Comparison of Master Information Block (MIB) in LTE and 5G-NR

Master Information Block (MIB) plays a foundational role in both LTE and 5G-NR, helping the UE acquire essential system-level information to begin communication with the network. While the fundamental objective of MIB remains consistent—assisting the UE in synchronization and system acquisition—the structure, content, and method of delivery in LTE and 5G-NR reflect the architectural and design differences of the two radio access technologies.

Purpose and Role of MIB

LTE:

In LTE, the MIB is transmitted on the Broadcast Channel (BCH) and delivered through PBCH mapped onto 6 central Resource Blocks. The main purposes are:

• Provide DL system bandwidth configuration
• PHICH (Hybrid ARQ Indicator Channel) configuration
• System Frame Number (SFN) for synchronization
• Help UE decode SIB1 and continue further configuration

5G-NR:

In 5G-NR, the MIB is transmitted via the PBCH, but it is embedded within the SS/PBCH block, which also contains synchronization signals (PSS/SSS). The evolved functions include:

• Subcarrier spacing configuration for SSB
• SSB subcarrier offset for accurate location of beams
• Beam-based operation awareness via SSB index
• PDCCH config for SIB1 decoding
• DMRS Type-A positioning and mobility control flags

Key Differences in MIB Structure

Synchronization & Decoding Flow

LTE:

1. UE detects PSS/SSS
2. Synchronizes to cell and frame timing
3. Decodes PBCH to extract MIB
4. Uses MIB info to decode SIB1

5G-NR:

1. UE detects SS Block (PSS + SSS)
2. Synchronizes and identifies SSB index
3. Decodes beamformed PBCH
4. Extracts MIB for subcarrier spacing, beam association, and CORESET#0
5. Uses CORESET#0 and SearchSpace#0 to decode SIB1 without blind decoding

SIB1 Decoding and Flexibility

LTE:

• MIB does not provide direct help in SIB1 decoding.
• SIB1 is blindly decoded based on predefined locations.

5G-NR:

• MIB carries the configuration for PDCCH ConfigSIB1, allowing precise decoding.
• This reduces decoding delay and supports URLLC and low-latency applications.

Beam-based Transmission and Multi-beam Awareness

5G-NR introduces beamforming at the synchronization level. This allows:

• SSB-based initial access (up to 64 beams in FR2)
• Unique association between each SSB and its MIB
• CRC masking with the SSB index for integrity checking

This feature is not supported in LTE, which uses omnidirectional broadcast.

Summary

While LTE’s MIB served well in the legacy mobile networks by delivering essential system configuration data, 5G-NR’s MIB reflects a future-proof, beam-aware design that caters to high-speed, low-latency, and flexible deployments. By embedding it in the beam-specific SS/PBCH block, and including additional configuration and mobility parameters, 5G-NR enhances initial access efficiency and reliability—especially crucial in mmWave and URLLC scenarios.