# Robust Symbol Timing Synchronization for Initial Access under LEO Satellite Channel
## 1 Introduction  
In the last decades, many private companies and public organizations have provided connectivity services based on satellite communication networks. In recent years, LEO satellite networks based on large multi-spot beams using Ku and Ka bands have been deployed or are being developed and will be operational over the coming years. The representative companies are OneWeb, Starlink of Space-X, Kuiper of Amazon, and Lightspeed of Telesat LEO [1]. They can offer higher data rates in the range of more than 150 Mbps per user and lower latency in a few tens of milliseconds. In light of improved data rates and reduced latency compared to conventional Geostationary Earth Orbit (GEO) satellite communication systems, recent research efforts have vigorously explored the integration of satellite-terrestrial networks [2, 3, 4, 5]. This endeavor aims to extend service coverage to underserved areas where terrestrial networks are not deployed. Meanwhile, according to the available literature, all other companies except for Telesat LEO have developed communication networks and user terminals utilizing proprietary specifications rather than adhering to commercial standard technologies such as 3 GPP and DVB. In the context of the 3 GPP NTN standard, it is noteworthy that the development of the first RAN standard to facilitate LEO satellite communication was initiated in release 17 and is currently in progress in the stage of release 18 [6]. Regarding the 3 GPP NTN standard activities, the specification works have been undertaken to support the compatibility through softwareupdates without modification of the existing hardware of TN's UEs. Typically, network timing and frequency synchronization are achieved with the aid of the assumption that satellite UEs of handheld or VSAT type are equipped with GNSS receivers. However, in the context of the TDMA-based DVB-S2x/ RCS2 standard, which has been mainly used for GEO satellite communication, network synchronization can be achieved by means of periodic broadcasting of internal highly accurate clock information from the gNB and proper acquisition from the UE without relying on the GNSS reference clock. Notably, the DVB standard has been adopted as the reference specification for Telesat's LEO (Low Earth Orbit) network [7], providing the proven Commercial On-The-Shelf (COTS) product in the market to satellite communication operators. Moreover, the potential to enable UEs for GEO satellite communication to support the coexistence with LEO satellites through software upgrades without new hardware is capable of supporting multi-orbit satellite constellations. Reviewing the DVB-RCS2 standard was published in 2011 for the GEO satellite [8]. It has been developed to enhance spectral efficiencies compared with the previous DVB-RCS system to satisfy the high demand for broadband service in the Ka band. In particular, the utilization of distributed pilot symbols and postamble, as well as preamble in a transmission packet, can be of help to the timing and frequency offset estimation accuracy. As a result, it can lead to Packet Error Rate (PER) performance improvement when operating the demodulator under much lower SNR conditions. The physical layer of DVB-RCS2 specification has two different modulation schemes. One is linear modulation like Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM), and the other is nonlinear modulation like Continuous Phase Modulation (CPM). Both schemes were selected as the mandatory technologies in the UTs due to their unique strengths in various service applications from a technical perspective. Meanwhile, the related research has been extensive, but there has been a scarcity of previous work focusing on the complete demodulator design. Most of the works have focused on the Carrier Recovery (CR) part, and there were limited descriptions of the demodulator in terms of functional block and performance results [9; 10; 11; 12; 13; 14].