# A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models
## 5 Discussions and Research Directions  
### Comparative Analysis: Emerging Technologies  
Besides the IS techniques mentioned throughout this survey to improve THz communications, other emerging technologies can be used for the same purpose. An emerging antenna technology with great potential to impact the development of 6G wireless networks is the fluid antenna system (FAS) [134]. It offers additional diversity and multiplexing benefits by exploiting dynamic radiating structures. As an example of a possible implementation, the antenna radiating elements can be installed in a tube-like structure within which a fluid can move freely among preset positions known as ports. Thus, the FAS structure changes its position and shape according to characteristics such as gain, radiation pattern, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **Performance Evaluation** & **System Model** & **Design Objective** & **Optimization Techniques** \\ \hline
[51] & Path loss and beam pattern & THz systems & Beamfocusing & Beamforming with physical optics channel model \\ \hline
[52] & Beam pattern & THz massive MIMO systems (downlink and uplink) & Beamforming design and channel estimation performance & Beamforming with physical channel model \\ \hline
[53] & SNR, ergodic capacity and OP & THz indoor systems downlink & Receiver beam misalignment and restore the transmitter/receiver connection blocked by obstacles & Electromagnetic theory \\ \hline
[54] & UAV trajectory, phase shift, THz band allocation and power control & UAV systems downlink & To maintain reliable THz communication and minimize average rate of users & SCA \\ \hline
[129] & Energy-efficient, SNR, latency and success rate & 3-D indoor THz wireless communication &
\begin{tabular}{c} Minimizes latency using ray tracing techniques to find the best THz signal propagation path \\ \end{tabular} & Ray searching and beam-selecting \\ \hline
[130] & Achievable rate, phase compensation and normalized array gain & IS-aided THz system & Achievable rate maximization for distributed IS-assisted THz communications & Analog beamforming, digital beamforming vector \\ \hline
[131] & Phase shifts, ergodic net throughput, blocking probability & CF mMIMO & Channel estimation and unblock links &
\begin{tabular}{c} OP and fading spatial correlation with increased element numbers \\ \end{tabular} \\ \hline
[128] & Misalignment fading & THz inter-satellite links &
\begin{tabular}{c} Compensation for the \\ high path loss associated \\ with high carrier \\ frequencies and to \\ improve SNR \\ \end{tabular} & Antenna theory \\ \hline
[132] & Coverage area and sum rate & mmWave or THz indoor communication & \
\begin{tabular}{c} Placement optimization to \\ maximize the long-term \\ sum rate and then \\ optimization of transmit \\ beamforming and \\ reflecting procedure in \\ real time \\ \end{tabular} &
\begin{tabular}{c} Antenna theory and \\ deep learning \\ \end{tabular} \\ \hline
[133] & BER and modulation speed & Full-wave simulation & \
\begin{tabular}{c} Eliminate on-chip signal \\ attenuation and \\ inter-symbol interference \\ \end{tabular} &
\begin{tabular}{c} Binary IS \\ optimization algorithm \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._and operating frequency bandwidth [135]. By exploring FAS flexibility in its shape and positions, the authors in [136] analyzed the OP to evaluate the FAS performance to combat high propagation path loss of THz communications. The research on FAS is still in its early stages, but it is advancing quickly. Some FAS limitations have already been pointed out and need to be overcome, such as the correlation model [137], how to achieve fast switching among the ports, and the optimal algorithm for the selection of the ports [138]. Similar to FAS, liquid crystal (LC) has been explored for THz communication applications due to their potential as a tunable dielectric material.