With the advancement of Internet applications, the global demand for broadband satellite communication services is incessantly escalating. Furthermore, there exists an exponential surge in the requisition for the capacity of satellite communication systems. In response to this evolving demand, the domain of satellite communication technology is progressively evolving toward the realm of high-throughput satellite (HTS) communication systems. The typical technical characteristics of HTS are: (a) the satellite adopts multibeam technology for beam overlapping coverage of the service area, which improves the quality of the wireless link while realizing the spatial dimension segmentation; (b) based on this, the system adopts multiple frequency multiplexing to improve the communication capacity of a single satellite; (c) gateway stations are tightly coupled with user beam clusters to complete two-hop communication, which makes multiple gateway stations share the communication resources of the same satellite, forming a spatial isolation of the gateway stations, and once again realizing the frequency multiplexing of the gateway station links. A key requirement for future multibeam broadband satellite communication systems is the ability to flexibly adjust beam capacity according to changes in business distribution, in order to meet time-varying business requirements. Beam hopping technology provides an efficient solution to achieve the efficient use of frequency resources and power resources. At the same time, the use of beam hopping makes the beam hopping of satellite payload need to match the business signals of ground signal stations, bringing about the need for synchronization of beam hopping between satellite and ground. In a review article recently published in Space: Science & Technology, scholars from Xidian University, CAST-Xi’an Institute of Space Radio Technology, Beijing Institute of Technology, Global Big Data Technologies Centre, University of Technology Sydney jointly analyze the synchronization problem of satellite to ground hopping beam, propose a signaling-assisted fast synchronization method of hopping beam for satellite to ground synchronization.
First, the basic principle of HTS hopping beam communication is introduced and the key technologies involved in the satellite hopping beam communication system is analyzed. Beam hopping in HTS system is a special way of load design, the on-satellite antenna is a full-coverage multibeam antenna, but the transponder resources are shared among multiple beams in time-sharing, which is essentially a signal hopping between different beams in time sharing on demand, and it is a beam-hopping system from the point of view of the user's use, and Fig. 3 gives a schematic diagram of the working principle of the hopping load of a typical forward link of HTS satellites. There are 3 key technologies of hopping beam communication in the HTS system. (1) Beam-hopping high-precision satellite-ground synchronization technology. Since beam-hopping is essentially a time-division system, if the service signal sent by the gateway station and the satellite beam-hopping load time are not synchronized, resulting in the mismatch between the satellite beam switching time and the forward service signal, it will directly affect the normal reception of the user terminal. (2) Low signal-to-noise demodulation of high-speed burst signals. In the beam-hopping communication system, beam-hopping causes the continuous time-division multiplexing (TDM) signal of the traditional forward link to become a burst TDM signal. As a result, the ground receiver needs to adapt to demodulation in burst mode, which necessitates fast synchronization of the ground demodulator. (3) Efficient resource management techniques for beam-hopping systems. The significant advantage of the hopping beam technology is that it can dynamically allocate resources according to the service volume of different wave positions, which makes the resource use efficiency of the whole system optimal. In order to give full play to the advantages of the hopping beam system, it is necessary to study the highly efficient resource allocation technology used in the hopping beam. Among the above key technologies, the hopping beam satellite-ground synchronization is the most important technology, which is the key to the successful operation of the system.
Then, a forward hopping beam synchronization method based on independent signaling carrier assistance is proposed. In order to complete high-precision forward hopping beam synchronization, simplify the complexity of hopping beam synchronization and enhance the flexibility of hopping beam application, the paper proposes a satellite-Earth hopping beam synchronization method with on-satellite synchronization to the ground gateway station, the basic principle of this method is to send an auxiliary hopping beam synchronization signal in one direction through the ground gateway station, and the on-board hopping beam controller will synchronize the hopping beam synchronization signal and then drive and control the switching of the hopping beam switch to achieve the star-ground synchronization. The whole signaling signal adopts burst transmission mode. The whole beam-hopping synchronization control signal consists of 2 parts: the first part is the fixed pseudo-random (PN) capture sequence with long length, and the second part is the control information. 1. A fixed-length PN sequence is used for satellite control signal burst detection, which requires good correlation; it is recommended that the length of the PN sequence is more than 128 bit. 2. Time synchronization sequence for assisting is to achieve high precision time deviation estimation. 3. The control information is recommended to be encoded by RM(7,64) in the DVB-S2X standard, which is characterized by a BER still better than 10e−9 at Es/N0=0dB. The block diagram of the on-satellite processing flow is given in Fig. 14. Step 1: Perform AD sampling and quadrature frequency conversion on the input signal to ensure 8 sampling points per symbol; Step 2: Perform serial-to-parallel conversion on the frequency-converted I/Q data to generate 8-channel parallel data; Step 3: Under the control of the demodulation master control module, the 8-channel parallel data are time-differentiated and then hard-judged; Step 4: correlating the 8-channel data sequence after the hard judgement of the dedifferencing with the local differential PN sequence in time, performing the detection of the correlation peak, comparing the size of the correlation value with the threshold value, and when the correlation value is larger than the threshold value, taking the point with the largest correlation peak among the adjacent 8 positions as the time reference point; Step 5: The data with the largest correlation peak all the way goes to the control information Reed-Muller (RM) decoding, and the beam control information is extracted.
Last, a high-precision synchronization method based on guide frequency assistance for HTS forward hopping beam star-Earth synchronization is proposed. Accurate timing deviation estimation is the key to the time synchronization of the hopping beam system, which affects the performance of the whole system. This paper adopts a light-weight high-precision timing deviation estimation method, which uses alternation as an auxiliary sequence to achieve high-precision timing deviation estimation while reducing complexity. The sequence is modulated by BPSK, and presents sinusoidal signal characteristics after root-raised cosine pulse shaping at the sender end and matched filtering at the receiver end, and the comparison of its waveform with the standard sinusoidal signal. Considering the transmission performance, the commonly used shaping factor parameter α = 0.35 can be selected, in which the normalized error between the shaped sequence waveform and the sinusoidal signal is less than 5e−3. A clock deviation 4-sample point estimation algorithm is given for a signal design as above. The four-sample point estimation algorithm for clock deviation in this paper is as:
τ = 1/π * angle[(A − jB)(A + jB)] = 1/π * angle(A2 + B2)
where A = x(4m) − x(4m + 2), B = x(4m + 1) + x(4m + 3), x(n) are four consecutive sampling points, and τ is an unbiased estimator of the timing deviation ε. Furthermore, simulation evaluation of the algorithm is carried out. The modulation method is selected as BPSK, the shaping coefficient is 0.35, the length of the sequence is 64, and the normalized simulation sampling parameters are set: TS = 100, T = 400, and the values of ε are taken in [0,1], and the values of δ are taken in [−0.5,0.5]. The simulation results in Fig. 22. Results show that the estimation accuracy of timing deviation can reach within 2% without frequency bias under the condition of Eb/N0=10dB. The carrier phase deviation has no effect on the algorithm. The estimation performance of the algorithm is consistent when the normalized frequency bias is in the range of ±10%. In conclusion, the algorithm given in this paper not only improves the time synchronization accuracy of the beam-hopping system, but also reduces the complexity of the processing, compared to that of the traditional method of adopting the digital squared filter, which has the significant advantages of high estimation accuracy and lightweight engineering implementation for the application background.
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