MIB- Master information block in 5G NR

In 5G the Master Information Block (MIB) is the very first system-level information that a UE must successfully decode after cell synchronization. It acts as the foundation stone of initial access, enabling the UE to progress from cell detection to system information acquisition.

Unlike higher-layer system information, the MIB is intentionally:

• Extremely compact

• Highly robust

• Broadcast periodically

• Independent of UE capability signaling

Its sole purpose is to provide just enough information for the UE to locate and decode SIB1, which then unlocks the rest of the system configuration.

No MIB → No SIB1 → No access to the cell

Position of MIB in the 5G NR Initial Access Procedure

The Master Information Block acts as the essential "bootloader" for a mobile device. Below is the step-by-step sequence where it appears:

• Cell Search: The UE searches for the PSS and SSS (Primary/Secondary Synchronization Signals).

• Downlink Sync: The UE achieves subframe/slot timing and identifies the Physical Cell ID (PCI).

• SSB Detection: These signals are part of the SSB (Synchronization Signal Block), which also houses the PBCH (Physical Broadcast Channel).

• PBCH Decoding: The UE decodes the PBCH to extract the MIB.

• The MIB "Bridge": The MIB provides the essential physical layer parameters (like Subcarrier Spacing and the Control Resource Set) needed to find SIB1.

• SIB1 Acquisition: Using the MIB’s instructions, the UE decodes SIB1 (System Information Block 1), which contains cell access parameters.

• RACH & Connection: The UE initiates the RACH Procedure to establish a connection.

The MIB is unique because it is the only system information carried on the PBCH (a physical channel), whereas all other SIBs are carried on the PDSCH (a shared data channel). This is why the MIB must come first—it tells the UE how to even begin looking for data channels.

What is a Synchronization Signal Block (SSB)?

In 5G New Radio (NR), the Synchronization Signal Block (SSB) is a fundamental physical-layer broadcast structure transmitted periodically by the gNB. It is the first downlink signal entity that a User Equipment (UE) attempts to detect when searching for a cell.

The SSB is designed to serve multiple purposes simultaneously:

• Enable time and frequency synchronization

• Allow cell identity detection

• Deliver essential system information through the MIB

Unlike LTE, where synchronization signals and PBCH were treated more independently, 5G NR tightly integrates them into a single, well-defined block called the SSB.

The SSB is the fundamental "heartbeat" of a 5G cell. It occupies 4 OFDM symbols in the time domain and 240 subcarriers (20 RBs) in the frequency domain.

Here is the breakdown of its components:

PSS (Primary Synchronization Signal):

• Located in Symbol 0.

• Used for initial timing and frequency synchronization.

• Helps the UE identify the cell's physical layer ID (part 1).

SSS (Secondary Synchronization Signal):

• Located in Symbol 2 (middle subcarriers).

• Used to detect the full Physical Cell ID (PCI).

• Provides subframe/frame timing.

PBCH (Physical Broadcast Channel):

• Spread across Symbols 1, 2, and 3.

• Carries the MIB (Master Information Block), providing critical parameters like the SIB1 configuration and System Frame Number (SFN).

PBCH DMRS (Demodulation Reference Signal):

• Interspersed within the PBCH resources (the red dots in the diagram).

• Acts as a reference for the UE to accurately decode the data inside the PBCH.

Benefits of Embedding MIB Inside SSB

• Beam-aligned broadcast: Each SSB beam carries the PBCH and MIB, ensuring that the UE receives correct system information corresponding to the detected beam without any configuration mismatch.

• Reliable decoding at low SNR: PBCH is designed with strong coding and repetition, allowing the UE to decode the MIB even at the cell edge, under mobility, or in challenging radio conditions.

• Fast cell discovery: Synchronization and system information acquisition occur together, eliminating the need to wait for separate broadcast channels and significantly reducing initial access latency in beamformed networks.

Physical Broadcast Channel (PBCH) and MIB Mapping

The Physical Broadcast Channel (PBCH) in 5G NR is specifically engineered to deliver the MIB with extremely high robustness and minimal decoding complexity. PBCH forms a critical part of the SSB and serves as the UE’s first source of system-level configuration information after synchronization.

PBCH Characteristics

• Modulation Scheme: PBCH employs QPSK modulation, chosen for its strong noise resilience and reliable performance under low Signal-to-Noise Ratio (SNR) conditions typical during initial cell access.

• Channel Coding: Polar coding is used for PBCH, aligning with 5G NR’s forward error correction framework for control channels. Polar codes provide excellent error correction capability, ensuring reliable decoding of broadcast information even at the cell edge.

• Payload Size: The PBCH carries a fixed payload of 24 bits, which includes all essential system information required for the UE to proceed with further system information acquisition.

• Transmission Robustness: PBCH is designed with very high robustness, leveraging strong coding, repetition, and beam-aware transmission to maximize decoding success probability.

• Beam-wise Transmission: The same PBCH payload is repeated across all SSB beams within an SS burst set, ensuring consistent system information delivery regardless of which beam the UE detects.

