# Cooperative SWIPT MIMO-NOMA for Reliable THz 6G Communications
## 4 System Model Design  
Recent academic and industrial research stated the assumption of deploying ultra-dense heterogeneous networks (UDHNs), with a large number of distributed base stations, access points, relays, and repeaters to the expected 6G densified infrastructure. The presented simple system in Figure 3 is suggested to solve the problems of the THz communications' lack of a coverage area, connection failures, and SIC complexity. Particular use cases are studied, e.g., rural areas, countryside, or any other place where IRS and other supporting equipment cannot be deployed, providing dynamic path selectivity in the case of the user's link failure. The system considers a small cell with MIMO-NOMA-based downlink transmission and two SWIPT-paired clustered users. The BS sends the superimposed signal simultaneously to both NOMA served users, the near user (NU) and the distant user (FU). At a certain moment, the BS to FU path is blocked by an obstruction, resulting in significant shadowing. As a result, the FU is unable to identify its obstructed signal. The NU, on the other hand, establishes a strong connection with the BS. According to NOMA principles, the NU is supposed to first decode the FU's signal before removing it by the SIC process, and then decode its intended signal. Hence, NU receives a copy of the FU's data, such that FU can rely on NU to connect to the BS by exploiting it as a DF relay. For relaying, the energy in the NU's battery is insufficient to convey the data to the FU. To this end, we recommended that NU makes use of EH's power-splitting protocol (SWIPT) to harvest energy from the surrounding RF energy to use it for this retransmission (relaying process). The transmission procedure is divided into two parts. The NU receives the BS superposed signal at the first phase, whereas the power-splitting procedure captures a portion of the NU's harvested power; however, the remaining power is used for data decoding. The captured energy is then used by NU to convey the FU's information to FU in the second phase. A Rayleigh fading channel with mean = 0 and variance = Transmission Distance\({}^{-}\)THz total losses was conducted in this analysis for all cases. We set additive white gaussian noise (AWGN)  
Figure 3: Cooperative SWIPT MIMO-NOMA in THz model. with mean = 0 and variance of \(\sigma^{2}\), representing the added noise. The probability density function (PDF) for the point z is given by  
\[f_{(z\sigma)}=\frac{1}{\sqrt{2\pi\sigma^{2}}}\exp\left(-\frac{z^{2}}{2\sigma^{2 }}\right). \tag{1}\]  
As space attenuation and molecule absorption are considered in the cooperative SWIPT NOMA signal, the path loss of the NLoS link is greater than that of the LoS link. If the LoS link prevails, then the NLoS impact can be ignored [22]. In free space, the THz total loss (\(\eta\)) is set as extraordinarily large. For the users \(k\), the channel gain is written as  
\[h_{k}=\sqrt{A}\sqrt{\frac{1}{\eta}}\ G, \tag{2}\]  
where \(A\) denotes the MIMO antennas, \(G\) denotes the gain of the antenna, and \(\eta\) represents the source-destination THz total losses, written as  
\[\eta=(\frac{4\pi fd}{C})^{2}e^{a(f)d}, \tag{3}\]  
where \(f\) denotes the THz, \(d\) denotes the source-destination distance, \(a(f)\) represents the absorbing coefficient, and \(C\) denotes the velocity of light. According to the proposed case, the closed-form derivations are presented below. Stage 1, BS to NU transmission:  
The BS superimposed signal is  
\[X=\sqrt{\mathrm{P}}\Big{(}\sqrt{\alpha n}\ xn+\sqrt{\alpha f}\ xf\Big{)}, \tag{4}\]  
where P denotes the transmit power, \(\alpha n\) and \(\alpha f\) denote the NU and FU powers, respectively, and \(xn\) and \(xf\) denote the NU and FU signals, respectively. However, FU cannot detect its signal due to obstruction.