Aiming at the problem of satellite power limitation, this paper proposes a distributed multi-satellite cooperative transmission (DMCT) scheme based orthogonal time frequency space (OTFS) for low earth orbit (LEO) satellite networks to achieve downlink power and transmission performance enhancement. Firstly, we construct the DMCT system model and channel model. Through theoretical analysis, it is found that the dynamic change of phase difference between each data stream from cooperative satellite cluster arriving at the terminal will cause serious time-varying fast fading. To this end, we design the OTFS transmission model for DMCT systems, and present the subspace pursuit (SP) channel estimation method and message passing (MP) data detection method taking advantage of the sparsity of the DMCT channel. Simulation results show that the proposed scheme can significantly enhance downlink power and transmission performance.
With the advancement of space technology and satellite communications, low-Earth-orbit (LEO) satellite networks have experienced rapid development in the past decade. In the vision of 6G, LEO satellite networks play an important role in future 6G networks. On the other hand, a variety of applications, including many delay-sensitive applications, are continuously emerging. Due to the highly dynamic nature of LEO satellite networks, supporting time-deterministic services in such networks is challenging. However, we can provide latency guarantees for most delay-sensitive applications through data plane traffic shaping and control plane routing optimization. This paper addresses the routing optimization problem for time-sensitive (TS) flows in software-defined low-Earth-orbit (LEO) satellite networks. We model the problem as an integer linear programming (ILP) model aiming to minimize path handovers and maximum link utilization while meeting TS flow latency constraints. Since this problem is NP-hard, we design an efficient longest continuous path (LCP) approximation algorithm. LCP selects the longest valid path in each topology snapshot that satisfies delay constraints. An auxiliary graph then determines the routing sequence with minimized handovers. We implement an LEO satellite network testbed with Open vSwitch (OVS) and an open-network operating system (ONOS) controller to evaluate LCP. The results show that LCP reduces the number of path handovers by up to 31.7% and keeps the maximum link utilization lowest for more than 75% of the time compared to benchmark algorithms. In summary, LCP achieves excellent path handover optimization and load balancing performance under TS flow latency constraints.
In this paper, we focus on the age of information (AoI) aware intelligent architecture of emergency service by caching placement optimization on satellite. Such an intelligent architecture can effectively improve the AoI of users when the cellular network are broken in a disaster area. Unlike the traditional method, our architecture can learn through interaction with the environment, continuously optimizing the cache policy to improve cache hit rate and system performance. Specifically, we first model the emergency service process of the satellite as a Markov process. Then, we apply the advantage actor-critic (A2C) algorithm to obtain the optimal satellite cache strategy. By extensive simulation results, we can verify that the proposed intelligent architecture outperforms the existing traditional methods.
In this paper, we propose a novel distributed multi- satellite cooperative transmission (DMCT) system model. Based on the analysis of the channel correlation between multiple antennas of multiple satellites and multiple antennas of a single smartphone in this model, we design three types of connection modes under different scenario configurations by drawing on terrestrial mature MIMO technologies. Then, we further discuss and analyze the application limitations and main characteristics of each connection mode. Finally, we discuss several key technologies that need to be conquered for the implementation of the proposed DMCT based system.
In this paper, a frequency offset independent timing synchronization method is proposed to solve the large frequency offset problem for the 5G integrated low-earth-orbits (LEO) satellite communication (SatCom) system. Firstly, a frequency offset independent timing metric is designed. Without changing the frame structure of NR and based on the conjugate symmetry of the primary synchronization signal (PSS) time-domain sequence, the received PSS sequence times its time-inverse sequence to construct a product sequence, each element of which has the same phase. And the phase can be canceled by the absolute value operation. Secondly, a delay-superposition method is used to improve the performance of the designed timing metric. The detection threshold can be improved further by accumulating the correlation peak value of each PSS in the SSB burst, and the number of superpositions can be selected flexibly to adapt the link-budget of the LEO SatCom system. The simulation results show that the proposed method maintains a high detection probability with increase of the frequency offset. Compared with the M-part PSS cross-correlation algorithm, the proposed method can improve 49% detection probability at −6dB when the frequency offset is as large as 600kHz.
We designed a FPGA based LDPC Decoder for decoding Type-I QC-LDPC codes with flexible code rate and code length, by which the channel adaptation could be realized with very fine granularity. In addition, the decoder shows good bit error rate (BER) performance whose performance loss is only O.6dB at the BER of 10 −6 , compared with the theoretical performance when decoding the same LDPC codes using normalized Min-Sum based layered decoding algorithm.
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