PBCH Payload

The 24-bit PBCH payload is structured as follows:

• 23 bits carry the actual MIB content, which includes essential parameters such as system frame number, common numerology, control channel configuration for SIB1, and access-related flags.

• 1 bit is reserved as a spare or choice bit, required by the ASN.1 message structure to support extensibility and maintain backward and forward compatibility.

• systemFrameNumber (6 bits)-Carries the most significant bits of the System Frame Number (SFN) to help the UE achieve frame-level time synchronization.

• subCarrierSpacingCommon-Indicates the subcarrier spacing used for common channels (such as SIB1), enabling the UE to apply the correct numerology during initial access.

• ssb-SubcarrierOffset (0..15)-Specifies the frequency offset of the SSB relative to the common resource grid, allowing accurate frequency-domain alignment.

• dmrs-TypeA-Position-Defines the symbol position of DMRS Type A for PDSCH/PUSCH, which is essential for proper channel estimation and demodulation.

• pdcch-ConfigSIB1 (8 bits)-Provides the CORESET#0 configuration used to locate and decode the PDCCH that schedules SIB1.

• cellBarred-Indicates whether the cell is barred or allowed for normal UE access during cell selection and camping.

• intraFreqReselection-Specifies whether intra-frequency cell reselection is permitted for UEs in idle mode.

• spare (1 bit)-Reserved for future use to support extensibility and backward compatibility.

MIB Periodicity and Position

• MIB in 5G NR is broadcast with a fixed periodicity of 80 ms, ensuring a predictable and standardized transmission interval across all deployments.

• Within each 80 ms period, the same MIB is repeated on the PBCH of every SSB beam in the SS burst set, allowing UEs to reliably decode system information irrespective of the detected beam.

• The MIB is mapped onto the PBCH, which is an integral part of the SSB.

• In the time domain, MIB transmission occurs within the PBCH symbols of the SSB, immediately after PSS and SSS enable synchronization.

• In the frequency domain, the MIB follows the SSB resource mapping, with its exact position derived using parameters such as the ssb-SubcarrierOffset.

Difference Between LTE MIB and 5G NR MIB

Feature	4G LTE MIB	5G NR MIB
Generation Periodicity	40 ms (repeated every 10 ms)	80 ms (repeated every 10–160 ms)
Channel Coding	Tail-biting Convolutional Coding	Polar Coding
Frequency Position	Always at the center 6 RBs	Anywhere on the sync raster (20 RBs)
Bandwidth Info	Included (3 bits)	Excluded (SIB1/RRC provides this)
Subcarrier Spacing	Fixed at 15 kHz	Flexible (15, 30, 60, 120 kHz)
SIB1 Pointer	Implicit (Fixed scheduling)	Explicit via pdcch-ConfigSIB1
Beamforming	None (Omnidirectional/Broad)	Supports Beam Sweeping (SSB index)
Total Bits	24 bits (14 useful, 10 reserved)	24 bits (23 MIB content + 1 spare)

PHY Layer Processing of MIB and Mapping of the 24-bit Payload to SSB

In 5G NR, the MIB is not transmitted directly over the air in its raw form. Instead, the 24-bit PBCH payload (23 bits of MIB content plus 1 spare/choice bit) undergoes a well-defined physical layer processing chain to ensure extremely robust and reliable delivery during initial access. This processing is tightly coupled with the PBCH and the SSB.

• MIB Formation: The Master Information Block is generated as a fixed 24-bit payload at the PHY input, carrying only the essential system information required for the UE to proceed toward SIB1 decoding, with the compact size minimizing latency and decoding errors.

• Channel Coding: The 24-bit payload is encoded using Polar coding, the standard 5G NR coding scheme for broadcast channels, providing strong error-correction performance under low-SNR, cell-edge, and mobility conditions.

• Rate Matching and Interleaving: The polar-encoded bits are rate matched and interleaved to fit the PBCH resources within the SSB, improving robustness against frequency-selective fading and localized channel impairments.

• Scrambling and Modulation: The coded bits are scrambled using a cell-specific sequence derived from the Physical Cell ID and then modulated using QPSK, ensuring high robustness and reduced inter-cell interference during initial access.

• PBCH Resource Mapping: The QPSK symbols carrying the MIB are mapped to predefined PBCH resource elements within the SSB, with deterministic time-frequency positioning known to the UE and exclusion of DM-RS resource elements.

• PBCH DM-RS Insertion: Demodulation Reference Signals (DM-RS) are inserted at fixed locations in the PBCH region to enable accurate channel estimation and coherent demodulation, which is critical for reliable MIB decoding.

• SSB Transmission and Beamforming: The complete SSB, containing PSS, SSS, and PBCH with the encoded MIB, is transmitted using beamforming, with the same MIB repeated across all SSB beams in an SS burst set.

• UE Reception and Decoding: The UE performs synchronization using PSS and SSS, extracts PBCH symbols, applies channel estimation, demodulation, descrambling, and polar decoding, and finally recovers the 24-bit MIB payload for further system information acquisition.