【5G RRC】Master Information Block （NR-MIB）

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I work for an internationally renowned terminal manufacturer and am responsible for the research and development of modem chips.
In the early days of 5G, he was responsible for the development of the terminal data service layer and the core network. Currently, he is leading the research on technical standards for 6G computing power networks.


The content of the blog mainly revolves around:
       5G/6G protocol explanation
       computing power network explanation (cloud computing, edge computing, end computing)
       advanced C language explanation
       Rust language explanation

Introduction to 5G MIBs

1. NR MIB features

• The MIB is transmitted through the BCH transport channel and the PBCH physical channel;
• QPSK modulation;
• It contains the necessary parameters needed to decode SystemInformationBlockType1 (SIB1) ;
• Its transmission cycle is 80 milliseconds, and repeated transmissions are performed within these 80 milliseconds;
• It is transmitted on symbols 1, 2, 3 of OFDM;
• According to TS 38.211, it uses subcarrier numbers 0~239 on symbols 1 and 3, and on symbol 2, uses subcarrier numbers 0~47 and subcarrier numbers 192~239;

2. MIB/SIB signaling call process

The MIB message is not sent through SRB and RLC-SAP TM (transparent transmission), but is sent from the network to the UE through the BCCH logical channel, BCH transport channel and PBCH physical channel, as shown in the following figure:
      

3. RRC parameters corresponding to MIB

MIB ::= SEQUENCE {
    
    

	systemFrameNumber                         	BIT STRING (SIZE (6)),
	subCarrierSpacingCommon               		ENUMERATED {
    
    scs15or60, scs30or120},
	ssb-SubcarrierOffset                        INTEGER (0…15),
	dmrs-TypeA-Position                         ENUMERATED {
    
    pos2, pos3},
	pdcch-ConfigSIB1                            INTEGER (0…255),
	cellBarred                                  ENUMERATED {
    
    barred, notBarred},
	intraFreqReselection                        ENUMERATED {
    
    allowed, notAllowed},
	spare                                       BIT STRING (SIZE (1))
}

The following is a log screenshot:
 

3.1 Parameter explanation

• systemFrameNumber : Similar to LTE, NR is also a 10-bit system frame number, from 0 to 1023. The MIB carries the 6 most significant bits (MSB), and the remaining 4 LSBs are transmitted in the PBCH transport block as part of the channel coding (i.e. outside of the MIB coding);
• subCarrierSpacingCommon : Indicates the subcarrier spacing (scs) of SIB1, message 2, 4 of initial access and system information message.For the carrier frequency less than 6GHz, its value is 15, 30kHz，For the carrier frequency greater than 6GHz, its value can be 60, 120kHz；
• ssb-subcarrierOffset：Expressed in the number of subcarriersBetween the SSB and the overall resource block gridfrequency domain offset. This field may indicate that this cell does not provide SIB1 and therefore has no common CORESET, in which case the pdcch-ConfigSIB1 field may indicate a frequency location where it is (not) possible for the UE to find an SS with a control resource set (CORESET) /PBCH and SIB1 search spaces;
• dmrs-TypeA-Position : indicates the position of the first downlink DM-RS. It corresponds to the parameter DL-DMRS-typeA-pos of L1 ;
• pdcchConfigSIB1 : It corresponds to RMSI-PDCCH-Config in TS 38.213 Section 4.1 . It is used to determine the bandwidth of PDCCH/SIB, common control resource set (CORESET), common search space and necessary PDCCH parameters. If the ssb-SubcarrierOffset field indicates that SIB1 is not present, the pdcch-ConfigSIB1 field indicates the frequency location where the UE may find SS/PBCH blocks with SIB1 or the frequency range where the network does not provide SS/PBCH blocks with SIB1 (see TS 38.213 para. Section 13);
• cellBarred : According to the specification TS 38.304, this field indicates whether the UE is allowed to camp in this cell;
• intraFreqReselection : Indicates whether intra-frequency cell reselection is allowed or not. According to TS 38.304, when the cell with the highest ranking is blocked or considered blocked by the UE, this field is used to control cell reselection within the same frequency;

4. MIB Acquisition and Processing

The MIB/SIB acquisition process varies from case to case. Here, we will discuss the whole MIB/SIB acquisition process defined in TS 38.331 NR-RRC specification.

4.1 The UE has just been powered on, no previous SIB messages are stored, and no On-Demand system information

• The UE is powered on;
• Cell search (PSS and SSS), decode PBCH to get MIB;
• Decode and store the MIB;
• Check whether the cell is Barred, if the cell is Barred, stop this process, otherwise continue the following process;
• Use the stored MIB parameters to decode SIB1, and store the decoded result;
• When SIB1 indicates that there is no demand SI, decode other system messages (OSIs);

4.2 The UE has just been powered on and has not stored the previous SIB message, but has On-Demand system information

• The UE is powered on;
• Cell search (PSS and SSS), decode PBCH to get MIB;
• Decode and store the MIB;
• Check whether the cell is Barred, if the cell is Barred, stop this process, otherwise continue the following process;
• Use the stored MIB parameters to decode SIB1, and store the decoded result;
• When SIB1 indicates that there is a demand SI, check the RRC status;
  • If the RRC state of the UE is RRC-IDLE or RRC-INACTIVE state:
    • Trigger the bottom layer of the UE to initiate the RACH process;
    • When receiving the acknowledgment message of the SI request, obtain the required SI message;
  • If the RRC state of the UE is RRC-Connected:
    • The protocol does not define the subsequent operations, it depends on the implementation;

5. MIB comparison between LTE and NR

parameter	LTE	NR (new radio)
broadcast channel	Transport Channel - BCH Physical Channel - PBCH	Transport Channel - BCH Physical Channel - PBCH
transmission cycle	The broadcast cycle is 40 milliseconds, and retransmission is performed every 10 milliseconds within 40 milliseconds	The broadcast period is 80 milliseconds, and multiple retransmissions are performed within 80 milliseconds
channel coding	Tail Convolutional Coding	Polar coding
modulation	QPSK	QPSK
Resource allocation	1. 6 RBs in the frequency domain (corresponding to 72 subcarriers) 2. 4 symbols 0, 1, 2 and 3 in the second slot of the first subFrame	1. It transmits on OFDM symbols 1, 2 and 3 2. It transmits on subcarriers 0~239 of symbols 1 and 3, subcarriers 0~47 and subcarriers 192~239 of symbol 2

6. SFN calculation during cell search in NR 5G

Similar to LTE, 5G NR also has a system frame number (System Frame Number, SFN) 0 ~ 1023, which requires 10 bits to represent SFN. The upper 6 bits of the SFN can be obtained from the MIB, and the remaining 4 lower bits can be obtained from the PBCH Payload .

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