This document studies if further enhancements to the 5G system are feasible or not for the support of Off-Network for Rail. This document collects all Off-Network use cases from SA1 Study on Future Railway Mobile Communication System (FRMCS) in order to refine them and new use cases to support Off-Network for Rail in clause 5 for Communication Applications. The potential new service requirements are also consolidated in clause 5. The results of gap analysis identify limitations, missing requirements and existing functionalities supported by 3GPP in clause 5. Following the gap analysis, the assessment of technical feasibility, complexity and overhead of normative 3GPP adjustments to support limitations, missing requirements and functionalities as required by Off-Network for Rail is considered in clause 8. Potential requirements are then consolidated in clause 9. The conclusions and recommendations are proposed in clause 10 to conclude if further enhancements to the 5G system are needed and feasible for the support of Off-Network for Rail.

The present document analyses Rail Off-Network use cases in order to derive potential requirements and investigate if normative adjustments are feasible or not.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

The present document is a 900 series Technical Report (TR) written by 3GPP TSG SA WG1 (SA1). Such a TR is not normative, i.e. cannot be used for implementation directly. It is primarily written by SA1 to summarise the high-level Off-Network communication needs of the railway community and to identify the corresponding requirements. Future rail communication is an important part of the digitalization of rail operations. In addition to network-oriented communication, this also includes that without involving the network. GSM-R already used this and was called Direct Mode to describe the ability of a User Equipment to communicate directly with each other, independent of the network. Off-Network is a new terminology for the Rail sector, already introduced within 3GPP MCX specifications. UIC will promote the use of Off-Network to replace Direct Mode. But unfortunately, the terminology has slight differences in the meaning. For example, the Rail sector considers use of Off-Network communications where no network services are available either due to network failure (e.g. coverage, capacity, etc.) or due to lack of coverage (remote area's). In 3GPP MCX specifications, Off-Network communications can be setup even when network services are available. In addition to voice communication, Off-Network communication will also increasingly be used for the exchange of train safety data in the future. Examples of use cases which require Direct Mode:

• Automatic Train Protection (ATP) data communication
• Automatic Train Operation (ATO) data communication
• Critical real-time video
• Virtual Coupling data communication

Virtual Coupling data communication is the only use case where Off-Network is the intended mode of operation (also called "default mode") even if the network is available. All other use cases shall consider Network-Oriented mode as the default mode when the network is available (for the first and following attachments) and shifting to Off-Network when the network is not available.