Description
Remaining Minimum System Information (RMSI) is a fundamental component of the 5G New Radio (NR) system information (SI) broadcast framework. It is transmitted via the Physical Downlink Shared Channel (PDSCH) and is scheduled by the Physical Downlink Control Channel (PDCCH) using a specific System Information-Radio Network Temporary Identifier (SI-RNTI). RMSI is not a single monolithic block but typically refers to the first System Information Block (SIB1) in NR, which contains the most crucial information a User Equipment (UE) requires after successfully decoding the Master Information Block (MIB) from the Synchronization Signal Block (SSB).
The content of RMSI is extensive and vital for initial access and cell selection/reselection. Key parameters include: the downlink and uplink cell bandwidth, the configuration of the Physical Random Access Channel (PRACH) including preambles and time/frequency resources, cell access restrictions (like cell barring and reservation for specific UEs), and the scheduling information list for other System Information Blocks (SIBs). The scheduling information tells the UE when and where to find other SIBs (like SIB2, SIB3, etc.), which contain information for mobility, measurement configurations, and common radio resource configurations. The transmission of RMSI is tightly coupled with the SSB; each SSB is associated with a specific RMSI configuration, allowing for flexible deployment in different frequency ranges (FR1 and FR2).
From a procedural perspective, a UE performs initial synchronization by detecting the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) within an SSB. It then decodes the MIB from the Physical Broadcast Channel (PBCH) within that same SSB. The MIB provides the absolute frequency location of the SSB, the system frame number, and most importantly, the necessary control resource set (CORESET) and search space configuration for the PDCCH that schedules the RMSI. Using this information, the UE monitors the designated PDCCH, decodes the Downlink Control Information (DCI) scrambled with the SI-RNTI, and then uses the granted resources to receive and decode the RMSI (SIB1) on the PDSCH. Only after acquiring RMSI can the UE proceed to the random access procedure to request a connection to the network.
Purpose & Motivation
RMSI was introduced with 5G NR in 3GPP Release 15 to address the limitations of the LTE system information broadcast architecture and to support the new, more flexible NR physical layer. In LTE, all essential system information was broadcast in a semi-static manner, which could be inefficient and inflexible. The 5G design separates the absolute minimum information (MIB) needed for initial synchronization and RMSI scheduling from the remaining critical information (RMSI), which is transmitted more dynamically on the shared channel.
This separation serves several key purposes. First, it enhances spectral efficiency. By transmitting only a tiny MIB on the always-on PBCH and scheduling the larger RMSI payload on-demand via the PDSCH, network resources are used more judiciously. Second, it enables greater flexibility and beamforming support. The RMSI can be beamformed along with its associated SSB, which is crucial for coverage in millimeter wave (mmWave) frequencies. Third, it allows for more dynamic updates. While the MIB is very static, parameters in RMSI can be changed more readily without impacting the fundamental synchronization process. This architecture is essential for supporting diverse 5G use cases, from enhanced mobile broadband (eMBB) to ultra-reliable low-latency communications (URLLC), by allowing network parameters to be tailored and updated efficiently.
Key Features
- Contains critical access parameters like PRACH configuration and cell barring info
- Scheduled dynamically on PDSCH via PDCCH using SI-RNTI
- Typically corresponds to SIB1 in the 5G NR system information structure
- Provides scheduling information for other SIBs (SIB2 and beyond)
- Transmission is associated with a specific Synchronization Signal Block (SSB)
- Enables beamforming of essential system information for mmWave deployments
Evolution Across Releases
RMSI was introduced as a core component of the 5G NR initial access procedure. Its initial architecture defined it as the set of system information (primarily SIB1) scheduled after MIB decoding, containing all necessary parameters for random access and acquisition of other system information blocks. It was designed for flexibility, supporting both sub-6 GHz and mmWave spectrums.
Defining Specifications
| Specification | Title |
|---|---|
| TS 37.910 | 3GPP TR 37.910 |
| TS 38.106 | 3GPP TR 38.106 |
| TS 38.133 | 3GPP TR 38.133 |
| TS 38.300 | 3GPP TR 38.300 |
| TS 38.523 | 3GPP TR 38.523 |
| TS 38.808 | 3GPP TR 38.808 |