How often is the broadcast channel transmitted?

In 5G NR, the SSB (SS/PBCH Block) containing the PBCH is transmitted periodically with a default period of 20 ms. During initial cell search, the UE assumes a 20 ms period, though the network can configure 5, 10, 20, 40, 80, or 160 ms periods after initial access. Each SSB occupies 4 OFDM symbols in time and 240 subcarriers (20 resource blocks) in frequency. In the FR1 band below 6 GHz, up to 8 SSBs can be beam-swept within each period; in FR2 above 24 GHz, up to 64 SSBs enable coverage through narrow millimeter-wave beams.

How does the 5G NR broadcast channel differ from LTE?

The 5G NR PBCH introduces several key differences from LTE: it uses polar coding instead of convolutional coding for better error correction at low SNR; it includes DMRS (demodulation reference signals) within the PBCH symbols for channel estimation rather than relying on cell-specific reference signals; and it supports beam sweeping through multiple SSB transmissions at different beam directions within each period. LTE broadcasts the MIB on a fixed omnidirectional beam every 40 ms with a 10 ms TTI, while NR can sweep up to 64 beams per period to support millimeter-wave deployment.

What is a Broadcast Channel? | BCH in Wireless Systems

Q: What information does the broadcast channel carry?

In 5G NR, the PBCH (Physical Broadcast Channel) carries the Master Information Block (MIB) containing essential cell access parameters: system frame number, subcarrier spacing for SIB1, CORESET#0 configuration for PDCCH, and cell barred status. The MIB is only 24 bits plus a 24-bit CRC, encoded with polar coding and QPSK modulation. Additional system information (SIBs 1-9) containing PLMN identity, cell selection criteria, neighboring cell lists, and service-specific parameters are carried on the PDSCH scheduled by SIB1.

Definition & Function

A broadcast channel (BCH) is a downlink physical channel that a base station uses to transmit essential system information to every user equipment (UE) within its coverage area. This information enables UEs to discover the network, synchronize timing, and initiate the random access procedure for connection establishment. The broadcast channel must be decodable by any UE without prior knowledge of the cell configuration, so it uses robust modulation (QPSK), low code rates, and fixed resource locations that the UE can find through blind detection.

In 5G NR, the Physical Broadcast Channel (PBCH) is part of the SS/PBCH Block (SSB), which also contains the Primary and Secondary Synchronization Signals (PSS/SSS). The SSB is the first signal a UE detects when searching for a cell, providing the physical cell identity, timing reference, and the MIB payload that tells the UE where to find SIB1 containing the remaining system configuration. The PBCH payload is intentionally minimal (24 information bits) to ensure reliable reception at the cell edge under extreme path loss conditions.

Key Parameters

PBCH Payload (5G NR): 24 info bits + 24-bit CRC = 48 bits total

Modulation: QPSK (2 bits/symbol)

Coding: Polar code (NR) | Convolutional (LTE)

SSB Size: 4 OFDM symbols × 240 subcarriers (20 RBs)

SSB Period: 5/10/20/40/80/160 ms (default 20 ms)

Max Beams per Period: FR1: 8 | FR2: 64

Broadcast Channel Comparison Across Generations

Parameter	5G NR PBCH	LTE PBCH	UMTS P-CCPCH	GSM BCCH
Payload	24 bits (MIB)	24 bits (MIB)	270 bits/frame	184 bits
Modulation	QPSK	QPSK	QPSK	GMSK
Channel Coding	Polar	Convolutional	Convolutional	Convolutional
Periodicity	20 ms (default)	40 ms (10 ms TTI)	20 ms	51 multiframe
Beam Support	Up to 64 beams	Omnidirectional	Omnidirectional	Omnidirectional
Reference Signal	PBCH DMRS	CRS	CPICH	N/A
Bandwidth	20 RBs (3.6 MHz)	6 RBs (1.08 MHz)	Full carrier	200 kHz

Practical Application

When a 5G NR smartphone powers on in an n78 (3.5 GHz) cell, it performs an SSB search by correlating against all 1,008 possible PSS/SSS sequences across the synchronization raster. Upon detecting the strongest SSB, it decodes the PBCH to extract the MIB, which indicates subcarrier spacing (30 kHz), the CORESET#0 configuration for finding SIB1 on the PDCCH, and the system frame number for timing alignment. The entire cell search and MIB decode takes 100-300 ms. In an mmWave n257 (28 GHz) deployment, the gNB transmits 64 SSBs per period across different beam directions, and the UE measures the RSRP of each beam to identify the best beam pair for subsequent communication.

Frequently Asked Questions

What information does the broadcast channel carry?

The 5G NR PBCH carries the 24-bit MIB: system frame number, SCS for SIB1, CORESET#0 configuration, and cell barred status. Additional SIBs (1-9) with PLMN identity, cell selection criteria, and neighbor lists are on PDSCH scheduled by SIB1.

How often is it transmitted?

Default 20 ms period, configurable from 5-160 ms. Each SSB occupies 4 OFDM symbols × 240 subcarriers. Up to 8 SSBs beam-swept per period below 6 GHz; up to 64 SSBs for mmWave.

How does 5G NR PBCH differ from LTE?

NR uses polar coding (vs convolutional), includes DMRS within PBCH symbols (vs CRS), and supports beam sweeping up to 64 directions (vs omnidirectional). NR SSB is 20 RBs wide vs 6 RBs for LTE.

Broadcast Channel