How 5G Works

Physical Broadcast Channel :

In 5G NR, similar to LTE, PBCH carries MIB (Master Information Block).

MIB Data arrives coding unit in the form of maximum one transport block for every 80 ms. That means the periodicity of MIB is 80 ms.

The interesting part of 5G NR MIB is the bit mapping to SFN.

From the above ASN, one can see the SFN bit string size as "6" that means the maximum SFN value can be 127 (0 .. 127). Is that the maximum permitted SFN in NR?

The answers is NO, LTE and NR have similar SFN range (0 ..1023). I explain below how SFN is configured by NW?

While generating the PBCH payload and additional timing related PBCH payload bits are generated (an octet) as shown below.

The above octet used for the calculation of SFN and SSB Carrier offset.

Above 4 bits are the 4th, 3rd, 2nd and 1st LSB of SFN respectively. A total of 10 bits for SFN.

SubCarrierSpacingCommon :

Inform the UE about the subcarrier spacing for SIB1, Msg 2/4, Paging and SI-messages. Unlike LTE, NR supports different SCS for different Control/Data channels.

Carrier Frequency < 6GHz, scs15or60 corresponds to 15 kHz else 60 kHz
Carrier Frequency < 6HHz, scs30or120 corresponds to 30 kHz else 120 kHz

CellBarred :

If the Cell Barred is set to Barred, then the UE is not allowed to camp on that cell. If the SIB1 indicates multiple PLMNs, all the PLMNs included in the SIB1 are barred (This IE is common for all the PLMNs)

When the cell status is "Barred",
1. The UE is not permitted to select/reselct the cell, not even for emergency calls.
2. The cell is treated as Barred, if the UE failed acquire MIB/SIB1
3. The UE shall not perform access attempts to decode the cell for 300 seconds in case of (2).

IntraFreqReselection:

When this IE is set to allowed, UE can reselect/ select any cell that satisfied the cell reselection or selection criteria though the highest ranked cell on the same frequency is Barred. Else all the cells having the same frequency as highest ranked barred cell, the UE should treat them as Barred.

ssb-SubcarrierOffset:

Corresponds to the frequency domain offset between SSB and the overall resource block grid in number of subcarriers (k_SSB ). The value range provided above is from 0 to 15 but for FR1 case k_SSB is 0 to 23 and for FR2 case k_SSB is 0 to 11_.

But the ASN above permits a value until 15, how do we configure 23 which is the possible case for FR1?

From the 8 bit timing PBCH payload,

The 5th (MSB) of Kssb is given by the above payload 6th bit i.e. bit A+5 .

For initial cell selection, a UE may assume that half frame with SS/PBCH blocks occur with a periodicity of 2 frames i.e. 20 ms.

UE upon detecting SS/PBCH block, determines that a CORESET for Type0-PDCCH CSS set is present if subcarrier offset is <= 23 for FR1 or <= 11 for FR2. If the Kssb > 23 for FR1 or Kssb >11 for FR2, the CORESET for Type0-PDCCH CSS set may be provided by PDCCH-ConfigCommon.

pdcch-ConfigSIB1:

PDCCH-ConfigSIB1 is used to configure CORESET#0 and Search space #0.

If Kssb indicates that SIB1 is not present (i.e Kssb>23 for FR1 or Kssb>11 for FR2), UE may find the SS/PBCH block frequency positions with SIB1 or the frequency range where the network does not provide SS/PBCH block with SIB1.