Satellite clustering has the potential to enhance inter-satellite cooperation, resist satellite malfunction, and enable more agile space-air-ground integrated applications. This paper investigates a promising model for clustered low Earth orbit (LEO) systems, in which one typical unmanned aerial vehicle (UAV) can assist one satellite cluster to serve one random terrestrial user. Particularly, intra-cluster satellites can communicate user, while inter-cluster satellites are regarded as interference. Two types of satellites and users are randomly deployed at three visible spherical spaces by adopting three independent spherical Poisson point processes. In the modeling, an adaptive selecting mechanism is proposed to pick the strongest received signal between direct and cooperative transmissions. To facilitate a simpler analysis, we firstly transform the three spaces into the three planes through modifying their respective density. Next, assuming that the shadowed-Rician fading is employed in the satellite channel, two Gamma random variables are utilized to approximately express the aggregated power of interference and noise received by the UAV and user, respectively. Subsequently, the exact conditional user association and approximate Laplace transform of the accumulated signal power are derived to further investigate the conditional coverage probability. Finally, simulation results illustrate that: 1) Moderate satellite cluster sizes combined with a UAV altitude of about 200m are beneficial for achieving higher coverage probability; and 2) The adaptive selection mechanism generally achieves comparable or better performance than traditional transmissions by leveraging spatial diversity.

The integration of non-orthogonal multiple access (NOMA) into low earth orbit (LEO) systems has the potential to facilitate the ubiquitous coverage with high spectrum efficiency. To characterize the fundamental limits of NOMA assisted LEO systems, this paper proposes a model for downlink NOMA-LEO system via modelling the locations of terrestrial users and LEO satellites as two homogeneous spherical Poisson point processes. In particular, a typical satellite uses NOMA to simultaneously serve the nearest and the farthest users within its visible range. The novelty of this paper is to first introduce an equivalent two-dimensional model that can significantly simplify the performance analysis of LEO systems. Then, considering that the satellite-terrestrial channel follows the Nakagami-mfading, the closed-form expressions of the user association and the visible probability are studied under the scenario where the number of users visible to a randomly selected satellite is greater than one. Subsequently, the derived results are utilized to analyze the approximate moments of the conditional success probability and the signal-to-interference-plus-noise ratio Meta distribution for both NOMA-LEO and orthogonal multiple access (OMA) LEO transmissions. Finally, the numerical results demonstrate that:1)Asymmetric target rates can achieve a performance gain of NOMA over OMA in terms of the link reliability and the coverage probability, while symmetric settings still have merit for NOMA if there is a low requirement for reliability; and2)Enhancements in the link reliability and the coverage probability are achievable through improvements in channel quality and reductions in orbital altitude and density. However, improving path loss develops the coverage probability but may not always yield an increase in the link reliability.

This paper proposes a downlink non-orthogonal multiple access (NOMA) low earth orbit (LEO) satellite system by modeling the locations of terrestrial users and LEO satellites as two homogeneous spherical Poisson point processes, respectively. Firstly, considering that the satellite-terrestrial channel follows the Nakagami-m fading, the closed-form expressions of the user association and the visible probability are studied under the scenario where the number of user visible to the random selected satellite is greater than one. Subsequently, the above results are adopted to analyze the approximate results for the moments of the conditional success probability and the signal-to-interference-plus-noise ratio Meta distribution. The numerical results demonstrate that the asymmetric target rates can realize a performance gain of NOMA over orthogonal multiple access in terms of the link reliability and the coverage probability, while symmetric settings still have a merit for NOMA when there is a low requirement for the link reliability.

The key idea of this paper is to leverage channel angle information to propose a low-complexity mode switching design in simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided multi-antenna non-orthogonal multiple access (NOMA) systems, which is suitable to both downlink and uplink transmissions by applying the concept of angle reciprocity. Particularly, a location-based assignment algorithm is firstly provided to perform NOMA pairing, then the equal gain transmission at the base station and the cophase matching criterion at STAR-RIS are adopted to improve the performance of the reflected user and the effective channel gain of the paired users, respectively. According to the proposed design, considering the difference in the Rician factors of the channel from the base station to STAR-RIS caused by the change of line-of-sight and non-line-of-sight components, two different cases for channel statistics of the paired users are rigorously characterized. Subsequently, we utilize the above results to analyze the closed-form expressions for the outage probability and ergodic rate of all users under downlink and uplink transmissions. Further, the diversity orders and the high slopes corresponding to all expressions are also determined to pursue more insights. Finally, numerical results are presented to demonstrate our analysis and reveal that: 1) the various system factors have significant impact on the performances of the proposed design; and 2) the low-complexity design can reduce the overheads of channel estimation and signal processing at the expense of extremely-low or even no performance loss in contrast to the traditional maximum ratio transmission based design.

