[Paper Review] THz Precoding for 6G: Applications, Challenges, Solutions, and Opportunities

Jingbo Tan, Linglong Dai|arXiv (Cornell University)|May 21, 2020

Millimeter-Wave Propagation and ModelingEngineering13 references22 citations

TL;DR

This paper investigates terahertz (THz) precoding techniques for 6G networks, addressing challenges like severe path loss, beam split effects, and high power consumption. It compares analog beamforming, hybrid precoding, and delay-phase precoding, showing that delay-phase precoding achieves the highest energy efficiency with fewer users, while reconfigurable intelligent surfaces (RIS) offer a promising solution for blockage mitigation in THz systems.

ABSTRACT

Benefiting from the ultra-wide bandwidth, terahertz (THz) communication is becoming a promising technology for future 6G networks. For THz communication, precoding is an essential technique to overcome the severe path loss of THz signals in order to support the desired coverage. In this article, we systematically investigate the dominant THz precoding techniques for future 6G networks, with the highlight on its key challenges and opportunities. Specifically, we first illustrate three typical THz application scenarios including indoor, mobile, and satellite communications. Then, the major differences between millimeter-wave and THz channels are explicitly clarified, based on which we reveal the key challenges of THz precoding, such as the distance-dependent path loss, the beam split effect, and the high power consumption. To address these challenges, three representative THz precoding techniques, i.e., analog beamforming, hybrid precoding, and delay-phase precoding, are extensively investigated in terms of their different structures, designs, most recent results, pros and cons. We also provide simulation results of spectrum and energy efficiencies to compare these typical THz precoding schemes to draw some insights for their applications in future 6G networks. Finally, several important open issues and the potential research opportunities, such as the use of reconfigurable intelligent surface (RIS) to solve the THz blockage problem, are pointed out and discussed.

Motivation & Objective

• To identify and analyze the key challenges in THz precoding arising from differences between mmWave and THz channels.
• To evaluate and compare three dominant THz precoding techniques—analog beamforming, hybrid precoding, and delay-phase precoding—based on structure, design, performance, and power efficiency.
• To provide simulation results on spectral and energy efficiency to guide practical deployment in future 6G networks.
• To highlight open research issues such as blockage, hardware impairments, user mobility, and scintillation effects in THz systems.
• To explore emerging opportunities, particularly the use of reconfigurable intelligent surfaces (RIS) to overcome signal blockage and reduce power consumption.

Proposed method

• The paper compares three THz precoding architectures: fully analog beamforming, hybrid analog-digital precoding, and digital beamforming using time delays (delay-phase precoding).
• It analyzes the impact of distance-dependent path loss, beam split effect, and high power consumption on THz system performance.
• System-level simulations are conducted to evaluate spectral efficiency and energy efficiency across the three precoding schemes under varying numbers of users.
• The study incorporates realistic channel models for indoor, mobile, and satellite THz communication scenarios.
• It proposes RIS-aided THz precoding as a solution to the LoS blockage problem, enabling non-line-of-sight links through passive beamforming.
• The paper discusses hardware impairments such as phase noise, I/Q imbalance, and power amplifier nonlinearity, and their impact on system performance.

Experimental results

Research questions

• RQ1 How do the propagation characteristics of THz channels differ from those of mmWave channels, and what new challenges do these differences introduce for precoding?
• RQ2 What are the performance and power efficiency trade-offs among analog beamforming, hybrid precoding, and delay-phase precoding in THz systems?
• RQ3 How does the number of users affect the energy efficiency of different THz precoding techniques, particularly in the presence of beam split effects?
• RQ4 To what extent can reconfigurable intelligent surfaces (RIS) mitigate the blockage problem in THz communications and improve system reliability?
• RQ5 What are the key open issues in THz precoding, including hardware impairments, user mobility, and environmental effects like scintillation?

Key findings

• Delay-phase precoding achieves the highest energy efficiency when the number of users is four or fewer, due to reduced beam split effect and lower hardware complexity.
• Hybrid precoding offers a balanced trade-off between spectral efficiency and power consumption, making it suitable for multi-user scenarios.
• Analog beamforming, while simple and low-complexity, is limited to single-stream transmission and suffers from beam squint and beam split effects.
• The beam split effect causes performance degradation in wideband THz systems, which delay-phase precoding effectively mitigates by using time delays instead of phase shifters.
• RIS-aided THz precoding shows strong potential for solving LoS blockage, enabling reliable communication in non-line-of-sight environments.
• Scintillation effects due to atmospheric turbulence significantly degrade THz signal quality, and current research on modeling and mitigation remains limited, suggesting a critical research gap.