The finalisation of 3GPP’s Release 17 has solidified space-based 5G NB-IoT and direct-to-device connectivity as a reality. Increasing interest in satellite-based 5G services places new demands on NB-IoT system architecture to achieve uninterrupted network coverage across vast geographical areas whilst addressing current connectivity challenges and constraints worldwide. GateHouse SatCom’s Product Director, Svend Holme Sørensen, and Senior Software Architect, Henrik Krogh Møller, introduce the architecture and system elements required for developing 5G NB-IoT NTN systems now and in the future.
GateHouse SatCom has developed leading-edge satellite communication systems for over 20 years. Today, the software-only company and waveform developer contributes its expertise to several commercial and military markets and recently became an Individual Member of ETSI and an active participant in 3GPP’s revolutionary Release 17 to standardize direct-to-device 5G NB-IoT specifications.
NT3GPP 組み込まれたことで、次世代のIoTデバイスは、モバイルネットワークと衛星ネットワーク間のローミングを実現するために、NTN対応チップセットへの更新のみで済む見込みである。
「デバイス直結型5Gサービス機能は、衛星通信における重要なマイルストーンであり、多くの魅力的な機会を提供します」と、GateHouse Satcom、スヴェンド・ホルム・ソレンセンは述べ、続けてこう語った：
従来、衛星通信専用に設計された大型端末や巨大なアンテナを通じて衛星接続を目にしてきました。現在当社が開発を支援するデバイスは地上波と衛星サービスの両方に対応し、通信が困難な地域を含む複数業界において、地上波と衛星ベースの5Gネットワークデバイスを広く途切れなく利用可能にします。
産業分野では、NB-IoT システムアーキテクチャの機能を活用し、地上波サービス向けに設計されたデバイスにも衛星駆動型5Gサービスを導入できる。
多くのデバイスは地上波接続のみをサポートしていますが、NB-IoT 応可能性は非常に高いです。物流業界はついに待ち望んでいたエンドツーエンドの可視性を実現できるでしょう。農業分野では、衛星通信を活用した家畜活動トラッカーやGPSトラッカーの導入が可能になります。同様に、農業分野では衛星通信による無線温度・湿度ロガーの採用が進むでしょう。可能性は無限大です。
However, Svend explains further that developing 5G NB-IoT system architecture to achieve optimal interoperability and connectivity between terrestrial networks, non-terrestrial networks and millions of IoT devices isn’t without its challenges.
遅れを取り戻し、適応する
3GPP originally created NB-IoT for terrestrial use in its Release 13 specifications, with the system architecture involving no satellite-driven signaling.
NB-IoT を介してネットワークに直接接続し、セルラーネットワーク内の地上設置型NodeBタワーに到達する。NB-IoT インターネットへデータを送信する際には、パケットコア（この場合は4G EPC）を経由する。NB-IoT 、Release 16事前調査およびRelease 17仕様書を通じた衛星駆動型NTN接続性の探求の基盤を形成した。
“Satellites now play a critical role in adapting NB-IoT NTN system architecture to existing terrestrial networks. Instead of a device connecting to a NodeB cellular tower directly, its service link will first pass through a satellite, forwarding a signal via a feeder link to a ground-based NTN NodeB that’s connected to the satellite operator’s EPC”, explains GateHouse SatCom’s Senior Software Architect, Henrik Krogh Møller.
Release 17 currently focuses on Transparent Mode (See Fig. 1), where the NodeB is ground-based. Regenerative Mode, which will be studied in Release 18 and again in Release 19, aims to implement NodeBs as payloads on the satellites. Implementation will require further device, NodeB and EPC adaption to increase connection speed and efficiency.
サービスおよびフィーダーリンク制約
Several service and feeder link challenges exist, including doppler, delay, signal changes and path loss, all of which depend on the satellite, its elevation angle and position in the sky.
High-orbit GEO satellites operating 36,000 km above the ground will experience significantly higher path loss than lower orbit LEO (NGSO) satellites at e.g., 600 km. Signal delay and NGSO-affected doppler will also vary as a satellite moves closer or further away.
“The device’s physical layer requires adaptation enabled by GNSS positioning information to compensate for these challenges. Satellites will broadcast their position and timing information in this compensation”, explains Henrik.
もう一つの焦点は、衛星が数百万の絶えず移動するIoTデバイスをどのように処理し、短期間で複数の追跡エリアを同時にカバーし、NGSOネットワークとの接続を維持するかである：
“Devices are given a list of tracking area identifiers similar to terrestrial networks. The NodeB will broadcast its current tracking identifier and a list of tracking areas in service, which will change over time as satellites move. This list will guide device paging, and only devices with an intersection between the lists can be paged. Devices may send their locations to the network, providing for optimization”.
Higher frequency equals greater signaling challenges
Release 17’s NB-IoT NTN specifications will begin operating on S-band frequencies which range between 2-4 gigahertz (GHz), crossing the conventional ultra-high frequency (UHF) and super high frequency (SHF) band boundaries at 3 GHz. Aviation, shipping and space industries utilize S-band for their optimized two-way communication and content delivery capabilities for mobile networks and handheld devices.
However, Svend states that higher frequencies “involve major technical challenges”, affecting satellite-connected 5G service quality, efficiency and reliability worldwide. This challenge primarily affects high-orbit GEOs operating at frequencies of above 6 GHz. “Discussions are ongoing between 3GPP and the International Telecommunication Industry (ITU) to address frequency spectrum challenges and how they affect NB-IoT connectivity. Unfortunately, these discussions haven’t been resolved, but we believe they will in the future”.