The application of distributed reconfigurable intelligent surface (RIS) to massive access multiple-input single-output (MISO) is significant to extend the communication coverage. In this paper, a novel framework is proposed in distributed RIS-aided massive access MISO systems with supporting non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) transmissions simultaneously, where a joint active and passive beamforming scheme is designed to fully eliminate the inter-cluster interference and improve the channel gains of prioritized users, respectively. Based on the proposed framework, firstly, we derive two exact channel statistics to characterize the equivalent channel gains of prioritized and non-prioritized users, respectively. Then, by taking into account the influence of imperfect channel state information (CSI) and successive interference cancellation (SIC), the approximate expressions of outage probability and ergodic rate for all users of one cluster under MISO-NOMA and MISO-OMA transmissions are analyzed to obtain their corresponding system throughput. Moreover, by utilizing the above results, we also determine the diversity order and high slope of these users to attain more viewpoints. Finally, simulation results prove our analyses and reveal that: 1) enhancing the estimated accuracy of CSI and the ability of SIC process can remarkably enhance the system performance; 2) the performance of priority users will be significantly improved with the increase of the number of reflecting elements and Rician factor; 3) heterogeneous quality of service requirements and deployment behaviors of users are beneficial for NOMA, while homogenous settings are competitive for OMA.

In this paper, we propose a low-complexity mode switching design in a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided multi-antenna non-orthogonal multiple access (NOMA) system via exploiting channel angle information only. Particularly, a location-based assignment algorithm is firstly provided to perform NOMA pairing, then the equal gain transmission at base station and cophase matching criterion at STAR-RIS are adopted to improve the performance of reflected user and the effective channel gain of paired users, respectively. On the basis of to the above design, we study the exact channel statistics in two different cases to further derive the outage probability of reflected and transmitted users. Numerical results are presented to demonstrate our analyses and reveal that: 1) the various system factors have markedly impacts on our proposed design; and 2) the low-complexity design can reduce the overheads of channel estimation and signal processing at the expense of extremely low performance loss.

In this paper, we investigate a distributed reconfigurable intelligent surface aided massive access in multipleinput single-output non-orthogonal multiple access system with imperfect channel state information (CSI) and successive interference cancellation (SIC). In particular, a novel active and passive beamforming scheme are designed to fully eliminate the intercluster interference and improve the effective channel gain of the prioritized users, respectively. To study the performance of the proposed scheme, the exact channel statistics are derived to further analyze the outage probability of each user within a cluster. Finally, simulation results are presented to prove our theoretical analyses and reveal that: 1) enhancing the estimated accuracy of CSI and the ability of SIC process can significantly enhance the outage performance; 2) the proposed zero-forcing based scheme can obtain a higher system throughput compared to previous designs.

In this paper, we study a distributed reconfigurable intelligent surface-aided downlink multiple-input multiple-output non-orthogonal multiple access systems with random distributed users, where the signal alignment can be achieved within the paired users by controlling discrete phase shifts. In particular, a unified precoder and decoder are provided to cancel inter-cluster interference. To evaluate the performance of proposed framework, the near and far users’ channel statistics with Nakagami-m fading are derived. Further, the approximate expressions of average outage probability are obtained by utilizing the proposed channel statistics, and the corresponding diversity orders can also be obtained to acquire more insights. Finally, simulation results reveal that not only discrete phase shifts design by selecting a 3bit resolution can realize the near optimal performance but also the diversity gain can be significantly improved by increasing the number of reflection elements.

The combination of reconfigurable intelligent surface (RIS) and non-orthogonal multiple access (NOMA) has been recognized as a critical method to improve the sixth generation networks performance. In this paper, a distributed RIS-aided downlink multiple-input multiple-output (MIMO) NOMA systems with discrete phase shifts are studied, where the channel directions from base station to paired users can be manipulated with the assistance of RISs by employing the concept of signal alignment. In order to ensure base station can flexibly serve some users with NOMA and others users with spatial division multiple access, a unified precoder and decoder are provide to cancel inter-cluster interference. Subsequently, the channel statistics over Nakagami-$m$fading channels are derived for near and far users. In particular, considering the cascade channel gain may exists two different cases, Beaulieu series is further adopted to characterize their corresponding cumulative distribution function. In what follows, the outage probability and ergodic rate for three situations within one cluster are derived by utilizing the obtained channel statistics, respectively. Based on the derived results, we also analyze the diversity order and high signal-to-noise ratio slope to provide essential insights into the considered systems. Finally, simulation results are presented to reveal that: 1) selecting the setting of 3-bits resolution can realize a near-aligned performance for our proposed systems; 2) the cascade channel statistic caused by RIS can be evaluated with any number of RIS element and any channel gain via Beaulieu series.