This work explores the statistical model for a 5G and beyond (5GB) multi-carrier signal candidate; spectrally efficient frequency division multiplexing (SEFDM), in which the orthogonality between its subcarriers is intentionally violated compared to conventional orthogonal frequency division multi-plexing (OFDM). A mathematical model describing the statistical characteristics of inter-carrier interference (ICI) is derived and verified through system model simulations. Results show that the ICI is of normal distribution, whose variance is dependent on both the level of bandwidth compression and on the signal level. Models developed are used to derive the error bounds of SEFDM signals in Gaussian noise channels, as well as the upper bound of SEFDM spectral efficiency.