In orthogonal frequency division multiple access (OFDMA) systems, the cyclic prefix (CP) length needs to be no less than the longest channel delay spread of many users, reducing bandwidth efficiency more significantly than in single user OFDM systems. We propose a novel receiver for OFDMA systems with an insufficient CP, exploiting the unused subchannels for a user. With the aid of computer simulations, we demonstrate that the proposed equalization method is superior to the conventional equalization method using a time domain channel shortening filter in terms of the achievable SNR and the error performance

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/spl les/N'/spl les/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.

We consider power adaptation strategies for binary phase-shift keying signals in Rayleigh fading channels under the assumption that channel state information is provided at both the transmitter and the receiver. We first derive a closed-form expression for the optimal power adaptation that minimizes average bit-error rate (BER) subject to average and peak transmission power constraints. Then, we analyze the average BER for channel inversion power adaptation with the same constraints. Our results show that the performance difference between the optimal power adaptation and the channel inversion becomes negligibly small as available average transmission power increases and/or peak-to-average power ratio decreases. We also find that an optimal peak-to-average power ratio exists that minimizes the average BER in the channel inversion scheme.