The work in [1] proposed a reconfigurable intelligent surface (RIS)-assisted downlink ISTN with NOMA, in which the energy efficiency was maximized by jointly optimizing the active beamforming vector at the BS and the passive beamforming matrix at the RIS. In [2], the authors investigated the joint optimization algorithm for the phase shift, transmit power, and receive beamforming of an uplink RIS-assisted NOMA-based ISTN. In the considered scenario, only one BS is deployed, and the direct UEs are served by space division multiple access technology. The authors of [16] investigated an uplink caching-based NOMA ISTN, where the satellite provides backhaul links for terrestrial BSs. In this work, a system utility function, which consists of the achieved terrestrial user rate and cross-tier interference caused by terrestrial BSs to satellite, was maximized by jointly optimizing user association, bandwidth assignment, and power allocation. In [17], a satellite-aerial-terrestrial uplink network was investigated. The authors considered a high-altitude platform as a grant-based entity serving multiple mobile terminals to realize satellite-terrestrial uplink communication. NOMA was also applied on the high-altitude platform to increase communication capacity. Although many studies have focused on resource allocation in NOMA-based ISTNs, a majority of these papers have only implemented NOMA for a portion of UEs. And joint resource optimization is mainly divided into two categories: One considers fixed user association, optimizing power, and beamforming, which means UEs cannot flexibly choose to access BSs or satellites according to their individual requirements. The other category allows UEs to choose to access BSs or satellites, optimizing user association and power allocation. However, such research often assumes that all communication nodes are equipped with a single antenna, and therefore does not need to consider the impact of beamforming on user association. The joint optimization of user association, beamforming, and power allocation in NOMA-based ISTNs still poses challenges. Motivated by this, we investigatedjoint resource optimization in an uplink NOMA-based ISTN, where NOMA is applied for all UEs and each user can choose to access either the satellite or terrestrial BSs based on their communication rate requirements. Note that this work differs significantly from the investigations in [2; 9; 16; 17], both in terms of the scenarios studied and the parameters optimized. The main contributions are summarized as follows:  
* We propose an uplink NOMA-based ISTN, where each user can access either BSs or the satellite. With the goal of maximizing the achievable sum rate of all the UEs, an optimization problem of jointly optimizing the transmit power, receiving beamforming, and user association is established by considering the backhaul link capacity of BSs and the satellite, as well as individual quality of service (QoS) constraints. * We develop a two-stage algorithm to solve the problem. At the first stage, each user is associated with a corresponding access point (AP), i.e., either a BS or the satellite, based on their preference list and the backhaul link capacity of the APs. At the second stage, the power allocation and the receiving beamforming vectors are optimized iteratively. Within each iteration, the closed-form solution for the transmit power is derived. * The simulation results demonstrate the superiority of the proposed scheme compared with the random power allocation scheme and the traditional OMA scheme. In scenarios where the backhaul link capacity of terrestrial BSs is sufficient, UEs tend to access these BSs. However, when the backhaul link capacity of terrestrial BSs is insufficient, the satellite can offer QoS guarantees to UEs. Moreover, the system's overall performance achieves its peak when the number of UEs in the system aligns with the number of receive antennas at the APs.