# Robust Symbol Timing Synchronization for Initial Access under LEO Satellite Channel
## 1 Introduction
Specifically, the references [9; 10; 13; 14] have dealt with practical or robust carrier synchronization to minimize the PER performance gap versus ideal conditions that work perfectly with STR in the AWGN channel. The reference [11] studied only the CPM modulation scheme in the DVB-RCS2 standard. The reference [12] described a complete DVB-RCS demodulator architecture that contains STR function but was considered a conventional scheme because mobility condition is not harsh in GEO satellite. The reference [14] showed a complete DVB-RCS2 demodulator structure and PER performance, but the STR scheme has been applied to employ the existing one that selects the maximum value as the optimum timing epoch position among 32 cross-correlation values within a symbol period. However, this approach did not consider the rapid timing and frequency phase variation that is characterized by GEO satellite conditions. The reference [15] has dealt with the demodulator of the Spread Spectrum (SS) mode to support DVB-RCS2 mobility. Under mobility conditions, a robust timing synchronization scheme is required to mitigate the Doppler effect. However, an SS scheme based on direct sequence can extend the number of chips in a packet by Spreading Factor (SF) length. It can provide the effect of extending the number of chips capable of estimating a time and frequency error. In general, the previous works did not address the description of Log-on Burst (LB) and Control Burst (CB) transmission for initial access or resource requests, respectively, which are characterized by short packet length and a large time and frequency offset condition. In this paper, we clarify the necessity of new technology and propose a hybrid STR structure to improve the PER performance for the initial access mode. Note that our proposed structure is verified for its robustness in terms of a link-level performance-oriented approach. The main motivation and contributions of our work can be summarized as follows. * We propose a robust STR scheme for a GW, which is not susceptible to a large Doppler offset caused by the LEO satellite. In the GEO satellite, most conventional schemes considered limited Doppler offset and timing uncertainty environment due to the motion of UTs. On the other hand, for LEO satellites, it is not simple to maintain network synchronization between satellite and ground UTs because the UTs are used to receive a variable Network Clock Reference (NCR) signal due to the severe Doppler offsets caused by satellite motion. Thus, UTs are required to continuously estimate the differential propagation delay based on the serving satellite's position and velocity within a beam. Furthermore, our scheme needs to consider the scenarios when UT should support high-speed vehicles such as airplanes or when UT should have a clock oscillator with low accuracy for cost-efficiency purposes [15]. Therefore, our proposed scheme can serve as a temporary solution until novel Forward Link Signaling (FLS) information, such as DVB System Information (SI), is developed to address Doppler and timing uncertainties in the DVB-RCS2 standard work for supporting LEO satellite communication [16]. * In the case of the 3 GPP NTN standard, UTs are capable to compute and pre-compensate for the delay and Doppler frequency offsets due to the LEO satellite velocity and position through ephemeris information from GW's and UT's positions through the GNSS receiver when triggering access to GW [17; 18]. Accordingly, there is a need to update the DVB-RCS2 standard, like the 3 GPP NTN standard, to accommodate LEO satellite communication [16]. Until the amendment in the DVB standard, the proposed scheme can be effective as a viable option through GW receiver implementation technology within the standard. Even in the 3 GPP standard, robust time and frequency synchronization enhancement based on non-GNSS operation has been required and discussed as a candidate technology for release 19.