Performance analysis of FSO/THz-RF dual-hop link based on NOMA

To compensate for the deficiencies of low spectrum efficiency (SE) and scarce spectrum resources in Radio Frequency (RF), the FSO/THz-RF dual-hop downlink based on Non-Orthogonal Multiple Access (NOMA) is proposed, which utilizes the superior SE of NOMA and the spectrum resources of Free-Space Optical (FSO) and Terahertz (THz). Closed-form expressions of outage probability (OP) and ergodic capacity (EC) are derived for single-user and NOMA system, accounting for joint effects of channel fading, pointing error, and path loss. Additionally, corresponding asymptotic OP and diversity gains provide valuable physical insights. Results show that asymptotic OP and diversity gains are equal for both single-user and NOMA system, with diversity gains dependent on the performance parameters of the first link, independent of the total number and order of users. Moreover, better system performance is achieved with a smaller power allocation coefficient (PAC) mean deviation and NOMA system integer coefficient (NSIC). Finally, numerical results illustrate the superiority of the system.