[4G&5G Topic-36]: Physical Layer-Synchronization Signal Block SSB and Cell Primary Synchronization PSS, Cell Secondary Synchronization SSS

In the article "[4G&5G Topic-32]: Physical Layer-Cell Time-Frequency Resources and the Process of Cell Phone Searching Cell Time-Frequency Resources (Cell Search Process)", the following conclusions are obtained:

Through the cell search process, the mobile terminal obtains the position of the base station's time-frequency resources in the frequency domain and time domain from the broadcast signals and channels provided by the cell of the base station, as well as the allocation relationship of the cell's time-frequency resources, that is, cell information.
The LTE terminal synchronizes the cell with the base station through 4 physical layer signals or channels provided by the base station: primary synchronization signal PSS, secondary synchronization signal SSS, cell reference signal CRS, physical broadcast channel PBCH, but these four "departments" are In the organization of time-frequency resources, they are independent and isolated from each other.

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In 5G NR, the four time-frequency resources corresponding to the signals and channels related to cell synchronization are organized in a structured manner to form a new resource block called the synchronization signal block SSB.
The LTE cell reference signal CRS is replaced by the demodulation reference signal DMRS. The CRS is at the cell level and the DMRS is at the channel level. Each channel has its own independent demodulation reference signal, which is similar to the cell reference signal.

This article focuses on the synchronization signal block SSB:

(1) The internal structure of the SSB and the channels and signals included

(2) Time-frequency resources where wireless channels and signals are located

(3) The content sent in the wireless channel and signal (that is, the role played)

1.3 The source and background of the synchronization signal block SSB

The main reason why 5G NR has made the above-mentioned changes is to organize the above-mentioned four physical synchronization signals and physical synchronization channels together. The main reasons are:

The bandwidth of 5G is very large, so the reference signal deviation of the whole cell level is relatively large, and signal quality measurement of smaller bandwidth is required.
The 5G terminal supports variable bandwidth BWP. Different BWPs need to have their own independent channel quality measurements, and different BWPs need independent reference signals instead of reference signals for the entire base station cell.
5G supports beamforming, different beams need to have their own independent channel quality measurement, and different beams need independent reference signals.

Therefore, understanding the above technical background helps to understand the difference in design between 5G synchronization and 4G synchronization.

1.4 The pre-knowledge required to understand this article

[4G&5G Topic-34]: Physical Layer-On the principle of m-sequence and its application in NR PSS

https://blog.csdn.net/HiWangWenBing/article/details/113759070

[4G&5G Topic-33]: Physical Layer-On the principle of ZC sequence and its application in LTE PSS

https://blog.csdn.net/HiWangWenBing/article/details/113459252

[4G&5G Topic-28]: Architecture-What is multi-antenna technology and 5G large-scale antenna array, beamforming, and high-order space division multiplexing?

https://blog.csdn.net/HiWangWenBing/article/details/113459252

[4G&5G Topic-26]: Architecture-What is a partial bandwidth BWP, asymmetric carrier bandwidth, and UE bandwidth adaptation?

https://blog.csdn.net/HiWangWenBing/article/details/113371451

Chapter 2 The basic structure of 5G NR SSB

2.1 Channel and signal composition structure of SSB sync block

SSB: A resource block containing multiple different synchronization signals and wireless resources of synchronization channels.

Including: 3 physical layer signals, 1 physical layer channel.

(1) Primary synchronization signal PSS: 10ms frame synchronization and the first part of the physical cell ID
(2) Secondary synchronization signal SSS: the second part of the physical cell ID number
(3) Channel reference signal: DMRS, used to replace the cell reference signal CRS
(4) Physical broadcast channel PBCH: MIB information

2.2 Time-frequency resource composition structure of SSB synchronization block

(1) Time domain

SSB occupies a total of 4 OFDM symbols in the time domain, numbered 0,1,2,3;

(2) Frequency domain

The frequency domain occupies a total of 240 subcarriers (20 PRBs), and the subcarrier numbers are 0~239.

