On the Achievable Spectral Efficiency of Non-Orthogonal Frequency Division Multiplexing

We investigate the spectral efficiency of non-orthogonal frequency division multiplexing (NOFDM). NOFDM is regarded as a frequency-domain faster-than-Nyquist (FTN) signaling with block transmission, where each subcarrier is compressed in the frequency domain by utilizing the discrete fractional Fourier transform (DFRFT). Since pulse shaping filters are not employed, the effective bandwidth of the NOFDM signal should take into account the resulting out-of-band (OOB) radiation. Therefore, we derive a closed-form expression for the power spectral density (PSD) of the NOFDM signal with windowing for OOB reduction. Through numerical comparison, it is shown that NOFDM outperforms OFDM in view of the effective signal bandwidth if they are compared under the same number of subcarriers. On the other hand, by the capacity analysis based on the asymptotic spectral efficiency with a large number of subcarriers, we elucidate that NOFDM may not outperform OFDM from an information-theoretic perspective.