SFN calculation in MIB

Hello Experts.

One question regarding SFN in MIB

MSB - 6bits LSB - 4 bits
1023 - 0011 1111 1111
0011 1111 → MSB
1111 → LSB

Remain 4 bits are derived from PBCH payload → i didn’t understood this point.

Why 4 bits are send separately?

And when it says PBCH payload, what does that mean?

MIB also goes through PBCH only ?

Also, why didn’t they make 6 or 5bits in LSB to send through PBCH payload?

It will reduce the MIB size further down.

Pbch load means th3 content of PBCH.
MIB is layer 3 content of layer 1 PBCH.

Because MIB in 5G is 24 bits, are needed for something else cannot be occupied by 5-6 bits of LSB of SFN.

4 is enough.

MIB is L3 generated by CU and PBCH is L1 generated by DU
Using this technique, CU will generate MIB less frequent since its contents will not change every frame