## 2 System and Signal Characterization  
The evolution of 6G communications is anticipated to meet stringent performance requirements, including a peak data rate of at least 1 Tbps for nomadic access--100 times greater than 5G--and 1 Gbps in mobile scenarios, which is 10 times higher than its predecessor. Additionally, energy efficiency is projected to improve by a factor of 10 to 100, while spectral efficiency is expected to be enhanced by 5 to 10 times in comparison to 5G. These advancements are expected to be realized through the implementation of LISs operating in the terahertz spectrum, combined with block transmission techniques. It is important to note that increasing the carrier frequency results in a larger channel coherence bandwidth, translated into a higher throughput. LIS technology is particularly compatible with terahertz frequencies due to the reduced wavelength, which enables the use of ultra-compact antennas, thereby simplifying the deployment of cost-effective LIS panels. However, as the number of antennas in an LIS grows--effectively functioning as a beyond-massive MIMO system--the associated signal processing demands also increase significantly. Consequently, there is a crucial need to explore receiver architectures with minimal complexity. This study links low-complexity receivers, which streamline the decoding process, aiming to optimize performance while keeping the computational complexity acceptable. To mitigate intersymbol interference, block transmission techniques are frequently employed. The SC-FDE method, in particular, offers the following key advantage over OFDM: it exhibits a lower PAPR [18; 19]. This characteristic makes it especially suitable for uplink scenarios and enhances the efficiency of power amplification [22]. Unlike conventional communications between user equipment (UE) and a base station (BS), LIS-based communication typically relies on line-of-sight connectivity. The LIS functions as a short-range beamformer, with antenna elements spaced at half-wavelength intervals--much shorter than the three to four wavelengths commonly used in traditional MIMO configurations. While individual LIS antenna elements communicate in the far field, the overall LIS array operates within the near field, meaning that communication occurs at distances shorter than the Fraunhofer limit, typically spanning just a few wavelengths. This unique property gives LISs distinctive capabilities. Whereas conventional beamforming is confined to azimuth and elevation adjustments, LISs extend this capability to the distance dimension as well. This enables interference mitigation from users who are aligned in azimuth and elevation but are positioned at varying distances. Focusing on Figure 1, the \(n\)th transmitted block--comprising \(N\) data symbols--from the \(t\)th UE is denoted as \(s_{n}^{(t)}\), while the received block received by the \(r\)th antenna of the LIS is denoted as \(y_{n}^{(t)}\). The relationship between the time-domain and the frequency-domain signals for the \(k\)th subcarrier (assuming that they remain constant throughout the transmission of a given block) is established via the Discrete Fourier Transform (DFT), such that \(DFT\left\{s_{n}^{(t)};n=0,1,\ldots,N-1\right\}=\left\{S_{k}^{(t)};k=0,1,\ldots,N-1\right\}\), i.e., the DFT is applied to the time-domain block. The same procedure, mutatis mutandis, is applied to the received signal block, channel, and noise. The received signal in the frequency domain, expressed in matrix-vector form, is given by [22], as follows:  
\[\mathbf{Y_{k}}=\left[Y_{k}^{(1)},\ldots,Y_{k}^{(R)}\right]=\mathbf{H_{k}S_{k} +N_{k}}. \tag{1}\]  
Here, \(\mathbf{Y}_{k}\) is the R-dimensional received vector at subcarrier \(k\), \(\mathbf{S}_{k}\) is the T-dimensional transmitted signal vector, \(\mathbf{H}_{k}\) is the R \(\times\) T channel matrix whose \((r,t)\)-th element is denoted as \(H_{k}^{(t,r)}\), and \(\mathbf{N}_{k}\) is the R-dimensional noise vector for the subcarrier [22].