PSS: 127 subcarriers located in the middle of symbol 0;
SSS: 127 subcarriers located in the middle of symbol 2;
Guard band: In order to protect PSS and SSS, there are a certain number of idle subcarriers at both ends respectively.
PBCH: Located at symbols 1, 2, and 3, of which symbol 1/3 occupies all subcarriers from 0 to 239, and symbol 2 occupies all subcarriers except for SSS occupied subcarriers and idle subcarriers for protecting SSS.
DMRS : Located inside the PBCH, it is the reference signal in the PCHB channel.

Chapter 3 Interpretation of 5G NR SSB Content

3.1 Physical cell ID PCI

(1) What is the physical cell ID

The full name of PCI is Physical Cell Identifier, which is the physical cell identifier. In LTE, the terminal distinguishes the wireless signals of different cells . The transmission of data on channels other than the PPS/SSS channel of the cell will be scrambled with this physical cell ID. It can be said that the cell ID is a means of initial identification of the data entering and leaving the cell, to avoid not being a cell. Data enters the cell to avoid interfering signals being treated as cell signals! ! ! It is somewhat similar to a cell pass. Only the carrier data of the cell communication pass will be recognized by the cell of the base station.

In reality, it is inevitable to multiplex PCI in networking, which may cause conflicts (PCI conflicts) for the same PCI due to too small multiplexing distance. Therefore, adjacent cells with the same frequency should avoid using the same PCI.

The purpose of PCI planning (physical cell ID planning) is to allocate PCI reasonably to each eNB cell, to ensure that the downlink signals of the same frequency and the same PCI cell will not interfere with each other, and to avoid affecting the correct synchronization of mobile phones and decoding the pilots of normal serving cells. channel.

(2) The composition of the physical cell ID

In 4G and 5G, the physical cell ID numbers are divided into N groups, and the group numbers are denoted as N(1)_ID. In each group, there is a group identification, denoted as N(2)_ID.

The PCI numbers of the 4G LTE system are divided into N(1)_ID=168 groups, and each group includes N(2)_ID= 3 different group IDs, providing a total of 168*3=504 IDs.

The PCI numbers of the 5G LTE system are divided into N(1)_ID=336 groups, and each group includes N(2)_ID= 3 different group IDs, providing a total of 336*3=1008 IDs.

Therefore, the physical cell ID (denoted as Ncell_ID ) can be calculated by the following formula:

among them,

The three types of identifiers N(2)_ID in the PCI group of the physical cell are carried by the primary synchronization sequence PSS,

The 1008 group numbers N(1)_ID are carried by the secondary synchronization sequence SSS.

3.2 The content of the main synchronization signal m/zc sequence PSS

(1) Data information carried by PSS: 3 types of identifiers N(2)_ID in the PCI group of the physical cell

PhyCellId

(2) PSS synchronization sequence

Three different binary sequences for synchronization: m sequence

[4G&5G Topic-34]: Physical Layer-On the principle of m-sequence and its application in NR PSS

https://blog.csdn.net/HiWangWenBing/article/details/113759070

[4G&5G Topic-33]: Physical Layer-On the principle of ZC sequence and its application in LTE PSS

https://blog.csdn.net/HiWangWenBing/article/details/113459252

3.3 The content of the auxiliary synchronization sequence SSS

(1) Data information carried by SSS: N(1)_ID part of PCI id of the physical cell.

