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

Abstract

The prospects of utilizing single-carrier (SC) and multi-carrier (MC) waveforms in future terahertz (THz)-band communication systems remain unresolved. On the one hand, the limited multi-path components at high frequencies result in frequency-flat channels that favor low-complexity wideband SC systems. On the other hand, frequency-dependent molecular absorption and transceiver characteristics and the existence of multi-path components in indoor sub-THz systems can still result in frequency-selective channels, favoring off-the-shelf MC schemes such as orthogonal frequency-division multiplexing (OFDM). Variations of SC/MC designs result in different THz spectrum utilization, but spectral efficiency is not the primary concern with substantial available bandwidths; baseband complexity, power efficiency, and hardware impairment constraints are predominant. This paper presents a comprehensive study of SC/MC waveforms for THz communications, utilizing an accurate wideband THz channel model and highlighting the various performance and complexity trade-offs of the candidate schemes. Simulations demonstrate that discrete-Fourier-transform spread orthogonal time-frequency space (DFT-s-OTFS) achieves a lower peak-to-average power ratio (PAPR) than OFDM and OTFS and enhances immunity to THz impairments and Doppler spreads, but at an increased complexity cost. Moreover, DFT-s-OFDM is a promising candidate that increases robustness to THz impairments and phase noise (PHN) at a low PAPR and overall complexity.

Localization as a key enabler of 6G wireless systems: A comprehensive survey and an outlook

“…For instance, new synchronization problems emerge and hardware needs to keep up with novel mmWave and THz-enabled system designs. Another already reported problem is the requirement for increased peak to average power ratio (PAPR) compared to 5G systems, which may render the classic orthogonal frequency division multiplexing (OFDM) inappropriate [130]. Another important aspect that needs to be taken into account in the development of next generation localization algorithms is the impact of near-field circumstances along with the beam-split effect (BSE) that can greatly influence the geometry of MIMO channel localization models.…”

“…The former appear to be advantageous in applications, where frequency-flat channels exist, while, in spite of the poor power efficiency and high complexity, multi-carrier systems are still favoured in frequency-dependent molecule absorption loss and multipath scenarios. The utilization of discrete-Fourier transform spread OFDM may be employed as a ready-made solution in order to mitigate the PAPR impact [130]. Spatial [441], index [442], hierarchical bandwidth [443], and orthogonal time-frequency space modulations [444] are also taken into account for specific scenarios as additional multi-carrier modulations.…”

Time-Domain Channel Estimation for Extremely Large MIMO THz Communications with Beam Squint

“…Although the problems of beam squint and channel estimation have been investigated for MC systems in THz communications, this is the first work that investigates the joint problem of beam-squint and channel estimation for SC transmissions. As previously mentioned, the low output power and the non-linearity induced by available THz PAs motivates the adoption of SC transmissions rather than OFDM [5]. Moreover, the first sub-THz standard (IEEE 802.15.3d [41]) describes a SC modulation mode to support long range and high rate wireless applications (such as 100 Gbps).…”

“…On the other side, the single-carrier (SC) waveform is known for having a lower PAPR compared to the OFDM waveform and being robust to system impairments and phase noise. Especially for THz communications, due to the low output power and the non-linearity effect induced by the available THz power amplifiers (PAs), it is preferable to use SC transmissions rather than OFDM [5].…”

TeraMIMO: A Channel Simulator for Wideband Ultra-Massive MIMO Terahertz Communications

“…Consider a downlink multi-carrier [29] THz-band system with an ultra-wide bandwidth 𝐵 sys and UM arrays at the Tx and Rx. The system has 𝐾 subcarriers, with the center frequency 𝑓 𝑐 and the frequency of the 𝑘th subcarrier…”