5G NR: PBCH and Master Information Block (MIB)


Cell search is the procedure for a UE to acquire time and frequency synchronization with a cell and to detect Physical layer Cell ID (PCI) of the cell.
During cell search operations which are carried out when a UE is powered ON, mobility in connected mode, idle mode mobility (e.g. reselections), inter-RAT mobility to NR system etc., the UE uses NR synchronization signals and PBCH to derive the necessary information required to access the cell.
Similar to LTE, two types of synchronization signals are defined for NR; Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). The Synchronization Signal/PBCH block (SSB) consists of PSS, SSS and Physical Broadcast Channel (PBCH).
The UE needs to first decode PBCH/MIB in order for it to receive other system information transmitted on PDSCH.
For more details about SSB, visit 5G NR: Synchronization Signal/PBCH block (SSB), for details about PSS, visit: 5GNR: Primary Synchronization Signal (PSS) and for SSS, visit 5GNR: Secondary Synchronization Signal (SSS).  

Physical Broadcast Channel (PBCH) and MIB
-   MIB is mapped on the BCCH logical channel and carried on BCH transport channel. BCH is then mapped on PBCH.
-   MIB is transmitted with a periodicity of 80ms and is repeated within the 80ms.
-   MIB provides the UE with parameters (e.g. CORESET#0 configuration) required to acquire SIB1, more specifically, information required for monitoring of PDCCH for scheduling PDSCH that carries SIB1.
Information carried within/by PBCH and MIB
PBCH carries critical information required for further system access (e.g. to acquire SIB1). In this section, all the information/fields included in MIB and the information that is carried by PBCH (excluding MIB contents) are discussed in detail.
MIB contents are same over 80ms period and same MIB is transmitted over all SSBs within the SS burst set. The information such as SSB index is unique and dedicated to an SSB, so MIB can’t carry such information and hence the approach of carrying some of the information over PBCH outside of MIB is adapted.
-   The information carried by MIB is shown below:








-   PBCH payload size including 24-bit CRC is 56-bits. The following table summarizes the number of bits occupied by the information/field within PBCH/MIB.


Information/field
Number of Bits
Total
Carried by MIB
    Carried by PBCH (excluding MIB contents)
     System Frame Number (SFN)
10
6
4
     Sub Carrier Spacing (for SIB1, Initial access Msg-2/4, paging, SI-messages)
1
1
0
     SSB Subcarrier Offset
    FR1
____   5
4
1
    FR2
____   4
0
     dmrs-TypeA-Position
1
1
0
     PDCCH Config for SIB1 
8
8
0
     Cell Barring Information flag
1
1
0
     Intra-Frequency Reselection allowed/not allowed flag
1
1
0
     SSB Index 
    FR1
____   0
0
0*
    FR2
____ . 3
3*
     half-frame bit
1
0
1
     Spare bits
1
1
0
     Reserved bits
    FR1
____   2
0
2
    FR2
____   0
0
     BCCH-BCH-MessageType indication
1
1
0
     CRC bits
24
0
24
     Total Number of bits
56 (FR1 or FR2)
24
32 (FR1 or FR2)
* 4 additional (LSB) bits are implicitly carried by PBCH scrambling sequence.
·    SFN (6-bits): Similar to LTE, SFN in 5G NR takes 10 bits and ranges from 0 to 1023. The 6 MSB bits of the 10-bit SFN are part of MIB. The 4 LSB bits of the SFN are conveyed in the PBCH transport block as part of channel coding.
·    subCarrierSpacingCommon (1-bit): Carried within MIB. Subcarrier spacing used for SIB1, Msg-2/4 for initial access, paging and broadcast of SI-messages. This bit indicates either 15 kHz or 30 kHz for FR1 and either 60 kHz or 120 kHz for FR2.
·    SSB Subcarrier Offset (4 or 5-bits): Corresponds to kSSB, which is the frequency domain offset between SSB and the overall resource block grid in number of subcarriers. For reception of SIB1, the UE needs to know where the overall resource block grid starts. This field takes 5 bits for FR1 and 4 bits for FR2.
-    Only 4 bits are carried by MIB parameter ssb-SubcarrierOffset.
-    For FR1, 4 LSBs of kSSB are obtained from MIB parameter ssb-SubcarrierOffset and an additional bit (MSB) is encoded within PBCH to represent 24 values (0, 1, 2, …,23).
-    For FR2, 4 LSBs of kSSB are obtained from MIB parameter ssb-SubcarrierOffset to represent 12 values (0, 1, 2, …,11).
-    This may also indicate that this cell does not provide SIB1 and that there is hence no CORESET#0 configured in MIB. In this case, the field pdcch-ConfigSIB1 indicates the frequency positions where the UE may find SSB with SIB1 or the frequency range where the network does not provide SSB with SIB1. The UE determines from MIB that a CORESET#0 is not present if kSSB > 23 for FR1 or if kSSB > 11 for FR2.
-    For more information about kSSB and its usage, visit 5G NR: Synchronization Signal/PBCH block (SSB).
·    dmrs-TypeA-Position (1-bit):  Carried within MIB. This field defines the position of first DM-RS symbol for downlink (PDSCH) and uplink (PUSCH);
-    For downlink, this bit is only relevant for PDSCH mapping Type A. The position of first DM-RS symbol is set to 3 if dmrs-TypeA-Position is set to pos3 and is set to 2 if dmrs-TypeA-Position is set to pos2.
-    For uplink, this bit is only relevant for PUSCH mapping Type A. The position of first DM-RS symbol is set to 3 if dmrs-TypeA-Position is set to pos3 and is set to 2 if dmrs-TypeA-Position is set to pos2.
·    pdcch-ConfigSIB1 (8-bits): Carried within MIB. This field is used to configure CORESET#0 and search space#0 (of the initial BWP) which is the most important information the UE should know in order for it to monitor for scheduling (PDCCH) of SIB1.
-    This CORESET configuration also provides and activates the initial bandwidth part in the downlink.
-    If the field ssb-SubcarrierOffset indicates that SIB1 is absent (explained above), the field pdcch-ConfigSIB1 indicates the frequency positions where the UE may find SSB with SIB1 or the frequency range where the network does not provide SSB with SIB1.
·    cellBarred (1-bit): Carried within MIB. This field indicates whether or not UEs in the cell are allowed to access the cell; ‘barred’ indicates, the UEs are not allowed to access the cell.
·    intraFreqReselection (1-bit): Carried within MIB. This field controls cell selection/reselection to intra-frequency cells when the highest ranked cell is barred (as indicated by cellBarred) or treated as barred by the UE.
·    SSB Index (0 or 3-bits): This information is not conveyed by MIB, instead, PBCH payload carries the required 3 bits. Index of the SSB within SSB burst set which is very important piece of information for achieving frame synchronization. The maximum number of candidate SSBs (Lmax) within an SS burst set depends upon the carrier frequency.
-    SSB Index for sub-6 GHz (Lmax = 8): Each one of the 8 PBCH scrambling sequences (section 7.3.3.1 from 38.211) used for PBCH scrambling implicitly indicates 1-out-of-8 SSB indices. In this case, there is no need of explicit bits to indicate SSB index.
-    SSB Index for above 6 GHz(Lmax = 64): Each one of the 8 PBCH scrambling sequence (section 7.3.3.1 from 38.211) used for PBCH scrambling implicitly indicates 3 LSB bits of SSB index. In order to represent 64 SSB indices, another 3 bits (MSB) are required which are explicitly carried by PBCH payload.
·    half-frame bit (1-bit): This bit is set to ‘0’ if SSB is transmitted in the first half-frame of the 10ms frame or set to ‘1’ if SSB is transmitted in the second half-frame of the 10ms frame.
-    The half-frame bit information together with SSB index are used to achieve frame synchronization.
PBCH time-frequency structure
-   The UE knows the timing of PBCH after detecting PSS.
-   PBCH/MIB is transmitted periodically at SSB periodicity.



















