# Cooperative SWIPT MIMO-NOMA for Reliable THz 6G Communications
## 1 Introduction  
The wireless communication field has witnessed a rapid rise with a significant expansion of smart gadgets, new technologies, and modern applications. The critical requirement for fast data transport combined with the global ubiquity of services has resulted in groundbreaking research. Energy efficiency (EE) and spectral efficiency (SE) are two distinct metrics that assess system performance [1, 2, 3, 4]. It is critically required to enhance EE and SE to fulfill the strict requirements of diverse emerging applications, adapting to the upgrades of the next era and eliminating power consumption of the required equipment [5, 6, 7, 8, 9, 10]. The efficient use of energy has become an inevitable demand in the next generation of 6G to support its practicability and satisfy its needs [11]. Terahertz (THz) communication [12] will have a significant impact on future generations. According to [PERSON]'s equation, SE is determined on the basis of bandwidth (BW) availability. THz represents the unexplored electromagnetic (EM) gap (0.1-10) THz between EM and optical bands; it has piqued the interest of the research community toward 6G because of its favored benefits. THz is regarded as an essential pier for 6G due to its capacity and capabilities to build anadequate system, supporting a wide range of applications due to its very short wavelength (\(\lambda\)) of 3000-30 \(\upmu\)m and other features, e.g., very high frequencies, ultrawide BW, very high data rates, enormous throughput, very low latency, and excellent directivity. THz has encouraged researchers to investigate its capabilities to enable THz communications as it outperforms the two adjacent frequency bands in some ways. THz transmitters/receivers are expected to be manufactured using electronic, photonic, and plasmonic technologies. It is expected to be integrated with optical networks, providing alternate signals for optical pathways in various cases, e.g., backhaul-backhaul, kiosk-node, data center, and intra-device interconnections [13]. As a result, IEEE Std. 802.15.3d-2017 specifies the disadvantages of visible water vapor absorption and path losses, dividing the THz gap across many sub-windows. However, these windows are being thoroughly investigated in order to accommodate 6G services, with certain exceptions in which some 6G services will be incompatible with THz bands [14]. The need for fully connected systems has arisen as a result of Internet of everything (IoE) requirements. There are restrictions to 5G systems that prevent any additions or enhancements to satisfy these needs [15, 16, 17, 18]. We aimed to improve wireless connectivity, resource management, scalability, and user fairness, as well as to enhance the overall performance of 6G wireless communications and reliability. Hence, we optimized the current wireless communication systems by utilizing MIMO-NOMA technology and THz frequencies, exploring the performance and gains obtained. The main contributions of this paper are as follows:  
1. We propose a reliable transmission with a non-LOS (NLoS) line of THz communications to overcome THz coverage shortage for open areas or any location where IRS cannot be deployed, in addition to using a simplified cost-effective DF relaying instead of IRS. 2. We design a system with low power consumption, complexity, and cost by integrating THz, NOMA, MIMO, cooperative networking, and EH (SWIPT) techniques, improving SE, EE, and other metrics compared to the state of the art. 3. We develop a path-selection mechanism for the potential NOMA-based far users to enhance the system performance and reliability. 4. Given the flexible structure, we recommend exploiting an existing user in the cell for relaying and not dedicating a fixed device, achieving a synergic coexistence with the expected mobile 6G infrastructure. 5. To enhance the SIC procedure, we suggest two-user clustering to prevent propagating errors, complications, and spectral deficiencies due to the burden of the additional operations. 6.