Introduction
Initial Access means a sequence of processes between UE and gNB (network) for UE to acquire uplink synchronization and obtain a specific ID for radio access communications. The initial access is referred to as RACH(Random Access Channel) procedure. The initial access may mean ‘Downlink Synchronization + RACH’. The main purpose of RACH is to achieve UPlink synchronization between UE and eNB and obtain the resource for message 3 (e.g RRC connection request). When we want to connect a UE to a 5G network, it has to synchronize in downlink as well as in uplink. Downlink synchronization is obtained after successfully decoding SSB, to establish uplink synchronization and RRC connection, UE has to perform RACH random access procedure.
PBCH (Physical Broadcast Channel) and MIB (Master Information Block)
Master information block is a very important message or information that is broadcasted irrespective of any user’s presence. The MIB is first among the other system information blocks or SIBs, which are also broadcasted by the eNodeB. PBCH and MIB cell search is the procedure for a UE to acquire time and frequency synchronization with a cell and detect the cell’s physical layer cell ID (PCI). The UE needs to first decode PBCH/MIB in order for it to receive other system information which is transmitted on the PDSCH (Physical Data Shared channel).
MIB is mapped on the BCCH logical channel and carried on the BCH transport channel. The BCH channel is further mapped on the PBCH. MIB is transmitted with a periodicity of 80msec and is always repeated after every 80 msec time duration. MIB provides the UE with parameters that are required to acquire SIB1. Information required for monitoring of PDCCH for scheduling PDSCH that carries SIB1. PBCH carries critical information which is required for system access i.e to acquire a SIB1. All the information/fields included in the MIB and the information that is carried by PBCH. the same MIB is transmitted over all SSBs within the SS burst set. The information such as the SSB index is unique and dedicated to an SSB, so MIB cannot carry such information.
PBCH carries critical information required for further system access e.g to acquire SIB 1. All the information/fields included in MIB and the information that is carried by PBCH.
MIB content
- System Frame Number (SFN): It provides the six most significant bits (MSB) of the current frame number.
- SubCarrier Spacing common: it defines the subcarrier spacing to be used for the reception of SIB 1, other broadcast system information, paging messages, and the MSG2/MSG4 transmitted during initial access.
- Ssb-subcarrier offset: it defines the four least significant bits of the sub-carrier offset.
- dmrs-TypeA-position: it specifies the first symbol used by the Demodulation Reference Signal (DMRS) when using ‘Mapping type A’. this information element applies to the DMRS for both the PDSCH and PUSCH.
- Cell barred: A UE is not permitted to complete cell selection or cell reselection onto a barred cell. So it will indicate whether the cell is barred or not.
- IntraFreqReselection: this is applicable when the current cell is to be treated or barred. A value of ‘allowed’ indicates that the UE is permitted to reselect another cell on the same frequency.
| 5G MIB | Description |
| SystemFrameNumber (SFN) | Data type/size : BIT STRING ( size = 8) |
| brz Transmission Period | Data type/size : ENUMERATED (5,10,15,20 ms)It defines the transmission period of the beam reference symbol and its value is in milliseconds.Periodicity is 5ms. 10ms, 20ms |
| ePBCHTransmissionPeriod | Data type/size : ENUMERATED (0,40,80,160 ms)Defines the transmission periodicity of ePBCH.Ms0 represents that ePBCH is not transmitted and ms40 represents a transmission period of 40msec. |
| Spare | BIT STRING (size = 2) |
Before user equipment (UE) can communicate with the network, it must perform a cell search and selection procedure and obtain the initial system information. The first steps in this process are acquiring frame synchronization, finding out the cell identity, and decoding the MIB and SIB1.
Comparison of LTE and NR MIB
| Parameters | LTE (Long term evolution) | NR (New Radio) |
| Broadcast channel | PBCH- Physical channelBCH- Transport channel | PBCH- Physical channelBCH- Transport channel |
| Resource allocation | 6 RBs (72 subcarriers)in frequency domain\4 symbols of the first subframe second slot symbol 0,1,2 and 3. | It is transmitted on OFDM symbols 1,2 and 3. It uses 0 to 239 subcarrier numbers on symbols 1 & 3, whereas on symbols 2 it uses sub carried numbers 0 to 47 and 192 to 239. |
| Modulation | QPSK | QPSK |
| Periodicity | 40msec – generation periodicity10msec- re-transmission periodicity | 80msec – generation periodicity 80msec – repetition periodicity |
| Channel coding | Tail bit convolution encoding | Polar coding |
Signaling call flow
- Signaling radio bearer: N/A
- RLC-SAP: TM
- Logical channel: BCCH
- Direction: Network to UE
MIB Acquiring and Processing
MIB/SIB acquisition processes vary case to case as defined in NR-RRC specifications.
Case 1: No on-demand system information, no previously-stored SIB & UE just powered on
- UE powered on
- Cell search process and PBCH decode to get the MIB
- Store the MIB
- Check if cell barred=barred
- Decode SIB1
- Decode other SIBs
Case 2: on-demand system information indication, no previously-stored SIB & UE just powered on
- UE powered on
- Cell search process and PBCH decode to get the MIB
- Store the MIB
- Check if cell barred=barred
- Decode SIB1
- Check RRC status
5G NR also has System Frame Number (SFN) 0 to 1023 and requires 10 bits to represent SFN. The 6 bits for SFN can be obtained from MIB and the remaining 4 bits are derived from the LTEBCH payload.
