Influence of Lattice Spacing in Disturbed Generalized Frequency Division Multiplexing Systems

Orthogonal Frequency Division Multiplexing (OFDM) is said to be extremely sensitive to synchronization offsets. Hence, non-orthogonal impulse shaping might be used in Generalized Frequency Division Multiplexing (GFDM) systems to increase robustness. In this paper GFDM with Gaussian pulse shaping is compared to a Cyclic Prefix (CP)-OFDM system where the receiver suffers from synchronization offsets in time and frequency domain. As the increased robustness due to non-orthogonal waveforms does not pay off, the GFDM system is clearly outperformed by the CP-OFDM system. Different results are obtained if a linear equalizer is added to the GFDM system. Even if only the interference caused by adjacent lattice points is treated by an equalizer, GFDM can be superior. Furthermore, we show that the performance of the non-orthogonal system can be adjusted dynamically to the channel conditions by changing the lattice spacing. Contrary to an orthogonal system this is possible without redesign of the transmit filters. The optimal lattice spacing follows from an interference/spectral efficiency trade-off that depends not only on the SNR but also on the modulation alphabet, where contradictory strategies are found for BPSK and 16QAM schemes.