrier transm~ssion systems have aroused great interest in recent years as a potential solution to the problem of transmitting high rate data over the wireless channel [l, 21. The principle of a multicarrier transmission is to split a high rate data stream into a number of lower rate streams, which are then simultaneously transmitted on a number of subcarriers. In a multicarrier system, when the channel state information (CSI) is available at the transmitter, the transmit power for each subcarrier can be adapted according to the CSI, to increase the channel capacity. In [3], the transmit power adaptation for a multicarrier system was performed in the frequency domain within the constraint of total transmit power. In [4], an adaptive multicarrier modulation technique was proposedin a fading channel, where transmit rate and power are adapted in both time and frequency to maximise data rates under a fixed BER constraint. In this Letter, we analytically derive the optimal transmit power adaptation method to maximise the channel capacity of a multicarricr system in a time-variant fading channel, To exploit the fading channel characteristics such as frequency selectivity and time variation, the optimisation process is performed jointly in the frequency and time domains under the constraint of average transmit power. Comparisons between the frequency-time domain power adaptation and the frequency domain power adaptation are made. The relationship between the channel capacity of a multicarrier system and that of a single-carrier system with optimal transmit power adaptation is also shown