Quadrature OFDMA systems based on layered FFT structure

In current OFDMA systems three major problems arise due to the large number of subcarriers, including high peak to average power ratio (PAPR), sensitivity to carrier frequency offset (CFO), and high complexity in users terminals. In this paper, based on an innovative concept of layered FFT structure, we propose novel Quadrature OFDMA (Q-OFDMA) systems which can overcome these problems. In particular, the proposed systems can achieve the same guard-interval overhead and same bandwidth occupation to conventional OFDMA systems, while with reduced PAPR and improved CFO robustness and frequency diversity. Q-OFDMA systems also promise low complexity in downlink receivers. Parameter configuration is investigated for both predefined and adaptive users data rates. Theoretical comparison of bit error rate (BER) performance between QOFDMA and OFDMA is conducted, and validated by simulation results. It is shown that Q-OFDMA systems could achieve better performance than OFDMA when signal to noise ratio (SNR) is above a threshold depending on the channel condition, and advanced equalizers, such as minimum mean square error equalizer, can significantly decrease this threshold due to the large frequency diversity in Q-OFDMA systems.