By Musbah Shaat and Marius Caus, CTTC, Spain
Current 3GPP Status for end-to-end
Non-Terrestrial Networks (NTN) has been integrated into 3rd Generation Partnership Project (3GPP) Release 17, following prior studies in Releases 15 and 16. Some key characteristics of NTN include the following end-to-end strategies:
- Direct access supports only transparent payload architectures.
- Specifications are confined to deployments in frequency range (FR)1 bands and frequency division duplex (FDD) mode.
- The User equipment (UE) is equipped with a global navigation satellite system (GNSS) receiver.
- Consideration of Earth-fixed tracking areas, with both fixed and mobile satellite cells.
- Satellite assistance information is transmitted in system information block 19 (SIB19).
- The network is responsible for managing the Doppler frequency shift experienced over the feeder link.
To address the challenges and overcome the impairments posed by the Geostationary Earth Orbit (GEO) and Low Earth Orbit (LEO) satellite communication systems, the normative work includes adaptations to the protocol. Procedures like random access, uplink timing and frequency synchronization, to mention a few, should consider the satellite delays and carrier frequency offsets induced by the orbital motion. TRANTOR aims to have an impact on 5G Advanced releases, including up to Release 20. In this article, we summarize the key aspects of future releases, i.e. Release 18 and Release 19 and then discuss TRANOTOR proposed fields of enhancement to Release 17.
Release 18 has started its progress in 2022. Concerning the system aspects, this release will study in 3GPP TR 23.700-28 the reduction of the no coverage period and introduce architecture enhancements for satellite backhaul in 3GPP TR 23.700-27. For instance, these enhancements aim to support backhaul with variable delay, which may be caused by inter-satellite links, and enable satellite edge computing services by hosting user plane functions on-board. Within the scope of the radio access network (RAN), the New Radio (NR) NTN enhancements work item will focus on the following areas:
- High throughput satellites with deployments above 10 GHz, prioritizing the Ka band.
- Mobility and service continuity enhancements.
- Location services to facilitate network verified UE location.
- Coverage enhancements for handheld terminals.
In 2022, study items targeting the 3GPP Release 19 were approved. The study item “Study on satellite access – Phase 3” will concentrate on new capabilities, such as the satellite access without GNSS receiver and communication between UEs under the same satellite footprint. To identify additional spectrum for NTN, a work item to support satellite services in the Ku band (Downlink: 10.7 – 12.75 GHz; Uplink 12.75-13.25 GHz & 13.75-14.5 GHz) will be drafted in 2023.
From the architectural standpoint, it is anticipated that satellite payloads will become regenerative in the coming years. The 3GPP Release 19 is expected to be the first release that supports regenerative architectures. Next generation satellite payload technology is expected to provide the required computational capabilities for hosting radio signal processing. In the logical architecture proposed by 3GPP, the gNB is split into three logical nodes: the central unit (CU), the distributed unit (DU) and the radio unit (RU). 3GPP defines eight functional split options, considering that RF functions must be handled by the RU. The most suitable functional split will be dictated by the network topology and the supported applications.
TRANTOR proposed fields of enhancement to Release 17
Doppler Compensation Schemes
Based on the recommendations submitted to 3GPP TR 38.811 and TR 38.821, TRANTOR addresses two compensation schemes:
- UE specific pre-compensation: where the UE possesses the capability to autonomously pre-compensate both the instantaneous Doppler effects and the RTT delay on the service link. This compensation requires that the UE acquire its location, the satellite ephemeris, and the partial common delay. To achieve this, the UE is equipped with a GNSS receiver to provide the location while the gNB broadcasts the satellite ephemeris and the partial common delay as part of the system information, which the UE can obtain during the downlink synchronization process.
- Beam specific pre/post-compensation: the gNB takes measures to pre-compensate and post-compensate the Doppler effects, respectively, at a specific reference point, such as the beam center.
The enhancement of both schemes requires the broadcasting of specific parameters in a dedicated system information blocks such as the architecture type (transparent or regenerative) and the partial common delay which represents the minimum round-trip time (RTT) between the satellite and the closest point on Earth within the satellite beam coverage.
To achieve DL synchronization, acquire cell ID and basic system information, the detection of the synchronization signal block (SSB) is necessary. However, the presence of phase uncertainty arising from the carrier frequency offset (CFO) can pose a major challenge in SSB detection. The overall frequency offset comprises four parts: UE oscillator offset, satellite oscillator offset, Doppler shift caused by the satellite movement and Doppler shift resulting from the UE movement. The most suitable SSB pattern, as defined in 3GPP TS 38.213, should be selected based on the expected maximum carrier frequency offset. To enable the operation in KU and Ka bands, a new synchronization signal (SS) raster must be specified.
To successfully detect the preamble, the uplink carrier frequency offset observed by the gNB should be lower than half the subcarrier spacing. This is a requirement posed by the preamble detector to achieve robustness to the detrimental effects induced by the residual Doppler. Beam specific pre/post compensation scheme is not suitable for deployments in the Ka band as it requires excessively high overhead with tenfold cyclic prefix (CP) extension. Remarkably, the UE specific pre-compensation scheme can be adopted even when the GNSS service is not available where a rough estimate of the position can be obtained by exploiting the satellite ephemeris and tracking information in the SSB. Accordingly, new design preamble principles are required to operate in presence of large positioning errors which considers the uplink CFO as well as the beam radius.
The waveform that is selected in NTN is based on the orthogonal frequency division multiplexing (OFDM) modulation to maximize synergies with terrestrial networks. However, OFDM may not be the optimal choice as it requires large subcarrier spacing to cope with the Doppler effects. However, when the subcarrier spacing is increased, the sampling frequency increases as well and thus, the CP becomes shorter, and the symbols become more sensitive to timing errors and multipath fading. OFDM has a high peak to average power ratio (PAPR) which affects the efficiency of the satellite power amplifier efficiency. Orthogonal Time Frequency Space (OTFS) has been suggested to work with much robustness against Doppler as it is specifically designed to work on doubly selective channels and that it can be transmitted with lower overhead than OFDM. Application of OTFS to NTN is at an early stage, there are still some aspects that need further study. Open research areas include the design of synchronization algorithms, access protocols, detection schemes and reference symbols, to mention a few.
TRANTOR is actively addressing the challenges and complexities discussed in this article. Stay tuned for more updates on the innovative solutions that will be developed.