(2) Auxiliary synchronization sequence: m sequence

N different binary sequences used for synchronization: m sequence

Remarks:

The data carried by the LTE secondary synchronization sequence SSS is: 168 types of group numbers N(1)_ID

The data carried by the secondary synchronization sequence SSS of NR is: 336 types of group numbers N(1)_ID

3.4 Physical cell broadcast channel PBCH

(1) The length of 5G PBCH information: 32 Bits

(2) The source of 5G PBCH information

Cell information provided by the RRC layer (L3 MIB: Master system information block)
The physical layer itself provides information (synchronization of L1)

(3) The content of 5G PBCH information

10ms system frame number SFN (high MSB, low LSB)
5ms half frame number 0 or 1
SSB beam index number: It consists of two parts, the lower part is carried in the DMRS, and the high part is carried in the PSS.
SIB1's location
Channel reference signal type

About PBCH is relatively independent, and there are more details, which will be discussed in the next article.

3.5 The content of the cell reference signal CRS (4G only)

3.6 Contents of demodulation reference signal DMRS (5G only)

Follow-up articles are discussed separately.

Chapter 4 Signal, Signal Coding and Modulation Method

4.1 xPSK modulation constellation diagram

PSK modulation refers to a digital modulation method with constant amplitude, constant frequency spectrum, and variable phase.

As shown in the above QPSK, each amplitude is the same, and the symbols with the same frequency and different phases represent 2 bits.

4.2 Different signals and signal modulation methods

(1) PSS: Binary m sequence

Modulation: BPSK

(2) SSS: Binary m sequence

Modulation: BPSK

（3）PBCH：

Encoding: Polar code
Modulation: QPSK

（4）DMRS：

Modulation: QPSK

Chapter 5 Position of SSB in the entire time-frequency resource

5.1 Location of LTE PSS/SSS in the frequency domain

The position of the PSS and SSS signals on the time-frequency resources is fixed, and they are located on 72 subcarriers of the center frequency of the cell , that is, on the center 6 RBs.

Among them, the center frequency point DC is not included (DC actually occupies one Sc, so for the lower layer, it should be accurate to 73)

5.2 The position of 5G SSB in the frequency domain

The 5G SSB occupies 20PBR time-frequency resources, and its position on the time-frequency resources of the entire bandwidth is not fixed. The main reasons are as follows:

The bandwidth of a single 5G physical cell is very large, up to 400M, and the number of sub-carriers has increased dozens of times.
5G supports partial bandwidth BWP, and different terminals have different bandwidths, and their center frequency points are not exactly the same.
5G supports beam forming, with a maximum of 64 beams, and different beams require their own SSB synchronization blocks.

There are two configurable parameters, which together determine the position of the SSB in the frequency domain:

offset-ref-low-scs-ref-PRB: determined $N_{SSB}^{CRB}$ position
Kssb: Determines the relative $N_{SSB}^{CRB}$ position of SSB

5.3 Definition of LTE synchronization signal transmission period in the time domain

PSS cycle: fixed 5ms

SSS cycle: fixed 5ms

PBCH cycle: fixed 10ms

5.4 Definition of the transmission period of the NR SSB synchronization block in the time domain

PSS/SSB/PBCH are together, so their transmission cycle is the same.

The sending cycle of SSB can be customized.
The default sending cycle of SSB is 20ms.
The number of continuously transmitted SSBs in a transmission cycle can be customized. In the schematic diagram above, 4 continuous SSBs are sent at a time , which is actually an SSB with 4 beams sent at a time.

Chapter 6 SSB Support for Beam

SSB supports different beams, and each beam has its own independent SSB.

SSB index number: PBCH DMRS goden sequence types (8 types) + SSB auxiliary index in PBCH MIB, which together constitute the SSB index number

Medium and low frequency: up to 8 beams, 3 bits, can be distinguished by using the goden sequence of the DMRS of the PBCH.
High frequency: 64 beams at most, distinguished by DMRS signal + MIB message in PBCH, DMRS (3bit) + MIB (3bit) = 6bits, forming 64 beams.

reference:

https://www.sohu.com/a/359560562_100191018

https://blog.csdn.net/u010658002/article/details/113067577

https://blog.csdn.net/qq_33206497/article/details/99209559