Transmission power adaptations in MC-CDMA communications over Rayleigh fading channels

We consider transmission power adaptations in multicarrier code-division multiple-access (MC-CDMA) communications, where the transmission power is adapted in frequency and/or time domain in response to channel variations. With frequency domain power adaptation, we propose to allocate the transmission power over N′ (1 ≤ N′ ≤ N) strongest subcarriers (having highest channel gains) rather than over all possible N′ subcarriers. We examine the effect of choosing N′ on the performance of the frequency domain power adaptation scheme, and find that there exists an optimal N′ that minimizes the average BER. In the time domain, we consider adapting the transmission power so that desired signal strength at the receiver output maintains at a fixed level. We analyze the BER performances of frequency domain, time domain, and simultaneous frequency-time domain power adaptations with an average transmission power constraint. Our results show that the frequency domain and the time domain power adaptation schemes outperform the nonadaptive (i.e. constant uniform power over all subcarriers) scheme especially for noise-limited and interference-limited regions, respectively. The combined frequency-time domain power adaptation is shown to have a significant performance gain over the power adaptation in only frequency or time domain as well as over the nonadaptive scheme.