Spectrally Efficient FDM Signals: Bandwidth Gain at the Expense of Receiver Complexity

This paper investigates the transmission of frequency division multiplexed (FDM) signals, where carrier orthogonality is intentionally violated in order to increase bandwidth efficiency. In analogy to conventional OFDM, signal generation relies on an inverse fractional Fourier transform (IFRFT) that can be implemented with O(N log₂ N) algorithmic complexity. Optimal maximum likelihood (ML) detection is overly complex due to the presence of substantial intercarrier interference (ICI). Consequently, we investigate an alternative detection mechanism based on the generalized sphere decoding (GSD) algorithm. We examine the bandwidth efficiency and the error performance in additive white gaussian noise (AWGN), for various FDM signal parameters. In particular, we show that it is possible to detect optimally and efficiently FDM signals, with 25% bandwidth gain with respect to analogous OFDM signals. This indicates that the transmission of spectrally efficient non orthogonal FDM signals is tangible.