Spectrally efficient frequency division multiplexing for improving photonics-aided terahertz wireless transmission capacity

Photonics-aided terahertz (THz) transmission leverages the abundant spectral resources of the THz band and enables seamless integration with optical fiber networks, representing a key pathway toward 6G communication. Meanwhile, spectrally efficient frequency division multiplexing (SEFDM) can improve spectral efficiency for optical and wireless communications, thereby further increasing the capacity of photonics-aided THz transmission systems, especially when the bandwidth of existing THz devices is limited due to cost constraints. In this work, we experimentally investigate and compare the performance of conventional SEFDM and the proposed non-orthogonal discrete Fourier transform spread (NO-DFT-S) SEFDM in a photonics-aided THz transmission system. In order to eliminate the inter-carrier interference (ICI) intentionally induced by bandwidth compression, three optional signal detection algorithms, including Volterra nonlinear equalization (VNLE), I/Q separation iterative detection (ID), and maximum a-posteriori (MAP)-Viterbi algorithm, are individually employed for performance comparison. Moreover, due to bandwidth compression relative to orthogonal frequency division multiplexing (OFDM) and a reduction in peak-to-average power ratio (PAPR) compared to conventional SEFDM, the capacity of NO-DFT-S SEFDM is increased by 31.3% over OFDM and 23.5% over conventional SEFDM under a BER threshold of 2.4e-2 @20% soft-decision forward error correction (SD-FEC). Specifically, by employing high-gain THz modules such as THz lenses and low-noise amplifiers (LNAs), a maximum line rate of 168 Gbit/s and a net rate of 129.2 Gbit/s are successfully achieved over a 100-m THz wireless transmission at 300 GHz using the NO-DFT-S SEFDM scheme.