DFT-Spread Orthogonal Time Frequency Space System with Superimposed Pilots for Terahertz Integrated Sensing and Communication

Abstract

Terahertz (THz) integrated sensing and communication (ISAC) is a promising interdisciplinary technology that realizes simultaneously transmitting Terabit-per-second (Tbps) and millimeter-level accurate environment or human activity sensing. However, both communication performance and sensing accuracy are influenced by the Doppler effects, which are especially severe in the THz band. Moreover, peak-to-average power ratio (PAPR) degrades the THz power amplifier (PA) efficiency. In this paper, a discrete Fourier transform spread orthogonal time frequency space (DFT-s-OTFS) system is proposed to improve the robustness to Doppler effects and reduce PAPR for THz ISAC. Then, a two-phase sensing parameter estimation algorithm is developed to integrate sensing functionality into the DFT-s-OTFS waveform. Meanwhile, a scheme of superimposed pilots is designed, which reduces the pilot overhead and improves the spectral efficiency. Based on the superimposed pilots, a low-complexity iterative channel estimation and data detection method is proposed to recover the data symbols of DFT-s-OTFS. The proposed DFTs-OTFS waveform can improve the PA efficiency by 10% on average compared to OTFS. Simulation results demonstrate that the proposed two-phase sensing estimation algorithm for THz DFT-s-OTFS systems is able to realize millimeter-level range estimation accuracy and decimeter-per-second-level velocity estimation accuracy. Moreover, the effectiveness of the iterative method for data detection aided by superimposed pilots in DFTs-OTFS systems is validated by the simulations and the bit error rate performance is not degraded by the Doppler effects.

Single- Versus Multicarrier Terahertz-Band Communications: A Comparative Study

“…DFT-spread-OTFS [48], [49] is recently proposed for THz ISAC to improve OTFS's PAPR characteristics and enhance the robustness to Doppler effects. Although this waveform seems to be a promising candidate for many THz applications, a detailed analysis of complexity, SE, TTI latency, and robustness to PHN and THz-specific impairments is still lacking.…”

“…We can perform DD domain equalization for delay-Doppler domain signal estimation first, using linear equalizers (MMSE or ZF), and then perform N-point IDFT to obtain the Tx symbols [48]. Other low-complexity solutions in [48], [49] do not guarantee a fair comparison with waveforms that use linear equalizer. As illustrated in Sec.…”

“…Thus, a viable OTFS solution with low-complexity implementation and good performance, such as unitary approximate message passing (UAMP) or variational Bayes (VB) detection, is a must [66]. A low-complexity solution, compared to linear MMSE, is proposed in [49] for DFT-s-OTFS, based on the conjugate gradient method, which has an overall complexity of O (𝑀𝑁log 2 (𝑀𝑁)); the solution still results in high complexity compared to TF MC schemes.…”

“…To meet the THz integrated sensing and communication (ISAC) requirements, the utilization of DFT-s-OFDM, with some modifications, is discussed in [47]. Most recently, a novel scheme called DFT-s-OTFS is proposed [48], [49] to address the severe Doppler effects and PAPR challenges of THz ISAC.…”

Terahertz Communications and Sensing for 6G and Beyond: A Comprehensive Review

“…2022 Wu [490] Proposed discrete Fourier transform spread OTFS (DFT-s-OTFS) system to improve the robustness to Doppler effects and reduce peak-to-average power ratio (PAPR) for THz ISAC.…”