-   It can be seen from the above figure that PBCH occupies two full OFDM symbols (second and fourth) spanning 240 subcarriers and in the third OFDM symbol spanning 48 subcarriers below and above SSS. This results in PBCH (including PBCH DM-RS) occupying 576 subcarriers across three OFDM symbols (240+48+48+240 = 576).
-   The total number of resource elements (REs) occupied by PBCH per SSB is equal to 576. This includes REs for PBCH as well as REs for DM-RS which is required for coherent demodulation of PBCH.
-   PBCH DM-RS occupies 144 REs which is one-fourth of total REs (576 REs) and the remaining bits are occupied by PBCH payload (432 REs)
-   The following table (from 38.211) summarizes resources within an SSB. The location of PBCH DM-RS depends upon PCI (v = NIDcell mod 4) of the cell (PCI is already determined by the UE using PSS/SSS).

   Channel or Signal 
    OFDM symbol number ‘l’ relative to the start of an SSB
Subcarrier number ‘k’
relative to the start of an SSB
PSS
0
56, 57, ..., 182
SSS
2
56, 57, ..., 182
Set to ‘0’
0
0, 1, ..., 55, 183, 184, ..., 239
2
48, 49, ..., 55, 183, 184, ..., 191
PBCH
1, 3
0, 1, ..., 239
2
0, 1, ..., 47, 
192, 193, ..., 239
  DM-RS for PBCH
1, 3
0+v, 4+v, 8+v,...,236+v
2
0+v, 4+v, 8+v,…,236+v
192+v, 196+v,...,236+v 

PBCH Payload Generation and other processing information
-   Layer-1, after receiving the BCH data (24-bits), appends Eight additional timing related bits to generate PBCH payload bits (32-bits).
-   A 24-bit CRC is appended to the PBCH payload which yields 56-bits in total.
-   These 56-bits would result in 512-bits after channel coding (Polar coding) which then becomes 864-bits after rate matching. Since QPSK is used, just 432 REs are needed to transmit 432 QPSK symbols (864-bits).
-   As discussed already, there are 576 REs dedicated for PBCH, remaining 144 REs (576-432) are used for PBCH DM-RS (see Table 7.4.3.1-1 from 38.211).
-   The following figure illustrates PBCH payload generation.













PBCH Misc.
-   For PBCH;
·     Polar coding is used.
·     QPSK modulation is used.
-   PBCH uses a single antenna transmission scheme.
-   PBCH, SSS and PSS within the same SSB uses the same antenna port for transmission.
Reference: 3GPP TS 38.211, 38.212, 38.213, 38.331, and 38.300