## 1 Introduction
[21] proposes combining SDMA with precoding to improve a system's capacity by reducing interference between users, while as the density of user distribution increases, more and more accurate channel state information (CSI) needs to be provided. In practical application scenarios, user distribution is uneven and random. Therefore, a dynamic allocation of beam bandwidth resources according to the user channel state [22; 23] and a beam-hopping algorithm (BH) based on service requirements are proposed [24; 25; 26; 27]. Although the resource allocation is time- and space-varying, it still cannot meet the needs of an exponential increase in the number of users. However, the combination of dynamically allocated resources and pre-emptive channels will lead to packet loss due to message squeezing [28]. In addition, ref. [29] proposes an adaptive grouping strategy based on the user density under each beam coverage to optimize the beam layout for ADS-B signal reception, providing s a way to adjust the beam layout based on the user distribution. This article mainly focuses on the study of the S-IoT using narrowband satellites to collect IoT information from various sensors. When faced with the demand for IoT terminal user access at the level of hundreds of millions of users under the condition of limited frequency resources, it is necessary to maximize spectrum utilization and reduce frequency idle time. However, most of the aforementioned research work is focused on satellite signal downlinking, and most of the solutions with high spectrum utilization adopt fixed resource allocations. This results in a situation where a few users occupy most of the frequency resources. The overall spectrum utilization of solutions based on users dynamically adjusting resources is low and cannot meet the needs of massive user access. Therefore, for uplink signals, we propose to improve the frequency reuse rate from the four dimensions of space, time, frequency, and code, and dynamically adjust the frequency resource allocation within the beam and the beam layout according to the user distribution in order to increase the user capacity of the S-IoT. The main contributions of this paper are summarized as follows:* To increase the user capacity of the S-IoT, a multi-dimensional hybrid multiple-access method for space-time-frequency-code division based on user distribution (MHSTFC-UD) is proposed. It divides a single beam cell into a central and edge region, with the edge region serving as a protection interval. Combining TDMA, FDMA, and CDMA, full-frequency multiplexing is possible between beams, which can increase the number of users by one to three orders of magnitude. * To improve spectrum utilization, we propose dynamically adjusting the allocation of frequency resources and the beam layout based on the user distribution. By adjusting the radius of the central area and the frequency allocation ratio between the central area and the edge area, the situation where allocated frequency resources are not used can be reduced. * We propose to use the genetic algorithm to optimize the radius of the central area and the proportion of frequency resources allocated. Compared with fixed resource allocation and layout methods, the MHSTFC-UD can increase user access by about 27.5%. The rest of this paper is organized as follows. Section 2 introduces the MHSTFC-UD model and the simulation of the terrestrial user distribution model. Section 3 introduces the beam layout and frequency allocation optimization algorithm based on user distribution and takes the user access amount as the optimization target. Section 4 conducts simulation comparison experiments. Finally, Section 5 summarizes the research results of this paper.