Signal transmission model and phase shift control for metasurface-enabled terahertz communication systems
With the gradual maturity of the fifth-generation (5G) mobile communication technology,research into the sixth-generation (6G) mobile communications has been underway. 5G technologies fail to meet the comprehensive communication needs of 6G, particularly in terms of system energy consumption and energy efficiency. Metasurface, comprised of numerous sub-wavelength units, are capable of controlling electromagnetic wave properties such as reflection, scattering, and polarization. They are well known for their low energy consumption, ease of fabrication, and low cost, and thus they can rebuild the wireless propagation environment as required, offering a new paradigm for wireless transmission. The application of metasurface has gained keen academic interest for their potential to cut energy consumption and enhance system energy efficiency in communication systems. High frequencies, such as terahertz, have garnered much attention due to their abundant bandwidths. However, they show inherent disadvantages, including a shorter effective transmission distance, severe path blockages, and increasingly apparent channel frequency-selective characteristics. A large number of metasurface units are needed to compensate for these performance losses. Consequently, near-field communication based on ultra-large-scale arrays is set to become a widespread application scenario. Near-field communication ensures most communication devices are located in the near-field of the array. Therefore, the signal transmission model based on the narrowband hypothesis in the far field is no longer applicable due to the near-field effect. Meanwhile, the phase-shift design based on the center frequency will introduce the beam squint effect owing to the wideband effect, leading to deterioration of the system performance. Solutions to issues above are proposed in this paper. The path loss model of metasurface-assisted communication systems from the perspective of electromagnetic scattering is first analyzed, with the consideration of both beam squint and near-field effects. The precise array manifold and near-field broadband signal transmission model of metasurfaces are derived, considering both spatial broadband effects and near-field effects. Furthermore, the maximization problem of achievable rates in near-field broadband systems is redefined, deriving a closed-form suboptimal solution for metasurface phase shift design. A phase shift design scheme based on non-uniform frequencies is proposed to mitigate the beam squint effect. For single-user scenarios, it derives a similar solution and proposes a phase shift design scheme to alleviate beam squint. Finally, simulation experiments are conducted to validate the accuracy of the proposed near-field broadband model and the superiority of the proposed metasurface phase shift design scheme in enhancing the achievable rates of near-field broadband. Our results validate the effectiveness of the proposed phase-shift control method in alleviating the beam squint effect. For example, compared with the central frequency-based phase shift design, the proposed scheme obtains more than 200％ improvement in the achievable rate while the transmitting power is 1 mW, the element number of base station antennas is 64 , and the elements number of metasurface is 1440 .
terahertz communicationsmetasurfaceperformance analysisphase shift control
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