Objective With the continuous development of small satellite technology, a large number of small satellite groups flying in formation have gradually replaced single large satellites and become a research hotspot in the field of aerospace. However, due to the constraints of size, mass, and power consumption of small satellite platforms, traditional RF and laser communication technologies can no longer meet the demand of large-capacity, small-delay, and high-reliability intersatellite communication links. As one of the most promising key technologies for the fifth wireless communication and beyond, visible light communication (VLC) has great potential in improving the spectrum efficiency and reducing the cost of spacecraft with many license-free spectrum resources. In practical applications, the narrow modulation bandwidth of LED seriously limits the system capacity and VLC communication rate. Thus, many high spectral efficiency technologies are used to improve the VLC system's data rate, including adaptive modulation, equalization technology, multiple input multiple output (MIMO), and multiple access technology. Among them, power domain NOMA (PD-NOMA) is suitable for downlink VLC systems to enhance the capacity and communication rate through power multiplexing.

Methods In this study, by combining the intersatellite VLC and NOMA technology, an intersatellite NOMA-VLC system consisting of a three-star formation configuration is constructed. Then, based on the analysis of the line of sight (LOS) link model of intersatellite and noise model at the receiver side, the model of signal transmission with power multiplexing is developed. Aiming at the problem of system communication rate optimization, an optimization model based on sum-rate maximization for the system is established. By transforming the nonconvex objective function into the convex function, an optimal power allocation strategy with low complexity is proposed to exploit the explicit optimal solution to the target problem using convex analysis.

Results and Discussions The performance of the NOMA scheme for the intersatellite VLC system is simulated and analyzed using MATLAB. The simulation results show that the sum-rate increases first, then decreases, and finally increases with the power allocation factors. Combined with the constraint conditions, the sum-rate achieves a maximum value of 124 Mbit·s ^-1 when the power allocation factor is 0.1 Fig. 4). Under the same conditions, the sum-rate will increase with the intersatellite distance ratio. It means that the greater the difference in user channel conditions, the more obvious the performance advantages brought by NOMA (Fig. 5). Besides, the average BER of 8-PPM modulation for the system will decrease with an increase in DC bias power; the BER of each curve decreases to the lowest at its optimal power allocation point. When DC power is 10 W, the average BER of the system can achieve 10 ^-3, while the average BER can achieve 10 ^-6 with 20 W (Fig. 6). Through the comparison of power allocation algorithms, it can be seen that the proposed power allocation algorithm performs better than the GRPA and FPA algorithms with a sum-rate factor of 0.5; its performance is almost the same as that of the LD algorithm. However, a convex problem solving during every iteration exacerbates the computational complexity of the LD algorithm; thus, the comprehensive performance of the proposed algorithm is optimal (Fig. 7).

Conclusions Based on the combination of intersatellite VLC and NOMA technology, this study investigates the power allocation of intersatellite NOMA-VLC systems. The simulation results show that the design of an efficient power allocation algorithm effectively improves the sum-rate of intersatellite VLC, which is similar to the NOMA technology in radio frequency communication. It also is shown that changing the DC bias power and power allocation factors of the algorithm will affect the system rate. With the increase in user channel difference, the performance advantage of NOMA will be more obvious, and the intersatellite VLC can increase the channel difference by increasing the intersatellite distance ratio. When the intersatellite distance ratio is too large, to ensure the system rate, the transmission power of one user satellite will be far lower than that of the other, which fails to guarantee fairness between users. Besides, through the comparison of power allocation algorithms, it can be seen that the comprehensive performance of the proposed algorithm is optimal.