Over-sampling effect in distributed Alamouti coded OFDM with frequency offset

Abstract

Unlike the conventional distributed Alamouti coded orthogonal frequency division multiplexing (OFDM), over-sampling substantially improves the performance of frequency asynchronous distributed Alamouti coded (FADAC) OFDM. This exclusive effectiveness of over-sampling in FADAC-OFDM comes from two factors. One is the unique characteristics of residual inter carrier interference term in FADAC-OFDM and the other factor is moving away the cyclic harmonic inter-carrier interference (ICI) by over-sampling. In addition, the authors confirm that over-sampling factor of 2, i.e. 2N point fast Fourier transform (FFT) is sufficient to move the cyclic harmonic ICI. From this investigation, the authors propose 2N point FFT FADAC-OFDM. The proposed scheme achieves the significantly improved performance not only in the flat fading channel but also in the selective fading channels. For example, while the non-oversampled FADAC-OFDM has the unacceptable error rate level near to or above 0.1 at the band edges even with high signal to noise ratio, the proposed scheme achieves near intersymbol interference-free performance. This leads the proposed scheme to achieve 50% reduction of the required number of null subcarriers to achieve ICI free performance compared with the non-oversampled FADAC-OFDM. Moreover, in terms of implementation feasibility and computational complexity, the proposed scheme is shown to be acceptable. © 2016 The Institution of Engineering and Technology.