SS/PBCH Block

Last updated on Mar 16, 2023

Introduction

The SS/PBCH block is a critical component of 5G NR (New Radio) physical layer. It carries essential synchronization and system information to enable user equipment (UE) to establish a connection with the 5G network. In this article, we will discuss the technical details of the SS/PBCH block, including its structure, encoding scheme, and modulation.

SS/PBCH Block Structure

The SS/PBCH block is a 240-symbol sequence that consists of two parts: the synchronization signal (SS) and the physical broadcast channel (PBCH). The SS part carries synchronization information to help the UE establish time and frequency synchronization with the 5G network. The PBCH part carries system information, including the cell identity (ID), the downlink control information (DCI) format, and other configuration parameters.

The SS part consists of two different signals: the primary synchronization signal (PSS) and the secondary synchronization signal (SSS). The PSS is a Zadoff-Chu sequence with a length of 127 symbols. The Zadoff-Chu sequence has good correlation properties, making it suitable for synchronization purposes. The PSS is transmitted in the middle of the SS/PBCH block and is used by the UE to establish the frame boundary and subframe boundary.

The SSS is a binary sequence that carries information about the cell ID group and the cell ID within the group. There are three different SSS sequences, each corresponding to a different group of cell IDs. The SSS sequence has a length of 62 symbols and is transmitted twice in the SS part of the SS/PBCH block. The UE can use the two SSS sequences to determine the cell ID group and the cell ID within the group.

The PBCH part of the SS/PBCH block carries the system information that the UE needs to establish a connection with the 5G network. The PBCH part consists of 4 consecutive slots, each with 4 OFDM (Orthogonal Frequency Division Multiplexing) symbols. The first OFDM symbol in each slot carries the PBCH data, while the other three OFDM symbols are used for channel estimation and other control information.

Encoding Scheme

The SS/PBCH block is encoded using a concatenation of turbo codes and polar codes. Turbo codes are used for the PSS and SSS signals, while polar codes are used for the PBCH data.

Turbo codes are a class of error-correcting codes that use two or more constituent convolutional codes in parallel. The constituent codes are interleaved, and the resulting encoded bit sequence is punctured to reduce its length. The decoder uses an iterative algorithm to decode the received bit sequence by exchanging extrinsic information between the constituent decoders. Turbo codes offer excellent error-correction performance and are widely used in 3G and 4G cellular systems.

Polar codes are a newer class of error-correcting codes that use the polar transformation to transform a set of independent input bits into a set of correlated bits. The polar transformation maps the input bits to a set of bit-channels, where each bit-channel has a different reliability. The reliability of each bit-channel is determined by the mutual information between the input bits and the output bits. The polar code encoder selects the most reliable bit-channels and encodes them using a binary linear code. The resulting encoded sequence has a length that is equal to the number of selected bit-channels. Polar codes offer excellent error-correction performance and are used in 5G NR for control channels and data channels.

Modulation

The SS/PBCH block is modulated using QPSK (Quadrature Phase Shift Keying) modulation. QPSK is a type of digital modulation that uses four different symbols, each representing two bits of information. The four symbols are represented by four different phase angles: 0 degrees, 90 degrees, 180 degrees, and 270 degrees. The QPSK modulator maps each pair of input bits to one of the four symbols by assigning the corresponding phase angle to the symbol. The resulting modulated signal is a complex-valued signal that can be transmitted over a wireless channel.

The QPSK modulation is used for both the PSS and SSS signals and the PBCH data. The PSS and SSS signals are modulated using QPSK symbols that are repeated several times to form the corresponding sequences. The PBCH data is modulated using QPSK symbols that are mapped to the OFDM subcarriers in each slot of the PBCH part of the SS/PBCH block.

Conclusion

The SS/PBCH block is a critical component of the 5G NR physical layer that carries essential synchronization and system information. The SS/PBCH block consists of a synchronization signal (SS) and a physical broadcast channel (PBCH) that are encoded using a concatenation of turbo codes and polar codes and modulated using QPSK modulation. The SS part carries synchronization information to help the UE establish time and frequency synchronization with the 5G network, while the PBCH part carries system information, including the cell identity (ID), the downlink control information (DCI) format, and other configuration parameters. The SS/PBCH block is a complex and highly optimized signal that enables efficient and reliable communication between the UE and the 5G network.